Reduced Ambient Air Cooling System

The reduced ambient air cooling system includes two cooling turbines and one heat exchanger. The air reduced for the refrigeration initially in system is bled off from the main compressor. This high pressure and high temperature air is cooled the heat exchanger. The air for cooling is taken from the cooling turbine which lower the high temperature of rammed air. the cooled air from the heat exchanger is passed through the second cooling turbine from where the air is supplied to the cabin. The work of the cooling turbine is used to drive the cooling fan (through reduction gears) which draws cooling air from the heat exchanger.

The reduced ambient air cooling system is used for very high speed aircrafts.

The T-s diagram for the reduced ambient air cycle cooling system is:

The various processes are as follows:

1. The process 1-2 represents isentropic ramming of air and the process 1-2' represents actual ramming of air because of internal friction due to irreversibilities.
2. The process 2'-3 represents isentropic compression in the main compressor and the process 2'-3' represents actual compression of air, because of internal friction due to irreversibilities.
3. The process 3'-4 represents cooling of compressed air by ram air which after passing through the first cooling turbine is led to the heat exchanger. The pressure drop in the heat exchanger is neglected.
4. The process 4-5 represents isentropic expansion of air in the second cooling turbine upto the cabin pressure. The actual expansion of air in the second cooling turbine is represented by the curve 4-5'.
5. The process 5'-6 represents the heating of air upto the cabin temperature T6.

If Q tonnes of refrigeration is the cooling load in the cabin, then the quantity of air required for the refrigeration purpose will be:

Power required for the refrigeration system is given by

and C.O.P. of the system