Refrigeration takes place in a continuous four-step process with a high-pressure and low-pressure side. In the refrigeration cycle, liquids with low boiling points are employed as primary refrigerants. The important thermodynamic properties of these liquids are displayed in the refrigerant chart. The chart displays a plot of the substances’ pressure against temperature as well as melting, triple, and critical points if they occur within the working range of the refrigerant. By using the chart, refrigeration design engineers can choose the appropriate refrigerant for the system of interest.
A home refrigerator can demonstrate the four steps of the refrigeration cycle. Three fluids are involved: the room air, the air circulated within the refrigerator, and the refrigerant — all exchanging heat through indirect contact. The refrigerant enters the evaporator, the coils on the back of the refrigerator, and absorbs heat from the warm air coming from the refrigerator, causing the refrigerant to boil; it is then recompressed and condensed into a high-pressure liquid after releasing heat to the air in the room. The high-pressure, warm liquid refrigerant is passed through an expansion valve, which causes a sudden drop in pressure and temperature. Liquid cold refrigerant reenters the evaporator, and the cycle is complete.
The thermodynamic properties of the refrigerant may be displayed either in a refrigerant chart or as a table of data. The chart is generally more useful, as many more refrigerants can be displayed in one easy visual reference. The refrigerant chart is sometimes referred to as a pressure-temperature chart or a PT chart. An engineer may use the chart to look for a refrigerant that operates at lower pressures to reduce the pressure rating required for a particular system. If pressures are too low, however, the compression ratio will become abnormally high, making the compression step unfeasible.
The refrigerant chart displays the boiling temperature and pressure of the refrigerant. It is desirable to operate above atmospheric pressure to avoid moisture and air leakage into the system. To do so, the refrigerant boiling point should be lower than the system temperature level. The critical temperature and pressure points are also plotted on the refrigerant chart. The system must operate well below the critical values above which liquid and gas phases have identical properties.
A refrigerant chart is available from vendors of refrigerants or refrigeration equipment. Different equipment types used in refrigeration systems, such as reciprocating equipment, absorption units, or stream jet units require different refrigerants. Most chemical engineering references include a refrigerant chart. Users unfamiliar with refrigerants should consult a professional engineer or qualified HVAC technician before replacement of a refrigerant due to the concerns of high-pressure equipment and the ongoing environmental regulations concerning chlorinated hydrocarbon refrigerants.