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440 14 Indoor Air Quality

14.3.2 Psychrometric Chart

In order to calculate the ventilation rates above, we can no longer assume that air is
always dry because moisture is a major concern in ventilation. The physical and
thermodynamic properties of the moist air at sea level can be found in psychro-
metric chart. The ASHRAE-style psychrometric chart, shown Figure A.2, was
pioneered by Willis Carrier in 1904 [9]. It depicts the parameters that are needed for
our calculation above such as dry bulb temperature (T), humidity ratio (w), relative
humidity (ϕ), specific sensible enthalpy (h), specific volumeðmÞ.
Example 14.3: Ventilation rate calculation
There are 20 people in a dinning room, each produces 200 W total heat. The carbon
dioxide concentration in the supply air is 500 ppmv. Assume the supply air tem-
perature is 15 °C and relative humidity 50 %. The room air is to be maintained at
22 °C and 60 % relative humidity. If the required maximum CO 2 concentration in
the room is 1000 ppmv, estimate the minimum ventilation rate based on the CO 2
concentration.

Solution
Based on the supply air temperature of 15 °C and 50 % relative humidity we can get
the specific volume of supply air using a psychrometric chart,

3
m s ¼ 0:822 m =kg dry air
The specific volume of exhaust air at 22 °C and 60 % relative humidity is

3
m e ¼ 0:855 m =kg dry air
The total heat production rate by 20 people in the dining room is,
THP ¼ 20 200 W ¼ 4000 W or 4kJ=s.

Then the mass production rate of CO 2 is estimated using Eq. (14.9)
THP 4
4
¼ q ¼ 1:83 ¼ 2:98 10 ðkg=sÞ
_ s CO 2
24;600 24;600
CO 2
Then the minimum ventilation rate fan is determined using Eq. (14.17)

m s c s m e c e
¼ Q e c e 1 1
_ s CO 2 ¼ Q s c s
m e c e m s c s
Since m s \m e , the minimum ventilation rate should be calculated based on the
exhaust air. However, we need to convert the unit of concentration c from ppmv to
3
kg CO 2 /m air.