Displacement Ventil Ventilation ation DESIGN GUIDE
SECTION J
Displacement Ventilation
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Contents
Displacemet Displacemet Vetilat Vetilatio io
Displacemet Vetilatio Itroductio .................... J8 - J10 J9 Typical Applications ............... ............... ............... ................ ...J9 Terminology ...........................................................................J10 Displacemet Vetilatio Characteristics ............ J11 - J14 Thermal Plume ......................................................................... J11 J11 Stratication Height ................................................................. J11 J11 J12 Room Airfow Pattern ..............................................................J12 J13 Diuser Air Flow Pattern .........................................................J13 J13 Contaminant Distribution .......................................................J13 J14 Temperature Distribution .......................................................J14 J14 Location o Returns .................................................................J14
Loadig Withi the Space ........................................... J24 – J25 J24 Loads ...................................................................................... J25 Sensible and Latent Loads .....................................................J25 Diuser Types................................................................ 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
Vetilatio Eectiveess ....................................................... J17
Desig Examples ............................................................J32 - J37 Small Oce Example .................................................J32 – J34 J35 – J37 Boardroom Example ....................................................J35
Heatig with Displacemet Vetilatio ............................. J18
Special Applicatios Supplemet .............................J38 - J52
Humidity Cotrol ........................................................... J19 – J20 J19 Design Suggestions ...............................................................J19 Direct Expansion Rootop Units .............................................J19 Dehumidication and Heat Recoery .............. ...........J19 – J20
Displacemet Vetilatio or Idustrial Applicatios.. ..J39 J40 – J43 Machine Shop Example ...............................................J40 Displacement ventilation ventilation and Schools .......................J44 – J45 J46 – J48 Classroom Example .....................................................J46
Acoustics ........................................................................ J21 – J22
Displacement ventilation ventilation and Healthcare...................J49 – J51
Desigig with AHUs ad RTUs .......................................... J23
Reereces ................................................................................ J52
Thermal Comort ........................................................... J15 – J16
Price Industries works hard to promote the use o sustainable building materials and innoatie air distribution technologies to improe the air quality and enironment integrity in the built enironment. Price has a long history o designing and promoting products systems that are energy ecient and ideal or use in “Green Building” designs. Price is committed to the continual introduction o new products and systems that urther the goals outlined by the USGBC. Price has partnered with Krantz Products USA Inc., North American Distributor or Krantz, the world leader in displacement entilation diusers, to oer a complete system or displacement entilation distribution. The Krantz Komponenten ® industrial diusers hae been the preerred specication or industrial applications since Krantz Komponenten® rst pioneered the North American market. Krantz Komponenten® diusers are oered to the U.S. and Canadian HvAC markets exclusiely through Price. © Copyright E.H. Price Limited 2007.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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N O I T A L I T N E v T N E M E C A L P S I D
Displacement ventilation Desig Guide
About this Desig Guide This document is intended to proide answers to common questions as well as proide some guidance or working though the most common issues when designing an Displacement ventilation ventilation system. Throughout the document you will nd helpul hints as well as Green Tips, Control Tips and Product Tips, an example o which is shown below.
Gree Tip Green Tips proide useul insight into some opportunities or making design decisions which might help in designing a sustainable building. Some pointers are proided proided or both the LEED® and the Green Globes® rating systems.
Cotrol Tip Control Tips are proided to maximize the understanding o all o the control opportunities and issues with Displacement ventilation systems. In some cases cases these will help reduce control complexity or optimize control eectieness.
Product Tip Product Tips proide a link between the design guide section and the product section to assist the design engineer in selecting product with the recommended characteristics. characteristics.
N O I T A L I T N E v T N E M E C A L P S I D
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All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation
Desig Guide
Displacemet Vetilatio Itroductio Overview Airfow in entilated spaces generally can be classied by two dierent types; mixing (or dilution) entilation and displacement entilation. Mixing entilation systems (Figure 1) 1) generally supply air in a manner such that the entire room olume is ully mixed.The mixed.The cool supply air exits the outlet at a high elocity, inducing room air to proide mixing and temperature equalization. Since the entire room is ully mixed, temperature ariations throughout the space are small while the contaminant concentration is uniorm throughout the zone.
Displacement ventilation ventilation systems (Figure (Figure 2) 2) introduce air into the space at low elocities which causes minimal induction and mixing. Displacement outlets may be located almost anywhere within the room, but hae been traditionally 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 contaminants and heat rom the occupied zone. By so doing, the air quality in the occupied zone is generally superior to that achieed with mixing entilation. Cocept Displacement entilation presents an opportunity to improe both the thermal comort and indoor air quality (IAQ) o the occupied space. Displacement entilation takes adantage o the dierence in air temperature and density between an upper contaminated zone and a lower clean zone. Cool air is supplied at low elocity into the lower zone. Conection rom heat sources creates ertical air motion into the upper zone where high leel return outlets extract the air as illustrated in Figure 3. 3. In most cases, these conection heat sources are also the contamination sources, i.e. people or equipment, thereby carrying the contaminants up to the upper zone, away rom the occupants.
Figure 1: Mixig (Dilutio) Vetilatio
Figure 2: Displacemet Vetilatio
Figure 3: Displacemet Flow Characteristics
Since the conditioned air is supplied directly into the occupied space, supply air temperatures must be higher than mixing systems (usually aboe 63 degrees F) to aoid creating uncomortable drats. By introducing air at eleated supply air temperatures and low outlet elocity a high leel o thermal comort can be achieed with displacement entilation.
© Copyright E.H. Price Limited 2007.
N O I T A L I T N E v T N E M E C A L P S I D
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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Displacement ventilation Desig Guide
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 was designed to hae a airly een load distribution and now has the loads concentrated to one side, the system is able to compensate as 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 warm and polluted air is extracted at the high return. When properly designed, there should always be a greater amount o resh air in the breathing 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,mayallowareductioninfanenergywiththeselection o a smaller an components. • Economizeroperatinghourscanbeincreasedtotakeadvantageoffreecoolingbecausesupplyairtemperaturesarehigherthan with oerhead air distribution systems. • Chillerefciencymaybeincreasedwhenthesystemisnotdehumidifying,asthereisalowersupplyairtemperatureandhigher return air temperature.
Typical Applicatios • Displacement ventilation is an effective method of obtaining good air quality and thermal comort in the occupied space. Spaces where displacement entilation has been successully used are:
- Schools
- Theaters
- Classrooms
- Casinos
- Hospitals
- Restaurants
- Dining Rooms
- Meeting Rooms
- Conerence Rooms
- Supermarkets
Figure 4: Classroom Application
- Industrial Spaces • Displace ment ventila tion is usually a good choice in the ollowing cases:
- Where the contaminants are warmer and/or lighter than the room air. - Where the supply air is cooler than the room air. Figure 5: High Ceiling Application
- Where the room heights are 9 eet or more. - Where low noise leels are desired. • OverheadAirDistributionmaybeabetterchoicethan displacement entilation in the ollowing cases: N O I T A L I T N E v T N E M E C A L P S I D
- Where ceiling heights are below 8 eet. - Where disturbances to room airfow are strong. - Where contaminants are colder and/or denser than the ambient air. - Where cooling loads are high and radiant cooling is not an option.
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All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
Termiology Adjacet Zoe
Percet Dissatised (PD)
The adjacent zone is dened as the distance rom the diuser ace to a point where the elocity o the airstream is reduced below to 40 FPM measured 1” aboe the foor.
ASHRAE denes the percent dissatised as the percentage o people predicted to be dissatised with their enironment due to drat.
Buoyacy
Predicted Mea Vote (PMV)
The ertical orce exerted on a olume o air that has a density lower than the ambient air. Breathig Zoe
The Predicted Mean vote, PMv, is an index that predicts the mean alue o the otes o a large group o persons in relation to a scale dened by ASHRAE [ASHRAE Standard 55 2004]
The estimated height at which occupants will inhale the surrounding air.
Predicted Percetage o Dissatised (PPD)
CFD
ASHRAE denes the predicted percentage o dissatised as “an index that establishes a quantitatie prediction o the percentage o thermally dissatised people determined rom PMv.” In real terms it is a measure o the thermal comort perormance in a space.
Computational Fluid Dynamics.The analysis o a space utilizing computers to simulate fuid motion. An example o output rom a CFD analysis is shown in Figure 8. Displacemet Vetilatio
Room entilation created by room air displacement, by introducing air at low leel in a space at a lower air temperature than the room air.
Upper Zoe
A sustainable building rating system rom the Green Building Initiatie (GBI). IAQ
Indoor Air Quality.
L
W
Straticatio
Unwanted local cooling o a body caused by moement o air.
Gree Globes ®
Figure 7
The air current rising rom a hot body. When the temperature o the space aries with height.
The eectie temperature based on the temperature and elocity o the supply air causing discomort.
L
Thermal Plume
Drat
Drat Temperature
Figure 6
Figure 8
The space aboe the occupied zone. Vetilatio Eectiveess
The ratio o contaminants in the exhaust to the contaminates at the breathing leel. An indication o how well a space is extracting contaminates, and an indication o IAQ. Width, Adjacet Zoe
The width o the adjacent zone is the width rom the diuser ace to a specied elocity, typically 40 FPM, reer to Figure 7.
LEED®
Leadership in Energy and Enironmental Design. A sustainable building rating program rom the US Green Building Council. N O I T A L I T N E v T N E M E C A L P S I D
Legth, Adjacet Zoe
The Length o the adjacent zone is the length rom the diuser ace to a specied elocity, typically 40 FPM, reer to Figure 6. Mixed Vetilatio
Air diusion where the mixing o supply and room air is intended. Occupied Zoe
An imaginary box in the room dened as 6 eet aboe the foor and not less than 24 inches rom the walls.
© Copyright E.H. Price Limited 2007.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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Displacement ventilation Desig Guide
Displacemet Vetilatio Characteristics Thermal Plume
Figure 9: Thermal Plume, [Source: ASHRAE Uderfoor Desig Guide]
As heat sources transer heat to the surrounding air, the air becomes more buoyant. This causes air to rise in the space and to be replaced by air rom the side or below, otherwise known as natural conection. As a thermal plume rises aboe the heat source, it entrains surrounding air and increases in size and olume as it loses momentum, moing away rom the heat source, as depicted in Figure 9. The maximum height to which a plume will rise is dependent on the heat source strength, as the initial momentum o the plume will increase. Also, a room with more stratication will reduce the relatie density o the plu me and, as a result, the height to which the plume will rise. The thermal plume generated rom a point source acts dierently than a thermal plume generated rom large objects in the space. For example, a cylinder produces a boundary layer and the conectie thermal plume takes a dierent shape. A point source type expansion o the thermal plume is still present, but at an altered height, and with the thermal plume boundary layer included, shown in Figure 10. The cylinder is a better approximation o an occupant in the space than a point source.
Figure 10: Thermal Plume o Cylider [Source: Skistad]
Straticatio Height
N O I T A L I T N E v T N E M E C A L P S I D
In Figure 11, q0 represents the supply airfow into the room rom a low side-wall di user, q1 is the upward moing airfow contained in thermal plumes that orm aboe heat sources, and q2 is the downward moing airfow resulting rom cool suraces. In terms o this simplied conguration, the stratication height will occur at a height (Yst) where the net upward moing fow, q1q2, equals q0. Clearly, an important objectie in designing and operating a displacement entilation system is to maintain the stratication height near the top o the occupied zone (1.8m [6 t]). I the building occupants are in a seated work position, a lower stratication height (e.g. 1.2m [4t]) may be acceptable. Gree Tip Lower stratication heights will result rom reduced airfow. This saes energy rom treating outdoor air as well as primary an energy.
Figure 11 - Straticatio Height
q1
q2 Yst q0
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All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
Displacemet Vetilatio Characteristics Room Airfow Patter Airfow patterns in a displacement entilation system are quite dierent than in a mixing system. Because o the low discharge elocity o displacement outlets, the room air motion is infuenced to a large degree by conection fows. The conection fows are created by heat sources such as people, equipment or warm windows, or by heat sinks such as a cold wall or window. The conection fows within the room cause the ormation o horizontal air layers. The warmest air layers are near the ceiling and the coolest air layers are near the foor as depicted in Figure 12.
Figure 12: Air Layers
Room air moes horizontally across the foor due to momentum rom the supply outlet and suction rom thermal plumes. vertical air moement (Figure 13) between layers is caused by stronger conection orces associated with heat sources or cold sinks. Heat sources such as people, computers, lights, etc. create a rising conection fow known as a thermal plume. The strength o the thermal plume is dependent on the power and geometry o the heat source. Depending on the strength o the thermal plume, the conection fows can rise to the ceiling or distribute at a lower height. Cold sinks such as an exterior wall or window can generate conection fows down the wall and across the foor.
