In high-speed aircraft flying at high altitudes, why is the cooling of the cabin required even if temperature outside is very low?

In high-speed aircraft flying at high altitudes, why is the cooling of the cabin required even if temperature outside is very low? Correct Answer Due to large internal heat generation

Explanation:

Aircraft, at high speed and at high altitude, even though the outside temperatures are very low at high altitudes, still cooling of the cabin is required due to:

• Large internal heat generation due to occupants, equipment etc.
• Heat generation due to skin friction caused by the fast-moving aircraft
• At high altitudes, the outside pressure will be sub-atmospheric. When air at this low pressure is compressed and supplied to the cabin at pressures close to atmospheric, the temperature increases significantly. For example, when outside air at a pressure of 0.2 bar and temperature of 223 K is compressed to 1 bar, its temperature increases to about 353 K. This effect adds heat to the cabin, which needs to be taken out by the cooling system.
• Presence of Solar radiation

For low-speed aircraft flying at low altitudes, a cooling system may not be required, because outside pressure will not be sub-atmospheric.

Additional Information

Even though the COP of air cycle refrigeration is very low compared to vapour compression refrigeration systems, it is still found to be most suitable for aircraft refrigeration systems as:

• Air is cheap, safe, non-toxic and non-flammable. Leakage of air is not a problem.
• Cold air can directly be used for cooling, thus eliminating the low-temperature heat exchanger (open systems) leading to lower weight
• The aircraft engine already consists of a high-speed turbo-compressor, hence a separate compressor for the cooling system is not required. This reduces the weight per kW cooling considerably. Typically, less than 50% of an equivalent vapour compression system.
• The design of the complete system is much simpler due to low pressures.
• Maintenance required is also less.