Airplanes have refrigeration systems that work differently from conventional refrigerators found in buildings or homes. In an aircraft, the refrigeration system doesn't rely on electricity or traditional vapor compression cycles, as you rightly pointed out that these systems might not work efficiently at high altitudes due to the lower pressure and lack of oxygen.
Instead, aircraft utilize what is known as an "air cycle refrigeration system" (also called an air cycle machine or ACM). The air cycle refrigeration system takes advantage of the principles of thermodynamics and the properties of compressed air to provide cooling without the need for a refrigerant or electric compressor.
Here's how an air cycle refrigeration system generally works:
Compressed Air Intake: The aircraft's engines compress large amounts of air during operation. Some of this compressed air is diverted and directed into the air cycle system.
Air Expansion and Cooling: The compressed air is allowed to expand rapidly, which causes its temperature to drop significantly. This cooling effect is a result of the thermodynamic principle known as the Joule-Thomson effect. As the expanded air cools down, it absorbs heat from the surrounding environment.
Air Conditioning: The cooled air is then channeled into the aircraft's cabin or other compartments that require cooling. It helps maintain a comfortable temperature for passengers and prevents critical electronic equipment from overheating.
Air Compression: After the cooling process, the air needs to be compressed back to its original state before it is expelled from the aircraft. This is typically achieved by using the aircraft's engine bleed air, which is at a higher pressure and can be used to re-compress the cooled air.
The advantage of using an air cycle refrigeration system is that it doesn't rely on the availability of electricity, and it can work effectively at high altitudes where conventional refrigeration systems might struggle. Additionally, it eliminates the need for refrigerants, making it more environmentally friendly.
While the air cycle system is efficient for providing cooling at moderate levels, it might not achieve extremely low temperatures like some electrically-powered vapor compression systems. However, it is well-suited for maintaining a comfortable cabin environment and cooling critical components in aircraft while flying at high altitudes.