David Butler

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Air conditioning using displacement ventilation to maximize free cooling

David Butler*

*David Butler is a Principal Consultant in BRE’s Environmental Engineering Centre.

SummaryDisplacement ventilation uses air supplied at a temperature, typically around 19°C for offices, which is only slightly cooler than the design temperature of the occupied zone.This is a much higher temperature than used for conventional mixed flow ventilationsystems and creates opportunities for using fresh-air based “free cooling” for a largeproportion of the year.

Conventional displacement ventilation design practice suggests that the cooling capacityof displacement ventilation in commercial offices with normal floor to ceiling heights (up

to 3 m) is limited to around 25 W/m2. For this reason it is fairly common in the UK tospecify chilled ceilings in conjunction with displacement ventilation to meet typical officeheat loads. However, recent BRE research has shown that displacement ventilation onits own using appropriate diffusers can deal with heat loads of around 50-55 W/m2 intypical office environments without causing thermal discomfort outside the outflow zone.This opens up the possibility of using fresh-air free cooling for large parts of the year.

Displacement compared with mixed flow ventilationDisplacement ventilation is essentially a buoyancy driven "displacement" process."Fresh" ventilation air is introduced at low velocity and at low-level into the occupiedzone at a temperature typically around 19°C, slightly cooler than the design room air temperature. Air is extracted at ceiling level. The supply air spreads out across the floor forming a reservoir of cool fresh air, as shown in Figure 1. Any heat source in the room,such as a person sat at a desk, will generate a positively buoyant thermal plume risingupwards. This plume will draw air from the reservoir of cool fresh air at low level in theroom. If the source of heat also represents a pollutant source, these pollutants will alsobe transported up out of the occupied zone.



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Return grill Return grillReturn grill Return grillReturn grill Return grill

Figure 1 Displacement ventilation – typical room air flow

Extensive investigations elsewhere (1-6) have shown that generally two distinct stratified

layers of air form, an upper zone containing warm polluted air and a lower zonecontaining cooler cleaner air separated by a boundary layer. This causes a vertical

temperature gradient to develop (typically 5-6°C between supply and extract), resultingin higher temperatures at ceiling level than with standard mixing ventilation systems.Displacement ventilation strategies exploit this vertical temperature gradient bymaintaining the lower occupied zone at comfort conditions while allowing the upper hotter zone to increase to temperatures in excess of accepted comfort limits. A risk withsuch a strategy is, however, that significant vertical temperature gradient between feetand head height can lead to discomfort and this must be considered when designingsuch a system.

Mixed flow ventilation is essentially a "mixing" process in which stale warm room air is

continually diluted by cooled fresh air. The air in the room is fully mixed and therefore air temperature and pollutant concentrations are uniform throughout the space. Fresh air isnormally introduced and removed at ceiling level and the interaction of upward warm air currents and horizontal supply air jets below the ceiling causes mixing and circulation tolower areas with relatively high air speeds (see Figure 2). This can lead to draughts anddiscomfort in the occupied area. Because air is supplied and extracted at ceiling levelsome of the supplied ‘fresh’ air may short circuit by being drawn into the extract grilleswhich effectively reduces its cooling and pollutant dilution potential.



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supply grill Return grillsupply grill Return grillsupply grill Return grillsupply grill Return grillsupply grill Return grillsupply grill Return grill

Figure 2 Mixed flow ventilation – typical room air flow

The air supply temperature for displacement ventilation systems is typically 19°Ccompared to 13°C to 14°C for most conventional mixing systems. Figure 3 shows the

variation in room air temperature for “ideal” displacement ventilation in a typical officeenvironment compared with mixing ventilation. Note that in this example there is

approximately 2°C air temperature pick-up between the supply and room air close to thefloor due to room air entrainment.

