Boot-strap Air Cooling System

This boot strap air cooling system has two heat exchangers instead of one, and a cooling turbine drives a secondary compressor instead of cooling fan. The air bled from the main compressor is first cooled by the ram air in the first heat exchanger. This cooled air, after compression in the secondary compressor, is led to the second heat exchanger where it is again cooled by the ram air before passing to the cooling turbine. This type of cooling system is mostly used in transport type aircraft.

The T-s diagram for a boot-strap air cycle cooling system:

The various processes are as follows:

1. The process 1-2' represents the isentropic ramming of ambient air from pressure P1, and temperature T1 to pressure P2 and temperature T2. The process 1-2' represents the actual ramming process because of internal friction due to irreversibilities.

2. The process 2'-3 represents the isentropic compression of air in the main compressor and the process 2'-3' represents the actual compression of air because of internal friction due to irreversibilities.

3. The process 3'-4 represents the cooling by ram air in the first heat exchanger. The pressure drop in the heat exchanger is neglected.

4. The process 4.5 represents compression of cooled air, from first heat exchanger, in the secondary compressor. The process 4-5' represents the actual compression process because of internal friction due to irreversibilities.

5. The process 5'-6 represents the cooling by ram air in the second heat exchanger. The pressure drop in heat exchanger in neglected.

6. The process 6-7 represents isentropic expansion of the cooled air in the cooling turbine upto the cabin pressure. The process 6-7' represents actual expansion of the cooled air in the cooling turbine.

7. The process 7'-8 represents the heating of air upto the cabin temperature T8.

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 refrigerating system,

and C.O.P. of the refrigerating system