Operational Characteristics of VAC Equipment, Plant & Material
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Transcript of Operational Characteristics of VAC Equipment, Plant & Material
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MCAST BTEC National Diploma in Building Services Engineering
Ventilation & Air Conditioning Design
Operational Characteristics of VAC Equipment, Plant & Materials
Joseph Gatt
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Contents
P.35.3-1 Air Terminal Devices ......................................................................... 4A. Diffusers, Grilles and Dampers.................................................................. 4
Linear Louver Diffuser .............................................................................. 4Circular Diffusers ..................................................................................... 4Domestic Grilles ....................................................................................... 5Linear Bar Grilles ..................................................................................... 6Standard Weather Louvers ........................................................................ 6Self-Drain Louvers ................................................................................... 8
B. Coanda Effect ......................................................................................... 9C. Exhaust Terminals .................................................................................. 9
P.35.3-2 Humidifiers ..................................................................................... 11Capillary Type Washers .............................................................................. 11
P.35.3-3 Air Filters & Maintenance .................................................................. 13Viscous Impingement ................................................................................. 13
P.35.3-4 Ductwork ........................................................................................ 14A. Ducting Material .................................................................................... 14B. Ductwork Shapes & Applications .............................................................. 15
Rectangular Ducting ................................................................................ 15Circular Ducting ...................................................................................... 15Flat Oval Ducting .................................................................................... 16
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C. Effect of Duct Space ............................................................................... 16P.35.3-5 Fans .............................................................................................. 17P.35.3-6 DX & Hydronic Heater/Cooler Batteries ............................................... 18P.35.4-8 Calculation ...................................................................................... 19References................................................................................................... 20
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P.35.3-1 Air Terminal Devices
A. Diffusers, Grilles and Dampers
Linear Louver Diffuser
(Brooke Air, 2011) states that an alternative form of
continuous air distribution system for variable air volume or
fan coil application is offered with the louver style ceiling
diffuser. In either single lengths or in section, with frame
alignment pins for continuous applications, the diffuser can
be supplied with one or two way cores. Removable cores
are offered as standard. Moreover, ease of installation and
service access to air filters or controls may be allowed by
the removable cores. The cores are retained with spring clips and are supplied
complete with detachable safety cords. The standard width increments rage from
150-400mm and can be supplied in single lengths up to 1200mm long. The core
section lengths are limited to 1200mm for continuous applications, although
frames can be manufactured up to 3m long.
Type:1 or 2 way blow
Control: Includes diffuser mounted opposed blade dampers or spigot mounted
flap dampers.
Options: A full range of plenums are available to suit a variety of installation
conditions.
Fixings:In order to provide an easy installation, the yoke strap fixing methods is
offered as standard.
Finish:A wide range of paint finishes are available. The standard finish, however,
is satin anodised aluminium.
Circular Diffusers
According to (Brooke Air, 2011), the circular diffusers are specifically designed to
suit applications such as atria, auditoria or industrial premises, due to their high
air capacity terminals. The core position is adjustable on a central screw thread,
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which gives conventional horizontal diffusion
with exposed duct or ceiling mounted
installations, or a progressively adjustable
vertical projection setting for spot cooling
and heating applications. The diffuser frames
and cores are manufactured from aluminium
spinnings and incorporate steel adjustment
mechanisms and core retainers.
Finish: The standard diffuser finish is stove enamelled silver with matt black
louver dampers, but a wide range of paint finishes is also available.
Sizes:Such diffusers are manufactured in ten standard duct sizes ranging from
150mm to 760mm.
Installation: For ceiling mounted applications, the cores are easily removed to
allow the diffuser neck to be riveted or screw fixed to the stub duct. When these
are installed with flexible ducting, it is advisable to fit a suitable length of sleeve to
prevent the damper blades catching the duct reinforcing wire. Otherwise, ensure
that there is adequate clearance throughout the blade travel. For ease of
commissioning, the damper adjuster can be accessed through the centre cone of
the diffuser.
Domestic Grilles
(Brooke Air, 2011) states that the range of
single deflection grilles has been specifically
designed for domestic usage or other
applications where occupants need to regulate
the air flow rate. The units incorporate a set of
rear damper blades which are easily adjustable
from the face operated lever, giving very low
leakage levels in the closed position. The grilles
can be supplied with either horizontal or vertical adjustable blades. Alternatively,
other forms of fixed blade core can be supplied to suit architectural requirements.
