Guide to Air Distribution Technology 2000.pdf

Guide to Air DistributionTechnology for the

Internal Environment

i

Page

Chapter 1 Introduction to air distribution technology 1

Chapter 2 Conventional air terminal devices and their normal functions 2

Chapter 3 Selection of air terminal devices 7

Chapter 4 Sound characteristics 19

Chapter 5 Duct entry conditions 26

Chapter 6 The fixing and installation of air terminal devices 29

Chapter 7 Measurement on site 33

Chapter 8 Regulation of air terminal devices 40

Bibliography 46

Supporting companies 47

Contents

1

Chapter One1 INTRODUCTION TO AIRDISTRIBUTION TECHNOLOGYThis booklet is intended as a guide todesigners or the contractors whoinstall air diffusion equipment and iscomplementary to prEN12238 - Airterminal devices - Aerodynamic testingand rating for mixed flow applications,prEN12239 - Air terminal devices -Aerodynamic testing and rating fordisplacement flow applications.

Although not specifically related to thefinal air diffusion within a conditionedspace, the following current and draftEuropean standards will serve as auseful reference for air distributionsystems in general, BS EN 1751 -Terminals - Aerodynamic testing ofdampers and valves, prEN12589 - Airterminal units - Aerodynamic testing ofconstant and variable rate terminalunits, prEN13182 - Instrumentation forventilated spaces, prEN13264 -Terminals - Floor mounted air terminaldevices - Tests for structuralclassification, CR1752 : 1998 - Designcriteria for the indoor environment.

Another useful document to consultwhen listing air terminal devices orequipment is HEVAC’s current GeneralSpecification and Product DirectoryFor Air Distribution Equipment IssueLevel 2.

The ultimate purpose of installing theextensive equipment and controlsinvolved in an air conditioning orventilation system is to provideadequate, controlled air movement atthe specified conditions in the treatedspace.

If the air distribution/system isinadequate or inefficient then thisreflects on the efficiency as a whole.The selection of air terminal devices istherefore of paramount importanceand it is hoped that the sectionsthroughout this guide will providecurrent best practice for referencepurposes.

Although the sections that follow givea generic overview of the variousdesigns that may be considered,manufacturers’ data should befollowed in order to obtain the bestresults from their particular products.

The performance data published in themanufacturers’ catalogues are theresults of extensive research andoperational studies. Where actualperformance on site falls short of thedesign criteria, this can generally beattributed to incorrect interpretation ofthe catalogue information or to faultyinstallations.

It should be understood that theconditions which apply in thelaboratories may be impossible toachieve on site, since structural orarchitectural features may determinehow an air terminal device is installed.However, if principles of airdistribution are applied, a satisfactorysolution to site problems can bereached.

The use of the following sections usedearly in the design stage will assistboth designers and contractors inobtaining a satisfactory air distributionsystem.

2

2 CONVENTIONAL AIR TERMINALDEVICES TOGETHER WITH THEIRNORMAL APPLICATIONS

2.1 INTRODUCTIONThis section will explore and list the currentrange of air terminal devices. It is hoped thatit is as comprehensive as possible and listseach product under two main air distributioncategories (a) mixed flow air distribution, (b)displacement flow air distribution.

In order to cover a wide range ofapplications, both supply, return and airtransfer, air terminal devices aremanufactured in a number of specificcategories. These main categories being asfollows:-

Grilles(square, rectangular and linear)adjustable, single or double deflectionfixed bareggcratenon-visionperforated, stamped or meshhigh security

Circular or Rectangular Diffusers(non-adjustable)multi-coneperforated platepan type

Circular or Rectangular Diffusers(adjustable)multi-coneperforated platepan type

Linear DiffusersSlot; non-adjustable, adjustableMulti-blade; non-adjustable, adjustable

Specialist Air Terminal Devicesnozzles/drumsdisc valvesair terminal luminairesventilated ceilingslaminar flow panelsdisplacement ventilation diffusers

Swirl Diffusers (non-adjustable)square typecircular type

Swirl Diffusers (adjustable)square typecircular typefloor swirl diffuserslinear high induction diffusers

Details of these various types of devicestogether with the systems normally usedwithin are further explained and detailed intable 2.1 pages 4 to 6.

Systems & ApplicationsIn order to select the correct air terminaldevice for (a) mixed flow air distribution

Chapter Two

Figure 2.1 Mixed flow

systems and (b) displacement/laminar flowair distribution system, the air distributioncategories will further be explained. Suitableproducts for these systems together withthe supply positions are also given in table2.1. Unit positions have therefore been splitas follows:-

(a) mixed flow - supply positions1. ceiling2. wall3. floor

(b) displacement/laminar flow -supply positions1. ceiling2. wall3. floor

(a) Mixed Flow Air DistributionSystemsThe principle of this type of air distributionsystem design is to inject air from thevarious types of air terminal devices into theconditioned space, thus generating highinduction and effective mixing with theexisting room air to provide comfortconditions within the occupied zone. Theeffectiveness of the system will normally bejudged by the occupants and theirperception of the room conditions,measured generally as room air movementand temperature differentials within theoccupied zone. High air change ratesthroughout the space make the selectionand positioning of air terminal devices moredemanding, with usually this selection takenfrom the range of high induction devices.A differential between the supply and roomair temperatures is normally limited to 100Cfor cooling, but will depend upon thedevices used and their positions. Referenceto table 2.1 will indicate the current type ofair terminal devices together with theirapplication and correct positioningthroughout the conditioned space. Thephilosophy of mixed flow systems is basedupon continual dilution of stale warm air bya fresh supply. A feature of this system isthat conditions within the enclosure areconsidered to be predominantly uniform. Asimple illustration of the air patterns fromsuch a system can be seen in figure 2.1.

3

(b) Displacement/laminar Flow AirDistribution SystemsWith these types of air distribution systemsair is introduced into a space in an evenpattern in order to generate gradualdisplacement of existing room air. This caneither be from a lower level which is normallyreferred to as displacement ventilation, orfrom a high level which is normallydescribed as laminar flow.

Displacement FlowIn this type of flow air enters through the airterminal units and spreads across the floorforming a reservoir of fresh cool air. Anysources of heat will generate a thermalplume rising upwards and thus entrainingthe surrounding air as illustrated in figure2.2. The fresh cool air at floor level will flowto replace that which is warm and lifted intothe convective plumes. By this means thefresh air continually replaces the air as it isused and contaminated air is lifted to ahigher level from which it is removed. Thisgives rise to stratification of temperature andcontaminant levels within the enclosure.Displacement ventilation works on lowertemperature differentials than mixed flowsystems (normally 3-60C cooling) and usesbuoyancy to distribute the air throughout thespace. This has the effect of potentiallybeing more energy efficient and at the sametime new fresh air is kept separate from staleair and subsequently the breathing airquality is substantially uncontaminated. Aircan be introduced at this lower level througheither side wall displacement units orgrille/swirl outlets.

Laminar FlowThis type of system introduces fresh air atgenerally a ceiling or side wall position andis used to remove contaminants fromworking areas by cascading cooler fresh airdown over the conditioned space.Contaminated air is then removed fromlower areas and therefore a continual flow ofair migrates over the working surface andavoids high mixing conditions. Generalapplications for this type of system areindustrial working areas, clean rooms andhospital theatre spaces. Air is normallyintroduced at a lower temperature than thesurrounding room air and thus falls throughthe conditioned zone to a lower level. Atypical illustration of such a system can beseen in figure 2.3. This type of systemintroduces air using proprietary laminar flowpanels or a full ventilated ceiling or wall.

Note: Unless total wall or ceiling surfaces are usedfor air supply, only local laminar flow areas will beachieved.

Figure 2.2 Displacement Flow

It is important to ensure that the supply airflow rate is at least equal or greater than theestimated air flow rate in the heat sourceconvective plumes, otherwise the systemcan change into mixed flow in the upperareas of the occupied zone.

Figure 2.3 Laminar flow

HighSecurity

4

MODEL DIAGRAM TYPE SYSTEM APPLICATION POSITION RECOMMENDED DESCRIPTIONM,D S,E,T C,W,F CORE

VELOCITIES*m/s

Quiet CommerciallyQuiet

upto upto

Adjustablesingle ordouble

deflection

Square/Rectangular/

LinearM

M

M

S, E

S, E

S, E

C, W

C, W

4 8

4 8

4 8

Frequently used grille, large freearea, in supply application hasdirectional control in one plane onlyby adjustment of aerofoil blades.

As 2, but gives direction control intwo planes

General robust grille with free areadependent upon application. Somedirectional control can be achievedby use of profiled blades or byusing additional rear adjustableblades which are perpendicular

Fixed bar

Square/Rectangular/

Linear

Square/Rectangular/

LinearC, W, F

Eggcrate

Non-Vision

Square/Rectangular/

LinearM

M,D

M,D

E

E,T

S, E,T

C, W

Wor

Door

4 8

3 6

3 6

Generally the largest free area grilleavailable.

Low free area designed to provideno through vision. Grilles whichpermit the predetermined passageof air from one treated space toanother. Grilles normally have oneset of non vision type fixed angle orchevron shaped blades whichobstruct direct line of sightthrough the grille core.

Simple form of grille relativelysmall free area.

Perforated,stampedor mesh

Square/Rectangular

Square/Rectangular/

LinearC, W, F

SYMBOLS

SYSTEM APPLICATION POSITION

M = Mixed Flow Air Distribution SystemsD = Displacement Air Distribution Systems

S = Used to supply air to conditioned spaceE = Used to extract air from conditioned space

T = Transfer of air from one conditioned space to another*based on any local control damper being fully open

C = CeilingW = WallF = Floor

Table 2.1 MAIN AIR TERMINAL DEVICES, SYSTEMS AND APPLICATIONS

6

Multi-cone

Perforatedface

Square/Rectangular/ M

M

M

M

S, E,T

S, E

S, E

S, E

C, W

C

3 6

4

4

8

8

3

Heavy and robust grille withgenerally low free area. Vandalproof grids or plates as part of theconstruction with additionalbalancing blades if required.

Radial discharge diffuser offeringgood air entrainment and allowinglarge quantities of air to be diffusedinto a room. In square orrectangular form can provide 1, 2,3 or 4 way discharge.

Radial discharge diffuser usingdeflectrol vanes or baffles behindperforated face for directionalcontrol. 1, 2, 3 or 4 way discharge.

Pan

circular orrectangulardiffusers

(non-adjustable)


(non-adjustable)


(non-adjustable)

C

CCentral baffle plate design, fixed inposition to generate horizontaldischarge of supply air.

Multi-cone

Perforatedface


(adjustable)

M

M

S, E

S, E

C

C

4 6

3 6

As multi-cone fixed geometryunits, but offers facility forhorizontal or vertical air discharge.

As perforated face fixed geometryunits with additional adjustablecontrol for horizontal or vertical airdischarge.


(adjustable)

Discvalves

Airhandling

luminaires

Specialist air terminal

devicesM

M

M

S, E

S

S, E

C, W

C

6 10

4 8

3 6

Disc valves have an adjustablecircular core complete withmounting sub-frame. The coreposition is adjustable for flow ratecontrol purposes and suitablenormally for exhaust air. Specialdesigns are necessary for supplyair conditions.

Air handling luminaires arecomplete with adjustable or fixedlinear slot diffuser within integralplenum box, incorporating flowcontrol blades to set the pattern forhorizontal one way or vertical airdiffusion. Fitted to either one ormore sides of a luminaire.

The void above the ceiling ispressurised and air is introduced atlow velocity through many singleholes or porous panels forming theceiling. Entrainment of room air isrestricted and natural currents inthe room can seriously affect roomair distribution.

Ventilatedceiling


devices


devicesC

5

8


VELOCITIES*m/sQuiet Commercially

Quietupto upto

Pan M

M

S, E

S, E 4 8

4

As pan fixed geometry units butoffering vertical positioning ofcentral baffle plate for horizontal orvertical air discharge control.

Linear


(adjustable)


(adjustable)

C

C,WFixed blade design offering eitherhorizontal or vertical discharge,single or multiple slot design.

SYMBOLS






LinearSlot

Diffusers

lineardiffusers

(adjustable)M

M

S, E

S

C,W 4 8

6 10

Offers vertical or horizontaldischarge single or multiple slotsavailable. Care must be taken withassociated plenum box design.Components normally used asceiling diffusers having acontinuous slot appearance andgenerally having an aspect ratiogreater than 10:1. Adjustablesingle or multi slot diffusers shallincorporate flow control blades toset the air discharge pattern forone of the following combinations.Discharge characteristics can beset as follows:-a) horizontal one wayb) horizontal two wayc) horizontal alternatingd) alternating between horizontaland vertical per slote) vertical

Nozzles/drums are air terminaldevices designed to give lowenergy loss and thus to produce amaximum throw with minimum airentrainment. These are generallydivided into three separate groupsrelated to function, (a) smallnozzles for personalventilation/spot cooling, (b) smallnozzles in arrays for mixed flowapplication, (c) large nozzles forprojecting air over long distances.Nozzles shall either be circular orrectangular, using a single unit ormultiple array and shall have thefollowing facilities as specified.Fixed or adjustable air pattern,fixed or adjustable air dischargedirection, and air flow rate control.

Nozzles/Drums

A

B

C

Specialistair

terminaldevices

C,WFree Space

Laminarflow

panels

Displacementventilationdiffusers


devicesM,D

D

M

S, E

S

S

C, W

W,F

3 6

3 6

4 8

Laminar flow panels have a largeperforated face plate complete withrear plenum box and spigot entry.Generally incorporating a damperfor flow control purposes and alldesigned to provide uniform airdischarge at 90o to the panel face.Normally used for supplyapplications but can be matchedfor appearance purposes forextract applications.

Displacement ventilation panelsare constructed with a largeface area normally using aperforated material. The remainingconstruction normally incorporatesa rear plenum box with spigotentry in which an air inlet dampercan be fitted for flow rate control ifrequired. Units are designed toprovide uniform air discharge atlow velocities over the face area,and to supply air directly into theoccupied zone of a conditionedspace. Note: Can be supplied inaddition to flat diffusers in 90o,180o and 360o configurations.

