Possibly one of the most complex systems in an aircraft, the air conditioning system works around the clock to keep passengers cool and comfortable. During its working process, the air conditioning system pulls air from the engine that has been heated up during the various compressor stages. This air is known as bleed air, and it is typically pulled from the low-pressure or intermediate stages of the high-pressure compressor. Occasionally, the air can be bled from the high-pressure stage of the compressor when lower stage bleed air is not sufficient. Typically, this bleed air is incredibly hot, which means it must be put through a pre-cooling stage before it can be dispensed into the cabin.
Pressurization air conditioning kits, or PACKS, feature a heat exchanger and a compressor turbine unit. Also called air cycle machines, they are intended to lower the engine bleed air temperature to a normal level, and the PACK is fed air through a one-way check valve, also called the PACK flow control valve. The heat exchanger inside the PACK features a primary and secondary heat exchanger, and when the bleed air moves through these heat exchangers, the temperature is reduced. It is helpful to look at heat exchangers as radiators because they draw in air through ram air inlets which are placed on the belly of the aircraft.
After air passes through the primary heat exchanger, it is then transferred to a compressor, which raises the air pressure and heats the air slightly. Then, the air moves through the secondary heat exchanger and lowers in temperature once again before it is passed through a turbine. Inside the turbine, the air expands and cools which helps to dramatically reduce its temperature. The ram air inlets are controlled by computers, and they can be closed when necessary to allow less cooling air to pass through the heat exchangers. For example, these ram air inlets occasionally close up for takeoff and landing in order to keep damaging debris out. Additionally, the compressor of the PACK runs a cooling fan that allows hot air built up in the heat exchangers to be sent overboard. This air is exhausted from the aircraft through ram air outlets, which are typically featured on the belly of the plane.
The PACK releases incredibly cold air which may be unfit for passengers and crew members, so a bypass valve exists inside the PACK which is temperature controlled by the pilots. Once the bypass valve opens, hot air coming from the primary heat exchanger is mixed in with the standard PACK outflow air. By comparing the temperature of this flow to the temperature of the air inside the cabin, the pilot is able to use specialized knobs and switches in order to set a temperature for the air flow. Then, the PACK control computers shift the bypass valve in order to meet the temperature value set by the pilots.
Air which moves throughout the cabin ends up being recirculated, which makes this system efficient because recycling air reduces the amount of bleed air necessary for the engines. During this process, recirculation fans or cabin fans are used to move the recycled air through a mixer unit where PACK air and recirculated air are combined prior to being moved into the cabin. Finally, an extraction fan draws air from the lavatories and galley areas to prevent strong smells from permeating throughout the cabin as it exits through outflow valves.
Smaller aircraft PACKs split up their load so that they supply different areas of the vehicle with conditioned air. Larger aircraft require more temperature regulation control, so they use trim air valves which are able to directly manipulate the air from the PACKs. These trim valves feature a direct engine bleed air feed which is powered by a hot bleed valve, so that when the pilot requests temperature changes in the cabin, the respective trim valve will open to supply hot air into the mixer unit. Trim valves are featured on many different zones of the aircraft to individually control the temperature of each zone.
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