Air conditioning using displacement ventilation to maximize ... - Cibse

Air conditioning using displacement ventilation to maximize free cooling
David Butler*
*David Butler is a Principal Consultant in BRE’s Environmental Engineering Centre.
Summary
Displacement 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 ventilation
systems and creates opportunities for using fresh-air based “free cooling” for a large
proportion of the year.
Conventional displacement ventilation design practice suggests that the cooling capacity
of displacement ventilation in commercial offices with normal floor to ceiling heights (up
to 3 m) is limited to around 25 W/m 2 . For this reason it is fairly common in the UK to
specify chilled ceilings in conjunction with displacement ventilation to meet typical office
heat loads. However, recent BRE research has shown that displacement ventilation on
its own using appropriate diffusers can deal with heat loads of around 50-55 W/m 2 in
typical 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 ventilation
Displacement ventilation is essentially a buoyancy driven "displacement" process.
"Fresh" ventilation air is introduced at low velocity and at low-level into the occupied
zone 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 rising
upwards. This plume will draw air from the reservoir of cool fresh air at low level in the
room. If the source of heat also represents a pollutant source, these pollutants will also
be transported up out of the occupied zone.
1

2
Return 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 zone
containing cooler cleaner air separated by a boundary layer. This causes a vertical
temperature gradient to develop (typically 5-6°C between supply and extract), resulting
in higher temperatures at ceiling level than with standard mixing ventilation systems.
Displacement ventilation strategies exploit this vertical temperature gradient by
maintaining 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 with
such a strategy is, however, that significant vertical temperature gradient between feet
and head height can lead to discomfort and this must be considered when designing
such 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 is
normally 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 to
lower areas with relatively high air speeds (see Figure 2). This can lead to draughts and
discomfort in the occupied area. Because air is supplied and extracted at ceiling level
some of the supplied ‘fresh’ air may short circuit by being drawn into the extract grilles
which effectively reduces its cooling and pollutant dilution potential.

3
supply grill Return grill
Figure 2 Mixed flow ventilation – typical room air flow
The air supply temperature for displacement ventilation systems is typically 19°C
compared 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 office
environment 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 the
floor due to room air entrainment.
Supply air temperature
= 14 o C
9 o C
Air temperature near
head height for seated
occupants = 23 o C
Supply air temperature
= 19 o C
Room & extract air
temperature = 23 o C
7 o C
Mixing
ventilation
12 14 16 18 20 22 24 26 28 30 o C
Extract air temperature
= 26 o C
Displacement
ventilation
Figure 3 Supply and extract air temperatures for mixed flow and displacement
ventilation (ideal situation)

“Free cooling” opportunities
Supplying 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 a
misleading 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 essential
in 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 may
be required, possibly in conjunction with heat pipe heat recovery and pre-cooling, or face
and 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 a
hot summer is also shown by plotting hours occurrence for 1976 which had an
exceptionally long hot summer. The data can be interpreted more easily by converting it
to 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 to
result 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 therefore
shows that fresh-air free-cooling can be used for 85% of the time for a displacement
system but only 53% of the time for a conventional mixing ventilation system.
4
Total hours occurence
350
300
250
200
150
100
50
0
-8
Five year average, 1976
to 1980
-5
-2
1
Dry Bulb T (07:00 - 18:00)
Kew 1976 to 1980
4
7
10
13
16
19
22
Dry Bulb Temperature (oC)
Figure 4 Bin analysis of outdoor dry bulb air temperature
25
28
31
34

5
Accumated frequency (% hours occurrence)
110%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
85%
53%
Dry bulb temperature (whole year, 0700 to 1800 hrs)
12 o C 18 o C
-5 0 5 10 15 20 25 30 35
Dry bulb temperature (oC)
1977 cool summer
1976 hot summer
1976 to 1980

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) have
also 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 cool
summer would increase the proportion of the year that free-cooling is available to about
90% of the total time.
Thermal comfort requirements
Thermal comfort requirements place limitations on the design parameters and the
maximum cooling performance achievable with displacement ventilation. For sedentary
occupancy such as office work, BS EN ISO 7730:1995 (7) recommends that air velocity
onto people should not normally exceed 0.15 to 0.2 m/s, depending on air temperature
and 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 function
of 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 ventilation
Conventional design practice and BRE investigations suggest displacement ventilation
systems in offices (with floor to ceiling heights below 3 m) are limited to around 20/25
W/m 2 cooling capacity. To increase cooling capacity either the supply air temperature
has 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 the
maximisation 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 occupied
space. Fan power and duct sizing will also be increased and therefore a life cycle
analysis of this compared with the reductions in cooling energy must be undertaken as
part of specific system designs.
Types of displacement diffuser
A wide range of diffusers can be used for displacement ventilation, including the
following:
6
• 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 are
not yet widely used and are rare in offices in the UK. Wall, corner and free-standing
diffusers typically consist of a perforated plate with an internal baffle to promote even air