The construction of this type ofdiffuser causes a swirling or highlyturbulent discharge whichaccelerates the mixing of thesupply air into the surroundingspace. This is generally greaterthan the mixing effect fromconventional square or circulardiffusers. The swirling motion ofthe discharge air is imparted byeither a device behind the diffuserface and/or the configuration of theface itself. The air direction can befixed generally for a horizontaldischarge, but may be vertical inspecial cases. Designs of this typeof device vary from open facevanes as illustrated to multiple slotarrays or perforated face units.

SquareAnd

circular


devices

Swirldiffusers

(non-adjustable)

C

6


VELOCITIES*m/s

Quiet CommerciallyQuiet

upto upto

SYMBOLS






SquareAnd

Circular

Linear

Swirldiffusers

(adjustable)M

M

M,D

S

S

S, E

C

C,W

4 8

4 8

4 8

As square or circular fixedgeometry swirl diffusers butoffering the additional control of airdischarge from horizontal tovertical by manual or motorisedmeans of control.

These units offer a similar highturbulent discharge to the squareor circular swirl diffusers whichaccelerates the mixing of thesupply and room air over andabove that offered by conventionallinear slot diffusers. Units can befixed or adjustable if required.

These are specifically designedswirl diffusers for floor dischargeapplication where the unit shallimpart a swirl motion of thedischarge air either by a device bythe diffuser face and/or by theconfiguration of the face itself. Thedischarge shall be vertical,horizontal or adjustable betweenthe two. Units should be of arobust design to take required floorloading requirements.

Square/Circular

Highinductiondesign

(fixed oradjustable)

Floor Swirldiffusers(fixed or

adjustable)

F

7

Chapter Three3 SELECTION OF AIR TERMINALDEVICES

3.1 INTRODUCTIONThe function of an Air Terminal Device (ATD)is to direct the incoming or exhaust air, insuch a way that comfortable and cleanconditions are maintained in the occupiedzone of the treated space. Failure to choosea suitable ATD, especially one used forsupply purpose, may well nullify all otherefforts to achieve such conditions in thetreated space. At the same time, the ATDinvariably needs to suit aestheticrequirements.

Acceptable conditions in any occupied zonewill depend upon the use of that zone, butsince it is the purpose of the supplied air(possibly together with other means) tomaintain these conditions, this will lead todefinite requirements for supply and exhaustair flow rates, noise level, and air movementin the occupied zone.

From these parameters, it is possible to putforward several solutions which, whilsteconomical in themselves, might not lead toa satisfactory overall scheme. Therefore, it isnecessary at the selection stage, to choosethe position (wall, ceiling, floor, or sill) of theATD’s together with the number, form andtype.

Before full technical proposals can be made,consideration must be given to theoccupancy and function of the space, and tointernal features, such as irregularities ofsurface, position of furniture, and anysources of heat loss or gain.

Consideration should be given to the methodof fixing, and to the finish of the ATD’s.

Finally, no ATD can compensate forincorrectly designed duct entry conditions.

3.2 CHARACTERISTICS OF AIR JETS

3.2.1 Mixed flow applicationsMany air terminal devices are designed tosupply air in a predominantly unidirectionalsense and with significant throw, hence it isimportant to understand the characteristicsof air jets.

The flow profile of an air jet depends uponthe temperature difference between supplyand room air as well as the proximity ofadjacent surfaces, however, the dischargecharacteristics of an isothermal air jet, asshown in figure 3.1, can generally bedescribed as follows :

Zone 1A short zone extending to a length ofapproximately four outlet diameters from thedischarge point where the centre line air-stream velocity remains practically constant.

Zone 2In most cases a transitional zone where thecentre line velocity decays inversely as thesquare root of the distance from the outlet.However, for high aspect ratio terminals thissection can extend up to a distance ofapproximately the width of the terminalmultiplied by four times the aspect ratio.

Zone 3This is a long zone where fully establishedturbulent flow exists and usually extendsfrom 25 to 100 outlet diameters from theoutlet. This zone is of great importance andwhere most ATD selection takes place. Thecentre line velocity decays inversely withincreasing distance from the outlet.

Zone 4The final zone where the centre line velocitydecays rapidly to a value below 0.25 m/s.

SymbolsA = Discharge AreaX = Distance from Discharge PointVx = Centre Line Air VelocityVo = Air Velocity at Discharge Point

Figure 3.1 Profile of a free air jet

8

3.2.1.1 ThrowThe distance measured from the face of theATD to the point at which the maximum axialvelocity reduces to a specific value, normally0.5m/s, is known as the throw. Values ofthrow for a range of ATD sizes andappropriate flow rates are given inmanufacturers' catalogue data, see figure3.2 for a typical characteristic.

Horizontal jetsThe trajectory of a horizontally projected airstream will curve upwards if the incoming airis warmer than room air and downwards ifcooler. The vertical deflection from thehorizontal plane is defined as the ‘drop’ or‘rise’ as shown in figure 3.4.

3.2.1.2 Effect of temperaturedifference

Vertical jetsIncoming air with a higher or lowertemperature than room air will have a loweror higher density respectively. This will havethe effect of increasing the throw of cooldownward or warm upward projected air.Conversely, the throw of a warm downwardor a cool upward projected air stream will bereduced (See Figure 3.3).

Note: The magnitude of these effects for verticaland horizontal jets is a function of the temperaturedifferential and the initial air stream velocity. Manymanufacturers include information on ATDperformance at specific temperature differentials,usually 10oC.

3.2.1.3 SpreadThe spread is defined as the divergence (or‘widening’) of the air stream in a horizontaland/or vertical plane after it leaves thesupply air terminal.

3.2.1.4 Effect of adjacent surfacesWhen an ATD is located in or close to asurface so that the discharged air streampasses along and adjacent to that surfacethe characteristics of the air stream will beaffected. The entrainment in the region closeto the surface will be inhibited and thereduction in velocity of the air jet will be lessrapid.

There will also be a tendency for the airstream to ‘cling’ to the surface and thisreduces the influence of temperaturedifferences which normally cause thetrajectory of a horizontally projected airstream to curve downwards (in the coolingmode).

This effect, often called ‘ceiling effect’ or‘Coanda effect’, is more pronounced inradial air streams and in those producedfrom linear devices because of the largeperimeter area in proximity to the surface.Thus by mounting circular and lineardiffusers in the ceiling and setting them todischarge horizontally, maximum advantageof this effect can be gained, allowing thethrow of the air stream to be somewhatlarger than an equivalent unbounded airstream. Similar effects will be evident withwall-mounted linear grilles located justbelow the ceiling level. To this end, wall-jetsare the normal means of air distribution inrooms with mixing flow systems.

Figure 3.2 Typical Characteristic for an ATD

Figure 3.4 Effect of temperature on ahorizontally projected air stream

Figure 3.3 Effect of temperature on a verticallyprojected air stream

Figure 3.5 Attraction to an adjacent surface

9

As the air leaves the diffuser, it falls to thefloor under the action of gravitational forcesand because the cooler supply air is denserthan surrounding air, see figure 3.6. The airspreads across the floor forming a reservoirof fresh cool air. Any sources of heat in theoccupied zone generate thermal plumesrising upwards entraining the surroundingair. The cool air at floor level flows to replacethat which was warmed and lifted into theconvective plumes. This process iscontinuous and gives rise to verticaltemperature and contaminant gradients,also known as ‘stratification’.

3.2.2.1 Near ZoneSince the air is supplied directly into theoccupied zone there will always be a ‘zoneof discomfort’ in the immediate proximity ofany supply diffuser. This region is known asthe “near zone” and is defined as the areaaround the air terminal unit in which thevelocity exceeds a given value that willcause discomfort to occupants (0.15m/s inwinter and 0.25m/s in summer, withreference to ISO 7730).

The extent of the near zone is a function ofair flow rate, the temperature of the supplyair relative to the room temperature (knownas the “under-temperature”), the air terminaltype and the location/proximity of the unit. Itshould also be noted that some units haveadjustable internal elements specificallydesigned to alter the size and shape of thenear zone. Generally, air terminals will differfrom manufacturer to manufacturer, hence itis important to contact them for up-to-dateinformation.

3.2.2 Displacement flow applicationsIn cooling displacement flow applications,the air terminals are located at low level inthe occupied zone. In order to avoid localdiscomfort due to draught, the supply airtemperature should only be slightly lowerthan the design temperature and withsignificantly lower momentum compared tomixing air flow systems.

The extent of the near zone will also beincreased if units are placed too closetogether. To keep the near zone small it isbetter to use more air terminals with low flowrates rather than have fewer terminals withhigher air flow rates.

3.3 TYPES OF ATD

3.3.1 Mixed flow applications Supply ATD’s fall within four main categorieseach with their own particularcharacteristics which may be used toadvantage in specific applications. Thesecategories are :

1. Sidewall mounted square and rectangulargrilles and nozzles

2. Sidewall mounted linear grilles

3. Ceiling or sidewall mounted slot andlinear diffusers

4. Ceiling mounted circular, rectangular andswirl diffusers

The characteristics of the air streamsdischarged from these devices are outlinedbelow:Figure 3.6 Schematic showing typical profile

from a displacement air terminal

3.3.1.1 Sidewall mounted square andrectangular grilles and nozzlesFrom these ATD’s the air discharges in athree-dimensional stream which normallyflows in a direction perpendicular or nearperpendicular to the face of the device. Aftera short distance from the ATD the velocity atthe axis of the air stream starts reducing asthe surrounding air is entrained into andmixes with it. This velocity reduces in directproportion to the distance from the ATDwhich corresponds to zone 3 in figure 3.1.The cross-sectional area of the air streamincreases so that its boundary diverges at anincluded angle of about 20o as shown infigure 3.7.

Figure 3.7 Air stream from a grille

10

Deflection characteristics of grillesThe most effective use of grillesincorporating air stream deflection usingadjustable blades is through good spacingdesign. The spacing between adjacentterminals depends upon the throw andspread characteristics of these ATD’s.

3.3.1.3 Ceiling or sidewall mountedslot and linear diffusersThese air terminals are mainly used inconjunction with attaining the ceiling effect.Air from these ATD’s discharge in a wide airstream which can be regarded as two-dimensional. The profile that is emitted fromthese diffusers are known as plane wall-jetsand have been studied extensively.Experimental tests have shown that the jetvelocity increases from zero at the wall toa maximum value and then decreasesto zero at some distance from the wall(see figure 3.12).

3.3.1.2 Sidewall mounted lineargrillesThese ATD’s have a length to width aspectratio of 10:1 or greater. The air dischargefrom these terminals is similar to grilles,however it forms a wider stream which canbe taken as a two-dimensional jet. Theprimary velocity reduction is in proportion tothe square root of the distance from theATD. Expansion of the air stream takes placemainly across the thinner section, again atan included angle of about 20o. There is littlesignificant increase in the width of the airstream (see figure 3.11).

Figure 3.8 With 0o deflection, the terminalspacing interval is equal to 1/3rd of the throw(based on 0.5m/s envelope)

Figure 3.9 With 22o deflection, the terminalspacing interval is equal to 1/2 of the throw(based on 0.5m/s envelope)

Figure 3.10 With 45o deflection, the terminalspacing interval is equal to the throw(based on 0.5m/s envelope)

Figure 3.12 Profile of a wall jet

Figure 3.11 Typical linear grille

3.3.1.4 Ceiling mounted circular,rectangular and swirl diffusersAir is discharged from these ATD’s in a thinstream. Entrainment and expansion takesplace rapidly, and as the stream progressesoutwards from the diffuser its velocityreduces in direct proportion to the distancefrom the diffuser. These diffusers are mainlyused in conjunction with attaining the ceilingeffect (see figure 3.13).

Swirl diffuser (figure 3.14) units usually havea circular pattern of radial vanes whichgenerate a swirling air motion when used insupply mode. This highly turbulent swirleffect allows the unit to introduce highvolumes of air into the conditioned space,taking advantage of the rapid entrainmentand intermixing characteristics. As a resultthe unit can deliver high room air changerates as compared to conventional diffusers.

Figure 3.13 Section through a circular orrectangular ceiling mounted diffuser

Figure 3.14 Schematic of a typical swirl diffuser

11

3.3.2 Displacement ventilationsystemsDisplacement air terminals, which arelocated in the occupied zone, shouldprovide air at low velocities and at atemperature only slightly lower than roomtemperature so as to avoid local discomfort.In practice, there will always be a ‘zone ofdiscomfort’ in the immediate proximity ofany supply diffuser placed in the occupiedzone. This zone is kept to a minimumthrough appropriate terminal selection, inother words one having large facial area withuniform velocity profile, and by dividing thetotal air flow demand over several wellplaced terminals. The total air flow deliveredto the space need not be more than 6 ac/hsince conditions above the occupied zoneare not critical. The essence of displacementventilation is to provide acceptableconditions in the occupied zone withconsiderably lower energy consumption dueto lower flow rates and higher supply airtemperatures. The terminal types used fordisplacement ventilation fall into thefollowing categories :

3.3.2.1 Wall mounted

Figure 3.16 shows a schematic view from awall mounted displacement air terminalwhereas figure 3.15 highlights thepossibilities in varying the emitted airflowprofile (check with the manufacturer forspecific details).

Figure 3.16 Wall mounted displacementair terminal

Figure 3.15 Typical profiles from wall mountedair terminal

3.3.2.2 Corner mounted

Figures 3.18 and 3.17 show the schematicand typical profiles for corner mounted unitsrespectively.

3.3.2.3 Free standingFigures 3.20 and 3.19 show the schematicand typical profiles for free standing unitsrespectively.

Figure 3.17 Typical profiles from corner airterminals having adjustable internal elements

Figure 3.18 Corner mounted displacementair terminal

Figure 3.19 Typical profiles for a free standingair terminal

Figure 3.20 Free standing displacementair terminal

12

3.3.2.4 Raised access floor mountedswirl diffusers

This type of air terminal is suitable for areasof high internal heat gain and is often usedin large open plan offices (typically as asupplement to perimeter units). As can beseen from figure 3.21 these units can usuallybe modified to offer alternative air profilesand introduce air with a high degree of swirlwhich generates rapid mixing with room airso as to reduce the extent of the near zone.

3.4 COMFORT CRITERIAThe main objective for room air distributiondesign is to provide a suitable thermalenvironment in terms of comfort andventilation effectiveness for the occupantsand processes to be undertaken inside.Acceptance of thermal environment and theperception of comfort are related tometabolic heat production, its transfer to theenvironment, and the resulting physiologicaladjustments and body temperatures. Theheat transfer is influenced by physicalactivity and clothing, as well as theenvironmental parameters; air speed, airtemperature, mean radiant temperature andhumidity. Of these, air speed and airtemperature are significantly dependent onthe air distribution system.