Fixing:Standard fixing method is by countersunk screw through the flange.
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Finish: Standard finish is satin anodised aluminium.
Sizes: The domestic grille can be supplied in sizes ranging from 150mm x
100mm, increasing in 25mm increments to a maximum of 500 x 300 or 300 x
300.
Linear Bar Grilles
According to (Brooke Air, 2011), the
linear grille offers a wide range of blade
formats to meet both architectural and
system requirements, and is ideally
suited to sidewall, bulkhead or cill
applications. Grilles can be supplied in
modular units or, for continuous
applications, in single sections up to 3m.
All core types are optionally available as
removable items, or alternatively, can be supplied without frames. Hinged core
sections can also be supplied to provide access to concealed control valves,
thermostats or switches. For continuous applications, make-up sections can be
manufactured to suit internal or external corner details. Grilles can also be curved
to suit architectural features.
Control: Opposed blade volume control damper or adjustable vertical rear blades.
Fixings: Standard fixing method is by countersunk screw through the flange.
Recommended secret fixing method is by removable core.
Finish: Standard finish is satin anodised aluminium.
Sizes:Linear grilles are available in standard heights ranging from 40 mm up to
600 mm and in single section lengths of up to 3m, depending on the height. Note
that grille heights less than 50 mm cannot be fitted with opposed blade dampers.
Standard Weather Louvers
(Brooke Air, 2011) explains that standard louvers have been designed to provide
weather protection to ventilation openings in buildings. In addition to the modular
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sizes shown it is possible to specify louvers for cladding, louver screens, access
doors and other architectural features.
These louvers are composed of
frame and blade assemblies,manufactured to suit the building
aperture size. Different formats are
available to suit aesthetic
requirements. Frame options include
flanged and recessed types. An
adjustable louver option is available
for applications requiring automatic
or manual closure/opening of the louver. Such louvers are manufactured from
extruded aluminium sections which conform to BS1474. Frames and blades are
screwed, riveted or cleated to form a robust structure. Louvers are conventionally
specified in terms of the building aperture size. The most common application of
such louvers is to protect the supply and exhaust duct openings through the
building fabric from the ingress of water. Louvers may also be used for transfer
applications where free flow ventilation of rooms is required to meet building
regulations. Standard finish stove silver special finishes are available to suit most
architectural requirements and include:
Natural satin anodised (AA25 micron) Colour Anodised
Polyester powder coat
Stoved epoxy
Stoved acrylic
Syntha Pulvin
PVF2
Primed
Mill
Bird or vermin guard galvanised mesh fixed to the rear of the louver section.
Insect screen in stainless steel fixed to the rear of the louver. Rain lip in the form
of an extended bottom blade.
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Self-Drain Louvers
According to (Brooke Air, 2011), these louvers
have been developed to provide maximum
weather protection to critical ventilationopenings in buildings. This is achieved by an
innovative blade design which drains any
surface water internally and thereby prevents
carry over by the intake air. In addition, the
deep format frame allows a shallow blade
angle (35 degrees) which increases louver free area and can reduce louver size.
The frame is ideal for modular louvers although it is possible to form curtain walling
via butt jointing. Louvers are composed of aluminium frame and blade assemblies
manufactured to suit the aperture size. Frame styles include recessed and flange
types. Louvers are constructed entirely from aluminium extrusions to BS1474.
Frames and blades are riveted or screwed to form a robust construction. Louvers
are conventionally specified in terms of the building aperture size. These louvers
are designed for maximum weather protection of supply and exhaust ventilation
openings in buildings, especially where water ingress must be kept to a minimum,
ideal for coastal and exposed areas. In addition, these louvers can be applied to
curtain walling applications around plant enclosures and rooms. Standard finish
stove silver special finishes are available to suit most architectural requirements
and include:
Natural satin anodised
Colour Anodised
Polyester powder coat
Stoved epoxy
Stoved acrylic
Syntha Pulvin
PVF2
Primed Mill
Bird or vermin guard galvanized mesh fixed to the rear of the louver section.
