BC 3722 HVAC Engineering Semester A 2003/04 Dr. Richard K K Yuen Department of Building &...

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


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


Wind Effect

Wind speed varies with height above the ground

This wind speed is related to wind pressure


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


am su u K z

Wind Effect

CIBSE provide Ks and a values:


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


0

212

p

r

p pC

u

Wind Effect

Volumetric flow rate through opening, V


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


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


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


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


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


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


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


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


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


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


( )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


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


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


Special CasesCase 2. Production of contaminants stopped atτ=0 if

Ci=0 and c=0


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


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)


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


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)


V

1 ( 1)exp /oC C V V

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

Special Cases (Con’t)

Case 5. Equilibrium condition


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


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


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


PTI The velocity flow field of a PTI in


Contaminant dispersion pattern in Indoor Shooting Range


THE END


BC 3722 HVAC Engineering Semester A 2003/04 Dr. Richard K K Yuen Department of Building &...

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