Displacement Ventilation
Transcript of Displacement Ventilation
Federation of European Heating, Ventilation and Air-conditioning Associations
Address:
Rue Washington 40
1050 Brussels
Belgium
www.rehva.eu [email protected] Tel: +32 2 514 11 71 Fax: +32 2 512 90 62
Federation of European Heating, Ventilationand Air-conditioning Associations
Federation of European Heating, Ventilation and Air-conditioning Associations
Use of REHVA Guidebook Power Point Presentations
• This Power Point Presentation can be freely used for training purposes by REHVA members.
• It is prepared by the main author to the REHVA Guidebook.
• Please refer the original author always when making the presentation.
• Inform REHVA secretariat each time the presentation is used: [email protected]
Federation of European Heating, Ventilation and Air-conditioning Associations
Displacement VentilationinNon-industrial Premises
an old principlewith 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- Disp.vent in a nutshell1. Terminology, symbols and
units2. Basic knowledge about
displacement ventilation3. Air diffusers4. Design procedures5. Energy aspects6. Automation and control7. Case studies
• Restaurant• Cell office• Auditorium• Meeting room• Class room
8. 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
The air rises because of the heat release of the human body
80 – 90 W
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 supplyunits
Air supply
The air floats along the floor like water and fills the room from below
Federation of European Heating, Ventilation and Air-conditioning Associations
The extract air flows along the ceiling
Air is extracted at ceiling level
The extract opening can be located anywhere in the highest part of the room
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
Semi-circular,corner-mounted Floor-
mounted
Circular, free-standing
Federation of European Heating, Ventilation and Air-conditioning Associations
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.
• 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.
Typical depth 20 cm
z
~ 2 - 5 cm
z
v
Acceleration region
Velocity decay region
vx
Federation of European Heating, Ventilation and Air-conditioning Associations
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 - RestaurantAir 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
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
q
qh
Federation of European Heating, Ventilation and Air-conditioning Associations
Basic principles – Thermal stratification
Hei
ght a
bove
flo
or [
m]
0 0,2 0,4 0,6 0,8 1 1,20,0
0,5
1,0
1,5
2,0
2,5
Temperature ratio -
- s
se
In practice, the air will stratify in many layers, making the temperature rise from floor to ceiling.
Federation of European Heating, Ventilation and Air-conditioning Associations
Basic principles – Thermal stratification
H
eigh
t abo
ve fl
oor,
z [m
]
0
0,5
1,0
1,5
2,0
2,5
Supply 0%
Occupied zone 50 - 90%
Extract 100%
18 °C
26 °C
50 - 90%
0 20% 40% 60% 80% 100%Contaminant
18 °C
26 °C
Hei
ght a
bove
flo
or, z
[m]
0
0,5
1,0
1,5
2,0
2,5
Supply 0%
Occupied zone 90% - 100%
Extract 100%
0 20% 40% 60% 80% 100%Contaminants
Breathing zone < 100 %
Federation of European Heating, Ventilation and Air-conditioning Associations
Less cooling is needed to obtain the desired temperature in the occupied space
16 18 20 22 24 26
Hei
gh
t ab
ove
flo
or,
z [
m]
0
0,5
1,0
1,5
2,0
2,5
Temperature [°C]
Supply 18 °C
Occupied zone 23 °C
Extract 26 °C
18 °C
26 °C
24 °C
Displacement
16 18 20 22 24 26
Hei
gh
t ab
ove
flo
or,
z [
m]
0
0,5
1,0
1,5
2,0
2,5
Temperature [°C]
Supply 16 °C
Occupied zone 24 °C
Extract 24 °C
16 °C
24 °C
24 °C
Mixing
Temperature distribution - normal rooms
Federation of European Heating, Ventilation and Air-conditioning Associations
Temperature distribution - Tall rooms
The cooling advantage is most pronounced for tall rooms
Supply air temperature
s =18,5°C
Extract air temperature
e= 27,5°C
[°C]18 20 22 24 261614
9°C
28
Displacement
Supply air temperature
s =14°C
Extract air temperature
e = 23°C
[°C]18 20 22 24 261614
Average temperature
in the occupied
space
oz = 23°C9°C
28
Mixing
Federation of European Heating, Ventilation and Air-conditioning Associations
Contaminant distribution in normal rooms
For the same ventilation rate, we may get better air quality with displacement ventilation
18 °C
26 °C
50 - 90%
Hei
gh
t ab
ove
flo
or,
z [
m]
0
0,5
1,0
1,5
2,0
2,5
Supply 0%
Occupied zone 50 - 90%
Extract 100%
0 20% 40% 60% 80% 100%Contaminant
16 °C
24 °C
100%
