Federation of European Heating, Ventilation
and Air-conditioning Associations
Address:
Rue Washington 40
1050 Brussels
Belgium
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www.rehva.eu
[email protected]
Tel: +32 2 514 11 71
Fax: +32 2 512 90 62
Federation of European Heating, Ventilation and Air-conditioning Associations
Use of REHVA Guidebook Power
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•
•
•
•
This Power Point Presentation can be freely used for
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Guidebook.
Please refer the original author always when making
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Inform REHVA secretariat each time the presentation
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Federation of European Heating, Ventilation and Air-conditioning Associations
Displacement Ventilation
in
Non-industrial Premises
an old principle
with new technology
Federation of European Heating, Ventilation and Air-conditioning Associations
Rehva Guidebook on
Displacement Ventilation
Aimed at:
 the practising engineer
Discussing:
 what is displacement ventilation?
 what are the benefits
 what are the limitations?
 where should it be used (and not
used)?
The theory is illustrated by case
studies showing how displacement
ventilation can be utilised in
practice
Federation of European Heating, Ventilation and Air-conditioning Associations
Contents of the book
1.
2.
3.
4.
5.
6.
7.
Disp.vent in a nutshell
Terminology, symbols and units
Basic knowledge about
displacement ventilation
Air diffusers
Design procedures
Energy aspects
Automation and control
Case studies
•
•
•
•
•
8.
Restaurant
Cell office
Auditorium
Meeting room
Class room
References
Federation of European Heating, Ventilation and Air-conditioning Associations
The basic idea
Warm air rises due to
buoyancy, and carries
the pollutants from
people up towards the
ceiling.
In industry….
….and non-industrial
premises
Federation of European Heating, Ventilation and Air-conditioning Associations
Man = heat source – in theory
80 – 90 W
The air rises because of the heat
release of the human body
Federation of European Heating, Ventilation and Air-conditioning Associations
Man = heat source – in practice
• Warm, polluted
air rises due to
buoyancy.
• Buoyancy is
created by the
heat from the
body.
Cigar smoke visualises
the rising air, but is is
the heat from the body
that drives the flow.
Federation of European Heating, Ventilation and Air-conditioning Associations
Air supply
Air
supply
units
The air floats
along the floor like
water and fills the
room from below
Federation of European Heating, Ventilation and Air-conditioning Associations
Air is extracted at ceiling level
The extract
opening can be
located
anywhere in the
highest part of
the room
The extract air
flows along the
ceiling
Federation of European Heating, Ventilation and Air-conditioning Associations
Air supply
The air is
supplied from
wall panels with
low velocity.
The supply air is
colder than the
room air.
Federation of European Heating, Ventilation and Air-conditioning Associations
Air supply can be arranged in
many ways
Plane,
wall-mounted
Circular,
free-standing
Semi-circular,
corner-mounted
Federation of European Heating, Ventilation and Air-conditioning Associations
Floormounted
Be aware of cold draught along
the floor!
•
•
•
When the supply air is colder
than the room air, it will fall to
the floor, and may cause cold
draught.
This may avoided by choosing
the right diffuser.
z
Typical
depth
20 cm
vx
v
z
Remember:
– Choose a diffuser that is suited
for the purpose
– Make sure that the adjacent
zone (the “draught zone”) does
reach places where people are
located permanently.
~ 2 - 5 cm
Acceleration
region
Federation of European Heating, Ventilation and Air-conditioning Associations
Velocity
decay
region
Diffuser types - Casino
Air diffusers behind columns
Federation of European Heating, Ventilation and Air-conditioning Associations
Diffuser types - Atrium
Air diffuser
Federation of European Heating, Ventilation and Air-conditioning Associations
Diffuser types - Restaurant
Air diffuser
Federation of European Heating, Ventilation and Air-conditioning Associations
Diffuser types - Department
store
Air diffuser
Federation of European Heating, Ventilation and Air-conditioning Associations
Basic principles – Convection
currents
q
q
H
q
q
The air that rises in the
convection current must
be replaced by new air.
This makes a two-layer
flow, where the polluted,
“used” air stratifies in
the upper layer.
Less supply air lower
the interface between
the two layers.
q
h
q
Federation of European Heating, Ventilation and Air-conditioning Associations
Basic principles – Thermal
stratification
Height above floor [m]
In practice, the air will stratify in many layers,
making the temperature rise from floor to ceiling.
