Refrigeration should be considered as the transfer of heat from one place, where it is in excess, to another, where it can be freely dissipated, without causing damage or other problems.
Heat travels spontaneously from a warmer body to a cooler one.
Heat can be transferred in three different ways:

• radiation: transmission of heat by infrared waves;
• conduction: transmission of heat through solid bodies; the molecules in the body transmit their own thermal energy from the heated part to all the other parts of the body;
• convection: transmission of heat that is typical of fluids (liquids or gases); the molecules in the fluid, in a state of thermal agitation, migrate through the environment in which the fluid is confined, generating so-called "convective currents".

The latter two types of heat transfer, in particular, are used in the design of refrigeration devices.

This transfer of heat is brought about materially by a refrigerant that circulates in a special circuit, called the REFRIGERANT CIRCUIT.
A refrigerant, therefore, must have properties whereby it evaporates at a low temperature and pressure, absorbing heat, and then gives up this heat by condensing at a higher temperature and pressure. This procedure requires the contribution of energy.

States of matter
In nature, substances are found in three different states: solid, liquid, gas.
When heating a substance at a constant pressure, its state changes from solid to liquid and from liquid to gas; when lowering the temperature, the reverse occurs (from gas to liquid, from liquid to solid).
In addition, when increasing the pressure at a constant temperature, the state of a substance changes from gas to liquid and from liquid to solid, while decreasing the pressure achieves the reverse (from solid to liquid, from liquid to gas).
The difference in the state of the substances depends on the relative distance between the molecules. Water is present in nature in all three states.


1. The compressor compresses the refrigerant in the gaseous state.
2. The refrigerant, in the gaseous state, condenses into liquid at high pressure; it then dissipates its heat through the coil at the rear of the refrigerator.
3. Subsequently, the refrigerant in liquid form at high pressure flows through the expansion valve (expansion or control device).
The expansion valve is a component fitted between the condenser and evaporator with the function of creating a significant drop in pressure. This valve performs two functions: first of all, it ensures the correct quantity of liquid refrigerant is sent to the evaporator, and in addition it creates a pressure differential that is essential for the completion of the cycle.

The pressure differential is very important in a refrigerant circuit, as it changes the boiling point of the fluid. Without this pressure change, no cooling would take place and the system would simply be a container of liquid refrigerant.

4. The refrigerant in liquid form immediately boils and evaporates, absorbing heat. This process makes the inside of the refrigerator cold. The refrigerant in the gaseous state then enters the compressor, and the cycle is repeated.

The relationship between the quantity of heat dissipated through the condenser and the quantity of heat expended due to mechanical operation, that is, the quantity of electrical energy consumed by the operation of the compressor, is called the COP (Coefficient of Performance) and depends on the evaporation and condensing temperatures and the intrinsic quality of the unit.

Heat pump operation
Reversing the flow of the refrigerant inside the refrigerant circuit creates a situation in which the evaporator is outside and the condenser inside. In this way, the heat is transferred to the air inside the space, heating it, while the outside is cooled.
Reversing the cycle requires the installation of a 4-way valve in the circuit. This valve sends the refrigerant to the indoor coil or the outdoor coil (no longer referred to as condenser and evaporator, as their functions have been exchanged) depending on whether cooling or heating is required. The latter (heating mode) is defined as heat pump operation, and means that the unit can also be used during winter.