Figure 13: Vertical Air Movemet
Airfow Penetration A displacement system supplying cool air through a diuser will delier air al ong the foor in a thin layer typically less than 8” in height. The supply air spreads across the foor in a similar manner to water fowing out o a tap, lling the entire space. I obstructions such as urniture or partitions are encountered, the air will fow around and beyond the obstruction illustrated in Figure 14. Een rooms with irregular geometries can be uniormly supplied with conditioned air (Figure 15). When the cool air meets a heat source such as a person or piece o equipment, a portion o the conditioned air is captured by the thermal plume o the heat source, while the remainder o air continues urther into the room. When designing the system to deal with the cooling demand o the space, the penetration depth o a displacement diuser can be 26 – 30 eet rom the ace o the diuser. For rooms exceeding 30 eet in length or width, diusers on seeral walls would normally be required. © Copyright E.H. Price Limited 2007.
Figure 14: Obstructio
Figure 15: Irregular Room Geometry
Couch Supply Air
Diffuser
Supply Air
Diffuser
Partition
Gree Tip Because displacement entilation systems are graity dri en, caution must be used in sloped applications. A theater with een seating, will require less diusers in the lower sections o the theater and more in the upper to compensate the natural moement o the air to the lower portion o the theater.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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N O I T A L I T N E v T N E M E C A L P S I D
Displacement ventilation Desig Guide
Displacemet Vetilatio Characteristics Diuser Airfow Patter
In order to aoid drat it is essential or the displacement diuser to uniormly delier the supply air across the entire diuser ace at low elocity. This requires an internal equalization bafe in combination with a low ree area ace. A displacement diuser supplying cool air will result in an air pattern resembling Figure 17. Due to the density o the cool Figure 17: Cool Air Supply
Cotamiat Distributio
supply air it alls towards the foor a short distance rom the diuser ace and continues along the foor at a depth o approximately eight inches.
with little or no penetration into the space. (Figure 19) For most applications supplying heated air through displacement outlets is not recommended.
When supply air is isothermal, when the supply air temperature equals room temperature, the fow will be distributed horizontally into the space per Figure 18. For a displacement diuser supplying heated air, supply air will rise towards the ceiling Figure 18: Isothermal Air Supply
Figure 19: Heatig Air Supply
Figure 20: Cotamiat Distributio
Contaminant distribution in a space is infuenced by seeral actors such as supply air method, contaminant source type, location within the space, heat sources, and space height.
N O I T A L I T N E v T N E M E C A L P S I D
Displacement entilation improes occupant air quality by reducing the contaminants in the lower portion o the room. The general upward motion o air causes contaminants to concentrate within the upper zone (Figure 20). With mixing entilation, contaminants are diluted with supply air and are distributed eenly throughout the space. The gure represents contamination distribution in a room supplied with mixing and displacement entilation or a typical case where the contaminant source is warm (a person, or example). For displacement entilation case, because the upward conection around a person brings clean air rom lower leel to the breathing zone, the air in the breathing zone is cleaner than the room air at the same height. Contaminants that are heaier than air need to be extracted at a lower leel i they present a saety concern.
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Uiorm distributio o cotamiats withi the mixed space.
Cotamiats are cocetrated at the upper portio o the space.
Gree Tip The potential IAQ increase and reduction in airborne illness transmission are substantial, but are typically not addressed in dollar gures. Recent publications and studies hae shown the increase in IAQ to i ncrease perormance in schools (Re. ASHRAE Journal volume 48 Number 10 Oct. 2006). A publicatio n rom Capital E shows the estimated costs with improed IAQ or schools, but could be applied to other areas as well. See the ventilation Eectieness section or urther discussion.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
Displacemet Vetilatio Characteristics Temperature Distributio Since cool air is introduced at low leel with a displacement entilation system, a temperature gradient exists between the foor and ceiling leel o the space. This ertical temperature gradient is known as stratication. Figure 21 illustrates a typical temperature prole or a room with displacement entilation.
Figure 21: Vertical Temperature Gradiet
The temperature prole, or stratication, is aected by seeral actors; most notably the supply air olume, room cooling load, location and type o heat source and height o the space.The greater the olume o air supplied into a room, the lower the temperature dierence between foor and ceiling. I heat sources are located in the lower part o the room, the temperature gradient is greater in the lower part o the room and lessens i n the upper part. Conersely, when heat sources are located in the upper part o the room the greatest stratication occurs near the ceiling (Figure 22). Controlling stratication in the occupied zone is critical to maintaining occupant comort. ASHRAE Standard 55 requires the temperature dierence between the head and oot leel o a standing person not to exceed 5°F. The ASHRAE Design Guide has determined a method to calculate the head to oot temperature stratication o a displacement system based on supply air olume and load distribution. This relationship was used to deelop a design procedure or displacement entilation systems. An explanation o how the calculation method was achieed is presented on page J30.The design procedure is presented on page J31. Using this design procedure an acceptable room temperature stratication leel can be achieed. Cotrol Tip Temperature stratication aboe the occupied zone is not a concern, as long as the ceiling is oer 8 eet. Ensure that the returns are extracting at a minimum o 9 eet to ensure stratication control.
For commercial displacement entilation systems, supply air temperatures ranging rom 65-68ºF can be expected. As well, the temperature dierence between return and supply in a stratiied system will general ly be greater than 13ºF.
Figure 22: Heat Source Locatio
9 8
Heat Sources In Upper Zone
7
Heat Sources In Lower Zone
6 5 4 3 2 1 64
66
68
70
72
74
76
78
80
Temperature °F Reference: REHVA Guidebook
be exhausted properly rom the space. I the exhaust is located lower than 7 eet there may be some polluted/hot air remaining within the occupied zone. For lower ceilings it is best to place the return aboe the heat source in the space.
Locatio o Returs
Returns should be located as high as possible in the space to remoe as much o the stratied zone as possible, ideally at ceiling height. I the return is located below the ceiling, the air aboe the return may not © Copyright E.H. Price Limited 2007.
Product Tip Where additional cooling is required, chilled ceiling systems can be used to remoe additional heat rom the space. For more inormation on chilled ceiling systems see the Radiant Systems section.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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N O I T A L I T N E v T N E M E C A L P S I D
Displacement ventilation Desig Guide
Thermal Comort Thermal Comort ASHRAE Standard 55 denes thermal comort as a “condition o the mind which expresses satisaction with the thermal eni ronment”. This denition is based on the act that each person denes what is thermally comortable based upon their own physiological and psychological states.
Figure 22: Acceptable range of operative temperature and humidity [ASHRAE Standard 55 2004]
Data based on ISO 7730 and ASHRAE STD 55
To no small part, a building occupants’ preerred thermal enironment is based upon what they are normally exposed to and 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 their preconceied expectations, a thermal complaint will occur.
.014
Upper Recommended Humidity Limit 0.012 humidity ratio .012
0.5 Clo
1.0 Clo
.010
IR A Y R D
D .006 N U O
Another issue aecting thermal comort is the act that when a person rst enters a new thermal enironment, they may not nd that enironment acceptable or a period o time i they hae experienced dierent thermal conditio ns or dierent actiity leels just prior to entering the space. This period o adaptation may take up to an hour beore the person becomes satised with the new thermal enironment. Unless the building is only occupied by one occupant and the occupant is in complete control o his or her thermal enironment, there will always be at least one occupant who will express dissatisaction with the building HvAC systems. As a result, ASHRAE denes the goal or the thermal enironme nt as an acceptance by a substantial majority (at least 80%) o the building occupants. Most building HvAC designers would preer to neer hear eedback rom the occupants o a building that they hae designed – no eedback would indicate a good design as most people will complain about being hot or cold, but rarely will a building occupant gie kudos or being thermally comortable.
P R
% 8 0 E P E R
% 8 0
U .005 T IS O
% 7 0 M S D
% 6 0
P .004
% 5 0 % 4 0
R
R M
P IT T
M .002
OI U T T A
A E Y
No Recommended Lower Humidity Limit
% 3 0 % 2 0 % 1 0
R
B H B Y R D
0 5
U L U
IDI T Y RELA TIVE HUM 5
ID E
5 6
0 6
5 7
0 7
5 8
0 8
0 5
9
0 9
1
PMV Limit 0.5
Figure 23: Effect of air velocity on acceptable range of operative temperature and humidity [ASHRAE Standa rd 55 2004]
Upper Recommended Humidity Limit 0.012 humidity ratio Data based on ISO 7730 and ASHRAE STD 55
.014
30 FPM .012
15 FPM
50 FPM
.010
•clothinginsulation
• radianttemperature
•airtemperature
• humidity
•airspeed
RI A Y R D
D .006 N U O P R
% 8 0
Most designers only consider the last three, but in reality, all six are o equal importance.
N O I T A L I T N E v T N E M E C A L P S I D
U O E
The actors that must be addressed when dening conditions or human thermal comort include: • metabolicrate
N ºF
ASHRAE Standard 55 denes a comort zone that may be determined or a gien range o humidity, air temperature, radiant temperature, air speed, metabolic rate, and clothing insulation. This comort zone is typically dened in terms o a range o operatie temperatures that will proide a thermal enironment that a specic percentage o occupants will nd acceptable. This method 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 between 0.5 clo and 1.0 clo o thermal insulation. Most oce spaces all within these limitations.
E P E R
% 8 0
U .005 T IS O
% 7 0 M S D
% 6 0
N ºF
U O -
P .004 E -
% 5 0 R
IO A
A
T
% 4 0
T R
R E
No Recommended Lower Humidity Limit
% 3 0 % 2 0 % 1 0
U
5
IT M ID E T
M .002 B U L H U B Y
T Y TIVE HUMIDI RELA 5
Y P
R 6
0
D 5 6
0 7
5 7
0 8
5 8
0 9
5 9
0 0 1
PMV Limit 0.5
Figure 22 shows the range o operatie temperatures or an 80% occupant acceptance. This range o operatie temperatures are based on a 10% dissatisaction criteria or whole body (general) thermal comort (based on the PMv – PPD index – see ASHRAE Standard 55 or a description o the PMv – PPD index) and an additional 10% dissatisaction or local thermal comort. Two zones are shown on these gures, one or 0.5 clo and o ne or 1.0 clo which is intended to be representatie o when the outdoor enironment is warm and cool, respectiely. Figure 23 shows the eect o air elocity on the operatie temperature.
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All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
Thermal Comort The predicted percentage dissatised is a measure o the thermal comort perormance in the built enironment. There are seeral actors which are rolled into this calculation including drat, radiant asymmetry and stratication. It is this measure that is typically reerenced with thermal comort is examined or discussed.
Figure 24: Effect of air velocity and temperature difference on thermal comfort for the neck region [source: ASHRAE Fundamentals]
The most common complaint due to thermal comort are either the occupant is too hot, or their hands and or eet are cold. You may hae both complaints in the same building on the same day. Too high an air elocity can be a signicant actor in generating these thermal complaints. The most sensitie part o the human body in the typical oce enironment is the back o the neck, which is shown in Figure 24. The ASHRAE design conditions o 70F, 50% humidity and 50 FPM, approximately 8% o the occupant will complain o a cool sensation. With a typical dead-band o a thermostat o +-2F, the complaint sensation may ary up to 25% occupant dissatisaction. A common complaint or cold sensation is the eet. Figure 25 shows the combined eect o air elocity and temperature or this body part. The ankles are demonstratiely not as sensitie as the back o the neck to the eect o elocity and only moderately impacted by mild temperature dierences rom set point. When a person experiences a drop in core body temperature, the body begins to restrict blood fow to the extremities, consering heat or the critical internal organs. This leads to physically cold hands and eet. The cold complaint caused by drat is more commonly experienced in oerhead air distribution than underfoor or displacement entilation. In an improperly designed oerhead air distribution system, the air may separate rom and ceiling may directly impact building occupants. For displacement air distribution, the occupant should not be closer than two eet to the displacement diuser. In displacement air distribution, the aerage room air elocity is just about 20 to 30 eet per minute. Since the natural conectie air elocity o the occupant is about 30 pm, this type o air distribution will not signicantly disturb the natural conectie air moement around the occupant. This will lead to higher occupant satisaction due to the signicantly lowered air elocity sensation and the sel balancing heat load o the occupant rom the low elocity cool resh air at the occupant’s eet.
100 NECK REGION
90
FEELING OF COOLNESS
80
70
40% 30%
60 P F , tyi
20% c lo
50 e
10%
V ri A
40 FEELING OF WARMTH
30
20
10
0 -7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
Temperature Difference, °F
Figure 25: Effect of air velocity and t emperature difference on thermal comfort for the ankle region [source: ASHRAE Fundamentals]
100 ANKLE REGION NECK REGION
40%
90
30%
80
20% 10%
M
70
60 P F , yt ol
c
i
50 e V A
ir
40
30
In stratication air distribution systems, to maintain thermal comort, it is important to maintain no more than a 5ºF (3ºC) temperature dierence between the occupants head and eet. A gradient larger than this alue may lead the occupant to become aware that his or her eet are cooler than their head.
