Refrigerant Diagrams

The characteristics of a refrigerant can be illustrated in a diagram using the primary properties as abscissa and ordinate.

For refrigeration systems the primary properties are normally chosen as energy content and pressure. Energy content is represented by the thermodynamic property of specific enthalpy – quantifying the change in energy content per mass unit of the refrigerant as it undergoes processes in a refrigeration system.

An example of a diagram based on specific enthalpy (abscissa) and pressure (ordinate) can be seen above. For a refrigerant the typically applicable interval for pressure is large – and therefore diagrams use a logarithmic scale for pressure.

The diagram is arranged so that it displays the liquid, vapour and mixture regions for the refrigerant. Liquid is found to the left (with a low energy content) – vapour to the right (with a high energy content). In between you find the mixture region. The regions are bounded by a curve – called the saturation curve. The fundamental processes of evaporation and condensation are illustrated.

The idea of using a refrigerant diagram is that it makes it possible to represent the processes in the refrigeration system in such a way that analysis and evaluation of the process becomes easy.

When using a diagram determining system operating conditions (temperatures and pressures) system refrigerating capacity can be found in a relatively simple and quick manner. Diagrams are still used as the main tool for analysis of refrigeration processes. However, a number of PC programmes that can perform the same analysis faster and with more details have become generally available.


Refrigeration process, pressure/enthalpy diagram

  • tc = condensing temperature
  • pc = condensing pressure
  • tl = liquid temperature
  • t0 = evaporating temperature
  • p0 = evaporating pressure

The condensed refrigerant in the condenser is in condition A which lies on the line for the boiling point of the liquid. The liquid has thus a temperature tc, a pressure pc also called saturated temperature and pressure.

The condensed liquid in the condenser is further cooled down in the condenser to a lower temperature A1 and now has a temperature tl and an enthalpy h0. The liquid is now sub-cooled which means that it is cooled to a lower temperature than the saturated temperature.

The condensed liquid in the receiver is in condition A1 which is sub-cooled liquid. This liquid temperature can change if the receiver and liquid is either heated or cooled by the ambient temperature. If the liquid is cooled the sub-cooling will increase and visa versa.

When the liquid passes through the expansion valve its condition will change from A1 to B. This conditional change is brought about by the boiling liquid because of the drop in pressure to p0. At the same time a lower boiling point is produced, t0, because of the drop in pressure.

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In the expansion valve the enthalpy is constant h0, as heat is neither applied nor removed.

At the evaporator inlet, point B, there is a mixture of liquid and vapour while in the evaporator at C there is saturated vapour. At the evaporator outlet 4. Refrigeration process, pressure/enthalpy diagram point C1 there is super-heated vapour which means that the suction gas is heated to a higher temperature than the saturated temperature. Pressure and temperature are the same at point B and at outlet point C1 where the gas is super-heated the evaporator has absorbed heat from the surroundings and the enthalpy has changed to h1.

When the refrigerant passes through the compressor its condition changes from C1 to D. Pressure rises to condensing pressure pc. The temperature rises to thot-gas which is higher than the condensing temperature tc because the vapour has been strongly superheated. More energy (from the electrical motor) in the form of heat has also been introduced and the enthalpy therefore changes to h2.

At the condenser inlet, point D, the condition is thus one of superheated vapour at pressure pc. Heat is given off from the condenser to the surroundings so that the enthalpy again changes to main point A1 . First in the condenser there occurs a conditional change from strongly superheated vapour to saturated vapour (point E), then a condensation of the saturated vapour. From point E to point A the temperature (condensing temperature) remains the same, in that condensation and evaporation occurs at constant temperature. From point A to point A1 in the condenser the condensed liquid is further cooled down, but the pressure remains the same and the liquid is now sub-cooled.