discharge through the perforated plate face. The active surface areas are usually quite
large 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 typically
produces 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 than
the other types of diffuser.
7
Corner displacement
diffuser
Wall mounted
displacement diffuser
Free standing “bin”
displacement diffuser
Figure 6 Outflow air profiles of typical displacement ventilation diffusers
Floor mounted swirl
displacement diffuser
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 ventilation
requirement for air to be introduced at low velocity. Their apparent popularity may be
due to the installation flexibility offered by floor terminals in conjunction with raised floors,

although there is a risk of low level thermal discomfort if occupants sit too close to them.
Limitations of conventional displacement diffusers
The primary limitation of displacement ventilation in dealing with heat loads has been
shown 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 generally
involves defining the distance from the diffuser at which the air speed has decreased to
between 0.15 and 0.2 m/s (the usual threshold for acceptable thermal comfort), and may
be measured at around 0.1 m above the floor.
BRE has tested a range of conventional displacement ventilation diffusers including a
swirl displacement diffuser and a floor-standing “bin” diffuser. The swirl diffuser had an
active area of 0.125 m 2 (overall diameter = 0.25 m) at a range of air supply volumetric
rates. The results for air speed at 1.07 m from the center of the diffuser are shown in
Figure 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 spacing
between diffusers (9 m 2 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 cooling
performance would be 24 W/m 2 .
8
Height (m)
0.30
0.25
0.20
0.15
0.10
0.05
Variation of mean air speed with height at 1.07 m from centre of diffuser
(0.25 m dia floor swirl)
maximum air
speed for
acceptable
thermal comfort
Diffuser A, 10 l/s
Diffuser A, 30 l/s
Diffuser A, 50 l/s
0.00
0.00 0.05 0.10 0.15 0.20
Mean air speed (m/s)
0.25 0.30 0.35 0.40
Figure 7 Swirl diffuser – air speed at 1.07 m from diffuser centre (0.05 m above the
floor)

The bin diffuser was constructed from perforated steel plate and had a height of 0.61 m
and a diameter of 0.20 m, giving an active face area of 0.38 m 2 . Figure 8 shows air
speed at a distance of 1.07 m from the center of the diffuser for a range of air volumetric
supply rates. It is clear that for acceptable thermal comfort (based on air speed in the
space at 0.05 m above the floor) the limit of this diffuser is 50 l/s. It is very unlikely that
in 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 same
assumptions for air temperatures as for the swirl diffuser the limit on cooling
performance is about 20 W/m 2 . Larger free-standing diffusers would allow higher air
volume supply rates and therefore provide a higher cooling performance than this but
would take up greater floor area.
9
Height (m)
0.30
0.25
0.20
0.15
0.10
0.05
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)
maximum air
speed for
acceptable
thermal comfort
Diffuser C, 30 l/s
Diffuser C, 50 l/s
Diffuser C, 100 l/s
0.00
0.00 0.05 0.10 0.15 0.20
Mean air speed (m/s)
0.25 0.30 0.35 0.40
Figure 8 “Bin” diffuser – air speed at 1.07 m from diffuser centre (0.05 m above
the 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 in
the 7.5 m x 5.5 m test cell was 1000 W or 24 W/m 2 .

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 traverse
line perpendicular to the diffuser). Air speed exceeds 0.2 m/s at 0.1 m height above the
floor 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.
Height above floor (m)
10
3
2.5
2
1.5
1
0.5
3.36 m from end of room
Maximum air
speed for
acceptable
thermal comfort
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
0
0 0.05 0.1 0.15 0.2
Air speed ( m/s)
0.25 0.3 0.35 0.4
Figure 10 Wall diffuser (130 l/s) – air speed

height above floor (m)
11
3
2.5
2
1.5
1
0.5
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
3.36 m from end of room
dT ~ 3 K
0
18 19 20 21 22
Air temperature (oC)
23 24 25 26
Figure 11 Wall diffuser (130 l/s) – dry bulb air temperature
A higher thermal load would require another diffuser. An earlier test with the same
diffuser 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/m 2 ) showed similar results, and that acceptable thermal comfort
could be achieved in the space outside the diffuser outflow zone. However, with a
smaller test cell floor area the relative size of the outflow zone is larger which reduces
the area that has acceptable thermal comfort for sedentary office work.
Displacement ventilation diffuser air entrainment
The degree of room air entrainment in the diffuser outflow is an important characteristic
of displacement ventilation diffusers and affects the volume and velocity of the outflow
beyond approximately 1 m from the diffuser face. For example, floor swirl diffusers and
wall 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 in
Figure 12. Each hole creates a small jet, which entrains some room air. For a given
supply 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 the
risk of cold temperatures around the legs and feet. However, the resulting higher levels
of room air mixing will tend to disrupt the stratified displacement ventilation room airflow
pattern 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 and
air turbulence intensity.