FEELING OF WARMTH
FEELING OF COOLNESS
20
10
0 -7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
Temperature Difference, °F
N O I T A L I T N E v T N E M E C A L P S I D
© 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-13
Displacement ventilation Desig Guide
Vetilatio Eectiveess Ventilation Eectiveness ventilation eectieness is a measure o the air distribution system’s ability to remoe airborne pollutants rom a building space. This remoal o airborne pollutants occurs through the injection o resh, clean air through a diuser into the space and remoal o the dirtier room air through a return grille. One measure o the entilation eectieness is to compare the contamination leel in the breathing zone to the contamination leel present in the return air grille or a zone. Where: ε= 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 entilation eectieness, ε= 1.0, the air in the occupied br eathing zone is as resh (or dirty) as the air in at the return grille. I ε > 1.0, the air in the occupied breathing zone is resher (cleaner) than the return air this indicates that the pollutants are being moed by the cleaner supply air away rom the occupied zone and toward the return grille. I ε < 1.0, this indicates that the occupied breathing zone is dirtier than the return air and this lowered entilation eectieness is typically caused by short-circuit ing o the supply air to the return grill and is considered a gross waste o pollution remoal potential. The type o supply diuser used will hae a direct impact in the entilation eectieness o the buil ding HvAC system. Typically, mixed entilation systems are oerhead air diusers and hae an aerage entilation eectieness o ε = 0.9. The oerall entilation eectieness o oerhead diuser systems may ary due to diuser type (0.7 < ε < 1.0 with aerage ε = 0.9) and mode o operation (heating or cooling). Well-designed displacement entilation air distribution systems hae a entilation eectieness that are at least ε = 1.2 and hae the potential or greater entilation eectieness when used in combination with dedicated outdoor air systems and radiant heating/cooling systems.
N O I T A L I T N E v T N E M E C A L P S I D
Gree Tip Using displacement entilation or schools is a great way to increase the entilation eectieness, indoor air quality o the space. CHPS credit EQ2.1: Thermal Displacement ventilation is a two point credit or the use o displacement entilation in the building. Also, both LEED® and Green Globes® hae IAQ credits or the implementation o displacement entilation.
Gree Tip The high entilation eectieness rom a properly designed displacement entilation system can earn credits in Green Globes and LEED® rating systems or higher indoor air quality.
Gree Tip The Green Building Initiaties: Green Globes® .1 section 7 G.1.3 requires the zone distribution eectieness Ez alue to be greater than 0.9, and proen with ASHRAE 129-1997 (12 points possible). This is achieable with a properly designed displacement system.
Table 1: Ventilation Effectiveness for Different Types of Air Distribution Systems [Source : Krantz™] Ceiling Outlets
Measurement point in room
Linear Displacement
Floor Twist Outlets
Twist Outlets
Slot Outlets
In ront o standing person
1.2 – 1.6
1.2 – 1.65
0.88 – 0.96
0.93 – 0.97
In ront o seated person
1.3 – 1.95
1. 3 – 2.0
0.90 – 0.97
0.92 – 0.96
Table 1 shows data collected by Krantz or the entilation eectieness o arious types o air distribution systems. Displacement diusers are shown in the Krantz laboratory to hae entilation eectieness higher than that o a ully mixed system.
J-14
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
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 preiously mentioned, supplying a space with hot air at the same fow elocities required by displacement entilation is not recommended. This is because the supply air does not hae enough orward momentum to oercome the eects o buoyancy, and will rise to the ceiling leel and be exhausted or returned, bypassing the occupied zone. There are seeral ways to oercome this and proide a comortable enironment. Fan Coils Fan coils may be incorporated into a displacement system as an alternatie heating source, as long as the an coil is located outside the 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 the sensible cooling demand and proide excellent comort conditions to a space. There are seeral methods or supplying radiant heat; perimeter radiation, radiant fooring, radiant panels (Figure 26), Sails, and Beams. For more inormation on this option reer to the Radiant Systems section o the catalog. Figure 26: Radiant Panel Gree Tip Heating and Cooling by using water or an antireeze mixture as the heat transport medium is much more ecient than by using air. Reer to the Radiant Products design guide or more inormation. Cotrol Tip A our-pipe system may be approximated by two separate hydronic systems. The use o panels or heat interspersed with Chilled Beams or cooling hae been used with good success. Reer to the Radiant Product design guide or more inormation.
Diusers with Integrated Heat There is a wide range o aailable diusers on the market that proide heating options. The DLE-H shown in Figure 27 utilizes an integrated heating element aboe the supply ace.The conectie orces rom the heating element are not substantial enough to draw the supply air into the ront intake opening or the heater, so short circuiting o the ai r is minimized. These diusers act like a perimeter radiation system common to northern climates.
Figure 27: DLE-H
Product Tip The DLE-H Displacement diuser proides a conectie heat source directly installed into the diuser and is ideal or perimeter treatment.
Seeral o the industrial models aailable rom Price oer a heating mode which, when supplied with warm air, will orce supply air down into the occupied space. The primary dierence with these diusers is the elocity being supplied to the space is much higher than what is acceptable in a commercial space, and as a result, some mixing will occur. Another alternatie is to use a special displacement diuser with dual aces. Shown in Figure 28, this displacement diuser proides cool, slow moing air through the top section and warm, ast moing air through the lower section in order to proide a localized mixed zone o warm air near the diuser.
N O I T A L I T N E v T N E M E C A L P S I D
Figure 28: Special Displacement Diffuser with integrated heating section
Product Tip There are many options or proiding heat through specialized displacement diuser. Price has a long history o manuacturing special designs to suit specic applications, including heating sections incorporated into displacement diusers. Contact your local Price representatie or details. © 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-15
Displacement ventilation Desig Guide
Humidity Cotrol Humidity control is extremely important and exists in most climates, not just the traditional hot and humid climates. Controlling humidity is the most common question when discussing the concepts o underfoor air distribution, or displacement air distribution. Controlling humidity means dierent things to dierent people as their personal perspecties are dierent. In the oce enironment, humidity control means limiting the upper humidity leel to the guidelines o ASRHAE Standard 55 in order to proide good thermal comort. Museums oten need humidity leels to be maintained in a narrow range to slow or preent decay in artwork and historical displays. Temperature control is automatically part o a building HvAC equipment design, while humidity control is not always automatically included in buildings located in areas that are not considered hot and humid. I humidity control is included, it may only be to maintain a humidity leel that does not exceed the recommended upper limit o ASRHAE Standard 55. In act, most buildings 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 humid climate, the amount o leakage is directly related to how much energy must be expended to control the humidity leel in the interior spaces. The ASRHAE Humidity Control Design Guide or Commercial and Institutional Buildings encourages designers to think o buildings as “ery leaky rerigerators”. This is an accurate analogy as most tight constructed buildi ngs hae been determined to leak around a minimum o one air changes in three hours. Poorly constructed buildings may experience two air changes an hour, or more. This leakage is a direct transer o moisture into or out o the interior zones and needs to be accounted or in the building moisture loads.
the conditioned air to achiee proper temperature and humidity prior to deliery to the displacement diuser. Only the outdoor air and a part o the return air are actually directed through the coil. This moisture control o the outside air will require the outside air to be cooled to a temperature below the dewpoint. In an underfoor or displacement air distribution system that will mean the supply air temperature rom the air handling equipment will be signicantly lower than the recommended design supply zone air temperatures. The air wil l need to be reheated to preent occupant 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 peak latent load is typically not connected to the peak sensible load. This means that the total load (sensible + latent) may peak when the outdoor dew point temperature is the highest, not the dry bulb temperature. Another option or humidity control i s the series type an powered terminal (Figure 29B). In this application the primary air is cooled to 55°F or less at the air handler to proide dehumidication. The an terminal is used to increase the supply air temperature to an acceptable leel beore entering the zone. Conditioned air is supplied to the primary ale o the terminal ia a supply duct. Return air is induced into the return air opening rom the return air plenum. The an deliers a constant air olume to the zone. The proportion o primary and return air is controlled to maintain a supply air temperature aboe 63°F. Figure 29A: Side Stream Bypass Humidity Control
When an open plenum return i s used in an exterior zone, the HvAC designer must take care to preent negatie pressurization in the plenum space. This negatie pressurization can and will cause air to inltrate through the building walls and will proide a transport o moisture rom the outside i the outside enironment has more moisture in the air than the interior does. The best solution to this issue is to use ducted returns. In an eort to minimize the ductwork in an underfoor or displacement designed building, the returns should be placed close to the air handling equipment, or duct chases.
Figure 29B: Series Fan Terminal
Desig Suggestios
N O I T A L I T N E v T N E M E C A L P S I D
The HvAC designer is responsible or the control o humidity leels and his selection o equipment will make or break the design. Although there are many actors which will aect the control o humidity in the space, this discussion will ocus on major issues and make some recommendations that will assist in building design. Pretreat Vetilatio Air
In a humid climate, the biggest source o moisture is typically the entilation air rom the outside. This typically accounts or about 50 to 80% o the building moisture load in typical commercial buildings. It is entirely likely that when this entilation air is pretreated or humidity control, the entire building humidity load will be controlled without any additional moisture remoal. Figure 29A illustrates one approach or humidity control commonly known as Side-Stream Bypass. The cooling coil is operated to produce 50-55ºF leaing air temperature or dehumidication. A portion o the return air is bypassed beore the coil and mixed with
J-16
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
Humidity Cotrol Direct Expasio Roo Top Uits
Load Reductio Equipmet
DX packaged roo top units may be used to condition raised foor caities and displacement entilation. Howeer, care must be exercised to select the proper sized equipment and controls to maintain moisture remoal. The issue is that at part-loads, the coil temperature is oten raised to preent sub-cooling the zone. This means that not enough moisture will be remoed by the cooling coil which will allow the humidity leels to rise in an uncontrolled manner. Simply sizing the coil or the highest total load will not preent this issue in latent capacity i the control is based upon only the zone dry-bulb temperature and not also the humidity leel.
It is outside the scope o this design guide to proide design criteria or the many dierent types o energy recoer/load reduction equipment aailable on the HvAC market today. Seeral dierent systems that maybe appropriate or the building design are:
Cotrol Tip When a DX system is oersized, the compressors will remoe the cooling load with ery little cycle time.Then the compressors shut down and the moisture on the coil wi ll re-eaporate and be added to the air. Additionally, the entilation air is still required and will also transport moisture into the zone. The net eect is a humid occupied zone. Dedicated Dehumidicatio ad Eergy Recovery
When the exhaust air exits the building at the same point as the supply air enters, a heat exchanger can be used to proide reheat to the supply air which will reduce the load on the equipment to proide the suggested supply air temperatures or underfoor and displacement air distribution. When moisture loads are high, it is oten cost eectie to use separate dehumidication equipment such as an actie Desiccant Dehumidier (dry wheel, or liquid system), or a Mechanical Dehumidier (condenser and eaporator coils in the air stream). Dehumidicatio
Dehumidication is actually quite simple. Merely place enough dry air into the building space to absorb the excess humidity. Haing said that, the issues complicated in that many dierent methods exist to take the moisture out o the air and many diculties exist in the control o this equipment.
Passive Desiccat Wheels – these wheels can transer between 10 and 90% o the heat and moisture dierence between two air streams. These wheels do not use heated air to remoe the moisture, but rely upon dry air. Active Desiccat Wheels – these wheels use heated air to remoe the moisture rom the desiccant and can deeply dehumidiy the air as a result. Heat Pipes – these are oten used to improe the operation o desiccant or mechanical dehumidiers. They are sealed tubes that contain some liquid and a gas a low pressure. The liquid in the bottom o the tube will bo il at low temperatures (cooling the air outside the tube) and drit upward where it will condense and reheat heat (heating the air outside the tube).These heat 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 separated by thin plates and the air passes through the exchanger in an “x” or “z” pattern. Plate heat exchangers are usually able to transer between 60 and 65% o the temperature dierence between the two air streams. Ecoomizer Cycle
For “ree-coolin g”, an econ omizer cycle is typical ly used. Unortunately, most are merely temperature controlled and may not preent humidity control issues year the entire year. Enthalpy control is oten used, but do not always sole this issue. The theory or enthalpy control is to use outdoor air when the total heat outdoors (the enthalpy) is lower than the total heat inside. This approach does not consider the dierence in dew points between inside and outside. Air with a lower enthalpy rom the outside may contain more moisture than is desired in the space. It is recommended that all economizer cycles are set so that the outdoor air is neer used when the outdoor dewpoint is higher than the interior dew point design point.
ASHRAE has seeral recommendations or dehumidication o a building: •Drytheventilationairrstasthebulkofthe moistureloadin buildings is due to the entilation air. •Lowerthedesigndewpointandraisetheinteriorsetpointdry bulb temperature. When the occupants o a building are in a dry climate, RH < 45%, they will hae the same perceied comort leel at 78F as they would at 74F and 50% RH. Interestingly, most people nd the dryer and warmer combination more comortable.
N O I T A L I T N E v T N E M E C A L P S I D
• Downsize the coolingequipment and use a dehumidier. If the cooling system is not required to remoe latent loads, it can typically hae a smaller cooling capacity. This will raise the oerall eciency o the HvAC system and allow or more l ocalized cooling in high sensible loadings such as call centers. This is a great approach or the use o an air columns in a raised foor application. •Remember toanalyzethe dehumidicationcycleat thepeak moisture remoal load as well as the peak temperature point.
