Simple Air Evaporative Cooling System

A simple air evaporative cooling system is shown below.

It is similar to the simple cooling system except that the addition of an evaporator between the beat exchanger and cooling turbine. The evaporator provides an additional cooling effect through evaporation of a refrigerant such as water. At high altitudes, the evaporative cooling may be obtained by using alcohol or ammonia. The water, alcohol and ammonia have different refrigerating effects at different altitudes. At 20000 metres height for example, water boils at 400 C, alcohol at 90 C and ammonia at - 70° C.

The T-s diagram for simple air cycle evaporative cooling system is shown below:

The various processes are same as in the simple air cooling system, except that the cooling process in the evaporator is shown by 4 - 4'.

If a Q tonnes of refrigeration is the Cooling load in the cabin then the air required for the refrigeration purpose,

Power required for the refrigerating system,

And C O.P. of the refrigerating system

The initial mass of evaporant (Me) required to be carried for the given flight time is given by

where Qe = Heat to be removed in evaporation in kJ/min, and Hfg = Latent heat of vaporisation of evaporant in kJ/kg

If cooling of 45 minutes duration or less is required, it may be advantageous to use evaporative cooling alone.