eng.harran.edu.treng.harran.edu.tr/~hbulut/introduction.docx · Web viewIntroduction The process by...

1

Introduction

The process by which fresh air is introduced and contaminated air is removed from an occupied space is termed ventilation.

Purposes of Ventilation

The purposes of ventilation are:

1. To provide a continuous supply of oxygen necessary for human existence.

2. To remove the products of respiration and occupation, that is; heat, moisture and carbon dioxide from people.

o At rest a normal adult inhales between 0.10 and 0.12 litre/s of air. o The exhaled breath contains between 3% and 4% of carbon dioxide, which is equal to 0.003 to 0.005 litre/s. o The amount of heat from occupants is about 100 Watts sensible and 40 watts latent heat from a sedentary worker. o The amount of moisture produced by a sedentary person is about 59g of water vapour per hour.

3. To remove contaminants such as:

Water vapour

Heat and smells from cooking

Gases and vapours from industrial processes.

Formaldehyde from; insulation foam, furnishings, wallpaper, carpets, resin in wood products and plasterboard.

2

Outdoor aerosol pollutants such as; smoke, soot, mist, fumes, pollen, plant fibres, mould spores, viruses and bacteria

Indoor aerosol pollutants such as; carpet fibres, furniture fibres, clothing fibres, skin flakes, mites, viruses and bacteria.

In practice most ventilation systems for rooms inhabited by people dilute contaminants such as Heat and Carbon Dioxide to acceptable limits for short and medium term exposure.

cal Ventilation System

A typical plant layout is shown below.

The photograph below shows a typical ventilation fan.

Incoming

Air from outside

Exhaust air to outside

Outgoing Air

Exhaust airIncoming Air

Room

Fan

3

Ventilation Rates

The building regulations require those habitable rooms and toilets are to be vented by natural or mechanical means.

Natural Ventilation

A habitable room requires one or more ventilation openings, the total area of which must not be less than 1/20 th. of the floor area of the room, and some part of the opening must be more than 1.7 metres above floor level.

4

When ventilation is by mechanical means, one air change per hour must be provided to habitable rooms and three air changes per hour to bathrooms and kitchens.

Design Criteria

To design a ventilation system, the engineer has to meet two basic requirements:

1. To supply fresh air for the occupants

2. To change the air in the room sufficiently so that smells, fumes and contaminants are removed.

Ventilation Rates in CIBSE guide

The following table gives Ventilation Rates for buildings.

Table 3.1 CIBSE Guide B2 (2001) Summary of recommendations

(Extract from Table)

Building sector Sectionnumber Recommendations

Animal husbandry 3.24.1 See Table 3.20Assembly halls 3.3 See Table 3.6Atria 3.4 See section 3.4.3Broadcasting studios 3.5 6 -10ACH (but heat gain should be assessed)Call centres 3.24.2 4 - 6 ACH (but heat gain should be assessed)

5

Catering (inc. commercial kitchens) 3.6 30 - 40 ACHCleanrooms 3.7 See Tables 3.11 and 3.12Communal residential buildings 3.8 0.5 - 1 ACHComputer rooms 3.9 See Table 3.13Court rooms 3.24.3 As for typically naturally ventilated buildingsDarkrooms (photographic) 3.24.4 6 - 10 ACH (but heat gain should be assessed)Dealing rooms 3.24.5 As offices for ventilation (but heat gain should be assessed)Dwellings (inc. high-rise dwellings) 3.10 0.5 - 1 ACHFactories and warehouses 3.11 See 3.11.1 for regulatory requirementsHigh-rise (non-domestic) buildings 3.12 4 - 6 ACH for office areas; up to 10ACH for meeting space.Horticulture 3.24.6 30 - 50 litres/s/m2 for greenhouses (45 - 60 ACH)Hospitals and health care buildings 3.13 See Table 3.15Hotels 3.14 10 - 15 ACH minimum for guest rooms with en-suite bathroomsIndustrial ventilation 3.15 Sufficient to minimise airborne contaminationLaboratories 3.16 6 - 15 ACH (allowance must be made for fume cupboards)Museums, libraries and art galleries 3.17 Depends on nature of exhibitsOffices 3.2 See Tables 3.2 and 3.3Plant rooms 3.18 Specific regulations apply, see section 3.18Schools and educational buildings 3.19 See Table 3.18Shops and retail premises 3.20 5 - 8 litres/s per personSports centres (inc. swimming pools) 3.21 See Table 3.19

Standards rooms 3.24.7 45 - 60 ACHToilets 3.22 Building Regulations apply; opening windows of area 1/20th of floor

area or mechanical ventilation at 6 litres/s per WC or 3 ACH minimum for non-domestic buildings; opening windows area 1/20th

6

of floor area (1/30th in Scotland) or mechanical extract at 6 litres/s (3 ACH in Scotland) minimum for dwellings.

Transportation buildings (inc. carparks) 3.23 6 ACH for car parks (normal operation)

10 ACH (fire conditions)

The Table below gives Ventilation rates required to limit CO2 concentration where level of activity is known.

Table 3.2 CIBSE Guide B2 (2001) Ventilation rates required to limit CO2 concentration for differing activity levels

Activity Minimum ventilation requirementLitres /s per person

0.5% CO2 limit 0.25% CO2 limitSeated quietly 0.8 1.8

Light work 1.3 – 2.6 2.8 – 5.6

Moderate work 2.6 – 3.9 N/A

Heavy work 3.9 – 5.3 N/A

Very heavy work 5.3 – 6.4 N/A

The following table gives fresh air rates.

Table 3.3 CIBSE Guide B2 (2001) Recommended outdoor air supply rates for sedentary occupants.

7

Level of Smoking Outdoor air supply rate (litre/s per person)

No smoking 8Some smoking 16Heavy smoking 24Very heavy smoking 36

The table below is an extract from Table 3.6 and gives rates for Assembly Halls and Auditoria

Design Requirements : Assembly halls and auditoria

Parameter Design requirement

Fresh air ventilation rates To suit occupancy levels

Air change rate

3 – 4 air changes per hour for displacement strategy

6 – 10 air changes per hour for high level mechanical strategy

Ventilation Calculations

8

The following formulae may be used:

1. For General Mechanical Ventilation

Ventilation rate (m3/h) = Air Change Rate (/h) x Room Volume (m3)

Air Change Rate (/h) comes from CIBSE Guide B2 Table 3.1

Ventilation rate (m3/s) = Ventilation rate (m3/h) / 3600

2. For Calculating Fresh Air Ventilation Rates

Fresh Air Rate (m3/s) = Fresh Air rate per person (l/s/p) x number of occupants

Fresh Air rate per person (l/s/p) comes from CIBSE Guide B2 Table 3.3.

9

Ventilation SystemsNatural ventilation cannot be relied upon to always provide enough fresh air to meet requirements.

Also more control can be obtained by using fans to supply air to a space or to remove contaminated air from a space.

Some mechanical ventilation systems use fans for both supplying and extracting air, thus mechanical ventilation systems may be classified as follows:

1. Supply system

2. Extract system

3. Balanced system.

10

1. Supply Ventilation System

Fresh air is supplied to a space from outside as shown below; this air provides oxygen for breathing and ventilation for occupants. Air is removed from the space by ‘natural’ means since the room is pressurised by the supply air.

Exhaust to outside

ExhaustSupply Air

Room

Supply Fan

Fresh Air In

11

In some cases it is advantageous to heat incoming air to offset fabric losses and avoid cold draughts in winter.This is known as a plenum system and is useful in large rooms with a high ceiling, which can be difficult to heat with radiators.

Exhaust to outside

Heater Battery

ExhaustWarm Supply Air

Room

Supply Fan

Fresh Air In

12

2. Extract Ventilation System

The principal function of an extract ventilation system is the removal of an unwanted contaminant, whether it is solid, gaseous or thermal.Air is extracted from the space and replaced by fresh air entering from outside; the space is under negative pressure; therefore air is naturally drawn into the building as shown below.

The photograph below shows a typical domestic extract hood.

Exhaust to outside

Supply Air

Room

Extract Fan

Fresh Air In

13

In industrial ventilation, airborne dust, toxic fumes, vapours and excessive heat have to be removed.

This is sometimes carried out at source, thus minimising the contamination of the occupied space. The spread of contaminant is decreased by installing hoods or canopies over the source and connecting these to the extract ventilation system as shown below.

14

The photograph below shows a typical commercial kitchen extract canopy.

15

Kitchen Extract

In kitchen extract ventilation it may be necessary to install a fire damper and enclose the duct which passes through upper floors in a fire resisting shaft.

This is because there is a possible danger of fire and smoke spreading throughout a building in the ductwork system.

Obviously when designing for the event of a fire in the building one should be careful to provide a safe and protective ventilation installation.

16

Consult all relevant standards and get as much advice as possible especially from the local Fire Authority.

A simple extract system for a kitchen is shown below.

The photograph below shows a typical commercial kitchen extract canopy incorporating lights and removable grease filters.

17

3. Balanced Ventilation Systems

A balanced system enables full control of ventilation to be achieved by the use of separate mechanical supply and extract systems.

It is usual to provide a surplus of supply air over extract air so as to maintain the pressure in the building at a slightly higher pressure than outside.

This minimises natural infiltration which reduces the likelihood of draughts.

A typical system is shown below:

18

It is a good idea to filter outside air so that atmospheric pollutants are excluded.

Also in winter, cold outside air may cause discomfort so the fresh air is heated. This can be used to offset heat losses thus providing the means of heating the building or room as shown below.

This is the same as a plenum heating system with return air system.

19

In most balanced systems, the supply air quantity, which is required, works out to be much more than that needed for fresh air supply to occupants.

It is possible therefore, to recirculate some of the extract air back into the supply duct to make use of the heat which it contains as shown below.

20

The amounts of fresh air in each section of ductwork are controlled by dampers, which can be set during commissioning so that the design quantity of air with the correct proportion of fresh air is supplied to the space.

21

Recirculation only works if the air has not been contaminated in the space. In kitchens, toilets, smoke filled spaces, etc., where the air contains odours or other contaminants all the extract air is removed and no recirculation takes place.

Filters are usually fitted in supply and balanced ventilation systems to remove any airborne particles in the fresh air intake duct.

A finer filter may be installed in a balanced ventilation system after the mix point to remove dust generated within the space.

The photographs below show typical filters.

The bag filters are for collecting fine particles of dust and are sometimes referred to as fine filters.

22

Natural Ventilation SystemsNatural ventilation is ventilation without the assistance of fans or other mechanical air moving equipment.

Natural ventilation uses no energy or little energy therefore reduces building running costs.

Air moves naturally due to the buoyancy effect when a temperature difference exists and less dense air rises.

This is called the stack effect.

Air also moves unassisted by wind.

These effects can be utilised in a building to create a ‘free’ ventilation system that requires no fans.

Some systems incorporate fans and are partially natural ventilation but with a greater degree of control.

Natural ventilation systems in use

In large buildings where large amounts of air need to be changed in rooms, natural ventilation can be used if special features are used.

Also is commercial building the ventilation system should be controlled to meet comfort criteria in rooms.

23

This can be achieved by careful building design and using technology to provide adequate ventilation.

The diagram below shows a building layout that uses the stack effect to increase natural ventilation.

Exhaust outlets at roof level can be disguised or used as a feature of building design as shown below.

24

Low level vents are shown below

25

Another natural ventilation system uses the sun to assist air movement.The vertical shafts in the building are glass fronted so that the sun heats up the air inside and causes it to rise out the openings at roof level. The high level openings in this case are stainless steel chimneys. As air flows out of the chimneys at roof level replacement air is drawn from the rooms into the shaft and thus naturally ventilated. This is shown below.

26

One of the difficulties with Natural Ventilation is that buildings have to be specially designed to incorporate features to assist air flow; these features may not look very good from the inside and outside and are sometimes disguised.

Effectiveness

The effectiveness of natural ventilation for commercial buildings depends on several criteria. These are wind strength and direction, size of openings, air temperatures and height of building. For effective controlled ventilation the designer should not rely solely on the wind but more on the stack effect and air controls.

Dampers can be used to control air entering and/or exiting a natural ventilation system. These dampers could be linked to occupancy sensors, temperature sensors, time switches and other weather sensors to give automatic control of ventilation which is the key to a useful system.The diagram below shows some of the features of a Natural Ventilation system for a four-storey building.

27

Through flow ventilation in rooms

Central Atrium

Naturally Ventilated Four Storey Building

....

....

....

....

Room outlet louvre with

Fresh air intake with motorised dampers

Sensor to operate dampers

..

..

..

..

..

..

..

..

Warm extract air

28

Designing Natural Ventilation Systems

CIBSE guide Applications Manual AM10 (1997) Natural Ventilation in non-domestic buildings gives design details. Section 4.2 of AM10 gives details of how to control Natural ventilation systems.Section 5 gives methods of calculating flow rates for wind driven and stack effect ventilation.

Stack Driven Ventilation Calculations

Stack ventilation calculations in the simplest form ignore wind effects, although these can be allowed for in a more complex analysis.

The pressures developed in stack systems can be determined from the following formula.

Ps = - ins . g . T ins (h2 – h1) (1/Tout - 1/Tins)

where;

Ps = Stack Effect Pressure (Pa)

ins = Air density inside stack (kg/m3)

g = Acceleration due to gravity (9.81 m/s2)

h1 = Height of inlet of stack above datum (m)

29

h2 = Height of outlet of stack above datum (m)

Tout = Temperature of air outside stack (oK)

Tins = Temperature of air inside stack (oK)

The equation below can be used to determine air flow rates in stack driven ventilation or the opening areas required.

Q = Cd . A [ ( 2 / insins . g . (hnpl – h ) (Tins - Tout / Tins ) ]

where;

Q = Air flow rate through a large opening (m3/s)

Cd = Discharge coefficient (0.61 for large openings)

A = Opening area (m2)



hnpl = Height of neutral pressure level above datum (m)

h = Height of opening above datum (m)



30

Neutral Pressure Level

This is where the outside pressure equals the internal pressure.

At this level there would be no flow of air in or out of the building.

This is usually high up in a building otherwise the stack effect would not work.

The neutral pressure level for most buildings is about 0.25 metres above the level of the top floor ceiling.

Temperatures

The internal room temperatures need to be calculated since in summer heat gains elevate the room temperature.

This can be done using software where summertime temperature can be predicted along with required air flow rates to keep room temperatures to acceptable levels.

The HEVACOMP software package and other programmes may be used.

Outside summer temperatures may be obtained from the CIBSE guide A section 2.

It would seem that the outdoor temperature in summer rarely exceeds 27oC, and if the temperature does rise above 27oC it is only for a maximum of 4 days in the south of England and less than one day in the north of the U.K.

If a solar chimney is used to assist stack suction pressure then the temperature inside the stack would have to be altered.

31

It is important to obtain accurate inside and outside temperatures since this difference creates the driving force inside the stack or the pressure difference to move air up the stack to outside.

Example 1

Calculate the ventilation opening area required in a Stack ventilation system for the building shown below.

Datum

2 mOutside air temperature 25oC

28oC

28oC

25oC

25oC

1 m

4 m ..

..

9 m

Stack

Warm extract air

Lecture Room 1

Lecture Room 2

Neutral Pressure Level0.25 m

..

..

Fresh air intake

Naturally Ventilated Two Storey Building with Stack

32

DATA:

The flow rate required each room is 4 air changes per hour.

Each lecture room measures internally 24 m x 10 m x 4m high.

ANSWER:

Air flow rate for each room Q = Room volume x Air change rate / 3600

Q = 24 x 10 x 4 x 4 / 3600

Q = 960 x 4 / 3600 = 1.07 m3/s

Rearranging above formula for Area (A) gives;

A = Q / Cd [ ( 2 / ins) ins . g . (h npl – h) (Tins - Tout / Tins ) ]

For Ground floor room;

A = 1.07 / 0.61 [ ( 2 / 1.1656 ) 1.1656 x 9.81 ( 9 – 1) ( 301 – 298 / 301 )

A = 1.07 / 0.61 [ 1.716 x 1.1656 x 9.81 x 8 x 0.00997 ]

33

A = 1.07 / 0.61 [ 0.19557 x 8 ]

A = 1.07 / 0.61 x 1.565

A = 1.121 m2.

For First floor room;

A = 1.07 / 0.61 [ ( 2 / 1.1656 ) 1.1656 x 9.81 ( 9 – 5) ( 301 – 298 / 301 )

A = 1.07 / 0.61 [ 0.19557 x 4 ]

A = 1.07 / 0.61 x 0.782

A = 2.242 m2.

Note: The upper floor has less stack suction pressure so openings are larger.

34

Example 2

Calculate the ventilation opening area required and the size of fresh air louvre required in a Stack ventilation system for the building shown below.

Datum

Outside air temperature 23oC

26oCNeutral Pressure Level

4 m

Warm extract air

26oC

23oC

1 m

3 m ..8 m

Class room

..Fresh air intake louvre

Naturally Ventilated Single Storey

Stack

35

DATA:

The flow rate required for the Class room is 10 air changes per hour.

The Class room measures internally 18 m x 10 m x 4m high.

The Fresh air louvre has a 50% free area.

ANSWER:


Q = 18 x 10 x 4 x 10 / 3600

Q = 720 x 10 / 3600 = 2.0 m3/s

Rearranging above formula for Area (A) gives;


Naturally Ventilated Single Storey

36

A = 2.0 / 0.61 [ ( 2 / 1.1605 ) 1.1605 x 9.81 ( 8 – 1) ( 299 – 296 / 299 )

A = 2.0 / 0.61 [ 1.723 x 1.1605 x 9.81 x 7 x 0.01003 ]

A = 2.0 / 0.61 [ 1.377 ]

A = 2.0 / 0.84

A = 2.38 m2 fresh air area required

The fresh air louvre has a 50% free area so the size of louvre is;

Louvre area = fresh air area / ( percent free area / 100 )

Louvre area = 2.38 / ( 50 / 100 )

Louvre area = 4.76 m2.

Stack Outlet

The opening at the top of a stack can be sized in a similar manner to the fresh air inlets.

37

The height difference in the formula is between the NPL and the stack outlet.

The flow through the stack outlet is the sum of all the flows through the rooms in a building feeding the stack.

A outlet = Q total / Cd [ ( 2 / ins) ins . g . (h – h npl) (Tins - Tout / Tins ) ]where;

Q total = Total air flow rate through stack (m3/s)

Cd = Discharge coefficient (0.61 for large openings)

A outlet = Stack outlet area (m2)



hnpl = Height of neutral pressure level above datum (m)

h = Height of stack outlet above datum (m)



38

Example 3

Calculate the stack outlet opening area required in the system given in Example 1.

Datum

Fresh air intake

2.38 m2 free area

2 mOutside air temperature 25oC

28oC

28oC

25oC

25oC

1 m

4 m ..

..

9 m

Stack

Warm extract air

Lecture Room 1

Lecture Room 2

Neutral Pressure Level0.25 m

..

..

Fresh air intake

4.76 m2 free area

Naturally Ventilated Two Storey Building with Stack

39

DATA:

The flow rate required each room is 4 air changes per hour.

Each lecture room measures internally 24 m x 10 m x 4m high.

ANSWER:

Air flow rate for each room Q = 1.07 m3/s (already calculated in Ex.1)

Total air flow rate Q total = 1.07 x 2 = 2.14 m3/s

A outlet = Q total / Cd [ ( 2 / ins) ins . g . (h – h npl) (Tins - Tout / Tins ) ]

A = 2.14 / 0.61 [ ( 2 / 1.1656 ) 1.1656 x 9.81 ( 11 – 9) ( 301 – 298 / 301 )

A = 2.14 / 0.61 [ 1.716 x 1.1656 x 9.81 x 2 x 0.00997 ]

A = 2.14 / 0.61 [ 0.19557 x 2 ]

40

A = 2.14 / 0.61 x 0.391

A = 8.97 m2.

Fittings Pressure Drop

In a rigorous analysis of a stack ventilation system the pressure drop from fittings such as intake and exhaust louvres should not exceed the driving pressure from the stack and the stack pressure drop.

The driving pressure (dPs) can be found from the formula at the beginning of this section and the pressure drop from the stack and fittings can be determined by the normal method for ductwork fittings.

Using Curves for Q/A

The following curves may be used to calculate air flow rates if the areas of openings are known.

42

Natural Ventilation SystemsExample 4

Size the fresh air intake and shaft outlet openings for the building shown below.

(a) Use the formulae given below to calculate the openings based on summer temperatures and compare this with the results given in Figure 5.3, given in the text.

(b) Use Figure 5.4 to estimate winter opening areas required for spring and winter conditions.

(c) Calculate air velocity rates at fresh air intakes in summer and winter.

43

28oC

4.0 m

2.0 m

4.0 m

Office 3

Office 2

25oC

..

..

..

..

Datum

2.0 mOutside air temperature 25oC

Warm extract air

0.25 m

28oC

28oC14.0 m

Stack

Office 1

25oC

Neutral Pressure Level

..

..

Fresh air intakes

Naturally Ventilated Three Storey Building with Stack

44

DATA:

The rooms measure 12 m x 12 m x 4m high.

Occupancy = 60 people

Summertime: The flow rate required for each Office is 8 air changes per hour in summer.

This is to reduce internal temperature rise due to heat gains.

Spring& Winter: The flow rate required for each Office is 3 air changes per hour.

This is to supply occupants with sufficient fresh air calculated as follows;

8 l/s/person x 60 people = 480 l/s = 0.48 m3/s = 1728 m3/h / room volume 576 m3 = 3 AC/h

The inside and outside spring temperatures are 10oC and 0oC respectively.

The inside and outside winter temperatures are 21oC and 0oC respectively.

ANSWER (a):


Q = 12 x 12 x 4 x 8 / 3600

Q = 576 x 8 / 3600 = 1.28 m3/s

45


Fresh Air Openings:

For Ground floor room;

A = 1.28 / 0.61 [ ( 2 / 1.1656 ) 1.1656 x 9.81 ( 14 – 2) ( 301 – 298 / 301 )

A = 1.28 / 0.61 [ 1.716 x 1.1656 x 9.81 x 12 x 0.00997 ]

A = 1.28 / 0.61 [ 0.19557 x 12 ]

A = 1.28 / 0.61 x 2.347

A = 0.89 m2.

For First floor room;

A = 1.28 / 0.61 [ 0.19557 . (14 – 6 ) ]

A = 1.28 / 0.61 [ 0.19557 x 8 ]

46

A = 1.28 / 0.61 x 1.565

A = 1.34 m2.

For Second floor room;

A = 1.28 / 0.61 [ 0.19557 . (14 – 10 ) ]

A = 1.28 / 0.61 [ 0.19557 x 4 ]

A = 1.28 / 0.61 x 0.782

A = 2.68 m2.

Stack Outlet Opening:

Air flow rate for each room Q = 1.28 m3/s

Total air flow rate Q total = 1.28 x 3 = 3.84 m3/s

47

A outlet = Q total / Cd [ ( 2 / ins) ins . g . (h – h npl) (Tins - Tout / Tins ) ]

A = 3.84 / 0.61 [ ( 2 / 1.1656 ) 1.1656 x 9.81 ( 16 – 14) ( 301 – 298 / 301 )

A = 3.84 / 0.61 [ 1.716 x 1.1656 x 9.81 x 2 x 0.00997 ]

A = 3.84 / 0.61 [ 0.19557 x 2 ]

A = 3.84 / 0.61 x 0.391

A = 3.84 / 0.2385

A = 16.1 m2.

A table can be drawn to accommodate data for summer time.

Inside to outside temperature difference = 28oC - 25oC = 3deg.C

Building LevelAir flow rate (Q)

Area (A) by Calculation

Height from opening to

Q/A from graph below A/Q

Area by Graph

= A/Q x Q (m2)

48

(m3/s) (m2)

NPL

(m) (m3/s/m2)

Ground floor fresh air opening 1.28 0.89 12 0.92 1.087 1.39

First floor fresh air opening 1.28 1.34 8 0.77 1.299 1.66

Second floor fresh air opening 1.28 2.68 4 0.55 1.818 2.33

Stack outlet 3.84 16.10 2 0.37 2.703 10.38

49

ANSWER (b):

50

A table can be drawn for spring and winter conditions.

The air flow rate from DATA = 0.48 m3/s

Spring inside to outside temperature difference = 10oC - 0oC = 10deg.C

Winter inside to outside temperature difference = 21oC - 0oC = 21deg.C

Building Level

Air flow rate (Q)

(m3/s)

Height from opening to

NPL

(m)

Condition

Q/A from graph below

(m3/s/m2)

A/Q

Area by Graph

= A/Q x Q (m2)

Compare summer Area by Graph

(m2)

Ground floor fresh air opening 0.48 12

Spring 1.75 0.571 0.271.39

Winter 2.50 0.400 0.19

First floor fresh air opening 0.48 8

Spring 1.40 0.714 0.341.66

Winter 2.05 0.488 0.23

Second floor fresh air opening 0.48 4

Spring 1.00 1.000 0.482.33

Winter 1.45 0.690 0.33

Stack outlet 1.44 2Spring 0.51 1.961 2.82

10.38Winter 0.75 1.333 1.92

51

Note that the fresh air and stack outlet areas required in winter are less than in spring.

Also the spring and winter areas are much less than the summer areas.

This means that automatic dampers would need to be used to change the air flows.

ANSWER (c):

Air velocity (m/s) = Volume flow rate (m3/s) / Cross sectional area (m2)

Building Level ConditionAir flow rate (Q)

(m3/s)

Area by Graph

(m2)

Air velocity at inlet

(m/s)

Ground floor fresh air openingSummer 1.28 1.39 0.92

Winter 0.48 0.19 2.53

52

First floor fresh air openingSummer 1.28 1.66 0.77

Winter 0.48 0.23 2.09

Second floor fresh air openingSummer 1.28 2.33 0.55Winter 0.48 0.33 1.46

It would be important to check that the velocity in winter does not exceed comfort criteria in the room.

The manufacturer’s catalogue for the fresh air intake louvres should give appropriate data.

Types of FansThere are several types of fan to choose from in ventilation.These are:

1. Propeller2. Axial flow3. Centrifugal4. Mixed flow

56

1. Propeller FanUsed in situations where there is minimal resistance to air flow.Typical outputs are; up to 4 m3/s and up to 250 Pa pressure.Fan efficiency is low at about 40%.Suitable for wall, window and roof fans where the intake and discharge are free from obstacles.Can move large volumes of air.Low installation cost.

2. Axial Flow FanHigh volume flow rate is possible with this type of fan with high efficiency, about 60% to 65%.Typical outputs are; up to 20 m3/s and up to 700 Pa pressure.The fan is cased in a simple enclosure with the motor housed internally or externally.

57

Aerofoil blades can be used to increase efficiency.Adjustable pitch blades can be used for greater flexibility.Ductwork can be simply connected to the flange at either end of the fan.

3. Centrifugal FanHigh pressure air flow is possible with this type of fan.Used in air handling units and other situations to overcome high resistance to air flow.The impeller is made of thin blades which are either forward or backward curved.The air changes direction by 90 degrees in a centrifugal fan so more space is required.

58

Usually the motor is placed external to the casing and a vee belt and pulley drive is commonly used.

Centrifugal BladesCentrifugal curved fan blades generally have higher efficiencies than if a plain flat blade is used. The efficiency of a fan with forward curved blades is about 50% to 60%.The forward curve has a scoop effect on the air thus a higher volume may be handled.

FORWARD CURVED BLADE

Blade

Direction of rotation

59

Backward curved blades offer even better efficiency, 70% to 75%.This improves airflow through the blade and reduces shock and eddy losses. High pressures can be developed with backward curved blades.Even further improvements may be made by using an aerofoil section blade in which case the efficiency may be 80% to 85%.Another feature of backward curved blades is their non-overloading characteristic.

A disadvantage is the high blade tip speed, necessary to obtain a comparable rate of discharge to forward curved blades, makes the fan noisy.

4. Mixed Flow Fan

Flat section non-overloading

Static Pressure Characteristic

Fan Power Characteristic

Pressure or

Power

Volume Flow rateFAN CHARACTERISTIC CURVES

60

Mixed Flow fans can be used for return air, supply, or general ventilation applications where low sound is critical. As compared to similarly sized axial fans, a mixed flow fan can be 5-20 dB quieter.

Characteristics of Axial Flow and Centrifugal FansAxial Flow Fans1. Axial flow and backward-curved centrifugal fans have similar characteristics as shown below.

Axial flow fan

EfficiencyPower

Power

Backward-curved

Centrifugal Fan2.0

1.0

Fan Power

kW

61

2. The axial flow fan is very convenient from an installation point of view, it can be directly duct mounted even in restrictive areas but they tend to be noisy. This is because they run at a higher speed compared to a centrifugal fan.

3. Like the Backward-bladed centrifugal fan, the Axial flow fan has a self-limiting power curve as shown above.

Centrifugal Fans4. The backward curved centrifugal fan runs at a higher speed than the forward curved fan for the same output.

5. A forward-curved centrifugal fan may be liable to overloading because the power rises as the volume increases. An example of this in practice is if the main dampers are left wide open when the fan is first started up, too much air will be handled and the excessive power absorbed will overload the driving motor.