© 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-17
Displacement ventilation Desig Guide
Acoustics Figure 30: Space Effect Factor [ASHRAE Fundamentals]
Acoustics Considerations There are typically at least e primary sources o sound generation in a displacement entilation application: an powered terminals; control ales; diusers; air-handling equipment and structuralborne sound. Fan Powered Terminals The rst and most commonly considered is the sound generated by an powered terminals. Fan powered terminals use either a PSC (permanent split-capacitor) motor or an ECM (electronically commutated motor) to drie a blower or vAv applications. These deices typically produce low to mid requency sound energy which, i not properly accounted or and treated, may cause discomort to the space occupants. The sound energy generated by terminals may be transmitted by three transmission paths into the occupied space: discharge sound rom the terminal outlet through the ductwork and out the diuser; and radiated sound rom the terminal/induction opening (i present) directly through the ceiling or foor tile and by ibration energy rom the an/motor through the casing into the ceiling support structure or foor slab. Manuacturers (including Price Industries) who participate in the Air-Conditioning and Rerigeration Institute (ARI), terminal certication program (ARI 880) are required to calculate NC (noise criteria) alues using predetermined sound transmission path attenuation actors in the ARI 885-98 Standard, Appendix E. Terminals – Discharge Sound Transmission As long as the duct downstream rom the terminal is lined, there will be some sound energy attenuation. The leel o sound attenuated depends upon the ductwork conguration. The ARI 885-98 Standard attenuation actors used to estimate the NC alues in the tabulated data or Price terminal units is based upon e eet o lined duct work, e eet o fex du ct, space eect and fow diision. A copy o the ARI 880 and 885 Standards may be downloaded at no cost rom www.ari.org.
10
B d 0 , r o t -10 c a F t -20 c e f f -30 E e c a -40 p S
SMALL OFFICE
LECTURE HALL AUDITORIUM
ARENA
-50 -60
1
3
10
30
100
300
1000
Distance from Source in Feet
Product Tip To estimate the actual NC alues present in the design space, the Price Quick Select program or terminals may be used with the attenuation actors shown inTables 1 and 2. This program uses the data in the ARI 885-98 Standard and allows the user to ‘build’ their ductwork conguration. Table 2: Sample Perormace Data or a DF3 Unit Size [Face Area, t ²] W x H x D 24 x 24 x 13 [7.7]
Inlet Size in.
Face Velocity FPM
Airfow CFM
10 10 10 10
20 30 40 50
155 232 310 387
Total Static Pres- Noise Pressure sure Criteria in.wg. in.wg. NC 0.02 0.04 0.06
0.01 0.02 0.03
Diusers The second most commonly considered is the sound generated by the air outlet. Interestingly, they are typically not at ault or any sound generation issues other than perhaps di rect radiated sound transmission rom a terminal or a control ale located near the diuser inlet. An example o diuser noise is shown in Table 2.
N O I T A L I T N E v T N E M E C A L P S I D
J-18
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
7
Displacement ventilation Desig Guide
Acoustics Diusers – Air Movemet Geerated Soud Largely due to their low pressure drop, displacement diusers do not typically hae NC alues aboe NC 30. The NC alues or the diusers are calculated using ASHRAE Standard 70. See Table 2, on the preious page, or NC leels associated with a DF3 displacement outlet. The let column indicates the NC leels lower than 15 as ---, and shows the quiet nature o displacement diuser.
ASHRAE Standard 70 assumes that all diusers are discharging air into a ‘typical’ space that will experience a sound absorption o 10dB in all bands. The tabulated NC alues may be corrected or the type o space in your design by using the ormula below and the SEF actor rom Table 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 generate an NC alues < 15 NC. The return grill would be selected to hae an NC alues o 19 or less as well.
Table 3: Suggested Discharge Soud Atteuatio (dB) (see ARI 885-98 Stadard or basic values ad methodology used).
Octave Band mid Frequency, Hz Component Description L i n e w d ( s t i e h D i e s n f c h o e x a t r e d g a u e ) c D t u c t D u U c n l ( s t i n e w e e i d t n o h D f i t e e s c h b x a ) d r u g c e t L i n ( s e e d e M n – o e t t e a l c D ) u c t
N O f e x d u c t
U n D l n u i c e t d ( s o e r e S – n o o l i d t e M d ) e t a l
N O f e x d u c t
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. Terminal 300 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
Small Terminal < 300 CFM
18
18
21
33
38
28
21
21
19
22
31
32
27
23
Med. Terminal 300 to 700 CFM
23
20
23
31
31
27
25
16
12
9
8
9
10
11
Large Terminal > 700 CFM
19
15
12
11
12
13
14
21
17
14
13
14
15
15
Small Terminal < 300 CFM
24
28
39
53
59
40
28
27
29
40
51
53
39
30
Med. Terminal 300 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
Small Terminal < 300 CFM
18
18
21
33
38
28
21
21
19
22
31
32
27
23
Med. Terminal 300 to 700 CFM
23
20
23
31
31
27
25
16
12
9
8
9
10
11
Large Terminal > 700 CFM
19
15
12
11
12
13
14
21
17
14
13
14
15
15
Note a: based on 5t lined duct (1in liner); 8 i nch branch duct with 5t o fex duct, occupant distance 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 distance rom sound sources o 5t and 1 diuser. Note c: based on 5t lined duct (1in liner); 8 i nch 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 (unlined), occupant distance rom sound sources o 5t and 1 diuser.
Product Tip These NC calculations are based on a ceiling present. I you do not hae a ceiling, you must correct or that lack o absorption. Please consult an acoustician or assistance with this issue.
© 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-19
N O I T A L I T N E v T N E M E C A L P S I D
Displacement ventilation Desig Guide
Desigig with AHUs ad RTUs Designing with Air Handling Units
Figure 31: Air Handling Unit (AHU)
When designing with Air Handling Units (AHUs), Figure 31, there are seeral options to consider, o which, some wil l be rom othe-shel AHUs, while others will require a custom package. AHUs in a displacement entilation systems must be able to supply an o-coil supply air temperature o ~65°F [18°C] in order to limit discomort. When climate permits, the use o an economizer is recommended. This can increase the energy eciency o the building while still creating the appropriate thermally comortable indoor enironment. Where dehumidication is required, side steam by-pass or heat recoery wheels can be used to bring the air back to the correct supply air temperature, see the humidity control section or urther inormation. variable speed dries on a vAv system,Figure 32, will help to sae energy under partial load conditions and will help to promote stratication in the space. I temperature reset systems are incorporated into the system the set point can be raised during low load conditions to extend the economizer cycle. Demand control entilation can be incorporated into the displacement system to help reduce the energy demand o the system in low load cases and still proide the proper space entilation. An example o this would be a buildi ng management system (BMS) in conjunction with CO2 sensors or demand control entilation schemes o partial use spaces.
Figure 32: Variable Frequency Drives [source: Siemens Building Technologies Ltd.]
For larger buildings, air handlers should eed each foor, or a range o 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 displacement entilation system due to their intended use o deliering 55°F supply air. This will almost certainly cause discomort in the zone and, thereore, should be used in combination with a heat recoery system. A large DX system with multiple compressors and temperature reset capabilities can be used to produce the cooling requirements more eciently, i the building can support a larger DX system.
N O I T A L I T N E v T N E M E C A L P S I D
J-20
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
Loadig Withi the Space Coolig Requiremets A traditional mixing system conditions the whole space to be an een temperature.The system then has be designed to cool the entire olume o the space (Figure 33).
With displacement entilation only the occupied zone, described by Figure 34 , needs to be conditioned to meet comort conditions. The reduction and calculation o these will be discussed in another section. The total load in the building remains the same, but when calculating the eect the loads hae on the occupied space, only a portion o the loads are considered rom the entire space. The latent portion o the loads in the system need to be remoed with the supply air.The sensible load needs to be remoed by either cool supply air or by radiant cooling.
Figure 33: Overhead Air Distributio
Figure 34: Occupied Zoe
Solar, Conduction, and Overhead Lighting Solar energy gain in the space is both radiant and conectie, the amount heating howeer, depends on the design o the window treatment. Without treatment, the majority o this load alls on the foor, as shown in Figure 35a . Shades at windows will reduce the amount o energy transerred to the space as the shades will absorb and refect the energy. Some o the energy wil l become a conectie energy gain outside o the occupied zone (Figure 35B). In the case o conduction and lighting loads, only a portion o the load remains in the occupied zone. Wall conduction, or example, shown in Figure 35c , will contribute a predicable amount o the heat to the occupied zone.Only the radiant component o oerhead lighting loads are considered because the conectie loads rom the lighting remains aboe the occupied zone by conecting directly to the upper zone. (Figure 35D). Radiant Eects The local surace temperature o objects within a space will cause occupants to either eel cooler or warmer, een i the space set point is constant. Cooling and heating can be eciently accomplished by using a radiant system in combination with a displacement entilation system. In this case the entilation air only needs to satisy entilation rates, and not cooling loads.
© Copyright E.H. Price Limited 2007.
1ft 1ft 6 ft
Figure 35: Exteral/Lightig Loads A
C
B
N O I T A L I T N E v T N E M E C A L P S I D
D
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
J-21
Displacement ventilation Desig Guide
Loadig Withi the Space People ad Equipmet Loads People and equipment transer heat to their surroundings by our heat transport methods: conduction, conection, radiation and eaporation. Each o which contribute to the heat gain o the space at dierent rates, as either sensible or latent loads. Conectie and radiatie heat transer rom a person are sensible heat gain to the space (Figure 36), while eaporatie heat transer are latent heat gains. ASHRAE 2005 Fundamentals Chapter 30, Nonresidential Cooling, and Heating Load Calculations, gie general heat load generated by people and equipment in arious states o actiities or both sensible and latent components.
Figure 36: Radiat ad Covective Portios o Heat Sources
Covectio
Sensible Heat The sensible heat gain to the occupied zone is only a portion o the total sensible load emitted rom the occupants. When using displacement entilation or cooling, only this portion is considered when sizing the air olume and supply air temperature.
Radiatio
People produce a conectie heat plume rom their bodies as they warm surrounding air as seen in Figure 37. The rate at which occupant heat is generated is dependent on seeral actors: • clothinglevels
Figure 37: Thermal Plume
• metabolicrate • environmentalconditions • activitylevel,etc.
ASHRAE 2005 Fundamentals Chapter 8, Thermal Comort, demonstrates the calculations or sensible heat generation rom people. A portion o conductie/conectie heat naturally transers to the upper zone, een without supply air. The radiation generated by the occupant is emitted to the space in all directions, with some radiating to the foor, walls, and ceilings. As a result only a portion o the sensible heat load need to be accounted or in a displacement entilation system. The actor applied to the total sensible heat gain to the space rom occupants is shown in the calculation section o this design guide. Latet Heat Unlike the sensible load, all o the latent load generated by people and equipment need to be accounted or in the air olume calculation. Eaporation rom occupants, humid air generated by certain equipment, and warm moist air exhaled by occupants all contribute to the space latent load. Control o the latent portion o the heat generated in the space is critical to controlling the relatie humidity o the space. For urther inormation on latent heat calculations see ASHRAE 2005 Fundamentals, Chapter 30, Nonresidential Cooling and Heating Load Calculations. N O I T A L I T N E v T N E M E C A L P S I D
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All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
Diuser Type A wide ariety o displacement air diuser types are aailable to suit the location restrictions and décor o a particular room or space. In some cases the diusers are custom abricated to meet an area’s unique architectural design.
Figure 38: Oe-Way Diuser
Figure 39: Three-Way Diuser
Figure 40: Wall Mouted
Figure 41: Recessed Diuser
Figure 42: Corer Diuser
Figure 43: Displacemet Liear Eclosure
Some common displacement diuser types are described as ollows: 1. Rectagular Uits Rectangular units are typically placed against a wall or partition. I only the ace o the unit is actie, a one-way pattern is produced as seen in Figure 38. I both the ace and sides are actie, a three-way pattern results (Figure 39). The three-way diuser has a higher air olume capacity than the one-way. Diuser inlets are usually at the top o the unit, although bottom or rear inlets are aailable.
One ersion o the rectangular units is designed to be integrated into the wall (Figure 40). A narrow plenum and rectangular inlet are characteristic o this design. Another ersion o a rectangular unit has no plenum or inlet and is designed or plenum eed applications. These units can be mounted in a stair riser, wall, cabinet, etc. and are supplied with a eld abricated plenum shown in Figure 41. 2. Corer Uits Corner units are specically designed to t into a 90° corner in a room. Supply i nlets can be located at the top or bottom o the unit. Flat or circular aces are aailable depending on the desired look. A 90° radial pattern is produced by corner units (Figure 42).These diusers are ideal or applications where wall space may be limited as corners are aailable or use. 3. Displacemet Liear Eclosure Linear enclosure can act as both a supply diuser and as a heating source. These enclosure are suited or perimeter locations. As a heating source they are designed so that the heating element does not interere with the air temperature or air fow patterns (Figure 43).
© Copyright E.H. Price Limited 2007.