Many attempts have been made to deviseindices for the specification anddetermination of the conditions for thermalcomfort. Laboratory research has led to theproduction of the predicted mean vote(PMV) thermal comfort index. The PMV hasbeen incorporated into internationalstandard ISO 7730.

Figure 3.21 Floor mounted swirl diffuser

Displacementflow

Mixed flow

3.4.1 Air temperatureAir temperature in a room generallyincreases from floor to ceiling. If this verticaltemperature gradient is large, local warmdiscomfort can occur at the head and / orcold discomfort at ankle height. Researchhas shown that the temperature differencebetween head and ankle height should beless than 3K. Thus the limiting temperaturegradient depends on whether the occupantsare considered to be standing or seated andgradients of 3K/m or 1.8K/m may be usedrespectively. It is also recommended that thelimits of variation of temperature across theoccupied zone of an enclosure should bewithin ±1.5K about the mean room airtemperature.

3.4.2 Air SpeedAir speed may cause unwanted local coolingof the body, known as draught. The risk ofdraught depends on the local air speed, theturbulence intensity, and the airtemperature. The draught risk may beexpressed as the percentage of peoplepredicted to be bothered by draughts. For atemperate climate it is recommended thatthe air speed be kept within the range of0.13 to 0.18m/s.

3.5 GENERAL GUIDE ONPRELIMINARY SELECTION OF CLASSOF AIR TERMINAL DEVICE.The following sections indicate guidelines togood practice.

3.5.1 Mixed flow applicationsFor a cooling temperature differential withinthe range of 10K (10oC), Table 3.1 givestypical air change rates, and air flow ratesper square metre of floor area. These shouldnot be taken as Max. or Min. but willgenerally result in average velocities withinthe above range. The figures in Table 3.1 arefor use as a general guide and have beenassessed from a number of publishedmanufacturers’ data. They have been basedon two room sizes of 3.6 x 3.6 x 2.5m highand 7.2 x 3.6 x 3.5m high, together with themounting positions shown in Table 3.2.Consideration must be given to the followingwhich may result in deviations from thevalues given in Table 3.1.

(a) Manufacturer.(b) Mounting position(c) Type of ATD.(d) Blade divergence.(e) Acceptable local velocities within

the room.

13

Table 3.1: General guide for mixed flowSee section 3.5.1

Class of ATD Air change rate per hour Air flow rate (litres/sec m2 floor area)

3.6x3.6x2.5m high 7.2x3.6x3.5m high

Grilles 8 5.6 8

Linear Grilles 10 7 10

Slot and Linear Diffusers 15 10.5 15

Rectangular Diffusers 15 10.5 15

Perforated Diffusers 15 10.5 15

Circular Diffusers 20 14.0 20

Table 3.2 Typical applications

3.5.1.1 Step by step procedures forselection and positioning of ATD’s.

3.5.1.1.1 Sidewall grilles (method I).

Figure 3.22 Room dimensions

Figure 3.23 Plan view and segment size

Given the total air flow rate, qt and roomdimensions (figure 3.22) determine thenumber of supply ATD’s (with blades set at0o deflection) that are needed.

14

Solution:1) It was stated in section 3.3.1.1, figure 3.8,

that the spacing between two adjacentair terminals should be 1/3rd of the throwof the air terminal when the grille bladesare set at 0o deflection.

2) The room is then divided along length Linto equal sections such that thewidth/length of each segment has anaspect ratio of not less than 1:3, seefigure 3.23.

3) In this example, the number of ATD’s is 4.

4) Then the air flow rate through each ATDcan be evaluated by the followingexpression:

Air flow rate per ATD = qt

4

5) Once the flowrate for each ATD is known,the exact throw can be determinedrelative to the segment length (W) fromthe manufacturers catalogue for selectedATD’s. This relationship is specifieddifferently by individual manufacturers.Reference should therefore be made tothe data published by the particularmanufacturer under consideration.

6) Next, determine the allowable drop suchthat the air flow envelope at a specifiedvelocity does not enter the occupied zone(normally up to 1.8m) causing draught,see figure 3.24, section 3.4 and table 3.2.

7) Referring to the manufacturers’ dataselect a suitable ATD size taking intoconsideration air flow, throw, temperaturedifferential and position of the ATD inrelation to the ceiling. It should be notedthat drop is made up of two components

a) drop due to temperature differentialb) drop due to vertical spread.

8) Note should also be taken of the resultantpressure and noise characteristics tocheck that they meet the environmentbeing served. If the above selection doesnot meet either the noise or pressurerequirement the following alternativeprocedures can be adopted (Methods IIand III). Both of these methods alleviatethis problem by dividing the room space

Figure 3.24 Allowable drop

into more segments, increasing thenumber of ATD’s such that each ATD hasa smaller duty.

3.5.1.1.2 Sidewall grilles (Method II)Divide the room area in two about thelongitudinal centre line in order to increasethe number of ATD’s and then proceed as inmethod I (section 3.5.1.1.1).

3.5.1.1.3 Sidewall grilles (Method III)1. Divide the room area into two halves about

the longitudinal centreline maintaining thesame segment length (W/2).

2. Set the ATD’s blades at 22o and thenproceed as before.

3. Note that the segment width to segmentlength ratio must not be less than 1:2according to the deflectioncharacteristics outlined in section 3.3.1.1which would correspond to a segmentwidth of no less than W/4. In this examplewe are maintaining a segment length ofW/3 which is acceptable.

Figure 3.25 Same example as method I but with16 ATD’S

Important noteShould the top of the core of the ATD be mountedwithin 200mm of the ceiling, an extension of thethrow can be anticipated (as described in section3.2.1.4) Information regarding this effect isnormally given by all manufacturers of ATD’s.

Figure 3.26 Same example as method I but with8 ATD’S

15

3.5.1.1.4 Linear grilles and registersGiven the total air flow rate (qt) and roomdimensions (as in figure 3.22) determine thenumber of supply air terminals and theirspacing.

Solution :1. The throw (Xm) relative to the sectional

length (W) or (L) is specified differently byindividual manufacturers. Referenceshould therefore be made to the datapublished by the particular manufacturerunder consideration

2. Determine the allowable drop such thatthe air jet does not enter the occupiedzone. This zone is normally taken as 1.8m from floor level. (See figure 3.24 andTable 3.2)

3. Referring to the manufacturer’s data,taking into consideration throw, drop,temperature differential relative to theceiling, establish the air flowrate per unitlength qm and the pressure and noisecharacteristics

4. Then determine the total active length (l)of the ATD by dividing the total air flowrate by air flow rate per unit length

Total active length (l) = qt

qm

5. This total active length can be divided tomeet the physical constraints and/orarchitectural requirements (see figure3.28). Each divided section should have aminimum aspect ratio of 1:5.

a) Total unit lengthsb) Number of individual active outlets

NoteShould the noise characteristics exceed theenvironmental requirement a reduction in thevelocity through the ATD may well result inpremature drop of the air envelope into theoccupied zone; it is therefore questionablewhether the type of ATD selected is suitable forthe application (See chapter 2, table 2.1).

3.5.1.1.5 Slot diffusersFrom the given air flow rate, qt and roomdimensions, as in figure 3.22, determine:

1) The throw (Xm) relative to the sectionaldistance to the boundary of the area to betreated (W or L) see figure 3.29. Thisrelationship is specified differently byindividual manufacturers. Referenceshould be made to the data published bythe particular manufacturer underconsideration.

2) The allowable drop such that the air flowenvelope at a specified velocity does notenter the occupied zone, see figure 3.24This zone is normally taken as 1.8m fromfloor level. In the event of the room heightbeing less than 2.5m reference should bemade to individual manufacturers’published data.

Figure 3.27 Throw

Figure 3.28 Total active length (l) dividedinto three sections

a b

l= a+b+c

c

Figure 3.29 Position of slot diffuser

6) Check that the noise characteristics of theselected ATD meet the requirements ofthe environment being served. Noteshould be taken of the manufacturer’scorrections for

16

3) Referring to the manufacturer’s data andtaking into account throw, drop (ifapplicable) and position of ATD in ceiling,establish air flow rate per unit length andwidth (number of slots) and slot width ofthe ATD to meet the throw and droprequirements. Note should also be takenof the pressure and noise characteristics.

4) The total active length of the ATD bydividing total air flow rate by air flow rateper unit length. This total length can thenbe divided to suit the physical limitationsand architectural requirements.

5) From the manufacturer’s data and theATD resulting from the procedure in theprevious paragraphs, select suitable ATDwidth (qualified by the number of slots)noting the noise characteristics.

6) Should architectural features indicatedifferent design requirements selectsuitable width (qualified by slot widths) tomeet throw requirements noting noisecharacteristics.

Note This type of ATD performs to themanufacturers’ published data with a flat ceilingapplication to ensure surface attachment (seeSection 3.2.1.4). With other than flat ceilingconditions it is advisable to check with theparticular manufacturer of the ATD underconsideration.

3.5.1.1.6 Circular, square and swirldiffusers (mounted at ceiling level)(First selection)

Given the total air flow rate, qt and roomdimensions, as in figure 3.22, determine:

1) Number of supply ATD’s by dividing theplan area into sections such that theirdimensions do not exceed three timesthe mounting height

2) Should rectangular sections be necessarytheir long dimensions should not exceed1.5 times the short dimension, this shortdimension not exceeding three times themounting height.

3) Example: Number of ATD’s = 4

4) Air flow rate for ATD by dividing the totalair flow rate by the number of supplyATD’s selected.

qt = Air flow rate / number of ATD’s

5) Determine radius of diffusion (Xm) relativeto the section dimensions. Thisrelationship is specified differently byindividual manufacturers. Referenceshould therefore be made to the datapublished by the particular manufacturerunder consideration.

6) Determine the allowable drop such thatthe air flow envelope at a specifiedvelocity does not enter the occupiedzone, see figure 3.24. This zone isnormally taken as 1.8m from floor level.(See Section 3.4.2. and Table 3.2)

7) Referring to manufacturer’s data select asuitable ATD from the preferred rangebased on air flow rate, radius of diffusiondrop and temperature differential. (Itshould be noted that drop is made up oftwo components):

(a) Drop due to temperature differential(b) Drop due to vertical spread

Figure 3.30 Plan room layout

Figure 3.31 Plan room layout

Figure 3.32 Radius of diffusion

17

8) Note should also be taken of the resultantpressure and noise characteristics.

9) If the above selection does not meeteither the noise or pressure requirements,review the implications in terms of usingthe next larger size or alternative model.Should this not meet the environmentrequirement, the following alternativeprocedures can be adopted.

Alternative selectionDivide each square or rectangular section(see section 3.5.1.1.6) into 2 or 4 smallersections within the guide lines set out in3.5.1.1.6. step 1 in order to increase thenumber of ATD’s ( see figure 3.33) and thusreduce the noise, throw and radius ofdiffusion requirements of each ATD and thenproceed as in Sections 3.5.1.1.6 steps 2 - 9.

2) Example: Number of ATD’s =5

Note Where opposing air streams meet the air willbe diverted downwards and this feature should betaken into account, see figure 3.35. For furtherinformation refer to manufacturer’s data

Alternative SelectionFor use with directional ATD’s only.

1) Position one additional ATD centrally ineach set of four ATD’s, as shown in figure3.24, in order to similarly reduce theradius of diffusion, the air flow rate perATD (qt /5) and the noise characteristics ofeach ATD.

Equalising grids and dampers where fitted inthe approach duct immediately above theceiling diffuser, can materially affect the totalpressure loss and noise level. Allowancesmust be made for such accessories andmanufacturer’s data should be consulted.

3.5.2 Displacement flow applicationsAir terminals should not be placed too closetogether since the combined effects willgenerate a near zone greater than the extentof a single unit near zone.

There should be a uniformly spaced layoutso that the near zone profiles do not collidesee figure 3.36. Reference should always bemade to the manufacturer’s literature.

It is particularly important with displacementflow applications to locate the exhausts athigh level.

3.5.3 Exhaust air terminal devicesThe selection and location of exhaust ATD’sis much less critical than that of supplyATD’s. This is because the air flow throughan exhaust ATD has very little influence onthe movement of air in a treated space. Theair approaching an exhaust ATD does sofrom many directions so that significantvelocities only occur close to the device.

Figure 3.33 Number of ATD’s= 16

Figure 3.36 Near zone profiles

Figure 3.35 Opposing airstreams

Figure 3.34 Adding one diffuser

18

For example, with a square exhaust grillemounted in a flat surface, the approachingair velocity at a distance equivalent to thelength of one of its sides is approximately10% of the core velocity. Thus it is onlywhere occupants are likely to be situatedclose to exhaust ATD’s that considerationneeds to be given to the air movementgenerated.

For typical core velocity see Table 3.3.

3.5.3.1 Pressure dropRefer to manufacturer’s data to selectmaximum allowable, whilst also taking intoaccount damper setting requirements, etc.

3.5.3.2 LocationThere are other factors, however, in additionto those mentioned above which influencethe choice of location of exhaust ATD’s.There are advantages in locating exhaustATD’s in the zone of the warmest room air forcooling applications or of the coolest air forheating applications. For similar reasonslocations close to areas of heat gain or lossmay be beneficial. To avoid short circuiting,the exhaust ATD should not be positionedtoo close to the supply ATD’s in the directionof air discharge.

For displacement ventilation systemsextracts should be placed at the highestlevel in order to extract the warmest air. Forlow ceiling areas, it is also preferable toposition extracts above large heat sources.This will minimise contaminant descent fromany recirculating air.

3.5.3.3 Grilles and DiffusersGiven the total air flow rate (qt), roomdimensions and the air flow pattern in thetreated space determine:

The number of ATD’s by consideration ofparagraphs 3.5.3.1. to 3.5.3.2. However, atleast one ATD should be provided pertreated space.

Air flow rate per ATD by dividing the total airflow rate (qt) by the number of ATD’sselected.

Referring to manufacturer’s data, selectsuitable size of ATD by using the flow rateper ATD and one or more of the parametersbelow:

(a) Noise(b) Core Velocity(c) Pressure Drop

NoteShort circuiting occurs when the supply air isexhausted from the treated space before, eitherreaching design room temperature, or providingsufficient energy to induce satisfactory room airmovement.

