The refrigeration cycle

Refrigerant circulation

In the following, the individual components of a simple refrigerant circuit will be described in order to finally explain the entire cycle understandable.

The evaporator

A liquid refrigerant absorbs heat during its evaporation. This phase change creates "cold" in a refrigerant circuit. When a refrigerant expands at ambient temperature through a restriction into the free atmosphere, it absorbs heat from the surrounding air and evaporates. If the atmospheric pressure changes, the refrigerant vaporizes at a different temperature, since the evaporation temperature is pressure-dependent. 
The component in which the described process takes place is the evaporator. Its task is to remove heat from the surrounding medium, ie to "generate" cold.

The compressor

The cooling process is, as already mentioned, a closed cycle process. Consequently, the refrigerant does not expand into the free atmosphere as described in the above example.

If the refrigerant coming from the evaporator is fed into a closed container, its internal pressure rises to the evaporation pressure. The refrigerant supply from the evaporator will thus gradually cease, and the temperature in both the evaporator and the container will gradually adjust to the ambient temperature.

In order to maintain a lower pressure and thus a lower temperature in the container, it must be deprived of steam. This is achieved by means of a compressor, which sucks steam from the evaporator. The compressor can be compared to a pump that delivers steam in the refrigerant circuit. 
In a closed circuit, pressure and temperature always seek equilibrium. If, for example, the compressor extracts more steam from the evaporator than is generated there, the pressure prevailing there and thus the temperature drop. Conversely, pressure and temperature increase as the evaporator load increases so that the amount of steam generated exceeds the capacity of the compressor.

The compaction process

Refrigerant exits the evaporator either in the saturated or superheated steam state and is compressed after entering the compressor. The compression is usually achieved by means of a piston and is therefore comparable to the operation of an internal combustion engine of a car. To be able to do his work, the compressor must be supplied with energy. This energy is in turn transferred to the refrigerant during the compression process. Due to the compression energy supplied, the refrigerant gas leaves the compressor at a higher pressure compared to the inlet state and in a strongly overheated state. The supplied compressor energy depends on system pressure and temperature. It is obvious that more energy needs to be expended,

The condenser

The refrigerant gives off heat in the condenser, which is supplied to another medium of lower temperature. The amount of heat released is composed of the energy absorbed in the evaporator and the compaction energy required for compaction.

The medium that absorbs this amount of heat may be water or air. The condition is that the temperature of the medium is lower than the condensing temperature. The liquefaction process is similar to the evaporation process, except that the state changes are in opposite directions, ie from the vapor phase to the liquid state.

The expansion process

Refrigerant liquid flows from the condenser to a collector that corresponds to the container mentioned in section 3.1.

Due to the pressure increase through the compressor, the collector pressure is much higher than the evaporator pressure. In turn, to lower the pressure to the level of the evaporator pressure, a throttle point must be installed, through which the refrigerant can expand. The corresponding component is usually an expansion valve.

Immediately before the expansion valve, the liquid refrigerant is slightly below its boiling point. The sudden drop in pressure causes a change of state. The refrigerant starts to boil and evaporates at a lower temperature. This evaporation takes place in the evaporator, which closes the described refrigeration cycle.

The high and low pressure sides of the refrigeration system

There are a number of different temperatures in the refrigeration cycle, as we are dealing with supercooled and saturated liquids as well as saturated and superheated steam. In principle, however, only two pressures prevail in the system; the evaporation and condensing pressure. Accordingly, the refrigeration cycle is divided into a low-pressure and a high-pressure side.