Page 457 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

438 14 Indoor Air Quality

d Vw Q s Q e
¼ _ w þ w s w e : ð14:7Þ
dt m m s m e
d
ð VcÞ ¼ _ s þ Q s c s Q e c e : ð14:8Þ
dt
where w is humidity ratio (kg of water vapor per kg of dry air), _ w is the water vapor
production rate in kg/s, c is the pollutant concentration in air (kg of pollutant per m 3
of air), _ s is the mass production rate of the particulate pollutant in kg/s, Subscripts
‘s’ and ‘e’ stand for supply and exhaust air, respectively.
If the unit of air pollutant concentration c is in ppmv, the unit conversion is
3
necessary for accurate calculation of ventilation rate with a unit of m air=sð Þ.
The total sensible heat transfer rate, _ q in Eq. (14.6), of the ventilated airspace is
the sum of all heat loss or gain within the airspace, including the sensible heat
production rate by occupants and indoor equipment (e.g. stove, lights), and the heat
transfer through building envelope. _ q can be positive or negative, which indicates
heating load or cooling load. Moisture production rate _ w in Eq. (14.7) depends on
the status of the indoor sources, for example, exhale of human beings or the capacity
of a working humidifier. For the occupants (human or animals) related data can be
found in handbooks and/or standards such as those published by ASHRAE [8].
It is relatively challenging to determine the pollutant production rate, _ s in
Eq. (14.8), which varies dramatically for different pollutants and sources. Among
all the typical air pollutants in indoor environments, carbon dioxide can often be
determined indirectly from the load of the occupants because the increase of CO 2
level in an indoor environment is mainly a metabolic product. From large scale
statistical analyses, it has been determined that the carbon dioxide production rate
3
of average human and animals is 1m of CO 2 per 24,600 kJ of total heat production
(THP) under normal indoor condition [8]. Converting to the mass production rate of
carbon dioxide in kg/s leads to
THP
¼ q :
_ s CO 2 ð14:9Þ
24;600
CO 2
¼ the mass production rate of CO 2 (kg/s), q ¼ the density of carbon
where _ s CO 2
CO 2
3
dioxide (1.83 kg/m at 20 °C and 1 atm), THP ¼ the total heat production of
occupants (kJ/s). Note that the total heat includes the sensible heat and latent heat,
whereas the heat production _ q in Eq. (14.6) only includes the sensible heat.
Q s
For steady state operation, the rate of mass of dry air entering ( = ) equals to
m s
that exiting ( = ) the room. Energywise, the total sensible heat contained in the
Q e
m e
supply air plus the net sensible heat transfer rate, must be equal to the total sensible