Insect screen in stainless steel. Where used in deep recessed applications, such
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louvers can be supplied with an extended bottom cill to carry water away from the
building facia.
B. Coanda Effect
According to (CIBSE Guide B, 2005), when a jet is discharged from a terminal
device to a flat surface, it entrains air from one side only, resulting in deflection of
the jet towards the surface. This phenomenon occurs due to frictional losses
between the jet and the surface. The effect reduces with distance from the
terminals as increasing volumes of air are entrained from the room side of the jet.
However, the Coanda effect is maintained despite temperature differences between
the jet and the room air. The selection and positioning of supply air terminals is a
critical factor, particularly for rooms with low ceilings. If the Coanda effect is not
present, the maximum throw for any terminal is reduced by approximately 33%.The main factors that influence whether or not the Coanda effect will occur are:
The distance between terminal and surface
The width of jet exposed to surface
The velocity of the jet
The presence of projections and other disturbing influences.
(CIBSE Guide B, 2005) p.2-54
C. Exhaust Terminals
(CIBSE Guide B, 2005) states that exhaust terminals may be positioned as follows:
In a stagnant zone where supply jet influence is limited
Close to a source of unwanted heat, e.g. above a bulb
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Close to an excessively cold surface to increase its surface temperature and
thereby reduce radiant losses and cold draughts
At a point of local low pressure, e.g. the centre of a ceiling diffuser
The following positions should be avoided:
Within the zone of influence of a supply air terminal since this allows
conditioned air to pass directly to exhaust without first having exchanged
heat with its surroundings; this results in very low ventilation efficiency
Close to a door or aperture which is frequently opened since this leads to the
exhaust handling air from outside the conditioned space
In a position which causes contaminated room air to be drawn through the
occupants breathing zone.
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P.35.3-2 Humidifiers
According to (CIBSE Guide B, 2005), humidifiers are classified as follows:
Direct humidifiers
o Hydraulic separation
o Compressed air separators
o Mechanical separation
o Vapour injection
Indirect type humidifiers
o Spray washers
o Capillary type washers
o Sprayed coils
o
Pan humidifierso Mechanical separators
o Steam humidifiers
o Rotating drum humidifiers
o Infrared evaporators
Capillary Type Washers
(CIBSE Guide B , 1986) p.B3-47
Capillary type washers are assembled up from unit cells. Each cell is packed with
filaments of glass, which is specially orientated to give the minimum resistance to
air flow with the highest efficiency. The cells are sprayed from nozzles at a gauge
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pressure of 40kPa. These cells produce droplets of water which, by capillary action,
produce a constant film of moisture over each glass filament. The air passing
through the cell is broken up into finely divided air streams. These provide
maximum contact between water and air and result in high efficiency of saturation.
Most dust particles down to 3m size are also eliminated from the air stream. It is
therefore necessary to provide a constant flush of water through the cells to
eliminate the danger of blockage. Alternatively, an irregular supply controlled by
time clock, may be used to flush the cells with water at programmed intervals. The
velocity through the washer chamber is 2.5 m/s with a maximum of 2 m/s through
the cells. Saturation efficiency of 97% can be achieved with as little as 0.8 litres of
water per 10 m3of air per second. Although a minimum of 4.5 litres per 10 m3of
air per second is required for flushing purposes. The cells have a maximum water
capacity of 11 litres per 10 m3of air per second. Capillary cells are arranged in
parallel flow formation, where the air and water pass through the cell in the same
direction, or in a contra-flow arrangement with water and air passing through the
cell in opposite directions. Selection is ruled by the humidifying or dehumidifying
duty required from each cell, and also the degree of cleanliness of the air handled.
Prevention of bacteriological and other contamination must also be considered.
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P.35.3-3 Air Filters & Maintenance
According to (CIBSE Guide B, 2005), air filters are classified as follows:
Viscous impingement
o Panel or unit
o Moving curtain
Dry
o Panel, bag, cartridge or unit with fabric or fibrous medium
o Absolute or diffusion [HEPA]
Electrostatic
o Charged plate
o Charged medium
Adsorption units Mechanical collectors
Viscous Impingement
A filter made up of a relatively loosely arranged medium, such that the airstream is
forced to change direction frequently as it passes through the filter medium. The
medium usually consists of spun-glass fibers, metal screens, or layers of curvy
expanded metal whose surfaces are coated with cheap oil.
