handbook displacement ventilation design guide booklet by price

8/2/2019 Handbook Displacement Ventilation Design Guide Booklet by PRICE

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Displacement VentilationDESIGN GUIDE

w w w . p r i c e - h v a c . c o m

Displacement VentilationSECTION J


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Copyright E.H. Price Limited 2007. All Metric dimensions ( ) are sot conersion.Imperial dimensions are conerted to metric and rounded to the nearest millimeter. J-3

Loadig Withi the Space........................................... J24 J25

Loads ...................................................................................... J24

Sensible and Latent Loads ..................................................... J25

Diuser Types................................................................ J26 J27

Diuser Layout ad Locatio ............................................... J28

DV Supply Air Methods ......................................................... J29

Compoet Selectio ad Istallatio ............................. J29

Air Volume Calculatios ............................................. J30 J31

Desig Procedure..................................................................... J31

Desig Examples ............................................................ J32 - J37

Small Oce Example ................................................. J32 J34

Boardroom Example ....................................................J35 J37

Special Applicatios Supplemet ............................. J38 - J52

Displacemet Vetilatio or Idustrial Applicatios....J39

Machine Shop Example ...............................................J40 J43

Displacement ventilation and Schools ....................... J44 J45

Classroom Example .....................................................J46 J48

Displacement ventilation and Healthcare................... J49 J51

Reereces ................................................................................ J52

Displacemet Vetilatio Itroductio .................... J8 - J10

Typical Applications ............... ............... ............... ................ ... J9

Terminology ........................................................................... J10

Displacemet Vetilatio Characteristics ............ J11 - J14

Thermal Plume .........................................................................J11

Stratication Height .................................................................J11

Room Airfow Pattern .............................................................. J12

Diuser Air Flow Pattern ......................................................... J13

Contaminant Distribution ....................................................... J13

Temperature Distribution ....................................................... J14

Location o Returns ................................................................. J14

Thermal Comort........................................................... J15 J16

Vetilatio Eectiveess....................................................... J17

Heatig with Displacemet Vetilatio ............................. J18

Humidity Cotrol........................................................... J19 J20

Design Suggestions ............................................................... J19

Direct Expansion Rootop Units ............................................. J19

Dehumidication and Heat Recoery .............. ........... J19 J20

Acoustics........................................................................ J21 J22

Desigig with AHUs ad RTUs .......................................... J23

Contents

Displacemet Vetilatio

Price Industries works hard to promote the use o sustainable building materialsand innoatie air distribution technologies to improe the air quality andenironment integrity in the built enironment. Price has a long history o designingand promoting products systems that are energy ecient and ideal or use inGreen Building designs. Price is committed to the continual introduction onew products and systems that urther the goals outlined by the USGBC.

Price has partnered with Krantz Products USA Inc., North American Distributor oKrantz, the world leader in displacement entilation diusers, to oer a completesystem or displacement entilation distribution. The Krantz Komponenten

industrial diusers hae been the preerred specication or industrial applicationssince Krantz Komponenten rst pioneered the North American market. KrantzKomponenten diusers are oered to the U.S. and Canadian HvAC marketsexclusiely through Price.


4/52J-4All Metric dimensions ( ) are sot conersion. Copyright E.H. Price Limited 2007.Imperial dimensions are conerted to metric and rounded to the nearest millimeter.

This document is intended to proide answers to common questions as well as proide some guidance or working though the mostcommon issues when designing an Displacement ventilation system. Throughout the document you will nd helpul hints as well asGreen Tips, Control Tips and Product Tips, an example o which is shown below.

Gree TipGreen Tips proide useul insight into some opportunitiesor making design decisions which might help in designing asustainable building. Some pointers are proided or both theLEED and the Green Globes rating systems.

Cotrol TipControl Tips are proided to maximize the understanding oall o the control opportunities and issues with Displacementventilation systems. In some cases these will help reducecontrol complexity or optimize control eectieness.

Product TipProduct Tips proide a link between the design guide sectionand the product section to assist the design engineer in selectingproduct with the recommended characteristics.

Displacement ventilationDesig Guide

About this Desig Guide


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Displacemet Vetilatio Itroductio

Figure 1: Mixig (Dilutio) VetilatioOverviewAirfow in entilated spaces generally canbe classied by two dierent types; mixing(or dilution) entilation and displacemententilation. Mixing entilation systems(Figure 1) generally supply air in a mannersuch that the entire room olume is ullymixed.The cool supply air exits the outlet ata high elocity, inducing room air to proidemixing and temperature equalization. Sincethe entire room is ully mixed, temperatureariations throughout the space are smallwhile the contaminant concentration isuniorm throughout the zone.

Displacement ventilation systems (Figure 2)

introduce air into the space at low elocitieswhich causes minimal induction and mixing.Displacement outlets may be located almostanywhere within the room, but hae beentraditionally located at or near foor leel.The system utilizes buoyancy orces,generated by heat sources such as people,lighting, computers, electrical equipment,etc. in a room to remoe contaminantsand heat rom the occupied zone. By sodoing, the air quality in the occupied zoneis generally superior to that achieed withmixing entilation.

CoceptDisplacement entilation presents an

opportunity to improe both the thermalcomort and indoor air quality (IAQ) o theoccupied space. Displacement entilationtakes adantage o the dierence in airtemperature and density between an uppercontaminated zone and a lower cleanzone. Cool air is supplied at low elocityinto the lower zone. Conection rom heatsources creates ertical air motion into theupper zone where high leel return outletsextract the air as illustrated in Figure 3. Inmost cases, these conection heat sourcesare also the contamination sources, i.e.people or equipment, thereby carrying thecontaminants up to the upper zone, awayrom the occupants.

Since the conditioned air is supplieddirectly into the occupied space, supply airtemperatures must be higher than mixingsystems (usually aboe 63 degrees F)to aoid creating uncomortable drats.By introducing air at eleated supply airtemperatures and low outlet elocity a highleel o thermal comort can be achieedwith displacement entilation.

Figure 2: Displacemet Vetilatio

Displacement ventilation

Desig Guide

Figure 3: Displacemet Flow Characteristics




Displacemet Vetilatio Itroductio

Beets

1. Flexibility as loads change within the space, a displacement system will be able to compensate. For example, i the space wasdesigned to hae a airly een load distribution and now has the loads concentrated to one side, the system is able to compensateas the buoyant orces drie supply system and will draw the air towards the loads.

2. IAQ Because resh supply air is pooling at the foor leel, personal thermal plumes draw resh air up the body. All o the warmand polluted air is extracted at the high return. When properly designed, there should always be a greater amount o resh air in thebreathing zone when compared to a conentional dilution system.

3. Both the LEED and Green Globes green building rating systems hae credits that are applicable to displacement entilation systems.See the Green Tips or urther inormation.

4. Energy Saings

Thelowerpressuredropassociatedwithdisplacementventilationoutlets,mayallowareductioninfanenergywiththeselectiono a smaller an components.

Economizeroperatinghourscanbeincreasedtotakeadvantageoffreecoolingbecausesupplyairtemperaturesarehigherthan

with oerhead air distribution systems. Chillerefciencymaybeincreasedwhenthesystemisnotdehumidifying,asthereisalowersupplyairtemperatureandhigher

return air temperature.

Typical Applicatios

Displacement ventilation isaneffectivemethodof obtaininggood air quality and thermal comort in the occupied space.Spaces where displacement entilation has been successullyused are:

- Schools - Theaters

- Classrooms - Casinos

- Hospitals - Restaurants

- Dining Rooms - Meeting Rooms

- Conerence Rooms - Supermarkets

- Industrial Spaces

Displacement ventilation is usually a good choice in theollowing cases:

- Where the contaminants are warmer and/or lighter than theroom air.

- Where the supply air is cooler than the room air.

- Where the room heights are 9 eet or more.

- Where low noise leels are desired.

OverheadAirDistributionmaybeabetterchoicethandisplacement entilation in the ollowing cases:

- Where ceiling heights are below 8 eet.

- Where disturbances to room airfow are strong.

- Where contaminants are colder and/or denser than theambient air.

- Where cooling loads are high and radiant cooling is not anoption.

Figure 4: Classroom Application

Figure 5: High Ceiling Application


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Termiology

Adjacet Zoe

The adjacent zone is dened as the distancerom the diuser ace to a point where theelocity o the airstream is reduced belowto 40 FPM measured 1 aboe the foor.

Buoyacy

The ertical orce exerted on a olumeo air that has a density lower than theambient air.

Breathig Zoe

The estimated height at which occupantswill inhale the surrounding air.

CFD

Computational Fluid Dynamics.The analysiso a space utilizing computers to simulatefuid motion. An example o output rom aCFD analysis is shown in Figure 8.

