General principles of cold production

Cold production is based on a fairly simple principle. The idea is to compress a steam obtained at low temperatures, in order for it to condense through the rejection of heat into the natural environment.

Practically, a refrigeration system allows to cool down an environment at a lower temperature than its surrounding atmosphere. Since naturally heat always travels from a warm body to a colder one, we can define a refrigeration system as a device that allows you to turn around heat conveyance. Energy then travels from a cold to a warmer environment. This reversed transfer will of course require some energy input.

The objective of a refrigeration system is then to transfer a heat quantity from a low temperature environment (the cold source) to a high temperature environment (the warm source).

The system absorbs heat at the cold source (thus produces cold) and transmits this heat to the warm source (thus produces heat). If the intended result is the production of heat, the device is then called a heat pump.

A heat pump is in fact a cooler that is used backwards. In a refrigerator, the intended effect is the production of cold, in other words the absorption of heat from the food placed inside the refrigerator. The cooling circuit allows this heat transfer. A heat pump works according to exactly the same principle as a refrigerator.

A refrigeration system allows to transfer heat from a cold environment to a warm environment thanks to a compression system. Such a refrigeration system usually consists of four elements:

  • a compressor
  • a condenser
  • an expansion valve
  • an evaporator

A refrigerant gas takes care of the thermal conduction between the different elements.

  1. The compressor: It first draws in the refrigerant gas at low temperature and low pressure. The mechanical energy given by the compressor will allow the temperature the pressure of the refrigerant to increase. This is called the compression phase.
  2. The condenser: The refrigerant at high temperature and high pressure coming from the compressor is then directed to the condenser. This equipment is a heat exchanger in which circulates, on one side, the external fluid to be heated (air or water) and on the other side, the refrigerant to be cooled down. The gas will transfer the heat to the external fluid that needs to be heated until the refrigerant temperature has reached the liquid-vapor balance temperature. The steam will then condensate at constant temperature and pressure. This is called the condensation phase.
  3. The expansion valve: The liquid formed inside the condenser is then released from high to low pressure. This expansion takes place in the expansion valve. During this expansion, a gaseous phase is formed.
  4. The evaporator: At low pressure conditions, the temperature of the liquid-vapor balance is lower. The evaporator is a heat exchanger in which circulates, on one side, the refrigerant coming from the expansion valve and on the other side, the external source from which the heat has to be removed (water or air). The liquid refrigerant coming from the expansion valve will start to boil due to the external source heat absorption. The formed gas is usually slightly heated by the external source. This phase is called superheating. The gas is then drawn in again by the compressor to carry out a new cycle.