Figure 12 Air flow pattern with perforated panel diffusers
12
Little air is entrained in
the primary flow
Air is is entrained entrained at at the the the
the
boundary surface between the
outflowing air and the room air.
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. Such
diffusers create very little air entrainment at the diffuser face, entraining a small volume
of room air at the boundary surface between the outflowing air and the room air. Overall
they produce low levels of room air entrainment which results in lower air speed and
turbulence 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 increase
cooling 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 rise
of the supply air as it is discharged from the diffuser. This mixing reduces the “undertemperature”
of air within the diffuser outflow zone. Under-temperature is defined as the
temperature difference between the air within the diffuser outflow zone and room

temperature (in the occupied region). However, when the aim is to supply the air at the
highest possible temperature to maximise free cooling a high entrainment diffuser is
undesirable as it raises the air temperature in the lower occupied zone. To maximise
free cooling it is therefore preferable to use a low entrainment diffuser.
13
height above floor (m)
3.0
2.5
2.0
1.5
1.0
0.5
Supply air
temperature
Little entrainment
Moderate entrainment
A lot of entrainment
Room air
temperature
Extract air
temperature
0.0
18 19 20 21 22 23 24 25 26 27 28
Air temperature (oC)
Figure 14 Variation of room air temperature with height (for diffusers with varying
room air entrainment) – based on practical experience
Extending the cooling performance of displacement ventilation systems
The results of BRE testing a range of existing displacement ventilation diffusers has
demonstrated that as the air supply rate is increased, above 30 l/s for a floor swirl
diffuser and 50l/s for a bin diffuser, the air velocities at approximately 1m from the
diffuser exceeded the maximum air velocity acceptable for thermal comfort. Such supply
airflow rates represent cooling loads of less than 25 W/m 2 for a practical office
configuration and temperature differential. Higher cooling loads could be met by
increasing the number of diffusers for a given floor area but this would reduce the usable
floor area with acceptable thermal comfort.
Tests with a conventional wall diffuser showed that 24 W/m 2 could be achieved without
causing 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 to
around 50 W/m 2 . The diffuser is nevertheless fairly large and takes up a significant
amount of wall space, and has a relatively large outflow zone in which occupants are
likely to experience thermal discomfort when involved in sedentary office work. A wall
diffuser would also be unsuitable for large open plan areas and in such situations
appropriately designed bin diffusers would need to be used instead.
Based on these results, therefore, the generally held belief that displacement ventilation
is only applicable to commercial buildings with modest cooling loads is only partially true.

It appears that appropriately designed and located diffusers are capable of meeting large
cooling loads, albeit with some impact on space utilisation flexibility due to the space
taken up by the diffusers and the impact on thermal comfort in the diffuser outflow
zones. Thermal comfort in the diffuser outflow zones is especially critical for sedentary
office work but may well be acceptable for circulation areas or areas around
photocopiers or drink dispensers which are only occupied for short times.
Fabric diffusers
Earlier work elsewhere (8) has shown considerable promise in the use of fabric diffusers
to provide higher air flow rates without causing thermal discomfort from draughts. The
results suggested that fabric diffusers positioned along the bottom of two opposite walls
of a room could deal with heat loads of around 50 W/m 2 without causing high air
velocities in the occupied space. A recent case study (9) used fabric diffusers in a
restaurant to overcome a high heat load from the occupants (56 W/m 2 ) and to provide a
high 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 to
supply 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 an
experimental fabric sock that was manufactured from polyester/cotton fabric with a
quadrant 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 high
supply-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 m
long, 200 mm high and 200 mm wide. It was retained in position by two sewn-in plastic
beads which clipped into channels fixed to the wall and floor. Air was supplied into the
sock through an opening at one end from a flexible duct. The fabric created sufficient
internal 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 to
48 W/m 2 in a BRE mock-up of a typical office space.
14
200 mm
200 mm

Figure 16 shows the vertical variation of air temperature at different distances from the
sock following a floor traverse perpendicular to the sock and starting about 0.5 m from
one 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 from
the 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.
height above floor (m)
15
3
2.5
2
1.5
1
0.5
0.5m from diffuser
1.2m from diffuser
2.2m from diffuser
2.7m from diffuser
3.2m from diffuser
4.5m from diffuser
2.4m from end of room
Traverse 0.5 m from end of diffuser
dT ~ 3.5K @ 1.2 m
(dT ~ 2.5K @ 2.2 m)
0
18 19 20 21 22 23 24 25 26 27 28
Air temperature (oC)
Figure 16 Variation of air temperature with height at 260 l/s (48 W/m 2 )
BS EN ISO 7730 relates the maximum acceptable air velocity to the air temperature and
turbulence intensity as well as the occupants' activity and clothing levels. Taking into
account these factors, and assuming typical office activity and clothing, the maximum
recommended air velocity is between 0.15 and 0.2 l/s. Figure 17 shows that air speed is
well within this limit.
Overall the fabric sock diffuser has provided acceptable thermal comfort in most of the
room for normal sedentary office work, apart from close to the centre of the diffuser.

