Air Cycle - Air Conditioning Systems
Have you ever wondered how air conditioning works on a commercial aircraft? Air cycle air conditioning is the most widely used process in commercial aviation. This system uses engine bleed air to pressurize the aircraft cabin. By manipulating bleed air through a few important components, this system is able to provide comfortable air flow to the cabin during flight.
Bleed air is too hot to pump directly from the engine system into the cabin. In order for the incoming air to begin the cooling process, it is routed into a primary heat exchanger, which reduces the temperature of the air by ducting controlled ram air through the exchanger. Ram air is provided by the aircraft pneumatic system.
The cooled air is then directed into an air cycle machine (ACM), or refrigeration unit, where it is compressed before flowing to a secondary heat exchange. The secondary exchanger works the same as the primary, and air is mixed with more ram air and cooled further. Meanwhile, excess cool air is fed from the secondary exchanger back to the ACM, where it drives the expansion turbine and cools the unit.
As the temperature of the air changes, air molecules cannot hold as much water as before. A fiberglass water separator is necessary to remove water from the air flow before it enters the cabin. Lastly, air at the proper temperature adjustment is delivered to the cabin through an air distribution system.
It is worth mentioning the following component parts, as they are integral to the air cycle process: a pack valve, bleed air bypass, and refrigeration turbine unit (ACM). A pack valve serves the same function as a supply shut off valve. It regulates pressurized bleed air entering the system and can open or close depending on the needed air flow. This part also operates as a redundancy measure for overheating and will shut off air supply if necessary. A bleed air bypass removes some pneumatic air and mixes it with cold air so that air enters the cabin at a comfortable temperature. The bypass is controlled by an auto temperature controller.
A refrigeration turbine unit, also referred to as an ACM, is the main cooling system. It is essentially a compressor that is driven by a turbine. Air from the primary heat exchanger flows to the compressor area of the unit. The air is then sent to the secondary heat exchanger for a second cooling process. The highly pressurized air allows for an easier exchange of heat energy when mixed with ram air. Excess cooled air is then directed from the secondary heat exchanger to the ACM to cool the turbine.