6. The backward–curved fan is less liable to over-loading than the forward-curved centrifugal fan and it is also able to deliver a relatively constant amount of air as the system resistance varies. The power of a backward curved fan reaches a peak and then begins to fall, this is called the self-limiting characteristic. This is shown below.

CENTRIFUGAL FAN CHARACTERISTICS

Forward-curved Centrifugal fan

PowerPower

Backward-curved

Centrifugal Fan

3.02.01.

2.0

1.0

Airflow

m3/s

Fan Power

kW

62

7. A backward-curved centrifugal fan must run at higher speed to deliver the same amount of air as a forward-curved fan because of the shape of the impeller blades and the direction of rotation.

8. The backward-bladed fan is used in high velocity systems where high pressures are required and is often made with aerofoil blades to increase efficiency.

9. Up to about 750 N/m2 fan pressure, the forward-curved centrifugal fan tends to be quieter and cheaper. Above this value of pressure backward-curved fans take over.

10. Choosing a Fan 11.12.To choose a suitable fan one must look at the performance curves.13.Performance curves are found in fan catalogues.14.15.These curves show the pressure developed by a fan at a given flow rate.16.The pressure to be developed by the fan is found from duct sizing data (See DUCT SIZING section) and the flow rate is

found from design data (See VENTILATION DESIGN section). 17.The operating point of the system is marked as a point on the curve.18.19. Example 1 20.21.The example below shows a system operating point of 250 Pascals (Pa) pressure and 0.48 (m3/s) flow rate.

63

22.

23.Go to the curve above the operating point, this is the fan curve for the appropriate fan.24.The fan size is chosen as a 250mm-diameter fan (1350 r.p.m. speed).25.26. Example 2 27.28.The example below shows a system operating point of 320 Pascals (Pa) pressure and 1.25 (m3/s) flow rate.

64

29.

30.The fan performance curve for a 400mm-diameter fan will be suitable for the requirements for this example since the curve is above the operating point.

31.32.The fan size is chosen as a 400mm-diameter fan (1350 r.p.m. speed).33.34. Example 3 35.36.An axial flow fan is required for a ventilation system for a Workshop.37.Four fans are represented below in the four curves – 2 green and 2 red curves.

65

38.The left-hand diagram shows fans with 4-pole electric motors, and the right hand diagram shows fans with 2-pole electric motors.

39.40.Four pole electric motors are slower than two pole motors, in this example 4-pole is at 1420 r.p.m. and 2-pole is at 2840

r.p.m.41.The system operating point requirements are 100 Pascals (Pa) pressure 42.and 0.60 (m3/s) flow rate.

43.

44. The fan size is chosen as a 350mm-diameter fan (1420 r.p.m. speed).The electric motor for this fan has a 4-pole winding and will run at 1420 r.p.m. which will be slower than a 2-pole motor and therefore quieter.

45.

66

46. Fan Laws and Running Costs 47.48.The Fan Laws are as follows:49.50. No.1 Speed / Volume 51.52.53.54. Where;55. N = Fan speed (rev. per minute or r.p.m.)56. Q = Volume flow rate of air (m3/s)57.58.This means that fan speed and volume flow rate of air are directly proportional.59.60. No.2 Speed / Pressure 61.62.63.64.65. Where;66. N = Fan speed (rev. per minute or r.p.m.)67. p = Fan pressure (N/m2)68.69.This means that as the fan speed is doubled, for example, the pressure developed is raised by a factor of 4.70.71. No.3 Speed / Power 72.73.74.

= Q2

Q1

N2

N1

2

= p2

p1

N2

N1

3

= P2

P1

N2

N1

67

75.76. Where;77. N = Fan speed (rev. per minute or r.p.m.)78. P = Fan power (Watts)79.80.This means that as the fan speed is doubled, for example, the power required to drive the fan is raised by a factor of 8.81.The above three laws may be written differently to aid calculations, as follows;82.83. No.1 Speed / Volume 84.85.86.87.88.89. No.2 Speed / Pressure 90.91.92.93.94.95. No.3 Speed / Power 96.97.98.99.100.101.102.103.

N1

N2=Q2 Q1

N1N2 =Q2

Q1

or

=p2 p1

N2

N1

2

or

=N2

N1

p2

p1

1/2

=P2 P1

N2

N1

3

or

=N2

N1

P2

P1

1/3

22222222222222222222222222222222222222222222222222222222222222222222222

33333333333333333333333333333333333333333333333333333333333333333333333

68

104. Fan Running Costs 105.106. Running costs can be examined as follows;107.108. Fan power (W) = Fan pressure (N/m2 or Pa) x Air Volume flow rate (m3/s) / Efficiency109.110.111. Annual running cost (£/annum) = Fan power (kW) x Hours run per annum x Electrical price £ per

kWh 112.113. For example; if a fan runs for 2496 hours per year and delivers 0.625 m3/s against a pressure of 200 Pa, then

the annual running costs can be calculated as follows;114. Take electricity cost at 11.2 p/kWh = 0.112 £/kWh.115.116. Fan power (W) = Fan pressure (N/m2 or Pa) x Air Volume flow rate (m3/s) / Efficiency117.118. Fan power (W) = 200 x 0.625 / (say) 0.6119.120. Fan power (W) = 208 Watts.121.122. Fan power (kW) = 0.208 kW.123.124. Annual running cost (£/annum) = 0.208 x 2496 x 0.112125.

69

126. Annual running cost (£) = £58.15 per annum

Ventilation Design

Design Methodology

When considering ventilation design the following approach could be adopted before sizing begins and the following questions should be considered:

1. What areas need ventilation?

The contaminants should be listed for these areas.

2. What type of system should be used, supply, extract or balanced?

3. Are there any alternative systems to consider?

4. Is air conditioning necessary in the building?

If air conditioning is necessary then should it be incorporated into the ventilation system?

5. Where should the fan(s) and plant be installed?

70

6. What type of fan(s) and plant should be used?

7. Is a separate heating system necessary?

8. What type of control system should be used?

9. What type of air distribution system should be used, upward or downward?

10. Have I considered what will happen in the event of a fire in the building?

11. Have I considered the noise from fans?

After all the above questions have been answered the sizing process may commence.

Sizing

The sizing procedure is as follows:

1. Calculate Ventilation rates.2. Decide on number of fans and grilles/diffusers.

3. Draw scale layout drawing:

71

Position fan(s).

Lay out ductwork.

Lay out grilles and diffusers.

Indicate flow rates on drawing.

4. Size ductwork

5. Size fan

6. Size grilles and diffusers.

Design Criteria

To design a ventilation system, the engineer has to meet two basic requirements:

1. To change the air in the room sufficiently so that smells, fumes and contaminants are removed. (See Table 3.1)

2. To supply fresh air for the occupants. (See Table 3.3)

72

1. Ventilation Rates



Building sectorSection

numberRecommendations

Animal husbandry 3.24.1 See Table 3.20Assembly halls 3.3 See Table 3.6Atria 3.4 See section 3.4.3Broadcasting studios 3.5 6 -10ACH (but heat gain should be assessed)Call centres 3.24.2 4 - 6 ACH (but heat gain should be assessed)Catering (inc. commercial kitchens) 3.6 30 - 40 ACHCleanrooms 3.7 See Tables 3.11 and 3.12Communal residential buildings 3.8 0.5 - 1 ACHComputer rooms 3.9 See Table 3.13Court rooms 3.24.3 As for typically naturally ventilated buildingsDarkrooms (photographic) 3.24.4 6 - 10 ACH (but heat gain should be assessed)Dealing rooms 3.24.5 As offices for ventilation (but heat gain should be assessed)Dwellings (inc. high-rise dwellings) 3.10 0.5 - 1 ACHFactories and warehouses 3.11 See 3.11.1 for regulatory requirementsHigh-rise (non-domestic) buildings 3.12 4 - 6 ACH for office areas; up to 10ACH for meeting space.Horticulture 3.24.6 30 - 50 litres/s/m2 for greenhouses (45 - 60 ACH)

Hospitals and health care buildings 3.13 See Table 3.15

73

Hotels 3.14 10 - 15 ACH minimum for guest rooms with en-suite bathroomsIndustrial ventilation 3.15 Sufficient to minimise airborne contaminationLaboratories 3.16 6 - 15 ACH (allowance must be made for fume cupboards)Museums, libraries and art galleries 3.17 Depends on nature of exhibitsOffices 3.2 See Tables 3.2 and 3.3Plant rooms 3.18 Specific regulations apply, see section 3.18Schools and educational buildings 3.19 See Table 3.18Shops and retail premises 3.20 5 - 8 litres/s per personSports centres (inc. swimming pools) 3.21 See Table 3.19Standards rooms 3.24.7 45 - 60 ACH

Toilets 3.22

Building Regulations apply; opening windows of area 1/20th of floor area or mechanical ventilation at 6 litres/s per WC or 3 ACH minimum for non-domestic buildings; opening windows area 1/20th of floor area (1/30th in Scotland) or mechanical extract at 6 litres/s (3 ACH in Scotland) minimum for dwellings.

Transportation buildings (inc. carparks) 3.236 ACH for car parks (normal operation)

10 ACH (fire conditions)

The following table gives fresh air rates.

Table 3.3 CIBSE Guide B2 (2001) Recommended outdoor air supply rates for sedentary occupants.

74

Level of Smoking Outdoor air supply rate (litre/s per person)

No smoking 8Some smoking 16Heavy smoking 24Very heavy smoking 36

The table below is an extract from Table 3.6 CIBSE Guide B2 (2001) and gives rates for Assembly Halls and Auditoria

Design Requirements : Assembly halls and auditoria

Parameter Design requirement

Fresh air ventilation rates To suit occupancy levels

Air change rate

3 – 4 air changes per hour for displacement strategy

6 – 10 air changes per hour for high level mechanical strategy

75

The following Table gives suitable duct air velocities in various buildings.

BuildingAir Velocity (m/s)

Main Duct BranchDomestic 3 2Auditoria 4 3Hotel bedroom, Conference hall 5 3Private office, Library, Hospital ward 6 4General office, Restaurant, Dept. store 7.5 5Cafeteria, Supermarket, Machine room 9 6Factory, Workshop 10-12 7.5

For Extract ventilation systems the rate in air changes per hour is obtained from Table 3.1 above.

76

A typical extract system is shown below.

For Balanced with No Recirculation ventilation systems the fresh air rate is obtained from Table 3.3 and the supply air rate in air changes per hour is obtained from Table 3.1.

The larger of the two rates is then chosen.

The Extract Rate may vary slightly from the supply air rate depending on whether a positive or negative pressure is desirable.

FanExhaust Extract

air rate

ROOM

EXTRACT VENTILATION

Exhaust Extract air rate

ROOM

Supply Air rate

Fresh Air rate

77

For Balanced with Recirculation ventilation systems the fresh air rate is obtained from

Table 3.3 and the supply air the rate in air changes per hour is obtained from Table 3.1.

The Recirculation Rate is the Supply Air Rate minus the Fresh Air Rate.

Extract air rate

Exhaust ROOM

BALANCED VENTILATION WITH RECIRC. AIR QUANTITIES

Supply Air Rate

Recirculation Air

Fresh Air Rate

78

For an Air Conditioning system the supply air flow rate for cooling is found from the following formulae:

m = H / (Cp x (tr –ts))where;

H = Sensible heat gain (kW)m = mass flow rate of air (kg/s)

Cp = Specific heat capacity of air (1.005 kJ/kg K)tr = room temperature (oC)ts = supply air temperature (oC)

Convert this to a volume flow rate:Volume flow rate (m3/s) = mass flow rate (kg/s) / density of air (kg/m3) Convert this to an Air Change rate for comparison.Supply Air Rate (AC/h) = Volume Flow Rate (m3/h) / Room Volume (m3)

If this rate less than the Air Change Rate given in Table 3.1 CIBSE guide, then use the higher value.

Heater Cooling

Extract air rate

Exhaust ROOM

AIR CONDITIONING SYSTEM WITH AIR QUANTITIES

Supply Air Rate

Recirculation Air

Fresh Air Rate

79

1. Ventilation Calculations


1.1 For General Mechanical Ventilation


Air Change Rate (/h) comes from CIBSE Guide B2 (2001) Table 3.1


1.2 For Calculating Fresh Air Ventilation Rates

Fresh Air Rate (m3/s) = Fresh Air rate per person (l/s/p) x number of occupants

Fresh Air rate per person (l/s/p) comes from CIBSE Guide B2 (2001) Table 3.3 (for most buildings).

80

2. Number of Fans and Grilles

Several fans are often better than one since its makes the ventilation system more flexible.

Also the air to be supplied or removed may be in different areas of a room or building where individual fans can be more effective.

The number of grilles or diffusers may depend on the ceiling layout, lighting layout and amount to air to be transferred.

Sometimes it is necessary to complete a preliminary grille size to decide on the final number in a room.

3. Drawings

Accurate, scaled plan drawings are necessary for installation, fabrication, estimating and commissioning a ventilation scheme.

Sometimes elevations, sections and details are also necessary especially in complicated installations.

Drawings should show:

1. Flow rates of air.

81

2. Ductwork to scale with sizes indicated.

3. Air flow direction

4. Items of plant

Other details such as; builder’s work, support details, fan specification, grille and diffuser details, louvre details, plant details, insulation, ductwork specification may be given on a drawing or in a specification document.

4. Size Ductwork

See duct sizing in PLANT SIZING section of these notes.

5. Size Fan

See fan sizing in FANS section of these notes.

6. Size Grilles and Diffusers.

82

See grille sizing in GRILLES section of these notes.

Good Ventilation DesignGood ventilation design should have the following features:

1. Not noisy2. Concealed3. No draughts4. Efficient fan5. Good control of air flow with dampers and appropriate diffusers.6. Good control of room temperature.7. Appropriate duct sizes.8. Well supported ducts and equipment.9. Prevent spread of smoke in the event of a fire with smoke/fire dampers.10.Ensure that supply air is clean by using a filter.11.Ensure that vermin cannot enter the duct system by using a bird/insect screen in the fresh air intake.12.Minimise risk of infection in some buildings (e.g. hospital) by having no recirculation duct.13.Use recirculation duct in some buildings to save energy.14.Use appropriate air change rates to meet room requirements.15.Use appropriate fresh air rate to meet room occupants’ requirements.16.Use suitable system to fit in with building aesthetics.17.Avoid duct leaks by using proper jointing method.

83

Ventilation (and Air Conditioning) of Large Rooms

Large rooms and buildings such as lecture rooms; theatres, conference rooms, exhibition halls, auditoria and halls for public gatherings need careful designing for ventilation or air conditioning.

Two of the most important design criteria are:o The ventilation rate.o The distribution of air in the space.

Ventilation Rate

The ventilation air change rate (Theatres) given in the CIBSE guide Table B2.3 is 6-10 air changes per hour. This table is in the Ventilation Design section of these notes. This is total air supply rate. We can choose a higher figure if necessary.

The amount of fresh air supplied per occupant is given in Table B2.2.This is 8 litres per second per person for non-smoking areas and 16 litres per second per person for smoking areas. If we assume that a building is non-smoking, therefore 8 l/s/p of fresh air is required for the fresh air amount.These two figures can determine the air flow rates in the ventilation system.


84



Example

The THEATRE building shown below requires mechanical ventilation.Determine the air flow rates in the system.CIBSE guide table B2.3 gives a supply ventilation rate of 6 - 10 air changes per hour.Use a supply air ventilation rate of 10 air changes per hour for this example.DATA:Room width = 20 metresOccupancy = 750 seats.

SECTION THROUGH LARGE HALL

Ceiling

Stage

1.8 m

9m

25 m

85

AIR FLOW RATESRoom Volume (m3) = L x W x HRoom Volume (m3) = 25 x 20 x 9Ventilation rate (m3/h) = 4500 m3

Ventilation rate (m3/s) = 12.5 m3/sVentilation rate (m3/h) = Air Change Rate (/h) x Room Volume (m3)Ventilation rate (m3/h) = 10 x 4500

= 45,000 m3/h

Choose an Outdoor Air Recommended minima Rate from Table B2.2 for a room with non-smoking = 8 l/s/p

Fresh Air Rate (l/s) = Number of occupants x Outdoor Supply air per Person (l/s)Fresh Air Rate (l/s) = 750 x 8 = 6000 l/sFresh Air Rate (m3/s) = Fresh Air Rate(l/s) / 1000Fresh Air Rate (m3/s) = 6000 / 1000 = 6 m3/s

For comparison convert Fresh Air Rate to an Air Change Rate.

Fresh Air Rate (m3/h) = Fresh Air Rate (m3/s) x 3600Fresh Air Rate (m3/h) = 6 x 3600 = 21,600 m3/h

Fresh Air Rate (AC/h) = Fresh Air Rate (m3/h) / Room Volume (m3) Fresh Air Rate (AC/h) = 21,600 / 4500 = 4.8 AC /hFresh Air Rate (AC/h) = 4.8 air changes of fresh air per hour

86

The supply air ventilation rate is 10 air changes per hour.These figures can now be put on a drawing for clarity.

AIR FLOW RATES FOR LARGE HALL

Recirculated Air

5.2 AC/h52%

Return air

10 AC/h

100%

Supply air

10 Ac/h

100%

Return air at ceiling grilles Air supplied from

under seats. Flexible branches are omitted for clarity

Exhaust Air

4.8 AC/h

Fresh Air

4.8 AC/h

Stage

87

Ventilation Rate if Air Conditioning is Used

The previous ventilation rate may change if air conditioning is necessary.

If air conditioning is to be incorporated into a design then the heat gains should be calculated to ascertain the supply air rate into the building.

The following formula may be used to calculate the supply air rate.

H = m x Cp x (tr -ts)Where;

H = Sensible heat gain (kW)m = mass flow rate of air (kg/s)

Cp = Specific heat capacity of air (1.005 kJ/kg K)tr = room temperature (oC)ts = supply air temperature (oC)

If the heat gain in the previous example is 135 kW, the room air temperature is 22 oC and the supply air temperature in summer is 14oC, then the mass flow rate of supply air is

m = H / (Cp x (tr –ts))m = 135 / (1.005 x (22 –14))m = 16.8 kg/s

Convert this to a volume flow rate:

Volume flow rate (m3/s) = mass flow rate (kg/s) / density of air (kg/m3) Volume flow rate (m3/s) = 16.8 / 1.2 kg/m3 =14m3/s

88

Volume flow rate (m3/h)= 14 x 3600 = 50,400 m3/h

Convert this to an Air Change rate for comparison.

Supply Air Rate (AC/h) = Volume Flow Rate (m3/h) / Room Volume (m3) Supply Air Rate (AC/h) = 50400 / 4500 = 11.2 ac/h This rate is more than the Air Change Rate given in Table 2.3 CIBSE guide of 10 ac/h.For good practice we will use the higher of the two values in the actual design which is: 11.2 Air Changes per hour.

Air Distribution

Air distribution in a large hall poses some difficulties which must be addressed.The object of good air distribution is to allow air to be supplied to all parts of the room, to avoid draughts and to have effective air mixing in a space.This means that all the areas in a room should have the benefit of cool air if required or warm air if required. Also all occupants should have a supply of fresh air.

In a heating system warm air will naturally rise so supplying at low level and extracting at high level is a good way to employ buoyancy effects. In an air conditioning system cool air may be supplied at high level and allowed to drop to low level to be extracted.The drawing below shows upward ventilation in an Auditorium.One advantage is that small adjustable louvres near seats can be used to give the occupants control over their environment.

89

The velocity of air supplied at low level is about 0.5 m/s to reduce draughts at floor outlets. Outlets may also be positioned at low level on side walls.

AIR DISTRIBUTION

Lecture Hall/

Theatre

Space for supply air ductwork

Space for return air ductwork

Air is returned from high level through ceiling grilles.

Air is supplied at low level, possibly under seats through small louvres. Velocity must be low to avoid draughts

90

The upward system can be cheaper to install since propeller fans can be used in the roof to extract the air.

The drawing below shows a typical example of Downward ventilation in a Concert Hall.One advantage of this type of air distribution is that there is no supply at seating level therefore no possibility of draughts and no difficulty in installing ductwork under seat areas. Another advantage is that if air conditioning is used, the cool supply air will tend to drop in the warmer room air, thus assisting distribution.

DOWNWARD AIR DISTRIBUTION

Space required

for ductwork

Low

level extract

Low

Level extract

High level extract

Lecture Hall/

Theatre

Space required

for ductwork

Space for ductwork

Air is supplied through diffusers mounted in the ceiling.

91

One disadvantage however, is that low level extract grilles are required and these can be difficult to assimilate into the building fabric. Also vertical ductwork is required from these low level grilles and this takes up floor and wall space and normally requires to be hidden from view.

In some halls with a very high ceiling the velocity of air supplied at high level needs to be high to get the air down into the occupancy space. The problem with high velocity at diffusers is noise. The diffusers must be carefully chosen to minimise the risk of noise yet maintain an adequate ‘throw’ to distribute the air down at low level. The air diffusers become like nozzles in a jet so that adequate velocity is reached.

An outlet velocity of about 3-5 m/s is available from jet diffusers. An example of this type of ventilation is used in Usher Hall Edinburgh where 65 jet diffusers are installed, each handling 280 litres/s at an outlet velocity of approximately 3 m/s.

Because of their aerodynamic design jet diffusers give a long throw even at high outlet volumes. In some diffusers the jet can be adjusted over 360 As these jets handle different supply temperatures the jet can be oriented upwards or downwards for heating or cooling mode. This can be achieved by hand or by electric motor.

92

The diagram below shows a sizing diagram and photo of a jet diffuser.

95

AIR DISTRIBUTION

Air is supplied into rooms via air diffusers.

These are used to direct the air in one, two, three or four directions if square or rectangular units are installed.

Return or Extract air is removed via a grille. These are not required to direct the air and are simpler in construction.

For continuity and aesthetic reasons diffusers may be used for both supply and extract.

It is difficult to supply or extract air at low level in rooms because draughts may result from low-level supplies and vertical ductwork in rooms is not generally feasible.

It is therefore usual to supply or extract air from the ceiling as shown below, but this means that air velocities at diffusers must be sufficient to ensure adequate mixing within the space otherwise short-circuiting of grilles may occur.

Short-circuiting

96

Circular diffusers may be used in some areas especially if they blend into the ceiling layout or room shape.

Another option is to use linear slot diffusers which can give a continuous long system of air distribution. This may be an advantage in some types of room.

In rooms with a very high ceiling it may be necessary to use a nozzle which acts like a jet to force the air down to the occupied space. One difficulty associated with high velocity jets is noise production. This is one of the aspects of air distribution which must be addressed by the engineer.

VENTILATION SYSTEM LAYOUT

When designing ventilation systems it must be remembered that since most of the ductwork is installed within ceiling spaces, it is a good idea to liaise closely with the Architect at the early stages of design so that space requirements are met.

Good mixing

97

Figure 1 below shows a typical balanced ventilation system layout. One method, which can be adopted, is to run main supply and return ductwork in the ceiling space above corridors and the branches into adjoining rooms.

Ceiling heights in corridors may be lowered to accommodate larger ductwork.

It is common practice to use flexible ductwork to grilles and diffusers. These have several advantages; sharp bends are eliminated, flexible ductwork has better sound adsorption qualities compared to sheet metal, it is easier to install especially in a congested area and it allows more freedom in positioning the grille or diffuser.

A plenum box can be used to connect the ductwork system to the grille or diffuser. This has a larger cross sectional area than the connecting duct and reduces the air velocity before it enters the diffuser thus giving better air distribution over total diffuser area. A less expensive method is to use diffusers with factory-fitted square or round necks, which can be fitted directly to the flexible duct connection.

Each grille or diffuser should have a damper to regulate flow of air. This damper can be an opposed blade type incorporated within the diffuser or a butterfly volume control damper (VCD) positioned in the branch duct. All dampers require access.

99

Figure 2 below shows a typical ventilation system using linear or slot diffusers. These have the advantage that air can be distributed over a wider area and the ‘coanda’ effect can be utilised. This is where cool air ‘sticks’ to the ceiling before falling gradually into the space below.

BUILDING REGULATIONS

The Regulations cover Natural and Mechanical Ventilation

The areas to which Mechanical Ventilation applies are:Domestic

Habitable room

Kitchen

Utility

Bathroom

Other Sanitary Accommodation separate from Bathroom

Non-domestic

Occupiable room

100

Kitchen

Bathroom including shower rooms

Other Internal Sanitary Accommodation

To implement the requirements of an EU Workplace Directive part K has been split into Domestic and Non-domestic buildings.As a consequence of this, the term occupiable rooms is introduced to provide for rooms such as:

Office, Workroom, Classroom, Hotel bedroom

INTERACTION OF MECHANICAL EXTRACT WITH OPEN-FLUED HEATING APPLIANCES.

Mechanical ventilation can suck flue gases back into a room from a heating appliance. This could happen at a leaking joint in a flue spigot or flue section or entrance into a chimney for example. Recommendations:

(a) Seal flue outlet from oil-fired boiler

(b) For gas-fired heating appliance - max. 20 litres/second mechanical ventilation rate.

(c) No mechanical extract in same room as solid fuel appliance.

101

DOMESTIC BUILDINGS

Use CONTROLLABLE BACKGROUND VENTILATION everywhere by installing:

Trickle Vents in windows

Air Bricks with “hit and miss” ventilator

Other

Use a PASSIVE STACK VENTILATION system.

Passive Stack Ventilation (PSV) may be used as an alternative to mechanical extract ventilation in:

KITCHENS

BATHROOMS

SANITARY ACCOMMODATION

Passive Stack Ventilation (PSV)System shall be designed and constructed in accordance with BRE information Paper 13/94.

DefinitionPassive Stack Ventilation (PSV) is a ventilation system using ducts

from the ceiling of rooms to terminals on the roof, which operate by a combination of the natural stack effect, ie the movement of air due to the difference in temperature between inside and outside, and the effect of wind passing over the roof of the dwelling.

102

OPEN FLUED HEATING APPLIANCESOpen-flued heating appliances take their combustion from the room where they are installed and so contribute to the

extract ventilation when in operation.Mechanical extract ventilation need not be provided in a room where there is either:

(a) an open flued appliance which provides the primary source of heating, cooking or hot water production, or

(b) an open flued appliance min. 125mm dia (approx) with permanently open air inlets.

OPENING WINDOW IN KITCHENS

An opening window is to be provided as a supplement to extract ventilation.

1/20 th of floor area.

103

VENTILATION FOR COMMERCIAL AND INDUSTRIAL - NON-DOMESTIC BUILDINGS

Note - Large Kitchens see CIBSE guide B2.3 and B2.11

For example B2.3 - Kitchens, hotel and industrial

Total air supply rate, not less than 17.5 litres per m2 offloor space, not less than 20 air changes per hour.

VENTILATION OF SPECIALIST ACTIVITIES

(a) School or other educational establishment.

(b) Workplaces

(c) Hospitals

(d) Building Services plant rooms

(e) Rest rooms where smoking is permitted

(f) Commercial Kitchens

104

(a) School or other educational establishment. Fume Cupboards - DFE Design Note 29

(b) Workplaces

Specific workplaces and work processes - HSE Guidance Note EH22

(c) Hospitals

Specific rooms in hospitals - DHSS Activity Data Base and Department of Health Building Notes 4,21 and 46.

(d) Building Services plant rooms If emergency ventilation is required to disperse contaminating gas releases

HSE Guidance Note EH22.

(e) Rest rooms where smoking is permitted

Min. 16 litres/second per person in addition to requirements of Table 3.1(f) Commercial Kitchens

Examples:For a Kitchen which measures 5 metres x 4 metres x 2.4 metres high, 60 litres per second is equivalent to 4.5 air changes per hour.For a bathroom which measures 3 metres x 2 metres x 2.4 metres high, 15 litres per second is equivalent to 3.75 air changes per hour.

105

Other guidelines:

C.I.B.S.E. Guide

BRE Digest 398 1994

Continuous Mechanical Ventilation in Dwellings:Design, installation and operation.

1. Discusses ducted extract and/or supply ventilation running continuously.

2. Less reliance on natural ventilation.

3. Better control of ventilation rate means less wasted energy.

4. Opportunity for heat recovery system as shown in diagram.

5. Dwelling must be fairly airtight.

6. A total ventilation rate for the whole house of 0.5 air changes per hour is typical.

106

BS 5925:1991

Code of Practice for

Ventilation principles and designing for natural ventilation

Continuous Mechanical Ventilation

Introduction

There are some advantages in having a ventilation system that operates continuously.Compared to natural ventilation, a continuous mechanical system has better control over air flows and air change rates. The air change rates vary in natural ventilation systems with wind direction, wind strength, air temperatures and height of building. This does not happen in mechanical systems.

A continuous system of ventilation is efficient at controlling humidity levels in buildings and in many cases is the answer for damp buildings.

The system runs for 24 hours per day continuously but does not consume a lot of electrical energy since fan power is small.

Operation

Air is extracted from areas of high contamination e.g. Toilets, Showers, Bathrooms, En-suites, Kitchen and Utility Rooms.

The air is removed and exhausted at high level usually through the roof.

107

Replacement air is either drawn in naturally as shown below or supplied by a fresh air supply fan.

Replacement air is normally able to be provided via structural air leakage with background ventilation openings like trickle vents not normally needed.