0 20% 40% 60% 80% 100%
Hei
gh
t ab
ove
flo
or,
z [
m]
0
0,5
1,0
1,5
2,0
2,5
Contaminant
Supply
Occupied zone 100%
Extract 100%Mixing
Displacement
Federation of European Heating, Ventilation and Air-conditioning Associations
Little air - less benefit
Hei
gh
t ab
ove
flo
or,
z [
m]
0
0,5
1,0
1,5
2,0
2,5
Supply 0%
Occupied zone 50 - 90%
Extract 100%
18 °C
26 °C
50 - 90%
0 20% 40% 60% 80% 100%Contaminant
18 °C
26 °C
Hei
gh
t ab
ove
flo
or,
z [
m]
0
0,5
1,0
1,5
2,0
2,5
Supply 0%
Occupied zone 90% - 100%
Extract 100%
0 20% 40% 60% 80% 100%Contaminants
Breathing zone < 100 %
20 l/s per person = very good
10 l/s per person = acceptable
< 10 l/s per person = insufficient
Federation of European Heating, Ventilation and Air-conditioning Associations
Contamination in inhaled airConcentration ratios measured at a manikin
Flow ratelitre/sperson
0,0
0,2
0,3
0,4
0,5
0,6
0,1
0 5 10 15 20
Con
c. in
inha
led
air
Con
c. in
air
at h
ead
leve
lThe rising flow around a person brings fresh air to the breathing zone
Source:Mats Sandberg, Sweden
Federation of European Heating, Ventilation and Air-conditioning Associations
Better air quality:
Yes – and No?When there are many people, and insufficient air, the contaminants stratify at lower levels
Hei
ght a
bove
flo
or [
m]
00,0
0,5
1,0
1,5
2,0
2,5
100 200 300 400 500 600 700 800
One personin the room
Three personsin the room
Two personsin the room
Increase in CO2-concentration
Federation of European Heating, Ventilation and Air-conditioning Associations
Passive smoking in crowded rooms with
Displacement Ventilation
0,0
0,5
1,0
1,5
2,0
2,5
3,0
0 1 2 3 4 5 6
Concentration ratio c/cR
Hei
ght [
m]
Ambient
Exposure, seated
exposure, standing
Source:Peter V- Nielsen, Denmark
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
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
Hei
gh
t ab
ove
flo
or,
z [
m]
0
0,5
1,0
1,5
2,0
2,5
Supply 0%
Occupied zone
Extract 100%
0 20% 40% 60% 80% 100%Contaminant
0 20% 40% 60% 80% 100%
Hei
gh
t ab
ove
flo
or,
z [
m]
0
0,5
1,0
1,5
2,0
2,5
Contaminant
Occupied zone 100%
Extract 100%
Supply 0%
Displacement Perfect mixing
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
Temperature [°C]
50% 50%
18 19 20 21 22 23 24 25 26 271716
appr. 6K
Hei
ght a
bove
flo
or [
m]
0
1
2
3
4
5
6
Extract air temperature e =26°C
Supply air temperature =16°C
Air temperature at floor level, 21°Cs oz
Federation of European Heating, Ventilation and Air-conditioning Associations
Auditoria – Recirculation
Supply air qs = 10 l/s per pers cs = 750 ppm
Outdoor air qp = 6 l/s per pers cp = 350 ppm
Extract air qe = 10 l/s per pers ce = 1350 ppm
Recirculation air qr = 4 l/s per pers cr = 1350 ppm
qCO2 = 0,006 l/s per pers.
Cooling unit
ce = 1350 ppm
cexp ~ 900 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 is ok when the cooling output of the ceiling is less than 40% of the total cooling.
Cooled ceilings, or cooling convectors, decrease the air quality benefit.
Mixing ventilation should be considered as an alternative.
0,00
0,50
1,00
1,50
2,00
2,50
Hei
ght a
bove
flo
or le
vel,
z [m
]
0,8 1,0 1,2 1,4
Relative air temperature(relative to temp. at 0,1 m above the floor)
= 0
= 0,4
= 0,5
= 0,6
= ratio of the cooled ceiling cooling output to the total cooling output (Tan 1998)
Cooled ceiling
Federation of European Heating, Ventilation and Air-conditioning Associations
Room heating
When the room is occupied:• Don't heat the room by the ventilation air !
At night:• Some people recommend heating by the ventilation air….
Short-circuiting:The warm, fresh air is pulled into the extract
Federation of European Heating, Ventilation and Air-conditioning Associations
Heating by radiator/convector
Warm air from radiator balancesmeets 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:
• Restaurants,
• Meeting rooms,
• Classrooms
• Tall rooms:
– Conference rooms,
– Theatres,
– Supermarkets, etc
Less suited for:
• Where surplus heat is the main problem, and not air quality.
• Where ceiling heights are lower than approximately 2,3 metres.
• When the problem is cooling in low rooms (in offices, consider mixing and cooling panels)
• Where disturbances to room air flow is strong.
• Where the contaminants are colder/denser than the ambient air.
Advantages:• Improved air quality• Most efficient in tall room• 1°C – 3°C lower temperature in
the occupied space for a given supply temperature
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 floordiffusers 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