2,5
2,0
1,5
1,0
0,5
0,0
0
0,2
0,4
0,6
0,8
Temperature ratio
Federation of European Heating, Ventilation and Air-conditioning Associations
1
 - s
e - s
1,2
Basic principles – Thermal
stratification
Height above floor, z [m]
Extract 100%
2,5
26 °C
2,0
1,5
Occupied zone
50 - 90%
1,0
0,5
50 - 90%
Supply 0%
18 °C
0
0
20% 40% 60% 80% 100%
Contaminant
Height above floor, z [m]
Extract 100%
2,5
26 °C
2,0
1,5
Occupied zone
90% - 100%
Breathing zone
< 100 %
1,0
0,5
Supply 0%
18 °C
0
0
20% 40% 60% 80% 100%
Contaminants
Federation of European Heating, Ventilation and Air-conditioning Associations
Temperature distribution normal rooms
Mixing
Height above floor, z [m]
Extract 24 °C
2,5
16 °C
2,0
24 °C
1,5
Occupied zone
24 °C
1,0
0,5
Less cooling
is needed to
obtain the
desired
temperature in
the occupied
space
24 °C
Supply 16 °C
0
16
18
20
22
26
24
Temperature [°C]
Height above floor, z [m]
Displacement
Extract 26 °C
2,5
26 °C
2,0
1,5
Occupied zone
23 °C
1,0
0,5
24 °C
Supply 18 °C
18 °C
0
16
18
20
22
26
24
Temperature [°C]
Federation of European Heating, Ventilation and Air-conditioning Associations
Temperature distribution - Tall
rooms
Extract air
temperature
 = 23°C
e
Mixing
Extract air
temperature
 = 27,5°C
e
Displacement
Average
temperature
in the
occupied
space
 = 23°C
oz
9°C
9°C
14 16 18 20 22 24 26 28
Supply air temperature
 =14°C
s
[°C]
14 16 18 20 22 24 26 28
Supply air temperature
 =18,5°C
s
The cooling advantage is most
pronounced for tall rooms
Federation of European Heating, Ventilation and Air-conditioning Associations
[°C]
Contaminant distribution in
normal rooms
Mixing
Height above floor, z [m]
Extract 100%
2,5
16 °C
2,0
24 °C
1,5
For the same
ventilation
rate, we may
get better air
quality with
displacement
ventilation
Occupied zone
100%
1,0
100%
Supply
0,5
0
0
20% 40% 60% 80% 100%
Contaminant
Height above floor, z [m]
Displacement
Extract 100%
2,5
26 °C
2,0
1,5
Occupied zone
50 - 90%
1,0
0,5
50 - 90%
Supply 0%
18 °C
0
0
20% 40% 60% 80% 100%
Contaminant
Federation of European Heating, Ventilation and Air-conditioning Associations
Little air - less benefit
Height above floor, z [m]
Extract 100%
2,5
26 °C
2,0
1,5
Occupied zone
50 - 90%
1,0
0,5
20 l/s per
person
= very good
50 - 90%
Supply 0%
18 °C
0
0
20% 40% 60% 80% 100%
Contaminant
10 l/s per
person
= acceptable
Height above floor, z [m]
Extract 100%
2,5
26 °C
2,0
1,5
Occupied zone
90% - 100%
Breathing zone
< 100 %
1,0
0,5
Supply 0%
18 °C
0
0
20% 40% 60% 80% 100%
Contaminants
Federation of European Heating, Ventilation and Air-conditioning Associations
< 10 l/s per
person
= insufficient
Contamination in inhaled air
Concentration
ratios measured
at a manikin
0,6
Conc. in inhaled air
Conc. in air at head level
0,5
0,4
0,3
0,2
0,1
0,0
0
5
10
15
Flow rate
The rising flow around a
person brings fresh air
to the breathing zone
Federation of European Heating, Ventilation and Air-conditioning Associations
20
litre/s
person
Source:
Mats Sandberg, Sweden
When there are many
people, and insufficient air,
the contaminants stratify at
lower levels
Height above floor [m]
Better air
quality:
Yes – and No?
2,5
One person
in the room
2,0
Two persons
in the room
1,5
1,0
Three persons
in the room
0,5
0,0
0 100 200 300 400 500 600 700 800
Increase in CO2-concentration
Federation of European Heating, Ventilation and Air-conditioning Associations
Passive smoking in
crowded rooms with
Displacement Ventilation
3,0
Height [m]
Ambient
2,5
Exposure, seated
2,0
exposure, standing
1,5
1,0
0,5
0,0
0
1
2
3
4
5
6
Concentration ratio c/cR
A standing person’s exposure may
be greater, but people are still
protected by the rising air current
around themselves
Federation of European Heating, Ventilation and Air-conditioning Associations
Source:
Peter V- Nielsen, Denmark
For protection
against
tobacco
smoke, see
REHVA
Guidebook
no 4:
“Ventilation
and
Smoking”.