Simple Continuous Extract Ventilation System

House

Extract Fan

Replacement fresh air through windows and doors

Extract through ceiling

Roof Space

Exhaust at high level

108

To provide more control over the system, fresh air may be delivered into the building by a fan as shown below.

Fresh Air Supply at Ground level ceiling

Fresh Air Inlet

Supply Fan with filter

Simple Continuous Supply and Extract Ventilation System

House

Extract Fan

Extract through

ceiling

Roof Space


109

It is normal to supply air, with no extract, to ‘dry’ rooms such as; bedrooms, dining rooms, living rooms, etc.

The extract grilles are installed in ‘wet’ rooms such as; kitchens, bathrooms, shower rooms, utility rooms and WC’s.

If there are not enough ‘wet’ rooms an extract terminal may be positioned in a large ‘dry’ room along with a supply diffuser.

Air Flow Rates

A whole house ventilation rate of 0.5 air changes per hour is recommended.

Rooms with an extract grille will have a high rate, typically 2 AC/h to 5 AC/h.

Additional ventilation is required when showers and cookers are in operation to minimise build-up of humidity levels.

Total air flow rates are divided among the rooms on a floor area basis.

Some extract fans have a boost facility to enable more ventilation in times when contamination is occurring such as when a shower or a cooker is switched on.

110

Airtightness

BRE Digest 398 – Continuous Mechanical Ventilation in dwellings, recommends the building being as airtight as practicable for efficient operation.

Heat Recovery

A heat exchanger can be installed in the exhaust system to recover heat in the air.

This heat is then transferred into the fresh air system in the Roofspace.

Modern heat exchangers can have an efficiency of over 90% which makes them worthwhile.

This is probably the best continuous ventilation system for houses since little heat is lost and the system is well controlled.

111

Fresh Air Inlet

Supply Fan with filter

Simple Continuous Ventilation System with Heat Recovery

House

Heat Recovery Unit with fans

Extract through

ceiling

Roof Space


112

Noise

The system should be very quiet in operation since it runs during the night as well as during the day.

Typical Continuous Mechanical Extract Ventilation Unit

The following are some features of a typical unit;

Low energy fan (often d.c. motor).

2 No. 120mm diameter extract ducts with balancing dampers.

4 No. 100mm diameter extract ducts with balancing dampers.

Acoustic noise insulation.

Fan motor to be speed-variable with boost facility.

Boost facility to be switched from more than one point ( Bathroom light switch or Kitchen switch)

113

Fan to have adjustable ‘run-on’ facility.

Easy-access filter to protect fan and motor.

Air Supply Only Systems

Air can be supplied into a building from the Roofspace to reduce dampness and surface condensation.

This is the opposite of the previous forms of ventilation.

Roofspace air tends to be warm and dry and can be recirculated down into the living space below to reduce condensation.

This is shown below.

Ventilation at eaves level

House

Supply Fan

Supply through ceiling

Roof Space

Air Intake at high level in roofspace

114

Transfer grilles may be installed in Bathroom doors to assist air circulation.

Note:

Building Regulations should be consulted when designing a continuous mechanical ventilation system.

In Building Regulations (England& Wales) part F (2006) information is given for whole house ventilation e.g. ventilator areas and sizing of ventilators for rooms.

Extract

Provide continuous extract from wet rooms at the following rates, making no allowance for infiltration:

Kitchen - 13 litres/second

Bathroom, utility room - 8 litres/second

Sanitary accommodation (WCs) - 6 litres/second.

Control of Ventilation Systems in the Event of a Fire

115

It is important in large buildings to control a ventilation system in the event of a fire.Smoke from a fire can spread quickly through a building and this spread of smoke can be assisted by the ductwork system if incorrectly designed.Some of the strategies to prevent spread of smoke are as follows;

1. Switch off all fans automatically when a fire alarm control panel is activated.2. Provide ductwork fire dampers between fire zones and provide a system of automatic closure of dampers.3. Fire stop all holes around ductwork.4. Provide smoke roof vents.5. Provide pressurising fans.6. Providing smoke extract fans.

See CIBSE Guide E (2003) Fire Engineering for more details.It is always a good idea to consult the local Fire Authority for advice on control of ventilation in the event of a fire in a building.

Smoke Vents

Fire venting is a means of preventing the spread of flames and smoke in the event of a fire.

This is appropriate to buildings with large open floor areas such as; factories, warehouses.

A series of vents are opened automatically when a fire starts and these vents direct flames and smoke into the atmosphere.

116

The normal position for fire vents is on the rooftop and they operate automatically when a signal is received from a fire detector.

The aim of these fire vents is to allow occupants to escape and to contain a fire until the fire brigade takes other measures.Since most fatalities from fire are a result of smoke and fumes causing asphyxiation, the control of such fumes is important in fire fighting. Smoke can be distributed throughout a building by the action of wind, stack pressure and fire pressure and by ventilation plant. Whereas fire vents are used mainly in large warehouses and factories, smoke control is more important in office blocks and high occupancy buildings.

Factories, warehouses, shopping centres and large buildings are prone to uninhibited smoke spread in the event of a fire. To remove smoke at source a roof vent can be used.The vents are equipped with a 74ºC fusible link for automatic operation. There are various methods of opening the vents quickly to remove smoke, these are;

1. Spring2. Compressed air 3. Electric motor.

The advantage of torsion spring drives is that no electric energy is needed to open the covers or louvres automatically. The automatic device can be reset after the fire is extinguished.Closing the vents may be done by pull handles, worm and screw manual device or electric motors.

117

The opening of a smoke vent will aid people in their escape and assist the fire service to see and promptly tackle the source of fire.The heat removed prevents risk of an explosion, flash-over and distortion to the structural steel frame.

Fire

Open smoke vent

Smoke vents in factory roof

Heat and smoke Screens to reduce spread of smoke

118

Grilles and Diffusers

A grille is a device for supplying or extracting air vertically without any deflection.

A diffuser normally has profiled blades to direct the air at an angle as it leaves the unit into the space, as shown below.

Grilles and Diffusers can be manufactured in:

Aluminium Mild steel Stainless steel Plastic

Airflow AirflowRound neck for duct fitting

Plenum box Curved blades

Ceiling level

2-way diffuser

Straight blades

Extract grille

119

Finishes can be white, any colour, brushed or anodised aluminium, stainless steel or chrome.

Grilles and diffusers may be mounted in ceilings, floors, walls, doors and in ducts. Some are suitable for horizontal or vertical mounting while floors grilles tend to be especially strong to withstand foot traffic.

Types of Grille and Diffuser

There are several types of grille and diffuser to choose from, as follows;

Egg grate grille Bar grille Transfer grille Louvre bladed diffuser Straight bladed diffuser Linear slot diffuser

Less commonly used diffusers are as follows;

120

o Swirl diffusero Floor outlet diffusero Jet diffusero Punkah diffusero Barrel diffusero Perforated diffusero Valveso Plain face diffuser

Egg Crate Grille

The Egg Crate Grille is probably one of the simplest and cheapest grilles.

In some cases the plenum box above the grille is visible from the room below.

If air is to be removed by an extract ventilation system then a diffuser with profiled blades to direct the air is not necessary and an egg grate grille can be used.

121

Bar Grille

The blades of this type of grille are shaped as a bar compared to a narrow blade.

The bar profile may be ‘T’ shaped to reduce ‘see through’ visibility.

122

Some linear bar grilles have adjustable blades or angled blades to reduce ‘see through’.

123

Transfer Grille

These are often used in doors and walls to provide ventilation but stop the spread of smoke and fire should it occur. An intumescent fire damper is incorporated in some Transfer grilles as a means of isolation in the event of a fire.

124

Louvre Bladed Diffuser

These are used to supply air at ceiling level.

The curved blades deflect air in one, two, three or four directions depending on where the diffuser is situated.

125

Diffusers may also be circular as shown below.

126

Straight Bladed DiffuserThese are cheaper than Louvre Bladed Diffusers.

Some types have adjustable bladed as shown below.

Linear Slot Diffuser

These are used for an alternative air distribution pattern and for aesthetic reasons.

Air can be delivered around the perimeter of a room as opposed to point sources interposed in a ceiling space.

Linear slots can be used for return air as well as supply.

In many cases blanking plates are used to create ‘dummy’ slots so that a continuous linear effect is created.

127

One to four slot widths are common depending on how much air is to be delivered.

128

Some proprietary plenum boxes are quite tall are require a large ceiling void, to overcome this a special box can be manufactured to fit the available space.

Some Slot Diffusers have an adjustable air pattern so that the air distribution can match the room shape.

If sound insulation is required in an air conditioning system then a lined plenum box may provide sufficient sound attenuation.

There are some specialised grilles and diffusers as described below.

Swirl Diffuser

129

High turbulence occurs immediately within the proximity of the diffuser, which allows high air change rates to be successfully supplied into the room. With careful selection swirl diffusers can handle up to 30 air changes per hour while still satisfying relevant comfort criteria.

Floor Outlet Diffuser

There is an advantage in supplying cool air conditioned air at low level.

This can be achieved by floor outlets.

Floor diffusers are also useful in areas with raised floors so that ductwork can be accommodated under the floor.

Another area for floor outlets is under large areas of glass so that condensation and cold down draughts are eliminated.

130

Jet Diffuser

Used in areas where the ceiling is high and large air velocities are needed to reach the occupied space.

131

The high volume and long throw of these diffusers makes them suitable for large halls

Some jet diffusers can be reversible and rotatable which allows the air jet to be adjusted for both pattern and deflection.

Punkah Diffuser

Used to give flexible air direction, sometimes in the back of seats so that occupants have control over flow and direction of air.

132

Perforated Diffuser

Perforated diffusers are suitable for installation in 600mm ceiling tiles.

The face plate may be pivoted down to gain access.

The diffusers are constructed from aluminium or stainless steel the flat face plates are perforated, this means that they are easily cleaned and are used in some kitchens.

Valves

Valves are used where the air flow needs to be regulated or throttled and are suitable for supply and exhaust applications.

133

Diffuser Terminilogy

There are several terms used in diffuser and grille systems as follows;o Isovel - A contour of equal velocity.o Throw - The distance from the terminal to the position where the velocity has decayed to 0.5 m/s.

The lower velocities of 0.5 m/s and 0.25 m/s are important because this is near the comfort zone.o Spread - This is the width of the 0·5 m/s isovel.o Drop - This is the vertical distance from the centre-line of the terminal to the bottom edge of the 0·25 m/s

isovel.

134

The Coanda effect occurs when an air stream is discharged along an unobstructed flat surface. Air near the ceiling tends to cling to the ceiling because it has a low pressure caused by the friction loss with moving air in contact with the ceiling material.A projection may well destroy the Coanda effect causing the air stream to become detached prematurely from the surface; throw is reduced by about one-third when the Coanda effect does not occur.

Elevation

Diffuser outlet

Throw

Isovels

Air Flow

Room

0.5 m/s Isovel

Air Flow from a Diffuser into a Room

135

Sizing Diffusers

When sizing air diffusers the room air flow rate for ventilation is required.

This can be ascertained from the VENTILATION RATES section of these notes.

The following information refers to supply diffuser design but can be applied to return air diffusers.

Design Criteria for Diffuser Design

There are three criteria that determine diffuser size and number.

These are:

o Throw

o Pressure loss

o Noise level

136

The throw is discussed on the previous page.

Pressure loss is the pressure resistance to air flow and is required in duct and fan sizing calculations.

The maximum Noise level is determined for various room types.

Noise Rating (NR) levels are given in section NOISE AND NOISE LIMITS in these notes.

Example 1

Design the diffuser system for the supply and return air ventilation for the room shown below.

DATA:

Air change rate from Table A3.1 in CIBSE guide B2 (2001) is 6 – 10 ACH. Choose 10 ACH for this room.

The Noise Rating level from Table 1.17 CIBSE guide for Television (Audience Studio) is 25dB.

The throw required is Ceiling height 3.0 minus 0.8m = 2.2 metres.

Keep the pressure drop below 20Pa.

Use Waterloo Aircell M Series 4-way diffusers.

137

Ceiling height = 3.0 PLAN

6 metres

10 metresBroadcasting

138

Answer.

1. Calculate ventilation rate for room in litres / second.


Ventilation rate (m3/h) = 10 ACH x 10 x 6 x 3

Ventilation rate (m3/h) = 10 ACH x 180 m3.

Ventilation rate (m3/h) = 1800 m3/h


Ventilation rate (m3/s) = 1800 m3/h / 3600

Ventilation rate (m3/s) = 0.5 m3/s

Ventilation rate (l/s) = Ventilation rate (m3/3) x 1000

Ventilation rate (l/s) = 0.5 m3/s x 1000

Ventilation rate (l/s) = 500 l/s

139

2. Look at Waterloo catalogue and choose suitable sizes of diffusers to provide air into the room and meet the design criteria.

The design criteria are; Throw 2.2m, NR 25 and 20Pa.

If two diffusers are used each flow rate is; 500 / 2 = 250 l/s

From the sizing chart below for a flow rate of 270 l/s, an M450/M500 gives 2.5m throw, 33NR, 33 Pa.

140

This is too noisy with too much pressure drop.

If three diffusers are used each flow rate is; 500 / 3 = 167 l/s

From the sizing chart below for a flow rate of 180 l/s, an M450/M500 gives 2.0m throw, 23NR, 15 Pa.

This is good, the throw is just slightly less than 2.3m.

If four diffusers are used each flow rate is; 500 / 4 = 125 l/s

From the sizing chart below for a flow rate of 135 l/s, an M450/M500 gives 1.5m throw, - NR, 9 Pa.

The throw is not enough but the other criteria are met.

On the basis of the above data three M450/M500 diffusers are chosen.

141

3. Show diffuser layout on a drawing.

Return air diffusers

Supply diffusers

1.5 m

1.5 m

3.0 m

1.67 m1.67 m 3.33 m3.33 m

PLAN

6 metres

10 metres

142

The final diffuser layout should consider ceiling layout and lighting layout.

If ceiling tiles are used the diffusers position will be determined by the tile grid pattern.

Careful co-ordination is required to install all the equipment into modern ceilings including warning devices and other services.

Kitchen Ventilation

Kitchen’s need adequate ventilation to remove;o Heato Smellso Steam and vapours.o Fumes from gas burners.

DW 171

The HVCA produce DW/171 Standard for Kitchen Ventilation Systems (1999)

143

This document gives details of;

Air change rates and air flow rates. Kitchen Canopies, grease filters, lights Materials for ductwork Fan details, attenuators. Fire suppression Heat recovery Air conditioning Smoke extract

Ductwork Design

The HVCA publishes guide DW/l44 Specification for Sheet Metal Ductwork Low, medium and high pressure/velocity air systems (1998).This document provides information on how ductwork and components are to be manufactured.

Ductwork Classification

Ductwork is classified according to static pressure of the air as follows;

DuctworkClass

Pressure Classification

Static Pressure Limit (Pa)

Positive Negative

A Low Pressure 500 500

B Medium Pressure 1000 750

144

C High Pressure 2000 750

Information in DW144

This guide specifies details such as;

Classification of ductwork. Jointing methods, joint seals, seams, screws, welds, Standard sizes. Materials, sheet thickness. Stiffeners, turning vanes, splitters. Section lengths Supports. Dampers Air leakage. Insulation Standard drawing details and abbreviations.

Duct Sections

The maximum length of a duct section depends on the size of the longer side. The sections can be flanged at each end, transported to site and bolted together in-situ.The table below gives typical section lengths for rectangular ducts using flanged joints in a Low Pressure system.

145

Standard Sizes of Ducts.

Rectangular ducts no longer have standard sizes since modern machines can fold sheet metal to any required size.

Circular ducts have typical standard sizes as follows in millimetres diameter;63, 80, 100, 125, 150, 160, 180, 200, 224, 250, 280, 300, 315, 355, 400, 450, 500, 560, 630, 710, 800, 900, 1000, 1120, 1250, 1500.

Joint Class and Minimum

angle size for flange

(mm x mm)

Type of sheet

Length of longer side (mm)

400 600 800 1000 1250 1600 2000

Maximum Spacing between Flanges (mm)

J1Plain sheet

3000

1600 1250 625

Stiffened sheet 3000 1250 625

J2Plain sheet 2000 1600 1250 625

Stiffened sheet 3000 1600 1250 625

J3 - 25 x 3J4 - 30 x 4J5 - 40 x 4J6 - 50 x 5

Plain sheet 2000 1600 1250 1000 800 800

Stiffened sheet 3000 2000 1600 1250 1000 800

146

Clean Rooms

Introduction

Clean rooms are used in areas where airborne particles are detrimental to the process or activity in the room.The semiconductor industry, pharmaceutical industry, food production establishments, scientific research industry and hospitals often have Clean rooms.

Ventilation System

The system usually adopted for Clean rooms is a piston ventilation system where outdoor air is introduced and this pushes the contaminated room air in front of it to the extract system.

To keep the Supply air from mixing with room air the air velocity is kept as low as possible.

Piston Ventilation System

Exhaust Air

Supply Air

Room

147

This means that the contaminated room air is expelled, turbulence is minimised and fresh Supply air replaces the expelled air, thus creating a ‘clean’ indoor environment.

This can be achieved by supplying air through ‘laminar flow’ panels containing high efficiency particulate air (HEPA) filters or ultra-low penetration air (ULPA) filters. These panels with the filters are placed on the ceiling or wall depending on whether a down flow or cross flow system is adopted, as shown below.The room air velocity in these systems is about 0.4 m/s to 0.5 m/s.

Filters

HEPA filters have a very high filtration efficiency of 99.97% on particle sizes of 0.3m. ULPA filter efficiency is even better at 99.999% on 0.12 m particles.

HEPA filters

Piston Ventilation System with HEPA Filters

Exhaust Air

Supply Air

Room

148

Classification

Clean rooms are classified by American Federal Standard 209E.The maximum number of particles of each size per cubic foot of air is given for each class of Clean room.

ClassMaximum number of Particles /ft3

Particle Size≥ 0.1 m ≥ 0.2 m ≥ 0.3 m ≥ 0.5 m ≥ 5.0 m

1 35 7.5 3 1 -10 350 75 30 10 -100 - 750 300 100 -1,000 - - - 1,000 710,000 - - - 10,000 70100,000 - - - 100,000 700

ISO 14644-1 also gives details of standards for Clean rooms as shown below.

ClassMaximum number of Particles /m3 air

≥ 0.1 m ≥ 0.2 m ≥ 0.3 m ≥ 0.5 m ≥ 1.0 m ≥ 5.0 m

ISO 1 10 2

ISO 2 100 24 10 4

ISO 3 1,000 237 102 35 8

ISO 4 10,000 2,370 1,020 352 83

ISO 5 100,000 23,700 10,200 3,520 832 29

149

ISO 6 1,000,000 237,000 102,000 35,200 8,320 293

ISO 7 352,000 83,200 2,930

ISO 8 3,520,000 832,000 29,300

ISO 9 35,200,000 8,320,000 293,000

Airflow Patterns

The airflow patterns can be classified as either unidirectional or non-directional.ISO 14644-4 (2001) suggests that airflow patterns for Clean rooms of ISO Class 5 and cleaner are often unidirectional while non-directional and mixed flow is typical for ISO Class 6 and less clean IN operation.

Unidirectional airflow may be either vertical or horizontal.

Filters

Unidirectional Horizontal Airflow

Exhaust Air

Supply Air

Room

Filters

Unidirectional Vertical Airflow

Exhaust Air

Supply Air

Room

150

Toilet Ventilation Example

Design a ventilation system for the toilets in an Auditoria as shown in the accompanying drawings.

The Male Toilet and Female Toilet should each have separate extract ventilation systems to avoid ‘cross-talk’ and spread of smoke within the ductwork system.

The mechanical ventilation rate from Table 3.1 CIBSE Guide B2 (2001) is at least 3 air changes per hour.

For the purposes of this exercise use a value of 8 AC/h.

MALE TOILET

The volume of the Male Toilet is: 4.5 x 3.0 x 2.7 m = 36.45 m3

The ventilation rate in m3/h is:

Ventilation Rate (m3/h) = Room volume (m3) x air change rate (ac/h)

Ventilation Rate (m3/h ) = 36.45 m3 x 8 AC/h

= 291.6 m3/h. Divide by 3600

The ventilation rate is = 0.081 m3/s.

A simple method of duct sizing would be: Duct area = volume flow rate / air velocity.

An appropriate maximum air velocity for a toilet is 4 m/s. (See Ventilation - Ventilation Design section)

Therefore duct area = 0.081 / 4 = 0.02025 m2.

If a square duct is used then the duct size is: ( 0.02025 )0.5 = 0.142 m x 0.142 m

The nearest standard size is 0.150 m x 0.150 m or, 150mm x 150mm.

The drawing below shows the ductwork layout for the Toilets.

153

The flexible ductwork may be sized using the same technique as above.

Volume flow rate through each flexible = 0.081 m3/s / 2 = 0.041 m3/s.

A simple method of duct sizing would be: Duct area = volume flow rate / air velocity.

An appropriate air velocity for a flexible in this situation is 4 m/s

Therefore duct area = 0.041 / 4 = 0.01025 m2.

Cross sectional area = . d2 / 4 d = ( 4 . CSA / )0.5

Therefore flexible diameter = ( 4 x 0.01025 / )0.5 = 0.114 m = 114 mm diameter.

154

The nearest standard size of PVC flexible is 125mm.

The flexible size should be smaller than the duct to which it is attached to enable a boot or branch to be spot welded into position, as shown below.

The Female Toilet may be sized using the same method but since the physical dimensions are so close to the Male Toilet then the same duct sizes can be used, this also standardizes the complete ventilation system so that the two fans are also identical.

Fans

To choose suitable fans for this example the duct pressure drop and fittings pressure drop would be calculated.

If the total pressure drop is assumed to be 200 Pa and the flow rate is 0.081 m3/s (81 litres/s) then a suitable catalogue can be used to pick the fan.

See http://www.flaktwoods.com. Go to Other Fans section.

A tube fan or boxed fan can be used for this project.

Round neck for fixing flexible with jubilee clip

Branch

BRANCH CONNECTION TO DUCT

Flange is spot welded into position

Branch or pop for circular flexible

Section Through DuctSide Of Duct

http://www.flaktwoods.com/

155

Kitchen Ventilation Example

The following example is for a Kitchen in a small Restaurant building.Air is extracted from kitchens from the source of contaminant, that is, the cooking island incorporating ovens, cookers, fryers, water boilers, etc.The fan can be located in the duct.

Table 3.1 CIBSE Guide B2 (2001) recommends 30 to 40 AC/h for a Commercial Kitchen.

The ventilation extract system for a kitchen should be kept separate from all other ventilation systems.

This is because contaminants (smells from cooking, water vapour, steam, heat) are being removed.

No recirculation is used in a Kitchen.

1. Calculate a suitable duct size based on an air velocity of 5m/s.

The ventilation rate to be extracted from the kitchen in the example is:

30AC/h x 75 m3 = 2250 m3/h

About 2.0

Extract hood at high level

Cooking Equipment

ELEVATION

Extract ductExtract duct

Extract hood at high level

Ceiling height = 5.0m

PLAN

5.0 m

3.0 m

Kitchen Extract System

156

Volume flow rate 2250 m3/h / 3600 = 0.625 m3/s

The ductwork can be sized if an air velocity (in the duct) is chosen.

For a main duct in a kitchen 7.5 m/s is recommended. (See Ventilation - Ventilation Design section)

Duct cross sectional area= Volume flow rate / air velocity

= 0.625 m3/s / 7.5

= 0.083 m2

If a square duct is used the dimensions are:

(0.083) 0.5 = 0.289 m x 0.289 m = 289 mm x 289 mm

The next standard size is: 300 mm x 300 mm.

2. Re-calculate the duct size based on 40ACH.

The ventilation rate to be extracted from the kitchen in the example is:

40 AC/h x 75 m3 = 3000 m3/h

Volume flow rate 3000 m3/h / 3600 = 0.833 m3/s

157

The ductwork can be sized if an air velocity (in the duct) is chosen.

For a main duct in a kitchen 7.5 m/s is recommended. (See Ventilation - Ventilation Design section)

Duct cross sectional area= Volume flow rate / air velocity

= 0.833 m3/s / 7.5

= 0.111 m2

If a square duct is used the dimensions are:

(0.111) 0.5 = 0.333 m x 0.333 m = 333 mm x 333 mm

The next standard size is: 350 mm x 350 mm.

Air Change Rate

One way to understand air change rates is to say 30 AC/h is one room volume changed every;

60 minutes / 30 = 2 minutes.

If 40 AC/h is used then the room volume is changed every 1.5 minutes which means that smells have less chance to linger in the room.

Another aspect of air change rates is the cost of installation and running.

The more air changes the more cost.

Running Costs

158

Running costs can be examined as follows;

For the above example at 30 AC/h, if the fan pressure to be developed to overcome resistances is 200 Pa, then the fan power can be calculated as follows:

Fan power (W) = Fan pressure (N/m2 or Pa) x Air Volume flow rate (m3/s) / Efficiency

Fan power (W) = 200 x 0.625 / (say) 0.6

Fan power (W) = 208 Watts.

If the fan is operating for 8 hours per day x 6 days per week x 52 weeks per year = 2496 hours/ annum, the annual running cost can be calculated.

Annual running cost (£/annum) = Fan power (kW) x Hours run per annum x Electrical price per kWh (assume 0.112 £/kWh)

Annual running cost (£/annum) = 0.208 x 2496 x 0.112

Annual running cost (£) = £58.15 per annum

If 40 AC/h is used then the fan running cost is;

Fan power (W) = 200 x 0.833 / (say) 0.6

Fan power (W) = 278 Watts.

159

Annual running cost (£/annum) = 0.278 x 2496 x 0.112

Annual running cost (£) = £77.72 per annum

For a kitchen of this size the increase in duct size and fan running costs is not significant between 30 AC/h and 40 AC/h.

The situation may be different for large kitchens with larger duct and fan sizes.

Fan Selection

To choose a suitable fan for this example the duct pressure drop and fittings pressure drop would be calculated.

Normally the pressure drop is calculated as shown in the Duct Sizing section.

If the total pressure drop is assumed to be 200 Pa and the flow rate is 0.833 m3/s then a suitable catalogue can be used to pick the fan.

See http://www.flaktwoods.com. Go to Products – Fans – Axial Fans – JM Aerofoil section.

To download a technical brochure go to; Downloads – Document Library (Fans – Axial Fans – JM Aerofoil)

Open – Express International JM Aerofoil pdf document.

An Axial Flow fan would be suitable for this project since it can be mounted directly into the ductwork system.

Fan Duty: 0.833 m3/s against 200 Pa pressure.

Go to 31JM to 35JM 2 pole fan. A 31JM 16/2/5/34 sound level 61dB meets the criteria.

This fan is a 2-pole fan running at 2840 rev/min.

A 4-pole fan runs slower and is therefore quieter. Try for a 4-pole fan.

http://www.flaktwoods.com/

160

A Flaktwoods 56JM 16/4/5/28 4-pole fan runs at 1420 rev/min and has a 58dB noise output.

The fan diameter is 560mm. Choose this fan.

161

Design Example - Workshop

Design a ventilation system for the building shown below.The building is a small workshop used for general metal fabrication and welding.No welding booths are installed.If the building gets too warm the door may be opened for free cooling.

DATA:Building dimensions : 15.0 metres long x 7.0 metres wide x 5.0 metres high to eaves.Roof ridge height is 7.5 metres.The air change rate for mechanical ventilation is to be 6 air changes per hour.

Roller Shutter door

WORKSHOP

162

The building volume may be calculated from:

V = ( L x W x H to eaves ) + ( 0.5 x W x perpendicular height from eaves to ridge x L )

V = ( 15 x 7 x 5 ) + ( 0.5 x 7 x 2.5 x 15 )

V = 525 + 131.25 = 656.25 m3.

DESIGN METHODOLOGY

1. What areas need ventilation? Answer: The total area of the workshop.

The contaminants are welding fumes and heat.

2. What type of system should be used, supply, extract or balanced?

Answer: Extract.

3. Are there any alternative systems to consider?

Answer: No, supply air is not required.

4. Is air conditioning necessary in the building?

Answer: No, external door to be opened.

5. Where should the fan(s) and plant be installed?

Answer: On the roof.

6. What type of fan(s) and plant should be used?

Answer: Roof mounted units with weatherproof cowl.

Anti-backdraught shutters on each fan. No ductwork.

7. Is a separate heating system necessary?

Answer: No, not for extract only.

8. What type of control system should be used?

Answer: Variable speed fan(s) for a flexible system.

163

9. What type of air distribution system should be used, upward or downward?

Answer: Upward for extract systems, with duct mounted extract grilles.

10. Have I considered what will happen in the event of a fire in the building?

Answer: The fans could remove smoke in the event of a fire.

11. Have I considered the noise from fans?

Answer: In a workshop, fan noise is not a problem.

SIZING


1. Calculate Ventilation rates.2. Decide on number of fans and grilles/diffusers.3. Draw scale layout drawing:

Position fan(s).

Lay out ductwork.

Lay out grilles and diffusers.

Indicate flow rates on drawing.

4. Size ductwork 5. Size fan

6. Size grilles and diffusers.

1. VENTILATION RATES

For Extract ventilation systems the rate in air changes per hour is obtained for some buildings in Table 3.1.

There is no explicit rate given for Factories and warehouses or Industrial ventilation due to the variations in building design and usage.

