Why We Need Cooling Systems In Any Aircraft ?

The human body is unable to withstand the effects of a low-pressure atmosphere, which makes the A/C and pressurization system a vital component of modern flight.

In an any modern aircraft, cooling systems are required to keep the cabin temperatures at a comfortable level, which provides a convenient environment for its passengers. . Even though the outside temperatures are very low at high altitudes, still cooling of cabin is required due to:

i. Large internal heat generation due to occupants, equipment etc.

ii. Heat generation due to skin friction caused by the fast moving aircraft.

iii. 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 (at10000 m altitude) is compressed to 1 bar, its temperature increases to about 353 K. If the cabin is maintained at 0.8 bar, the temperature will be about 332 K. This effect is called as ram effect. This effect adds heat to the cabin, which needs to be taken out by the cooling system.

iv. Solar radiation

For low speed aircraft flying at low altitudes, cooling system may not be required,however, for high speed aircraft flying at high altitudes, a cooling system is a must.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:

i. Air is cheap, safe, non-toxic and non-flammable. Leakage of air is not a problem.

ii. Cold air can directly be used for cooling thus eliminating the low
temperature heat exchanger (open systems) leading to lower weight.

iii. The aircraft engine already consists of a high speed turbo-compressor,hence separate compressor for cooling system is not required. This reduces the weight per kW cooling considerably. Typically, less than 50% of an equivalent vapour compression system.

iv. Design of the complete system is much simpler due to low pressures.Maintenance required is also less.

These components provide conditioned air via the following step by step process:

  1. First, outside air enters the airplane engine.
  2. Next, compressors within the engine compress this low-density air.
  3. Hot compressed air from the compressor (bleed air) is then transported via ducts to the A/C packs.
  4. Prior to entering the air conditioner units, the bleed air passes through the pack flow control valve, which regulates the flow of air entering the conditioning packs.
  5. Within the A/C unit, two air-to-air heat exchangers are installed that supply outside air via a pack inlet scoop and the air exits through an outlet duct.

  6. As the cold air exits from the conditioning pack, it is mixed with warm air.
  7. The desired air temperature is achieved by regulating the amount of hot air mixed with the cold conditioned air exiting from the packs through a by-pass valve.
  8. The regulated air is then fed to a mixing unit which transports the air further on into the cabin and the cockpit.
  9. The by-pass valve, pack flow control valve, inlet scoop and outlet duct are all operated by, and connected to, a pack controller.

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