9oC

Supply air temperature= 14oC

Room & extract air

temperature = 23oCMixingventilation

7oC

Supply air temperature= 19oC

Extract air temperature= 26oC

Displacementventilation

12 14 16 18 20 22 24 26 28 30oC

Air temperature near

head height for seatedoccupants = 23oC

Figure 3 Supply and extract air temperatures for mixed flow and displacementventilation (ideal situation)



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“Free cooling” opportunitiesSupplying air at 19°C to a displacement ventilation system opens up the possibility of using fresh air “free-cooling” for large parts of the year. “Free cooling” is rather amisleading term as energy will always be needed for fan (and in some cases pump)operation. In cold weather a heat reclaim thermal wheel or heat exchanger is essentialin order to minimise the need for pre-heating of the fresh-air supply. Where there is a

requirement for dehumidification in warm weather a separate low temperature coil maybe required, possibly in conjunction with heat pipe heat recovery and pre-cooling, or faceand bypass dampers.

A bin analysis of annual outdoor dry bulb temperature allows the potential of fresh air free-cooling to be investigated. Figure 4 shows the number of hours occurrence of outdoor air temperatures for normal working hours (0700 to 1800) during a whole year,based on the average of the five year period 1976 to 1980. The effect of a year with ahot summer is also shown by plotting hours occurrence for 1976 which had anexceptionally long hot summer. The data can be interpreted more easily by converting itto an accumulated frequency “s-curve”, shown in Figure 5. This allows the proportion of the total hours that were at and below a certain threshold temperature to be read directly

from the graph. To take account of heat pick up from the air distribution fan (assumed toresult in a 1°C rise in the air stream) it is appropriate to take 12°C for a conventional

mixing ventilation system and 18°C for a displacement system. The s-curve thereforeshows that fresh-air free-cooling can be used for 85% of the time for a displacementsystem but only 53% of the time for a conventional mixing ventilation system.

Dry Bulb T (07:00 - 18:00)

Kew 1976 to 1980

0

50

100

150

200

250

300

350

- 8 - 5 - 2 1 4 7 1 0 1 3 1 6 1 9 2 2 2 5 2 8 3 1 3 4

Dry Bulb Temperature (oC)

T o t a l h o u r s o c c u r e n c e Five year average, 1976

to 1980

Figure 4 Bin analysis of outdoor dry bulb air temperature



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Dry bulb temperature (whole year, 0700 to 1800 hrs)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

110%

-5 0 5 10 15 20 25 30 35

Dry bulb temperature (oC)

A c c u m a

t e d f r e q u e n c y ( % h o u r s o c c u r r e n c e )

1977 cool summer

1976 hot summer

1976 to 1980

12oC 18

oC

53%

85%



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Figure 5 S-curves for occurrence of outdoor dry bulb temperatures

Additional s-curves for years with a hot summer (1976) and a cool summer (1977) havealso been plotted. These show that a hot summer would reduce the time that fresh air

can be used for the displacement system from 85% to about 78% of the time. A coolsummer would increase the proportion of the year that free-cooling is available to about90% of the total time.

Thermal comfort requirementsThermal comfort requirements place limitations on the design parameters and themaximum cooling performance achievable with displacement ventilation. For sedentaryoccupancy such as office work, BS EN ISO 7730:1995 (7) recommends that air velocityonto people should not normally exceed 0.15 to 0.2 m/s, depending on air temperatureand turbulence intensity. The maximum ankle to head temperature gradient (between

0.1 m and 1.1 m above the floor) should not normally exceed 2°C to 3°C in summer.The dry resultant temperature in the occupied region should also normally be between

23°C and 26°C. For displacement ventilation systems air velocity is principally a functionof the air supply volumetric flow rate and temperature, and the degree of room air entrainment at the diffuser. The ankle to head temperature gradient is also a function of the supply air volumetric flow rate and the degree of room air entrainment at the diffuser.