Federation of European Heating, Ventilation and Air-conditioning Associations
Contaminant distribution in tall
rooms
Perfect mixing
Displacement
2,5
2,0
1,5
Occupied zone
100%
1,0
0,5
0
Extract 100%
Height above floor, z [m]
Height above floor, z [m]
Extract 100%
2,5
2,0
1,5
Occupied
zone
1,0
0,5
0
0
Supply 0%
20% 40% 60% 80% 100%
Contaminant
20% 40% 60% 80% 100%
Contaminant
Supply 0%
0
Federation of European Heating, Ventilation and Air-conditioning Associations
Auditoria – air supply
Air supply in
front of the room
Air supply under
the seats
Federation of European Heating, Ventilation and Air-conditioning Associations
Auditoria – air flow pattern
Thermal and contaminant stratification
Federation of European Heating, Ventilation and Air-conditioning Associations
Auditoria – air flow pattern
Supply air is contained
between the rows.
Supply air is floating
down the stairways.
Federation of European Heating, Ventilation and Air-conditioning Associations
Auditoria – temperature
distribution
Extract air temperature
e =26°C
50%
50%
Height above floor [m]
6
5
4
3
appr. 6K
2
1
0
16 17 18 19 20 21 22 23 24 25 26 27
Supply air
temperature
s =16°C
Air temperature at
floor level,

oz21°C
Temperature [°C]
Federation of European Heating, Ventilation and Air-conditioning Associations
Auditoria – Recirculation
Outdoor air
qp = 6 l/s per pers
cp = 350 ppm
Recirculation air
qr = 4 l/s per pers
cr = 1350 ppm
ce = 1350 ppm
Extract air
qe = 10 l/s per pers
ce = 1350 ppm
qCO2 =
0,006 l/s per pers.
Cooling
unit
cexp ~ 900 ppm
Supply air
qs = 10 l/s per pers
cs = 750 ppm
cs = 750 ppm
Displacement ventilation gives benefits to recirculation in
tall rooms – for details, see the book
Federation of European Heating, Ventilation and Air-conditioning Associations
Cooled ceiling - high
temperature cooling
Cooled ceiling
Cooled ceiling is ok
when the cooling
output of the ceiling is
less than 40% of the
total cooling.
2,50
= 0
Height above floor level, z [m]
 = 0,6
2,00
 = 0,5
1,50
 = 0,4
1,00
 = ratio of the cooled
ceiling cooling output to
the total cooling output
(Tan 1998)
0,50
Cooled ceilings, or
cooling convectors,
decrease the air
quality benefit.
0,00
0,8
1,0
1,2
1,4
Relative air temperature
(relative to temp. at 0,1 m above the floor)
Mixing ventilation
should be considered
as an alternative.
Federation of European Heating, Ventilation and Air-conditioning Associations
Room heating
Short-circuiting:
The warm, fresh air is
pulled into the extract
When the room is
occupied:
• Don't heat the room
by the ventilation air !
At night:
• Some people recommend
heating by the ventilation
air….
Federation of European Heating, Ventilation and Air-conditioning Associations
Heating by radiator/convector
Warm air from
radiator balances
meets the cold air
from window/wall
Federation of European Heating, Ventilation and Air-conditioning Associations
Floor heating - low
temperature heating
Floor heating is ok as long as the floor
temperature is moderate
(i.e. less than appr. 25° C)
Federation of European Heating, Ventilation and Air-conditioning Associations
Useful or useless?
Best suited for:
Less suited for:
•
Restaurants,
•
•
Meeting rooms,
Where surplus heat is the main
problem, and not air quality.
•
Classrooms
•
•
Where ceiling heights are lower
than approximately 2,3 metres.
Tall rooms:
•
When the problem is cooling in
low rooms (in offices, consider
mixing and cooling panels)
•
Where disturbances to room air
flow is strong.
– Conference rooms,
– Theatres,
– Supermarkets, etc
Advantages:
•
• Improved air quality
• Most efficient in tall room
• 1°C – 3°C lower temperature in
the occupied space for a given
supply temperature
Where the contaminants are
colder/denser than the ambient
air.
Federation of European Heating, Ventilation and Air-conditioning Associations
..and please remember:
Displacement Ventilation is no miracle,
and should not be used everywhere.
It has lost much of its reputation in
several countries due to incompetent or
over-optimistic use.
Problems:
draught along the floor
diffusers require much wall space
..but used with skill in the right places,
it has definite advantages.
Federation of European Heating, Ventilation and Air-conditioning Associations
Thank you
for your
attention
To order the REHVA guidebooks :
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Federation of European Heating, Ventilation and Air-conditioning Associations