3.5.3.4 Transfer grillesWhere exhaust ATD’s are used in order thatair can pass from one treated space toanother, such devices are termed transfergrilles and are normally mounted in doors.

The selection and positioning of transfergrilles, in relation to the first treated spacecan be dealt with generally in accordancewith paragraph 3.5.3.

Location Core Velocity

4m/s and above

3-4m/s

1-3m/s

Within occupied zoneaway from occupants

Within occupied zonenear occupants

1-2m/sDoor Transfer

Above occupied zone

Table 3.3 Typical core velocities

Figure 3.38 Transfer grille

Figure 3.37 Exhaust air velocity profile

19

Chapter Four4 SOUND CHARACTERISTICS

4.1 INTRODUCTIONThe intention of this chapter is to coversufficient acoustic technology to helpengineers in the selection of terminaldevices and the interpretation ofmanufacturers’ literature. There are anumber of text books giving introductoryinformation concerning sound power, soundpressure level, frequency and rating system.

In building design acceptable noise levelsrank equally with other major environmentalrequirements in defining an acceptablespace or room criteria.

In designing a ventilation or air conditioningsystem consideration is normally given tothe control of noise within the ductworksystem, if necessary, by the use of ductsilencers. In addition, air terminal devicessuch as grilles and diffusers, can themselvesgenerate some degree of noise related tothe velocity of the air passing through them.However, being the component directlyconnected to the room or treated space, theair terminal device must be selected not onlyto provide the desired aerodynamicperformance but also to ensurecompatibility with the environmental noiserequirements. Failure to do this can result inan unacceptable noise level with theresulting necessity of changing the airterminal device to one of different size orshape which can entail obvious penalties ata later stage in the building project.

The success or failure of an installation candepend as much on acoustic performanceas on achieving good room air diffusion.Thus it is important that the engineerselecting air terminal devices should havean understanding of the basic acousticprinciples affecting their installation, theirsize and their type.

4.2 SOUND IN ENCLOSED SPACEGiven a knowledge of the physical nature ofsound and of the subjective rating of soundpressure levels, one can consider the soundpower level generated by an air terminaldevice and radiating from it into a givenspace.

Three main factors are involved in theconversion of sound power to soundpressure.

i) Distance from the ATD - see Figure 4.1.This is based on spherical radiation ofsound waves and thus as the distancefrom the source increases then the soundpressure level decreases.

ii) Directivity - see Figure 4.2. This is relatedto the size of the ATD and the number ofadjacent boundary surfaces which causea restriction in the arc over which soundwaves can radiate. This locally increasesthe sound pressure level at a fixed pointin space. This characteristic is a functionof frequency.

iii) Room constant - see Figure 4.3. Thisdescribes the capability of the boundaryof the room and furnishings to absorb orreflect sound waves. It is frequencydependent and related to the averageacoustic absorption coefficient of allsurfaces. For the same source a hardsurfaced room will have a higher soundpressure level than a room with carpetand furnishings.

Figure 4.1 Spherical Radiation from asource in free space

Figure 4.2 Directivity

Figure 4.3 Room Constant Reverberant Effect

20

Factors (i) and (ii) are usually referred to asthe direct component of the sound pressurelevel and factor (iii) as the reverberantcomponent. The resultant sound pressurelevel at a point in space is the logarithmicsummation at each octave band of thesetwo components.

Figure 4.4 shows a typical relationship of theabove as a function of distance from thesource.

The above phenomena can be described inmathematical terms enabling calculations ofthe resultant sound pressure level as afunction of frequency. This is beyond thescope of this booklet, and in cases wherenoise level requirements are very low orcritical a specialist in this field should beconsulted. However, there are approximatemethods that can be employed in the initialdesign stages and these are described inparagraph 4.4.

The previous equation may then be re-written as:

LwNR = K1 log Vv + 10 log A + K2

Where K1 and K2 are constants, similar to KX

and KY, but relate to the subjective NR rating(see table 4.1).

LwNR = NR rating of sound power levels.

LwNC = NC rating of sound power level

LwdBA = dBA rating of sound power level

Typically -

LwNR approximately equal to LwNC

LwNR approximately equal to LwdBA - 5

Also all the foregoing data are based on airflow dampers being fully open.

As has already been indicated, controldampers on the back of ATD’s are onlyintended to provide a fine adjustment of theair flow quantities by imposing additionalpressure drop, which results in additionalturbulence and, hence, noise. See Figure 4.5.

The increase in noise level due to closure ofopposed blade, flap or radial vane damperscan be approximated from the followingequation:

For supply ATD’s ∆dB = 30 log P1

P0

For extract ATD’s ∆dB = 16 log P1

P0

Where ∆dB = increase in overall sound powerlevel or NR sound power level.

P0 = total pressure drop through thedamper plus grille - damper fully open.

P1 = total pressure drop through damperplus grille - damper partially closed.

Figures 4.6.1 and 4.6.2 gives plots of Logfunctions for velocity, area and damperpressure drop ratio, this in conjunction withthe constants given in Table 4.1 provides asimplified means of noise prediction.

From examination of Figures 4.6.1 and 4.6.2the following should be noted:

i) With the same ATD doubling of dischargevelocity increases the noise level byapproximately 18 dB.

Figure 4.5 Mechanism of Noise Generation byDampers and Grilles

Figure 4.4Variation of Sound Pressure Level in Roomas a function of Distance from the Source

4.3 NOISE GENERATIONCHARACTERISTICS OFAIR TERMINAL DEVICESNoise generation is caused by the increasein air velocity and turbulence as the air flowpasses around solid obstructions such asdeflecting vanes, etc. (See Figure 4.5). Thesound power level generated by thesemechanisms can be approximatelyexpressed as follows:

LW = KX Log VV + 10 Log A + KY

WhereVv = Discharge velocity in m/s

A = Aerodynamic free area of ATD in m2

LW = Overall sound power level in dB re 10-12

watts

KX and KY are constants, being a function ofthe sound, frequency, ATD type and design.

Sound power levels can be evaluated usinga number of subjective rating systems forexample NR, NC or dBA. This is particularlyuseful when determining room noise levels.See section 4.4.

21

Figure 4.6.2 Increase in ATD noise due to closing Damper

Figure 4.6.1 Data for Basic ATD Noise Prediction

ii) With the same discharge velocity doubling the ATD area the noise level increases by approximately3 dB, i.e. this is very insensitive in comparison with the velocity characteristic.

iii) A doubling of total pressure drop through an ATD damper combination by means of partial closureof the damper results in (a) supply + 9 dB, (b) extract + 5 dB.

22

Type of ATD K1 K2

Grilles and Linear Grilles (Supply Air) 60 -3

Grilles and Linear Grilles (Extract Air) 60 -3

Circular & Square Diffusers 60 -3

Linear Diffusers 60 0

Slot Diffuser 60 +10

Perforated Plate Diffuser 80 -8

Swirl Diffusers 63 +3

Floor Diffusers - vertical Discharge 60 +6

Floor diffusers - Horizontal Discharge 60 +9

NR Sound Power Level:LwNR= K1 log V + 10 log A + K2 (Damper fully open)LwNC approximately equal to LwNRLwdBA approximately equal to LwNR +5

Displacement DiffusersFor sidewall or free standing units the noisecharacteristics are a strong function of internaldiffuser design. This varies significantly frommanufacturer to manufacturer and hence nounique values for K1 and K2, may bedetermined.

Figure 4.7 Effect of Blade Divergence

BLADE DIVERGENCE1st Row 2nd Row

INCREASE IN NOISE LEVELdB

0

1

3

2

6

0

0

0

22.5o

45o

0

22.5o

45o

22.5o

45o

2nd Row

1st Row

When supply grilles with movable blades areemployed, there is additional increase innoise due to blade deflection, showntypically in Figure 4.7.

4.3.1 Example of Subjective SoundPower Level PredictionGiven: Supply air grilleFlow rate= 0.45m3/secDischarge velocity= 4.5 m/sAerodynamic free area= 0.1m2

Opposed blade damper fully open P0= 15 Pa

Required pressure drop at 0.45 m3/s P1= 30 Pa

From Table 4.1:LwNR = 60 log V + 10 log A - 3

From Figure 4.6:LwNR = 39 - 10 - 3 = 26dB

P1 = 30 = 2P0 15

∆dB = 9dB

LwNR = 26 + 9

= 35dB

4.4 DETERMINATION OF SPACE NOISELEVELS DUE TO ATD SELECTIONHaving determined the sound powercharacteristics either from manufacturers’data or from the approximate method givenin Section 4.3., it is now necessary to takeaccount of the room characteristics todetermine sound pressure levels at a givendistance from the ATD. The principlesinvolved have been described in Section4.2.

Method 1 - Specialist Approach -Within 3 dBA detailed analysis of the direct andreverberant components as a function ofoctave bands of frequency can be carriedout. This is normally required in cases wherenoise level requirements are critical.Reference should be made to an acousticsengineer for this service.

Method 2 - Graphical - Within 6 dBThis is based on a graphical solution andcan be undertaken by evaluating thefollowing relationships.

Room NR level = LwNR + C1 + C2 + C3

Again - Room NC level approximately equalto NR level

Room dBA level approximatelyequal to NR level -5

Table 4.1 Constants for the prediction ofsubjective ratings of sound power levels

23

Assume an office block with a room 10m x5m x 3m carpeted floor, acoustic tiled ceiling.3 off ATD’s serve the space positioned in thewalls at ceiling/wall junction. Take theconditions given in the example in 4.3.1., i.e.

Subjective Rating of Sound Power LevelLwNR = 35dB

Where C1, C2 and C3 are corrections basedon room volume and acousticcharacteristics, the distance, from ATDlocation and number of ATD’s and can bedetermined as follows:

Determine NR level at 2.0m from ATD’s.

1. Acoustic rating of space - see Table 4.2.

Due to carpet and acoustic tiled ceilingtake room as Medium Dead.

2. Room Volume.

Volume = 10 x 5 x 3 = 150m

3. C1 from Figure 4.8 using space ratingroom volume and Distance from ATD.

C1 = -8 dB

4. Directivity Correction C2 which dependson ATD location. See Table 4.3.

C2 = +6 dB (at wall/ceiling junction)

5. C3 from Figure 4.9 based on number of ATD’s

C3 = + 5 dB (3 off ATD’s)

NR Rating = 35 - 8 + 6 + 5 = 38NC Rating approximately equal to 38dBA Rating approximately equal to 38 + 5= 43

RATING USE OF ROOM

Radio and TV Studiosaudiometric rooms

Restaurants, offices andboardrooms withabsorbent ceiling andfloor covering, hotelbedrooms

Standard offices,libraries, hospital wards(rooms with no specialacoustic treatment)

School rooms, lecturetheatres, art galleries andpublic houses

Churches, swimmingbaths, factories, operatingtheatres, large canteensand gymnasiums

Table 4.2: Typical room characteristics

Table 4.3: Directivity index

Dead Room

Medium Dead

Average Room

Medium Live Room

Live Room

MOUNTING POSITION C2

Free space 0dB

Flush with one surface +3dB

Junction with two surfaces +6dB

Junction with three surfaces +9dB

Method 3The final method is one which is quite oftenused in manufacturers’ literature to give avery approximate idea of room subjectiveratings as follows:

Room subjective rating = Sound PowerLevel subjective rating - 8 dB

Where 8 dB is referred to as the roomcorrection.

Care must be taken when using thistechnique as when more than one ATD isinvolved in large spaces the actual roomcorrection can be in the region of 2 to 3 dB.If noise is considered important then eithermethods 1 or 2 should be adopted.

A general indication is given in Table 2.1 ofthe relationship between various types ofATD and what, subject to check by theabove procedures, is normally considered tobe ‘quiet’ and ‘commercially quiet’.

If after calculating the space noise level thisshould prove to be too high, then thefollowing possible courses are open toimprove the situation.

i) Increase the sizes of the ATD - whilst agreater area will cause a slight increase innoise, the resultant velocity reduction forthe same air flow will give an overall noisereduction. If this course is followed, theaerodynamic performance should thenbe rechecked.

ii) Increase the number of ATD’s of the samesize - whilst the increased numbers willresult in a small increase in noise level,the resultant reduction in velocity througheach individual ATD will againpredominate to give an overall noisereduction. Again the aerodynamicperformance should be rechecked.

iii) Use an ATD with a greater free area -again the reduction in air velocity willpredominate to give an overall noisereduction.

Where large number of ATD’s of similar typeare to be checked for noise characteristics,it is usually more convenient to establish amaximum discharge velocity assuming thesiting and number of terminals serving aspecific space. This is acceptable because

24

the velocity effect on noise is dominant. Ifwhen using this method the selectionapproaches the limiting air velocity, then thecase should be subjected to the morerigorous analysis in method 2 above.

When using a damper in conjunction with anATD, it is good practice to allow anadditional 2 to 3 dB before finalisingselection.

4.5 EFFECT OF SYSTEM DESIGNIt is important to realise that the noiseapparently radiated from an ATD can eitherbe as a result of:

a) Noise generated at the ATD itself, or

b) Noise propagated down the ductworkfrom upstream sources such as maincontrol dampers, constant and variableflow rate devices, or

c) Noise generated from poor plenum boxdesign

Figure 4.8 Correction for receiving space

Figure 4.9 Correction for number of ATD’s C3

4.5.1 Duct and Plenum DesignAs already indicated, air velocity through anATD provides the dominant contribution tothe noise level generated at that device.With a poorly designed plenum or approachductwork connected to a supply ATD, it isquite possible, for example, for the bulk ofthe air flow to pass through half the ATDarea, thereby doubling the dischargevelocity. Not only would this be undesirablefrom a room air diffusion viewpoint but alsothe effective noise generation wouldincrease by approximately 15 to 20 dB.

In cases where plenum boxes are fitted tothe back of ATD’s, linear grilles, slot diffusersetc., care must be taken to ensure that noisegenerated in the plenum box, due to flowdischarge from the spigot, does not make asignificant contribution to the overall noisegeneration.

25

Typical methods of avoiding this type ofproblem are:

i) Air velocity in the spigot should notexceed the ATD discharge velocity.

ii) Maximum spigot velocities for NC/NR 35-40 should not exceed 4 m/s.

iii) If there is doubt concerning plenum noisegeneration, the plenum box should belined acoustically on the inside with anadequate thickness of fibrous material,suitably covered and installed to preventparticle migration.