Obtaining higher cooling performance with displacement ventilationConventional design practice and BRE investigations suggest displacement ventilationsystems in offices (with floor to ceiling heights below 3 m) are limited to around 20/25W/m2 cooling capacity. To increase cooling capacity either the supply air temperaturehas to be reduced, or the volumetric air supply rate must be increased. Reducing the air

supply air temperature below the normal 19°C is not an option when the aim is themaximisation of free-cooling, and could lead to unacceptable thermal discomfort in the

ankle region. Raising the volumetric air supply rate allows a 19°C supply air temperature to be maintained, but increases the risk of high air speed in the occupiedspace. Fan power and duct sizing will also be increased and therefore a life cycleanalysis of this compared with the reductions in cooling energy must be undertaken aspart of specific system designs.

Types of displacement diffuser A wide range of diffusers can be used for displacement ventilation, including thefollowing:

• Wall mounted diffusers

• Corner mounted diffusers

• Free-standing "bin" diffusers• Floor mounted swirl diffusers

• Fabric “sock” diffusers.

Figure 6 shows typical air outflow profiles with these diffusers. Fabric sock diffusers arenot yet widely used and are rare in offices in the UK. Wall, corner and free-standingdiffusers typically consist of a perforated plate with an internal baffle to promote even air



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discharge through the perforated plate face. The active surface areas are usually quitelarge without taking up excessive amounts of floor space.

Floor mounted diffusers can normally provide either vertical or horizontal discharge.Horizontal discharge is usually used for displacement applications and this typicallyproduces a horizontal circular swirling jet. These diffusers are often 150 mm or 200 mm

diameter. This restricted size leads to relatively higher discharge or face velocities thanthe other types of diffuser.

Corner displacement

diffuser

Wall mounted

displacement diffuser

Free standing “bin”


Floor mounted swirl


Figure 6 Outflow air profiles of typical displacement ventilation diffusers

Floor diffusers appear to be a very popular choice for new commercial office buildings,despite the fact that their relative small size conflicts with the displacement ventilationrequirement for air to be introduced at low velocity. Their apparent popularity may bedue to the installation flexibility offered by floor terminals in conjunction with raised floors,



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although there is a risk of low level thermal discomfort if occupants sit too close to them.

Limitations of conventional displacement diffusersThe primary limitation of displacement ventilation in dealing with heat loads has beenshown to be the amount of air that can be supplied through conventional wall or floor

mounted displacement diffusers without causing discomfort due to draughts.

The design procedure for the specification of displacement ventilation diffusers generallyinvolves defining the distance from the diffuser at which the air speed has decreased tobetween 0.15 and 0.2 m/s (the usual threshold for acceptable thermal comfort), and maybe measured at around 0.1 m above the floor.

BRE has tested a range of conventional displacement ventilation diffusers including aswirl displacement diffuser and a floor-standing “bin” diffuser. The swirl diffuser had anactive area of 0.125 m2 (overall diameter = 0.25 m) at a range of air supply volumetricrates. The results for air speed at 1.07 m from the center of the diffuser are shown inFigure 7. This shows that the diffuser air supply limit for acceptable thermal comfort

(maximum air speed of 0.15 m/s at 1.07 m from the diffuser and 0.05m above the floor)is about 30 l/s. Assuming that the diffuser is installed in a space with 3.0 m spacingbetween diffusers (9 m2 floor area per diffuser) and a supply and extract air differential of

6°C (19°C air supply, 25°C extract and about 24°C in the occupied region) the coolingperformance would be 24 W/m2.

Variation of mean air speed with height at 1.07 m from centre of diffuser

(0.25 m dia floor swirl)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Mean air speed (m/s)

H e i g h t ( m )

Diffuser A, 10 l/s

Diffuser A, 30 l/s

Diffuser A, 50 l/s

maximum air

speed for acceptable

thermal comfort

Figure 7 Swirl diffuser – air speed at 1.07 m from diffuser centre (0.05 m above thefloor)