Displacemet Vetilatio

Room entilation created by room airdisplacement, by introducing air at lowleel in a space at a lower air temperaturethan the room air.

Drat

Unwanted local cooling o a body causedby moement o air.

Drat Temperature

The eectie temperature based on thetemperature and elocity o the supply aircausing discomort.

Gree Globes

A sustainable building rating system romthe Green Building Initiatie (GBI).

IAQ

Indoor Air Quality.

LEED

Leadership in Energy and EnironmentalDesign. A sustainable building ratingprogram rom the US Green BuildingCouncil.

Legth, Adjacet Zoe

The Length o the adjacent zone is the lengthrom the diuser ace to a specied elocity,typically 40 FPM, reer to Figure 6.

Mixed Vetilatio

Air diusion where the mixing o supplyand room air is intended.

Occupied Zoe

An imaginary box in the room dened as6 eet aboe the foor and not less than 24inches rom the walls.

L

W

Figure 7

Figure 8

Percet Dissatised (PD)

ASHRAE denes the percent dissatisedas the percentage o people predicted tobe dissatised with their enironment dueto drat.

Predicted Mea Vote (PMV)

The Predicted Mean vote, PMv, is an indexthat predicts the mean alue o the oteso a large group o persons in relationto a scale dened by ASHRAE [ASHRAEStandard 55 2004]

Predicted Percetage o Dissatised(PPD)

ASHRAE denes the predicted percentageo dissatised as an index that establishesa quantitatie prediction o the percentageo thermally dissatised people determinedrom PMv. In real terms it is a measure o thethermal comort perormance in a space.

Thermal Plume

The air current rising rom a hot body.

Straticatio

When the temperature o the space arieswith height.

Upper Zoe

The space aboe the occupied zone.

Vetilatio Eectiveess

The ratio o contaminants in the exhaust tothe contaminates at the breathing leel. Anindication o how well a space is extractingcontaminates, and an indication o IAQ.

Width, Adjacet Zoe

The width o the adjacent zone is the widthrom the diuser ace to a specied elocity,typically 40 FPM, reer to Figure 7.

L

Figure 6




Displacemet Vetilatio Characteristics

Thermal Plume

As heat sources transer heat to thesurrounding air, the air becomes morebuoyant. This causes air to rise in thespace and to be replaced by air rom theside or below, otherwise known as naturalconection. As a thermal plume rises aboethe heat source, it entrains surroundingair and increases in size and olume asit loses momentum, moing away romthe heat source, as depicted in Figure 9.The maximum height to which a plumewill rise is dependent on the heat sourcestrength, as the initial momentum o theplume will increase. Also, a room with more

stratication will reduce the relatie densityo the plu me and, as a result, the height towhich the plume will rise.

The thermal plume generated rom a pointsource acts dierently than a thermalplume generated rom large objects in thespace. For example, a cylinder produces aboundary layer and the conectie thermalplume takes a dierent shape. A point sourcetype expansion o the thermal plume is stillpresent, but at an altered height, and withthe thermal plume boundary layer included,shown in Figure 10. The cylinder is a betterapproximation o an occupant in the spacethan a point source.

Straticatio HeightInFigure 11, q0 represents the supply airfowinto the room rom a low side-wall diuser,q1 is the upward moing airfow containedin thermal plumes that orm aboe heatsources, and q2 is the downward moingairfow resulting rom cool suraces. Interms o this simplied conguration, thestratication height will occur at a height(Yst) where the net upward moing fow, q1-q2, equals q0. Clearly, an important objectiein designing and operating a displacemententilation system is to maintain thestratication height near the top o theoccupied zone (1.8m [6 t]). I the buildingoccupants are in a seated work position, a

lower stratication height (e.g. 1.2m [4t])may be acceptable.

Figure 9: Thermal Plume, [Source: ASHRAE Uderfoor Desig Guide]

Gree TipLower stratication heights will resultrom reduced airfow. This saes energyrom treating outdoor air as well asprimary an energy.

Figure 10: Thermal Plume o Cylider [Source: Skistad]

Figure 11 - Straticatio Height

Yst

q1

q2

q0


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Figure 12: Air LayersRoom Airfow PatterAirfow patterns in a displacement entilationsystem are quite dierent than in a mixingsystem. Because o the low dischargeelocity o displacement outlets, the roomair motion is infuenced to a large degreeby conection fows. The conection fowsare created by heat sources such as people,equipment or warm windows, or by heatsinks such as a cold wall or window. Theconection fows within the room causethe ormation o horizontal air layers. Thewarmest air layers are near the ceiling andthe coolest air layers are near the foor asdepicted in Figure 12.

Room air moes horizontally across the foordue to momentum rom the supply outletand suction rom thermal plumes.

vertical air moement (Figure 13) betweenlayers is caused by stronger conectionorces associated with heat sources orcold sinks. Heat sources such as people,computers, lights, etc. create a risingconection fow known as a thermal plume.The strength o the thermal plume isdependent on the power and geometry othe heat source. Depending on the strengtho the thermal plume, the conection fowscan rise to the ceiling or distribute at a lowerheight. Cold sinks such as an exterior wall

or window can generate conection fowsdown the wall and across the foor.

Airfow Penetration

A displacement system supplying cool airthrough a diuser will delier air along thefoor in a thin layer typically less than 8in height.

The supply air spreads across the foor in asimilar manner to water fowing out o a tap,lling the entire space. I obstructions suchas urniture or partitions are encountered,the air will fow around and beyond theobstruction illustrated in Figure 14. Eenrooms with irregular geometries can beuniormly supplied with conditioned air

(Figure 15).When the cool air meets a heat sourcesuch as a person or piece o equipment, aportion o the conditioned air is capturedby the thermal plume o the heat source,while the remainder o air continues urtherinto the room.

When designing the system to deal with thecooling demand o the space, the penetrationdepth o a displacement diuser can be 26 30 eet rom the ace o the diuser. Forrooms exceeding 30 eet in length or width,diusers on seeral walls would normallybe required.

Figure 14: Obstructio

Diffuser

Partition

Couch

Supply Air

Figure 15: Irregular Room Geometry

Supply Air Diffuser

Gree TipBecause displacement entilation systems are graity drien, caution must be usedin sloped applications. A theater with een seating, will require less diusers inthe lower sections o the theater and more in the upper to compensate the naturalmoement o the air to the lower portion o the theater.

Figure 13: Vertical Air Movemet





Diuser Airfow Patter

In order to aoid drat it is essential or thedisplacement diuser to uniormly delierthe supply air across the entire diuser aceat low elocity. This requires an internalequalization bafe in combination with alow ree area ace.

A displacement diuser supplying coolair will result in an air pattern resemblingFigure 17. Due to the density o the cool

with little or no penetration into the space.(Figure 19) For most applications supplyingheated air through displacement outlets isnot recommended.

Figure 17: Cool Air Supply Figure 18: Isothermal Air Supply Figure 19: Heatig Air Supply

Cotamiat Distributio

Contaminant distribution in a space isinfuenced by seeral actors such as supply

air method, contaminant source type,location within the space, heat sources, andspace height.

Displacement entilation improesoccupant air quality by reducing thecontaminants in the lower portion othe room. The general upward motion oair causes contaminants to concentratewithin the upper zone (Figure 20).With mixing entilation, contaminants arediluted with supply air and are distributedeenly throughout the space. The gurerepresents contamination distributionin a room supplied with mixing anddisplacement entilation or a typical casewhere the contaminant source is warm (a

person, or example).For displacement entilation case, becausethe upward conection around a personbrings clean air rom lower leel to thebreathing zone, the air in the breathingzone is cleaner than the room air at thesame height.

Contaminants that are heaier than air needto be extracted at a lower leel i they presenta saety concern.

Figure 20: Cotamiat Distributio

Uiorm distributio o cotamiats

withi the mixed space.

Cotamiats are cocetrated at

the upper portio o the space.

Gree TipThe potential IAQ increase and reduction in airborne illness transmission aresubstantial, but are typically not addressed in dollar gures. Recent publicationsand studies hae shown the increase in IAQ to increase perormance in schools (Re.ASHRAE Journal volume 48 Number 10 Oct. 2006). A publication rom Capital E showsthe estimated costs with improed IAQ or schools, but could be applied to otherareas as well. See the ventilation Eectieness section or urther discussion.

supply air it alls towards the foor a shortdistance rom the diuser ace and continuesalong the foor at a depth o approximatelyeight inches.

When supply air is isothermal, whenthe supply air temperature equals roomtemperature, the fow will be distributedhorizontally into the space per Figure 18.

For a displacement diuser supplying heatedair, supply air will rise towards the ceiling


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Figure 21: Vertical Temperature GradietTemperature DistributioSince cool air is introduced at low leelwith a displacement entilation system, atemperature gradient exists between thefoor and ceiling leel o the space. Thisertical temperature gradient is knownas stratication. Figure 21 illustrates atypical temperature prole or a room withdisplacement entilation.