N O I T A L I T N E v T N E M E C A L P S I D
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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Displacement ventilation Desig Guide
Diuser Type 4. Semi-circular Uits Shown in Figure 44 Semi-circular units are typically placed against a wall or pillar. Supply inlets can be located at the top, bottom or rear o the unit. A 180° radial pattern is produced.
Figure 44: Semi Circular Diuser
Figure 45: Circular Diuser
Figure 46: Roud Floor Grille
Figure 47: Liear Floor Grille
5. Circular Uits Circular units can supply high olumes o air to a space because the air is distributed in a complete 360° radial pattern (Figure 45). The supply inlet can be located at the top or bottom o the units. Circular units are typically placed ree-standing in large interior spaces such as halls, lobbies, walkways, lounges, etc. 6. Floor Mouted Uits Displacement diusers are aailable or integration with a raised foor distribution system. The round foor displacement unit produces a low elocity radial pattern across the foor as seen in Figure 46.
The foor mounted grilles can proide a li near pattern rom the grille ace. Displacement foor grilles can also be an assisted (Figure 47) when additional air olumes are required and a an terminal is not economical. 7. Idustrial Diusers For the industrial enironment diusers need to be able to withstand impact rom moing equipment, or able to be mounted aboe the working space and designed to supply air deep into the space (Figure 48). The Price industrial fat diuser is intended to be placed on the industrial foor space and proide supply air.The robust design allows this diuser to withstand the impact orces common to the industrial sector.The Krantz line o industrial diusers are designed to be mounted aboe the occupied zone, and hae integrated heating and cooling supply air modes.
Figure 48: Idustrial Diuser
N O I T A L I T N E v T N E M E C A L P S I D
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© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
Diuser Layout ad Locatio Due to the ariety o diuser types aailable, displacement outlets can be mounted in numerous locations and congurations.
Figure 49: Wide Rooms
The ollowing are some general recommendations or supply diusers. •Rectangular or semi-circular unitsare oftenlocated onwalls opposite to the exterior windows and walls. •Forlarge rooms wider than 30 feet, consider mounting the diuser on two opposite walls as seen in Figure 49 . •Forroomslongerthan15feet,considerseveraldiffusersalong the wall per Figure 50A. •Forlargeopenspaces,roundorrectangulardiffuserscanbe placed in the mid o the space (Figure 50B). •Placediffusersnocloserthan2feetfromoccupantsasshown in Figure 51.
30 ft
•Avoidplacinglargeobstaclesnearthediffuserface. •P lace more diffusers in areas which have a higher cooling load.
When ducting rom below a diuser it is important to supply the diuser with a base or easy connection to the diuser.
Figure 50A: Log Rooms
When mounting displacement diuser; in the ceiling or at an eleated location, it is i mportant to locate these aboe aisle ways or along perimeters. Placing a diuser directly oer an occupant will lead to occupant discomort. Locating the diusers along perimeters will help to reduce the heat gain and entrainment o pollutants as the air passes down through the stratied layers.
Figure 50B: Large Ope Rooms
When mounting displacement diusers along the walls it is important to proide support, due to the weight o the outlets. In installations where the ductwork is supplied rom aboe the diuser and needs to be hidden, ensure that the coer will properly conceal the ductwork. Perorated coers may require the ductwork to be painted to conceal it completely. When supplying displacement air to a room with a sloped foor, or ramp, place more o the diusers at the upper leel o the space. The cool air will want to fow down to the lower.
15 ft
Figure 51: Distace rom Diusers
Regarding return air outlets, it is essential that they be placed at high leels either on the wall or in the ceiling. The same model types or mixing systems would apply to displacement systems. Locating the return air outlets aboe strong heat sources such as windows will ensure the ecient remoal o the heat and contaminants generated by the thermal plume. Product Tip To conceal ductwork rom the ceiling to a foor mounted diuser, a duct coer may be used.These coers are designed to match the look o the diuser or a consistent architectural nish.
© Copyright E.H. Price Limited 2007.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
N O I T A L I T N E v T N E M E C A L P S I D
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Displacement ventilation Desig Guide
DV Supply AIr Methods Ducted Coectio
Figure 52: Ducted Coectio
The most common method to supply air to a displacement diuser is ia a ducted connection. Diusers can be connected rom the top, bottom, and sides depending on the unction and design o the diuser. Product Tip When selecting a bottom duct diuser ensure that the diuser has a base i the product is foor mounted. This will make the diuser easier to install in the eld.
Balancing dampers required or diusers should be mounted at least 3 duct diameters away rom the inlet connection on the diuser, in a ducted conguration. Product Tip When selecting a bottom duct diuser and a balancing damper is required, special designs may be required to accommodate the balancing o the diuser. Allow or a base where possible.
Figure 53: Pressurized Pleum
Pressurized Pleum:
When utilizing a pressurized plenum with a displacement diuser, ensure that the plenum is properly sealed . Adantages o using a pressurized plenum are reduced ductwork easier bal ancing and quicker installation. For urther inormation on pressurized plenum designs see the UFAD design guide. Product Tip The DF1R, the DLE/DLE-H and the ARFHD all require pressurized plenums, they do not come standard with ducted connections.
Compoet Selectio ad Istallatio Figure 54: Typical Wall Moutig Compoet Selectio
Displacement diusers designed or the commercial sector hae a recommended maximum ace elocity o 40 eet per minute to ensure comort in the space. In transitional spaces such as lobbies, 50 eet per minute ace elocities are acceptable.
N O I T A L I T N E v T N E M E C A L P S I D
The air olume, return air temperature, and supply air temperature are calculated alues based on the room dynamics, see the Calculation section or ull equations. The type o di user is typically selected to match architecture or other space constraints. See the DiuserType section or summary descriptions o the diusers oered by Price.
Mounting Plate
H Distance X X From Floor to Screw Location
Figure 55: DF1R Istallatio
Istallatio
Generally foor mounted diusers are proided with wall mounting strips, the DR360 is proided with a foor mounting ring. (Figure 54) The DF1W installs into a engineered ducted plenum, proided with the diuser. The DF1R displacement diuser is designed to t into a pressurized plenum. Mounting fanges are proided or a tamper proo installation as shown in Figure 55.
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© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
Displacemet Vetilatio Air Volume Calculatios Coolig Flow Rate Due to the higher supply air temperatures inherent with displacement entilation, it is oten assumed that the cooling supply air fow rate will be signicantly greater when compared to a traditional oerhead mixing system. Howeer, since a proportion o the heat sources in a room are exhausted directly without impacting the occupied space, displacement entilation fow rates can be equal or een lower than mixing systems, depending on room layout and space type (Figure 56).
Figure 56: Coolig Loads
Conduction and Solar
Due to stratication, each heat source will hae a dierent eect upon the loads in the space. Since some heat sources are aboe the occupied zone, we can include a l oad actor or their contribution to the total space load. Typical Heat sources hae both radiant and conectie components so it is important to assign loads to the occupied zone and the upper zone, depending on the load type.
Overhead Lighting
Occupant and Equipement
In determining the air olume requirements or an all air displacement entilation system the ASHRAE Design Guide has deeloped 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,Q oe (Btu/h). • Theoverheadlighting,Q l (Btu/h). • Theheatconductionthroughtheroomenvelopeandtransmitted solar radiation, Qex (Btu/h).
Such that, the total cooling load is:
Actual zone air fow rate is the maximum o the cooling air fow and the entilation rate. The supply air temperature is calculated rom:
Qt = Qoe + Ql + Qex
Load actors or the aboe catagories hae been determined by ASHRAE research project RP-949. • Occupants,desklampsandequipment,a oe = 0.295 Approximately 1 / 3 o this cooling load enters the space between oot and head leel. The other 2 / 3 enters the upper space ia the thermal plume and radiation. • Overheadlighting,al = 0.132 Less than 15% o the total lighting load is radiated to the occupied space. • Heatconductionandsolarradiation,a ex = 0.185 More than 80% o the external loads enter the upper space ia the thermal plume and radiation.
Based on the coecients aboe, the ASHRAE Design Guide lists the ollowing equations or determining summer cooling fow rates or a typical oce space.
Where the temperature dierence between head and oot is gien by:
and:
Using αr and αc = 0.95 BTU/h*t²*°F rom ASHRAE undamentals. The exhaust air temperature can be calculated rom:
Determination o the required air fow rate or summer cooling: Where: • ρ is the air density (lb/t³),
Determination o the entilation rate:
• Cpisthespecicheatoftheairatconstantpressure(BTU/lb•°F), • Histheheightoftheceiling(ft),
vr is determined rom ASHRAE Standard 62-2004 based on room application. Local codes may not allow the discount or the entilation eectieness, or may hae stricter requirements. Reer to ASHRAE Standard 62.1 or recognized alues o entilation eectieness.
• Aistheareaofthespace(ft²), • ΔTh is the temperature dierence rom heat to oot leel (°F), • η 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), • Θ is a dimensionless temperature, • Te is the exhaust air temperature (°F)
© Copyright E.H. Price Limited 2007.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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N O I T A L I T N E v T N E M E C A L P S I D
Displacement ventilation Desig Guide
Displacemet Vetilatio Air Volume Calculatios Depending on the application, these ormulae can be used in arious combinations. A ull calculation is possible using the aboe ASHRAE ormulae, but at least one or two o the ollowing alues must be predetermined, usually set by codes, standards, or experience: n , v, ΔTh, Tsp, or Ts.
V h = 0.076 Qoe + 0.034 Ql + 0.048 Qex 3.6
Typically we suggest using ΔTh=3.6 or people in a seated position as per ASHRAE Standard 55-2004, but this alue can change i the occupants are not seated. Standard air has a density, ρ = 0.075 lb/t² and Cp = 0.24 BTU/lb*°F. Using these alues, we can reduce the equations to the ollowing:
A• Qt 2.33V 2 + 1.8 A•V
Desig Procedure The ollowing step by step desig n procedure is oered as a simplied approach to determine entilation rate and supply air temperature or typical displacement entilation applications. The procedures presented are based on the ndings o ASHRAE Research Project RP-949 and the procedure outlined in the ASHRAE Design Guide. For a complete explanation and deriation o the assumptions and equations used to deelop this procedure, please reer to the ASHRAE Design Guide.The design procedure applies to typical North American oce spaces and classrooms. These procedures should be used with care when applied to large spaces such as theaters or atria, a computational fuid dynamic analysis (CFD) o large spaces is recommended to optimize the air supply olume. Only the sensible loads should be used or the preceding calculations. These calculations are only or determining the airfow requirements to maintain the set point in the space, the total building load 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 to determine the design cooling load o the space in the summer. I possible, assume a 1°F/t. ertical temperature gradient in the space in the computer simulation as the room air temperature is not uniorm with displacement entilation. Itemize the cooling load into the ollowing categories:
Step 3: Determie Flow Rate o Fresh Air
Standard 62-2004 ventilation Rate Procedure includes deault alu es or entilation eectieness. From standard 62-2004: Equation 6-1 is used to determines the Breathing Zone Outdoor Air Flow vbz and Equation 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 Rate
Choose the greater o the required fow rate or summer cooling and the required entilation rate as the design fow rate o the supply air,
Step 5: Determie Supply Air Temperature
The supply air temperature can be determined rom the ASHRAE Design Guide equations and simplied to:
• Theoccupants,desklampsandequipment,Q oe (Btu/h) • Theoverheadlighting,Q l (Btu/h) • Theheatconductionthroughtheroomenvelopeandtransmitted solar radiation, Qex (Btu/h). N O I T A L I T N E v T N E M E C A L P S I D
Step 2: Determie the Coolig Load Vetilatio Flow Rate, V h
The fow rate required or summer cooling, using standard air, is:
V h = 0.076 Qoe + 0.034 Ql + 0.048 Qex
Step 6: Determie Exhaust Air Temperature
The exhaust air temperature can be determined by the ollowing method:
Step 7: Selectio o Diusers
The goal is to maximize comort in the space and minimize the quantity o diusers. At a maximum, ASHRAE suggests a 40 pm ace elocity, but this alue may increase or decrease depending on the space and comort requirements. A CFD simulation can alidate the design and is recommended or l arger spaces, contact your local Price representatie about CFD modeling.
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Displacement ventilation Desig Guide
Small Oce Example Space Desig
The owner o an oce building is renoating and would li ke to consider using displacement entilation in the oce areas. This example examines 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 rom 13:00 to 17:0 0.The space is designed or 2 occupants, a computer with LCD monitor, T8 forescent lighting, and has a control temperature o 72°F. 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 with wall 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 the maximum combination o elocity and temperature in the occupied zone, PPD due to drat, as well as the stratication in the space. In an oce, 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/ft² • Computerloadis308BTU/h(CPUandLCDMonitor) • Conductionthroughthewindowandwallis5BTU/h/ft² • Thespecicheatanddensityoftheairorthisexamplewillbe0.24BTU/lb°Fand0.075lb/ft³respectively.