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The bin diffuser was constructed from perforated steel plate and had a height of 0.61 mand a diameter of 0.20 m, giving an active face area of 0.38 m2. Figure 8 shows air speed at a distance of 1.07 m from the center of the diffuser for a range of air volumetricsupply rates. It is clear that for acceptable thermal comfort (based on air speed in thespace at 0.05 m above the floor) the limit of this diffuser is 50 l/s. It is very unlikely thatin a commercial office space free-standing diffusers could be as close as the 3 m

spacing assumed for the swirl diffusers. Assuming a wider spacing of 4 m and the sameassumptions for air temperatures as for the swirl diffuser the limit on coolingperformance is about 20 W/m2. Larger free-standing diffusers would allow higher air volume supply rates and therefore provide a higher cooling performance than this butwould take up greater floor area.

Variation of mean air speed with height at 1.07 m from centre of diffuser

(0.61 m (h) x 0.2 (d) "bin" diffuser)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Mean air speed (m/s)

H e i g h t ( m )

Diffuser C, 30 l/s

Diffuser C, 50 l/s

Diffuser C, 100 l/s

maximum air


thermal comfort

Figure 8 “Bin” diffuser – air speed at 1.07 m from diffuser centre (0.05 m abovethe floor)

A typical wall diffuser, shown in Figure 9, (dimensions 1.2 m high and 0.75 m wide) has

been tested. The diffuser was supplied with 130 l/s of air at 19°C and the thermal load inthe 7.5 m x 5.5 m test cell was 1000 W or 24 W/m2.



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Figure 9 Typical perforated plate wall diffuser

Figure 10 shows the vertical variation of air speed at various distances from the diffuser and Figure 11 shows the vertical variation of dry bulb air temperature (along a traverseline perpendicular to the diffuser). Air speed exceeds 0.2 m/s at 0.1 m height above thefloor within 2.7 m from the diffuser. Vertical air temperature gradient between ankle and

seated head height exceeds 3°C close to the diffuser but is 3°C or lower further away.

3.36 m from end of room

0

0.5

1

1.5

2

2.5

3

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Air speed ( m/s)

H e i g h t a b o v e f l o o r ( m )

0.5m from side wall

1.2m from side wall

2.2m from side wall

2.7m from side wall

3.2m from side wall

4.5m from side wallMaximum air


thermal comfort

Figure 10 Wall diffuser (130 l/s) – air speed



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0

0.5

1

1.5

2

2.5

3

18 19 20 21 22 23 24 25 26

Air temperature (oC)

h e i g h t a b o v e f l o o r ( m )

0.5m from side wall

1.2m from side wall

2.2m from side wall

2.7m from side wall

3.2m from side wall

4.5m from side wall

dT ~ 3 K

Figure 11 Wall diffuser (130 l/s) – dry bulb air temperature

A higher thermal load would require another diffuser. An earlier test with the samediffuser in a 3.2 m x 6.5 m test cell with a similar thermal load but a smaller floor area(equivalent to 54 W/m2) showed similar results, and that acceptable thermal comfortcould be achieved in the space outside the diffuser outflow zone. However, with asmaller test cell floor area the relative size of the outflow zone is larger which reducesthe area that has acceptable thermal comfort for sedentary office work.

Displacement ventilation diffuser air entrainmentThe degree of room air entrainment in the diffuser outflow is an important characteristicof displacement ventilation diffusers and affects the volume and velocity of the outflowbeyond approximately 1 m from the diffuser face. For example, floor swirl diffusers andwall diffusers with small perforations tend to produce higher levels of room air entrainment. The outflow air volume for a typical perforated panel diffuser is shown inFigure 12. Each hole creates a small jet, which entrains some room air. For a givensupply volumetric airflow rate reducing the degree of perforation increases the level of entrainment and therefore the volume of the air outflow at 1 m from the diffuser.

The effect of entrainment is to increase the amount of mixing between supply and room

air and this raises the temperature of the low level cool air pool. High levels of entrainment can allow a lower air supply temperature without significantly increasing therisk of cold temperatures around the legs and feet. However, the resulting higher levelsof room air mixing will tend to disrupt the stratified displacement ventilation room airflowpattern and result in a largely mixed lower zone. This may cause a reduction in air quality, and there may also be a risk of draught discomfort due to higher air speeds andair turbulence intensity.