The temperature prole, or stratication, isaected by seeral actors; most notably thesupply air olume, room cooling load, locationand type o heat source and height o thespace.The greater the olume o air suppliedinto a room, the lower the temperature dier-

ence between foor and ceiling. I heat sourcesare located in the lower part o the room, thetemperature gradient is greater in the lowerpart o the room and lessens in the upper part.Conersely, when heat sources are located inthe upper part o the room the greateststratication occurs near the ceiling (Figure 22).

Controlling stratication in the occupiedzone is critical to maintaining occupantcomort. ASHRAE Standard 55 requires thetemperature dierence between the headand oot leel o a standing person not toexceed 5F.

The ASHRAE Design Guide has determineda method to calculate the head to oot

temperature stratication o a displacementsystem based on supply air olume andload distribution. This relationship wasused to deelop a design procedure ordisplacement entilation systems. Anexplanation o how the calculation methodwas achieed is presented on page J30. Thedesign procedure is presented on page J31.Using this design procedure an acceptableroom temperature stratication leel canbe achieed.

For commercial displacement entilationsystems, supply air temperatures rangingrom 65-68F can be expected. As well,the temperature dierence between returnand supply in a stratiied system willgenerally be greater than 13F.

Locatio o Returs

Returns should be located as high aspossible in the space to remoe as mucho the stratied zone as possible, ideally atceiling height. I the return is located belowthe ceiling, the air aboe the return may not

Figure 22: Heat Source Locatio

Heat SourcesIn Upper Zone

Heat SourcesIn Lower Zone

Temperature FReference: REHVA Guidebook

64 66 68 70 72 74 76 78 80

9

8

76

5

4

3

2

1

be exhausted properly rom the space. I the exhaust is located lower than 7 eet theremay be some polluted/hot air remaining within the occupied zone. For lower ceilingsit is best to place the return aboe the heat source in the space.

Cotrol TipTemperature stratication aboe theoccupied zone is not a concern, as long asthe ceiling is oer 8 eet. Ensure that the

returns are extracting at a minimum o 9eet to ensure stratication control.

Product TipWhere additional cooling is required, chilled ceiling systems can be used to remoeadditional heat rom the space. For more inormation on chilled ceiling systemssee the Radiant Systems section.




Thermal Comort

Thermal ComortASHRAE Standard 55 denes thermal comort as a condition o themind which expresses satisaction with the thermal enironment.This denition is based on the act that each person denes whatis thermally comortable based upon their own physiological andpsychological states.

To no small part, a building occupants preerred thermalenironment is based upon what they are normally exposed toand as a result, expect to nd in any space they enter. Today,most building occupants expect a narrow range o temperature,air elocity and humidity and i the enironment is out o theirpreconceied expectations, a thermal complaint will occur.

Another issue aecting thermal comort is the act that when aperson rst enters a new thermal enironment, they may notnd that enironment acceptable or a period o time i they haeexperienced dierent thermal conditions or dierent actiity leelsjust prior to entering the space. This period o adaptation maytake up to an hour beore the person becomes satised with thenew thermal enironment. Unless the building is only occupiedby one occupant and the occupant is in complete control o hisor her thermal enironment, there will always be at least oneoccupant who will express dissatisaction with the building HvACsystems. As a result, ASHRAE denes the goal or the thermalenironme nt as an acceptance by a substantial majority (at least80%) o the building occupants.

Most building HvAC designers would preer to neer hear eedbackrom the occupants o a building that they hae designed noeedback would indicate a good design as most people willcomplain about being hot or cold, but rarely will a buildingoccupant gie kudos or being thermally comortable.

The actors that must be addressed when dening conditions orhuman thermal comort include:

metabolicrate clothinginsulation

radianttemperature airtemperature

humidity airspeed

Most designers only consider the last three, but in reality, all sixare o equal importance.

ASHRAE Standard 55 denes a comort zone that may bedetermined or a gien range o humidity, air temperature, radianttemperature, air speed, metabolic rate, and clothing insulation.This comort zone is typically dened in terms o a range ooperatie temperatures that will proide a thermal enironmentthat a specic percentage o occupants will nd acceptable. Thismethod may be used in spaces where the occupants Met leels

are within 1.0 Met to 1.3 Met and clothing has a clo alue between0.5 clo and 1.0 clo o thermal insulation. Most oce spaces allwithin these limitations.

Figure 22 shows the range o operatie temperatures or an 80%occupant acceptance. This range o operatie temperatures arebased on a 10% dissatisaction criteria or whole body (general)thermal comort (based on the PMv PPD index see ASHRAEStandard 55 or a description o the PMv PPD index) and anadditional 10% dissatisaction or local thermal comort. Twozones are shown on these gures, one or 0.5 clo and one or 1.0clo which is intended to be representatie o when the outdoorenironment is warm and cool, respectiely. Figure 23 shows theeect o air elocity on the operatie temperature.

Figure 22: Acceptable range of operative temperature andhumidity [ASHRAE Standard 55 2004]

80%

80%

70%

60%

50%

40%

30%

20%

10%

RELATIVEHUMIDITY

55

60

65

70

75

80

85

90

95

100

DRYBULBTEMPERATURE-F

HUMIDITYRATIO-POUNDSMOISTUREPERPOUNDDRYAIR

.014

.012

.010

.006

.005

.004

.002

PMV Limit 0.5

No Recommended

Lower Humidity

Limit

Upper Recommended Humidity Limit 0.012 humidity ratio

Data based on ISO 7730

and ASHRAE STD 55

1.0 Clo 0.5 Clo

Figure 23: Effect of air velocity on acceptable range ofoperative temperature and humidity [ASHRAE Standard55 2004]

80%

80%

70%

60%

50%

40%

30%

20%

10%

RELATIVEHUMIDITY

55

60

65

70

75

80

85

90

95

100

DRYBULBTEMPERATURE-F

HUMIDITYRATIO-POUNDSMOISTUREPERPOUNDDRYAIR

.014

.012

.010

.006

.005

.004

.002

PMV Limit 0.5

No Recommended

Lower Humidity

Limit

Upper Recommended Humidity Limit 0.012 humidity ratioData based on ISO 7730

and ASHRAE STD 55

15 FPM 50 FPM

30 FPM


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Thermal Comort

Figure 24: Effect of air velocity and temperature differenceon thermal comfort for the neck region [source: ASHRAEFundamentals]

0

10

20

30

40

50

60

70

80

90

100

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5

Temperature Difference, F

AirVelocity,FP

FEELING OF

COOLNESS

FEELING

OF

WARMTH

30%

NECK REGION

40

20%

10%

Figure 25: Effect of air velocity and temperature differenceon thermal comfort for the ankle region [source: ASHRAEFundamentals]

0

10

20

30

40

50

60

70

80

90

100

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5

Temperature Difference, F

AirVelocity,FPM

FEELING OF

COOLNESS

FEELING

OF

WARMTH

NECK REGION

30%

40

20%

10

The predicted percentage dissatised is a measure o the thermalcomort perormance in the built enironment. There are seeralactors which are rolled into this calculation including drat, radiantasymmetry and stratication. It is this measure that is typicallyreerenced with thermal comort is examined or discussed.

The most common complaint due to thermal comort are eitherthe occupant is too hot, or their hands and or eet are cold. Youmay hae both complaints in the same building on the same day.Too high an air elocity can be a signicant actor in generatingthese thermal complaints. The most sensitie part o the humanbody in the typical oce enironment is the back o the neck,which is shown in Figure 24. The ASHRAE design conditions o70F, 50% humidity and 50 FPM, approximately 8% o the occupantwill complain o a cool sensation. With a typical dead-band o athermostat o +-2F, the complaint sensation may ary up to 25%

occupant dissatisaction.

A common complaint or cold sensation is the eet. Figure 25shows the combined eect o air elocity and temperature orthis body part. The ankles are demonstratiely not as sensitie asthe back o the neck to the eect o elocity and only moderatelyimpacted by mild temperature dierences rom set point. Whena person experiences a drop in core body temperature, the bodybegins to restrict blood fow to the extremities, consering heator the critical internal organs. This leads to physically cold handsand eet.

The cold complaint caused by drat is more commonly experiencedin oerhead air distribution than underfoor or displacemententilation. In an improperly designed oerhead air distributionsystem, the air may separate rom and ceiling may directly impactbuilding occupants. For displacement air distribution, the occupant

should not be closer than two eet to the displacement diuser. Indisplacement air distribution, the aerage room air elocity is justabout 20 to 30 eet per minute. Since the natural conectie airelocity o the occupant is about 30 pm, this type o air distributionwill not signicantly disturb the natural conectie air moementaround the occupant. This will lead to higher occupant satisactiondue to the signicantly lowered air elocity sensation and the selbalancing heat load o the occupant rom the low elocity coolresh air at the occupants eet.

