bc 3722 hvac engineering semester a 2003/04
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BC 3722HVAC EngineeringSemester A 2003/04
Dr. Richard K K Yuen
Department of Building & Construction
Aims of Ventilation System
To remove any undesirable odours or contaminants in a space or living place
To ensure the CO2 produced by respiration is being replaced by an adequate supply of O2
BC 3722 HVAC Engineering
Types of Ventilation
Natural Ventilation No mechanical means of supply and extraction of air Allow air to enter or leave the building due to pressure
differences between outside and inside air Open windows, ventilators, shafts
Mechanical Ventilation With mechanical means of supply and extraction of air Control the amount of air entering and leaving Fans
BC 3722 HVAC Engineering
Natural Ventilation
Two basic driving forces:
Wind Effect:
Pressure on the face of a building due to rapid deceleration of the wind of the outside surface
Stack Effect
Buoyancy effect due to the temperature difference between the outside and inside air
BC 3722 HVAC Engineering
Wind Effect
Wind speed varies with height above the ground
This wind speed is related to wind pressure
BC 3722 HVAC Engineering
m
s
u is mean wind speed at height z
u is mean wind speed at heigth of 10m in open country
K and a are parameters dependednt on terrain
am su u K z
Wind Effect
Roughness => depends on the type of bldgs From the eqt.,
z ↑ u ↑
However, this is impossible for the wind speed to be infinity. To explain this, basic boundary theory is adopted.
Basic boundary theory:When wind speed approaches the ambient wind speed, the
height become the thickness of the boundary layer
BC 3722 HVAC Engineering
am su u K z
Wind Effect
CIBSE provide Ks and a values:
BC 3722 HVAC Engineering
Terrain Ks a
Open, flat country 0.68 0.17
Country with scattered windbreaks
0.52 0.20
Urban 0.35 0.25
City 0.21 0.33
Wind Effect
A non-dimensional coefficient, Cp
where p = mean pressure at any point on bldg surface
p0 = pressure in the undistributed air stream
ur = mean wind speed at a height equal to the bldg height
ρ= density of air at temp. of outside air
BC 3722 HVAC Engineering
0
212
p
r
p pC
u
Wind Effect
Volumetric flow rate through opening, V
BC 3722 HVAC Engineering
2
12
/ 2
where A is the cross-sectional area of opening
is pressure difference across opening
(2 / )
d
d d
V C Au
u p
p
C Au C A p
Wind Effect
For a number of openings in the same face of bldg, since the pressure difference is the same for each opening, and assuming the same value of Cd for each opening
p
BC 3722 HVAC Engineering
12( )(2 / )dV C A p
Wind Effect
Assume air enters through openings on windward side of bldg and leave bldg through openings on leeward side
In steady state, and assume
BC 3722 HVAC Engineering
12
1 1 1 1
12
2 2 2 2
( ) 2( ) /
( ) 2( ) /
d i
d i
V C A p p
and V C A p p
1 2 wV V V 1 2
2
1 2 2 22
1 2
1 1
2w
d
Vp p
C A A
Wind Effect
BC 3722 HVAC Engineering
21 1 0
22 2 0 r
2 21 2 1 2
2 2
22
2 2 2
1 2
1 2
1( )
21
( ) where u is the wind speed2
1( ) / 2
2
Thus, 2 2
1 1 1 where
Therefore,
p r
p r
p p r p r
p r w
d w
w
w d w r p
C p p u
C p p u
p p C C u C u
C u V
C A
A A A
V C A u C
Stack Effect – Buoyancy flow
BC 3722 HVAC Engineering
1 2
1 2
1 1 2 2( )
o o o
i i i
o i o i
p p g z
p p g z
p p gz p p
1
2
ti
ρi
to
ρo
ur
to
ρo
A1 and A2 = area of openings separate by a height z
ρi and ρo = mean density of outside and inside air respectively
Δρ = difference in density between outside and inside air
Stack Effect
BC 3722 HVAC Engineering
In steady state, mass flow entering the bldg at section 1 is equal to the mass flow leaving the bldg at section 2
1 2
1 12 2
1 1 1 2 2 2
21
2 2 1 1 22
21
1 1 1 1 22
( ) 2( ) / ( ) 2( ) /
( )By rearranging, ( )
( )
( )Thus, ( )
( )
o i
o d o i o i d i o i
oi o o i
i
oo i o i
i
V V
C A p p C A p p
Ap p p p
A
Ap p gz p p
A
1 1 2 21 2
1 ( ) ( )o i
o i
gzp p
A A
Stack Effect
BC 3722 HVAC Engineering
1 2
1 1 1
1 2
1 2 21 2
2 21 2
Volume rate entering the bldg by natural ventilation
2( ) /
2
1 ( ) ( )
2
1
N d o i o
N d
o o i
N d
oo
i
V C A p p
gzV C A
A A
gzV C
A A
