Page 458 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

14.3 IAQ Control by Ventilation/Dilution 439

heat removed by the exhaust air. With this minimum requirement, the d=dt terms
are zeros and aforementioned equations become, respectively,
d
¼ 0 ð14:10Þ
dt

Q s Q e
¼ : ð14:11Þ
m s m e
Q s Q e
_ q þ h s ¼ h e : ð14:12Þ
m s m e
Q s Q e
_ w þ w s ¼ w e : ð14:13Þ
m s m e
_ s þ Q s c s ¼ Q e c e : ð14:14Þ

Solving these equations leads to the relationship between volumetric ventilation
rates and sensible heat balance (or temperature balance) control at steady state

_ q Q s Q e
¼ ¼ : ð14:15Þ
h e h s m s m e
By similar approaches, we can get the moisture balance ventilation requirement
at steady state. Combination of the conservation of mass equation and the con-
servation of moisture equation gives volumetric ventilation rates for moisture (or
relative humidity) balance control at steady state
_ w Q s Q e
¼ ¼ : ð14:16Þ
w e w s m s m e

Applying the mass balance to pollutant balance one can get the pollutant balance
ventilation requirement. At steady state,

m s c s m e c e
_ s ¼ Q e c e 1 ¼ Q s c s 1 : ð14:17Þ
m e c e m s c s
The analysis above defines different ventilation rates for the control of temper-
ature, moisture, and a specific air pollutant.
There is a minimum ventilation rate that is required to maintain an acceptable
indoor environmental condition (temperature, humidity, and a specific pollutant
concentration). This minimum value must be the greatest value of the six venti-
lation rates for balances of sensible heat, moisture, and the given pollutant defined
by Eqs. (14.15)to(14.17). Otherwise, at least one of the requirements cannot be
met. In reality, the HVAC equipment of a building has an upper limit in capacity
and there is a maximum ventilation rate. Therefore, we have to be careful in
selecting the right HVAC equipment.