In stratication air distribution systems, to maintain thermalcomort, it is important to maintain no more than a 5F (3C)temperature dierence between the occupants head and eet. Agradient larger than this alue may lead the occupant to becomeaware that his or her eet are cooler than their head.

ANKLE REGION




Vetilatio Eectiveess

Ventilation Eectiveness

ventilation eectieness is a measure o the air distributionsystems ability to remoe airborne pollutants rom a buildingspace. This remoal o airborne pollutants occurs through theinjection o resh, clean air through a diuser into the space andremoal o the dirtier room air through a return grille.

One measure o the entilation eectieness is to compare thecontamination leel in the breathing zone to the contaminationleel present in the return air grille or a zone.

Where:

= Cpe/Cpbz

Cpe = concentration o pollutants in the exhaust

Cpbz = concentration o pollutants at breathing leel

I the zone (or room) is 100% mixed, the entilationeectieness,= 1.0, the air in the occupied breathing zoneis as resh (or dirty) as the air in at the return grille.

I > 1.0, the air in the occupied breathing zone isresher (cleaner) than the return air this indicates thatthe pollutants are being moed by the cleaner supplyair away rom the occupied zone and toward the returngrille.

I < 1.0, this indicates that the occupied breathing zoneis dirtier than the return air and this lowered entilationeectieness is typically caused by short-circuiting o thesupply air to the return grill and is considered a grosswaste o pollution remoal potential.

The type o supply diuser used will hae a direct impact in theentilation eectieness o the building HvAC system. Typically,mixed entilation systems are oerhead air diusers and hae anaerage entilation eectieness o = 0.9. The oerall entilationeectieness o oerhead diuser systems may ary due to diusertype (0.7 < < 1.0 with aerage = 0.9) and mode o operation(heating or cooling).

Well-designed displacement entilation air distribution systemshae a entilation eectieness that are at least = 1.2 and haethe potential or greater entilation eectieness when used incombination with dedicated outdoor air systems and radiantheating/cooling systems.

Table 1 shows data collected by Krantz or the entilationeectieness o arious types o air distribution systems.Displacement diusers are shown in the Krantz laboratory tohae entilation eectieness higher than that o a ully mixed

system.

Measure-ment point

in room

LinearDisplace-

ment

Floor Twist

Outlets

Ceiling Outlets

Twist

Outlets

Slot

Outlets

In ront ostandingperson

1.2 1.6 1.2 1.65 0.88 0.96 0.93 0.97

In ronto seatedperson

1.3 1.95 1. 3 2.0 0.90 0.97 0.92 0.96

Table 1: Ventilation Effectiveness for Different Types ofAir Distribution Systems [Source: Krantz]

Gree TipUsing displacement entilation or schools is a great way toincrease the entilation eectieness, indoor air quality o thespace. CHPS credit EQ2.1: Thermal Displacement ventilationis a two point credit or the use o displacement entilation inthe building. Also, both LEED and Green Globes hae IAQcredits or the implementation o displacement entilation.

Gree TipThe high entilation eectieness rom a properly designeddisplacement entilation system can earn credits in GreenGlobes and LEED rating systems or higher indoor airquality.

Gree TipThe Green Building Initiaties: Green Globes .1 section 7G.1.3 requires the zone distribution eectieness Ez alue tobe greater than 0.9, and proen with ASHRAE 129-1997 (12points possible). This is achieable with a properly designeddisplacement system.


15/52


Diusers with Integrated Heat

There is a wide range o aailable diusers on the market thatproide heating options. The DLE-H shown in Figure 27 utilizes anintegrated heating element aboe the supply ace.The conectieorces rom the heating element are not substantial enough todraw the supply air into the ront intake opening or the heater,so short circuiting o the air is minimized. These diusers act likea perimeter radiation system common to northern climates.

Seeral o the industrial models aailable rom Price oer aheating mode which, when supplied with warm air, will orcesupply air down into the occupied space. The primary dierence

with these diusers is the elocity being supplied to the spaceis much higher than what is acceptable in a commercial space,and as a result, some mixing will occur. Another alternatie isto use a special displacement diuser with dual aces. Shown inFigure 28, this displacement diuser proides cool, slow moingair through the top section and warm, ast moing air throughthe lower section in order to proide a localized mixed zone owarm air near the diuser.


Heatig with Displacemet Vetilatio

Heating

Displacement entilation relies on the principle that thermal plumes drie the moement o the air within the space and, as preiouslymentioned, supplying a space with hot air at the same fow elocities required by displacement entilation is not recommended. Thisis because the supply air does not hae enough orward momentum to oercome the eects o buoyancy, and will rise to the ceilingleel and be exhausted or returned, bypassing the occupied zone. There are seeral ways to oercome this and proide a comortableenironment.

Fan CoilsFan coils may be incorporated into a displacement system as an alternatie heating source, as long as the an coil is located outsidethe occupied zone and is used to treat perimeter walls and glass, without mixing the occupied zone.

Radiant Systems

Utilizing radiant systems has benets beyond supplying heat or a displacement system, they can be used to compensate or thesensible cooling demand and proide excellent comort conditions to a space. There are seeral methods or supplying radiant heatperimeter radiation, radiant fooring, radiant panels (Figure 26), Sails, and Beams. For more inormation on this option reer to theRadiant Systems section o the catalog.

Figure 26: Radiant Panel

Figure 27: DLE-H

Gree TipHeating and Cooling by using water or an antireeze mixtureas the heat transport medium is much more ecient than byusing air. Reer to the Radiant Products design guide or moreinormation.

Cotrol TipA our-pipe system may be approximated by two separatehydronic systems. The use o panels or heat interspersedwith Chilled Beams or cooling hae been used with goodsuccess. Reer to the Radiant Product design guide or moreinormation.

Product TipThe DLE-H Displacement diuser proides a conectie heatsource directly installed into the diuser and is ideal orperimeter treatment.

Product TipThere are many options or proiding heat through specializeddisplacement diuser. Price has a long history o manuacturingspecial designs to suit specic applications, including heatingsections incorporated into displacement diusers. Contact yourlocal Price representatie or details.

Figure 28: Special Displacement Diffuserwith integrated heating section




Humidity Cotrol

Humidity control is extremely important and exists in most climates,not just the traditional hot and humid climates. Controlling humidityis the most common question when discussing the concepts ounderfoor air distribution, or displacement air distribution.

Controlling humidity means dierent things to dierent people astheir personal perspecties are dierent. In the oce enironment,humidity control means limiting the upper humidity leel to theguidelines o ASRHAE Standard 55 in order to proide goodthermal comort. Museums oten need humidity leels to bemaintained in a narrow range to slow or preent decay in artworkand historical displays.

Temperature control is automatically part o a building HvACequipment design, while humidity control is not alwaysautomatically included in buildings located in areas that are notconsidered hot and humid. I humidity control is included, it mayonly be to maintain a humidity leel that does not exceed therecommended upper limit o ASRHAE Standard 55. In act, mostbuildings experience a drit o humidity leels rom hour to hour.

Buildig Shells Are Sources o Humidity

All buildings leak air through the building shell. In a humidclimate, the amount o leakage is directly related to how muchenergy must be expended to control the humidity leel in theinterior spaces. The ASRHAE Humidity Control Design Guide orCommercial and Institutional Buildings encourages designers tothink o buildings as ery leaky rerigerators. This is an accurateanalogy as most tight constructed buildings hae been determinedto leak around a minimum o one air changes in three hours.Poorly constructed buildings may experience two air changes anhour, or more. This leakage is a direct transer o moisture intoor out o the interior zones and needs to be accounted or in the

building moisture loads.

When an open plenum return is used in an exterior zone, the HvACdesigner must take care to preent negatie pressurization in theplenum space. This negatie pressurization can and will cause airto inltrate through the building walls and will proide a transporto moisture rom the outside i the outside enironment has moremoisture in the air than the interior does. The best solution to thisissue is to use ducted returns.

In an eort to minimize the ductwork in an underfoor ordisplacement designed building, the returns should be placedclose to the air handling equipment, or duct chases.

Desig Suggestios

The HvAC designer is responsible or the control o humidityleels and his selection o equipment will make or break the

design. Although there are many actors which will aect thecontrol o humidity in the space, this discussion will ocus onmajor issues and make some recommendations that will assistin building design.

Pretreat Vetilatio Air

In a humid climate, the biggest source o moisture is typically theentilation air rom the outside. This typically accounts or about50 to 80% o the building moisture load in typical commercialbuildings. It is entirely likely that when this entilation air ispretreated or humidity control, the entire building humidity loadwill be controlled without any additional moisture remoal.

