Question

In a tropical country, it is desired to provide processing air conditions of 15°C and 80% RH. The ambient air is at 31.5°C and 90% RH. If the chosen method is to cool the air to condense out enough water to reduce the water content of the air sufficiently, then to reheat if necessary, determine the temperature to which the air should be cooled, the quantity of water removed and the amount of reheating necessary. The processing room has a volume of 1650 m3 and it is estimated to require six air changes per hour.

Answer 1

Using the psychrometric chart (normal temperatures):
Initial humidity is 0.0266 kg kg^-1.
Final humidity is 0.0085 kg kg^-1.
Saturation temperature for this humidity is 13°C.
Therefore the air should be cooled to 13°C
At the saturation temperature of 13°C, the enthalpy is 33.5 kJ kg^-1
At the final conditions, 15°C and 80 % RH, the enthalpy is 37 kJ kg^-1
and the specific volume of air is 0.827 m³ kg^-1.

Assuming that the air changes are calculated at the conditions in the working space.
       Mass of air to be conditioned = (1650 x 6)/0.827
          = 11,970 kg h^-1
       Water removed per kg of dry air DY
          = 0.0266 - 0.0085
          = 0.018 kg kg^-1
       Mass of water removed per hour
          = 11,970 x 0.018
          = 215 kg h^-1

       Reheat required DH              = (37 - 33.5)
          = 3.5 kJ kg^-1
       Total reheat power required    = 11,970 x 3.5
          = 41,895 kJ h^-1
          = 11.6 kJ s^-1
          = 11.6 kW.