operational characteristics of vac equipment, plant & material

8/12/2019 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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Joseph Gatt Page 2 of 20

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.



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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



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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



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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

operational characteristics of vac equipment, plant & material

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