Figure 29A illustrates one approach or humidity control commonlyknown as Side-Stream Bypass. The cooling coil is operated toproduce 50-55F leaing air temperature or dehumidication. Aportion o the return air is bypassed beore the coil and mixed with

the conditioned air to achiee proper temperature and humidityprior to deliery to the displacement diuser. Only the outdoorair and a part o the return air are actually directed through thecoil. This moisture control o the outside air will require theoutside air to be cooled to a temperature below the dewpoint. Inan underfoor or displacement air distribution system that willmean the supply air temperature rom the air handling equipmentwill be signicantly lower than the recommended design supplyzone air temperatures. The air will need to be reheated to preentoccupant dissatisaction rom the temperature o the supply air.

Designers typically size the cooling coils on peak sensible load(the hottest part o the weather cycle). Unortunately, the peaklatent load is typically not connected to the peak sensible load.This means that the total load (sensible + latent) may peak whenthe outdoor dew point temperature is the highest, not the dry

bulb temperature.

Another option or humidity control is the series type an poweredterminal (Figure 29B). In this application the primary air is cooledto 55F or less at the air handler to proide dehumidication. Thean terminal is used to increase the supply air temperature toan acceptable leel beore entering the zone. Conditioned air issupplied to the primary ale o the terminal ia a supply duct.Return air is induced into the return air opening rom the returnair plenum. The an deliers a constant air olume to the zone.The proportion o primary and return air is controlled to maintaina supply air temperature aboe 63F.

Figure 29A: Side Stream Bypass Humidity Control

Figure 29B: Series Fan Terminal


17/52



Humidity Cotrol

Load Reductio Equipmet

It is outside the scope o this design guide to proide design criteriaor the many dierent types o energy recoer/load reductionequipment aailable on the HvAC market today. Seeral dierentsystems that maybe appropriate or the building design are:

Passive Desiccat Wheels these wheels can transer between10 and 90% o the heat and moisture dierence between twoair streams. These wheels do not use heated air to remoe themoisture, but rely upon dry air.

Active Desiccat Wheels these wheels use heated air to remoethe moisture rom the desiccant and can deeply dehumidiy theair as a result. Heat Pipes these are oten used to improe theoperation o desiccant or mechanical dehumidiers. They aresealed tubes that contain some liquid and a gas a low pressure.

The liquid in the bottom o the tube will boil at low temperatures(cooling the air outside the tube) and drit upward where it willcondense and reheat heat (heating the air outside the tube).Theseheat pipes are usually capable o transerring between 45 and 60%o the temperature dierence between two air streams.

Plate Heat Exchagers hot and cold air streams are separatedby thin plates and the air passes through the exchanger in an xor z pattern. Plate heat exchangers are usually able to transerbetween 60 and 65% o the temperature dierence between thetwo air streams.

Ecoomizer Cycle

For ree-cooling, an economizer cycle is typical ly used.Unortunately, most are merely temperature controlled and maynot preent humidity control issues year the entire year. Enthalpycontrol is oten used, but do not always sole this issue. The

theory or enthalpy control is to use outdoor air when the totalheat outdoors (the enthalpy) is lower than the total heat inside.This approach does not consider the dierence in dew pointsbetween inside and outside. Air with a lower enthalpy rom theoutside may contain more moisture than is desired in the space.It is recommended that all economizer cycles are set so that theoutdoor air is neer used when the outdoor dewpoint is higherthan the interior dew point design point.

Direct Expasio Roo Top Uits

DX packaged roo top units may be used to condition raisedfoor caities and displacement entilation. Howeer, care mustbe exercised to select the proper sized equipment and controlsto maintain moisture remoal. The issue is that at part-loads,the coil temperature is oten raised to preent sub-cooling thezone. This means that not enough moisture will be remoed bythe cooling coil which will allow the humidity leels to rise in anuncontrolled manner. Simply sizing the coil or the highest totalload will not preent this issue in latent capacity i the controlis based upon only the zone dry-bulb temperature and not alsothe humidity leel.

Dedicated Dehumidicatio ad Eergy Recovery

When the exhaust air exits the building at the same point as thesupply air enters, a heat exchanger can be used to proide reheatto the supply air which will reduce the load on the equipment toproide the suggested supply air temperatures or underfoorand displacement air distribution.

When moisture loads are high, it is oten cost eectie touse separate dehumidication equipment such as an actie

Desiccant Dehumidier (dry wheel, or liquid system), or aMechanical Dehumidier (condenser and eaporator coils in theair stream).

Dehumidicatio

Dehumidication is actually quite simple. Merely place enough dryair into the building space to absorb the excess humidity. Haingsaid that, the issues complicated in that many dierent methodsexist to take the moisture out o the air and many diculties existin the control o this equipment.

ASHRAE has seeral recommendations or dehumidication oa building:

Drytheventilationairrstasthebulkofthemoistureloadinbuildings is due to the entilation air.

Lowerthedesigndewpointandraisetheinteriorsetpointdrybulb temperature. When the occupants o a building are in a dry

climate, RH < 45%, they will hae the same perceied comort leelat 78F as they would at 74F and 50% RH. Interestingly, most peoplend the dryer and warmer combination more comortable.

Downsize thecoolingequipmentanduse adehumidier. Ifthe cooling system is not required to remoe latent loads, itcan typically hae a smaller cooling capacity. This will raise theoerall eciency o the HvAC system and allow or more localizedcooling in high sensible loadings such as call centers. This is agreat approach or the use o an air columns in a raised foorapplication.

Remembertoanalyzethedehumidicationcycleatthepeakmoisture remoal load as well as the peak temperature point.

Cotrol TipWhen a DX system is oersized, the compressors will remoe

the cooling load with ery little cycle time.Then the compressorsshut down and the moisture on the coil will re-eaporate and beadded to the air. Additionally, the entilation air is still requiredand will also transport moisture into the zone. The net eect isa humid occupied zone.




Acoustics

Acoustics Considerations

There are typically at least e primary sources o sound generationin a displacement entilation application: an powered terminals;control ales; diusers; air-handling equipment and structural-borne sound.

Fan Powered Terminals

The rst and most commonly considered is the sound generatedby an powered terminals. Fan powered terminals use either aPSC (permanent split-capacitor) motor or an ECM (electronicallycommutated motor) to drie a blower or vAv applications. Thesedeices typically produce low to mid requency sound energywhich, i not properly accounted or and treated, may causediscomort to the space occupants.

The sound energy generated by terminals may be transmitted

by three transmission paths into the occupied space: dischargesound rom the terminal outlet through the ductwork and out thediuser; and radiated sound rom the terminal/induction opening(i present) directly through the ceiling or foor tile and by ibrationenergy rom the an/motor through the casing into the ceilingsupport structure or foor slab.

Manuacturers (including Price Industries) who participate inthe Air-Conditioning and Rerigeration Institute (ARI), terminalcertication program (ARI 880) are required to calculate NC (noisecriteria) alues using predetermined sound transmission pathattenuation actors in the ARI 885-98 Standard, Appendix E.

Terminals Discharge Sound Transmission

As long as the duct downstream rom the terminal is lined, therewill be some sound energy attenuation. The leel o soundattenuated depends upon the ductwork conguration. The ARI885-98 Standard attenuation actors used to estimate the NCalues in the tabulated data or Price terminal units is based upone eet o lined duct work, e eet o fex duct, space eect andfow diision.

A copy o the ARI 880 and 885 Standards may be downloaded atno cost rom www.ari.org.

DiusersThe second most commonly considered is the sound generated bythe air outlet. Interestingly, they are typically not at ault or anysound generation issues other than perhaps direct radiated soundtransmission rom a terminal or a control ale located near thediuser inlet. An example o diuser noise is shown in Table 2.

Figure 30: Space Effect Factor [ASHRAE Fundamentals]

Distance from Source in Feet

1 3 10 30 100 300 1000

10

0

-10

-20

-30

-40

-50

-60

Space

EffectFactor,

dB

SMALL OFFICE

LECTURE HALL

AUDITORIUM

ARENA

Product TipTo estimate the actual NC alues present in the design space,the Price Quick Select program or terminals may be used withthe attenuation actors shown inTables 1 and 2. This programuses the data in the ARI 885-98 Standard and allows the userto build their ductwork conguration.

Table 2: Sample Perormace Data or a DF3

Unit Size[Face

Area, t]W x H x D

Inlet Sizein.

FaceVelocity

FPMAirfow

CFM

TotalPressure

in.wg.

Static Pres-sure

in.wg.

