Chimney Design and Simple Flue Calculations

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Overview of RequirementsFunctions

Definitions, Classifications & Designations

System Requirements

Legal Requirements

Introduction toGeneral Principles, Flue Dynamics

Chimney Design

Chimney Construction, Materials

Draught Influence & Control Methods

Presentation Summary

Examples of Chimneys. Domestic implies 50kW appliance output ref Doc J.Definition.


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Solid Biofuel Domestic Heating

ChimneysChimneys

Examples of Chimneys. Domestic implies 50kW appliance output ref Doc J.Definition.


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Primarily

1) To discharge the products of combustion intothe enviroment without causing inconvenience

2) To safely discharge products of combustionthrough the structure of the building

3) To provide assistance or total force for

combustion air input

FUNCTIONS

References; 1) Building Regs Approved Doc J:2002, Requirements J2, J3

2) 45 kW and above - BS 5854:1980, Requirement 5.1

3) European Standard BS EN 1443:1999


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1) Products of combustion; products resultingfrom the combustion of fuel (Gaseous, liquid &solid constituents)

a) Flue gas; gaseous portion of products ofcombustion conveyed in a flue

b) Liquid; condensates, (possible rain ingress)

c) Solid ; soot, smuts, fly-ash

DEFINITIONS

Reference; European Standard BS EN 1443:1999, Chimneys- General requirements


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a) System Chimneys-installed in accordance, using a combination of compatiblecomponents obtained or specified from one manufacturingsource, with product responsibility for the whole chimney; or

b) Custom Made Chimneys-

installed as above using compatible components from morethan onesource

EN Classifications & Designations Apply Installers Product Liability Insurance ?

Designers professional indemnity insurance applies

DEFINITIONS

Ref; BS EN 1443:1999 Chimneys general requirements

BS EN 12391 Metal chimneys - Execution


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ChimneysChimneys

DEFINITIONS1 Chimney 2 Flue 3 Flue Liner 4 Thermal Insulation5 Outer wall 6 Enclosure or cladding7 Chimney section8 Multi-wall chimney (eg; Flue

Block)9 Chimney fitting10 Connecting flue pipe11 Heating appliance

Terminal

Reference; EN 1443:1999


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ChimneysChimneys

CLASSIFICATIONChimneys are classified in

accordance with the following

performance characteristics:

G

Minimum specification for wood fuel: T400 N2 D 3 Gxx


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ChimneysChimneys

1. Discharge at sufficient height for convenient dispersalof products of combustion, including any airbornesolids!

2. Discharge total volume of flue gases, p.o.c + entrained air

3. Facilitate removal of solid residues eg; fly-ash

4. Allow for dispersal of any liquid products eg; condensates

5. Structural Integrity and Stability eg; loadbearing

6. Material Durability eg; resistance to temperature & corrosion

7. Protection of Building eg; Risk of Fire

8. Protection of Occupants eg; Risk of CO poisoning

General

Requirements


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ChimneysChimneys

Building Control Approval -Prior applicationto & inspection by local authority OR installation byCompetent Person eg H.E.T.A.S followed bynotification

Planning Permission -Local Authorities,National Parks, Conservation areas, listed buildingsetc.

Clean Air Act -Smoke Control Areas Environmental Health Large non-domestic

systems Neighbour complaints

Legal Requirements


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ChimneysChimneys

Approved Document J - April 2002

England & Wales The Requirements

J1 Air Supply

J2 Discharge of products of combustion

J3 Protection of Building

J4 Provision of Information

Building Regulations

Different Regulations and guidance in SCOTLAND and NI.


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ChimneysChimneysGeneral Guidance

J1 Air Supply

Applies to all fuels


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ChimneysChimneysSpecific Guidance

J1 Air Supply

Section 2Applies to solid fuels

Closed appliance no draught stabiliser550mm of permanently open free area

Of ventilation for every kW above 5.0 kW

Closed appliance with draught stabiliser

300mm for the first 5 kW and 850mm ofpermanently open free area of ventilation

for every kW above 5.0kW


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Specific Guidance


ChimneysChimneys

J2 Discharge of theproducts of combustion

Lateral Support Stays usually required 1.5m above roof line


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Specific Guidance


ChimneysChimneysJ3 Protection ofBuilding

Ventilated Firestops sometimes required. No combustible surrounding surfaces toexceed 100C in any test conditions.


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Specific Guidance


ChimneysChimneysJ4 Provision ofinformation

Data Plate


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Natural draught

Usually gives Negativepressure

within flue

Induced draught

Usually gives Negativepressure

within flue

Forced draught

by blower fan which usually givescombustion air, can causePositive pressure within the flue

Measurement by draught gaugeto achieve manufactures target

Draught types

Definitions ref Doc J


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Flue Draught

also important in naturally

draughted appliances to control

fuel/air mix of combustion.

