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The purpose of the pack cooling fans in the air conditioning system is to...

  • A
    supply the heat exchangers with cooling air during slow flights and ground operation.
  • B
    supply the Passenger Service Unit (PSU) with fresh air.
  • C
    cool the APU compartment.
  • D
    supply the heat exchangers with cooling air during cruise flight.

Refer to figure.

Air Cycle Machines are ideally suited for turbine aircraft due to the supply of already compressed bleed air. Often referred to as "air conditioning packs".
In Air Cycle Machines, high-pressure bleed air from the engines is first pre-cooled in the primary heat exchanger. The bleed air is then compressed in the compressor section and is then fed through to the secondary heat exchanger to remove as much heat as possible before being expanded and introduced to the turbine section which is used to drive the compressor. After passing through the turbine section, the air is then ducted through a water separator to remove excess moisture from the conditioned air before it enters the cabin and cockpit via a one way check valve. Finally, the air passes through a humidifier to increase the moisture content of the conditioning air.

  • As discussed above, the temperature reduction of the air inside the system is accomplished by air passing through heat exchangers. Heat exchangers normally use ram air as the cooling medium. In flight, ram air doors are modulated to increase or decrease ram air flow to the exchanger. During slow flight or when on the ground, a "ground cooling fan" is responsible to draw air through the ram air duct so that the heat exchange is possible when the aircraft is stationary.

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