The Cycle

In the vapor-compression refrigeration cycle, heat is transferred from a lower temperature source to a higher temperature heat sink. Heat naturally flows in the opposite direction and due to the second law of thermodynamics, work is required to move heat from cold to hot. A food refrigerator or freezer works in much the same way; it moves heat out of the interior into the room in which it stands. This most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile, the compressor is usually driven by a belt connected to a pulley on the engine’s crankshaft, with both using electric motors for air circulation. Since evaporation occurs when heat is absorbed, and condensation occurs when heat is released, air conditioners are designed to use a compressor to cause pressure changes between two compartments and actively pump a refrigerant around. A refrigerant is pumped into the low pressure compartment (the evaporator coil), where despite the low temperature, the low pressure causes the refrigerant to evaporate into a vapor, taking heat with it. In the other compartment (the condenser), the refrigerant vapour is compressed and forced through another heat exchange coil, condensing into a liquid and rejecting the heat previously absorbed from the cooled space. The heat exchanger in the condenser section (the heat sink mentioned above), is cooled most often by a fan blowing outside air through it, but in some cases can be cooled by other means such as water.