Comparison of Various Air Cooling Systems used for Aircraft

Dry Air Rated Temperature (DART): The concept of Dry Air Rated Temperature is used to compare different aircraft refrigeration cycles. Dry Air Rated Temperature is defined as the temperature of the air at the exit of the cooling turbine in the absence of moisture condensation. For condensation not to occur during expansion in the turbine, the dew point temperature and hence moisture content of the air should be very low, i.e., the air should be very dry. The aircraft refrigeration systems are rated based on the mass flow rate of air at the design DART. The cooling capacity is then given by:

where, m is the mass flow rate of air, T DART and Ti are the dry air rated temperature and cabin temperature, respectively.

The performance curves for the various air cooling systems used for aircrafts are shown above. These curves show the dry air rated turbine discharge temperature (DART) against the Mach number. From the figure, we see that the simple air cooling system gives maximum cooling effect on the ground surface and decreases as the speed of aircraft increases. The boot strap system on the other hand, requires the air plane to be in flight so that the ram air can be used for cooling in the heat exchangers. One method of overcoming this drawback of boot strap system is to use part of work derived from turbine to drive a fan which pulls air over the secondary heat exchanger, thus combining the features of a simple and boot strap system .As the speed of aircraft increases, the temperature of ram cooling air rises and the ram air becomes less effective as a coolant in the heat exchanger. In such cases, a suitable evaporant is used with the ram air so that the cabin temperature does not rise.

For high speed aircrafts, the boot strap evaporative or regenerative systems are used because they give lower turbine discharge temperature than the simple cooling system. In some cases, aeroplanes carry an auxiliary gas-,turbine for cabin pressurisation and, air conditioning. From the chart, we see that the turbine. Discharge temperature of the air is variable. Therefore, in order to maintain the content temperature of simply air to the cabin, it requires some control system.

Summary:

i. DART increases monotonically with Mach number for all the systems except the reduced ambient system

ii. The simple system is adequate at low Mach numbers

iii. At high Mach numbers either bootstrap system or regenerative system should be used

iv. Reduced ambient temperature system is best suited for very high Mach number, supersonic aircrafts