Influenced by

Properties & Design of chimney

& by the action of wind amongst

other factors

Flue draught


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Stack Effect , Displacement

Heated flue gas expands, becomesless dense & is pushed up thechimney by a colder, densercolumn of surrounding air of equalvolume i.e. Buoyancy

The greater the temp. differencethe greater the pressure differenceor draught

The taller the columns the greater

the pressure differential &combined with the above thegreater the natural draught


ChimneysChimneysNatural draught


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Chimney Design StrategyCombine energy conservation with low resistance

Route through structure,

Tend to the vertical

Limits to offset bends & inclined runs

Limits to horizontal runs

Cleaning Access provision (whole chimney)

To Debris collection space

For Rods, Brushes, Vacuum hoses

Terminal

Additional Accessories e.g.;

Condensate or rain drains

Draught Control devices

Flue Draught Stabilisers

Special Cowls


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Sizing of Flue Height minimum recommended 4.5m (ADJ) but must also be

Sufficient for dispersal

Be discharged at a required rate dependant on combustion of

fuel

Diameter of Connecting Flue Pipe may differ to remainder of

chimney

Remember

Adopt Guidance from Building Regs

Adhere to system manufacturers recommendations if require largerthan regulations state

Calculations can be applied to provide smaller flues if carried out in a

recognised method

Chimney Design


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Considerations include;

System & Material choice (matching appliance and

chimney designations) Lateral & Vertical support

Fire protection

Weathering details

Lightning conduction & earthing !

Access, scaffolding

Materials handling

Chimney Construction


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By design of terminal cowl

This may be possible if windstrongly influences thedraught.

Draught Control


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Additional ventilation required

Types, Sizes

Positioning considerations

Same room as combustion air

Distance from appliance

Distance from combustibles

Flue Draught

Stabilisers


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Clay Liners, Concrete

Pumice Liners &

Modular Systems

Metal;

Cast Iron 3 mm. , Mild

steel 1 mm.

Stainless steel Grade

316Vitreous enamelled low

carbon steel

Flue materials


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Sweeping

at least 2 x each heatingseason

(6 x in Austria as instructedby the sweep in Germany)

Visual Inspection

Testing; smoke, clearance

RepairsLining, re-pointing orreplacement components

Maintenance


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BS 5854 BS EN 13384

CIBSE Design Guide B (The most common method)

All requires complex calculations

Spreadsheet analysis easier

Designing a chimney to provide the correct amount of naturaldraught involves a number design factors, many of which requiretrial-and-error reiterative methods

As a "first guess" approximations can be used to estimate thenatural draught/draft flow rate by assuming that the molecularmass (i.e., molecular weight) of the flue gas and the external airare equal and that the frictional pressure and heat losses arenegligible

Sizing of Flue Methods


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Pressure difference ( P) is the driving force for thestack effect and it can be approximately calculated withthe equation below

For flue gas stacks and chimneys, where air is on theoutside and combustion flue gases are on the inside, theequations will only provide an approximation of theheight (h)of the chimney.

Where:P= available pressure difference in PaC= 0.0342a= atmospheric pressure, in Pah= height, in mTo= absolute outside temperature, in KTi= absolute flue gas temperature, in K

Simple calculation- P


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Draft or draught flow rate (Q) which is induced by thestack effect can be calculated with the equation below

For flue gas stacks or chimneys, where air is on theoutside and combustion flue gases are on the inside, theequation will only provide an approximation. Also, thecross-sectional flow area (A) and is the height (h) of theflue gas stack or chimney.

Where:Q= stack effect draft/draught flow rate, m/sA= flow area, m

C= discharge coefficient (usually taken to be from 0.65 to 0.70)g= gravitational acceleration, 9.807 m/sh= height, mTi= average inside temperature, KTTo= outside air temperature, K

Simple calculation- Q


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Appliance maximum and minimum rated outputs

Appliance efficiency (gross)

Flue outlet temperatures at both maximum and

minimum outputs

Minimum and maximum draught required at the

boiler outlet.

Full method Appliance

parameters


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Fuel type (Gas, oil type of solid fuel)

Calorific value

Required excess oxygen level

Required optimum oxygen consumption

Fuel analysis including carbon, hydrogen

and sulphur content.

Full method

Fuel parameters


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Proposed inner surface material

Proposed height

U values for heat loss

Proposed diameter

Proposed eflux velocity

Full method

Chimney parameters


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Flue terminal location

Ambient temperatures (max and min

expected)

Topography

Full method Environmental

factors


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Pressure difference

Average flue gas temperature in flue

Flue surface area

Flue cross sectional area

Volume flow rate of gases

Velocity of gases

Full method - Outputs

Chimney Design and Simple Flue Calculations

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