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Figure 12 Air flow pattern with perforated panel diffusers

Little air is entrained inthe primary flow

Air is entrained at the the

boundary surface between the

outflowing air and the room air.

Little air is entrained inthe primary flowLittle air is entrained inthe primary flow







Figure 13 Air flow pattern with filter-mat or fabric type diffuser

An alternative to perforated plates is to use fabric or filter mat based diffusers. Suchdiffusers create very little air entrainment at the diffuser face, entraining a small volumeof room air at the boundary surface between the outflowing air and the room air. Overallthey produce low levels of room air entrainment which results in lower air speed andturbulence in the occupied region (see Figure 13). However, because of the lack of mixing with room air, the cool pool of air at floor level will be closer to the supply air temperature, which may result in discomfort at foot/ankle level. This may be increased if

the supply air temperature is reduced or the volumetric supply rate is raised to increasecooling performance.

The effect of varying diffuser room air entrainment on the air temperature in the lower occupied zone is shown in Figure 14. High entrainment increases the temperature riseof the supply air as it is discharged from the diffuser. This mixing reduces the “under-temperature” of air within the diffuser outflow zone. Under-temperature is defined as thetemperature difference between the air within the diffuser outflow zone and room



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temperature (in the occupied region). However, when the aim is to supply the air at thehighest possible temperature to maximise free cooling a high entrainment diffuser isundesirable as it raises the air temperature in the lower occupied zone. To maximisefree cooling it is therefore preferable to use a low entrainment diffuser.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

18 19 20 21 22 23 24 25 26 27 28



Little entrainment

Moderate entrainment

A lot of entrainment

Supply air

temperature

Room air

temperature

Extract air

temperature

Figure 14 Variation of room air temperature with height (for diffusers with varyingroom air entrainment) – based on practical experience

Extending the cooling performance of displacement ventilation systemsThe results of BRE testing a range of existing displacement ventilation diffusers hasdemonstrated that as the air supply rate is increased, above 30 l/s for a floor swirldiffuser and 50l/s for a bin diffuser, the air velocities at approximately 1m from thediffuser exceeded the maximum air velocity acceptable for thermal comfort. Such supplyairflow rates represent cooling loads of less than 25 W/m2 for a practical officeconfiguration and temperature differential. Higher cooling loads could be met byincreasing the number of diffusers for a given floor area but this would reduce the usablefloor area with acceptable thermal comfort.

Tests with a conventional wall diffuser showed that 24 W/m2 could be achieved withoutcausing discomfort in the occupied space outside the diffuser outflow zone. Space

permitting a second diffuser could be used to allow the thermal load to be increased toaround 50 W/m2. The diffuser is nevertheless fairly large and takes up a significantamount of wall space, and has a relatively large outflow zone in which occupants arelikely to experience thermal discomfort when involved in sedentary office work. A walldiffuser would also be unsuitable for large open plan areas and in such situationsappropriately designed bin diffusers would need to be used instead.

Based on these results, therefore, the generally held belief that displacement ventilationis only applicable to commercial buildings with modest cooling loads is only partially true.



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It appears that appropriately designed and located diffusers are capable of meeting largecooling loads, albeit with some impact on space utilisation flexibility due to the spacetaken up by the diffusers and the impact on thermal comfort in the diffuser outflowzones. Thermal comfort in the diffuser outflow zones is especially critical for sedentaryoffice work but may well be acceptable for circulation areas or areas around

photocopiers or drink dispensers which are only occupied for short times.