NoiseCriteria

NC

24 x 24 x 13

[7.7]

10 20 155 - - -

10 30 232 0.02 0.01 -

10 40 310 0.04 0.02 -

10 50 387 0.06 0.03 7


19/52



Acoustics

Table 3: Suggested Discharge Soud Atteuatio (dB) (see ARI 885-98Stadard or basic values ad methodology used).

Diusers Air Movemet GeeratedSoudLargely due to their low pressure drop,displacement diusers do not typically haeNC alues aboe NC 30. The NC alues orthe diusers are calculated using ASHRAEStandard 70. See Table 2, on the preiouspage, or NC leels associated with aDF3 displacement outlet. The let columnindicates the NC leels lower than 15 as ---,and shows the quiet nature o displacementdiuser.

ASHRAE Standard 70 assumes thatall diusers are discharging air into atypical space that will experience a sound

absorption o 10dB in all bands. Thetabulated NC alues may be corrected orthe type o space in your design by usingthe ormula below and the SEF actor romTable 3.

Catalog NC = Room NC 10 + SEF 10*log10N

Where:

RoomNCisthedesigngoal

SEFisthecorrectionfactorfromFigure1

Nisthe#ofoutletsinthespace

Example:

Priate Oce space (Design NC = 30).

10 t x 10 t with 2.5 CFM/SF (295 CFM)

From Figure 1, the SEF = 5.

Number o supply diusers is 1 (250 CFM)and the number o return diusers is 1.

Catalog NC = 30 10 + 5 10*log10(1+1)

Catalog NC = 22

DF1 Diuser (48x24x13) at 295 CFM generatean NC alues < 15 NC.

The return grill would be selected to haean NC alues o 19 or less as well.

Product TipThese NC calculations are based ona ceiling present. I you do not haea ceiling, you must correct or thatlack o absorption. Please consult anacoustician or assistance with thisissue.

Component Description

Octave Band mid Frequency, Hz

125 250 500 1000 2000 4000 8000

Small Terminal< 300 CFM

24 28 39 53 59 40 28

27 29 40 51 53 39 30

Med. Terminal300 to 700 CFM

29 30 41 51 52 39 32

22 22 27 28 30 22 18

Large Terminal> 700 CFM

25 25 30 31 33 25 21

27 27 32 33 35 27 23


18 18 21 33 38 28 21

21 19 22 31 32 27 23


23 20 23 31 31 27 25

16 12 9 8 9 10 11


19 15 12 11 12 13 14

21 17 14 13 14 15 15


24 28 39 53 59 40 28

27 29 40 51 53 39 30


29 30 41 51 52 39 32

22 22 27 28 30 22 18


25 25 30 31 33 25 21

27 27 32 33 35 27 23


18 18 21 33 38 28 2121 19 22 31 32 27 23


23 20 23 31 31 27 25

16 12 9 8 9 10 11


19 15 12 11 12 13 14

21 17 14 13 14 15 15

LinedDischargeDuct

withfexduct

(seenotea)

UnlinedDischarge

D

uctwithfexduct

(seenoteb)

NO

fexduct

LinedMetalDuct

(seenotec)

NOfexduct

UnlinedorSolidMetal

Duct(seenoted)

Note a: based on 5t lined duct (1in liner); 8 inch branch duct with 5t o fex duct, occupantdistance rom sound sources o 5t and 1 diuser.

Note b: based on 5t unlined duct; 8 inch branch duct with 5t o fex duct, occupant distancerom sound sources o 5t and 1 diuser.

Note c: based on 5t lined duct (1in liner); 8 inch branch duct with 8 inch round solid metal duct(unlined), occupant distance rom sound sources o 5t and 1 diuser.

Note d: based on 5t unlined duct 8 inch branch duct with 8 inch round solid metal duct (un-lined), occupant distance rom sound sources o 5t and 1 diuser.



Desigig with AHUs ad RTUs

Designing with Air Handling Units

When designing with Air Handling Units (AHUs), Figure 31, thereare seeral options to consider, o which, some will be rom o-the-shel AHUs, while others will require a custom package.

AHUs in a displacement entilation systems must be able tosupply an o-coil supply air temperature o ~65F [18C] in orderto limit discomort.

When climate permits, the use o an economizer is recommended.This can increase the energy eciency o the building whilestill creating the appropriate thermally comortable indoorenironment.

Where dehumidication is required, side steam by-pass or heatrecoery wheels can be used to bring the air back to the correctsupply air temperature, see the humidity control section or

urther inormation.variable speed dries on a vAv system, Figure 32, will help to saeenergy under partial load conditions and will help to promotestratication in the space. I temperature reset systems areincorporated into the system the set point can be raised duringlow load conditions to extend the economizer cycle.

Demand control entilation can be incorporated into thedisplacement system to help reduce the energy demand o thesystem in low load cases and still proide the proper spaceentilation. An example o this would be a building managementsystem (BMS) in conjunction with CO2 sensors or demand controlentilation schemes o partial use spaces.

For larger buildings, air handlers should eed each foor, or a rangeo foors, depending on the size or design o the building.

Designing with Packaged Rootop Units (RTUs)

Generally, it is dicult to use packaged rootops with a displacemententilation system due to their intended use o deliering 55Fsupply air. This will almost certainly cause discomort in thezone and, thereore, should be used in combination with a heatrecoery system.

A large DX system with multiple compressors and temperaturereset capabilities can be used to produce the cooling requirementsmore eciently, i the building can support a larger DX system.

Figure 31: Air Handling Unit (AHU)


Figure 32: Variable Frequency Drives [source: SiemensBuilding Technologies Ltd.]


21/52


6 ft

1ft

1ft


Loadig Withi the Space

Coolig RequiremetsA traditional mixing system conditions thewhole space to be an een temperature.Thesystem then has be designed to cool theentire olume o the space (Figure 33).

With displacement entilation only theoccupied zone, described by Figure 34,needs to be conditioned to meet comortconditions. The reduction and calculation othese will be discussed in another section.The total load in the building remains thesame, but when calculating the eect theloads hae on the occupied space, only aportion o the loads are considered romthe entire space. The latent portion o the

loads in the system need to be remoedwith the supply air. The sensible load needsto be remoed by either cool supply air orby radiant cooling.

Solar, Conduction, and Overhead LightingSolar energy gain in the space is bothradiant and conectie, the amount heatinghoweer, depends on the design o thewindow treatment. Without treatment, themajority o this load alls on the foor, asshown in Figure 35a. Shades at windowswill reduce the amount o energy transerredto the space as the shades will absorb andrefect the energy. Some o the energy willbecome a conectie energy gain outside

o the occupied zone (Figure 35B).In the case o conduction and lightingloads, only a portion o the load remainsin the occupied zone. Wall conduction,or example, shown in Figure 35c, willcontribute a predicable amount o theheat to the occupied zone.Only the radiantcomponent o oerhead lighting loadsare considered because the conectieloads rom the lighting remains aboe theoccupied zone by conecting directly to theupper zone. (Figure 35D).

Radiant EectsThe local surace temperature o objectswithin a space will cause occupants toeither eel cooler or warmer, een i the

space set point is constant. Cooling andheating can be eciently accomplished byusing a radiant system in combination witha displacement entilation system. In thiscase the entilation air only needs to satisyentilation rates, and not cooling loads.

Figure 35: Exteral/Lightig Loads

A

C

B

D

Figure 33: Overhead Air Distributio

Figure 34: Occupied Zoe



Figure 36: Radiat ad Covective Portios o HeatSources

Loadig Withi the Space

People ad Equipmet LoadsPeople and equipment transer heat to their surroundings by ourheat transport methods: conduction, conection, radiation andeaporation. Each o which contribute to the heat gain o the spaceat dierent rates, as either sensible or latent loads. Conectie andradiatie heat transer rom a person are sensible heat gain to thespace (Figure 36), while eaporatie heat transer are latent heatgains. ASHRAE 2005 Fundamentals Chapter 30, NonresidentialCooling, and Heating Load Calculations, gie general heat loadgenerated by people and equipment in arious states o actiitiesor both sensible and latent components.

Sensible HeatThe sensible heat gain to the occupied zone is only a portion othe total sensible load emitted rom the occupants. When usingdisplacement entilation or cooling, only this portion is considered

when sizing the air olume and supply air temperature.

People produce a conectie heat plume rom their bodies as theywarm surrounding air as seen in Figure 37. The rate at whichoccupant heat is generated is dependent on seeral actors:

clothinglevels

metabolicrate

environmentalconditions

activitylevel,etc.

ASHRAE 2005 Fundamentals Chapter 8, Thermal Comort,demonstrates the calculations or sensible heat generation rompeople.

A portion o conductie/conectie heat naturally transers to theupper zone, een without supply air. The radiation generated bythe occupant is emitted to the space in all directions, with someradiating to the foor, walls, and ceilings.