In cases where very low noise levels arerequired, i.e. below NR30, then it is wise tolocate the dampers not adjacent to the ATDbut some distance upstream followed bysecondary attenuation. Care should betaken in the design of the attenuator and theduct connecting the attenuator outlet to theATD to ensure that local velocity high spots

Figure 4.10 Uneven Velocity Profiles

1) Shut down in rotation the various plantsserving the space in question to identifywhich system is creating the noise. Takecare not to be misled by the possible airimbalance created within the space, i.e.supply only creating noise due to airescaping through doors, etc.

2) On the noisy system, check position ofATD dampers. If these are well closed,open dampers. If the noise level isreduced then damper generated noise isthe probable cause of the problem.Solution - consider introduction ofdampers as far as possible upstream ofthe ATD to provide required pressuredrop for balancing. This may also involvethe use of secondary silencersdownstream of the new damper.

3) If opening and closing the ATD damperdoes not result in any significant variationin noise level then either the noise isentering the space through some otherpath, i.e. structure borne vibration, directtransmission through walls, ceiling orfloor, duct breakout, etc., or ispropagating down the ductwork fromanother noise source.

4) Remove ATD plus associated damperfrom duct. If the noise level reduces thenthe sources of the problem may well bethe ATD and damper. However, if thenoise level increases significantly, thenagain, it is likely that duct borne noise ispropagating down the ductwork systemfrom some upstream source such asmain control damper, primary fans, poorlydesigned ductwork junctions etc.

Should the above investigation indicate thatthe noise problem is associated with theATD damper plenum box combination, thenconsideration must be given to suchremedial measures as:

i) Replacing ATD with one of greater freearea.

ii) Replacing ATD with a larger device.

iii) Redesign the plenum box (possiblyincreasing the number of spigots).

iv) Increase the number of ATD’s.

As can be seen from the above, the costimplications of remedial action should thenoise problem occur with an ATD,emphasises the importance of adequateconsideration being given in the designstages to the correct selection of ATD’s toensure compliance with noise specification.

do not occur at the ATD which would againresult in increased noise level. Figure 4.10illustrates a number of points made above.

4.6 OTHER NOISE SOURCESIf a noise level problem in a space has beenidentified when a project has reachedcommissioning stage, it is recommendedthat the following basic procedures areemployed as a process of elimination inorder to determine whether or not theproblem relates to the ATD.

26

Chapter Five5. DUCT ENTRY CONDITIONS

5.1 GENERALAir terminal devices are selected on theassumption that there are no irregularities ofthe velocity profile at the face of the airterminal device. It follows that duct entryconditions are equally as important as thesize and type of air terminal device.

Unequal velocity profiles at the face of an airterminal device can give rise to the following:

■ Excessive turbulence

■ Unpredictable throw and spread.

■ Breakdown of wall/ceiling attachment.

■ High noise levels.

■ Extremely difficult balancing procedures

Recommendations to assist in the correctentry condition design to air supply devicesare shown in the subsequent sections.

The following designs can be used even insituations where there is limited duct spaceavailability.

Each design illustrated employs theprinciple of the Total Pressure to VelocityPressure ratio Pt /Pv, this principle beingbased on simple Bernoulli theorem. Thisallows the correct balance of Pv to Ps (staticpressure) by introducing Pt as a vectorcomponent. (Pt = Pv + Ps)

Although dampers or pressure reducingdevises are not always shown, their effectsshould always be taken into considerationwhen establishing the Pt /Pv ratio.

The following are used in the subsequentfigures.

TPu - Total pressure in duct before take-off

VPu - Velocity pressure in duct before take-off

TPb - Total pressure in plenum box

VPb - Velocity pressure in plenum box

Vu - Average duct velocity before take-off

Vc - Average velocity in connector duct

Vb - Average velocity in plenum box

Øn - Neck diameter for circular diffusers orequivalent dia for rectangular diffusers

Vn - Average velocity in neck

Exhaust air devices have not been specificallyindicated, there being insufficient datacurrently available for the losses of fittingsused for return air. The friction and dynamiclosses for straight duct and duct elements arethe same on the return side as the supply, butat junctions of branch and main ducts thedynamic losses result from the jointing ratherthan the dividing of air streams. The presentpractice is to employ the same methods as forsupply units but with reverse airflow.

5.2 DIFFUSER CONNECTIONS

Figure 5.1 Branch connection (long)

Figure 5.2 Branch connection (short)

Figure 5.3 Branch connection (flap deflector)

Figure 5.4 Flexible connection

Figure 5.5 Flexible connection - good practiceto ensure correct results

Figure 5.9 With change section connector

27

End connections are as branch connectionswith identical pressure ratio restrictions.

N.B. The length of straight duct between a 90°1.5d radiused bend (radius to centre line) and thediffuser depends on the pressure ratio for branchconnections.

Figure 5.8 After rectangular or radiused bends

5.3 GRILLE CONNECTIONS

Figure 5.6 End connection

Figure 5.7 Connection for use where height isrestricted

Figure 5.10 Longitudinal section with plenumheader box

Figure 5.11 From branch duct

Figure 5.12 From branch with deflector

5.4 LINEAR GRILLE/DIFFUSERCONNECTIONS

5.4.1 Straight boxesWith few exceptions, these should bedesigned as plenum header boxes. Thecross sectional aspect ratio of the headerboxes should not exceed 4:1.

Figure 5.13 Straight box side entry (entry view)

28

Figure 5.14 Straight box side entry (side view)

Figure 5.15 Straight box top entry (side view)

The average velocity in the connection (Vc)should not exceed 4.5 m/s.

The cross sectional area of the box shouldbe such that the TPb/VPb ≥ 3 at each side ofthe connection.

TPb should be the TP of the air terminaldevice plus 15%.

If the box exceeds 2m in length, multipleconnections can be used, but at not greaterthan 2m distances.

If the air may impinge directly on the airterminal device a perforated plate ofapproximately 50% free area should beused.

5.4.2 Tapered boxesTapered boxes maintain the velocitypressure and keep the TPb/VPb ratioconstant.

Figure 5.16 Tapered box (side view)

Figure 5.17 Tapered box air pattern

The air stream is not perpendicular to the airterminal device and a spreading patternresults.

5.5 DISPLACEMENT FLOW DIFFUSERSThe duct entry conditions for a displacementflow diffuser depend upon the internaldesign of the specific model. This variesconsiderably from manufacturer tomanufacturer and a generic condition is notapplicable. Please refer to manufacturers’data for the relevant information for themodel selected.

29

Chapter Six6 THE FIXING AND INSTALLATION OFAIR TERMINAL DEVICESThere is a wide range of air terminal devicesand applications for them and a wide varietyof fixing methods have therefore evolved.The principal current fixing methods areillustrated in the following sub sections. It isadvisable to use fixing details issued by themanufacturer where these are available.

In addition to the following groups, there areATDs such as nozzles, disc valves, etc.,which have their own proprietary fixingmethods and therefore have not beenincluded in this section.

In general, ATDs are shown in their normalorientation of use, i.e. ceiling, wall or floormounted. Where details are specifically notsuited for other orientations this is madeclear in an accompanying note.

6.1 SQUARE, RECTANGULAR ANDLINEAR GRILLES

6.1.1 Visible fixing

6.1.2 Concealed fixing

Figure 6.1 Flange screw fixing

Figure 6.2 Sub-frame/quick release fastenerNormally not recommended for ceiling use

Figure 6.3 Beading (transfer grilles)

Figure 6.4 Sub-frame/locking device

Figure 6.5 Sub-frame/Rear bracket

Figure 6.6 Sub-frame/Spring clipNot recommended for use in ceilings or doors

Figure 6.7 Spring clipNot recommended for use in ceilings or doors,or where distortion of surround can occur

30

6.2 LINEAR AND SLOT DIFFUSERS

Figure 6.8 Rear bracket or strap

Figure 6.9 Rear bracket and saddle bracket

Figure 6.10 Saddle bracket into duct

Figure 6.11 Hidden screw fixing

Figure 6.12 Bridge or saddle bracket

Figure 6.13 Flange screw fixing

Figure 6.14 Rear support bracket

Figure 6.15 Internal suspension bracket

Figure 6.16 Angle brackets from plenum box

Figure 6.17 Spring clip to plenum box

31

6.3 CIRCULAR AND RECTANGULARDIFFUSERS

Figure 6.18 Wire supportNot for use in walls

Figure 6.19 Hidden screw fixing

Figure 6.20 Suspension bolts and brackets

Figure 6.21 Flange screw fixingCeiling must be of a suitable material

Figure 6.22 Rear support anglesa) Suspension method

Figure 6.22 Rear support anglesb) Angle support from Tee bar ceiling

Figure 6.23 Rear suspension bracketsCeiling must be of a suitable material

Figure 6.24 a) Internal suspension bracket

Figure 6.24 b) Screw access either through coreor with core removed

Figure 6.25 Spring edge clip

Figure 6.26 Drop in angles support(Linear grilles only)

6.4 FLOOR MOUNTED GRILLESAND DIFFUSERS

32

Figure 6.27 Drop in recessed support

Figure 6.30 Swirl diffuser (circular)c) Back collar fixing

Figure 6.31 Floor grille with jacks

Figure 6.28 Drop in frame support

Figure 6.29 Drop in spring fixing

Figure 6.30 Swirl diffuser (circular)a) Lay in core

Figure 6.30 Swirl diffuser (circular)b) Core and trim ring/cover plate

6.5 INSTALLATIONIt is important that all ducts or fixingopenings are regular in shape and correctlydimensioned.

When carrying out installation ensure that:

■ Specific manufacturer’s instructions arefollowed.

■ The ATD fits snugly into the openingwithout distortion.

■ There is sufficient material around thefixing points to give a secure fixing intothe duct, ceiling or building fabric.

■ Suitable fixings are used for the materialto which the ATD is to be fixed.

■ All joints are correctly sealed using selfadhesive foam strip or similar to avoid airleakage.

■ Mastic or similar is not used except onsub-frames which are not intended to beremovable.

■ Visible edge joints have a neat finish.

■ Floor mounted ATDs should be of anappropriate design to reduce risk oftripping.

■ Sharp edges and protruding screws arenot left where they could pose a safetyrisk in normal use or buildingmaintenance.

■ Once fitted, all moving parts operatecorrectly and removable cores can betaken out and replaced.

■ Safety wires with quick release ends mustbe used on removable cores where ATDsare installed in high level or ceilingapplications.

33

Chapter Seven7 MEASUREMENT ON SITEAlthough there now exists a large amount ofsophisticated instrumentation that makessite measurement easier, quicker and morecomprehensive there is a lot of older, morebasic, equipment still in everyday use. Insome instances, for example resolution ofdispute, it is common to revert back tofundamental devices to obtain the definitivemeasurement. For these reasons thissection attempts to cover both old andnewer technology instruments.

7.1 MAINTENANCE AND USEOF INSTRUMENTATIONAll test equipment shall be in accordancewith the specifications laid down in BS 4773Part 1 (aerodynamic testing) and ISO DIS5220.

Attention should also be paid to CEN pr EN13182 and subsequent revisions

To ensure a high level of confidence in testdata, instruments must have been checkedand calibrated within the recommendedtime period.

It is the responsibility of the test engineer tomake sure that, where a choice is available,the best instrument is selected to suit themeasurement to be taken.

For consistency it is advisable that the seriesof readings taken during a test from one or anumber of instruments should be completedby the same individual.

Vibration will upset the accuracy of mostinstruments. Care must be taken to ensurethat all instruments are placed on a vibrationfree surface.

Many instruments are temperature and/orambient pressure sensitive. Adequate timemust be allowed, before readings are taken,for the conditions to stabilise. Recognition ofthe appropriate correction factors should bemade.

Ancillary equipment, such as lights, tables,platforms, should be arranged in such amanner that the best possible resolution ofreading can be obtained from the instrumentin use.

7.2 PRESSURE MEASUREMENT There are two basic types of pressuremeasuring instruments used in the H & Vindustry:

(i) The liquid filled manometer in which theapplication of pressure is counter-balanced by the weight of a column ofliquid.

(ii) The dry gauge in which the application ofpressure causes the mechanicaldisplacement of a diaphragm or bellowswhich is sensed electronically or counter-balanced by a spring system with anattached pointer.

7.2.1 Liquid filled instrumentsThe fundamental instrument for pressuremeasurement is the vertical or inclined liquidin glass manometer. The sight glass must beof precision bore in either glass or plastic material. The liquid must be of knownrelative density, low viscosity and lowsurface tension.

Vertical manometers are made with eithersingle or double limbs. In the double limbarrangements both sides of the ‘U’ arevisible. The pressure is read by summing thechange of liquid height in each limb either byreading against a fixed scale or bymicrometer pointers adjusted to just touchthe liquid surfaces (Hook Gauge).

Figure 7.1 U tube manometer

Figure 7.2 Electronic instrument

Figure 7.3 Dial gauge, mechanical

34

In the single limb arrangement only one limbof the ‘U’ tube is visible. The “hidden” limbis usually of very much larger cross-section(a tank) and the measuring scale divisionsare calculated to take into account the levelchange in the tank as well as the relativedensity of the liquid.

Inclined manometers are usually of thesingle limb arrangement and offer the optionof various angles of inclination.

Pressures below 500Pa should be read onan inclined manometer. The instrumentshould be frequently checked for level andzero where these facilities are available.

When a choice of “incline” is provided, a trialrun will ascertain the lowest incline (andhence the reading of highest resolution) thatmay be safely used without over ranging theinstrument with consequent loss ofmanometer fluid.

Check for polarity and overload, since thesemay lead to a discharge of liquid into themanometer pipelines, which if not clearedwill cause false readings. Great care shouldbe taken to ensure that the pipelines arealways clear, dry and as short as practical.

When reading a concave meniscus (e.g.,paraffin, alcohol, etc.) always measure to thelowest part of the liquid surface. Poorillumination can cause dark line shadowswithin the meniscus which, if misread, canlead to inaccurate results.

A variation of the single limb verticalmanometer incorporates a transparent scalesuspended from a float in the smallersection limb. The change in height of thescale resulting from the application ofpressure is viewed through a high poweredtelescope. (Betz)

A further variation of the ‘U’ tube allows forthe change in liquid level in twointerconnected reservoirs, caused by theapplication of pressure, to be restored to adatum mark by raising or lowering one of thereservoirs by a calibrated screw thread.