164

The rate for this example is given in the question as 6 air changes per hour.


FOR GENERAL MECHANICAL VENTILATION

Ventilation rate (m3/h) = Air Change Rate (/h) x Room Volume (m3)= 6 x 656.25= 3937.5 m3/h

Ventilation rate (m3/s) = Ventilation rate (m3/h) / 3600= 3937.5 m3/h / 3600= 1.094 m3/s.

Three extract roof units are used so each will remove 1.094 / 3 = 0.365 m3/s.The resistance to air flow is negligible since there is no ductwork or grilles connected to the fans.It is a good idea to look at several types of extract unit to ascertain the best option for the job. A typical unit is shown below.

CHOOSING A FAN

To choose a suitable fan one must look at the performance curves.These curves show the pressure developed by a fan at a given flow rate.

The operating point of the system is marked as a point on the curve. To accurately establish a fan size the air flow resistance would have to be calculated (see DUCT SIZING).

165

In the absence of duct resistance data we can look at typical fans for the job. There are several options to choose from, for example, from the Woods Air Movement Ltd catalogue:

REF. FAN TYPESIZEDia.

(mm)

VOLUME FLOW RATE(m3/s)

PRESSURE(Pa)

SPEED(rpm)

SOUNDLEVEL

(dB)

Woods DSP 315 Propeller 315 0.37 32 1420 48

Woods DSM 330 Mixed flow 330 0.37 230 1300 57

Woods DSC 250 Centrifugal 250 0.37 135 1350 51

Woods DSJ 315 Axial flow 315 0.37 55 1420 49

From the above table, the propeller type fan and the axial flow fan produce 32Pa and 55 Pa pressure which is sufficient for the proposed system.The noise output from both fans is similar.I would choose the DSP 315 propeller type unit since a speed controller is available. The ductwork resistance can be calculated later to ascertain an accurate fan size.

3No. fan speed controllers

3No. Roof mounted extract units

167

Design Example – Restaurant

Scenario

The drawing below shows a Restaurant building with four rooms.It is the ground floor of a three storey building in a terrace of buildings.Other rooms such as Toilets are upstairs and have a working ventilation system already installed. The two external walls are shown on the drawing.The heat gains are not excessive and cooling by air conditioning can be avoided.A plastered suspended ceiling is to be incorporated into the Restaurant and Conference room.The ceiling in the Corridor, Kitchen and Food Prep. is suspended with 600mm square tiles.

Data

The level of cooking is Medium.

Complete the following tasks;

(a) Design a suitable ventilation system for the Restaurant shown below.(b) Select suitable air terminal devices for the ventilation system.(c) Size the Kitchen extract canopy.(d) Select suitable duct sizes and submit a duct sizing table for the above system.(e) Select suitable fans for the above system.(f) Design a suitable control system.(g) Discuss the builder’s work necessary to complete the ventilation system.

Corridor

Rear door and fire escape in street.

External wall

Kitchen Extract canopy

Food Prep. RoomKitchen

8m

2m 8m 8m

168

Answer

Design Methodology

Following the methodology as described in previous sections;

1. What areas need ventilation? All rooms

2. What type of system should be used, supply, extract or balanced?Choose one separate extract system for each room since this gives a very flexible design. Also no smells or noise can spread across the ducts from one room to the next.

3. Are there any alternative systems to consider?A balanced system could be considered to make up the air that is removed. This would require heater batteries to heat incoming air in winter to avoid discomfort. The cheaper alternative is an extract system with replacement air coming in through doors and openable windows.

4. Is air conditioning necessary in the building?No.

5. Where should the fan(s) and plant be installed?Fans should be located in accessible positions; in the corridor, above ceilings. Consider asking Architect to drop ceiling in Corridor, Kitchen and Food Prep. Room to facilitate fan positioning. See drawing below.

6. What type of fan(s) and plant should be used?In-duct axial flow fans take up less space than centrifugal fans.Possible use of wall mounted fan in Food Prep. Room.

7. Is a separate heating system necessary?Yes. Possibly a wet system with under floor heating or radiators.

8. What type of control system should be used?Fan speed control on all fans to give flexibility.

9. What type of air distribution system should be used, upward or downward? Upward with diffusers installed in suspended ceilings.

10. Have I considered what will happen in the event of a fire in the building?Use separate extract systems so that smoke will not spread from room to room through ducts. Consider fire dampers in ducts.

11. Have I considered the noise from fans?Yes, fans are positioned as far away from Conference room and restaurant as possible. Choose fans from catalogue with decibel levels less than 85dB if possible.

170

Kitchen Canopy

Extract Fans

Extract Fans

15m

8m

2m 8m 8m

171

After all the above questions have been answered the sizing process may commence.

Sizing


1. Calculate Ventilation rates.2. Decide on number of fans and grilles/diffusers, size canopy.3. Draw scale layout drawing:

Position fan(s).Lay out ductwork.Lay out grilles and diffusers.Indicate flow rates on drawing.

4. Size ductwork 5. Size fan6. Size grilles and diffusers.

1. Ventilation Rates



Building sector Sectionnumber Recommendations

Catering (inc. commercial kitchens) 3.6 30 - 40 ACH

Section 3.6 CIBSE Guide B2 (2001) gives details of ventilation requirements in Catering and Food Processing areas.

172

Table 3.9 gives Hood face velocities for Kitchen Extract Canopies.

Cooking Duty Hood Face Velocities (m/s)

Light 0.25Medium 0.40Heavy 0.50

More information may be obtained from Table B2.3 (CIBSE 1986) Ventilation requirements for a range of building types.

173

Room or building type Recommended fresh air supply rate

Recommended total air supply rateAir changes/hour unless otherwise

stated

Boardrooms, conference rooms

As required for occupants (with allowance for smoking)

6-10

Canteens As required for occupants 8-12

Dining Halls, Restaurants As required for occupants 10-15

Kitchens: hotel and industrial

As required for appliances

Not less than 17.5 litre/m2 of floor space, nor less than 20 air changes/h

Ventilation Calculations


For General Mechanical Ventilation


Air Change Rate (/h) comes from CIBSE Guide Table 3.1 (2001) and Table B2.3 (1986)


174

Restaurant

Room volume = 15 x 10 x 3.2 = 480 m3.Extract rate = 15 AC/h x 480 m3.Extract rate = 7200 m3/ hExtract rate = 7200 m3/ h / 3600 = 2.0 m3/sEstimate of Extract duct area (m2) = Volume flow rate (m3/s) / Air Velocity (m/s/) Extract duct area (m2) = 2.0 / 5 = 0.4 m2.Duct diameter = ( 4 x CSA ) / )0.5

Duct diameter = ( 4 x 0.4 ) / )0.5

Duct diameter = 0.714 mDuct diameter = 714 mmNearest standard diameter = 750mm.

Ventilation Food Prep Room

Volume = 8 x 8 x 3.2 = 204.8 m3

Ventilation Rate = 8 AC/hr (assume).Ventilation Rate = 8 x 204.8 = 1638 m3/hVentilation Rate = 0.46 m3/s (460 l/s)

Pressure Drop in ductwork:Total pressure drop = Pressure drop in straight duct + Pressure drop in fittings.Assume for this example total PD is 200 Pa.Extract FanFan Duty: Volume flow rate of 0.46 m3/s against a pressure of 200 Pa.Flakt-Woods Single Boxed fan type 355H.

175

Dimensions: 600mm x 600mm x 467mm high. 355mm diameter duct spigot.Will fit into 800mm ceiling space. Check steelwork drawings.Access through ceiling tile. Leave sufficient space on top to remove cover.Supported from concrete floor above with screwed rod.

Fire/Smoke: Only extracting from one room, no branches to other rooms.

Noise: No cross talk since only one room on system.Over 80 dB would be a noisy fan. This fan has an output of no more than 72dB at 125 Hz frequency.

Flexible Design: Use speed controller ME1.6.

Electrical Data: 220v-230v ,50Hz.,1-phase motor rated at 0.91 kW, starting current 8.1 amps. Motor speed 1150 rpm.

Extract DiffusersCeiling mounted 2No. Each extracting 230 l/s2No. Waterloo Aercell polymer diffusers 2600 (600mm x 600mm).Pressure drop = 20Pa.

176

Drawings for Ventilation Systems.A good drawing should show as much detail as is necessary and be clear and easy to read.It is a good idea to remember what the drawing is for or who is going to read it to obtain information.

The drawing may be used by; An installer or ventilation contractor. A draughtsman /CAD designer to produce a more detailed drawing. A main contractor or builder. A sub-contractor – electrical, controls, heating/plumbing. An estimator or quantity surveyor. A clerk of works. A commissioning engineer.

The following information and details should be produced in a ventilation drawing;

Drawing information

1. Draw outline of building.2. Show cavity wall3. Show internal partitions, doors and windows.4. Draw diffusers / grilles.5. Show kitchen canopy.6. Show ducts as 2 lines.7. Show branches as; shoes / boots / pops.8. Show direction of air flows.9. Add notes for; Duct sizes and air flow rates.10. Position fans11. Add note for fan size.

177

12. Add Index run identification i.e. sections.13. Add Title block showing;

Building Services Consultants name. Job, job address. Client Drawing description Drawn by Date Drawing Number Scale Revisions.

Further information to be given.1. Show other building details such as; stairs, lifts, structure, roof, roofspace, ceiling spaces, void spaces, shafts.2. Show steelwork details where appropriate.3. Show flexible ductwork.4. Show grid pattern for ceiling tiles and building grid if appropriate.5. Show table of diffusers and grilles.6. Show table of External louvers.7. Show table of Fans with duty and specification 8. Show furniture.9. Show Volume Control Dampers (VCD’s)10. Show fire dampers.11. Show Attenuators if appropriate.12. Show what symbols represent in a legend e.g. FD – Fire damper.13. Show any other equipment such as; AHU (Air Handling unit), Chiller, cassette unit, air cooled condenser, heater battery, cooling

coil, fan coil unit, etc.14. Installation Notes and fixing methods.15. References to other drawings e.g. steelwork drawing.

178

16. Drawing details of important areas e.g. fixing, weatherproofing.

Some of the above details may be given in a specification but it is a good idea to produce the same information again on a drawing if space permits to give a full picture of the job. One of the reasons for this is that the drawings or drawings that are used for installation purposes often don’t tell the whole story and mistakes can be made. Also installers don’t always have access to specification documents.One must remember that if changes are made to equipment then both specification and drawings should be updated.

179

Heating SystemsMost heating systems for buildings use hot water which is pumped through pipework from a boiler (or boilers) to heat emitters in the rooms.

This has proved to be cheaper than warm air heating because installing pipework is less expensive than ductwork and more equipment is necessary in warm air heating which increases installation costs.

It is possible to heat large spaces with warm air using fan convectors fed with hot water but these have several disadvantages, one of which is they tend to be noisy when running at high speed.

Radiators have proved to be the most common type of heat emitter for small to medium sized spaces, although under floor heating systems are becoming more popular.

Most radiators act more like natural convectors because of the extended finned surface which is readily obtainable.

2-Pipe Heating Systems

Older heating systems sometimes used a 1-pipe system of distribution. This has been superseded by the more effective 2-pipe system as shown below.

RETURNFLOW

Bottom return connection with lock shield valve for setting.

Top connection to radiator with hand wheel valve

Flow branch to radiator

Single pipe

RadiatorRadiatorRadiator

180

The 2-pipe heating system is most common today and uses a pump to circulate water at about 80oC through heat emitters as shown below.

Steel or copper pipework can be used; copper for small installations and steel for larger sizes (above 50mm diameter pipe) or where there is risk of damage to copper pipework.

Recent developments have made possible the use of 'plastic' polypropylene pipework and this has the advantage that it will not corrode.

Single pipe

181

Taking Care of ExpansionAs water heats up it expands by about 3% if it’s original volume.To allow for this increase in volume an expansion vessel is required.This is a steel cylinder that has a flexible diaphragm inside that allows for water movement.

Isolating Valves

Pipework

Pump Space Heating

Emitters

Safety valve

Expansion vessel

Flexible connection

Mains water supply

Isolating Valve

Pressure Gauge

Double Check Valve

Return

Flow

2-Pipe Heating System

Boiler

182

The system is filled from a Mains Water connection and is slightly pressurised to about 1 bar pressure.

This pressure does not help the water circulation but is there so that negative pressures can never exist in the system when it cools.

Sub-zero pressures are dangerous since steam can be formed.

Because the heating system is sealed and pressurised, a safety valve is required.

If excess pressure builds up in the system, a spring on the safety valve lifts and releases an amount of water to drain.

When a system is commissioned and filled with water, the flexible connection can be removed.

This is to ensure that heating water cannot back feed into the clean mains water supply.

Advantages of Pressurised Heating Systems

There are many advantages in pressurising a heating system. Some of the benefits are:

1. No Feed & Expansion vessel required.2. No vent pipe required.3. No cold feed pipe required.4. No roof space area required.5. System cannot pump over.6. System is completely sealed, reducing evaporation losses.7. Air cannot be sucked into the vent pipe.8. No ball valve and float required in tank reducing maintenance.9. In commercial heating systems pressurisation allows the system operating temperature to be above 100oC if necessary.

A typical small-scale pressurised system is shown below.

183

Also known as a sealed system.

There are some practical recommendations for these systems.

1. Fit expansion vessel close to inlet side of pump to take advantage of pump suction pressure.

184

2. Fit safety valve on top of boiler or above boiler.3. Fit pressure gauge near filling point.

Combination Boilers

The combination boilers take pressurisation one stage further.In these boilers the pressure vessel and all safety devices are incorporated inside the boiler casing. This makes the unit very compact and some can even fit into a kitchen cupboard. Another advantage of the combination boiler is the ability to produce hot water for washing and cooking as well as hot water for a heating system. This is achieved by a heat exchanger, which is divided into two sections or sometimes by a diverting valve.The diagram below shows a simplified combination boiler.

186

The photograph below shows a white cased combination boiler installed in a kitchen.

The flue outlet for a Combination boiler can be quite small approximately 80mm to 100mm diameter because the products of combustion are expelled with the assistance of a fan.

187

Open Vented Systems

The traditional method of allowing for the expansion volume in a heating system was to install a small tank at high level, usually in a roof space in a domestic system.

To avoid pressure building up in the system an open vent is added and the water level is allowed to rise on heat up in the small tank called a feed & expansion (F & E) tank.

Also a cold feed pipe is used to fill the system automatically on commissioning or in the event of a leak.

188

The F & E tank is equipped with a valve and ball float arrangement so that mains water can enter and fill up to a level in the tank determined by the ball float.

The open vent should be clear of all valves and obstructions and any valves should be positioned near the pump for isolation and pump removal.

The isolating valve on the cold feed pipe is best positioned in domestic installations in the roof space where it cannot be shut off and forgotten about when the system is running.

C

Feed & Expansion Tank (F&E)

B

A

Space Heating Emitters

Isolating Valves

NOTE:

No valves on vent pipe

Cold Feed pipe

Cold levelWater level when system has heated up

Open Vent pipe

RETURN

FLOW

Pipework

Pump

Open Vented 2-Pipe Heating System

Boiler

189

Although the open vented system has been extensively used in the past there are some difficulties associated with it.

One of the main problems is pumping over; this is where water may be pumped into the F&E tank via the vent pipe especially if the pump is a high head variety.

If the pump is installed on the return pipe near the boiler or on the flow before the vent at positions A and B then pumping over could occur.

The pressurised or sealed heating system is therefore recommended for small or large heating system.

If the pump is installed at position C then pumping over will not occur.

190

Commercial / Industrial Heating SystemsSmall Installations

A typical small commercial heating system is shown in the schematic diagram below.

191

It is common to have more than one boiler in commercial systems.

This helps to make the system more efficient during part-load conditions.

Single pumps are shown for each circuit for this installation.

The system is pressurised. Safety valves are shown.

Safety valves are installed to relieve high pressure in the system and are usually sized to discharge at 21/2 times normal working pressure.

The header pipes are large diameter pipes from which branches or circuits are installed.

A header is a good location for installing; isolating valves, gauges, drains, spare connections and other items.

Isolating valves are used to isolate and remove an item of plant for maintenance or replacement.

Larger Installations

In larger installations it is normal to have pump sets in which one pump is running when required and one pump is standby.

192

These may be switched over periodically to ensure equal running life.

A non-return valve is installed after each pump so that no back-flow of water occurs and no short-circuiting is possible through the pumps.

Sometimes in small commercial installations the pumps are not duplicated to save installation costs.

To achieve good control over the heating system 3-port control valves may be used. Two types of control valve are in common use; a Mixing valve which gives variable temperature (between 70oC and 80oC) to radiator circuits or a Diverting valve which gives a variable flow rate of water to convectors or the hot water cylinder.

193

Flow measuring valves or Regulating valves can also be used to balance heating circuits and achieve the desired water flow rate for each circuit. Double regulating valves (DRV) are in common use.For more information on heating system controls see CONTROLS section.

The items mentioned above are shown in the diagram below.

Best Domestic Heating System

The following discussion is for northern European countries where air conditioning is not required in summer.

The criteria that I would use for the best type of domestic heating system would be as follows:

http://www.arca53.dsl.pipex.com/index_files/cont1.htm

194

1) Cheapest running costs.2) Cheapest installation costs.3) Efficient4) Individual room temperature control5) Different zones for Bedrooms and Living Spaces6) Hot Water only facility in summer7) Pressurised system for heating and for hot water.8) Quick heat-up from cold.9) Even heat distribution throughout building.10) Robust system.

1. Cheapest running costs

In the U.K. at present oil or natural gas are the cheaper of the common energy sources so an oil-fired or natural gas-fired boiler is required.

Other sources such as wood pellet boilers could be considered.

2. Cheapest installation costs.

A wet system with pipes and emitters is cheaper to install than an all air system with ducts, so conventional water boilers, pipes and radiators are the cheapest.

Under floor heating systems tend to be slightly more expensive to install than radiator systems

195

3. Efficient

Most oil and gas-fired boilers are well over 80% efficient; choose the boiler with the highest efficiency rating.

This may mean a condensing boiler but the extra cost for 93% efficiency may not be worth it.

4. Individual Room Temperature Control

To achieve this, Thermostatic Radiator Valves (TRV’s) should be used in all or most rooms.

This means that any room can be set at a room temperature to suit the activity and room usage.

5. Different Zones for Bedrooms and Living Spaces

The heating circuits should be divided into two sub-circuits.

Each sub-circuit should have a separate heating pump as shown below in Figure 1 below.

Each pump can be switched on and off by the time clock and room thermostat so that the bedrooms can have heat at different times to the Living Spaces.

See Controls section of these notes for a schematic diagram with controls.

6. Hot Water only facility in Summer

This can be achieved by having a separate sub-circuit with a separate pump for the primary flow and return to the hot water cylinder as shown in Figure 1.

196

Alternatively if a combination boiler is used, as shown in Figure 2, then no separate hot water cylinder is required.

Always check the hot water flow from a combination boiler to see if it matches the demand.

7. Pressurised system for heating and for hot water.

An expansion vessel for the boiler and pressurised hot water cylinder is used as shown below in Figure 1.

An oil or gas-fired combination boiler can be used to obtain pressurised heating and hot water.

A combination boiler achieves criteria No’s. 1, 2, 3 and 6.

Motorised valves are required to achieve criterion No.5 as shown below in Figure 2 below.

8. Quick heat-up from cold

This is achieved by using convector radiators as heat emitters, correctly sizing radiators and boiler and insulating exposed pipework.

Aluminium radiators heat up quickly and are physically smaller compared to steel panel radiators but are from 200% to 300% more expensive to purchase.

In some dwellings more expensive radiators can be used to augment the interior décor.

9. Even heat distribution throughout building.

The best system to meet this criterion is under floor heating, but to achieve a compromise between quick heat-up and even heat distribution a carefully designed radiator system in a well-insulated house should provide fairly even distribution of heat and meet

197

No.2 criterion.

10. Robust system.

Steel boilers can corrode over time but the extra expense of a cast iron boiler (up to 150% more) probably isn’t warranted.

Steel and aluminium radiators are robust enough for the domestic market.

Motorised control valves are not as robust as domestic heating circulators (pumps) but are useful to convert a Combination (Combi) boiler installation to two separate heating zones as shown in Figure 2.

198

Thermostat

Room Thermostat

T

Oil or GasBOILER

Hot Water Service

Combined Expansion VesselSafety valve

and drain

Safety valve

and drain

Pressure Reducing Valve

3 separate

sub-circuits

Expansion

Vessel

Flexible connection

Isolating Valve

Drain valve

Flow pipe to Living Space radiators

Flow pipe to

Bedroom radiators

Return pipe

Return pipe

Flow pipe

Primary flow pipe to Hot Water Cylinder

Pumps

PRESSURISED

HOT WATER

CYLINDER

Figure 1 - Heating System with Three Pumped Circuits using Conventional Boiler

Room

Room Thermostat

T

Two 2-port motorised valves to separate Bedrooms and Living Space.

Flow pipe to

Flow pipe to Bedroom radiators

Thermostatic Radiator Valves (TRV)

Heating Pump incorporated in

199

WATER AS HEATING MEDIUMWater as a heating medium offers many advantages: among the various heat transfer liquids, it has a high heat capacity and a high thermal conductivity. This means that water can transport high amounts of heat and can readily transfer that heat to air via heat emitters.

Within the temperature range 20-200oC, although the specific heat capacity rises by about 8%, the specific mass falls by about 13% with the result that the volumetric heat capacity varies by only 5%. This is another useful property of water since a large expansion volume requires large tanks or vessels to contain the increase in water quantity as the temperature rises.

Properties of Water:

TemperatureoC

Specific Masskg/m3

Heat Capacity kJ/kgK

Volumetric Heat CapacitykJ/litre K

20 998.2 4.183 4.12570 977.7 4.191 4.09880 971.8 4.199 4.080100 958.3 4.219 4.043150 916.9 4.322 3.961180 886.9 4.422 3.920

The heat capacity of air varies as follows:

At 20oC the specific heat capacity Cp = 1.0044 kJ/kg K, at 40oC Cp = 1.005 kJ/kg K.

This means that air has roughly one quarter of the heat capacity of water and thus can transfer much less energy per unit mass.

Pipework to transfer water from heating source to emitter takes up much less space than ductwork and associated equipment.

Another advantage of water as a heating medium is its flexibility in that various water temperatures can be used to meet the requirements of the heating system. Transporting water is also easier since pumping is more efficient than moving air with a fan.

Fans tend to generate noise in the air system whereas pumped water systems are quiet.

One of the main disadvantages of a wet system is that the response time for heating up is greater since water heats up pipework and emitters before heating the air whereas a ducted system of warm air heating will heat air in a room directly.

Room Flow pipe to Heating Pump incorporated in

200

The Table below shows the design Temperatures for Water Heating Systems:

System Flow Temp. oC

ReturnTemp.oC

Low temperature warm water 40-70 35-60

Low temperature hot water 80 70

Medium temperature hot water 110-130 85-110

Most small and medium systems use Low Temperature Hot Water (L.T.H.W.) with a design flow temperature of 80oC and return temperature of 70oC. These are design conditions and in practice boiler thermostats may be set at lower temperatures especially in mild weather. Also in domestic heating systems it may be dangerous to have high emitter surface temperatures and a lower water temperature is used.

In large commercial and industrial heating systems more heat can be transmitted using higher water temperatures; therefore Medium Temperature Hot Water is sometimes used. This means that emitters such as radiant panels, radiant strips and radiant ceilings work much more effectively.

In some countries air conditioning is required in the summer, even in houses. It is therefore normal to use this air conditioning system for heating during the winter period, thus making a separate wet system unnecessary. However, it should be remembered that air conditioning systems are more expensive to install and to run when compared to a water based heating system. If the cold period only extends for a short time, then an electric heater in the air conditioner may be suitable.

The diagram below shows a typical heating system using water in pipes to convey the heat to Radiators and an indirect coil inside a Hot Water Cylinder. Not all systems are installed this way, but this is a typical example. Some pipework such as vent and cold feed are not shown for clarity.

201

Water Flow Rate

It is necessary to calculate the water flow rate in heating systems to; size pipes, size pumps and size control valves.

For heating systems using water the mass flow rate in (kg/s) is found from the following expression:

H = m x Cp x t

Where;

H = Heat load (kW)

m = Mass flow rate of water (kg/s)

Flow pipe to Ground floor

Flow pipe to heating system

Flow pipe to First floor radiators

Return pipe Return pipe

Flow pipe

Primary flow pipe to Hot Water Cylinder

Pumps

HOT WATER

CYLINDER GROUND FLOOR

RADIATORS

BOILER

FIRST FLOOR

RADIATORS

Heating System with two pumped circuits

202

Cp = Specific heat capacity of water 4.187 kJ/kg degC - approx. 4.2 kJ/kg degC.

t = The temperature difference between flow and return water. (degC)

This can be re-written as;

H

m = ___________________

Cp x (tflow - treturn)

Where;

m = Mass flow rate of water (kg/s)

H = Heat load (kW)

Cp = Specific heat capacity of water 4.187 kJ/kg degC - approx. 4.2 kJ/kg degC.

tflow - treturn = 80oC - 70 oC = 10 deg C for LTHW systems.

H

This approximates to: m = _____

42

Example 1

Calculate the mass flow rate of water from a boiler in a Low Temperature Hot Water (LTHW) domestic heating system with a boiler output of 20kW.

203

H 20

m = _____ = _____ = 0.476 kg/s.

42 42

204

Example 2

Calculate the mass flow rate of water through the Secondary pumps, the Primary pumps and the Low loss header in the LTHW system shown below.

Secondary Pump

Secondary Pump

Boiler

Boiler

Flow Pipe

Primary Pump

3-port motorised

Safety Valves

Low Pressur

Heat Output = 25 kW

Heat Output = 32 kW

205

Mass flow rate through Secondary pump set No.1;

H 32

m = _____ = _____ = 0.762 kg/s

42 42

Mass flow rate through Secondary pump set No.2;

H 25

m = _____ = _____ = 0.595 kg/s.

42 42

Total heat output = 32kW + 25kW = 57 kW.

206

Assume each primary pump has half the total flow rate of water passing through it.Heat load through each boiler = 57kW / 2 = 28.5 kW

H 28.5

m = _____ = _____ = 0.679 kg/s.

42 42

The flow rate through the Low Loss Header is;

H 57

m = _____ = _____ = 1.357 kg/s.

42 42

The flow rates are shown on the drawing below.

207

Heating – Gravity Circuits

Gravity Circuits

Gravity heating systems use the fact that hot water will tend to rise and circulate in a pipework system if cooling water becomes less buoyant and descends.

Gravity heating systems were installed before circulating pumps were used and do not act quickly enough for modern systems.

For gravity circulation to work best the heat emitters are higher than the boiler. Pipes also have a gradient to assist circulation.

A Gravity system can be used for a solid fuel heating system because the hot water cylinder can receive heat at night when the fire is banked and act as a heat leak. This is a safety measure.

It is recommended that a heat leak radiator is also installed on the same circuit; this is usually the radiator in the bathroom.

0.679 kg/s

0.679 kg/s

1.357 kg/s 1.357 kg/s

0.762 kg/s0.595 kg/s

0.595 kg/s

0.762 kg/s

Secondary Pump

Secondary Pump

Boiler

Boiler

Flow Pipe

Primary Pump

3-port motorised

Safety Valves

Low Pressure

Heat Output = 25 kW

Heat Output = 32 kW

208

To assist circulation an 'Injector Tee' can be used as shown below.

Another method of ensuring safe operation of a solid fuel appliance is to pump both radiators and hot water cylinder primary circuit and install a high limit pipe thermostat on the flow pipe at the boiler outlet. This means that if the flow temperature gets dangerously high (say 90oC) then the pump is automatically switched on by the pipe stat. to relieve a build up of temperature in the boiler.

Gravity circulation is not recommended in most modern heating systems.This is because gravity systems are difficult to control and pumps can circulate water much faster and heat up a house much quicker.

209

ZONING

A heating system may be divided into sub-circuits or zones for several reasons. These are:

1. Different floors

2. Different usage

3. Consideration of sun path and solar radiation on building.

4. Different clients in one building

5. Internal gains vary.

6. Heat losses vary throughout the building.

7. In tall buildings heat rises so that upper floors can be overheated.

Certain areas of a building have common characteristics and hence similar heat requirements and these may be grouped into one zone.

An example of this is where southern facing rooms gain heat from the sun while northern facing rooms do not have solar heat gain.

The building may initially be divided into two zones; south and north zones.

The southern zone will require less heating when the sun is shining and a control valve will determine the reduced heat input into this zone.

In some buildings various rooms are used at different times so a separate zone may be required to accommodate this.

In tall buildings the upper floors may be zoned separately to allow for the build up of heat from below.

210

Lecture theatres can become occupied quickly and internal gains may rapidly increase therefore a separate heating zone would be a good idea.

The ultimate solution to good control of heating is to have a zone for every room although this is rather expensive to install and maintain.

A compromise solution is to group rooms together that have similar ‘heating’ characteristics.

Typically three to five zones are used for small to medium sized buildings.

Another factor to consider is that the more zones are installed, the more pumps and/or control valves are required.

There are usually space limitations in plant rooms and the number of pump sets is usually limited to space available.