Fabric diffusersEarlier work elsewhere (8) has shown considerable promise in the use of fabric diffusersto provide higher air flow rates without causing thermal discomfort from draughts. Theresults suggested that fabric diffusers positioned along the bottom of two opposite wallsof a room could deal with heat loads of around 50 W/m2 without causing high air velocities in the occupied space. A recent case study (9) used fabric diffusers in arestaurant to overcome a high heat load from the occupants (56 W/m2) and to provide ahigh fresh air flow rate to overcome indoor air pollution from smokers (30 l/s/person).Fabric diffusers were selected on the basis that they provided a high surface area tosupply a high flow rate without risk of excessive draught or noise.

Fabric socks can be manufactured in virtually any shape and size. BRE has tested anexperimental fabric sock that was manufactured from polyester/cotton fabric with aquadrant cross section so that it would fit into the skirting board position at the bottom of a long wall, see Figure 15. The aim was to provide a large surface area to allow highsupply-air volumetric flow rates without causing excessively high discharge air speeds or noise, but without taking up too much floor space. The sock dimensions were 4.25 mlong, 200 mm high and 200 mm wide. It was retained in position by two sewn-in plasticbeads which clipped into channels fixed to the wall and floor. Air was supplied into thesock through an opening at one end from a flexible duct. The fabric created sufficientinternal pressure for the discharge air flow to be essentially constant along the length of the sock.

Figure 15 Fabric sock diffuser (smoke visualisation of outflow shown on right)

The sock was tested at two air flow rates, 130 l/s and 260 l/s and 19°C air temperature.At the higher air flow rate the test room heat load was 2000 W which was equivalent to48 W/m2 in a BRE mock-up of a typical office space.

2 0 0 m m

2 0 0 m m



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Figure 16 shows the vertical variation of air temperature at different distances from thesock following a floor traverse perpendicular to the sock and starting about 0.5 m fromone end of the sock. The temperature gradient between ankle and head height (0.1 to

1.1 m for a seated person) was 3.5°C at 1.2 m from the sock and 2.5°C at 2.2 m fromthe sock. This is on the limit of what is recommended as being acceptable by BS EN

ISO 7730 for normal office activity and clothing levels. The traverse from the centre of the diffuser showed a higher temperature gradient (4.5°C at 1.2 m and 3.0°C at 2.2 m).Therefore according to standard BS EN ISO 7730 occupants sitting close to the diffuser may experience some thermal discomfort.

2.4m from end of room

0

0.5

1

1.5

2

2.5

3

18 19 20 21 22 23 24 25 26 27 28



0.5m from diffuser

1.2m from diffuser

2.2m from diffuser

2.7m from diffuser

3.2m from diffuser

4.5m from diffuser

Traverse 0.5 m from end of diffuser

dT ~ 3.5K @ 1.2 m

(dT ~ 2.5K @ 2.2 m)

Figure 16 Variation of air temperature with height at 260 l/s (48 W/m2)

BS EN ISO 7730 relates the maximum acceptable air velocity to the air temperature andturbulence intensity as well as the occupants' activity and clothing levels. Taking intoaccount these factors, and assuming typical office activity and clothing, the maximumrecommended air velocity is between 0.15 and 0.2 l/s. Figure 17 shows that air speed iswell within this limit.

Overall the fabric sock diffuser has provided acceptable thermal comfort in most of theroom for normal sedentary office work, apart from close to the centre of the diffuser.



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0

0.5

1

1.5

2

2.5

3

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Air speed ( m/s)


0.5m from diffuser

1.2m from diffuser

2.2m from diffuser

2.7m from diffuser

3.2m from diffuser

4.5m from diffuser

Traverse 0.5 m from end of diffuser

Maximum air

speed for

acceptable

comfort

0.15 to 0.2 m/s

Figure 17 Variation of air speed at 260 l/s (48 W/m2)

BRE also tested an experimental fabric bin diffuser which was constructed from thesame polyester/cotton fabric as the sock, see Figure 18. The dimensions of the bin were700 mm diameter and 740 mm high which gave a similar active area as the fabric sock.Supply air was fed through the bottom of the diffuser by a flexible duct run through theunderfloor void. The bin was located in the centre of the room.