7.2.2 Dry ManometersThe basic form of the dry gauge convertsthe deflection of a diaphragm or bellows bymechanical linkage to the movement of apointer over a calibrated scale.

An alternative linkage with less resistance tomovement is obtained by magnetic means.

More sophisticated versions of the drygauge convert the diaphragm movementinto an electrical signal (transducer) whichmay be remotely displayed in eitheranalogue or digital form. More commonlyinstruments of this type contain an integralelectronic pressure sensing element. Inaddition to display of pressure reading manyinstruments are also capable of giving directindication of velocity when used with a pitot

Figure 7.4 Hook gauge

Figure 7.5 Single limb manometer showingvarious inclinations

Figure 7.6 Floating scale manometer

Figure 7.7 Null gauge

35

static tube, or similar device. Volumetric flowrates can also be displayed by someinstruments with the additional input of acharacteristic area.

Particular care must be taken to ensure thatgauges of this type are shielded from anyform of shock loading or vibration duringuse.

The calibration can be upset by the incorrectorientation of this type of gauge. It isadvisable to ascertain the plane in whichcalibration was carried out.

As with liquid filled instruments it isadvisable to keep pressure connecting pipelines as short as practical.

7.2.3 Other methods of pressuremeasurementA variation of the transducer type of drygauge depends upon the measurement ofleakage flow rate through a calibrated orificeby means of either the rate of heat loss froma heated element or the mechanicaldeflection of a pivoted spring loaded vane.

7.3 FLOW RATE MEASUREMENTThe preferred methods of flow ratemeasurement are specified in BS EN ISO5167-1 and employ either the orifice plate,venturi tube, nozzle or nozzle venturi tocreate a pressure differential signalproportional to flow.

It is advised that the calculations andtolerances laid down in BS EN ISO 5167-1will only apply provided all the dimensionsand conditions up and down stream asspecified are observed. If theserecommendations cannot be followed orgreater accuracy is required then in situcalibration will be necessary.

The range of flow rates accuratelymeasurable by these methods is limited bythe published Reynolds Number. In the caseof the orifice plate method this can beovercome by preparing a series of easilyexchangeable orifice plates whose sizes canoffer overlapping flow rate ranges. Caremust be taken to ensure accuratecentralising and that airtight sealing isobtained when plates are changed.

Flow Grids

Figure 7.8 Volume flow indicating instrument

Classic venturi

Nozzle venturi

Figure 7.10 Wilson flow grid

Orifice plate assembly

NozzleFigure 7.9 Differential pressure volume flowmeasuring devices

A less costly device for in duct measurementis the flow grid. These devices range fromtwo simple cross tubes, to multiple tube andmanifold devices, which sample the flow

36

with upstream and downstream facingsensing holes. Either differential pressure, orbypass flowmeter, techniques are used tomeasure volume flow.

Resistance to flow in ducts is less with thesedevices than orifice or venturi meters.

Since the flow is sampled at the sensingholes, which are distributed across the ductcross section, good mean flow values aremeasured. For this reason such devices areoften more tolerant of less than optimummeter siting within ducts followingdisturbances such as bends, branch ductsetc.

7.3.2 Capture Hoods

For flow rate measurement with air terminaldevices capture hoods provide a convenientmethod. These devices are pushed againstthe surface surrounding the air terminaldevice thus making a seal and capturing theentire flow. The flow is channelled throughthe measurement section of the devicewhere it is measured via a grid section andeither a differential pressure method orbypass flow meter.

The capture hood is capable of makingmeasurements in either supply or exhaustmode.

Some capture hoods, particularly those whichuse mechanical sensing methods, may beorientation sensitive. Care should therefore beexercised to ensure calibration data is valid inuse whether this be for measuring verticalflows, from ceilings or floors, or horizontalflows from wall grills or diffusers.

The flow patterns associated with differentgrilles and diffusers can also significantlyalter the calibration factor of some capturehoods. Swirl diffusers can give rise toparticular problems.

If in any doubt about calibration validityensure that the measuring device iscalibrated under conditions as near aspossible to those encountered in use.

In order to reduce the effect themeasurement device has upon theventilation system care should be exercisedto ensure the capture hood chosen does notexert excessive back pressure on thesystem at the flow rates to be measured.

7.4 FLOW RATE MEASURED BYVELOCITY AREA METHODS(see BS 1042 Section 2.1 and section 8.4.2of this guide)

The air flow rate in a duct may be assessedby averaging the velocity readings obtainedby traversing across the duct in the logTchebychef distribution of points. (seemethod 10 of BS 1042 section 2.1)

Care must be taken to locate the velocitymeasuring head precisely at the pointsspecified.

7.4.1 Flow rate measurement byvane anemometerThe vane anemometer may be usedsuccessfully to assess the air flow rate in aduct and at the entry/exit of a duct by thepoint traverse method.

It may also be used with a calibrated hood,provided due allowance is made for theadditional resistance imposed by the hoodon the system.

Due to the blockage effects and thedifficulties of taking readings close to theduct wall the anemometer is rarely suitablefor accurate measurement in small ducts,but may be used to detect proportionalchanges in flow rate following accuratemeasurement by other means.

Where the anemometer head forms part ofthe duct run and swirl conditions areeliminated it may be used as a permanentflow monitor when suitably calibrated.

If the anemometer projected face area isgreater than 1/100 of the duct area themanufacturer’s blockage correction factorshould be applied.

Note should be taken of the manufacturer’scalibration data, particularly at low speeds.

7.5 VELOCITY MEASUREMENTThere are four types of velocitymeasurement instruments readily suitablefor use in the HEVAC market:

(i) Pitot static tube.(ii) Heated element anemometer.(iii)Vane anemometer(iv)Ultrasonic anemometer.

Figure 7.11 Capture hood

37

7.5.1 Pitot static tubesThe primary standard instrument formeasuring velocities above 2.5 m/s is thepitot static tube. The use of the instrumentand the calculations involved are laid out inBS 1042, section 2.1, IS0 3966

Inspect the tube carefully before use fordamage or blockage of the small holes in thehead.

Always ensure that the head is correctlyaligned in the airstream.

The differential pressure output signal fromthe pitot static tube may be connected toeither a suitable manometer for directreading or a transducer, etc., for processing.Both manometery and electronic pressurereading instruments are available with directread out in velocity.

If excessive pressure fluctuations exist andmake reading of the manometer difficultadditional damping may be obtained byusing a resistance, which is linearlyproportional to velocity in the pressure lines.Alternatively when using an electronicmanometer damping factors are sometimesavailable.

7.5.2 Heated element anemometerThe most suitable instrument for measuringvelocities below 2.5 m/s is the heatedelement instrument. The element is usuallyof the fine wire type or thermistor type. Bothrely for their operation on the measurementof rate of energy loss caused by the flow ofair past the element.

This type of instrument is very sensitive tolow velocities but natural convection effectscan become significant at flow velocitiesbelow 0.25m/s.

The instrument is also sensitive to changesin ambient temperature and pressure. Thetemperature variations can either be allowedfor internally or by use of a correction chart.Pressure variations usually have to becorrected from manufacturers data.

Heated element anemometers are alsodirection sensitive and care should be takento correctly align the device with thedirection of flow to be measured to ensurereproduction of the instruments calibratedperformance. This can be particularlyrelevant where trying to measure a specificvelocity vector with an omnidirectionalprobe where significant polar calibrationvariations often exist.

The elements are usually very fragile andsubject to drift in calibration. Frequentcalibration checks are advised for accurateresults.

Care should be taken to ensure that theelement is clean since trapped particles of

dust and dirt can affect the heat transfercharacteristic and give false readings.

Versions of thermal anemometers areavailable where a characteristic area may beinput and volume flow displayed.

7.5.3 Vane anemometersThe vane type anemometer (alreadyconsidered when discussing flow ratemeasurement) overlaps both the abovevelocity ranges.

The direct reading (electronic) instrumentmay be used to measure velocities both inlarge airways and at duct inlets/outlets. Caremust be taken when observing lowvelocities to allow the vane assembly toattain full speed before taking a reading.Similarly where the instrument is designedto time average, care must be taken to holdthe vane assembly in the airstream for thewhole of the time averaging period.

The mechanical instrument may be used tomeasure a velocity by holding it at therequired station for a suitable time interval.With instruments where the stop/start leveris close to the path of the airstream beingmeasured, disturbance may be caused byits operation. It is then advisable to note thetime between a predetermined number ofcounts whilst the instrument is left running.

The vane anemometer should be regardedas an obstruction in the air path. The usualcalibration assumes that the instrument willbe used in free space. If the instrument isused with a hood assembly or in a confinedspace such as a duct whose cross-sectionalarea is less than 100 times the projectedface area of the anemometer, thenknowledge of special calibration andpressure drop will be necessary for accurateresults.

Versions of vane anemometers are availablewhere a characteristic area may be inputand volume flow displayed.

7.5.4 The Pivoting Vane AnemometerThe principle of a pivoting vane anemometeris dependent on the velocity pressuredetected by the instrument probe forcing asmall flow of air along a large bore flexibletubing in order to impinge on a lightlypivoted vane within the instrument case. Thedeflection of the vane is related to the bleedflow rate and the velocity is indicated by apointer moving across a calibrated scale.The range of the instrument is varied byrestricting the bleed flow rate.

Several different shapes of probe are usuallyavailable with this instrument. Selection isdependent upon the site situation and isadequately described in the manufacturer’sliterature.

38

Care must be taken to hold the instrumentcase in the plane in which it has beencalibrated.

Care must be taken to ensure that theflexible tubing is not so bent or “kinked” asto restrict the bleed flow rate.

The instrument must be handled with caresince the vane assembly is very delicate andthe calibration may be impaired byaccidental impact.

7.5.5 Ultrasonic Anemometer

Ultrasonic anemometers are capable ofmaking velocity measurements over a widerange of velocities since they have aninherently linear response to velocity. Theydo not suffer from thermal device lowvelocity natural convection errors, vaneanemometer low velocity bearing friction ordifferential pressure device high velocitycompressibility effects.

Ultrasonic measurements, if configured tomake corresponding measurements in boththe upstream and downstream directions,are also insensitive to changes in the flowmedia such as; temperature, pressure,humidity or gas composition.

Due to the very fast sensing activity thesedevices are also able to measure turbulencevalues.

7.6 TURBULENCE INTENSITYMEASUREMENTPersonal comfort within an environmentdepends not only upon limited velocity of air,especially in the occupied zone, but also thelevel of turbulence intensity associated withthe air flow.

Turbulence intensity is defined as the ratio ofthe standard deviation of the air velocity to

the mean air velocity and is expressedas a percentage. Hence in areas ofdisturbed air, with no significant mean valuevelocity vector, turbulence intensity valuescan reach values of several hundredpercent. Personal comfort depends upon anacceptable combination of air temperature,mean velocity and turbulence intensity. SeeCEN CR 1752:1999

7.7 TEMPERATURE MEASUREMENTThere are three standard sensors fortemperature measurement:

(i) Mercury-in-glass thermometer.

(ii) Resistance thermometer.

(iii)Thermo-couple.

All readings taken may be classified aseither surface temperature or airtemperature.

When measuring air temperature careshould be taken that the temperature sensoris fully immersed in the airstream. When thesensor is used inside a protecting sleeve thetemperature difference due to heat lossdown the sleeve should be allowed for bycalibration.

When measuring surface temperatures careshould be taken that the sensor is inpressure contact with the surface to bemeasured and covered on the exposed sideto reduce heat losses. It is not advisable touse the mercury-in-glass thermometerwithout preparing a pocket or recess whichwill completely contain the mercury bulb.

When measuring metallic surfacetemperature the contact (open circuit)thermo-couple may be used. Care must betaken that the probe and the surface arethoroughly clean to ensure good electricalcontact.

It is always necessary to allow sufficient timefor the sensor to attain a steady value of thesurface or fluid (air) temperature beingmeasured before taking a reading.

Figure 7.12 Ultrasonic Anemometer

39

TYPE OPERATION MEASURES RANGES CALIBRATION SETTING ACCURACYOR RELAYS (TYPICAL) IN FIELD

Liquid filled manometer

Adjustable inclined orvertical liquid column isdisplaced by appliedpressure

Various types using drydiaphragms, spring andmagnet assembles todeflect a face pointer

Specially designedcoaxial probe

Rotating vanes relayimpulses to electronicsignal processor(remote head)

Rotating vanes relayimpulses to electronicsignal processor(remote head)

Remote jets relay airpressure to casemounted deflecting vanecalibrated for velocity

Hot-wire or thermistorprobe exhibitsresistance/temperaturecharacteristic which iselectronically calibratedfor air velocity/coolingeffect

Ultrasonic anemometerUltrasonic pressurewave times of flightused to compute flowvelocity

Pressure

Pressure

Static and totalvelocity pressure

Velocity

Velocity

Velocity

Velocity

Velocity

0-125Pa0-2500-5000-2500

0 – 100to

0-5000(Pa)

4-82 m/s

0.12 – 2.5to

0.12 – 25(m/s)

0.3 – 5to

0.3 – 20(m/s)

0 – 10 – 100 – 20(m/s)

0 - 0.5to

0 – 20(m/s)

0 – 50(m/s)

By manufacturer

By approved testagent (annually)

None




By Manufacturer

By Manufacturer

Level instrumentand set zero

Set zero

None

Set requiredrange

Set requiredrange

Set zero Setrange

Set zero Setrange

None

± 1.0% ofreading or ± 1 Pawhichever is the

greater

± 2% of full scaledeflection

± 2% of reading(with suitablemanometer)

± 2% of readingor ± 0.1 m/s

whichever is thegreater.

± 5% of readingor ±0.2 m/s

whichever is thegreater

± 10% full scaledeflection

± 3% of readingor ± 0.1 m/sec

whichever is thegreater

± 1% of reading

Diaphragm pressuregauge

Pitot static tube

Rotating vaneanemometer electronic& mechanical 75-125mm

Miniature vaneAnemometer electronic10-25mm

Deflecting vaneanemometer

Thermo-electricanemometers

Ultrasonic anemometer

Table 7.1. Instrumentation for velocity and performance measurement

Care must be taken, when making measurements approaching zero velocity, that the instrumentcalibration is valid.

