What is the principle of refrigeration?

The principle of refrigeration is to remove heat from one area (i.e inside your fridge) and locate it to another area (i.e outside of your fridge).

Air is not brought in from the outside of the fridge the heat is absorbed by the evaporator inside the fridge which has refrigerant inside it, this refrigerant at low pressure is at low temperature inside the evaporator so the heat from the product inside the fridge is absorbed by the evaporator (as heat always transfers from the hotter object to the colder object) which has a fan to circulate the air around the fridge.

Then the refrigerant is pushed around the pipework by the compressor to the condenser where the refrigerant is hot from the heat out of the fridge, because the outside air will be lower than that of the pressurized refrigerant the heat is absorbed by the ambient air which leaves the refrigerant cooler and lower pressure so when its back into the evaporator it can absorb more heat and expel it into the ambient air.

There are 5 main components in a normal refrigeration system like on your fridge:-

Compressor

Condenser

Expansion Device or Capillary tube

Evaporator

Thermostat

The compressor compresses the refrigerant gas. This raises the refrigerant's pressure and temperature, so the heat-exchanging coils outside the refrigerator allow the refrigerant to dissipate the heat of pressurization. As it cools, the refrigerant condenses into liquid form and flows through the expansion valve.

When it flows through the expansion valve, the liquid refrigerant is allowed to move from a high-pressure zone to a low-pressure zone, so it expands and evaporates. In evaporating, it absorbs heat, making it cold. The coils inside the refrigerator allow the refrigerant to absorb heat, making the inside of the refrigerator cold. The cycle then repeats.