Stack Effect
BC 3722 HVAC Engineering
1 2
2 2 21 2
2
1 1 1where
N d N
N
gz TV C A
T
A A A
Two assumptions can be made with good accuracy:
(i) Density if air at mean of the inside and outside temp, ρ, can be put equal to the densities, ρo and ρi
(ii)Ratio Δρ/ρ can be put equal to ΔT/T, where T (in K) is the mean temp. of the absolute temp. of the inside and outside air
Mechanical Ventilation
Air infiltration air leakage through cracks around windows, doors Uncontrolled and undesirable
Displacement ventilation Air is supplied at low velocity through large number of
openings in ceiling or floor, and push across the cross-section of the space taking all the contaminants with it
Dilution ventilation Air is introduced through jets into the room, jet set up a
mixing pattern in room, thus diluting the contaminant in space before extract
BC 3722 HVAC Engineering
Threshold Limiting Value (TLV)
1. As a time-weighted average concentration for a working day, or working week, to which workers may be subjected without adverse effects
2. As a maximum concentration to which workers can be subjected for a short time period, say up to 15 minutes
* Lower explosive limit: concentration which must never be exceeded at any time
BC 3722 HVAC Engineering
Dilution of contaminants
Concentration, C, at any time τ
{CiV + Vc – C(V+Vc)}dτ=d(CV)
{ ( )} 0i c co
CdC d
CV V C V V VC
BC 3722 HVAC Engineering
( )1ln
( ) ( )i c c
c i c o c
CV V C V V
V V CV V C V V V
• Then, the concentration, C, at any timeτis given by,
exp ( ) /i c i co c
c c
CV V CV VC C V V V
V V V V
( )exp{ ( ) / }
( )i c c
ci c o c
CV V C V VV V V
CV V C V V
BC 3722 HVAC Engineering
Dilution of contaminants (Cont’d)
Special Cases
Case 1. No contaminant in incoming air (i.e. Ci=0)
exp ( ) /c co c
c c
V VC C V V V
V V V V
BC 3722 HVAC Engineering
exp ( ) /i c i co c
c c
CV V CV VC C V V V
V V V V
0 0
Special Cases
Eqn(1.62) case1
Eqn(1.64) case3
Eqn(1.63) case 2
Eqn(1.65) case 4
1
Cc=CiV+Vc /V+Vc
Co
Con
cent
rati
on,
C
Time, t
BC 3722 HVAC Engineering
Special CasesCase 2. Production of contaminants stopped atτ=0 if
Ci=0 and c=0
BC 3722 HVAC Engineering
V
exp ( ) /i c i co c
c c
CV V CV VC C V V V
V V V V
000
exp /oC C V V
0
0
0
0
0
Special Cases
BC 3722 HVAC Engineering
Eqn(1.62) case1
Eqn(1.64) case3
Eqn(1.63) case 2
Eqn(1.65) case 4
1
Cc=CiV+Vc /V+Vc
Co
Con
cent
rati
on,
C
Time, t
Special Cases
Case 3. Space is uncontaminated at time τ (i.e. Co=0)
BC 3722 HVAC Engineering
1 exp ( ) /i c cc
c
CV VC V V V
V V
exp ( ) /i c i co c
c c
CV V CV VC C V V V
V V V V
0
Special Cases
BC 3722 HVAC Engineering
Eqn(1.62) case1
Eqn(1.64) case3
Eqn(1.63) case 2
Eqn(1.65) case 4
1
Cc=CiV+Vc /V+Vc
Co
Con
cent
rati
on,
C
Time, t
Special Cases (Con’t)Case 4. Sudden failure of ventilation system at time
τwhen conc. is Co ( =0)
BC 3722 HVAC Engineering
V
1 ( 1)exp /oC C V V
0
Special Cases
BC 3722 HVAC Engineering
Eqn(1.62) case1
Eqn(1.64) case3
Eqn(1.63) case 2
Eqn(1.65) case 4
1
Cc=CiV+Vc /V+Vc
Co
Con
cent
rati
on,
C
Time, t
Special Cases (Con’t)
Case 5. Equilibrium condition
BC 3722 HVAC Engineering
e
( ) 0
Equilibrium concentration,
Hence, ventilation flow rate required to restrict the conc. to C is
(1 )
i c e c
i c ce
c
c e
e i
CV V C V V
CV VC
V V
V CV
C C
Special Cases
BC 3722 HVAC Engineering
Eqn(1.62) case1
Eqn(1.64) case3
Eqn(1.63) case 2
Eqn(1.65) case 4
1
Cc=Ci V+Vc /V+Vc
Co
Con
cent
rati
on,
C
Time, t
Design of Ventilation system Ventilation rate
By the eqt.
Ideal volumetric flow rate Distribution of Air
Effects of: Mixing Dilution Removal
Cannot be derived from the eqt. shown above
BC 3722 HVAC Engineering
exp ( ) /i c i co c
c c
CV V CV VC C V V V
V V V V
Design of Ventilation system Ventilation can be assessed through
Experimental measurement
Computational Fluid Dynamics (CFD) Examples:
1. Public Transport Interchange (PTI)
2. Shooting Range
BC 3722 HVAC Engineering
PTI The velocity flow field of a PTI in
BC 3722 HVAC Engineering
Contaminant dispersion pattern in Indoor Shooting Range
BC 3722 HVAC Engineering
THE END
BC 3722 HVAC Engineering
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