In summary, it is a good idea to divide a heating system into zones for better control but the final number of heating zones is a compromise.

Zone Design

There are several ways to design zones.

Two popular methods are; to have a separate pump set for each zone or to have one large pump set with control valves downstream.

This is shown below.

Several Pump Sets

One Pump with Control Valves

211

Factors that Influence the Choice of Heating System

The following is a list of some of the factors that influence the type of heating system that may chosen:

o Cost o Fuel or Heat Source o Safetyo Type of Buildingo Comfort o Power Supply o Space o Vandalism o Security of Supply of Heat Source o Let Buildings o Environmental Issues o District Heatingo Outside Conditionso Fluctuating Heat Demando Appearanceso Industrial Waste Heat

These factors are explained on the next page.

212

Factors that Influence the Choice of Heating System

The following is a list of some of the factors, which influence the type of heating system:

1. Cost

Installation cost - Pipes are cheaper than ducts.

Running cost - Oil, Coal or Gas or Economy 7 electricity or wood products.

Life Cycle costs - Reliable system, long working life e.g. Cast Iron boiler.

Maintenance costs - Coal as a fuel may be expensive to maintain. Other systems have less maintenance requirement.

- Gas burns cleaner than oil and there is less soot to clean out of a gas boiler and flue.

2. Fuel or Heat Source

There is a choice in most countries between;

Oil, Coal, LPG (Liquid Petroleum Gas), natural gas, Economy 7 electrical heating, Ordinary Rate electrical heating, Wood products.

In some countries peat, lignite or soft brown coal is available.

The economics of burning this on a large scale would have to be considered.

There are various grades of oil, some of the more viscous (heavier) oils are cheaper but require specially heated burners and heated pipes.

Calculate the less expensive option.

3. Safety

213

Some open gas and coal fires and paraffin heaters have a poor safety record.

Ensure all apparatus is approved and meets standards and regulations.

Systems that use steam should be inspected annually to ensure that pressure vessels are safe and safety valves function.

4. Type of Building

There are many types of building encountered in building services, the following are a few suggestions:

Large areas benefit from the quick warm-up of air heating.

Ventilation systems with ductwork require ceiling void space.

For Warehouse radiant heating may be a suitable option since the air temperature need not be high.

Hospitals require clean environment; thus filtered air heating may be necessary, usually in a full air conditioning system.

Museums and Archive Stores require constant control of room temperature and humidity - air-conditioning may be necessary.

In some buildings it is difficult to run services through e.g. stone walls, solid concrete slabs, therefore electrical heating may be used.

In buildings with large occupancy a ventilation system may be necessary to provide adequate fresh air for occupants e.g. concert hall, auditoria.

In buildings with high heat gains air-conditioning may be necessary to maintain comfort levels.

Schools have limited wall space so underfloor heating or low temperature ceiling heating is sometimes used.

In some buildings like nursery schools and nursing homes, if radiators are utilised, it is advisable that low surface temperature radiators are used.

214

In wet areas like shower rooms and bathrooms underfloor heating has an advantage in that it keeps the floor dry.

Some buildings like churches may be intermittently used so electrical heating may not completely ruled out.

High temperature roof mounted quartz electric heaters have been used in this type of building.

Prestigious areas may have full comfort air conditioning to reflect the importance of the room e.g. board room.

5. Comfort

To maintain adequate comfort conditions a controllable heating system will be necessary e.g. automatic controls on oil or gas-fired system or electrical heating system.

A solid fuel system cannot be easily controlled.

Wood pellet boilers are automatically controlled in the same way as other boilers.

A comfortable heating system may incorporate some radiant heating as well as convective.

Radiant heating is not always achievable but radiators produce about 70% convective and 30% radiant heating.

It may be difficult to obtain comfort levels in an office if a purely radiant system is used such as radiant panels so a mixture of convective and radiant heating is desirable.

If noise levels in a room such as a Library are to be at a minimum then fan convectors are not a good option and some other quieter form of heating is better such as radiators, underfloor heating, natural convectors or a radiant ceiling.

6. Power Supply

When using electrical heating there must be an adequate electrical power supply.

For a large building or group of buildings this may mean a new or upgraded electrical sub-station has to be provided.

215

7. Space

Plant requirements; room for plant and equipment, storage space for fuel.

Some construction methods do not provide adequate space for large plant e.g. a trussed roof space is awkward to use for services plant.

A basement plant room can be compromised if the area is prone to flooding.

An apartment or flat may not have sufficient room for water tanks or boiler.

An inner city building may have no space for fuel storage therefore electrical heating could be the option if natural gas in unavailable.

8. Vandalism

Some systems do not stand up to abuse.

Keep walls clear of pipes in some buildings e.g. prisons, detention centres.

Use steel instead of copper pipework in exposed areas.

Some emitters are not robust e.g. economy 7 electrical heaters.

Heavy-duty radiators can be used e.g. cast iron.

Some types of steel panel radiators are suitable for flush fitting in a wall if a recess is provided.

Prison cells can be heated with surface mounted low-level pipes.

Temperature sensors should be protected.

9. Security of Supply of Heat Source

Some fuels at certain times may be liable to unsecured supply e.g. oil prices can fluctuate during a Middle East crisis.

It may be advisable to have a dual fuel system so that burners can easily be changed over to burn the cheaper or more readily available fuel.

216

Alternative sources of energy are not always secure e.g. the wind doesn't always blow on a wind farm.

The sun doesn’t always shine if the system relies on solar panels.

A hybrid system is more secure or back-up boilers can be used.

10. Let Buildings

Most landlords prefer the tenant to look after payment of their own heating bills.

Individual meters for gas or electricity in a block of flats means that the tenants are responsible for the payment of bills.

In a large office building with several tenants, economy 7 electrical or natural gas heating may be used otherwise it is difficult to divide up a wet heating system serving a whole building so that suitable payments can be made for heating.

Some heat meters are expensive and not always reliable.

Some billing arrangement needs to be in place to charge tenants for heating.

11. Environmental Issues

The products of combustion of oil, coal and gas pollute the atmosphere.

Coal is probably the worst offender since carbon dioxide contributes to the greenhouse effect and sulphur dioxide causes acid rain.

Smoke causes urban smog and soot and ash add to the problem.

Oil produces contaminants to a lesser extent and gas is probably the best of the three.

Using electricity is of little benefit because power stations burn fuel to produce electricity or use nuclear fusion or fission as a source of heat which has its own impact on the planet.

A totally 'green' source of heat may be wind power or wave power or solar energy if you live in an area with plenty of sunshine.

Wood products such as pellets have zero Carbon emission since trees can be replanted to replace this fuel source.

217

Wood pellets boilers use pellets from an on site storage facility.

12. District Heating

If un-used hot water from a power station or other industrial plant is utilised for domestic and commercial heating then the system could be designed to utilise this cheap source of energy.

13. Outside Conditions

In some countries the outside temperature in winter is very low i.e. minus 10 decC to minus 30 degC.

Because of high emitter output requirements it may be better to heat with warm air as opposed to hot water.

In some temperate climates it is not worth the expense of having a wet heating system and electrical heaters are suitable for occasional use.

If a building has high internal heat gains, even in winter, then an electrical heater battery can be used in the air conditioning system instead of a wet system if occasional use is envisaged.

In rooms or buildings, which have an intermittent net heat gain and heat loss, then a heat pump may be used.

A typical use of heat pump is in a heavily glazed building where in a sunny period the heat pump is in cooling mode and if the outside temperature drops the heat pump switches to heating mode.

14. Fluctuating Heat Demand

In some buildings the demand for heat fluctuates widely throughout the day.

To meet this demand economically, a modular boiler system is a good option.

This means that the required number of boilers is automatically switched on to meet the demand.

In some circumstances it is recommended that condensing boilers can be used to meet the base heating load and non-condensing boilers can be utilised to meet the peak loads.

218

Condensing boilers squeeze more energy out of the fuel by taking extra heat out of the flue gases with a heat exchanger, Efficiency can be 98%

15. Appearances

In some rooms or buildings the designer may require the heating system to be totally hidden e.g. underfloor heating, heated ceiling or air heating.

In some buildings the designer may wish to make a feature of the heating system or heat emitters e.g. warm air ductwork system painted a bright colour in a swimming pool hall or sports hall, Victorian cast iron radiators in a period building.

16. Industrial Waste Heat

In some factories heat is available from the process e.g. condensate or steam available as a by-product.

Steam can be used directly in a warm air heater or ‘stepped’ down to low temperature hot water in a heat exchanger for use in the normal L.T.H.W. emitter system.

There are many ways in which waste heat can be utilised to pre-heat water or up-grade in heat pumps for further use in space heating.

Hot water from Combined Heat and Power (CHP) systems can be used to heat a factory or sold to neighbours.

A useful tool for designers is to make up a table of various systems suitable for different buildings.

The table below is my attempt at doing this; you could think of some amendments.

Heating / Cooling Systems and Buildings

Building Heating System / Emitters Heating Media

Hospital - Ward

Full air conditioning central plant Air Used where air needs to be clean.

Draughts of cool air may be a problem.Plenum heating Air Can be used if heat gains are minimal.

Radiant ceiling Water Cleaner than radiators. High radiant temperatures may cause discomfort.

219

Hospital – Operating Theatre

Full air conditioning central plant Air Clean air essential. Special high efficiency filters

required.

Chilled ceiling / beams. Chilled water Advantage no air – no bacteria. Condensation may be a problem.

Large Workshop,Industrial Building,Factory

Industrial warm air heaters Oil/gas fired to air Cheap to run. May be noisy. May use up floor space if floor mounted.

Unit heatersSteam, HTHW, MTHW, LTHW to air.

A unit heater is simply a heat exchanger. Compact. May be noisy.

Radiant tubesSteam / HTHW / MTHW /direct gas fired.

No noise, especially if non-direct fired. Suitable for between isles heating.

Radiant panels Steam, HTHW, MTHW

Usually roof mounted. May be cheaper to operate than heating air.

Small Workshop,Garage

Industrial warm air heaters Oil/gas fired to air Cheap to run. May be noisy.Unit heaters MTHW, LTHW to air Compact. May be noisy. Fan convectors MTHW, LTHW to air. Suitable for smaller workshops. There are various types.

Office Building,Public Building

Air conditioning central plant Air Prestigious offices. Inner city areas to reduce pollution.Fan coil units Hot/chilled water to air Not quiet running. Individual room control.Room air conditioners Hot/chilled water to air Individual room control. Plenum heating Air Used in low heat gain areas. Cheaper than air

conditioning.Radiators LTHW to air Require wall space. Easy to control.Natural convectors LTHW to air Require wall space. Quite bulky.Fan convectors LTHW to air May be noisy.

Underfloor heating LTHW No floor space required. Heat output may not be sufficient

Storage heaters E7 electricity to air Easy to charge occupier. Can be expensive to run.

Large Public Hall,Auditoria

Air conditioning central plant Air Calculate fresh air requirement. Air distribution is important.

Plenum heating Air Cheaper than air conditioning.

Church,Ecclesiastical Building,Library

Radiators LTHW to air High output usually required for large buildings.Underfloor heating LTHW If no wall space is available.Quartz lamp heaters Electric quartz tube Electric heating may be economical for occasional use.High temperature panels Electric metal plate. Small heat emitters. Can be roof mounted.Low temperature panels Electric elements in plate Can be roof mounted.

Skirting heating Electric element to air Low output. May be used with other systems.

Tubular heaters Electric element to air Mounted at low level.

Pipe coils LTHW to air Heaters under pews.

Department Store,Supermarket

Air conditioning central plant Air Large Prestigious store. Use in areas of high heat gain.

Room air conditioners Hot/chilled water to air Good control possibility. Compact.

Plenum heating Air Used if heat gains are minimal.

220

Radiators LTHW to air Use in small store.Fan convectors LTHW to air Use in small store. May be noisy.

School,College,University.

Radiators LTHW to air Can be easily controlled with thermostatic rad. valves.Radiators LST(Low Surface Temperature) Warm water to air In Nursery schools limit water temperature for safety.

Radiant ceiling Water High radiant temperatures may cause discomfort. No wall space required

Underfloor heating LTHW No wall space required. Warm air or air conditioning. air In larger areas e.g. lecture rooms.

Hotel

Radiators LTHW to air Easy room control. Bedrooms.

Air conditioning central plant Air Large hotel or in large rooms. Use in areas of high heat gain.

Room air conditioners Hot/chilled water to air Good control possibility. Compact.

Plenum heating Air Used if heat gains are minimal.

Fan convectors LTHW to air Use in areas requiring quick heat up e.g. foyer. May be noisy.

Natural convectors Electric element to air. Possible key entry system which operates power to room.

House

Radiators LTHW to air Different types and materials. Efficient.Underfloor heating LTHW Invisible system.

Air conditionerElectrically operated refrigerant to air.

Areas of high summertime temperatures. Use quieter systems.

Flat,Apartment

Storage heaters E7 electricity to air Easier to charge client. Can be more expensive to run. Difficult to control.

Underfloor heating Electric cables Expensive to run if not a suitable tariff. Invisible system.Underfloor heating Water Invisible system. Requires suitable floor with insulation.

Radiators LTHW gas /oil-fired Natural gas easier to charge. Efficient.

The first system with a pump set for each zone and a 3-port Mixing valve for each zone works were the heat emitters are radiators since 3-port Mixing Valves are used in radiator circuits.

The second system with one large pump set and a 3-port Diverting Valve for each zone works for the other emitter types such as; convectors, heater batteries, under floor heating, trench heating and radiant panels.

Variable temperature circuits use first system.

Variable flow rate systems use second system.

221

Some Variable flow rate systems require a Mixing Valve to adjust the water temperature to emitters incorporated into a Compensator system.

A compensator controller adjusts water temperature between 70oC to 80oC depending on how cold it is outside.

The system shown below is a way to achieve compensating control for a variable flow zone system.

222

Introduction

There are many types of heat emitter used for space heating requirements.

Most of the common ones in use are listed below.

Ref. Heating System Emitter Types

1. RadiatorsSteel PanelCast IronAluminiumBathroom

2. Warm Air Heaters

Natural ConvectorsFan ConvectorsIndustrial Warm Air HeatersUnit HeatersSkirting HeatersTrench Heating

3. Radiant HeatersMetal Radiant PanelsMetal Radiant StripsMetal Radiant CeilingsGas Radiant Heaters

4. Underfloor Heaters Piped Underfloor HeatingElectrical Underfloor Heating.

5. Electrical HeatersElectrical Tubular HeatingStorage HeatersHigh Temperature Heaters

Heating -Heating Emitters - Page 1 2 3 4 5 6 7

RadiatorsRadiators do not strictly speaking 'radiate' all their heat into the space but up to 80% may be convected, typically for a double panel radiator about 30% of total heat output is radiated and 70% is emitted by convection.

http://www.arca53.dsl.pipex.com/index_files/emitters7.htm







http://www.arca53.dsl.pipex.com/index_files/heat.htm



223

Radiators are used in a wide variety of buildings to provide central heating in rooms.

These emitters are usually positioned at low level, typically under windows, although other positions can be used.

Heat outputs vary up to around 3 kW.

A typical radiator height for a house is 600mm, but other sizes are used depending on location.

Radiators can be described by various means but the type of material used in the manufacture is the main method of distinction.

Six radiator types as listed below show various methods of manufacture and style to suit different conditions.

{1} Steel Panel - Simple convoluted panel which may be single, double or more.

{2} Cast Iron - Column type.

{3} Aluminium - Extruded sections.

{4} Tubular - Using vertical and horizontal steel tubes.

{5} Bathroom Radiator - Various shapes and materials some for drying towels.

{6} Low Surface Temperature - Usually steel.

Steel Panel

224

Cast Iron

Cast iron sections are bolted together.

Robust, heavy radiator.

Good heat transfer but expensive.

225

Aluminium Sectional

A selection of aluminium radiators are shown below.

Aluminium radiators are more expensive than steel panel but are light with high heat output for size.

The material used and production techniques ensure a clean smooth finish but one of the problems with using aluminium is corrosion of the metal in contact with hot water which may have a small quantity of air absorbed in it.

An inhibitor can be provided as a capsule inserted in the radiator during installation or special additives can be added to the water during commissioning of the system to overcome this problem.

226

Tubular

In some types steel tubes are welded to top and bottom headers.

Shown in photograph above.

Decorative appearance, useful where tall, narrow radiators are required.

Easier to clean than steel panel.

The photo below shows a radiator with horizontal flat tubes.

227

Bathroom Radiator

Bathroom radiators are made in various materials and shapes, some are also used to dry towels.

Low Surface Temperature (LST)

Some manufacturers make low surface temperature (LST) radiators for use in hospitals, old people’s homes, nursery schools, and kindergartens.

These prevent injury if hot radiators are used in these areas.

One method of limiting the surface temperature of a radiator to about 45oC is to cover the hot metal parts with an outer casing as shown below.

228

Warm Air Heaters1. Natural Convectors

Convectors are used to heat up spaces more quickly than radiators.

Typical locations may be entrance hall, foyer, kitchen, bathroom, small hall or auditoria, small workshop.

A natural convector has no fan but has more output than most radiators.

2. Fan Convectors

A fan-assisted convector has even more output and is more common but in some areas fan noise can be a nuisance.

Convectors can be recessed into walls so that they appear to be part of the fabric of a building and may have a decorative panel on the front to add to their appearance.

Convectors operate by heating air, which passes over the finned pipe through which warm/hot water passes.

The fins are mechanically fixed to the tube(s) and extend the heating surface so that all the heat output is purely convective. The heating tubes are enclosed in a cabinet

229

with louvres at the bottom to allow cooler air to enter and louvres at the top to emit heated air into the space.

The convector may also have some form of control such as a damper to alter the flow of air as shown in the diagram below.

Natural convectors rely on air movement over the heating element by natural convection and forced convectors use a fan or fans to assist the movement of air.

Small convectors called ‘kickspace’ heaters are sometimes used in domestic kitchens or hallways.

230

3. Industrial Warm Air Heaters

The type of unit illustrated below is used for space heating and has the advantage of being simple in both construction and operation. Hot gases from an oil (or gas) fired burner are directed over a heat exchanger and then exhausted through a flue to outside.

Air is passed over the heat exchanger by a fan or fans and is subsequently heated to about 30oC to 40oC. The warm air is supplied into a space through outlet diffusers, which can direct the air where it is required and throw heated air up to about 10 metres from the point of discharge.

231

These units are suitable for large areas, which require heating since large amounts of heated air can be supplied. Warehouses, factories, workshops and supermarkets use this type of heating and outputs range from 30kW to 400kW.

It may be advisable to provide a protective area around floor standing units so that it is not tampered with or machines such as forklift trucks don’t cause damage.

In some areas it is best to position industrial warm air heaters at high level or at roof level to avoid possible damage and to give uninterrupted floor space. The unit may be supported from or suspended from the roof structure with the nozzles arranged to blow warm air into the space below.

One disadvantage of these units is that they can be noisy since a fuel burner and fan are incorporated in the casing.

In some cases ventilation ductwork can be fitted to the outlet nozzles to heat other areas such as offices in a factory.

The photo below shows a typical industrial floor standing warm air heater.

232

4. Unit Heaters

Unit heaters are very like fan convectors in operation in that they blow out warm air from a heat exchanger. The heat exchanger uses steam or hot water to heat air, which is forced over the tubes and fins by a powerful fan.

Since the steam or water temperature may be high e.g. over 100oC and the fan may develop a high volume of air, the output is 10kW to 300kW.

Unit heaters may be used in factories, workshops and warehouses.

One advantage of this convective form of heating is that a relatively small unit can produce a high heat output, but fan noise has to be considered.

233

The photograph below shows an all-electric unit heater, which obviates the need for pipework but is more expensive to run than wet oil and gas fired systems.

5. Skirting Heating

This form of low level convective heating can be used in areas where unobtrusive emitters are required.

Since the heat output per metre linear run is low then a substantial skirting length is required in each room to offset heat losses.

The heat output is about 450 Watts per metre.

The units consist of one or two finned tubes inside a casing, which emits slow moving warm air through a linear outlet at the top.

234

One disadvantage is that efficiency is reduced by dust collecting in the fins.

Skirting heating can be used as perimeter heating below glazing or for background heat in some areas.

6. Trench Heating

This type of heating is useful for some areas where perimeter heating at floor level is required.

Trench heaters consist of finned tube elements which are fully or partially recessed into a steel casing within a concrete floor.

A trench is required around the perimeter of the room into which the tubular heater is installed.

This has been used successfully in airports where large areas of perimeter glass require an up-current of warm air to cancel out heat losses.

235

Trench heaters do not take up wall space and require a floor grille to withstand foot traffic.

Some aluminium floor grilles can be rolled up for cleaning.

Outputs up to 1000 Watts per metre can be achieved.

Electric trench heating units have a fan in the trench which gives a higher heat output compared to natural convention systems.

236


Radiant Heaters1. Metal Radiant Panels

Radiant panels are a good way to heat up large spaces such as factory, workshop and warehouses because the air is not heated directly but the surfaces below the panels are heated.

This is a less expensive way to heat large volumes.

High temperature water or steam is passed through a series of pipes that are connected to steel panels. The panels heat up to about 100oC to 150oC and radiate heat downwards into the occupied space.

Panel sizes are usually several metres long by about 1 metre wide.

They can be suspended from the roof or from a wall at high level, either vertically or at an angle to direct radiant energy into the space below.

2. Metal Radiant Strips

For workshops, strip heaters can be used when supplied with high or medium temperature hot water or steam.

The radiant strip shown below uses a single pipe, other systems use 2, 3 or 4 pipes to increase the radiant heat output.











237

Strips may be up to 1 metre wide but usually are continuous throughout the length of a factory or workshop.

They are also suitable for mounting between stacking rows in a warehouse.

To enhance the effectiveness of the system a fin is added to increase the hot surface area, in the diagram below the fin is a profiled aluminium plate that is clipped to the pipe.

Some high outputs can be achieved with radiant strips e.g. 3kW per metre at 100oC.

The photo below shows two radiant strip heaters at high level in a factory.

3. Metal Radiant Ceilings

238

This type of radiant heating system incorporates the whole ceiling.

One type uses a ceiling made from metal plates above which are clipped pipes containing hot water.

The pipes heat the metal ceiling below which in turn heats the rooms by radiation.

Typical heat output is about 160 Watts per square metre of floor area.

To make the system work effectively, some insulation is added on top of the pipes, as shown below.

Radiant ceilings can be used in a wide variety of buildings such as: schools and offices.

The system is silent but requires careful temperature control to ensure a comfortable environment.

Some radiant ceilings are invisible from beneath as the photograph below shows. This type uses ceiling tiles with pipes above.

239

4. Gas Radiant Tubes

The main elements of Radiant Tubes are; a burner, a steel tube of about 60-70 mm diameter, a reflector plate and an extractor fan discharging to outside the building.

Gas is burnt and the products of combustion are drawn through the tube, which reaches a temperature of about 540oC.

The units are in lengths of about 4-7 metres with outputs of 12-14 kW.

Radiant Tubes are used in areas with high ceilings or roofs because the hot tubes must be well away from the working area.

They are used in workshops and factories.

240

5. Ceramic Gas Heaters

The domestic gas fire operates when the radiant heat output from a hot ceramic is reflected into a room.

Some units have a boiler attached so that a central heating system with emitters can be connected to heat other rooms.

NOTE: All permanent gas appliances must have a permanent flue and adequate fresh air supply from outside.

241

Larger ceramic gas radiant heaters are designed primarily for commercial and industrial applications in buildings with a high ceiling and are piped to a natural gas or LPG supply.

For permanent installations when wall mounted or suspended from a roof, they may be rated up to 30kW but a range between 3kW and15kW is more common.

Construction of one type takes the form of a heavy-duty rectangular reflector with refractory elements and a burner array fitted behind a safety guard.

Portable versions are mounted on a telescopic stand and a propane cylinder is mounted at the base.

Flue connections are not usually provided and manufacturers quote minimum rates of fresh air to be provided for safe use.

In the case of portable units, these are usually sited in temporary positions within open buildings, stockyards and construction sites.

Most other types of infrared heaters are used where it is too expensive to heat all of the air in a building and local heating by radiation is acceptable.












242

Underfloor HeatersUnderfloor heating is suitable in areas where wall space is not available for other emitters and where a warm floor is not a disadvantage.

In some cases underfloor heating cannot be used due to the nature of the floor and the type of materials proposed.

One of the disadvantages is that it may take some time before the benefit is felt in a room particularly if a concrete slab or other materials have a high thermal capacitance.

This time lapse is called thermal lag.

One area where underfloor heating is useful is in shower or changing areas, where the floor feels comfortable to stand on and is kept dry.

Two types of underfloor heating system are detailed below, they are:

(1) Piped Underfloor Heating (Wet System)

(2) Electrical Underfloor Heating

1. Piped Underfloor Heating

This consists of 15-20mm bore plastic pipes, laid without joints at 150-450mm centres.

The pipes can be laid above a solid concrete slab and within a graded floor screed not less than about 75mm thick, or under a suspended timber floor.

243

Copper tube is sometimes used (soft copper pipe to BS 2871 Table Y) but more often Polyethylene tube having outside diameters of 17mm and 20mm, with 2mm thick walls, in coil lengths of up to 120 metres is the preferred material.

When fixing polyethylene tube metal strips holding plastic clips are laid on the base slab, at right angles to the coil line, to form a locating grid. An emulsifying agent is added to the screed mix to improve contact with the coils.

A typical tube layout is shown below.

A section through a solid floor is shown below with pipe clips fixed to a grid and sand / cement screed covering.

244

It is generally accepted that a floor surface temperature of 24oC should not be exceeded where occupants are static, 27oC where they are able to move about and about 30oC in corridors and halls.

A variety of finishes may be used over heated floors and almost all types of hard material, marble, slate, stone, terrazzo and brick are suitable provided that provision is made for expansion and that no cavities are left in the finish to impede heat transmission.

In the case of softer materials, wood blocks may be used if properly seasoned, cork tiles are satisfactory and carpets can be used if not foam backed.

2. Electrical Underfloor Heating

Most electrical forms of Underfloor heating use off-peak electricity and are therefore storage-heating systems.

An alternative direct system can be used were heating elements are laid on a solid floor with an output of around 150 W/m2, laid close to the finished floor surface with about 50mm of insulating material under the coil elements.

245

Such an arrangement is suitable for a building used intermittently and for short periods such as churches, etc.

Floor warming can be successfully carried out by using Economy 7 off-peak electricity to heat elements in the floor.

There are three system designs as shown below:












246


Electrical Heaters1. Electrical Tubular Heaters

These are steel or aluminium tubes usually round or oval in section as shown below.

They consist of an electrical heating element, which extends from end to end and is surrounded by air.

The surface temperature is about 80oC. A single tube at 50mm diameter has an out put of about 180 Watts per metre length and tubes may be mounted in banks, one above the other, for higher outputs.

An electrical skirting heater with an output of 400 Watts per metre run is typical of some installations requiring background or low level heating.

Tubular heaters are used in churches, under pews, in greenhouses, conservatories and foyers. They can be placed at the bottom of high windows to prevent downdraughts of cold air or be set to prevent frost in greenhouses or conservatories.











247

2. Storage Heaters

Electrical storage heaters store heat overnight in thermal material and release the heat the next day to heat a building.

One of the advantages of electrical storage heaters is that cheaper electricity can be used at night to heat up thermal storage material from which heat is emitted later the next day.

In the U.K. this cheaper tariff is called Economy 7 because it is available for seven hours during the night-time.

There are several methods of storage and several types of room electrical storage heater, such as; storage radiators, storage fan heaters and warmed floors or walls.

Heat energy can also be stored centrally in several devices such as warm air units, dry-core boilers, wet-core boilers and thermal storage cylinders.

One of the advantages of using a central system of storage is it is possible to obtain better control of the heating system in a large building if for example a wet-core boiler is used and conventional hot water controls are utilised.

248

NOTE: A wet core boiler uses electricity to heat water in a steel or cast iron boiler. An electric current is passed between electrodes through the water, which due to its resistance, becomes heated. An electrode boiler is about 98% efficient.

Domestic Storage Heaters

Storage heaters or storage radiators comprise a number of sheathed elements enclosed within blocks of refectory material or Feolite to form the heated core, which is surrounded by insulation.

The surface temperature of the casing reaches a maximum of about 80oC at the end of the charging period and this reduces to about 40oC during the following day. Output is both radiant and convective in almost equal proportions.

Heater ratings vary with different makes but are usually 1.7, 2.55 and 3.4kW, the seemingly odd figures being related to rounded 7 hour charge acceptances of 12,18 and 24kWh.

Storage heaters are used in houses, flats, apartments and office buildings.

They have the disadvantage that the heat output during the day is not easily controlled and may not match the heat loss in a building for any given period. Also the lower electricity tariff (Economy 7) may not work out to be cheaper than oil or gas. See fuels section of the notes.

3. High Temperature Heaters

There are several types of Electrical High Temperature Heater:

249

(1) Infra red heater

(2) Quartz lamp heater

(3) High temperature panel

3.1 Infra Red Heater

The electrical elements used are similar to those fitted to luminous fires but, for a given rating, are commonly longer as shown below, and arranged to operate at about 900oC.

Wall or ceiling models of these are suitable for kitchens and bathrooms, ratings are up to 3 kW.

