The refrigeration machine of a mechanical system is that component that removes thermal energy (heat) from one location and transfers it to another location by inducing a cycle of heat exchange. 

By far, the most common system of refrigeration is the vapor-compression refrigeration cycle, which utilizes a refrigerant. A refrigerant is a working fluid which undergoes a repeated phase-change from a liquid to a vapor and back again. Through the cyclical phase-change, an effective amount of thermal energy is exchanged.

The following is primer for the vapor-compression refrigeration cycle.


Two principles of vapor-compression refrigeration

The vapor-compression refrigeration cycle is based on two phenomena:

1. A large quantity of thermal energy (via heat of vaporization) must be added to change a liquid into a gas; however, the same amount of thermal energy (via heat of condensation) is released when the gas condenses back into a liquid.
 2. The boiling/condensation point temperature of any material varies with pressure. When pressure is reduced, the boiling/condensation point temperature is also reduced.


Figure: The vapor-compression refrigeration cycle.
 Image by Daniel Overbey.


Step-by-state of vapor-pressure refrigeration

Compressor: The compressor pumps cool, low-pressure gas from the evaporator coil and applies pressure (i.e., it compresses the gas) to the gas. Adding pressure increases the temperature of the gas. Hot, high-pressure gas leaves the compressor and enters the condenser coil.

Condenser coil: As the hot, high-pressure gas runs through the condenser coil, the coil releases heat. This heat output can be transferred by mechanical systems in several ways. As heat is released, the gas cools down to the boiling/condensation point and turns into a warm, lower-temperature (but still high-pressure) liquid and collects at the bottom of the condenser coil.

Expansion valve: This warm, high-pressure liquid is drawn through the expansion valve, which lowers the pressure (i.e., allows the liquid to decompress) of the liquid; therefore, lowering the temperature of the liquid considerably in the process. 

Evaporator coil: The compressor pumps fluid through the evaporator coil. Within the coil, the cold, low-pressure liquid vaporizes due to its lower boiling/condensation point. As the vaporization occurs, it absorbs heat input from the evaporator coil - this creates the very cooling process that is the goal of the refrigeration cycle. Cool, low-pressure gas is drawn to the compressor. 


Added capability to serve as a heat pump

When the process is reversed, the vapor-compression refrigeration cycle has the added capability of functioning as a source of heat - referred to as a heat pump.