40

Chapter Eight8 REGULATION OF AIR TERMINALDEVICES

8.1 APPLICATIONThis section gives guidance on theregulation of air flow rates through airterminal devices. The more comprehensivetask of commissioning is covered in theCIBSE Commissioning Code A for airdistribution systems, and is defined as “theadvancement of an installation from thestage of static completion to full workingorder within specified requirements”.

These procedures may be applied to thoseparts of supply and exhaust systems whichhandle air at low velocity.

The instruments and techniques consideredare those in common use at the time ofpublication and represent good orrecommended practice.

8.2 MEASUREMENT,INSTRUMENTATION AND EQUIPMENTRegulation of air flow rate passing throughan air terminal device entails themeasurement of a characteristic air velocityor pressure, which may be measured byvarious techniques.

The instruments detailed in Table 7.1 arethose most commonly used for fieldmeasurements and have in the past provedreasonably reliable, accurate, robust and portable.

It is important that all instruments areregularly maintained and recalibrated by thesupplier or an approved agent and thatsuitable test records are kept. If instrumentsare well maintained, they will provide thecommissioning staff with accurate testresults and thus ensure an acceptablesystem balance (see Section 7).

8.3 MEASURING TECHNIQUESAll the following flow rate measurementtechniques require the measurement of airvelocity or pressure at a characteristiclocation either in a main, branch or stubduct or at the air terminal device itself.

A velocity reading, or more usually theaverage of a set of velocity readings, maybe used to calculate air flow rate, thus:

TECHNIQUES

1. V

elocit

y tra

vers

e

2. P

oint

velo

city

read

ing

3. F

ace

velo

city

4. C

alibr

atio

n ho

od

5. C

hara

cter

istic

velo

city

calib

ratio

n

6. C

alibr

ated

term

inal

loss

coe

fficie

nt

MEASUREMENTSTATION

Table 8.1 Air flow measuring techniques

Main/branch ducts

Connecting stubducts to terminals

Supply Grilles

Exhaust grilles

Exhaust slots

Swirl diffusers

Linear slot supplydiffusers

Perforated facediffusers

✓

✓ ✓ ✓

✓ ✓ ✓

✓✓

✓ ✓✓

✓

✓ ✓

✓ ✓ ✓

✓✓ ✓ ✓

q = v x A

where q = air flow rate (m3/s)

v = average air velocity (m/s)A = area of air flow in measurement

Plane (m2)

The area “A” may be duct cross-sectionalarea, hood outlet area, nominal grille outletarea or effective terminal outlet area,depending on the technique used. It ispossible to measure the absolute air flowrate or a proportional or indicated air flowrate.

8.3.1 Absolute Air Flow RateAir velocities are measured at a plane ofknown cross-sectional or effective area andtheir product is equal to the actual air flowrate, which may be corrected, if necessary,for standard air density.

8.3.2 Proportional Air Flow RateWhere a number of similar terminals areserved by the same branch duct it isunnecessary to measure the absolute airflow rate; it is adequate to regulate air flowrate on one terminal as a proportion ofthe air flow rate at a reference terminal.In this way, each of the terminals onthe branch duct are in balance and itonly remains to regulate the branch flow ratewith respect to the main duct flow rate (Theproportional balancing method is detailedfurther in Section 8.6.) The “indicated airflow rate” is the product of the measuredvelocity (v) and the terminal area (A).

q = v x A

8.4 IN-DUCT VELOCITY TRAVERSEBy this method, point velocity readings aretaken at defined locations within an air ductusually across a fixed plane, as shown inTable 8.2. This method is a time consumingbut reasonably accurate technique w h i c his generally reserved for air flowmeasurements in main or branch ducts. It is

41

Figure 8.1 Pitot static tube traverse

Table 8.2 Velocity measurement positions

Measuring stations for square or rectangular ductwork havinga cross-sectional area less than 0.2m2

Measuring stations for square or rectangular ductworkhaving a cross-sectional area greater than 0.2m2

Measuring stations forcircular ductwork

however sometimes necessary with certainair terminal devices to measure the velocityat connecting or stub ducts where face orhood velocity readings are suspect.

8.4.1 Pitot Tube TraverseThe pitot tube is connected to an inclinedmanometer, or other suitable differentialpressure gauge, and the velocity pressure isrecorded at each of the designatedmeasuring stations shown in Table 8.2.

Average duct velocity which is derived fromvelocity pressure measurements is equal tothe arithmetric average of all the velocityreadings (it is not sufficient to average thevelocity pressure readings and convert theresult to a single velocity reading). For fielduse, it is generally adequate to use thevelocity scale supplied with the manometeras this considerably reduces themeasurement and calculation time.

The pitot tube/manometer measuringtechnique will produce field accuracy of±5% with a reasonably uniform duct velocityprofile. Accuracy of the method will dependon the velocity profile in the duct and

readings should not be taken unless atleast five equivalent diameters are availableboth upstream and downstream of themeasuring station.

As accuracy will be drastically affected bynon-uniform duct flow, it is thereforerecommended that a preliminary ducttraverse is made with the pitot tube, prior totaking any readings; if air flow is particularlyuneven, an alternative measurement stationor technique should be used. Air velocitiesless than 4 m/s should not be measured withthe common pitot tube and manometer.Further reference may be made to BS 1042,section 2.1, IS0 3966 “Methods for theMeasurement of Fluid Flow in Pipes”.

42

Measurements may be taken with a pointreading probe which presents a minimumobstruction to air flow rate, e.g., pitot statictube

Figure 8.2 Point reading

8.5 FACE VELOCITY READINGSFace velocity readings are used forproportional balancing of a series of ATD’s ofthe same type, size and blade setting andinvolves recording the air velocity at the faceof a supply or exhaust terminal.

The proportional rate may be calculated from

q = v x A

where A = effective core area (m2)

v = mean velocity reading (m/s)

With a large rotating vane anemometer, it issufficient to take one reading for each 150 x150mm face area up to a maximum of 16readings equally spaced. The anemometerhead should be set to face the supply orexhaust flow as required and may besituated directly on the face or as much as25mm from the face of the ATD

Figure 8.3 Face velocity measurement with avane anemometer

This method is only valid if all measuredvelocities are in the same direction.

A throttled damper behind a supply grillecauses a series of higher velocity jets which

For non standard air conditions, the formulabecomes

8.4.1.1 Corrections forStandard Air DensityAir Velocity may be calculated from thevelocity pressure readings according to thefollowing formula (for standard air density =1.2 kg/m3)

V = 1.29 Pv

V = 1.29 1013Po

T293

Pvx x

where

V = air velocity (m/s)Pv = velocity pressure (Pa)Po = absolute static pressure at point

of measurement (millibars)T = absolute temperature (K)

(OoC = 273K)

Alternatively, multiply the measurementvelocity by the correction factor from Table8.3 based on the atmospheric pressure andtemperature.

PRESSURE mb AIR TEMPERATURE oC

0 10 20 30 40 50 60800

820

840

860

880

900

920

940

960

980

1000

1013

1020

1040

1060

1080

1100

1.088

1.073

1.060

1.048

1.028

1.024

1.013

1.002

0.992

0.982

0.972

0.965

0.962

0.953

0.944

0.935

0.926

1.106

1.093

1.079

1.067

1.055

1.043

1.031

1.020

1.010

0.999

0.989

0.983

0.980

0.970

0.951

0.952

0.943

1.125

1.112

1.098

1.085

1.073

1.061

1.049

1.038

1.027

1.017

1.007

1.000

0.997

0.987

0.978

0.969

0.960

1.144

1.130

1.117

1.104

1.091

1.079

1.057

1.058

1.045

1.034

1.024

1.017

1.014

1.004

0.994

0.985

0.978

1.163

1.149

1.135

1.122

1.109

1.097

1.085

1.073

1.062

1.051

1.040

1.034

1.030

1.020

1.011

1.001

0.992

1.182

1.167

1.153

1.140

1.127

1.114

1.102

1.090

1.079

1.068

1.057

1.050

1.047

1.038

1.027

1.017

1.008

1.200

1.185

1.171

1.157

1.144

1.131

1.119

1.107

1.095

1.085

1.073

1.068

1.063

1.052

1.042

1.033

1.023

8.4.2 Point Velocity ReadingsWhere flow conditions in a connecting orstub duct to an ATD are particularly even itmay be adequate to record a singlecentreline point measurement. For theproportional balance method, no correctionis necessary; however, an approximation ofair flow may be obtained from:

q = v x A x 0.8

where

v = centre-line velocity reading (m/s)

A = duct area (m2)

Table 8.3 Measured velocity multiplicationcorrection factor

43

may result in erroneous readings and in thissituation, unless correction factors aregiven, it may be necessary to use abalancing hood. Alternatively a short length,approximately two characteristic diameters,of stub duct can be constructed, usingcardboard or similar material, outside of thegrille to facilitate measurement.

8.5.1 Balancing HoodThis method is most practically applied toair flow measurement with most ATD’s,particularly where duct entry and facevelocity readings are unreliable.

Wherever a balancing hood is used, themeasured air flow rate may be less than theactual air flow rate, due to the additionalpressure loss imposed by the hood itself.

The hood may be of two types:

(i) Calibrated Type: usually a fairlysophisticated venturi shape having aknown pressure loss co-efficient andprovision for fitting an anemometer orgrid and differential pressuremeasurement, or by-pass flow meter,device. The hood should be precalibratedsuch that its pressure loss characteristicis known and designed so that entry flowirregularities into the hood are reduced atthe point of measurement.

(ii) Uncalibrated Type: can be made on sitefrom available materials and is used toproduce a reasonably uniform outletvelocity profile, with a minimum pressureloss. This hood may produce incorrectresults if used with fixed vane diffuserswhich tend to divert the air to the sides ofthe hood.

Figure 8.4 Calibrated device

Figure 8.6 Measurement within ATD

Figure 8.5 Uncalibrated application

8.5.2 Characteristic TerminalArea FactorsAlthough not recommended, this techniqueentails measurement of velocities at orwithin the ATD which combined with aquoted area factor allows an assessmentof air flow rate to be made:

q = vK x AK

where vK = average outlet velocity (m/s)

AK = area factor (m2)

The type of instrument, sensing headlocation and area factor must be supplied bythe ATD manufacturer. Accuracy is generallyof a low order and this method should onlybe used where no other technique ispossible.

8.5.3 Calibrated Terminal PressureLoss FactorWith certain ATD’s, particularly slot, squareand circular diffusers, the terminal pressureloss may be used as a calibration of flowrate. With smooth (not turbulent) air flowprofiles, a duct wall tapping, pitot/static tubeor hypodermic probe reading in the neck ofthe ATD will provide a stable pressure lossreading of which the square root multipliedby the manufacturer’s loss co-efficient “K”equals air flow rate. This technique relies ona reasonable pressure reading combinedwith stable and uniform duct entryconditions to ensure a usable accuracy.

Using an anemometer to measure hoodoutlet velocity by point or multipointreadings, the flow rate is obtained from:

q = v x AH

where v = average outlet velocity (m/s)

AH = outlet hood area (m2) Figure 8.7 Uncalibrated application

44

Where a large number of one type and sizeof terminals are served by a single ductsystem, the loss co-efficient systemprovides a fast method of proportionalbalancing.

It is essential that the terminal loss co-efficient is constant, and this entails thesetting of air pattern controls to the designrequirements and the location of the volumecontrol dampers before the pressuremeasurement station.

8.6 REGULATION AND BALANCINGPROCEDURERegulation is defined as “the process ofadjusting the rates of air flow in a ductsystem within specified tolerances”.

Balancing the system consists of “settingthe correct proportional air flow rate at eachterminal or junction without regard forabsolute air flow rate measurements”.

A balanced system is then finally regulatedat the main duct damper so that the designsystem air flow rate is achieved.

8.6.1 Preliminaries(i) Ensure system cleanliness.

(ii) Check correct installation of ductturning vanes, dampers and air terminaldevices.

(iii) Check that all dampers are set fullyopen.

(iv) Set all adjustable ATD’s for the specifiedoperating arrangement.

(v) Check that test access is provided insuitable locations as necessary.

(vi) Check for obvious air leakage,particularly at such locations asbuilders’ ducts, access panels, etc.

(vii) Check that the duct system to beregulated has sufficient air flow foraccurate measurement and is isolatedfrom interaction with other systems,e.g., exhaust.

8.6.2 MethodUse consistently, any of the measurementmethods indicated in Sections 8.3 to 8.5 andadopt the following procedure:

(i) Within the branch duct system to beregulated, locate the terminal handlingthe lowest “percentage air flow rate”(i.e., the lowest ratio of the measured tothe design air flow rate). This is theindex terminal which will be used as areference to balance all terminals on thebranch duct (see Section 8.6.4.1.).

(It is necessary for the last terminal tobe, or to be equal to, the index terminal;if it is not, adjust it until it is handling thesame percentage air flow rate as theindex.)

(ii) Measure the indicated air flow rate atthe next terminal and calculate thepercentage air flow rate. If it is within thetolerances specified in Table 8.4 then nobalancing is necessary. If not, slightlyreset the damper and determine thenew percentage flow rate.

(iii) Return to the index terminal anddetermine its new percentage air flowrate.

(iv) The two terminals will be in balancewhen the compared percentage air flowrates are within the tolerance indicatedin Section 8.6.3.

(v) Repeat the procedure set out in (ii), (iii)and at the subsequent terminals.

(vi) Adjustments to terminal dampersremote from the index have little effecton the index flow rate. In practice,therefore, it is seldom necessary toreturn at each stage to re-measure theindex terminal flow rate.

8.6.3 System Regulation andBalancing TolerancesTable 8.4 provides a guideline for realisticaccumulated tolerances which can beapplied to low velocity supply and exhaustair systems.