3.2 Quartz Lamp Heater

The elements of this type of heater operate at about 2000oC and consist of a tungsten wire coil sealed within a quartz tube containing gas and a suitable halide

- rating of elements about 1.5kW.

Some quartz lamp heaters are shown below.

250

These are used in large spaces either where the requirement is intermittent or where only local areas require spot heating.

3.3 High Temperature Panels

These consist of either a vitreous enamelled metal plate or a ceramic tile behind which a resistance element is mounted within a casing.

Panels of this type operate at a temperature of about 250oC and have ratings in the range 750 W to 2 kW; they are normally used in washrooms in industrial situations.











251

Heat Transfer from EmittersHeat transfer in emitters occurs in various forms as shown below.

The media for heating is the substance used to transfer heat from a central point to the emitters or within the emitter to the space.

Some heaters are ‘Direct’ in that they consume fuel or electrical energy in the heater and heat the surrounding space.

Ref: Heating System Emitter Types Media

1 Radiators Radiators - all types water

2 Warm Air Heaters

Convectors - all types water

Industrial Warm Air Heaters air

Unit Heaters water

Skirting Heating water

Trench Heating water

3 Radiant Heaters

Metal Radiant Panels, Strips and Ceilings

water

Gas Radiant Tubes direct

Ceramic Gas Heaters direct

4 Underfloor HeatersPiped Underfloor Heating water

Electrical Underfloor Heating. direct

5 Electrical Heaters

Electrical Tubular Heating direct

Storage Heaters direct

High Temperature Heaters direct

Heat transfer may be by three methods or any combination of the three as follows:

1. Conduction

2. Convection

3. Radiation

252

Conduction of heat is generally a small proportion of the total heat output from equipment and is not useful heat transfer since most of the conducted heat ends up in ceiling voids or through external building fabrics as a heat loss.

Most heaters are either purely Convective or purely Radiant or a combination of the two – for example, with added fins on radiators the percentage convective heat transfer can be up to 80%, the remaining 20% being radiant heating.

Types of Boilers

Boilers may be defined by the material for manufacture as follows;

Steel Cast Iron

Also boilers can be classified by fuel used as follows;

Oil-fired Gas-fired Coal-fired Electric boiler Wood pellets

Another classification is used for large boilers as follows;

Shell type boiler Packaged boiler Water tube boiler

Steel and Cast Iron Boilers

Small domestic and commercial boilers can be made using welded steel construction with a water way between two steel plates.

253

A more robust and longer life boiler is made from cast iron sections that are bolted together.

Cast iron boilers sometimes as long as 30 years or more.

Water connections

Baffles to reduce flue gas rate for better heat transfer

Waterways between double skin steel casing

Double skin welded steel case

Combustion chamber

Burner mounting

Flue outlet

Steel Domestic Steel Boiler

254

Classification by Fuel

Large oil and gas-fired boilers can be similar in construction.

Some boilers can have dual fuel burners to reduce running costs.

Coal-fired boilers have special methods of admitting coal into the combustion chamber.

This may be by using pulverised fuel or using a hopper and a chain grate stoker for example.

The simplest of small coal fired boilers with a stoking system uses a gravity feed arrangement so that the hopper is filled periodically with grains or small easy flow coal particles.

One of the best ways to heat water using electricity is by utilising the electrode boiler.

Pellet boilers are fed small wood pellets with a slow moving auger as a cheap form of fuel. See Energy Sources – Wood Burning Systems.

Shell, Packaged and Water Tube Boilers

The shell boiler may incorporate one or more furnace tubes within a pressure shell.

The flue gases pass inside these furnace tubes and heat water inside a steel shell as shown below.

Combustion Chamber

Water

Furnace tubes

Burner

Flue

255

In the water tube boiler the tubes contain water.

Special Boilers

These include;

Back boiler Condensing boiler Combination boiler

Back Boiler

Back boilers can be attached to a fire front to provide heat to water.

The back boiler is usually positioned behind or at the back of a fire front.

256

This means that flue gases from a coal or gas fire can be utilised to heat hot water in a small steel boiler which can heat a hot water cylinder or radiators.

Some gas-fired units can heat the back boiler without the front fire being lit.

Some solid fuel stoves have a high performance back boiler that is designed to take maximum heat from the combustion area and from flue gases.

This ‘wrap around’ back boiler is shown below.

Back Boiler

Back boiler inside stove

Outer stove covers

Pipe connections

Wrap around Back Boiler

Flue outlet

Double skin steel wrap around water ways

Flue gases

Back BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack BoilerBack Boiler

257

Condensing Boiler

A secondary heat exchanger is used in a Condensing boiler to extract more heat out of the flue gases.

This means that water in the flue gases condenses in a stainless steel heat exchanger and drains away at the bottom of the boiler, latent heat is given up when a liquid condenses.

The efficiency of a condensing boiler can be 15% more than a conventional boiler with figure about 93% to 96% quoted.

When the flue gases are cooled after the secondary heat exchanger the temperature is reduced so that the natural buoyancy is minimised.

This means that a fan is required to remove the flue gases.

Some designs split the heat exchanger into two parts and use an ordinary non-condensing copper heat exchanger for the first, and an aluminium condensing heat exchanger for the second. Aluminium doesn’t react well with water so it is lined with copper.

258

In practical terms you should expect a condensing boiler to be 'A' rated (which means a stated efficiency greater than 90%) and the gains depend on what you compare it with.

Current law in England and Wales

From 1 April 2005 all central heating boiler installations fall under the control of building regulations (the change does not apply to oil-fired central-heating boilers until 1 April 2007). The new legislation states that all gas boilers fitted in both new and existing homes must be condensing boilers with either an ‘A’ or ‘B’ efficiency rating (A= greater than 90%, B= 86%-90%). There are several important points which need to be noted:

Combination Boiler

259

A combination boiler heats water for the central heating system together with water for washing and cooking.

This means that a separate hot water cylinder is not required.

The hot water for the taps is fed directly from the mains rather than from a hot water storage cylinder.

An advantage of the combination boiler, or a "combi boiler" as it is usually called, is that they eliminate the need for both an expansion tank in the roof space and a hot water cylinder.

Another advantage is that the continuous flow of hot water is delivered at mains pressure or a reduced mains pressure so that showers have adequate flow, unlike gravity fed shower systems.

260

Diagrammatic drawing of Combination Boiler

Control panel

Hot water service (HWS)

Heating flow and return

Exhaust Fan

(gas boilers)

Gas supply

Flue outlet to wall or roof

Heat exchanger

Pressure gauge

Safety Valve

Expansion vessel

Mains water supply (MWS)

261

Pump Types

Common Types of Heating Pump

The table below shows commonly used pumps for heating systems.

Pump Type Construction & Installation Use

Centrifugal In-line Pump is connected in line with the pipes and may be supported on feet as well as by pipe.

Pipeline mounted for LTHW and MTHW.

Centrifugal End Suction Pump and Motor are placed on a concrete base.The fluid enters the horizontal inlet and discharges out the vertical outlet.In larger installations pump and motor may be separate and the shaft may have to be aligned.

LTHW and MTHW.Can handle more flow than In-line pumps.

Multi-stage Centrifugal Several centrifugal impellors connected to one vertical shaft. Often stainless steel is used for impellors.

Feed water to steam boilers.

Smaller pumps tend to be in-line, that is, installed in the pipeline.Larger pumps may be seated on a concrete base, these are often end suction pumps where the water is sucked into the pump end and comes out at 90 degrees at the outlet.

End Suction PumpCentrifugal In-line Pump

262

A positive displacement pump with piston inside a cylinder was the first type of pump to be used for water.When using a ‘bicycle pump’ the bottom of the pump gets warm indicating inefficient pumping.The more efficient Centrifugal pump has now superseded the positive displacement pump.Centrifugal pumps account for about 90% of the pumps used in industry.

The photos below show four different centrifugal pumps.

Centrifugal Action

In the centrifugal pump, the impellor rotates usually at motor speed and increases the water velocity.The impellor with backward curved blades rotates inside a casing or volute.The volute has an increasing area so that most of the velocity energy is converted into pressure energy.

Water outlet

Impellor with backward curved blades

Volute or Casing

Centrifugal Action

263

Pump Installations

In a plant room it is sometimes a good idea to have all the heating pump sets together.Access is easier if all pumps are mounted on brackets on a wall or on a concrete plinth at floor level.This means they are easier to maintain and monitor.The diagram below shows pumps mounted on a wall bracket.

Vertical support bracket

Plant room wall

Wall fixings Vertical steel channel support

Pipes

Anti-vibration pad

Steel channel bracketFlexible pipe connectors

Isolating valves

Non-return valve not shown

In-line pumps

Electrical Isolators

F.F.L.

264

Rubber Flexible connections between pumps and pipework minimises vibration in the pipes.

Domestic Heating Pumps

Small Centrifugal In-line pumps are used. Domestic pumps are sometimes called circulators.That is because the pressure or head developed by the pump only overcomes resistance in the pipework and fittings.A high pressure is not necessary in domestic heating installations, usually about 3 metres head is more than sufficient for most houses.Most manufacturers require domestic pumps to be installed with the motor shaft in the horizontal position as shown below.

Pump Maintenance

Pumps can either be glanded or glandless.The domestic canned rotor pump (circulator) is glandless, that means there is no shaft seal or anywhere for water to leak out.The glanded type pump has a replaceable mechanical seal or gland that may need to be replaced if it leaks.A seal is required since the rotating shaft must pass through a casing to the electric motor.Some manufacturers make seal replacement easy by using split couplings.If a canned rotor pump is not used for several months the impellor may stick.This can be remedied by unscrewing the plug on the shaft and turning the shaft with a screwdriver.

Motor Shaft

Domestic Centrifugal In-line Pump

265

Variable Speed Pumps

Pump pressure and water flow rate can be altered by changing the pump speed.When a system is being commissioned the pump speed is set to give the correct pressure (head) and flow rate of water.Domestic Pumps usually have several speeds (often 3 speeds) and can easily be set by a controller attached to the motor. Variable speed drives are used for larger 3-phase electric motor driven pumps.This flexibility in pump speed can be used as a control mechanism so that when less heat is required the pump speed slows down.See Controls section – Energy Saving Devices for more information on VSD’s.

See Plant Sizing section – Pump Sizing, for details of how to size pumps.

266

IntroductionRefrigeration is necessary for the cooling process in full air-conditioning as well as process cooling and freezing.

The refrigeration cycle takes in energy at a relatively low temperature and discards it at some relatively higher temperature. For example in the domestic 'fridge the evaporator takes in heat from the body of the fridge or the 'ice box' and discards heat at the back of the fridge through a series of coiled pipework called the condenser.

Basis of Mechanical RefrigerationThe principle of mechanical refrigeration is that a liquid is made to 'boil' at a low temperature.When a liquid boils a significant amount of energy is required to make the molecules vibrate enough to break free of the surface and change state into vapour.

If ethyl alcohol is poured onto a sheet of glass the alcohol will evaporate quickly at room temperature and the heat required for this vapourisation is taken from the glass thus cooling the glass, as shown below.

Also, if petrol is poured onto your skin (not recommended) after a while the skin feels cooler because the petrol has evaporated. This principle is adopted for mechanical refrigeration. A highly volatile liquid is passed through a heat exchanger where it boils when in contact with warmer air or water. Heat is extracted from the air or water thus cooling it down.

VapourAlcohol

Glass

VAPAPORISING LIQUID

267

This heat exchanger is called an EVAPORATOR since the volatile liquid is evaporated into the vapour state from the liquid state as shown below.

In the case of water it takes about 2500 kJ of energy per kg of water at 100oC to change water into steam. This is also known as the latent heat of vapourisation.

For a continuous cycle where the volatile liquid doesn’t need to be replaced, the liquid can be condensed back to a liquid and then re-boiled in the evaporator. The CONDENSER is another heat exchanger where the vapourised gas is turned back into liquid.

If we call the volatile liquid the REFRIGERANT and ethyl alcohol is used as in the previous example given, then the vapourised alcohol needs to come in contact with something cool to condense it back to the liquid state. If mains water at 10oC was used as shown below a simple condenser can be made.

Vapour condenses on cold pipes

Cold mains water in

Vapourised alcohol in

Highly Volatile Liquid in

Gas or vapour out

Warm water in

Chilled water out

Evaporation

SIMPLE EVAPORATOR

268

If the evaporator shown above is put in a system with the condenser then a continuous refrigeration cycle is the result as shown below. A pump or compressor can be used to circulate the refrigerant.

Refrigeration Cycle

Cold water in

SIMPLE REFRIGERATION CYCLE

Refrigeration pipework

Expansion valve or Pressure reducing valve or capillary tube.

Heat Input

Heat is rejection to cold water

Compressor

Evaporator

Condenser

Heat

RejectedCooling

Condenser Evaporator

Compressor

269

Refrigerant is like a sponge, compress it and heat comes out of it. Let it expand, and it will soak up heat.

This soaking up of heat can be used to cool air or anything else, like water to form ice.

The compressing and releasing of the refrigerant takes place between the compressor and the evaporator in the standard air-conditioning system. It is the evaporator that does the cooling.

In most air conditioning systems the evaporator is indoors and the condenser is outdoors.

Low-Pressure, Low-Temperature Liquid.High-Pressure, High-Temperature

Liquid.

High-Pressure, High-Temperature Vapour.

Low-Pressure, Low-Temperature Vapour.

HeatCooling

Condenser Evaporator

Expansion Valve

Compressor

270

Two copper pipes connect the outdoor compressor to the indoor evaporator. One pipe brings in the compressed refrigerant in its liquid form to the evaporator, where it is slowly released and allowed to expand. When it expands, the liquid changes back to a vapour, and in doing so, absorbs large amounts of heat from the surrounding air. In other words it cools the surrounding air.

Meanwhile, the expanded refrigerant gas, with its new warmth, is being sucked out of the evaporator into the compressor through the second pipe. This pipe is larger than the one supplying liquid to the evaporator, because the refrigerant has expanded and needs more room, it is known as the suction pipe. The compressor then compresses this refrigerant vapour and pumps it through the condenser.

A condenser in refrigeration is just a piece of apparatus to remove heat out of the system. There are several types of condenser.The air cooled condenser has a fan, which drags outdoor air over the refrigerant. The outside air is warm in summer, but still cools enough to "chill" the refrigerant and help it return to the liquid state.

When the cycle is complete then the refrigerant goes around to be compressed, condensed, expanded and evaporated again and again to achieve cooling in homes, refrigerators, freezers and in commercial buildings.

271

Refrigerants

A refrigerant is the working fluid in a refrigeration system.

At some stages it is a gas and others it is liquid.

An ideal refrigerant should;

Be non-toxic.

Be non-flammable.

Have a low boiling point, boil or evaporate easily.

Condense easily.

Not mix with oil since compressors are lubricated.

Have a high latent heat capacity to transport energy around the ‘fridge system.

Operate at moderate pressures to reduce compressor work and leakage.

Be relatively cheap to produce and store.

Most modern refrigerants, except ammonia incorporate chlorine, fluorine and carbon and are called fluorocarbons.

R11 is a single Chlorofluorocarbon (CFC) compound. It has high chlorine content.

CFC’s are ozone depleting.

R22 is a single Hydrochlorofluorocarbon (HCFC) compound. It has a low chlorine content and low ozone depleting potential.

272

Most older refrigeration systems use either R12 for refrigeration or R22 for air conditioning.

Ammonia is used in large ice making plant.

Modern refrigerants (HFC’s) hydroflurocarbons are mixtures and are used to reduce harmful ozone depleting properties of some older refrigerants.

The tables below give properties of common refrigerants.

273

Older Refrigerant Properties

Refrigerant FormulaBoiling temp.

oC

Condensing pressure

(bar gauge)30oC condensing temperature

Operating pressure

(bar)at 40oC

C.O.P

cfm per 1TR

Critical temp.

oCProperties

R11 C Cl3F 23.8 0.24 9.1 5.0 36.5 198 Non flammable, non corrosive, non toxic, stable, faint odour

R12 C Cl2F2 -30.0 6.4 4.7 5.8 112Little odour, colourless as gas or liquid, non flammable, non corrosive of ordinary metals, stable

R22 CH Cl F2 -40.8 10.8 14.4 4.7 3.6 96Little odour, colourless as gas or liquid, non toxic, non irritating, non flammable, non corrosive, stable

R500C Cl2 F2

(73.8%)CH3 CH F2

(26.2%)-33 7.7 4.7 5.0 -

A mixture of R12 and R152a.Similar to R12Refrigerating effect 18% greater than R12.

R502

C Cl F2

(48,8%)C Cl

F2 - CF3

(51.2%)

-45.6 12.0 4.4 3.6 90A mixture of R22 and R115Non flammable, non toxic, non corrosive, stable.High condensing pressures.

AmmoniaR717

NH3 -33 10.5 4.8 3.4 133

Penetrating odour, soluble in water. Harmless in concentration up to 1/30.Non flammable, explosive.Very efficient refrigerant.Attacks copper.Ammonia has excellent environmental credentials, no ODP and no GWP. It has been extensively used for many years in industrial refrigeration, food processing and breweries. Ammonia has only recently found (limited)Usage in air conditioning applications.Despite its advantages ammonia has some significant drawbacks; it isHighly toxic, concentrations of 0.5% v/v can be rapidly fatal. It is also flammable at concentrations of between 15 to 28 per cent (v/v) in air. Whilst the highly noxious smell of ammonia aids leak detection its extremeunpleasantness can also induce panic. The new European Standard EN378 imposes limits on the use of ammonia.

274

Abbreviations

TR = Ton of refrigerating effect, i.e. the refrigerating effect of 1 ton of ice melting over 24 hours. 1 TR = 3.5 kJ/s.

cfm per 1TR = Cubic feet per minute of refrigerant flow per ton of refrigerating effect i.e. the rate of flow of refrigerant needed for a given cooling duty.

ODP = Ozone Depleting Potential.

GWP = Global Warming Potential

HFC = Hydroflurocarbons

CFC = Chlorofluorocarbon

HCFC = Hydrochlorofluorocarbon.

COP = Coefficient of Performance.

HydroFluoroCarbon (HFC) Refrigerants

HFC’s such as R134a, R404A, R407C and R410A have dominated the replacement of CFC’s and HCFC’s, mainly because they broadly possess similar characteristics as well as having been extensively marketed by manufactures.

275

HFC Refrigerant Properties

RefrigerantOperating pressure

(bar)at 40oC

Boiling Temperature

oC

Maximum cycle

efficiency(%)

GWP(100 yr) Properties

R134a 9.2 -26oC 83% 1300

Non-toxic,

No chlorine content,

Non ozone depleting,

No restrictions on use

Low pressures, high volume.Centrifugal and high speed screw compressors.Widely used as a refrigerant in centrifugal chillers and automobile air conditioningIts low operating pressures make it particularly suitable for use in heat pumps.Good COP’s.Advantages over R407C in service and maintenance procedures.

R404A

R-125/143a/134a 18.0 -46oC 75% 3260High pressures, low volume suits positive displacement compressors.Limited application range because of low critical temperature.Low efficiency

R407C

R-32/125/134a14.4 -44oC 80% 1520

High pressures, low volume suits positive displacement compressors.High critical temperature allows wide application range.Used between The specific heat of vaporisation and the volumetric characteristics are similar to R22, but the heat transfer performance is poorer during both evaporation and condensation.As a zeotrope, R407C exhibits a phenomenon known as ‘glide’, that is changing state (evaporating or condensing) over a range of temperatures.Glide has an adverse effect on the performance of conventional, mixed flow, shell and tube heat exchangers. For best performance, R407C should be used in plant with counterflow plate heat exchangers.Glide can also increase the risk of freeze-up, in the event of a loss of flow or a localised restriction in the evaporator.

R410A

50% R32, 50% R125

23.0 -52oC 76% 1720

Very high pressures suits positive displacement compressors.Low critical temperature.Small systems below 20 kW.Used in some split systems.Very high refrigeration effect and operating pressures that are approximately 50% higher than R22. Heat transfer performance is also better than that of R22, R134a and R407C.R410A has rapidly replaced R407C as the preferred refrigerant in split systems, mini chillers and some packaged units.

276

Zeotropic and Azeotropic Mixtures

Mixtures fall into two categories; Zeotropic and Azeotropic.

A zeotropic liquid mixture shows no maximum or minimum when vapour pressure is plotted against the composition at a constant temperature.

An Azeotropic mixture is a constant boiling mixture.

When an azeotropic mixture is boiled, the resulting vapour has the same ratio of constituents as the original mixture of liquids. Because composition is unchanged by boiling, azeotropes are also known as constant boiling mixtures.

Azeotropes are refrigerants with R5xx designation and zeotrope with R4xx designation.

In flooded systems, zeotropes will change their composition due to the different vapour pressures in the system.

As the system operates, the chemicals with a lower boiling point accumulate within the high pressure-side, and higher boiling point chemicals shift to the low pressure side of the cycle.

Consequently compression ratio increases, refrigerating capacity reduces leading to lower system efficiency.

In flooded systems zeotropes are not recommended for use.

Domestic refrigeration

277

Following the phase-out of R12, R134a was adopted within the domestic refrigeration sector but since then R600a has become widely used because it enables lower noise levels to be achieved.

In addition, R600a offers a slight efficiency improvement.

Commercial Refrigeration

Commercial refrigeration includes stand-alone units such as vending machines, ice cream freezers and bottle coolers, to remote systems such as those used for cold stores and display cabinets in retail outlets.

Integral units generally use R134a (mainly for medium temperature applications) and R404A (mainly for low temperatures).

Air Conditioning

Air conditioning systems includes; small window and split units, multi-split systems and central chillers that provide cooling water to air handlers.

Most integral and split systems previously used R22 until R407C was introduced, but the larger manufacturers are now adopting R410A due to smaller components and marginal gains in efficiency.

R290 is also being used because of its favourable environmental characteristics and good efficiency, and for similar reasons systems using CO2 are being investigated.

Multi-split systems are following the trend of shifting from R407C to R410A for much the same reasons.

278

Most chillers for air conditioning within Europe are positive displacement machines, using reciprocating, scroll or screw compressors. R22 had been the primary choice for these chillers, but the most common refrigerants are now R134a and R407C.

Heat pumps had almost exclusively used R22, but recently most manufacturers have offered units with a selection of refrigerants

including R290, R407C and lately R410A.

The Future

It may be that three refrigerants will dominate commercial air conditioning over the next few years.

R407C – a Zeotropic blend

R134a – a single substance

R410A – an azeotropic blend

279

p-H Diagrams

The pressure enthalpy (p-H) diagram is a useful way to show changes in system pressure and energy changes.

The refrigerant exists as a mixture of vapour and liquid under the Saturated Liquid and Saturated Vapour line.To the left of the Saturated Liquid line the refrigerant exists as a liquid.To the right of the Saturated Vapour line the refrigerant exists as a superheated vapour.On the diagram, the refrigeration cycle is represented by the line 1-2-3-4.1-2 is where the gas is compressed causing a rise in pressure and enthalpy which equalsthe energy put into the gas by the compressor, all in the superheat region. 2-3 is where the gas is condensed to a liquid.3-4 is where the liquid / vapour is passed through an expansion device, the pressure is reduced without any enthalpy change.4-1 is where the liquid / vapour is evaporated completely to a gas and where enthalpy is extracted from surroundings. This is the REFRIGERATION or COOLING effect as shown below.

Saturated Liquid Line

Saturated Vapour Line

Liquid & Vapour Region

Superheated Vapour Region

Sub-cooled Liquid Region

Expansion

Evaporating

Condensing

Compression

p-H Diagram of Refrigeration Cycle

Enthalpy (kJ/kg)

Pressure (Pa or bar)

4

3 2

1

280

Line 2-2’ represents cooling of the superheated gas in the condenser down to the saturated vapour temperature.The remainder of the condensing takes place from 2’-3 where latent heat is removed.If the condenser can sub-cool the refrigerant to a temperature less than saturation vapour temperature then extra Cooling Effect will result as shown below.The work input at the compressor can also be determined from the p-H diagram as shown below.

Sub-Cooling



Sub-cooled Liquid RegionPressure (Pa or bar)

4

3 2

1

2’

Cooling Effect


Saturated Vapour Line

Liquid & Vapour Region



Expansion

Evaporating

Condensing

Compression

Enthalpy (kJ/kg)

Pressure (Pa or bar)

4

3 2

1


281

A typical p-H diagram can be shown for refrigerant R134a.

Compressor Work

It can be seen from the above diagram that the compressor compresses refrigerant from 3.5 bar to 10 bar.The suction pressure is therefore 3.5 bar.

31524268

3.5 bar

10 bar

4oC

34oC

30025020015050 100

3



Enthalpy

(kJ/kg)

Pressure

(bar)

4

2

1

p-H Diagram of Refrigeration Cycle for 134a

282

The delivery pressure is 10 bar.The Work Input to the compressor is 315 kJ/kg - 242 kJ/kg = 73 kJ/kg.The Compressor Work can be calculated as follows;

W comp = m ref (h2 - h1)

Where;W comp = Compressor work (kW)m ref = Mass flow rate of refrigerant (kg/s)h2 = Specific enthalpy at point 2 (kJ/kg)h1 = Specific enthalpy at point 1 (kJ/kg)

If the refrigerant flow rate in the above example is 0.3 kg/s then the compressor work is;

W comp = m ref (h2 - h1) W comp = 0.3 (73) W comp = 21.9 kW

Refrigeration Effect

The Refrigeration Effect can also be determined from the above diagram by using the following formula;

RE = m ref (h1 – h4)Where;

RE = Refrigeration or Cooling Effect (kW)m ref = Mass flow rate of refrigerant (kg/s)h1 = Specific enthalpy at point 1 (kJ/kg)h4 = Specific enthalpy at point 4 (kJ/kg)

RE = 0.3 (242 – 68)RE = 0.3 ( 174)RE = 52.2 kW.

Coefficient Or Performance

The Coefficient of Performance is an indication of how efficient a refrigeration system is.

COP = Refrigeration Effect / Work Inputor

COP = RE / W comp

283

In this example the COP is;

COP = 52.2 / 21.9 = 2.38

Efficient Running

For efficient running the Evaporator temperature should be as high as possible. This is restricted by the dew-point temperature in an air conditioning application. The Condenser temperature should be as low as possible. Maximum cooling is generally required in the hottest summer weather when the condensing arrangements are least efficient and caution is thus necessary in selecting an appropriate temperature.

Further Examination of p-H Diagram

A p-H diagram with some more detail is shown below.

The stages in the cycle are as follows:Stage 1 to 2: the superheated vapour is compressed.Stage 2 to 3: the hot superheated vapour enters the condenser where the first part of the process is desuperheating.

EXPANSION DEVICECOMPRESSOR

CONDENSER

EVAPORATOR

7

5 3

Cooling Effect

6 1Saturated

Liquid Line

Pressure (Pa or bar) Superheated

Vapour Region


Enthalpy (kJ/kg)

4 2


284

Stage 3 to 4: the hot vapour is condensed back to a saturated liquid.Stage 4 to 5: the liquid is subcooled before it enters the expansion valve (this may occur in the condenser, a second heat exchanger or in the pipework connecting the condenser with the expansion valve).Stage 5 to 6: the high pressure liquid passes through an expansion device.Stage 6 to 7: low pressure liquid refrigerant in the evaporator absorbs heat from the air or water being cooled and evaporates to become dry saturated vapour.Stage 7 to 1: the refrigerant vapour absorbs more heat while in the evaporator and while in the pipework joining the evaporator to the compressor, to become a superheated vapour.

285

Chillers

Introduction

For large installations the Condenser, Evaporator, Compressor and Expansion device can be purchased as a package unit, known as a Chiller. The usual package consists of electrically driven compressor(s) mounted on top of two shell and tube heat exchangers, one for the evaporator and the other for the condenser. The cooling coil(s) are piped up to the chiller in the conventional manner as shown below.

Chiller

Fans

Chilled

Water

Chilled

Water

Control Valve

Pump

-

Refrigerant Liquid + Vapour

Refrigerant Gas

4

3 2

1

Expansion valve

Heat Input

Heat Rejection

Evaporator

Condenser

Cooling Coil(s)

286

In some countries the Cooling tower is the preferred method of removing heat from the system.Cooling towers that are open to atmosphere are not often used since the water may become contaminated. A closed cycle cooling tower or evaporative cooler can be used to reduce contamination risk.

Condenser Water

Cooling Tower

Chiller

Chilled

Water

Chilled

Water


Refrigerant Gas

4

3 2

1

Expansion valve

Heat Input

Heat Rejection

Evaporator

Condenser

287

Alternative arrangements are shown for smaller installations where the condenser may be mounted on the roof or external wall of a building and cooled by outside air.The evaporator may be installed directly into the ductwork or air handling unit (AHU) for smaller installations.This is known as a Direct Expansion (or DX) coil.A typical Direct Expansion (or DX) system is shown below.This avoids using condenser water and chilled water in the system and installing the accompanying plant.

Fans


Refrigerant Gas

3 2

Expansion valve

Heat Rejection

Condenser

288

Chilled-water System

In larger buildings and particularly in multi-story buildings, the split-system approach begins to run into problems. Either running the pipe between the condenser and the air handler exceeds distance limitations (runs that are too long start to cause lubrication difficulties in the compressor), or the amount of duct work and the length of ducts becomes unmanageable. At this point, it is time to think about a chilled-water system. In a chilled-water system, the entire air conditioner is situated on the roof or behind the building. It cools water to between 4.0oC and 8.0oC.

