Many years ago, refrigeration systems commonly used three basic refrigerants: chlorofluorocarbon (CFC)-502 for low-temperature systems; CFC-12, mainly for medium-temperature systems but sometimes for low-temperature systems; and hydrochlorofluorocarbon (HCFC)-22, mainly for high-temperature systems and sometimes medium-temperature systems.
Today, however, there is a vast selection of refrigerants, and sometimes a technician has a choice in selecting a refrigerant for a particular system. Knowing some of the basic properties of refrigerants can assist a technician in making that selection.
One of the most basic properties is a refrigerant’s saturation temperature/pressure. This is the pressure of a refrigerant at a given temperature when both its liquid and vapor forms are present together. The saturation pressure must be within an acceptable range for use in both the condenser and evaporator. If the pressures are too high or too low for the required temperature range, then the refrigerant is not a suitable choice.
For example, the saturation pressure for hydrofluorocarbon (HFC)-134a is 19 psig at a temperature of 21ºF and 135 psig at a temperature of 105º. This makes HFC-134a very suitable for use in medium-temperature refrigeration systems. On the other hand, CFC-11 has a saturation pressure of 21.08 inches of mercury (Hg) in a vacuum at a temperature of 20º, which makes it a poor choice for use in medium-temperature refrigeration systems. The low-side pressure would need to operate in a vacuum that would make it very prone to drawing in air and moisture, which would lead to service problems for
Another refrigerant property is its specific volume, which is the volume 1 pound of a refrigerant vapor occupies. It’s advantageous for a refrigerant vapor to have a low specific volume. Most refrigeration compressors pump a constant volume per minute. The lower the specific volume of a refrigerant, the more refrigerant a compressor can pump.
Toxicity is an important refrigerant property. It’s the degree to which a substance is toxic (poisonous). ASHRAE’s Standard 34 provides the toxicity rating of refrigerants. It classifies a refrigerant with either an A (lower) or B (higher) toxicity rating. ASHRAE’s Standard 34 also provides the flammability rating of a refrigerant. Refrigerants are assigned to one of three classes (1, 2, or 3) and one optional subclass (2L) based on lower flammability limit testing, heat of combustion, and the optional burning velocity measurement.
Refrigerants also have an ozone depletion potential (ODP). Scientists have proven that some refrigerants, when released into our atmosphere, will chemically react with the earth’s ozone layer and cause it to become depleted. The ODP is a number derived to express the risk a refrigerant has for damaging the ozone level — the higher the number, the greater the risk. CFC refrigerants have a high ODP number. Some refrigerants have a zero ODP.
GLOBAL WARMING POTENTIAL
The global warming potential (GWP) of a refrigerant is another rating of a refrigerant’s hazard to the environment. Scientists believe certain refrigerants, when released into the atmosphere, prevent the sun’s long-wave (infrared) radiation from escaping, thus causing a gradual warming of the Earth. The GWP is a number derived to express the risk a refrigerant has for trapping heat in the atmosphere; the higher the number, the greater the risk. CFC refrigerants have a high GWP number.
Another very important property of a refrigerant is its miscibility with lubricants. A refrigerant should be able to mix well with the refrigeration oil used in a system at all the expected temperatures. Refrigeration oil travels with refrigerant through the system and relies on the refrigerant to help carry the oil back to the compressor. The refrigerant and oil must mix well in order for the oil to return to the compressor. When selecting a refrigerant for a system, it must be miscible with the oil used.
Understanding these basic properties will help technicians choose the best refrigerant to use for a particular application or system.
Publication date: 10/05/2015