The refrigeration cycle uses low boiling point liquids as primary refrigerants in a four-stage process. A graph or table displays the thermodynamic properties of the refrigerant, which can help engineers choose the appropriate refrigerant for a system. A home refrigerator demonstrates the four stages of the cycle. The refrigerant graph shows the boiling temperature and pressure of the refrigerant, as well as critical points. Different types of equipment require different refrigerants, and it’s important to consult a professional before replacing a refrigerant due to high pressure equipment and environmental regulations.
The refrigeration takes place in a continuous four-stage process with a high-pressure and a low-pressure side. In the refrigeration cycle, liquids with a low boiling point are used as primary refrigerants. The important thermodynamic properties of these liquids are displayed in the table of refrigerants. The graph shows a graph of media pressure versus temperature, as well as melting, triple, and critical points if they occur within the working range of the coolant. Using the table, refrigeration designers can choose the appropriate refrigerant for the system of interest.
A home refrigerator can demonstrate the four stages of the refrigeration cycle. Three fluids are involved: the room air, the air circulating inside the refrigerator and the refrigerant, which exchange heat through indirect contact. The refrigerant enters the evaporator, the coils at the back of the refrigerator, and absorbs heat from the hot air coming from the refrigerator, causing the refrigerant to boil; it is then recompressed and condensed into a liquid at high pressure after releasing heat to the ambient air. High pressure hot liquid refrigerant is passed through an expansion valve, which causes a sudden drop in pressure and temperature. The cold liquid refrigerant re-enters the evaporator and the cycle is complete.
The thermodynamic properties of the refrigerant can be displayed in a refrigerant graph or as a data table. The graph is generally more useful, as many more refrigerants can be displayed in one simple visual reference. The refrigerant graph is sometimes referred to as a pressure-temperature graph or a PT graph. An engineer can use the table to look for a refrigerant that operates at lower pressures to reduce the pressure rating required for a particular system. If the pressures are too low, however, the compression ratio will become abnormally high, making the compression stroke impossible.
The refrigerant graph shows the boiling temperature and pressure of the refrigerant. It is preferable to operate above atmospheric pressure to avoid loss of moisture and air into the system. To do this, the boiling point of the coolant must be lower than the temperature level of the system. The temperature and pressure critical points are also shown on the refrigerant table. The system must operate well below the critical values above which the liquid and gaseous phases have identical properties.
A table of refrigerants is available from suppliers of refrigerants or refrigeration equipment. Different types of equipment used in refrigeration systems, such as reciprocating equipment, absorption units or jet flow 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 replacing a refrigerant due to concerns regarding high pressure equipment and ongoing environmental regulations regarding chlorinated hydrocarbon refrigerants.
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