289

This chilled water is then piped throughout the building and connected to the cooling coils in air handlers as needed. There is no practical limit to the length of a chilled-water pipe if it is well-insulated.

Chilled Water Temperatures

Typically chilled water flow and return temperatures to cooling coils is generally between 7oC and 12oC, depending upon the dew point to be maintained.When this water is pumped through the evaporator section of the chiller this water temperature will be lowered by about 4oC to 6oC.In order that the necessary heat transfer may take place, the refrigerant must be at some temperature below that of the leaving water but, at the same time, it must generally be slightly above freezing point.

In a typical case, the following water temperatures may be used:

Apparatus dew point 12°C Cooling coil outlet 10°C

290

Cooling coil inlet 6°C Water at evaporator outlet 5.5°C

The refrigerant in the evaporator would in this case be maintained at about 1°C giving a differential for 4.5°C for heat transfer.As will be appreciated, this small temperature potential means that the cooling surface of a simple tubular type would need to be very extensive: a variety of devices has been developed to augment the transfer rate.

Ethylene/glycol solutions may be used in cooling coils in order to allow lower air temperatures to be obtained.The temperatures of the fluid circulating may be -7°C from the evaporator and -3°C returning to it, or lower as required.

In instances where cooling for an air-conditioning system is provided from a refrigeration machine by direct expansion, the refrigerant is piped directly to cooling coils in the air stream which thus become the evaporator. The surface temperature of the coils is a function of the leaving air temperature required, the form of the coil surface and the velocity of the air flow. Refrigerant temperatures much below freezing point are inadmissible owing to the risk of build-up of ice on the coil surface when dehumidification is taking place. An apparatus dew point of 3°C is normally considered as the practical minimum for such coils if frosting is to be avoided.

Thermosyphon Cooling

Water chilling may be accomplished without using the compressor under favourable, cool, outside conditions. If the condenser is at a higher level than the evaporator the compressor may be by-passed and the evaporator connected directly to the condenser. Superheated refrigerant vapour leaves the evaporator and migrates upwards to the condenser. If the condenser coolant (air or water) is cold enough, the refrigerant will condense and sub-cool. The expansion valve or the float valve is also bypassed and sub-cooled liquid refrigerant drains by gravity into the evaporator and is available for chilling water. The performance is like a heat pipe and the compressor is not used.

291

Absorption Chillers

The vapour absorption refrigeration cycle uses two fluids in solution, one a refrigerant and the other an absorbent. The affinity of the absorbent for the refrigerant on the one hand and the application of heat on the other, are used to vary the strength of the concentration of refrigerant in the absorbent. An absorber and a generator take the place of the compressor (heat being supplied at the generator instead of power at the compressor), but the evaporator and the condenser remain as the pieces of equipment where heat is removed from the water being chilled and where surplus heat in the process is rejected to the outside.

Water chillers using lithium bromide as the absorbent and water as the refrigerant are commercially available with capacities from 350 kW to 6000 kW of refrigeration employing steam at absolute pressures from 115 kPa to 1293 kPa as the source of thermal energy for the generator. MTHW and HTHW have been used, instead of steam; but have not given satisfaction because of the thermal expansion associated with the difference between the flow and return temperatures.

Because of fundamental thermodynamic limitations, coefficients of performance exceeding 1.0 are not really practical in Absorption Chillers. Realistic coefficients at design conditions are 0.6 to 0.72 and steam consumptions are about 0.9 g/s for each kW of refrigeration. The refrigerant may crystallize because of control mal-function, air in the system, failure of the expansion valve, or interruption to the electrical supply (needed to operate certain pumps in the plant). Crystallization is a nuisance rather than a disaster but water-cooled machines are preferred over air cooled because there is less risk of crystallization.

292

Refrigeration – Heat Pumps - Page 1 2 3 4 5 6

IntroductionThe heat pump is used to produce heat from a low-grade energy source such as outside air, a lake, pond or seawater.The refrigeration cycle is utilised so that the heat rejected at the condenser is used to heat a building as shown below. This is the opposite function of the condenser when the ‘fridge cycle is used for air conditioning, in which case the heat rejected is lost to the atmosphere.

The compressed gas from the compressor is passed to the condenser where heat is removed for use, and in the evaporator the refrigerant absorbs heat at a relatively low temperature from the heat source.


Expansion device

Heat Input from HEAT SOURCE

Heat output

Compressor

Evaporator

Condenser

Heat Pump Refrigeration Cycle

http://www.arca53.dsl.pipex.com/index_files/hpfrig5.htm

http://www.arca53.dsl.pipex.com/index_files/gshp.htm





http://www.arca53.dsl.pipex.com/index_files/fridge.htm


293

A typical example would be an air to air heat pump, which extracts heat from outside air even at very low winter temperatures by using the vapour compression refrigeration cycle in reverse mode. This can produce warm air at the condenser to heat a building.Heat pumps work best when a building needs to be heated during winter and cooled in summer.

One of the disadvantages of a heat pump is the amount of electrical energy used to drive the compressor. This can be overcome in large installations by running the compressor from a gas driven engine. Another difficulty is noise and several heat pumps can be linked to one remote compressor to reduce local noise. This has the added advantage that compressor maintenance is centralised.


Heating and Cooling

The diagram below shows a refrigeration plant with a reversing valve to reverse the direction of flow of refrigerant. The reversing valve is set for the HEATING mode.










294

The indoor heat exchanger becomes a condenser giving out heat whereas the outdoor heat exchanger is the evaporator and takes in heat energy from a low grade source.

NRV

NRV

Expansion valve for heating mode

Reversing valve

Heat output

Indoor heat exchanger

Expansion valve for cooling mode


Heat Input

Compressor

Outdoor heat exchanger

Heat Pump in Heating Mode

295

The diagram below shows the same plant with the reversing valve set for the COOLING mode. The refrigerant travels around the system in the opposite direction and arrives at the Outdoor heat exchanger first after the compressor. This is effectively the condenser where heat is rejected to atmosphere. The refrigerant passes through a non-return valve and expansion valve where it then passes through the Indoor heat exchanger, which is the evaporator or cooling coil.

The reversing valve is an effective method of using one item of plant to heat and cool.

NRV

NRV


Reversing valve

Heat Input

Indoor heat exchanger



Compressor

Outdoor heat exchanger

Heat Output

Heat Pump in Cooling Mode

296

This is appropriate for rooms such as glazed foyers where the temperature is low in the morning and high when the sun shines.

Heat pumps are similar to room air conditioners in that the same components are used with the addition of a reversing valve. This means that the designer could consider up-grading a room air conditioning scheme to a heat pump scheme. A split system can be used for heat pumps as in air conditioning so that indoor and outdoor units are similar to those used in air conditioning. See Air Conditioning section of the notes.


Coefficient of performance (COP)

This is used to define heat pump efficiency.

COP = T1 / ( T1 - T2 )

Where;

COP = coefficient of performance

T1 = condensing temperature (oK).

T2 = evaporating temperature (oK).

It is normal to express this coefficient as the ratio of energy output to energy input, rather than use temperatures, as shown below.

COP = Q / W






http://www.arca53.dsl.pipex.com/index_files/hpfrig.htm




297

Where;


Q = high grade energy output (kWh or kJ).

W = electrical or mechanical energy input (kWh or kJ).

Typically, a heat pump operating from a source temperature of 5°C and designed for heating only would have a COP of about 3.0, whereas a reversible machine for the same conditions would have a COP of about 2.6.

The diagram below shows typical COP’s for a heat pump. It can be seen that the COP is increased with higher evaporating temperatures, this means that the higher the low grade energy source temperature the higher the COP. In winter water sources can be at a higher temperature than outside air, these include; lakes, deep ponds and rivers. Other sources of low grade energy are sub-soil, and waste products from industry such as effluent and power station waste water.

-10 oC

2.8

4.2

3.6

2.6

3.8

4.4

4.0

3.4

3.0

45oC

50oC

55oC

High grade heat output or condensing temperature (oC)

+5 oC0 oC-5 oC

3.2

COP

TYPICAL HEAT PUMP PERFORMANCE

Low grade source or evaporating temperature (oC)

298

In summary, the coefficient of performance (COP) is improved if the temperature of the cold side (heat source) is raised. For this reason heat pumps can be seen as devices able to convert low grade thermal energy to useful heat.


Types of Heat Pump

Heat pumps may be classified under the following types;

1. Air-to-air (air source to air heating system).

2. Air-to-water (air source to water heating system).

3. Water-to-water (water source to water heating system).

4. Water-to-air (Water source to air heating system).

5. Ground source to water.










299

The diagram below shows a heat pump using pipes buried in sub-soil as the low grade energy heat source.

Hot/Warm water flow

Pump

Radiators

EVAPORATOR

CONDENSER Pipework

Expansion Valve

Compressor

Drive unit e.g. electric motor

Heat input from source - water pipes laid in subsoil.

HEAT PUMP USING SUB-SOIL AS HEAT SOURCE AND REFRIGERANT TO WATER CONDENSER

Heat rejection to rooms via heat emitters

300

It is possible to use solar energy in the heat pump system, this is suitable in countries where solar radiation is not at consistently high levels throughout the year and the low temperatures achieved from solar panels can be upgraded to a higher more useable temperature for space heating.

De-icing

Air to air heat pumps can be effective in their use of energy. But one problem with most heat pumps is that the coils in the outside air collect ice. The heat pump has to melt this ice periodically, so it switches itself back to air conditioner mode to heat up the coils. To avoid pumping cold air into the building in air conditioner mode, the heat pump also switches on electric heaters to heat the cold air that the air conditioner is pumping out. Once the ice is melted, the heat pump switches back to heating mode.

301

Refrigeration - Heat Pumps - Page 1 2 3 4 5 6

Ground Source Heat Pump

Introduction

In this country the earth, a few meters below our feet, is at a constant temperature of about 11-12oC throughout the year.

Because of the ground's high thermal mass, it stores heat from the sun during the summer.

Ground source heat pumps (GSHP) can pump this heat from the ground into a building to provide space heating.

For every unit of electricity used to pump the heat, 3-4 units of heat are produced.

There are three important elements to a GSHP:

• Ground loop - Comprises lengths of plastic pipe buried in the ground, either in a borehole or a horizontal trench.

The pipe is a closed circuit and is filled with a mixture of water and antifreeze, which is pumped round the pipe absorbing heat from the ground.










302

• Heat pump - A heat pump works by using the evaporation and condensing of a refrigerant to move heat from one place to another.

In this case, the evaporator the takes heat from the water in the ground loop; the condenser gives up heat to a hot water storage tank, which feeds the distribution system.

A compressor, which uses electricity, moves the refrigerant around the heat pump.

It also compresses the gaseous refrigerant to increase the temperature at which it condenses, to that needed for the distribution circuit.

• Heat distribution system - Consists of underfloor heating or radiators for space heating.

Some systems can also be used for cooling in the summer.

Distribution System inside building e.g. Warm water flow to radiators or to underfloor heating pipes.

Pump

CONDENSERPipework

Expansion Valve

EVAPORATOR Compressor

303

Installation Costs

The installed cost of a GSHP ranges from about £800-£1,200 per kW of peak heat output, excluding the cost of the distribution system.

Trench systems tend to be at the lower end of this range.

304

The installed cost of a typical 8kW system would therefore vary between £6,400-£9,600 plus the cost of the distribution system.

Earth Loops

Horizontal loops are common in some areas. A 1.2 to 2.0 metre deep trench is dug using a backhoe or chain trencher. The pipe is laid out, sealed and pressure tested. Then the trench is filled. The land area required ranges from 75 to 150 square metres per ton (3.5kW) of refrigeration.

Vertical loops require less pipe than horizontal loops. Well-drilling equipment bores small diameter holes 15 to 60 metres deep. Two pipes joined with a U-bend are inserted into each hole. The system can use one deep hole or several shallow ones.

Verticals can be installed almost anywhere. The cost of drilling is often greater than trenching. Pond loops reduce excavating costs by placing much of the loop in a pond, lake or stream. In most cases, 1/4 to 1/2 acre of water surface and a minimum depth of 2 to 2.5 metres is required.

The Heating Cycle

In the heating cycle, the ground water, the antifreeze mixture, or refrigerant (which has

Ground loop pipes

Earth Loop

305

circulated through the underground piping system and picked up heat from the soil), is pumped back to the heat pump unit inside the building. It then passes through the refrigerant-filled primary heat exchanger for groundwater or antifreeze mixture systems. In DX systems the refrigerant enters the compressor directly, with no intermediate heat exchanger.

The heat is transferred to the refrigerant, which boils to become a low temperature vapour. In an open system, the ground water is then pumped back out and discharged into a pond or down a well. In a closed-loop system, the anti-freeze mixture or refrigerant is pumped back out to the underground piping system to be heated again.

The reversing valve sends the refrigerant vapour to the compressor. The vapour is then compressed which reduces its volume, causing it to heat up. Finally, the reversing valve sends the now-hot gas to the condenser, where it gives up its heat to a water heating system. Having given up its heat, the refrigerant passes through the expansion device, where its temperature and pressure are dropped further before it returns to the first heat exchanger or to the ground in a DX (direct expansion) system, to begin the cycle again.

Underfloor heating pipes or heat emitters at 35

Primary Heat Exchanger

Refrigerant

NRV

NRV


Reversing valve

Condenser



Compressor

306

Cooling Cycle

A heat pump can also be used for air conditioning in summer.The cooling cycle is basically the reverse of the heating cycle. The direction of the refrigerant flow is changed by the reversing valve. This works best if a ground source to air heat pump is used instead of a ground source to water unit.The refrigerant picks up heat from the house air and transfers it to the outside into a water body or return well (in the case of an open system), or into the underground piping (in the case of a closed-loop system).

Environmental Issues

Because heat pumps consume less primary energy than conventional heating systems, they are an important technology for reducing gas emissions that harm the environment, such as carbon dioxide (CO2), sulphur dioxide (SO2) and nitrogen oxides (NOx).

However, the overall environmental impact of electric heat pumps depends very much on how the electricity is produced.

Heat pumps driven by electricity from, for instance, hydropower or renewable energy reduce emissions more significantly than if the electricity is generated by coal, oil or gas-fired power plants.

Ground Source Heat Pump linked toUnderfloor heating system

307


Heat Pump Economics

Coefficient of Performance (CoP)

In order to transport heat from a heat source to a heat sink, external energy is needed to drive the heat pump.

Theoretically, the total heat delivered by the heat pump is equal to the heat extracted from the heat source, plus the amount of drive energy supplied.

Electrically-driven heat pumps for heating buildings typically supply 100 kWh of heat with just 20-40 kWh of electricity.

Many industrial heat pumps can achieve even higher performance, and supply the same amount of heat with only 3-10 kWh of electricity.

The efficiency of a Heat Pump system is measured by the Coefficient of Performance (CoP).

This is the ratio of the number of units of heat output for each unit of electricity input used to drive the compressor and pump.

Typical CoPs range between 2.5 - 4.0.

The higher end of this range is for underfloor heating, because it works at a lower temperature (35-45oC) than radiators (60-80oC).









308

In simple terms, if a heat pump has a COP of 3.0 then for every 100 KWh of electrical energy input there will be 300 kWh of heat energy available, since;

COP = Q / W

Where;


Q = high grade energy output (kWh or kJ).

W = electrical or mechanical energy input (kWh or kJ).

Rearranging Q = COP x W

Q = 3 x 100

Q = 300 kWh

Running Costs

Based on current fuel prices, assuming a CoP of 3-4, a GSHP can be a cheaper form of space heating than gas, oil, LPG and electric storage heaters.

An approximate evaluation is given by;

Normal tariff Electric cost = 15.03 p/kWh. (2009) (see Fuel Costs section)

If the heat pump CoP is 3 then, cost of heat is 15.03 / 3 = 5.01 p/kWh.

This is cheaper than oil at current price of; 5.44 p/kWh and gas at 6.59 p/kWh. (2008) (see Fuel Costs section)

309

If grid electricity is used for the compressor and pump, then an economy 7 tariff usually gives the lowest running costs.

Current E7 electricity cost is 6.60 p/kWh, therefore cost of heat with GSHP is; 6.60 / COP 3 = 2.20 p/kWh.

This is significantly cheaper than any other heating energy source but installation cost is higher than oil or gas fired heating.

This assumes that all the heating can be achieved at night time and there is a storage system so that hot water can be pumped around the underfloor heating system when required.

This does not include the increased maintenance costs of the compressor and associated equipment in a heat pump.

The economics for heat pumps are even better if low cost electricity is used or the compressor is driven by a gas or oil engine using very cheap fuel.

Also the installation cost may be justified if air conditioning as well as heating is necessary.

If 2 hours of ‘topping up’ is required from the grid on very cold days then the cost is;

Assume the heat pump is sized to heat the building during 7 hours of off-peak electricity.

7 hours x 6.60 p / kWh = 46.20 p / day / kWh

2 hours x 15.03 p/ kWh = 30.06 p / day / kWh

Total = 76.26 p / day / kWh

Average cost of electricity per day = 76.26 p / kWh / 9 hours

310

= 8.47 p / kWh

= 8.47 p / kWh / COP 3

= 2.82 p / kWh

This makes the system still less expensive than oil or gas heating at about 2.8 p/kWh.

The extra installation and maintenance costs of a heat pump and ground source pipe coil may defer some of these savings.

A storage vessel (or buffer vessel) may be incorporated into the heating system that stores heat at nighttime in readiness for use the next day.










311

Comparison of Fossil Fuels, Biomass and Electricity for Heating Buildings

We will look at four fossil fuels, one biomass fuel and two common forms of electricity available in most countries:

Coal Oil Natural Gas Liquid Petroleum Gas (LPG) Wood Chips and pellets Electricity Ordinary rate Electricity Low Tariff rate.

There are several ways to compare the various sources of energy.The following discussion looks briefly at some of the factors.The factors are:

1. Calorific value2. Electrical tariffs3. Efficiency4. Availability & Security of supply5. Security6. Payment7. Handling8. Safety9. Environmental issues10. Other uses11. Running costs

1. Calorific Value

The calorific value of a fuel is the quantity of heat energy released in Mega Joules per kg of fuel as a result of combustion.The gross calorific value (GCV) includes the latent heat of any water in the fuel as it turns to steam and the net calorific valve (NCV) excludes this extra small amount of heat energy.

312

Anthracite may contain up to 3% moisture, light oils and gas contain less than 0.5% moisture.Generally the GCV is used in cost calculations.

The GCV of coal is around 30 MJ/kgFor oil the GCV is about 46 MJ/kgThe GCV of wood pellets is around 18 MJ/kg and wood chips about 12 MJ/kg.

It can be seen that oil has a higher calorific value than coal (about 53% higher) and wood pellets and chips have calorific values much less than coal.It is not so easy to compare the Calorific value of a gas since figures are normally in Mega Joules per cubic metre or Mega Joules per litre if the gas is under pressure and liquefied as in Liquid Petroleum Gas.

The GCV for natural gas is about 38 MJ/m3

The GCV for LPG is about 25 MJ/litre under pressure at 6 bar.

Fuel Gross Calorific Value (MJ/kg) Density (kg/m3)

Wood chips (30% moisture content) 12.6 250Log wood (stacked - air dry: 20% m.c.) 14.7 350-500

Wood (solid - oven dry) 19 400-600

Wood pellets 17-18 600-700

Miscanthus (bale - 25% m.c.) 13 140-180

House coal 27-31 850

Anthracite 33 1,100

Heating oil 46 845

Natural gas (NTP) 54 0.7

LPG 49.7 510

Lignite 20-24 650-780

Peat – sod (35% m.c.) 13 350

Peat – briquettes (15% m.c.) 17 750

2. Electrical Tariffs

313

Some electricity suppliers provide different tariffs for the user.A tariff is the cost per unit of electricity used.A unit of electricity is a Kilowatt hour or 1 Kilowatt for one hour.One typical tariff is for weekend use; another is for night-time use.This is because electricity-generating plant can’t really be switched off at night-time or periods of very low demand, so companies try to sell ‘unwanted’ electricity as best they can.Some tariffs in use are:

1. Domestic Tariff2. Small Business Tariff3. Industrial Tariff4. Off-peak tariff5. Seasonal Tariff6. Weekend Tariff (Commercial)

Some tariffs have limitations e.g. Domestic Tariff (estimated maximum demand not exceeding 70kVA).The off-peak tariff can be also known as Economy 7 tariff because it is available for seven hours at night-time. In some cases, if a boost during the day is required this may be charged to the user at a higher rate. Other charges can be made by electricity suppliers such as standing charges; these have not been included in any cost analysis.Some suppliers also divide up the cost to the user on a volume basis e.g. for the first 250 units the cost is 10 pence per unit and for additional units the cost is 9 pence per unit.

3. Efficiency

The efficiency of boilers, stoves, fires and furnaces may be a subject of debate.Modern oil and gas boilers have efficiencies above 80%.That is 80% of the energy in the fuel is used to heat water or air.Coal fired plant generally has a lower efficiency. Modern wood pellet and chip boilers have efficiencies as high as 80-90%.An open fire has a very low efficiency but let’s assume that solid fuel is burnt in some kind of boiler with an efficiency of 70%.Condensing boilers have an efficiency usually above 90%.

Electricity can be taken as 100% efficient but storage heaters are normally oversized to allow for the difference between the rate of electrical energy input and the slower rate of heat release from the storage material.

314

Also storage heaters are not as 'controllable' as other forms of heating since the storage blocks heat up at night whether or not their full capacity is used the next day. It may be necessary in cold weather to have a top-up of electricity during the latter part of the day at a higher cost than the low tariff or Economy 7 rate. This means that in practice the low tariff or Economy 7 heating process can have a much lower efficiency than 100% and the actual figure will vary for each project. For this costing exercise we will assume 100% efficiency but a more detailed study should be made for greater accuracy.The efficiency of plant may deteriorate over time; therefore in the Comparison of Fuel Costs section rather conservative figures have been used.

4. Availability and Security of Supply

Sometimes there can be problems with the supply of fuels. Oil supply is dependent on variable world prices on the ‘spot’ market and a war or unrest can upset this price. Union strikes can also lead to supply problems.Sometimes to be safe, a dual fuel burner can be used to burn oil or gas, and so the cheaper or available fuel can be used. Also large boilers can have two different sets of burners e.g. to burn oil or pulverised coal.

The supply of coal in a country can also be unsecure and suffers some of the same problems as oil e.g. union strikes, closure of inefficient coalmines.Bad weather, union strikes, and lack of primary fuel or faults can interrupt electricity supply in the system.Liquid Petroleum Gas (LPG) is a by-product of the oil refining industry and the price is linked to the world crude price. The production of LPG is also to some extent dependent on security of oil supply although local storage is a benefit.

The amount of primary fuel left on the planet is a debating point. Several decades ago it was thought that the world reserves of oil might run out in our lifetime but new finds have been discovered and new ways to extract oil have been used, sometimes in the most difficult circumstances. Although we shouldn’t be complacent, there seems to be enough primary fuel to give us time to find and develop new ways to produce energy for heating and other uses.

Wood pellets and wood chips are a relatively new source of heat in the UK but not in some other European countries.

315

The industry that supplies wood products is growing rapidly but not mature enough to supply to a high proportion of users.It defeats the sustainability of wood products if they are transported over long distances.The wood supply industry has some catching up to do to meet an ever growing demand for pellets and wood chips.

5. Storage

One of the benefits of using piped natural gas and electricity is that a storage facility is not required. Oil, coal, LPG and wood pellets/chips require appropriate storage installations.In the case of oil a tank or tanks is the usual way to store the fuel.Tanks can be made in plastic (polyethylene) or steel for larger installations.

Ultraviolet protected Polyethylene will not rust and does not require to be painted.Oil tanks for larger installations may be installed underground to free up space.It is a good idea to protect oil tanks from vandalism with a fence or wall.Coal bunkers vary from small concrete, plastic types to large constructions. If large quantities of coal are stored in a confined space (e.g. a ship’s hold) it is possible, given the right conditions, that self-combustion can occur if heat builds up.

Storage rooms and bunkers are required for wood pellets and chips.Smaller boilers may have a hopper beside the boiler but an area is required for long term storage of wood products.Space is also required for augers to transport pellets and chips to the boilers.

Liquid Petroleum Gas (LPG) installations vary from a cylinder to several tanks. LPG can be stored at around 7 bar pressure in the liquid form with pressure reducing stations at the tank and as the supply enters the building. The pressure is further reduced at the gas appliance.Gas tanks are steel welded and usually installed above ground.LPG is heavier than air so there should be no drain in the vicinity of the gas tanks. LPG tanks are normally placed in a remote area or as far away from buildings as possible; there are regulations for plant location and safety distances for LPG tanks and cylinders.

6. Payment

In some circumstances it is important that an easy payment system in available for a building occupier. If an office building has a central heating system and is divided up and let to several clients, it is quite difficult to charge the occupier for the amount of fuel used.

316

In this type of building it may be appropriate to use natural gas or electricity to heat the building in a de-centralised system so that occupiers can be charged directly by the gas or electricity supplier.It is worth noting that some suppliers of fuel and energy give discount for prompt payment.There is an obvious benefit for large users of fuel and electricity to negotiate a fixed price over a given time period.

7. HandlingOf all the fuels, coal (in its many forms) is the most difficult to handle.To get coal from the bunker into the boiler presents some problems.Mechanical conveyors are used in large plant and automatic feed systems have been developed to supply coal in the right quantity to a boiler.

One system uses pulverised coal, which is easier to transport since it moves more like a fluid than a solid. Also pulverised coal burns more efficiently since more air is in contact with the smaller particles of fuel. Two of the difficulties in conveying coal are dampness causing the fuel to become clogged in the system and coal dust causing electric motors to fail and bearings to wear prematurely.

Oil can be conveyed over vertical and horizontal distances with pumps. The heavier oils need to be preheated in the tank to make them fluid enough to move in the pipeline. Trace heated pipes are also a feature when using heavy oil and the burner also needs to be heated in some cases.

Class E,F and G oils need to be heated to at least 10oC, 30oC and 45oC respectively to enable them to move in pipework.

Gas moves from the storage tank or gas main under its own pressure, so of all the fuels it ranks with electricity as easier to handle.If a vapourising gas supply pipe is too small and a high pressure drop is experienced with a high volume flow rate then it can be possible to cause freezing of the pipeline but this is unusual in the building services field of engineering.

Wood Pellets are relatively easily handled with electrically driven augers.

8. Safety

317

It is difficult to say which of the energy sources has the best or the worst safety record.There are potential difficulties with any of the fuels; oil, coal, natural gas, wood pellets/chips and LPG all have a fire risk, although if a gas leak does occur, the results can be devastating.There are various safeguards, which must be designed into a fuel storage system.The following are some of the safeguards we use:

o Vent on oil tank.o Overfill alarm on large oil tank.o Careful siting of plant and pipework. o Fire wall in fuel storage area. Protect oil tanks in hot countries.o Fire wall between wood storage facility and pellet/chip boilers.o Pellet /chip augers that prevent fire and smoke spread into storage

areas.o Level alarms on oil daily service tank.o Automatic or Fire Authority foam system in basement oil tank rooms.o Inert gas (e.g. Halon) burner protection system.o Fire authority approval of storage site.o In the event of a fire, adequate fire-fighting systems available near fuel

storage.o Heat/smoke detector at burner or fusible link at burner.o Electrical continuity and earthing of LPG tanks.o Safety valves on oil/gas supply. Visible safety shut-off valves.o Flame failure device and other safety devices incorporated into oil / gas

burner.o Burner purging system to remove un-burnt fuel.o Pressure relief valve on LPG tank.o Manholes on large LPG tanks for inspection.o Shut-off valve and emergency shut-off valves on LPG tanks.o LPG tanks should be protected from corrosion.o Appropriate marking and information on LPG tankso Pressure and vacuum relief valves on LPG tanks.o Gas pressure regulator at appliances.o Electrical earthing systems for steel tanks, pipes, etc.o Gas leak detection system.o Appropriate signage and warning signs.o Appropriate pipework and pipework jointing system. Tests on systems.o Electrical supply protection devices.o Adequate air supply to burner.

318

o Plant room safety – alarm, emergency shut-off, fire extinguisher systems.

Incomplete combustion can lead to production of carbon dioxide and carbon monoxide. Burners should be well maintained and have adequate air supply. Solid fuel appliances in homes should be supplied with air for combustion and an appropriate chimney system. Rooms, in which there is a device for burning fuel, should be well ventilated. Carbon monoxide gas, if produced, does not have a smell and is lethal. Most countries have regulations for room heaters, fires and stoves and the engineer should be aware of all the design issues.

9. Environmental IssuesWhen fuel is burnt there are some by-products.Some of the gaseous by-products are carbon dioxide, carbon monoxide, sulphur dioxide, the Nox gases and water vapour.Some of the liquid by-products, which can occur, are dilute sulphuric acid and condensation.