(CIBSE Guide B, 2005) p.2-122
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P.35.3-4 Ductwork
A. Ducting Material
(CIBSE Guide B, 2005) states that materials most suitable for the formation of
ductwork are:
Metal
o Steel
o Galvanized steel
o Black steel
o Stainless steel
o Aluminium
o Copper
Non-metal
o Resin bonded glass fibre
o Glass reinforced plastic [GRP]
o Polypropylene
o Polyester (textile or fabric ducting)
o Polyvinyl chloride [PVC]
o Polyisocyanurate
o Concrete
o Synthetic rubber
(CIBSE Guide B, 2005) p.3-37
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B. Ductwork Shapes & Applications
Rectangular Ducting
Rectangular ducting is most common for low pressure systems because:
It is readily adapted to fit into the space available.
It can be readily joined to such component items as heating and cooling
coils and filters.
Branch connections are made more easily.
The aspect ratio should be close to 1:1
since high aspect ratios increase the
pressure loss, the heat gains/losses and
the overall costs. This should be done for
the overall economy and performance.
These ducts are rarely used in ceiling
zones due to space limitations because
ducts with a 1:1 aspect ratio require a
deep service area. For high pressure
systems, rectangular ducting should not be the first choice. It requires
strengthening of the flat sides and needs to be sealed to make it suitable for this
application (CIBSE Guide B, 2005).
Circular Ducting
Circular ducting increase its economy by the
spirally wound ducting and a standard range of
pressed and fabricated fittings. Economy gets
even better in low pressure systems having a
relatively small proportion of fittings.
Particularly for the main runs of ductwork, it is
likely to be easier to install. Due to its high
inherent stiffness, circular ducting is preferable
for high pressure systems and for systems
operating at high negative pressures. At high negative pressure, additional
stiffening rings may be necessary (CIBSE Guide B, 2005).
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Flat Oval Ducting
Where there is a limitation on one of the
dimensions in the space available for the duct
run, flat oval ducting provides an alternative tocircular ducting. Since flat oval ducting can fit
in spaces where there is insufficient room for
circular ducting, and can be joined using the
techniques for circular duct assembly, it
combines the advantages of circular and
rectangular ductwork. Flat oval ducting
requires less reinforcement than the corresponding size of rectangular duct
because it has considerably less flat surface that is subject to vibration. Flat oval
duct may be suitable for both positive and negative pressure applications
depending the parameters.
C. Effect of Duct Space
According to (Newnes, 2004), thoughts should be given to the following issues:
The energy in overcoming duct resistance can be reduced by approximately
18% if the duct diameter is increased by 5%.
The resistance of the duct can be reduced to 12.5% if air is fed into the
middle of a duct instead of from one end.
It is recommended that air is distrusted into several ducts in ceiling voids
where space is restricted. This, however, enables the air velocity, hence
reducing the resistance.
The horizontal duct resistance can be reduced by up to 87% if the location
of the AHU would be central in the area to be served.
Duct resistance mostly occurs due to duct fittings. It is therefore
recommended to enable duct runs to be as straight as possible to avoid
many joints. It should be noted that high priority should be given to
ductwork when coordinating multiple services.
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P.35.3-5 Fans
(CIBSE Guide B, 2005) p.2-130
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P.35.3-6 DX & Hydronic Heater/Cooler Batteries
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P.35.4-8 Calculation
Q: A multipurpose hall with dimensions 20m x 18m x 5m high has 75 occupants. 8
litres per second and 25lt/s of re-calculated air are supplied to the theatre for each
person. If 10% of the supply volume leaks out of the theatre, calculate the roomair change rate and the air volume flow rate for the various parts of the ventilation
system i.e.: leakage, extraction, fresh air, re-calculated and exhaust.
1. Supply air quantity:
2. Air change rate:
3. Leakage from the theatre:
4. Quantity of air extracted from the theatre:
5. Quantity of fresh air entering the ductwork:
6. Quantity of re-circulated air:
7. Exhaust air quantity:
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References
Name; Edition; Author