Occupants
2
Set Point
72 °F
Floor Area
120 t²
Exterior Wall Area
90 t²
volume
1080 t³
Qoe
1012 BTU/h
Ql
819 BTU/h
Qex
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 t² X 6.82 BTU/h/t² = 819 BTU/h Qex = 90 t² X 5 BTU/h/t² = 450 BTU/h QT = 2077 BTU/h Total cooling load or this space (QT) is 2077 BTU/h, and approximately 17.31 BTU/h/t². N O I T A L I T N E v T N E M E C A L P S I D
ASHRAE Standard 62-2004 requires 0.06 CFM/t² outdoor airfow rate required per unit area, Ra, and 5 CFM/Person outdoor airfow rate required per person, Rp, be deliered to the space or moderately actie oce work appli cations. For displacement entilation a entilation eectieness, or zone air distribution eectieness (Ez), assumed to be 1.2 (table 6-2, ASHRAE Standard 62-2004).
© Copyright E.H. Price Limited 2007.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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Displacement ventilation Desig Guide
Small Oce Example Determie the Airfow Rate to meet the Coolig Load
Determie the Fresh Air Flow Rate ad Breathig Zoe Vetilatio Eectiveess
Note: Some local codes may not allow the discount or vE, or may hae stricter requirements, and they should be used instead o this calculation. Example: Title 24 i n Caliornia. The total supply air olume or cooling is then the maximum alue between vh and vr.
Calculate the Supply Air Temperature
Determie the Retur Air Temperature
N O I T A L I T N E v T N E M E C A L P S I D
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Displacement ventilation Desig Guide
Small Oce Example Selectio o Diusers
For this application we are limited to wall mounted or corner diusers at the request o the owner. Traditional displacement diusers are limited to 40 pm ace elocity in standard commercial applications in order to meet comort criteria. With a supply air rate o 111 CFM and a ace elocity o 40 pm, 2.78 t² o diuser ace area is required. For the DF1W, DF1R or DF1C a 24”x18”diuser will proide a ace area o 3 t². A DR90 unit with an 18” di ameter and 30” tall will proide a ace area o 2.94 t². Layout o the Oce
The corner diusers could be placed in any o the corners to supply this room, as long as the occupant i s not within 2 eet o the diuser ace. The wall diusers can be placed on any o the walls in the room, again ensure that sedentary occupants will be at least 2 eet rom the diuser.
Flow Visualizatio
A CFD analysis was run or this example using the conditions, calculated airfow and supply air temperature or the small oce with the DF1W to gie a isual representation o the temperature distribution, air moement, and drat temperatures in the space. Figure 57 shows the temperature proles across the space. The DF1W produces the predicted temperature stratication in the space. Also isible are the heat plumes o o the occupants and computer.The seated occupant is experiencing ambient air temperatures rom 69º to 72º F, and the standing occupant 69º to 75º F. Both are within the thermal stratication comort conditions set by ASHRAE. Figures 58 and 59 depict the elocity proles. From the elocity prole images, slow moing air is isible throughout the space. The images also show the plumes o o the occupants and computer as well as the general shape o the air pattern leaing the diuser. Figure 60 shows the predicted drat temperature or the space. The range in which people will eel the most comortable, is indicated in green. The DF1W diuser seems to produce a thermally comortable enironment, and reiterates that occupants need to be located at an appropriate distance rom the diuser. Figure 57: DF1W Temperature Prole
Figure 58: DF1W Velocity Prole
Figure 59: DF1W Velocity Prole
Figure 60: DF1W Drat Temperature N O I T A L I T N E v T N E M E C A L P S I D
© Copyright E.H. Price Limited 2007.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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Displacement ventilation Desig Guide
Boardroom Example Space Desig
The owner o a new oce building wants to use a displacement entilation system or all occupied spaces. This example examines a priate boardroom that is located in the center o the building without any exterior suraces. The space is designed or 8 occupants, a computer with LCD monitor, a proj ector,T8 forescent lighting, and has a control temperature o 72°F. The room is 24 t wide, 14 t long, and 10 t rom foor to ceiling. There is a large w hite board at the west end o the room and cabinets along the south and east end o the room. The owner and architect want the displacement diusers in the space to t seamlessly into the room. Space Cosideratios • TheheadtofootgradientrecommendedbyASHRAEis3.6°Ffromheadtofootforseatedoccupants. • Someoftheassumptionsmadeforthespaceareasfollows: • Loadperpersonis250BTU/h • Lightingloadinthespaceis6.82BTU/h/ft² • ComputerandLCDloadis308BTU/h • Projectorloadis188BTU/h • Thespecicheatanddensityoftheairorthisexamplewillbe0.24BTU/lb°Fand0.075lb/ft³respectively.
Occupants
8
Set Point
72 °F
Floor Area
336 t²
volume
3360 t³
Qoe
2496 BTU/h
Ql
2294 BTU/h
Qex
0 BTU/h
QT
4790 BTU/h
The loads are broken down as ollows: Qoe = (8 People X 250 BTU/h) + 308 BTU/h + 188 BTU/h = 2496 BTU/h Ql = 336 t² X 6.82 BTU/h/t² = 2294 BTU/h Qex = 0 BTU/h QT = 4790 BTU/h Total cooling load or this space (QT) is 4790 BTU/h, and approximately 14.26 BTU/h/t².
N O I T A L I T N E v T N E M E C A L P S I D
ASHRAE Standard 62-2004 requires 0.06 CFM/t² outdoor airfow rate required per unit area, Ra, and 5 CFM/Person outdoor airfow rate required per person, Rp, be deliered to the space or moderately actie oce work applications. For displacement entilation a entilation eectieness, or zone air distribution eectieness (Ez), is assumed to be 1.2 (table 6-2, ASHRAE standard 62-2004).
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© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
Boardroom Example Determie the Airfow Rate to meet the Coolig Load
Determie the Fresh Air Flow Rate ad Breathig Zoe Vetilatio Eectiveess
Note: Some local codes may not allow the discount or vE, or may hae stricter requirements, and they should be used instead o this calculation. Example: Title 24 in Caliornia. The total supply air olume is then the maximum alue between vh and vr.
Calculate the Supply Air Temperature
Determie the Retur Air Temperature
N O I T A L I T N E v T N E M E C A L P S I D
© Copyright E.H. Price Limited 2007.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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Displacement ventilation Desig Guide
Boardroom Example Selectio o Diusers
For this application we hae three goals set by the owner: 1. Quiet operation 2. Thermal comort to the space 3. Diusers must be hidden Inherently, displacement entilation diusers are quiet, but care has to be taken to limit the sound generated rom the HvAC air supply. Price recommends limiting the duct elocity to 1200 pm, to minimize noise rom ductwork. For thermal comort a ace elocity o 40 pm is required. At 126 CFM a diuser ace area o 6.68 t² would be required. To make these diusers as unobtrusie as possible there a two options; mount them in the wall, or as part o the urniture. Layout o the Boardroom
For a concealed look, the DF1R displacement diuser could be installed at the base o the cabinets, or in the wall under the whitebo ard in a pressurized plenum . Two diusers at 60”x8” will be able to meet the 40 pm requirement. The diusers can be placed on any o the walls in the room, but they must ensure that sedentary occupants will be located at least 2 eet rom the diuser.
Flow Visualizatio
A CFD analysis was run or this example using the conditions, calculated airfow and supply air temperature or the small oce with the DF1W to gie a isual representation o the temperature distribution, air moement, and drat temperatures in the space. Figure 61 shows the temperature proles across the space and a reasonable temperature stratication is predicted. Also isible are the heat plumes o o the occupants and computer. The seated occupant is experiencing ambient air temperatures rom 69º to 72º F, and the standing occupant 69º to 75º F. Both are within the thermal stratication comort conditions set by ASHRAE. Figures 62 depict the elocity prole. The images show the plumes o o the occupants and computer as w ell as the general shape o the air pattern leaing the diuser and slowly entering the zone. Figure 63 depicts the drat temperature or the space. Again, the range in which people will eel the most comortable is indicated in green. The DF1R diusers produce a thermally comortable space.
Figure 61: DF1R Temperature Prole
N O I T A L I T N E v T N E M E C A L P S I D
Figure 62: DF1R Velocity Prole
Figure 63: DF1R Drat Prole
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© Copyright E.H. Price Limited 2007.
SECTION J:
DISPLACEMENT VENTILATION
SPECIAL APPLICATION SUPPLEMENTAL
Machine Shops Schools Hospitals
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Displacement ventilation Desig Guide—Special Supplemetal
Idustrial Displacemet Vetilatio Idustrial Applicatios Displacement ventilation is i deally suited or the industrial sector. In act, modern displacement entilation was rst used in actories o Europe. The increased entilation eectieness and inherent pollution control o displacement entilation ensures a clean breathing zone or occupants.
Figure 64: Heavy Pollutat Cocetratio
Diuser Locatio In most industrial applications, the foor plan is set up with a certain degree o fexibility. As the enironment eoles, there is oten the need to recongure the space. With this in mind, it is adantageous to mount displacement diuser s beside permanent xtures such as roo supports and o o the foor in order to acilitate reconguratio n and to protect the diusers rom damage rom orklits, equipment, and materials moing throughout the acility. Weight o Pollutats In industrial enironments with pollutants that are lighter than air, the same principles apply as in the commercial application o displacement entilation, where the supply diusers are typically supplying at a low leel and returns are located high within the space.
For enironments with pollutants that are heaier than air, such as carbon monoxide (CO), Ozone (03), some paint vOCs, there is the possibility that these pollutants can collect in the lower portion o a space (Figure 64). It is recommended that diusers supply entilation rom aboe the occupant and exhaust is located at a low leel. The low leel exhaust will sere as an outlet or these gasses. When a low leel return is used, diuser placement is critical in an eort to reduce short circuiting. The displacement diuser should be mounted aboe the occupied zone so that supply air has a chance to trael through the breathing zone and the occupied zone and carries with it some pollutants. Figure 65 shows a layout or an automotie garage. An automotie garage may hae umes rom exhaust which linger due to their density. In this example, there are outlets at a high leel as is typical o displacement entilation but there is also an outlet near the foor leel to exhaust any umes being stored.
Figure 65: Pollutat Extractio
Diuser Selectio Unlike commercial displacement entilation diusers, the industrial diuser is selected based on adjacent zone.The object is to proide as much quality air to the occupants and ensure stratication , while still ensuring a comortable space. Adjacent zones should not oerlap, and should come as close to each other as possible, as the goal is to proide resh air eenly throughout the space. The diusers are not limited by the 40 pm ace elocity, and hae a wide range, depending on the diusers unctionality. N O I T A L I T N E v T N E M E C A L P S I D
J-36
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide—Special Supplemetal
Machie Shop Example Space Desig
A machine shop owner has outgrown his existing operations and is renoating an older building. To proide good internal air quality and sae on energy the owner wants to supply air in the shop with displacement entilation. This example examines the shop foor in this space. The shop foor is designed or 35 occupants, 40 machines, Mercury vapor Light Fixtures, and has a control temperature o 75°F. The space is 88 t wide, 90 t long, 15 t rom foor to ceiling. The owner doesn’t want any o the diusers to take up foor space, but will allow diusers to be placed on columns, and wants the diusers to operate in both heating and cooling modes. Space Cosideratios
In this machine shop, pollutants generated in the space are lighter than air, so the exhausts need to be located high in the space and the diusers in or near the occupied zone. with the assumptions made or the space are as ollows: • Loadperpersonis375BTU/h • Lightingloadinthespaceis10.234BTU/h/ft² • Machineloadis: • 6x5-axisCNC’sat9435BTU/heach • 4xHorizontalturningmill’sat7609BTU/heach • 5xVerticalmill’sat7636BTU/heach • 5xLatheat6210BTU/heach • 10xDrillat6439BTU/heach • 10xSawat3388BTU/heach • The“U”valuesforthewallis.045BTU/h/ft²•°F,andfortheroofis.031BTU/h/ft²•°F • Thespecicheatanddensityoftheairorthisexamplewillbe0.24BTU/lb°Fand0.075lb/ft ³ respectiely.
Occupants
35
Set Point
75 °F
Floor Area
7920 t²
Exterior Wall Area
2670 t²
volume
118800 t³
Qoe
314591 BTU/h
Ql
81053 BTU/h
Qex
6582 BTU/h
QT
402226 BTU/h
The loads are broken down as ollows: Qoe = (35 People X 375 BTU/h) + (33880 + 64390 + 31050 + 38180 + 30436 + 56610) BTU/h = 267671 BTU/h Ql = 7920 t² X 10.234 BTU/h/t² = 81053 BTU/h N O I T A L I T N E v T N E M E C A L P S I D
Qex=(2670ft²X.045BTU/h/ft²•°FX18°F)+(7920ft²X.031BTU/h/ ft²•°FX18°F)=6582BTU/h QT = 355306 BTU/h Total cooling load or this space (QT) is 355306 BTU/h, and approximately 44.86 BTU/h/t². ASHRAE Standard 62-2004 requires 0.18 CFM/t² outdoor airfow rate required per unit area, Ra, and 10 CFM/Person outdoor airfow rate required per person, Rp, be deliered to the space machine shop applications. For displacement entilation a entilation eectieness, or zone air distribution eectieness (Ez), assumed to be 1.2 (table 6-2, ASHRAE standard 62-2004).
