
slowly if the air is in contact with massive structures that were cooled down
before, for example, by strong airing during the cool night.
Mechanical cooling is needed to keep the surfaces in contact with the air
cold, and to get continuous air drying and cooling. Warm, humid air is first
cooled down when passing through a refrigerated heat exchanger (horizontal
‘cooling’ line in Figure 5.3) until it reaches its dew point. Then it is dried by
losing the water that condenses on the heat exchanger (‘drying’ curve) until
it reaches the required specific humidity, at a new lower dew point. It should
then be reheated to the required temperature.
Numerical values for this process are given in Table 5.1. The largest change
in enthalpy is when drying, since 2500 J should be withdrawn from the heat
exchanger to condense each gram of water.
The energy required to reheat the dry, cold air can be provided by various
means:
Table 5.1 Humidity ratio and specific enthalpy of warm, humid air cooled down
and dried as shown in Figure 5.3
Temperature
Relative
humidity
Humidity
ratio
Specific
enthalpy
Enthalpy
increase
Process (8C) ’ (%) x (g/kg) h (J/(kg K)) h (J/(kg K))
Cooling 30.0 70 18.8 78,756 6941
Drying 23.9 100 18.8 71,815 44,244
Heating 9.3 100 7.3 27,571 11,153
20.0 50 7.3 38,724
0
5
10
15
20
25
30
0 102030
Water content [g/kg]
°C
Sat.
90%
80%
70%
60%
50%
40%
30%
20%
10%
Reheating
Cooling
Drying
10
20
30
40
50
60
70
80
90 kJ/kg
Figure 5.3 Paths in the psychrometric chart for heating outdo or air in
winter or cooling it in summer to reach 208C and 50 pe r cent relative
humidity
82 Ventilation and Airflow in Buildings
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