The fabric bin was tested at the same air flow rates and supply air temperature as for the

fabric sock, 130 l/s and 260 l/s and 19°C air temperature. At the higher air flow rate thetest room heat load was 2000 W which was equivalent to 48 W/m2. Air speed and air temperature were measured throughout the room space by traversing with multipletransducer poles.



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Figure 18 Fabric bin diffuser (smoke visualisation of outflow shown on right)

Figure 19 shows the vertical variation of air temperature at various points along atraverse across the width of the test room. The middle of the traverse wasapproximately 1.5 m from the diffuser. The measured temperature gradient betweenankle and head height (0.1 to 1.1 m for a seated person) at the closest position to the

diffuser (1.5 m) was 3.2°C. This is just on the limit of what is recommended as beingacceptable by BS EN ISO 7730 for normal office activity and clothing levels. However,

elsewhere the temperature gradient was around 2°C which is within the comfort limit.

2.4m from end of room

0

0.5

1

1.5

2

2.5

3

18 19 20 21 22 23 24 25 26 27 28



0.5m from side wall

1.2m from side wall

2.2m from side wall2.7m from side wall

3.2m from side wall

4.5m from side wall

Middle of traverse ~1.5 m from diffuser

dT ~ 3K at point

nearest to diffuser

Figure 19 Variation of air temperature with height at 260 l/s (48 W/m2)

Figure 20 shows the vertical variation of air speed. Air speed was well below therecommended maximum value given by BS EN ISO 7730.



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Figure 20 Variation of air speed at 260 l/s (48 W/m2)

Comparison of the test results from these two diffusers showed that overall the fabric bindiffuser provided a higher level of thermal comfort in the test room than provided by thefabric sock diffuser. This was largely due to the greater vertical temperature gradientclose to the middle of the sock diffuser. The regions of high temperature gradient shownto exist close to both of the diffusers are a direct result of the low levels of room air entrainment in this region. The air temperature at a height of 1.1m is relatively constant

throughout each of the test mock-ups and is a function of supply airflow rate and thermalload in the space. Therefore, the vertical gradient is greatest in the diffuser outflow zonewhere the cool supply air enters the room and undergoes little mixing with the room air.The diffuser outflow zone is largest at the middle of the sock diffuser as it is essentially asingle sided diffuser whereas the bin has an unrestricted radial discharge. This effect isillustrated in Figure 21 below.

The size of the outflow zones, and therefore the size of the areas around the diffuserswith relatively poor thermal comfort, would be smaller with lower cooling loads. Thecooling load assumed in this study is 48 W/m2 which is deliberately on the high side of the cooling loads found in typical office buildings.


0

0.5

1

1.5

2

2.5

3

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Air speed ( m/s)


0.5m from side wall

1.2m from side wall

2.2m from side wall

2.7m from side wall

3.2m from side wall

4.5m from side wall

Middle of traverse ~1.5 m from diffuser

Maximum air

speed for

acceptable

comfort

0.15 to 0.20 m/s



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Limit of zone withhigh verticaltemperature gradient

1.5 m

Limit of zone withhigh verticaltemperature gradient

1.5 m

Figure 21 Outflow size from fabric sock and bin diffusers

ConclusionsIt has been shown that the limitations of many existing displacement diffusers for meeting high cooling loads include the maximum volumetric air supply rate that can be

achieved without causing discomfort to the occupants, and the maximum number of diffusers that can be installed in a given space without reducing the area that can beoccupied by people. Large wall diffusers have been shown to be capable of satisfyingcooling loads as high as 50-55 W/m2 without causing thermal discomfort outside thediffuser outflow zone. However, the diffuser outflow zone will be relatively large and thiswould reduce the practicality of using such a diffuser for large loads, especially inbuildings where the occupants were mainly involved in sedentary office type work.Alternatively it might be possible to arrange for the outflow zones to be used for generalcirculation areas which have less stringent criteria for acceptable thermal comfort. Acooling performance of 50-55 W/m2 would be sufficient for the cooling requirements inthe core zones of most normal office buildings. Bin diffusers would have to be usedinstead of wall diffusers in large open plan areas and the space that they would take up

would further reduce the usable floor area.