© Copyright E.H. Price Limited 2007.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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Displacement ventilation Desig Guide—Special Supplemetal
Machie Shop Example Determie the Airfow Rate to meet the Coolig Load
Determie the Fresh Air Flow Rate ad Breathig Zoe Vetilatio Eectiveess
Note: Some local codes may not allow the discount or vE, or may hae stricter requirements, and they should be used instead o this calculation. Example: Title 24 i n Caliornia. The total supply air olume or cooling is then the maximum alue between vh and vr.
Calculate the Supply Air Temperature
Determie the Retur Air Temperature
N O I T A L I T N E v T N E M E C A L P S I D
J-38
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide—Special Supplemetal
Machie Shop Example Selectio o Diusers
The DR360i-HC will be the best choice or this space, as it can perorm the heating and cooling, it can be mounted aboe the occupied zone and along columns. Heat cool changeoer may be accomplished by an actuator, controlled though the BMS or by Bowden cable. Unlike the commercial applications, the ace elocity is not limited to the 40 pm ace elocity. Adjacent Zone (Cooling)
Placed 3 m above groud: Coolig Unit Size (Face Area)
Face Vel pm
Airfow cm
Total Pressure in WG
Static Pressure in WG
SPL dBA
∆T = 18°F
10 [6.5 ft2]
75 100 125 150 75 100 125 150 100 125 150 175 150 175 200 225
488 650 813 975 870 1160 1450 1740 1470 1838 2205 2573 3090 3605 4120 4635
0.214 0.386 0.608 0.882 0.156 0.281 0.445 0.645 0.214 0.329 0.467 0.628 0.249 0.339 0.441 0.557
0.164 0.297 0.470 0.683 0.115 0.208 0.330 0.480 0.171 0.262 0.370 0.496 0.198 0.269 0.350 0.442
48 56 62 67 45 52 59 64 53 58 62 66 56 60 64 67
Note 1 Note 1 Note 1 Note 1 Note 1 Note 1 Note 1 Note 1 Note 1 Note 1 Note 1 Note 1 Note 1 Note 1 Note 1 Note 1
14 [11.6 ft2]
18 [14.7 ft2]
25 [20.6 ft2]
t.
Adjacent Zone (Heating)
Placed 3 m above groud: Heatig Unit Size (Face Area)
Face Vel pm
Airfow cm
Total Pressure in WG
Static Pressure in WG
SPL dBA
∆T = 5°F
∆T = 10°F
t.
t.
14 [11.6 ft2]
50 75 100 125 75 100 125 150 100 125 150 175
580 870 1160 1450 1103 1470 1838 2205 2060 2575 3090 3605
0.068 0.156 0.281 0.445 0.123 0.214 0.329 0.467 0.112 0.174 0.249 0.339
0.050 0.115 0.208 0.330 0.099 0.171 0.262 0.370 0.089 0.138 0.198 0.269
36 45 52 59 47 53 58 62 47 52 56 60
16 28 32 45 27 38 45 52 32 37 42 47
<6 21 26 37 19 31 36 42 24 29 36 41
18 [14.7 ft2]
25 [20.6 ft2]
Perormance Notes: 1. The maximum supply air penetration depth or foor mounted diusers when cooling largely depends on the number and intensity o the heat sources. Under normal circumstances a max supply air penetration o 32 t with 10 inch unit and 82 t with 25 inch unit.
In this example, diusers are placed on the columns, to keep the foor area clear.The entire foor surace must be sericed by the diusers, so an appropriate adjacent zone must be selected. To proide the proper foor coerage an adjacent zone o 20 eet is selected. The 14” diameter diuser proides the 20 eet adjacent zone or the smallest pressure drop and noise leel at 1165 CFM. For the 14025 CFM the space requires a total o 12 diusers, each with a supply air olume rate o 1169 CFM. For urther instructions on heating mode operation see the industrial diusers section o this design guide. Layout o the Shop
The diusers should be mounted on columns eenly throughout the space about 9 eet aboe foor leel. For the diusers with heat/ cool changeoer, the Bowden cable should be mounted i n close proximity to the diuser and needs to be accessible to the occupants. As with commercial applications, returns should be located as high as possible in the space.
© 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-39
N O I T A L I T N E v T N E M E C A L P S I D
Displacement ventilation Desig Guide—Special Supplemetal
Machie Shop Example Flow Visualizatio Airfow in the machine shop can be isualized through a CFD simulation or physical mock-up. A CFD simulation can ensure that the outlet locations, sizes and quantities are reasonably selected and help ensure that the space is optimized or energy perormance and thermal comort.
Figure 66: Temperature Prole
A CFD analysis was run or this example using the conditions and calculated airfow and supply air temperature. Figure 66 and 67 show the temperature proles throughout the space. The space is stratied and the design set point o 74 was met. Figure 68 shows the temperature surace o 74 colored or elocity, to show how the air is deliered rom the diusers to the occupied zone. Figure 69 shows the elocity prole o the space, the space doesn’t show jets or major moements o air. Figure 70 shows the drat temperature. With the temperatures and elocities in the space, there are not any areas within the occupied zone where the drat temperature is excessie.
Figure 67: Temperature Prole
Figure 68: Diuser Airfow Patter
Figure 69: Velocity Prole
Figure 70: Drat Temperature Prole
N O I T A L I T N E v T N E M E C A L P S I D
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All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide—Special Supplemetal
Displacemet Vetilatio ad Schools High Perormace Schools
As operating costs or schools continue to increase, there is an increased demand to construct schools that are more resource ecient, sustainable, and hae improed learning and health benets. As this demand increased, seeral studies hae been conducted and organizations ormed to meet the need or inormation and establish baselines. Most notably the Collaboratie or High Perorming Schools (CHPS) was initiated in Caliornia, based on the USGBC LEED® Program, to meet the needs o school design. The CHPS program is a sel-certication and recognition program designed to “acilitate the design o high perormance schools: enironments that are not only energy ecient, but also healthy, comortable, well let; and containing the amenities needed or a quality education”. Gree Tip Using displacement entilation or schools is a great way to increase the entilation eectieness in a classroom. CHPS credit EQ2.1:Thermal Displacement ventilation gies two credit points or the use o displacement entilation in the building.
Beets rom a Displacemet Vetilatio System
As described throughout this design guide, there are seeral benets to applying a displacement type system to commercial spaces. The major benets are: • Longer “Free Cooling” periods due of higher supply air temperatures • LowerSystemPressureRequirements • Higherindoorairquality • Improvedthermalcomfort • Lownoisegenerationfromdiffusers
These benets correlate to the cost o running a bui lding as well as the health and well being o the occupants. In certain markets, the longer ree cooling periods allow or an economizer to run longer during the day, leading to a lower operating equipment cost o the building. As well, the lower system pressure can allow or a reduction in the size o the an or motor driing the an to that zone, which will lead to lower energy consumption or the building during operation. In a displacement entilation system, the natural thermal plumes o each occupant drie the moement o air, and each person is deliered the amount o cooling that they require.Because this air has not been mixed with room air the air moing up the occupant is cool and unpolluted. This results in a stratied enironment, with resh, cool air at the lower portions and warm, polluted air a high leel in the space. This leads to the high i ndoor air quality, as the occupants thermal plumes dictate the required amount o air deliered to the occupant. These benets hae been translated into cost saings, in a 2006 report by Gregory Kats at Capital E, the thermal comort and air quality o schools were presented as a dollar and perormance alue. Furthermore, this report collected the ndings o arious researchers demonstrating the reduction in transmission o airborne illnesses. Figures 72 and 73 show these reductions by increasing the entilation rate and applying pollutant source controls, respectiely.
Figure 71: Airfow Patter with Displacemet Vetilatio
Table 4: Applicability o Displacemet Vetilatio Applicable Spaces Classrooms Library Multi-Purpose/ Caeteria Gym Corridors Administration Toilets Other
Climates South Coast North Coast Central valley Mountains Desert
Figure 72: Icreased Outdoor Air 100%
s m o t p m y S n i n o i t c u d e R / t n e m e v o r p m I %
87.3% Flu
90% 80%
67.0% SBS
70% 60% 50%
46.0% Resipatory
40%
35.0% SBS
33.6% SBS
33.0% SBS
30% 20.0% Resipatory
20% 10%
Drinka et al 1996
Jaakkda Brundage Fisk & Bourbeau & et al Rosendfeld et al Miettinen 1985 1995 1997 1995
Sundell 1996
Fisk & Rosenfeld 1995
Figure 73: Pollutat Source Cotrols 100%
s m o t p m y S n i n o i t c u d e R / t n e m e v o r p m I %
90%
N O I T A L I T N E v T N E M E C A L P S I D
85.0% Colds
80% 70%
61.5% Asthma, Allergies
60%
47.0% SBS
50% 40% 30%
23.6% Asthma
21.4% Asthma, Mucosal
Wieslander et al 1997
Jaakkda et al 1994
20% 10%
Beijing Residence Liu et al 1996
© Copyright E.H. Price Limited 2007.
Whe to Cosider DV Programming Schematic Design Deelopment Contract Docs. Construction Commissioning Operation
Australian Wargocki Residence 1998 Liu et al 1996
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
J-41
Displacement ventilation Desig Guide—Special Supplemetal
Displacemet Vetilatio ad Schools With the increase o indoor air quality, there is a positie impact on health, most notably a reduction in asthma complications, fu, sick building syndrome, respiratory problems, and headaches. An increase to the IAQ o a school has been shown to reduce the incidents inoling Asthma by 25% and the reduction o cold and Flu cases by 51% on aerage. The reduced time that teachers and onsite health care workers spend attending to sick children translates directly into costs.The time that parents take o to proide care to their children can be translated into costs as well. With a teacher away or ewer days due to sickness, there is a reduced need or substitute teaching sta. With a reduction o sick day taken requirements in a green school, there is an estimated saings o $2/t² (Table 5). The increased IAQ led to an increase in student test scores, and teacher retention, while reducing absenteeism. Green schools hae been shown to increase aerage test scores by 3-5% and reduce teacher turn-oer rates by 5%.
Table 5: Fiacial Beets rom Gree Schools [Source: Capital E] Fiacial Beets o Gree Schools ($/t²)
Energy Emissions Water and Wastewater Increased Earnings Asthma Reduction Cold and Flu Reduction Teacher Retention Employment Impact TOTAL COST OF GREEnInG nET FInAnCIAL BEnEFITS
$9 $1 $1 $49 $3 $5 $4 $2 $74 ($3) $71
Figure 74: DFXi
Other studies hae shown that an increase to the entilation rate or an oerhead mixing system can improe the speed at which students complete work without an increase in the amount o errors produced rom students. The increase in entilation rate is related to the entilation eectieness o the mixed system, shown here. Eq. 6.2 rom ASHRAE Standard 55-2004
Rp = outdoor airfow rate required per person by space Pz = zone population Ra = outdoor airfow rate required per area by space Az = zone area Ez = zone air distribution eectieness voz is the required outdoor air fow rate The zone entilation eectieness is approximately 1.0 or an ideal oerhead air distribution system, and conseratiely estimated to be 1.2 or a displacement entilation. A displacement entilation system will require at least 20% less outdoor air than a oerhead mixing system, so a system designed with displacement entilation will require less outdoor air to increase the perormance within schools.
N O I T A L I T N E v T N E M E C A L P S I D
Product Tip The DFXi Industrial Displacement Diuser , shown in Figure 74 is a more robust diuser and can be incorporated into a commercial design. This is desirable in spaces such as gymnasiums and high trac areas such as hallways and oyers. See the industrial displacement entilation product section or urther detail.
The low noise leels generated by a displacement system is also a desirable eature or school applications. Spaces such as auditoriums, band rooms, libraries or any space that demands acoustical perormance are the perect application or this quiet system.
J-42
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide—Special Supplemetal
Classroom Example The Space Desig
The local school board is looking or ways to improe the IAQ o the classrooms, as is has been shown to increase student perormance and reduce absenteeism.They want to use displacement entilation in the spaces due to a recognized increase in entilation eectieness while ensuring comort within the rooms. This example examines a typical classroom in this school. The classroom is designed or 25 children and 1 teacher, three computers with LCD monitors, T8 forescent lighting, and has a control temperature o 74°F. The room i s 25 t wide, 30 t long, and 10 t rom foor to ceiling leel. There is one exterior wall, acing NW, with 100 t² o window, and the room has an exposed ceiling. The Space Cosideratios
As preiously discussed, ASHRAE standard 55-2004 stipulates the maximum combination o elocity and temperature in the occupied zone, ppd due to drat, as well as the stratication in the space. In a classroom, the occupants tend to be seated throughout the majority o the day; the stratication or a sedentary seated person according to ASHRAE standard 55-2004 is 3.6F. Some o the assumptions made or the space are as ollows: Load per person is 250 BTU/h Lighting load in the space is 6.826 BTU/h/t² Computer loads are 308 BTU/h each Aerage Solar/Conduction load through the exterior wall is 14.6 BTU/h/t² The specic heat and density o the air or this example will be 0.24 BTU/lb°F and 0.075 lb/t³ respectiely Occupants
26
Set Point
74 °F
Floor Area
750 t²
Exterior Wall
300 t²
volume
300 t³
Qoe
7500 BTU/h
Ql
7424 BTU/h
Qex
4381 BTU/h
QT
17044 BTU/h
The loads are broken down as ollows: Qoe = (26 People X 250 BTU/h) + (3 Computers X 308 BTU/h) = 7424 BTU/h Ql = 750 t² X 6.826 BTU/h/t² = 5120 BTU/h Qex = 300 t² X 14.6 BTU/h/t² = 4381 BTU/h QT = 17044 BTU/h Total cooling load or this space (QT) is 16925 BTU/h, and approximately 22.6 BTU/h/t². N O I T A L I T N E v T N E M E C A L P S I D
ASHRAE Standard 62-2004 requires 0.12 CFM/t² outdoor airfow rate required per unit area, Ra, and 10 CFM/Person outdoor airfow rate required per person, Rp, be deliered to the space or classroom applications. For displacement entilation a entilation eectieness, or zone air distribution eectieness (Ez), is assumed to be 1.2 (table 6-2, ASHRAE standard 62-2004).