SYSTEM

Systems where allterminals on anysub-branch serveone area

+ 20%

- 0%

+ 10%

- 0%

+ 10%

- 0%

Systems where theterminals on any onesub-branch serve morethan one area

+ 15%

- 0%

+ 10%

- 0%

+ 10%

- 0%

TERMINAL BALANCE(proportion of percentage

flow at index terminal)

BRANCH BALANCE(proportion of percentageflow at index sub-branch)

MAIN DUCT BALANCEDesign flow

Table 8.4 Typical balancing tolerances

45

8.6.4 Typical Examples

8.6.4.1 Calculation of PercentageAir Flow Rate

Index Terminal– design flow rate = 720 l/s– indicated flow rate = 680 l/s– therefore percentage flow rate (680/720)= 94%

Terminal being balancedFor a system where all terminals on a sub-branch serve one area, balancing tolerancefrom Table 8.4 is:

Upper limit = + 20% (94% x 120% = 113%)Lower limit = - 0% (94% x 100% = 94%)

Design flow rate of terminal to be balanced= 550 l/s

Indicated flow rate should be regulated towithin:

Upper limit = 550 x 1.13 = 622 l/sLower limit = 550 x 0.94 = 517 l/s

8.6.4.2 Balancing ProcedureComplete preliminaries outlined in Section8.6.1. Adopt procedure as shown in 8.6.2.

Figure 8.8 Typical branch/sub branch example

Table 8.5 TYPICAL BRANCH/SUB BRANCH MEASUREMENT & BALANCING PROCEDURETerminals being in balance, the sub-branch duct flow rate can be regulated with respect to the branch duct.

TERMINAL DESIGNDUTY (l/s)

MEASURED(INDICATED)DUTY (l/s)

% DESIGN REGULATIONTOLERANCE

UPPERLIMIT

LOWERLIMIT

COMMENT

Index terminalClose damper

2 slightlyRemeasure 3& 2 flow rate

Close damper1 slightly

remeasure 3& 1 flow rate

Withintolerance

Index terminal

Index terminal

Withintolerance

3

3

2

1

1

3

2

200

200

300

200

200

200

300

210

230

390

260

240

220

345

105%

115%

130%

130%

120%

110%

115%

+ 15%- 0%

+ 15%- 0%

+ 15%- 0%

+15%- 0%

121%

126%

132%

126%

105%

110%

115%

110%

8.7 DESIGNING FOR COMMISSIONING

8.7.1 DocumentationProvide the commissioning engineer withcomplete, clear installation drawings and ifpossible, schematic line drawings. Providedetails of flow rates through terminal, eachbranch and the main duct; terminal pattern,etc., duct and terminal sizes; damperlocations and measurement stations. Alldocumentation should have clearidentification coding for zones and terminallocations. This is particularly important in thecase of large projects.

Results should be presented in a clear andconcise manner. Standard forms such asthose presented in the BSRIA “Manual forRegulating Air Conditioning Installations”(1/77) are recommended.

8.7.2 Design ConsiderationsGood duct design and installation savesenergy and minimises noise generation;remember design faults may be difficult orimpossible to rectify at the commissioningstage.

Well designed stub ducts (see Section 5) willminimise the need for excessive terminaldamper throttling; faulty design in thiscontext can modify the ATD performancedrastically with respect to noise generation,pressure loss and throw.

Rationalisation of the ATD range selected forinstallation throughout the project willprovide the opportunity for the application ofsimple effective balancing techniques.

Provide adequate flow rate control dampersand test access or alternative flowmeasuring stations.

Ensure that good access to dampers, testholes, inspection panels and to othercontrols is included in the initial design.

46

Bibliography on Air Distribution Technology

BS EN ISO 5135: 1999Acoustics - Determination of sound power levelsof noise from air-terminal devices,air-terminal units, dampers and valves bymeasurement in a reverberation room

BS ISO 10294: Part 1: 1996Fire resistance tests. Fire dampers for airdistribution systems. Test method

BS ISO 10294: Part 2: 1999Fire resistance tests - Fire dampers for airdistribution systems - Part 2: Classification,criteria and field of application of test results

BS ISO 10294: Part 3Fire resistance tests. Fire dampers for airdistribution systems. Classification

BS EN 1366: Part 2Fire resistance tests for service installations.Fire dampers

BS EN 1751: 1999Ventilation for buildings - Air terminal devices -Aerodynamic testing of dampers and valves

ISO 3258: 1976Air distribution and air diffusion - vocabulary.

ISO 5219: 1984Air distribution and air diffusion - Laboratoryaerodynamic testing and rating of air terminaldevices.

ISO 5221: 1984Air distribution and air diffusion - rules to methodsof measuring air flow rate in an air handling duct.

ISO 7730: 1984Moderate Thermal Environments, determinationof the PMV and PPD indices and specification ofthe conditions for thermal comfort

BS 4718: 1971Methods of testing for silencers for air distributionsystems.

BS 4733: Part 1: 1977Methods of testing and rating air terminal devicesfor air distribution systems.

BS 4857: Part 1: 1972Methods for testing and rating terminal reheatunits for air distribution systems Part 1 Thermaland aerodynamic performance

BS 4954: Part 1: 1973Methods for testing and rating induction unitsfor air distribution systems. Part 1. Thermal andaerodynamic performance.

BS 4979: 1986Aerodynamic testing of constant and variable dualor single duct boxes, single duct units andinduction boxes for air distribution systems.

BS 4979: Part 1: 1973Methods for testing and rating air control devicesfor air distribution systems.

CR 1752: 1998Design criteria for the internal environment (CENreport)

prEN 12238Air terminal devices – aerodynamic testing andrating for mixed flow applications

prEN 12239Air terminal devices – aerodynamic testing andrating for displacement flow applications

prEN 12589Air terminal devices – aerodynamic testing ofconstant and variable rate terminal units

prEN 13182Instrumentation for ventilated spaces

prEN 13264Terminals – floor mounted air terminal devices –tests for structural classification

ISO DIS 5220: 1996Methods for aerodynamic testing of constant andvariable dual or single duct boxes, single ductunits and induction boxes for air distributionsystems

CIBSEGuide A, Section 1; Environmental criteria fordesign, 1999.

HVCADW144 Specification for sheet metal ductwork,1998

ASHRAEHandbook Fundamentals - SI Edition 1997;Chapter 7 - Sound and Vibration and Chapter 31- Space Air Diffusion

ASHRAEHandbook HVAC Applications - SI Edition 1999;Chapter 46 - Sound and Vibration Control

Holmes, M. J. and Sachariewicz, E.Effect of ceiling beams and light fittings onventilating jets. BSRIA Laboratory Report No. LR79, 1973

Jackman, P. J. Air distribution in naturally ventilated offices.BSRIA February 1999, Technical Note TN 4/99

Jackman, P. J.Air movement in rooms with ceiling mounteddiffusers. BSRIA, Laboratory Report LR 81, 1973

Jackman, P. J.Design recommendations for room air distributionsystems BSRIA 1990, Technical Note TN 3/90

Lloyd, S.Fire dampers BSRIA 1994, Technical Note 6/94

Mathisen, H. M. and Skaret, E.Efficient ventilation of small rooms (Proc. 16th Int.Congr. Refrig., Paris 1983) 1984, vol.5, 199-205

Nielsen, P. V.Air diffusion in rooms with ceiling-mountedobstacles and two-dimensional isothermal flow(Proc. 16th Int. Congr. Refrig., Paris 1983) 1984,vol.5, 161-168

Rajaratnam, N.Turbulent jets; Elsevier Scientific PublishingCompany, 1976.

Salemi, R. and Alamdari, F.Flow characteristics of grilles and diffusers.BSRIA March 1995, Research Report RR 13/95

Shepherd, KVAV Air Conditioning Systems, BlackwellScience Ltd

Skistad, HDisplacement Ventilation, Research StudiesPress Ltd

Whittle, G. E.Room air distribution - design and evaluation.BSRIA Technical Note TN 4/86, May 1986

47

This publication has been sponsored by the following companies:

ActionairJoseph Wilson Industrial Estate,South Street, Whitstable, Kent CT5 3DUTel: +44 (0) 1227 276100Fax: +44 (0) 1227 [email protected] of fire and smoke dampers (LPCBcertificated), air control dampers, smoke dampercontrol systems, waterside and airside controlledFan Coil Units.Registered to BS EN ISO 9002

Advanced Air (UK) Ltd3 Cavendish Road, Bury St. Edmunds,Suffolk IP33 3TETel: +44 (0) 1284 701356Fax: +44 (0) 1284 [email protected] of fire, smoke and controldampers, VAV terminal units, duct access doorsand air distribution products, all tested to therelevant UK, European and Internationalstandards. Fire and smoke dampers are includedin the LPC Approved Products List.Registered to BS EN ISO 9002

Air Diffusion LtdBirchwood Trading Estate,164 Great North Road, Hatfield, Herts AL9 5JNTel: +44 (0) 1707 272601Fax: +44 (0) 1707 [email protected] and manufacturers of a full range ofaluminium air terminal devices, metal and glasslouvres and the VCD range of fire, smoke andcontrol dampers.Registered to BS EN ISO 9002

Airflow Developments LtdLancaster Road, Cressex Business Park,High Wycombe, Bucks HP12 3QPTel: +44 (0) 1494 525252Fax: +44 (0) 1494 [email protected] of industrial fans and blowers,domestic ventilators and air flow measuringinstruments. Also distributors of extensive rangesof ventilation supplies.Registered to BS EN ISO 9001

BSB Engineering Services LtdUnit E, Tribune Drive, Trinity Trading Estate,Sittingbourne, Kent ME10 2PDTel: +44 (0) 1795 422609Fax: +44 (0) 1795 [email protected] of air control and fire/smokecontrol products including control panels andaccessories.Registering to BS EN ISO 9002 (in process at timeof print)

Gilberts (Blackpool) LtdClifton Road, Marton, Blackpool, Lancs FY4 4QTTel: +44 (0) 1253 766911Fax: +44 (0) 1253 767941sales@gilbertsblackpool.comwww.gilbertsblackpool.comManufacturers of a comprehensive range ofgrilles, diffusers, VAV and constant volume units,volume control, fire and smoke dampers, louvres.Registered to BS EN ISO 9001

Halton Products Ltd5 Waterside Business Park, Witham,Essex CM8 3YQTel: +44 (0) 1376 507000Fax: +44 (0) 1376 [email protected] of air distribution equipment, VAVsystems, marine dampers, kitchen canopies,displacement ventilation, ventilated cooledbeams, volume control dampers, grilles anddiffusersRegistered to BS EN ISO 9001

Roof Units LtdBlackbrook Road, Narrow Boat Way, Dudley,West Midlands DY2 0NBTel: +44 (0) 1384 418600Fax: +44 (0) 1384 [email protected] ventilation products including AirHandling Units, fans, grilles, louvres and fire,smoke and balancing dampers.Registered to BS EN ISO 9002

Royair LtdHeathpark Industrial Estate, Honiton,Devon EX14 1SPTel: +44 (0) 1404 41651Fax: +44 (0) 1404 [email protected] range of grilles, diffusers andlouvres manufactured and finished in-house. Allwelded and powder paint construction.Supported by own test facilities, selectionsoftware and technical team.

Senior Air SystemsOldfields Industrial Estate, Birrell Street, Fenton,Stoke on Trent, Staffordshire ST4 3ESTel: +44 (0) 1782 599995Fax: +44 (0) 1782 599220sales@seniorairsystems.co.ukwww.seniorairsystems.co.ukManufacturers of air distribution productsincluding Grilles; Diffusers; VAV; Mechanical,Acoustic and Architectural Louvres; Air HandlingUnits; Fan Coil Units; Chillers and Condensers.Registered to BS EN ISO 9001

Trox (UK) LtdCaxton Way, Thetford, Norfolk IP24 3SQTel: +44 (0) 1842 754545Fax: +44 (0) 1842 [email protected] of air distribution, air filtration andnoise control products and constant and variablevolume equipment, louvres, commercial dampers,fire and smoke dampers, industrial dampers,chilled ceilings and chilled beams, silencers, bag,panel HEPA and ULPA filter systems.Registered to BS EN ISO 9002

Waterloo Air Management plcMills Road, Aylesford, Kent ME20 7NBTel: +44 (0) 1622 717861Fax: +44 (0) 1622 [email protected] Distribution Products including grilles,diffusers, louvres, VAV and Aircell engineeringpolymer products, fire, smoke, and volumecontrol dampers: Air Handling Units, Fan CoilUnits and Chilled Ceiling Products; FanConvectors, Trench and Perimeter Heating;Acoustics and Air Filters.Registered to BS EN ISO 9001

This Guide to Air Distribution Technology for the Internal Environment was produced by members of theHEVAC Air Distribution Group. It supercedes the HEVAC Air Diffusion Guide which has largely beenincorporated into this new guide.

The Air Diffusion Guide was first produced in 1982, subsequently revised in 1988 and has remainedunchanged until this new publication.

Contributions to this publication are gratefully acknowledged from the following people:

Paul White Actionair

John Rochester Advanced Air (UK) Ltd

Peter Downing Airflow Developments Ltd

Tariq Abbas Building Services Research and Information Association

Paul Compton Colt International Ltd

John Mawdsley Gilberts (Blackpool) Ltd

Alan Green Trox (UK) Ltd

David White Waterloo Air Management LtdJon Fermor

Mike Duggan HEVAC Technical Manager

Published by the Heating Ventilating and Air Conditioning Manufacturers Association (HEVAC).HEVAC is the Building Services division of the Federation of Environmental Trade Associations (FETA).

© Federation of Environmental Trade Associations 2000All rights reserved. Apart from any fair dealing for the purposes of private study or research allowedunder applicable copyright legislation, no part of the publication may be reproduced, stored in aretrieval system, or transmitted in any form by any means, electronic, mechanical, photocopying,recording or otherwise, without the prior permission of the Federation of Environmental TradeAssociations, Henley Road, Medmenham, Marlow, Bucks SL7 2ER.

FETA uses its best efforts to promulgate Standards and Guidelines for the benefit of the public in thelight of available information and accepted industry practices but do not intend such Standards andGuidelines to represent the only methods or procedures appropriate for the situation discussed. FETA,and the individual contributors, do not guarantee, certify or assure the safety or performance of anyproducts, components, or systems tested, installed or operated in accordance with FETA’s Standardsor Guidelines or that any tests conducted under its Standards or Guidelines will be nonhazardous orfree from risk.

FETA, and the individual contributors, disclaim all liability to any person for anything or for the conse-quences of anything done or omitted to be done wholly or partly in reliance upon the whole or any partof the contents of this booklet.

Supported by

Association of Ductwork Contractorsand Allied Services (ADCAS)

Building ResearchEstablishment (BRE)

Building Services Research andInformation Association (BSRIA)

Heating and Ventilating Contractors’Association (HVCA)

Henley Road, Medmenham, Marlow, Bucks SL7 2ER.Tel +44 (0) 1491 578674 Fax: +44 (0) 1491 575024

Email [email protected] Internet www.feta.co.uk

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