As a result only a portion o the sensible heat load need to beaccounted or in a displacement entilation system. The actorapplied to the total sensible heat gain to the space rom occupantsis shown in the calculation section o this design guide.

Latet HeatUnlike the sensible load, all o the latent load generated bypeople and equipment need to be accounted or in the air olumecalculation. Eaporation rom occupants, humid air generated bycertain equipment, and warm moist air exhaled by occupants allcontribute to the space latent load. Control o the latent portiono the heat generated in the space is critical to controlling therelatie humidity o the space. For urther inormation on latentheat calculations see ASHRAE 2005 Fundamentals, Chapter 30,Nonresidential Cooling and Heating Load Calculations.


Figure 37: Thermal Plume

Covectio

Radiatio


23/52



Diuser Type

Figure 38: Oe-Way DiuserA wide ariety o displacement air diusertypes are aailable to suit the locationrestrictions and dcor o a particular roomor space. In some cases the diusers arecustom abricated to meet an areas uniquearchitectural design.

Some common displacement diuser typesare described as ollows:

1. Rectagular UitsRectangular units are typically placedagainst a wall or partition. I only the aceo the unit is actie, a one-way pattern isproduced as seen in Figure 38. I both theace and sides are actie, a three-way patternresults (Figure 39). The three-way diuserhas a higher air olume capacity than theone-way. Diuser inlets are usually at thetop o the unit, although bottom or rearinlets are aailable.

One ersion o the rectangular units isdesigned to be integrated into the wall (Figure40). A narrow plenum and rectangular inletare characteristic o this design.

Another ersion o a rectangular unit hasno plenum or inlet and is designed orplenum eed applications. These units canbe mounted in a stair riser, wall, cabinet,etc. and are supplied with a eld abricatedplenum shown in Figure 41.

2. Corer UitsCorner units are specically designed to tinto a 90 corner in a room. Supply inlets canbe located at the top or bottom o the unit.Flat or circular aces are aailable dependingon the desired look. A 90 radial pattern isproduced by corner units (Figure 42). Thesediusers are ideal or applications wherewall space may be limited as corners areaailable or use.

3. Displacemet Liear EclosureLinear enclosure can act as both a supplydiuser and as a heating source. Theseenclosure are suited or perimeter locations.As a heating source they are designed sothat the heating element does not interere

with the air temperature or air fow patterns(Figure 43).

Figure 39: Three-Way Diuser

Figure 40: Wall Mouted Figure 41: Recessed Diuser

Figure 42: Corer DiuserFigure 43: Displacemet LiearEclosure




Diuser Type

4. Semi-circular UitsShown in Figure 44 Semi-circular unitsare typically placed against a wall or pillar.Supply inlets can be located at the top,bottom or rear o the unit. A 180 radialpattern is produced.

5. Circular UitsCircular units can supply high olumes oair to a space because the air is distributedin a complete 360 radial pattern (Figure 45).The supply inlet can be located at the topor bottom o the units. Circular units aretypically placed ree-standing in largeinterior spaces such as halls, lobbies,walkways, lounges, etc.

6. Floor Mouted UitsDisplacement diusers are aailable orintegration with a raised foor distributionsystem. The round foor displacement unitproduces a low elocity radial pattern acrossthe foor as seen in Figure 46.

The foor mounted grilles can proide a linearpattern rom the grille ace. Displacementfoor grilles can also be an assisted (Figure47) when additional air olumes are requiredand a an terminal is not economical.

7. Idustrial DiusersFor the industrial enironment diusersneed to be able to withstand impact rommoing equipment, or able to be mounted

aboe the working space and designed tosupply air deep into the space (Figure 48).The Price industrial fat diuser is intended tobe placed on the industrial foor space andproide supply air.The robust design allowsthis diuser to withstand the impact orcescommon to the industrial sector. The Krantzline o industrial diusers are designed tobe mounted aboe the occupied zone, andhae integrated heating and cooling supplyair modes.

Figure 44: Semi Circular Diuser Figure 45: Circular Diuser

Figure 46: Roud Floor Grille Figure 47: Liear Floor Grille

Figure 48: Idustrial Diuser


25/52



Diuser Layout ad Locatio

Figure 49: Wide RoomsDue to the ariety o diuser types aailable, displacement outletscan be mounted in numerous locations and congurations.

The ollowing are some general recommendations or supplydiusers.

Rectangularorsemi-circularunitsare oftenlocatedonwallsopposite to the exterior windows and walls.

Forlarge roomswider than 30 feet, consider mounting thediuser on two opposite walls as seen in Figure 49 .

Forroomslongerthan15feet,considerseveraldiffusersalongthe wall per Figure 50A.

Forlargeopenspaces,roundorrectangulardiffuserscanbeplaced in the mid o the space (Figure 50B).

Placediffusersnocloserthan2feetfromoccupantsasshown

in Figure 51.Avoidplacinglargeobstaclesnearthediffuserface.

Place more diffusers in areaswhich have a higher coolingload.

When ducting rom below a diuser it is important to supply thediuser with a base or easy connection to the diuser.

When mounting displacement diuser; in the ceiling or at aneleated location, it is important to locate these aboe aisle waysor along perimeters. Placing a diuser directly oer an occupantwill lead to occupant discomort. Locating the diusers alongperimeters will help to reduce the heat gain and entrainment opollutants as the air passes down through the stratied layers.

When mounting displacement diusers along the walls it isimportant to proide support, due to the weight o the outlets.

In installations where the ductwork is supplied rom aboe thediuser and needs to be hidden, ensure that the coer will properlyconceal the ductwork. Perorated coers may require the ductworkto be painted to conceal it completely.

When supplying displacement air to a room with a sloped foor, orramp, place more o the diusers at the upper leel o the space.The cool air will want to fow down to the lower.

Regarding return air outlets, it is essential that they be placed athigh leels either on the wall or in the ceiling. The same modeltypes or mixing systems would apply to displacement systems.Locating the return air outlets aboe strong heat sources suchas windows will ensure the ecient remoal o the heat andcontaminants generated by the thermal plume.

30 ft

Figure 50A: Log Rooms

15 ft

Figure 50B: Large OpeRooms

Figure 51: Distace rom Diusers

Product Tip

To conceal ductwork rom the ceiling to a foor mounted diuser,a duct coer may be used.These coers are designed to matchthe look o the diuser or a consistent architectural nish.




DV Supply AIr Methods

Figure 52: Ducted CoectioDucted Coectio

The most common method to supply air to a displacement diuseris ia a ducted connection. Diusers can be connected rom thetop, bottom, and sides depending on the unction and design othe diuser.

Balancing dampers required or diusers should be mountedat least 3 duct diameters away rom the inlet connection on thediuser, in a ducted conguration.

Pressurized Pleum:

When utilizing a pressurized plenum with a displacement diuser,ensure that the plenum is properly sealed . Adantages o usinga pressurized plenum are reduced ductwork easier balancing andquicker installation. For urther inormation on pressurized plenumdesigns see the UFAD design guide.

Compoet Selectio

Displacement diusers designed or the commercial sector haea recommended maximum ace elocity o 40 eet per minute toensure comort in the space. In transitional spaces such as lobbies,50 eet per minute ace elocities are acceptable.

The air olume, return air temperature, and supply air temperatureare calculated alues based on the room dynamics, see the

Calculation section or ull equations.The type o diuser is typically selected to match architecture orother space constraints. See the DiuserType section or summarydescriptions o the diusers oered by Price.

Istallatio

Generally foor mounted diusers are proided with wall mountingstrips, the DR360 is proided with a foor mounting ring. (Figure 54)

The DF1W installs into a engineered ducted plenum, proidedwith the diuser.

The DF1R displacement diuser is designed to t into a pressurizedplenum. Mounting fanges are proided or a tamper prooinstallation as shown in Figure 55.

Product TipWhen selecting a bottom duct diuser ensure that the diuserhas a base i the product is foor mounted. This will make thediuser easier to install in the eld.

Product TipWhen selecting a bottom duct diuser and a balancingdamper is required, special designs may be required toaccommodate the balancing o the diuser. Allow or a basewhere possible.

Product Tip

The DF1R, the DLE/DLE-H and the ARFHD all requirepressurized plenums, they do not come standard with ductedconnections.

Figure 53: Pressurized Pleum

Compoet Selectio ad Istallatio

Distance XFrom Floor toScrewLocation

Mounting Plate

X

H

Figure 54: Typical Wall Moutig

Figure 55: DF1R Istallatio


27/52


Actual zone air fow rate is the maximum o the cooling air fowand the entilation rate.