Height above floor (m)
16
3
2.5
2
1.5
1
0.5
2.24 m from end of room
Traverse 0.5 m from end of diffuser
0.5m from diffuser
1.2m from diffuser
2.2m from diffuser
2.7m from diffuser
3.2m from diffuser
4.5m from diffuser
Maximum air
speed for
acceptable
comfort
0.15 to 0.2 m/s
0
0 0.02 0.04 0.06 0.08 0.1
Air speed ( m/s)
0.12 0.14 0.16 0.18 0.2
Figure 17 Variation of air speed at 260 l/s (48 W/m 2 )
BRE also tested an experimental fabric bin diffuser which was constructed from the
same polyester/cotton fabric as the sock, see Figure 18. The dimensions of the bin were
700 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 the
underfloor 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 the
test room heat load was 2000 W which was equivalent to 48 W/m 2 . Air speed and air
temperature were measured throughout the room space by traversing with multiple
transducer poles.

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 a
traverse across the width of the test room. The middle of the traverse was
approximately 1.5 m from the diffuser. The measured temperature gradient between
ankle 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 being
acceptable 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.
17
height above floor (m)
3
2.5
2
1.5
1
0.5
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
2.4m from end of room
Middle of traverse ~1.5 m from diffuser
dT ~ 3K at point
nearest to diffuser
0
18 19 20 21 22 23 24 25 26 27 28
Air temperature (oC)
Figure 19 Variation of air temperature with height at 260 l/s (48 W/m 2 )
Figure 20 shows the vertical variation of air speed. Air speed was well below the
recommended maximum value given by BS EN ISO 7730.

Figure 20 Variation of air speed at 260 l/s (48 W/m 2 )
Comparison of the test results from these two diffusers showed that overall the fabric bin
diffuser provided a higher level of thermal comfort in the test room than provided by the
fabric sock diffuser. This was largely due to the greater vertical temperature gradient
close to the middle of the sock diffuser. The regions of high temperature gradient shown
to 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 thermal
load in the space. Therefore, the vertical gradient is greatest in the diffuser outflow zone
where 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 a
single sided diffuser whereas the bin has an unrestricted radial discharge. This effect is
illustrated in Figure 21 below.
The size of the outflow zones, and therefore the size of the areas around the diffusers
with relatively poor thermal comfort, would be smaller with lower cooling loads. The
cooling load assumed in this study is 48 W/m 2 which is deliberately on the high side of
the cooling loads found in typical office buildings.
18
Height above floor (m)
3
2.5
2
1.5
1
0.5
2.24 m from end of room
Middle of traverse ~1.5 m from diffuser
Maximum air
speed for
acceptable
comfort
0.15 to 0.20 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
0
0 0.05 0.1 0.15 0.2
Air speed ( m/s)
0.25 0.3 0.35 0.4

19
Limit of zone with
high vertical
temperature gradient
1.5 m
Limit of zone with
high vertical
temperature gradient
Figure 21 Outflow size from fabric sock and bin diffusers
Conclusions
It 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 be
occupied by people. Large wall diffusers have been shown to be capable of satisfying
cooling loads as high as 50-55 W/m 2 without causing thermal discomfort outside the
diffuser outflow zone. However, the diffuser outflow zone will be relatively large and this
would reduce the practicality of using such a diffuser for large loads, especially in
buildings 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 general
circulation areas which have less stringent criteria for acceptable thermal comfort. A
cooling performance of 50-55 W/m 2 would be sufficient for the cooling requirements in
the core zones of most normal office buildings. Bin diffusers would have to be used
instead 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 performance
around 50 W/m 2 could be achieved. This performance was in part achieved by utilising
the low entrainment characteristic of fabric as part of the diffusers, which allows
relatively high volumetric supply rates without causing unacceptably high air speeds. A
disadvantage of minimal mixing and entrainment of the room air with supply air at the
diffuser 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 area
around 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 when
choosing 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 entrainment
diffusers would require detailed physical mock-ups to confirm suitability before
installation.
The supply air temperature used throughout the testing was 19°C, which should allow
free air cooling for about 85% of the year in south-east England. This might drop to
1.5 m

about 78% in a year with a very hot summer, but should be higher in more northerly UK
locations.
The improvement in displacement ventilation cooling performance was achieved by
increasing 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 taken
up. 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 tests
showed that this produces low entrainment air discharge and may offer a practical
alternative to socks and bins.
Acknowledgements
Some 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
20
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