Aircraft air refrigeration is required due to heat transfer from many external and internal heat sources which increase temperature of air in cabin. Following are the major sources:
1. External Heat Sources
- Heat is transferred to the cabin in heavy quantity due to sunlight through windows and also heats top surface of plane.
- The pressure required in passenger cabin is atmospheric pressure at sea level. So it is necessary to compress ambient air to this pressure because with increase in altitude, pressure of atmosphere decreases. Therefore the temperature of air entering the cabin will be higher than ambient temperature due to this compression.
- Due to the high speed of the aircraft, skin friction occurs all over its surface such that a high temperature air layer will always be in contact with the external surface of aircraft and transfers heat to the cabin.
2. Internal Heat Sources
- Heat generated from human body is continuously dissipated into the cabin air at a rate of 400 kJ per hour.
- Heat generated by electrical and electronic equipment used for control system.
- Heat from the engine parts of the plane which are subjected to high temperature throughout the flight, reaches the cabin through conduction, convection and radiation.
All above sources add heat to cabin. For human comfort, temperature of air should not exceed 22°C and hence cooling of aircraft is desirable to maintain this temperature. Still lower temperature of about -5°C is also required for food and drinks preservation.