The most common type of air conditioning system found in aircraft is the aircraft vapor cycle system. Within this system, there are six major components: the refrigerant, receiver dryer, expansion valve, evaporator, compressor, and condenser. In this blog, we will discuss each component and its role within the system as a whole.
Refrigerant is a fluid substance or mixture used in the heat pump and refrigeration cycle to rapidly absorb heat energy from all surrounding matter. For many years, the standard refrigerant used in aircraft vapor cycle air conditioning systems has been dichlorodifluoromethane, also known as R12. While R12 is still in use today, it has been found to have a negative effect on the earth’s protective ozone layer. As such, it has largely been replaced by tetrafluoroethane (R134a), which is better for the environment.
The receiver dryer serves as the reservoir of the vapor cycle system and is located downstream of the condenser and upstream of the expansion valve. Liquid refrigerant flows from the condenser into the receiver dryer, where it passes through filters and a desiccant material to remove any foreign particles and water in the refrigerant.
After exiting the receiver dryer, refrigerant flows to the expansion valve. This thermostatic valve has an adjustable orifice through which the correct amount of refrigerant is metered to obtain optimal cooling capabilities. This is done by controlling the temperature of the gaseous refrigerant at the outlet of the cycle’s next component, the evaporator. The expansion valve should only allow an amount of refrigerant spray into the evaporator that can be completely converted into a vapor. Ultimately, the temperature of the cabin air being cooled will determine the amount of refrigerant the expansion valve should spray into the evaporator.
The next part of the cycle is the evaporator. Most evaporators are constructed of copper or aluminum tubing coiled into a compact unit. The evaporator is equipped with fins to increase the surface area and facilitate rapid heat transfer between the cabin air blown over the outside of the evaporator with a fan and the refrigerant inside. The evaporator’s valve releases high-pressure, high-temperature liquid refrigerant into the evaporator. As the refrigerant absorbs heat from the cabin air, it changes into low-pressure vapor. From here, the vapor is discharged from the evaporator and into the next component of the system, the compressor.
The compressor is the heart of the system. It is responsible for circulating the refrigerant throughout the vapor cycle system. It receives low-pressure, low-temperature refrigerant vapor from the outlet of the evaporator and subsequently compresses it. This increases the pressure, also increasing the temperature. The refrigerant then flows out of the compressor to the condenser, where it gives off the heat to the outside air.
The final part of the vapor cycle is the condenser. This part is a radiator-like heat exchanger situated such that the outside air flows over it to absorb heat from the refrigerant received from the compressor. A fan is usually included to draw the air through the compressor during ground operation. On some aircraft, outside air is ducted to the compressor, while in others, the condenser is lowered into the airstream via a hinged panel.
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