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An air condenser is a component used in many air conditioning systems. More broadly, it is a step in heat pump and refrigeration cycles that exchange heat. In a typical air conditioner, a fluid called a refrigerant circulates between the indoor space and outside air. The air condenser is one part of this path, and it serves to transfer heat from the refrigerant to the outside atmosphere. It is called a condenser because the refrigerant condenses, or changes phase from gas to liquid, during this step.
All matter, including liquids and gases, carry some amount of heat energy within. Whenever heat is used to raise the temperature of a substance, the heat is stored within the substance. Likewise, whenever a substances changes phases—such as from gas to liquid—, heat is either released or absorbed. Therefore, the internal heat of a refrigerant can be used to carry heat away from an inside space and towards the external environment. Causing the refrigerant to change from gas to liquid can allow that internal heat to be transferred from the refrigerant to the outside air.
Air conditioners make use of a pressure difference between different parts of the refrigerant circulation path to keep the fluid moving and allow it to change phases. This pressure differential is created using an air compressor powered by either electricity or, in a vehicle, a combustion engine. High-pressure refrigerant, which is mostly vapor at this point in the cycle, is then pushed through a tube into thermal contact with the outside environment. A fan may be used to ensure fresh outside air is passing over the tube full of refrigerant.
At this point, the refrigerant enters the air condenser. Fluids at higher pressures tend to have a boiling/condensation point at lower temperatures. Air conditioners make use of this property to transfer refrigerant heat to the outside environment—even if external air is already very hot. The air condenser, consequently, functions to condense refrigerant vapor into liquid, thereby releasing heat. The heat released during gas-liquid phase changes is called the enthalpy of vaporization.
To complete the air conditioning cycle, the primarily-liquid refrigerant passes through a choke point called an expansion valve. This valve contains the high pressure of the air condenser and allows a new region of low pressure. The low-pressure region has a component analogous to the air condenser, except that refrigerant evaporates instead of condenses. Accordingly, this evaporation absorbs—rather than releases—heat. The evaporator sucking heat from the air is what makes air from an air conditioner feel cold.