Page 456 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

14.3 IAQ Control by Ventilation/Dilution 437

production rate is _ w, and the pollutant production rate is _ s. The densities of the air at
the intake, indoor space, and the exit are denoted as q ; q; and q , respectively.
e
s
Since it is assumed that indoor air is completely mixed, the properties of the exhaust
air are the same as those of the room air. For example,
m ¼ m e

h ¼ h e ð14:3Þ

c ¼ c e
Then, mass balance for dry air leads to,

d V Q s Q e
¼ ð14:4Þ
dt m m s m e
where m is specific volume of moist air, which is defined herein as the volume of the
moist air (dry air plus the water vapor) containing one unit of mass of “dry air” (m 3
air/kg dry air). As such the unit of both sides of the equation is kg dry air/s and that
3
of Q’sism air/s.
Equation (14.4) shows that the difference between the mass flow rates of the
supply dry air and the mass flow rate of exhaust dry air equals the change of mass of
dry air within the airspace. In this equation, we use specific volume of the air instead
of the density because specific volume the of the air defines the volume of the
mixture (dry air plus the water vapor) containing one unit of mass of “dry air”. The
3
SI unit of specific volume of air is (m air per kilogram of dry air). Note that it is
different from the inverse of air density.
1 6¼ q
m ð14:5Þ
3
The unit of air density is kg of air/m air. Similarly, we can describe the energy
balance based on the sensible heat as follows

d Vh Q s Q e
¼ _ q þ h s h e ð14:6Þ
dt m m s m e
where h ¼ sensible heat for supply air and exhaust air (kJ/kg of dry air). It implies
that the difference between the sensible heat of the supply air and that of the exhaust
air equals is the total sensible heat production plus the change of sensible heat in the
room air.
Similarly, the moisture mass balance and a specific pollutant mass balance can
be described using the following two equations, respectively,