Learning the Basic Refrigeration Cycle

The basic refrigeration cycle functions by removing the heat from a colder medium and ejecting it to a medium with higher temperature. This is done by using the refrigerant’s latent heat properties. Simply stated, the transfer of heat energy may be accomplished when the refrigeration system provides a refrigerant temperature just below the temperature of the medium that needs cooling, and when it raises the refrigerant’s temperature to a certain level just above the temperature level of the medium that will be used for rejection. Although the whole chiller package is more complicated, the components needed for the basic refrigeration cycle are the evaporator, compressor, thermostatic expansion valve, and condenser.

The P-H chart is a very important tool in understanding and learning the different property changes that take place in each basic refrigerant cycle phase. The horizontal lines found on the P-H chart correspond to the constant pressure while the vertical lines correspond to the constant heat energy or enthalpy. The line that says “Saturated Vapour Line” and “Saturated Liquid Line” corresponds to the plots of the enthalpy vs. pressure for the saturated state of a refrigerant. The P-H chart is divided into three separate regions. The left area is called the sub-cooled region, the right is called the superheated region, and the centre is the mixture state or the wet region.

The lines for the constant temperatures are horizontal and they are in the mixture region to indicate the phase change occurring at constant pressure. Similarly, the gas expansion takes place at constant heat energy or enthalpy. During the evaporation phase, the refrigerant enters the evaporator as a liquid and vapour mixture by being metered with the use of a thermostatic expansion valve, which lowers the pressure and the temperature. Because the temperature of the refrigerant is lower than the process fluid, the refrigerant absorbs the heat coming from the process fluid and boils and changes the liquid phase to the gas phase. In order to change the refrigerant’s state, heat energy must be taken in. During the heat energy transfer, only the latent heat is absorbed and this results in the refrigerant having a constant temperature. After the latent heat is absorbed during the evaporation and superheating phase, the refrigerant gas undergoes compression from a low to high pressure gas. During this process, the refrigerant gas becomes capable of absorbing the additional heat called the Heat of Compression, which is the result of the molecule friction being forced rapidly into the confined space.

Superheat is the heat added to the produced vapour; however, this is beyond what is needed to vaporize the available liquid. Therefore, superheat is not latent but it is sensible to heat and is measurable in degrees. While an amount of superheat is necessary for the protection of the refrigeration system and the prevention of the liquid from going into the compressor, more than necessary superheat can possibly lead to increased system downtime and oil breakdown.

Sub cooling is another important part of the basic refrigeration cycle. This is the cooling process of the condensed gas beyond what the condensation process requires. Sub cooling is measured in degrees and is sensible to heat. It can cause a dramatic effect in the refrigeration system’s capacity by increasing the refrigerant’s capacity to absorb heat for similar compressor KW input during the evaporation phase.

The basic refrigeration cycle can be very complex but learning and understanding it provides you with a lot of benefits.