© 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-43
Displacement ventilation Desig Guide—Special Supplemetal
Classroom Example Determie the Airfow Rate to meet the Coolig Load
Determie the Fresh Air Flow Rate ad Breathig Zoe Vetilatio Eectiveess
Note that some local codes may not allow the discount or vE, or may hae stricter requirements, and they should be used instead o this calculation. Example: Title 24 in Caliornia. The total supply air olume is then the maximum alue between vh and vr.
Calculate the Supply Air Temperature
Determie the Retur Air Temperature
N O I T A L I T N E v T N E M E C A L P S I D
Selectio o Diusers
Because foor space is limited in a classroom, and wall space is typically coered with teaching material, either a ceiling mounted or corner mounted diuser would be the ideal diuser choice. As stated in preious examples, or comort reasons, the di users or this space are limited to 40 pm. With a supply air olume o 946 CFM and a ace elocity o 40 pm, 23.65 t² o diuser ace is required. Three 30”x 42” DFIC corner diusers will supply 946 CFM at 36.0 pm, and occupants will need to be located at least 2 eet or more rom the diuser.
J-44
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide—Special Supplemetal
Classroom Example Layout o the Classroom
The corner diusers could be placed in any o the corners to supply this room, as long as the occupant is not within 2 eet o the diuser ace. CFD Represetatio o the Space
A CFD analysis was run or this example using the conditions, calculated airfow and supply air temperature with the DF1C selected in the example or a isual representation o space dynamics. Figure 75 shows the temperature prole in the space. The DF1C’s unction as predicted and creates an een stratication in the space. The seated occupants are experiencing the desired set point and the stratication in the space is not outside the comort criteria. Figure 76 shows the temperature prole in the space with a isoplane showing the set point temperature o the space. All o the occupants are near or within the desired temperature, but at ull load in the space, some oer cooling may occur. Figure 77 shows the elocity prole in the space. The DF1C’s produces a prominent air pattern predicted to be up to 60 eet per minute, shown in red on the foor. Figure 78 shows the drat temperature prole in the space. The DF1C diusers create the appropriate stratication in the space, but placement o the diuser with respect to the occupants is critical. There is a prominent uncomortable space directly in ront o the diusers. Figure 75: Temperature Prole
Figure 76: Temperature Prole with a 74º F Isoplae
Figure 77: Velocity Prole
Figure 78: Drat Prole
N O I T A L I T N E v T N E M E C A L P S I D
© 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-45
Displacement ventilation Desig Guide—Special Supplemetal
Displacemet Vetilatio ad Healthcare Figure 80: Exam Room
Healthcare Air Distributio
Air Distribution or Healthcare acilities is much more critical and specialized than or a typical air conditioned oce space. In addition to accurate control o temperature and elocity in the space to maintain acceptable comort o the occupants the air distribution system must be able to dilute and eectiely remoe contamination (odor, airborne microorganisms and iruses) rom the space. Traditionally oerhead mixing systems hae been used to supply sucient quantity o entilation air to dilute and carry away contaminants. ASHRAE proides guidelines or minimum air changes o total and outside air or the arious types o spaces in a healthcare acility (ASHRAE HvAC Applications Handbook). Displacement entilation has the potential to signicantly improe contaminant remoal as well as proide superior thermal comort leels in the space. On page J-12 o this design guide it has been pointed out that the entilation eectieness o Dv systems are greater than oerhead systems with typical entilation eectieness o 1.2 or higher. Higher entilation eectieness translates directly to contaminant remoal resulting in a healthier, cleaner occupied space. Two areas in health care acilities particularly well suited or displacement entilation are patient rooms and waiting rooms.
Figure 81: Pla view o Exam Room
Patiet Rooms
The air distribution system in a patient room must maintain thermal comort, aoid objectionable drats and remoe contaminants to protect both the patient and isitor rom inection. By supplying the entilation air at low elocity and eleated temperature the Dv system maintains comort conditions. As the supply air is drawn to and up the occupants and equipment contaminants are eectiely captured by the thermal plume and carried out o the occupied zone. I the patient room contains a large amount o medical equipment with high heat output, the room load may exceed the limit or the Dv system (38 BTU/hr/t2). In this case the Dv system can be combined with radiant cooling to counteract the loads. In this case the Dv system is sized to distribute the entilation air requirement while the radiant cooling system deals with the cooling load.
Figure 79: Model o the Patiet Room
Figure 82: Model o the Waitig Room
N O I T A L I T N E v T N E M E C A L P S I D
Diuser Loads
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All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
Diuser
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide—Special Supplemetal
Displacemet Vetilatio ad Healthcare Waitig Room
Table 6: Patiet Room
Waiting rooms pose a special challenge to the air distribution system. Occupancy in the room can ary greatly and the health status o the occupant is not known. People in the waiting area could be extremely inectious or on the other hand extremely susceptible to inection. Obiously inection control is a priority consideration o the air distribution system but comort condition must also be maintained. Displacement entilation is again ideally suited to this application. Fresh clear air is distributed directly to the occupants and contaminants remoed with the thermal plumes. Unless seated directly in ront o a displacement outlet occupants will also experience a high leel o thermal comort. Mock-upTestig
Mock-up tests conducted in the Price Laboratory in co-operation with Stantec Consulting Ltd and Mazzetti & Associates support the application o displacement entilation in hospital patient and waiting rooms. Three rooms were mocked-up and run at seeral conditions to determine comort and entilation eectieness. Comort was determined by Percent o People Dissatised (PPD) as dened by ASHRAE Standard 55. A PPD alue less than 20 is deemed acceptable. For a detailed report o the Healthcare Air Distribution Mock-up contact your Price representatie. 1. Patiet Room:
Figure 79 illustrates the room layout which included an exterior window simulated with an enironmental chamber and two Dv outlet locations. Table 6 demonstrates excellent entilation eectieness or both outlet locations een at ACHs below current code requirements. Outlet location B proides superior comort as it is urther away rom the occupants. 2. Exam Room:
Figures 80 & 81illustrate the room layout which included an exterior window simulated with an enironmental chamber and radiant ceiling panels or supplemental heating and cooling.Tests were run at seeral outside temperature conditions. Table 7 demonstrates excellent entilation eectieness at all conditions, een at ACHs below current code requirements. By comparison, oerhead mixing systems usually are limited to a entilation eectieness o 0.8 – 0.9 when operating in the heating mode. In all modes o operation, acceptable comort conditions were maintained. 3. Waitig Room:
Figure 82 illustrates the room layout and outlet location. Table 8 conrms that comort is maintained een with air change rates as high as 12. Table 9 illustrates entilation eectieness alues or the Dv system are superior to oerhead mixing een at lower air change rates.
Test
1
2
Method
Dv A
Dv B
Supply CFM
190
190
Supply ACH
4
4
SA Temp
68
66
OP Temp
76
74
RA Temp
78
76
Air Velocity, FPM
10 to 67
10 to 40
PPD %
6 to 22
2 to 17
Vet E
1.4
1.3
Table 7: Exam Room Test
1
2
3
Method
Dv
Dv
Dv
Supply CFM
80
80
80
Supply ACH
4
4
4
Outside Temp
91
0
-22
SA Temp
68
66
69
OP Temp
75
74
75
RA Temp
76
77
79
Air Velocity, FPM
10 to 25
12 to 20
12 to 28
PPD %
10 to 12
8 to 16
7 to 10
Vet E
1.3
1.1
Table 8: Waitig Room Comort
Test Method Supply CFM Supply ACH SA Temp OP Temp RA Temp Air Velocity, FPM PPD %
1
2
3
Dv
Dv
Dv
1800 12 65
1200 8 64
900 6 65
70
71
72
74 10 to 46
74 12 to 34
76 12 to 30
10 to 20
5 to 14
5 to 12
Table 9: Waitig Room Vet E
© Copyright E.H. Price Limited 2007.
Test
1
2
3
Method
Ceiling
Dv
Dv
Supply CFM
1800
1200
900
Vet E
0.9
1.1
1.0
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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N O I T A L I T N E v T N E M E C A L P S I D
Displacement ventilation Desig Guide—Special Supplemetal
Displacemet Vetilatio ad Healthcare CFD Aalysis
The test data gathered rom the Laboratory mock-up was used to create CFD models o the space. Figure 83 is the output or the CFD simulation o the patient room example at the cooling design conditions with oerhead air distribution.Figure 84 illustrates the temperature prole o the patient room with Dv. A distinct stratication is obsered with warmer temperatures at the ceiling. Temperatures in the occupied zone are in the 70° - 74ºF comort range. Any lighter than air contaminants would be concentrated in the higher warm air layer. By contrast the oerhead system (Figure 83) produces a airly uniorm temperature distribution as expected. Figure 85 illustrates the elocity prole o the patient room with Dv. Generally elocities in the space are less than 20pm (0.33 ps). The oerhead system (Figure 86) demonstrates higher elocities under the diuser illustrating the induction o room air as well as some higher elocities projecting down the ar wall.
Figure 83: Temperature Pro ile with overhead air distributio
Figure 84: Temperature Proile with disp lacemet vetilatio
Figure 85: Velocity Prole with displacemet vetilatio
Figure 86: Velocity Prole with overhead air distributio
N O I T A L I T N E v T N E M E C A L P S I D
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All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.
Displacement ventilation Desig Guide
Reereces 1. 20 01 ASHRAE Fundamentals Handbook, Space Air Diusion, Chapter 32.11 2. Hakon Skistad, Displacement ventilation, 1994, Control o the Built Enironment series; 1, John Wiley & Sons Inc., England. 3. Hakon Skistad, Elisabeth Mundt, Peter Nielsen, Kim Hagstrom, and Jorma Railio, Displacement ventilation in Non-Industrial Premises, Reha Guidebook No 1, Federation o European Heating and Air Conditioning Associations, 2002. 4. Qingyan Chen and Leon Glicksman, Perormance Ealuation and Deelopment o Design Guidelines or Displacement ventilation, Final Report to ASHRAE on Research Project RP-949. 5. Shiping Hu, Qingyan Chen, Leon Glicksman, Comparison o Energy Consumption between Displacement and Mixing ventilation Systems or Dierent U.S. Buildings and Climates, ASHRAE Transactions 1999, v. 105, Pt. 2. 6. Xiaoxiong Yuan, Qingyan Chen and Leon Glucksman, A Critical Reiew o Displacement ventilation, ASHRAE Transactions 1998 v. 104, Pt. 1. 7. Xiaoxiong Yuan, Qingyan Chen and Leon Glucksman, Perormance Ealuation and Design Guidelines or Displacement ventilation, ASHRAE Transactions 1999, v. 105, Pt.1. 8. U.S. Green Building Council, LEEDTM (Leadership in Energy & Enironmental Design) - Green Building Rating System or New Construction & Major Renoations (LEED-NC) version 2.1, Noember 2002 - reised 3/14/03, www.usbgc.com/leed. 9. ASHRAE Standard 62. 10. Qingyan Chen and Leon Glicksman, “System Perormance Ealuation and Design Guidelines or Displacement ventilation”,ASHRAE, Inc, 2003. 11. ASHRAE Standard 55. 12. Pawel Wargocki and Daid Wyon, Research Report on eects o HvAC on Student Peormance, ASHRAE Journal, vol 48, No. 10, 2006. 13. Morton Blatt, Adanced HvAC Systems or Improing Indoor Enironmental Quality and Energy Perormance o Caliornia K-12 Schools: Applications Guide or O-the-Shel Equipment or Displacement ventilation Use Consultant Report prepared or Caliornia Energy Commission, 2006. 14. Gregory Kats, Greening America’s Schools, Costs and Benets, Capital E Report, October 20 06.
N O I T A L I T N E v T N E M E C A L P S I D
© Copyright E.H. Price Limited 2007.
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
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Underfoor Air Distribution Desig Guide
notes
N O I T A L I T N E v T N E M E C A L P S I D
J-50
All Metric dimensions ( ) are sot conersion. Imperial dimensions are conerted to metric and rounded to the nearest millimeter.
© Copyright E.H. Price Limited 2007.