The solid by-products from coal burning are ash, clinker and soot.To minimise these products from boilers the engineer can choose low sulphur fuels or some of the ‘cleaner’ fuels such as natural gas. Natural gas is 95% methane and contains no sulphur. It burns cleanly and is relatively easy to combust.It is quite difficult to burn coal, even good quality coal, as ‘cleanly’ as the other fuels. Pulverised coal burning in a fluidised bed is an attempt to reduce the emissions and make the combustion process more efficient.In some urban areas normal coal burning is prohibited in domestic appliances and ‘smokeless’ coal has to be used.A hybrid system, which uses alternative or sustainable energy sources as well as fossil fuels, is a step ahead in terms of reduced environmental impact.Using high efficiency boilers e.g. condensing boilers reduces the amount of fuel used and therefore the amount of harmful emissions.Ash produced from wood pellet and chip boilers is minimal and can be used to improve soil quality.

10. Other UsesIf natural gas or LPG is used for heating, it may also be used for cooking and refrigeration/air conditioning.

319

Some householders prefer to cook with gas and it is the popular fuel for commercial kitchens. The savings in running costs are quite substantial even for domestic use as demonstrated in the section – Comparison of Fuel Costs.

The heat absorption refrigeration cycle uses gas as a heat source instead of a compressor driven mechanical refrigeration process. It should be pointed out, however, that the heat absorption cycle is not as efficient as the more commonly used mechanical refrigeration system.

Natural gas can also be used to drive internal combustion engines for a Combined Heat & Power scheme where electricity is generated and rejected heat is utilised to heat a building. Diesel fuel can be used in a similar manner or in larger scale CHP schemes a gas driven turbine may be the prime mover.

11. Running CostsThe running costs of a heating system depend on various factors. The following is a list of some of these factors:

Purchased price of fuel. May be negotiated. Efficiency of process. Usually in the range 70% to 100%. Maintenance cost. Totally automatic systems require less looking after. Coal-fired systems require more maintenance. Domestic systems may be inspected once a year. Disposing of by-products. Ash, clinker from solid fuel plant. Flue gas clean up. Large-scale systems using coal or heavy oil may need flue gas

scrubbing plant. Heavy oil plant trace heating and tank heating costs. If electric heating is used this

may be a substantial cost.

SummaryIn summary, a fuel with; low purchase price, high efficiency and low maintenance would be ideal for heating systems. This usually narrows the choice to; Kerosene (28 sec), Gas Oil (35 Sec), Natural gas, Wood Chips/pellets or LPG for most installations in most countries. If the off-peak electricity tariff is low enough and the overall daily efficiency is above 80% then one could add off-peak electricity to the above choice.

320

Comparison of Domestic Fuel Costs

There are several ways to compare fuel costs for heating systems.

A similar calculation can be done for air conditioning systems.

One method is to compare on a price per kilowatt-hour basis; another is on a price per Mega joule of available heat from the fuel basis.

A Mega joule (MJ) is a unit of energy.

The comparison below uses price per kilowatt-hour (In the U.K. this is pence/kWh).

We will take into consideration the differing Calorific Values of fuels and different efficiencies of boilers and combustion processes.

We will look at seven sources of energy available in most countries, which are:

1. Oil2. Coal3. Natural Gas4. Liquid Petroleum Gas (LPG)5. Electricity Ordinary rate6. Electricity Low Tariff rate7. Wood Pellets

The table below shows the current fuel prices in Northern Ireland.

The prices are for Domestic use with no discounts.

For high volume commercial/industrial use, prices may vary considerably.

You could compare prices in your own area.

321

Current Fuel Prices

DATE : 31st October 2011

Fuel or Heat Source Type Net Price Price including5% VAT Additions

Oil Kerosene 28 secs. £0.58 per litre

Coal Anthracite £15.00 per 50kg bag

Natural Gas - Domestic Tariff

MethaneFirst 2000 kWh 6.095 p/kWh 6.40 p/kWh

Direct debit discount is £22.05 (inc. VAT) for consumption above 2000 kWh per year.Methane

After 2000 kWh 4.065p/kWh 4.259 p/kWh

Natural Gas – Industrial and Commercial Tariffs

MethaneFirst 2000 kWh 6.095 p/kWh

MethaneFrom 2000 kWh to 73,200 kWh consumption per year.

4.065p/kWh

MethaneMore than 73,200 kWh consumption per year.

3.75 p/kWh

LPG -Stored Gas. Propane 57.6 p/l

322

ElectricityOrdinary Domestic rate – Home Energy Tariff

15.31 p/kWh http://www.nihe.gov.uk/latest_tariffs

Electricity Economy 7 rate(1am to 8am) 7.50 p/kWh Standing charge 11.96 pence per day.

Electricity Small Business Popular Tariff

Wood PelletsWood by-product, Bulk delivery

£200 / tonne

http://www.nihe.gov.uk/latest_tariffs

323

Let us look at each fuel and fuel price in some more detail.

1.0 Heating Oil

In commercial and industrial buildings 35second oil is used, that is, oil with a viscosity of 35sec Redwood No.1.

In domestic premises 28second oil is used, this is sometimes known as Kerosene.

Usually 35second oil is about 0.5 pence per litre cheaper than 28second oil but the saving is out-weighed by the slightly lower calorific value.

Price Comparison

28 sec. Price 58 p/litre

GCV = 45.5 MJ/kg and Relative Density = 0.835 kg/m3

Therefore GCV per litre = (45.5 X 0.835 X 1000) \ (1000) = 38 MJ/l.

since ( p/l) / (MJ/ I) = p/MJ

so 58 / 38 = 1.53 p/MJ

If an oil-fired boiler is 85% efficient then the price per MJ can be weighted for the efficiency of the process.

324

Therefore cost is: 1.53 p/MJ / 0.85 = 1.80 p/MJ.

1 kWh = 1 kJ/s . h = 1 kJ/s.3600 s = 3600 kJ = 3.6 MJ.

Therefore 1 MJ = 1 / 3.6 kWh = 0.278 kWh

The actual cost per kWh is therefore: 1.80 / 0.278 = 6.46 p / kWh.

2.0 Coal

The fuel chosen for comparison is Anthracite.

Anthracite is suitable for combustion in several types of fire or stove (furnace) and burns well with little clinker and ash.

Also Anthracite has a fairly good calorific value of 32 MJ/kg.

You may have figures for other types of solid fuel to compare costs.

Price Comparison

Anthracite costs: £15.00 per 50kg bag = 30 p / kg

GCV Anthracite = 32 MJ/kg.

325

since ( p/kg) / (MJ/ kg) = p/MJ

so 30 / 32 = 0.938 p/MJ

If a coal-fired boiler is 70% efficient then the price per MJ can be weighted for the efficiency of the process.

Therefore cost is: 0.938 p/MJ / 0.7 = 1.34 p/MJ.

1 MJ = 1 / 3.6 kWh = 0.278 kWh


3.0 Natural Gas

Price Calculation

The price in pence per kWh is calculated as follows:

Price: 6.40 p/kWh for the first 2000 kWh, and 4.259 p/kWh after the first 2000 kWh in Northern Ireland. (the GCV of natural gas is 38.2 MJ/m3)

326

To allow for the natural gas lower tariff after the first 2000 kWh we need to estimate annual energy usage.

The average annual heating energy bill for a household is £780 (2004)

Divide this by the 2004 cost per kWh of gas @ 4.38 p/kWh.

78,000 p / 4.38 p/kWh = 17,808 kWh annual energy usage.

The average natural gas price is as follows;

6.40 p/kWh x 2000 kWh = 12,800 p

4.259 p/kWh x (17,808 – 2000 = 15,808 kWh) = 67,326 p

80,126 p

Divide by total annual energy consumption; 80,126 / 17,808 = 4.50 p/kWh (average)

If a gas-fired boiler is 90% efficient then the price per kWh can be weighted for the efficiency of the process.

Therefore actual cost is: 4.50 / 0.90 = 5.00 p/kWh

327

4.0 Liquid Petroleum Gas (LPG) Propane

Price Calculation

Price 57.6 p/litre.

This is a price per litre as supplied bulk delivery and pressurised to about 6 bar.

The GCV of gas is 25.5 MJ/litre of liquid gas under pressure.


so 57.6 / 25.5 = 2.26 p/MJ

If an gas-fired boiler is 85% efficient then the price per MJ can be weighted for the efficiency of the process.

Therefore cost is: 2.26 p/MJ / 0.85 = 2.51 p/MJ

1 MJ = 1 / 3.6 kWh = 0.278 kWh


328

5.0 Ordinary Rate Electricity

Price Calculation

1 unit =1 kWh

Cost per unit = 15.31 p = 15.31 p/kWh

Take electricity at 100% efficient for heating.

The actual cost per kWh is therefore: 15.31 p / kWh.

6.0 Economy 7 or Low Tariff Electricity

Price Calculation

Cost per unit = 7.50p = 7.50 p/kWh

Electricity can be taken as 100% efficient but storage heaters are normally oversized to allow for the difference between the rate of electrical energy input and the slower rate of heat release from the storage material.

329

Also storage heaters are not as 'controllable' as other forms of heating since the storage blocks heat up at night whether or not their full capacity is used the next day. It may be necessary in cold weather to have a top-up of electricity during the latter part of the day at a higher cost than the E7 rate.

This means that in practice the Economy 7 heating process can have a much lower efficiency than 100% and the actual figure will vary for each project. For this costing exercise we will assume 100% efficiency but a more detailed study should be made for greater accuracy.

Add standing charge of 11.96p per day = £43.65 per year.

Average annual energy consumption = 17,808 kWh

Standing charge = 4365 p /17,808 = 0.245 p/kWh

Add standing charge = 7.50 + 0.245 = 7.745 p/kWh

Cost per unit; 7.75 p/kWh

7.0 Wood Pellets

330

Wood pellets have a gross calorific value of 18 MJ/kg, source; http://www.biomassenergycentre.org.uk/

Price Comparison

Pellets cost: £130 to £250 per tonne

Say cost is £200 per tonne bulk delivered in Northern Ireland

£200 / tonne = 20 p / kg source

GCV pellets = 18 MJ/kg.

since ( p/kg) / (MJ/ kg) = p/MJ

so 20 / 18 = 1.111 p/MJ

If a pellet-fired boiler is 80% efficient then the price per MJ can be weighted for the efficiency of the process.

Therefore cost is: 1.111 p/MJ / 0.85 = 1.307p/MJ.

1 MJ = 1 / 3.6 kWh = 0.278 kWh

The actual cost per kWh is therefore: 1.307 / 0.278 = 4.70 p / kWh = 4.70 p / kWh.

http://www.biomassenergycentre.org.uk/

331

Table of Fuel Costs (p/kWh)

Fuel 2006 2007 2008 2009 2010 2011 2012p/kWh

Oil 3.97 4.40 5.44 5.44 6.40 7.02 6.46

Coal 4.44 4.44 4.82 4.82 4.82 4.82 4.82

Natural Gas 5.06 4.32 6.59 4.16 4.90 5.46 5.00

Stored Gas (L.P.G.) 5.24 5.24 8.30 8.30 8.70 9.03 9.03

Electricity Ord. Rate 11.57 11.22 15.81 15.03 15.80 17.10 15.31

Electricity E7 Rate 4.49 4.34 6.64 6.60 7.20 7.83 7.75

Wood Pellets 2.66 3.75 3.75 3.75 4.20 4.70 4.70

332

Bar Chart of Fuel Costs (Year 2009)

Oil

Coal

Natural Gas

Stored Gas (LPG)

Elec.Ord.Rate

Elec.E7

Wood Pellets

18.0

17.0

16.0

15.0

14.0

13.0

12.0

11.0

10.0

9.0

8.0

7.0

6.0

5.0

4.0

3.0

2.0

1.0

333

Summary

It would appear, at present, that Wood Pellets at approx. 4.70 p/kWh are the least expensive of the seven energy sources.

Coal is a little more expensive than pellets at 4.82 p/kWh, but there are difficulties in controlling and maintaining coal-fired systems.

Natural gas price is 5.00 p/kWh.

Oil, Stored Gas and Economy 7 are more expensive and are above 5.00 p/kWh.

If the security of supply and installation cost of Wood Pellets systems is a problem then Oil or Natural Gas would be the next fuels to choose if available.

If natural gas is used for cooking; this costs much less than if electricity is used for ovens and may be advantageous for natural gas running costs.

334

Alternative Energy for BuildingsIntroductionThere are several ways to use alternative energy or renewable energy sources in buildings.

The ultimate alternative energy is electricity because it is a high grade energy source and can be easily transported and if necessary converted back to heat.

However, it is not always feasible to produce electricity and some energy sources convert energy to heat only.

The table below gives a summary of alternative energy and renewable energies.

Technology Brief Description Comments

Biomass

Plant material is used to produce gas or heat.Typical Biomass fuels are;Wood chips, wood pellets, wood logs, bioethanol from sugar beets and grains, biodiesel from rapeseed and sunflower seeds, biogas (methane) from a large range of waste products such as animal wastes.

Requires no external energy source.

Combined Heat & Power (CHP)

Generates both heat and electricity from a gas or diesel engine. Larger systems use gas or steam turbines.

Requires an external fuel source to drive the engine.

Fuel Cell Electrochemical device that produces electricity and heat. Requires no external fuel source.

Ground Source -air

Heat from the ground is used to heat air. This heated air can be used to preheat fresh air intake into a building.

Requires no external energy source other than pumps.

Ground Source - water

Underground water sources are used to provide cool water for cooling a building. Requires no external energy source other than pumps.

Ground Source - Heat pump

Ground heat is used and upgraded in a refrigeration cycle (heat pump) to provide central heating.

Requires an external energy source to drive the compressor in the heat

335

pump and the water pumps.

Photovoltaics Sunlight is converted to DC electrical supply and is then inverted to AC. Requires no external energy source.

Solar air heating Air is warmed by the sun and this is then used to heat a building. Requires no external energy source other than fans.

Solar cooling Heat from the sun is used in an Absorption refrigeration cycle to provide cool air. Requires no external energy source other than fans.

Solar Water heating

Heat from the sun is used to heat water usually in a hot water cylinder for domestic hot water supply.

Requires no external energy source other than a pump.

WindA wind turbine is used to generate electricity.The electrical output can be connected to the building and connected to the grid or used to charge batteries.

Requires no external energy source.

See BSRIA Guide – A Illustrated guide to Renewable Technologies (2008) BG 1/2008.

The alternative energy sources discussed below include:1. Heat pumps2. Solar systems3. Water turbines4. Combined heat & power5. Biomass6. Wind generators7. Geothermal energy8. Fuel cell9. Tidal energy systems and Wave power systems

336

One or several of the systems may be incorporated into a building to produce heat, electrical energy or mechanical energy for a process.

A heat pump works by converting energy from a low-grade source such as sub-soil to a higher-grade heat that would be suitable to warm up a building.

Heat pumps move warmth from one place to another, transferring heat from the soil to the house in winter and from the dwelling into the ground in summer if it can be reversed for cooling.

The refrigeration cycle is utilised so that the heat rejected at the condenser is used to heat a building as shown below.

EVAPORATOR

CONDENSER Pipework

Expansion Valve

CompressorDrive unit e.g. electric motor

Heat input from source such as pipes laid in subsoil.

Heat rejection or heat to rooms via heat emitters

VAPOUR COMPRESSION REFRIGERATION CYCLE

337

Solar panels attract heat from the sun and transfer this to water or if photovoltaic cells are used then transfer the energy directly to electricity.

Another type of solar system is a passive system and this has a high mass, high thermal capacitance wall that heats up when the sun shines on it and passes that heat in a controlled fashion into the heated spaces.

SOLAR PANEL

Radiation from sun

Pipes to heat emitter

Solar Collector

mounted on roof Control damper

ROOM

Warm air

PASSIVE SOLAR HEATING

Glazed

area

High mass, high thermal

capacitance wall

Solar Radiation

338

Water turbines can be used to generate electricity or drive a machine.

The site for a hydro scheme should be carefully selected to ensure that an adequate flow of water is obtainable throughout the year.

339

The type of turbine can be selected after the site has been evaluated.

In some buildings it is economical to produce electricity using a generator by installing a Combined Heat & Power scheme.

This is where an electricity generator is used to meet all the electric demand of a building and the heat that is produced as a by-product is utilised to heat all or part of the building.

An example is shown below where a diesel driven generator set produces 850 kW of electricity and heat is also produced at the water cooling system and even in a flue heat recovery heat exchanger.

Flue gasHeat recovery

unit

340

Biomass can be used where a source of decomposable matter is available.

Willow trees, sugar cane, wood, rice or soybean can be grown, harvested and used and converted into energy.

The technologies include a variety of thermal and thermo chemical processes for converting biomass by combustion, gasification, and liquefaction, and the microbial conversion of biomass to obtain gaseous and liquid fuels by fermentative methods.

341

An example of microbial conversion is the anaerobic digestion of willow plants to yield a relatively high-methane-content fuel gas (biogas) which can then be used to drive a gas engine coupled to a generator to produce electricity.

Wind generators can be a method of converting ‘free’ energy to useful electricity.

If a suitable windy site is available then a wind turbine can be installed to provide some or all of electric demand of the building.

Controls are required to adjust the rotor into the wind, feather the blades in a storm and regulate the electricity produced so that it can be used in buildings.

This may involve rectifying the voltage and smoothing the frequency of the electricity produced.

342

Geothermal energy is to be found underground in some parts of some countries.

Geothermal heat is a renewable energy source primarily produced when ground water descending from the Earth's surface meets molten magma rising toward it.

Some of this geothermal water circulates back up through faults and cracks and reaches the Earth's surface as hot springs or geysers, but most of it stays deep underground, trapped in cracks and porous rock

A bore hole can be sunk to reach hot rocks where water can be heated and pumped to the surface where it is used to heat building through heat exchangers or if steam is produced then a power station can be used to generate electricity.

In some cases heat pumps can be used. The geothermal heat pump is one of the most efficient and non-polluting home heating / cooling systems available.

Other sources of energy are being developed such as fuel cells.

The hydrogen fuel cell has been researched for some years since hydrogen is abundant and clean burning.

One way of obtaining hydrogen is by cracking a water molecule into the components hydrogen and oxygen.

The hydrogen is then fed into a fuel cell, a battery-like device that generates DC current.

It supplies electricity by combining hydrogen and oxygen electrochemically without combustion.

Unlike a battery, however, a fuel cell does not run down or require lengthy recharging.

It will produce electricity and heat as long as hydrogen and oxygen are supplied.

343

The oxygen comes from ambient air, but the hydrogen comes from a system called a reformer, which produces the gas by breaking down a fossil fuel.

Reformers do release pollutants as they break down the hydrocarbons to release hydrogen.

Fuel cells are basically electrochemical engines that produce electricity by harnessing the reaction of hydrogen and oxygen.

The only byproducts of the cell itself are clean water and useful heat.

Tidal energy works on the same principal as the water wheel.

The difference in water elevation is caused by the fluctuation between low and high tides.

A dam is built across an estuary to block the incoming or outgoing tide.

When the water level on one side of the dam is higher than the level on the other side due to a tidal change, the pressure of the higher water increases.

The water is then channelled through a turbine in the dam, which produces electricity by turning an electric generator.

Wave power could be harnessed to produce electricity.

One system being studied is one in which the energy from a wave is converted to compressed air and the air compressor drives an electrical generator.

The machine is positioned at sea and anchored to the sea bed.

One drawback is the hazardous environment in which the machinery has to operate.

344

Combined Heat & Power

IntroductionIt is feasible in some buildings to provide heat and electricity from plant rooms to meet demands as they occur.

A Combined heat and power facility will produce heat for central heating as well as electrical power.

There are several ways to achieve this and the following are prime movers used in CHP; diesel and gas engines, gas turbine coupled or steam turbine each coupled to an alternator.

The following table gives information about the prime movers.

Prime Mover Heat to power ratio Notes

Diesel and gas engines 1.5 : 1 Used for similar heat and power building energy demands.

Gas turbine 3 : 1Noisy.The ratio can be increased by adding supplementary boilers or waste heat recouperators.

Steam turbine 10 : 1 Used for installations with high heat demand

345

The first task of the engineer is to match the equipment with the building heat and power demands.

Diesel & Gas Engines

With modern diesel and gas engines in which both the cooling water and the exhaust gases are at high temperatures, it becomes possible to use waste heat produced as steam or hot water, or both. When compared with the other prime movers, diesel engines have excellent thermal efficiencies. Up to 39% of the calorific value of the fuel can be converted to shaft power. There are three sources from which heat can be recovered, when operating diesel and gas engines. They are:

1. Exhaust gases2. Jacket water heat extraction3. Lubricating oil cooler.

Water jacket

Hot water to heating system.

(via storage vessel)

Hot Water flow

Hot Water flow

Electrical Energy Output

Flue gas

Heat recovery

unit

Fuel input AlternatorDieselEngine

346

Small Scale Economics

Small Scale Economics

If a small scale diesel engine was used to drive a generator and produce electricity for a small building, how much would it cost to run the plant and would it be cheaper than buying electricity from NIE at about 10.5 p/kWh?

35 sec. Gas oil Price 39 p/litre

GCV = Gross Calorific Value or heat in the fuel which can be liberated by combustion.GCV = 46 MJ/kg and Relative Density = 0.835 Actual density = 0.835 x 1000 = 835 kg/m3

Therefore GCV per litre = (45.5 x 835 ) \ (1000) = 38 MJ/I.


so 39 / 38 = 1.026 p/MJ

A diesel engine has a thermal efficiency of 39% at best.An alternator has an efficiency of about 80%.If a value of 35% is taken for the diesel engine efficiency and 80% for the alternator then the overall efficiency is 0.35 x 0.8 = 0.28. (28%)The price per MJ can be weighted for the efficiency of the process.

Therefore actual cost is: 1.026 p/MJ / 0.28 = 3.66 p/MJ.

To covert to pence per kilowatt hour (p/kWh) divide by 0.2777. (1/3.6)

Water jacket

347

3.66 / 0.2777 = 13. 18 p/ kWh

say 13.2 p/ kWh

This is more expensive than buying electricity from the grid at 10.5 p/kWh but we have the advantage of recoverable heat from the flue gases and lubricating oil.The above calculation does not consider the part load efficiency of running plant at less than peak load. The purpose of the exercise is to evoke discussion on economics of small scale production of electricity.

If the gas oil price is 31 p/litre then the cost to run a CHP plant is 10.5 p/kWh.

It would be cheaper to run the CHP plant on natural gas if it was available.Natural gas costs about 3.20 p/kWh (corrected for domestic consumer using average amount per year – see Energy Sources – Fuel Costs section)

Using the efficiency as above;

Therefore actual cost is: 3.20 p/kWh / 0.28 = 11.43 p/kWh.

The above figure is closer to the grid price for electricity at 10.5 p/kWh.

348

Example 1

On commissioning a CHP scheme it was found that the oil flow rate for a 400 kW diesel engine was 12.5 x 10-3 kg/s.

The electrical power output was 150 kW and the heat output was measured as water flow rate and temperature.

DATAFuel GCV = 46 MJ/kg

Hot water temperature = 80oC flow & 70oC return.

Water flow rate = 2 kg/s

(a) Calculate the heat to power ratio(b) Calculate the overall efficiency – fuel in to power out.(c) Calculate the percentage of heat recovered over possible recoverable heat.

Electrical Power Output

Exhaust gas

Hot water

Cooling water heat exchanger

Exhaust gas heat exchanger

350

Example 1 - Answer:

Electrical Power Output 150kW

Exhaust gas

Hot water

Cooling water heat exchanger

Exhaust gas heat exchanger

Oil flow INAlternatorDiesel Engine

rated 400kW

351

(a) The electrical power output from the alternator is 150 kW.

The heat output can be calculated from:

H = m x Cp x T

where: H = Heat output (kW)

m = Water mass flow rate (kg/s)

Cp = Specific heat capacity of water , 4.2 kJ/kgdegC

T = Temperature difference (degC)

352

H = 2.0 x 4.2 x (80 - 70)

H = 84 kW

Therefore the heat to power ratio is:

Heat Power

85 kW : 150 kW

1 : 1.77

(b) The overall efficiency from fuel input to electrical power output can be calculated from:

Toto Energy in fuel input (kW) = m x GCV

x 100%

Energy in fuel (kW)

Power output (kW)Efficiency =

353

where: GCV = Gross Calorific Value of the fuel (kJ/kg)

m = fuel mass flow rate (kg/s)

Therefore: GCV = 46 x 1000 = 46,000 kJ/kg

Mass flow fuel = 12.5 x 10-3 kg/s

Energy in fuel input (kW) = 12.5 x 10-3 kg/s x 46,000

= 575 kW

Therefore:

Efficiency = 26%

(c) Calculate the percentage heat recovered over the possible maximum theoretical heat recoverable.

x 100%150 (kW)

575 (kW)Efficiency =

% Heat recovered =x 100%

Actual useful heat recovered (kW)

Maximum theoretical heat recoverable (kW)

354

The maximum theoretical heat recoverable = Heat input in fuel - Engine rating.

= 575 kW - 400 kW

= 175 kW

% Heat recovered = = 48%

Example 2

A CHP scheme comprised a 1 MW diesel engine.The alternator power output is 850 kW.The heat is recovered from cooling water.

DATA:Heat recovery system water flow rate = 5.622 kg/s

Water temperatures = 80oC flow, 62oC return.

Diesel oil mass flow rate = 175.057 kg/h.

Diesel oil G.C.V. (Gross Calorific Value) = 46 MJ/kg

Exhaust gas temperature rise = 300 KExhaust gas specific heat capacity = 1.1 kJ/kgK

x 100% 84 kW175 kW

355

Exhaust gas flow rate (air to fuel ratio) = 14 : 1

Neglect heat in the combustion air into engine.

Complete the following:

(a) Calculate the heat to power ratio.

(b) Calculate the overall efficiency from fuel input to electrical power output.


(d) Draw an energy flow diagram of the processes.

Water flow

Exhaust gas

Heat recovery unit

Fuel input Output 850kW

Alternator Diesel

Engine

1 MW

356

Heat recovery unit

357

Example 2 - Answer:

(a) The electrical power output from the alternator is 850 kW.

The heat output can be calculated from:

H = m x Cp x T

where: H = Heat output (kW)

m = Water mass flow rate (kg/s)

Cp = Specific heat capacity of water , 4.2 kJ/kgdegC

T = Temperature difference (degC)

H = 5.622 x 4.2 x (80 - 62)

H = 425 kW

Therefore the heat to power ratio is:

Heat Power

425 kW : 850 kW

1 : 2

358

(b) The overall efficiency from fuel input to electrical power output can be calculated from:

Energy in fuel input (kW) = m x GCV

where: GCV = Gross Calorific Value of the fuel (kJ/kg)

m = fuel mass flow rate (kg/s)

Therefore: GCV fuel = 46 x 1000 = 46,000 kJ/kg

Mass flow fuel = 175.057 / 3600 = 0.04863 kg/s

Energy in fuel input (kW) = 0.04863 x 46,000

= 2236.84 kW

Therefore:

x 100%

Energy in fuel (kW)

Power output (kW)Efficiency =

Efficiency = x 100%850.00 (kW)

2236.84 (kW)

359

Efficiency = 38%


DATA:

Exhaust gas temperature rise = 300 KExhaust gas specific heat capacity = 1.1 kJ/kgK

Exhaust gas flow rate (air to fuel ratio) = 14 : 1

Neglect heat in the combustion air into engine.

The percentage heat recovered over the possible maximum theoretical heat recoverable is found from the following:

The maximum theoretical heat recoverable = Heat input in fuel - Engine rating.

= 2237 kW - 1000 kW

2236.84 (kW)

% Heat recovered =x 100%

Actual useful heat recovered (kW)

Maximum theoretical heat recoverable (kW)

360

= 1237 kW

% Heat recovered = = 34.4%

(d) An energy flow diagram of the processes can be drawn if a breakdown of the energy flows is calculated.

Heat lost in exhaust gas = m x Cp x t

The exhaust gas mass flow rate = 14 parts air + 1 part fuel

The fuel flow rate = 0.04863 kg/s

Therefore the exhaust gas flow rate = 0.04863 x 15 parts= 0.7294 kg/s

Heat lost in flue gas = 0.7294 x 1.1 x 300

= 240.7 kW say 241 kW.

Energy Flows

x 100% 425 kW1237 kW

361

(given) Energy to electrical power = 850 kW

(1000 - 850) Alternator losses = 150 kW

______________

(given) Diesel Engine rating = 1000 kW

(calculated) Heat lost in exhaust gas = 241 kW

(812 - 241) Heat lost from engine = 571 kW

________________

(1237 - 425) Total Heat losses = 812 kW

(calculated) Heat recovered to water = 425 kW

________________

(2237 - 1000) Total heat flows = 1237 kW

(calculated) Energy input = 2237 kW

Energy Flow Diagram

362

The energy flow diagram helps to understand where the energy goes in the CHP plant.

Energy Input in Fuel

2237 kW812 kW Heat

Losses

1000 kW Diesel Engine Rating

Heat recovered to water

425 kW

Heat lost from engine

571 kW

Heat loss in flue gas

241 kW

Alternator losses

150 kW

Electrical Power

850 kW

eng.harran.edu.treng.harran.edu.tr/~hbulut/introduction.docx · Web viewIntroduction The process by...

Documents

Transcript of eng.harran.edu.treng.harran.edu.tr/~hbulut/introduction.docx · Web viewIntroduction The process by...