The supply air temperature is calculated rom:

Where the temperature dierence between head and oot isgien by:

and:

Using r and c = 0.95 BTU/h*t*F rom ASHRAE undamentals

The exhaust air temperature can be calculated rom:

Where:

is the air density (lb/t),

Cpisthespecicheatoftheairatconstantpressure(BTU/lbF),

Histheheightoftheceiling(ft),

Aistheareaofthespace(ft),

Th is the temperature dierence rom heat to oot leel (F),

is the entilation eectieness o the space,

V is the required resh air rate or displacement entilation(CFM),

Vr is the fow required or acceptable indoor air quality (CFM)

Ts is the supply air temperature (F),

is a dimensionless temperature,

Te is the exhaust air temperature (F)

Figure 56: Coolig Loads

OverheadLighting

Conductionand Solar

Occupant andEquipement


Displacemet Vetilatio Air Volume Calculatios

Coolig Flow RateDue to the higher supply air temperatures inherent withdisplacement entilation, it is oten assumed that the coolingsupply air fow rate will be signicantly greater when compared toa traditional oerhead mixing system. Howeer, since a proportiono the heat sources in a room are exhausted directly withoutimpacting the occupied space, displacement entilation fow ratescan be equal or een lower than mixing systems, depending onroom layout and space type (Figure 56).

Due to stratication, each heat source will hae a dierent eectupon the loads in the space. Since some heat sources are aboe theoccupied zone, we can include a load actor or their contributionto the total space load. Typical Heat sources hae both radiant andconectie components so it is important to assign loads to theoccupied zone and the upper zone, depending on the load type.

In determining the air olume requirements or an all airdisplacement entilation system the ASHRAE Design Guide hasdeeloped a procedure to calculate the cooling supply fow rate,taking into account the stratied loads.

Loads can be diided into the ollowing three catagories:

Theoccupants,desklampsandequipment,Qoe (Btu/h).

Theoverheadlighting,Ql (Btu/h).

Theheatconductionthroughtheroomenvelopeandtransmittedsolar radiation, Qex (Btu/h).

Such that, the total cooling load is:

Qt = Qoe + Ql + Qex

Load actors or the aboe catagories hae been determined byASHRAE research project RP-949.

Occupants,desklampsandequipment,aoe = 0.295Approximately 1/3 o this cooling load enters the space betweenoot and head leel. The other 2/3 enters the upper space ia thethermal plume and radiation.

Overheadlighting,al = 0.132 Less than 15% o the total lightingload is radiated to the occupied space.

Heatconductionandsolarradiation,aex = 0.185 More than 80%o the external loads enter the upper space ia the thermalplume and radiation.

Based on the coecients aboe, the ASHRAE Design Guide liststhe ollowing equations or determining summer cooling fowrates or a typical oce space.

Determination o the required air fow rate or summer cooling:

Determination o the entilation rate:

vr is determined rom ASHRAE Standard 62-2004 based on roomapplication. Local codes may not allow the discount or theentilation eectieness, or may hae stricter requirements. Reerto ASHRAE Standard 62.1 or recognized alues o entilationeectieness.



Step 3:

Determie Flow Rate o Fresh Air

Standard 62-2004 ventilation Rate Procedure includes deault aluesor entilation eectieness. From standard 62-2004: Equation 6-1is used to determines the Breathing Zone Outdoor Air Flow vbz andEquation 6-2 Is used to determine the Zone Outdoor Air Flow voz

where Ez = 1.2 or displacement entilation per Table 6.2 .

Step 4:

Determie Supply Air Flow RateChoose the greater o the required fow rate or summer coolingand the required entilation rate as the design fow rate o thesupply air,

Step 5:

Determie Supply Air Temperature

The supply air temperature can be determined rom the ASHRAEDesign Guide equations and simplied to:

Step 6:

Determie Exhaust Air TemperatureThe exhaust air temperature can be determined by the ollowingmethod:

Step 7:

Selectio o Diusers

The goal is to maximize comort in the space and minimize thequantity o diusers. At a maximum, ASHRAE suggests a 40 pmace elocity, but this alue may increase or decrease dependingon the space and comort requirements. A CFD simulation canalidate the design and is recommended or larger spaces, contactyour local Price representatie about CFD modeling.


Displacemet Vetilatio Air Volume Calculatios

Depending on the application, these ormulae can be used in ariouscombinations. A ull calculation is possible using the aboe ASHRAEormulae, but at least one or two o the ollowing alues must bepredetermined, usually set by codes, standards, or experience: n,v, Th, Tsp, or Ts.

Typically we suggest using Th=3.6 or people in a seated positionas per ASHRAE Standard 55-2004, but this alue can change i theoccupants are not seated. Standard air has a density, = 0.075lb/t and Cp = 0.24 BTU/lb*F. Using these alues, we can reducethe equations to the ollowing:

Desig Procedure

The ollowing step by step design procedure is oered as a simpliedapproach to determine entilation rate and supply air temperatureor typical displacement entilation applications. The procedurespresented are based on the ndings o ASHRAE Research ProjectRP-949 and the procedure outlined in the ASHRAE Design Guide.For a complete explanation and deriation o the assumptionsand equations used to deelop this procedure, please reer to theASHRAE Design Guide.The design procedure applies to typical NorthAmerican oce spaces and classrooms. These procedures shouldbe used with care when applied to large spaces such as theaters oratria, a computational fuid dynamic analysis (CFD) o large spacesis recommended to optimize the air supply olume.

Only the sensible loads should be used or the precedingcalculations. These calculations are only or determining the airfow

requirements to maintain the set point in the space, the total buildingload remains the same as with a mixing system.

Step 1:

Determie the Summer Coolig Load

Use a cooling load program or the ASHRAE manual method todetermine the design cooling load o the space in the summer.I possible, assume a 1F/t. ertical temperature gradient in thespace in the computer simulation as the room air temperature isnot uniorm with displacement entilation. Itemize the cooling loadinto the ollowing categories:

Theoccupants,desklampsandequipment,Qoe (Btu/h)

Theoverheadlighting,Ql (Btu/h)

Theheatconductionthroughtheroomenvelopeandtransmittedsolar radiation, Qex (Btu/h).

Step 2:

Determie the Coolig Load Vetilatio Flow Rate, Vh

The fow rate required or summer cooling, using standard air, is:

Vh= 0.076 Qoe + 0.034 Ql + 0.048 Qex

Vh= 0.076 Qoe + 0.034 Ql + 0.048 Qex

3.6A Q t

2.33V2+ 1.8 AV


29/52



Small Oce Example

Space Desig

The owner o an oce building is renoating and would like to consider using displacement entilation in the oce areas. This exampleexamines a small oce in this space. The oce is a north acing room, used primarily during the hours rom 8:00 to 12:00, and rom13:00 to 17:00. The space is designed or 2 occupants, a computer with LCD monitor, T8 forescent lighting, and has a control temperatureo 72F. The room is 10 t wide, 12 t long, and 9 t rom foor to ceiling. The owner expressed interest in supplying the oce spaces withwall mounted displacement diusers or corner displacement diuser as space is limited.

Space Cosideratios

One o the primary considerations when using a Dv system is comort. As preiously discussed, ASHRAE standard 55-2004 stipulates themaximum combination o elocity and temperature in the occupied zone, PPD due to drat, as well as the stratication in the space. In anoce, the occupants tend to be in a seated position; the stratication or a sedentary seated person according to ASHRAE 55 is 3.6F.

The assumptions made or the space are as ollows:

Loadperpersonis250BTU/h

Lightingloadinthespaceis6.82BTU/h/ft

Computerloadis308BTU/h(CPUandLCDMonitor) Conductionthroughthewindowandwallis5BTU/h/ft

Thespecicheatanddensityoftheairorthisexamplewillbe0.24BTU/lbFand0.075lb/ftrespectively.

Occupants 2

Set Point 72 F

Floor Area 120 t

Exterior Wall Area 90 t

volume 1080 t

Qoe 1012 BTU/h

Ql 819 BTU/hQex 450 BTU/h

QT 2281 BTU/h

The loads are broken down as ollows:

Qoe = (2 People X 250 BTU/h) + 308 BTU/h = 808 BTU/h

Ql = 120 t X 6.82 BTU/h/t = 819 BTU/h

Qex = 90 t X 5 BTU/h/t = 450 BTU/h

QT = 2077 BTU/h

Total cooling load or this space (QT) is 2077 BTU/h, andapproximately 17.31 BTU/h/t.

ASHRAE Standard 62-2004 requires 0.06 CFM/t outdoor airfowrate required per unit area, Ra, and 5 CFM/Person outdoorairfow rate required per person, Rp, be deliered to the spaceor moderately actie oce work applications. For displacemententilation a entilation eectieness, or zone air distributioneectieness (Ez), assumed to be 1.2 (table 6-2, ASHRAE Standard62-2004).


30/52J-30All Metric dimensions ( ) are sot conersion. Copyright E.H. Price Li

handbook displacement ventilation design guide booklet by price

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