A refrigeration unit consist of:
a refrigerant to transfer heat,
a compressor to circulate and add pressure to the refrigerant,
an evaporator where the refrigerant turns from a liquid to a gas and absorbs heat,
a condenser where the refrigerant turns from gas to liquid and disperses heat,
a thermostat to regulate temperature by switching the compressor on or off.
Size/capacity of the refrigeration unit is selected to provide effective chilling at a reasonable cost without compromising storage space, vehicle power or weight .
For lower temperatures (ie. for frozen goods) a higher capacity refrigeration unit is often required for efficient operation.