Testing of a range of innovative fabric diffusers showed that a cooling performancearound 50 W/m2 could be achieved. This performance was in part achieved by utilisingthe low entrainment characteristic of fabric as part of the diffusers, which allowsrelatively high volumetric supply rates without causing unacceptably high air speeds. Adisadvantage of minimal mixing and entrainment of the room air with supply air at thediffuser is high ankle to head temperature gradients which could exceed the limit of acceptability for people engaged in sedentary office type work in quite a large floor areaaround the diffuser (the diffuser “outflow zone”).

The degree of under temperature and the size of the diffuser outflow zone, as well as the

air velocity are all issues that need to be addressed very carefully be designers whenchoosing a diffuser for an application. At present little information exists on the extent or range of acceptable under temperature regions and adoption of low air entrainmentdiffusers would require detailed physical mock-ups to confirm suitability beforeinstallation.

The supply air temperature used throughout the testing was 19°C, which should allowfree air cooling for about 85% of the year in south-east England. This might drop to



about 78% in a year with a very hot summer, but should be higher in more northerly UKlocations.

The improvement in displacement ventilation cooling performance was achieved byincreasing the air supply volumetric rate, which entails increased ventilation fan power and larger duct sizes. This must be taken into account when determining the overall

benefit of using displacement ventilation for cooling.

A practical consideration of using bin and sock fabric diffusers is the floor space takenup. In large open plan areas the bin diffuser could be located around structural columns,and there is no reason why fabric diffusers could not be located inside partition walls.Another possibility is to incorporate fabric into floor grill type diffusers. Preliminary testsshowed that this produces low entrainment air discharge and may offer a practicalalternative to socks and bins.

AcknowledgementsSome of the work described in the paper has been supported by the DTI’s Partners in

Innovation research programme and industry sponsors, including, Arup Research &Development, CIBSE, IPS Ventilation Ltd, Gilberts (Blackpool) Ltd, Halton Products Ltd,Hoare Lea, Oscar Faber, Troup Bywaters & Anders, Trox (UK) Ltd and Waterloo Air Management PLC.

References

1. Wyon D P and Standberg M. Thermal Manikin Prediction Of Discomfort Due ToDisplacement Ventilation. ASHRAE Trans., 96 (17), 67-75, 1990.

2. Alamdari F, Bennett K M, Rose P M. Displacement Ventilation Performance –office space application. Technical Note TN 3/93, BSRIA, 1993.

3. Alamdari F and Eagle N. Displacement Ventilation And Chilled Ceilings.Technical Note TN 2/96, BSRIA, 1996.

4. Breum N O. Ventilation Efficiency In An Occupied Office With DisplacementVentilation – A Laboratory Study. Environmental International, 18, 353-361,1992.

5. Breum N O, Helbo F and Laustesen O. Dilution Versus Displacement Ventilation – An Intervention Study. Annals of Occupational Hygiene, 33(3), 321-329, 1989.

6. Ørchede E, Breum N O and Skov T. Perceived And Measured Indoor ClimateWith Dilution Versus Displacement Ventilation: An Intervention Study In ASewing Plant. Indoor Air, 1996.

7. BS EN ISO 7730: 1995. Moderate thermal environments – Determination of thePMV and PPD indices and specification of the conditions for thermal comfort.

British Standards Institute, London, 1995.8. Geens A J, Graham M S and Alamadari F. Displacement ventilation applications

– an alternative view. 1997 CIBSE National Conference.9. Cullen N. High performance displacement ventilation using fabric diffusers – a

case study. 2000 ASHRAE CIBSE joint conference, Dublin, 2000.

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David Butler