Servicing a mechanical refrigeration system often involves recovering the refrigerant charge, making repairs, and evacuating and recharging the system. Therefore, a service technician must know what refrigerant is in a system before servicing it.
Refrigerants are divided into three classifications (Figure 3-5). The first group is the chlorofluorocarbons, called CFCs. This group of refrigerants is made up of Chlorine, Fluorine, and Carbon atoms. It includes CFC-11, CFC-12, CFC-500, and CFC-502. CFCs are the most hazardous to the environment. For this reason, they are under the strictest regulation and are being phased out of production. Production of CFCs will cease in the United States on December 31, 1995.
The second group is the hydrogenated chlorofluorocarbons, called HCFC s. Th is group of refrigerants is made up of Hydrogen, Chlorine, Fluorine, and Carbon atoms. HCFC refrigerants are considered as short-term alternates to replace CFC refrigerants when applicable. HCFCs are considered far less damaging to the environment than CFCs. Their ozone depletion capability ranges from 25% to 1% of that for the CFCs. Some HCFC refrigerants in use are HCFC-22, HCFC-123 and HCFC-124- As of 1996, HCFC refrigerant production is frozen to historic usage levels with eventual phase-out scheduled for the year 2030.
The third group is the hydrogenated fluorocarbons, called HFCs. They are made up of Hydrogen, Fluorine, and Carbon. These refrigerants are considered to be least damaging to the environment. HFC refrigerants HFC-134a and HFC-125 are being used mainly to replace refrigerants CFC-12 and CFC-502, respectively. HFC-134a is used primarily in commercial and residential medium-temperature refrigeration and residential and transportation air conditioning equipment. HFC-125 is used for low-temperature supermarket refrigeration and commercial food transportation.
A refrigerant will be either a single chemical compound (pure refrigerant) or a blend o f two or three refrigerants. CFC-12 is an example of a pure refrigerant. Blends are classified as either azeotropes or zeotropes.
An azeotropic refrigerant blend behaves like a compound when evaporating or condensing. It has a constant volume and saturation temperature as it evaporates or condenses at a constant pressure. The boiling point of the mixture is different from either of the base refrigerants. Azeotropic blends typically have identifiers in the 500 series. CFC-500 (Freon 500™ and Gentron 500™) and CFC-502 (Freon 502™ and Gentron 502™) are examples of azeotropic blends in common use. Since these are CFC refrigerants, they are being phased out of production and use.
Pure refrigerants and azeotropic refrigerants have only one temperature at which they evaporate or condense associated with one given pressure. Most zeotropic blends are ternary blends; i.e., a blend of three refrigerants. Zeotropic blends, unlike azeotropic blends, never mix chemically; therefore, they have a temperature glide when they evaporate or condense. Temperature glide is a range of temperatures within which evaporating or condensing takes place at a given pressure. The exact amount of glide is determined by the system design and chemical makeup of the refrigerant blend. Because of temperature glide, the methods used to calculate subcooling and superheat for zeotropes are somewhat different than those used with pure refrigerants or azeotropes. The methods used for calculating subcooling and superheat are described in Section IV of this manual.
Another property of zeotropic blends is fractionation. Fractionation means that one or more refrigerants o f the same blend leak at a faster rate than the other refrigerants in the same blend.
This different leakage rate is caused by the influence a given pressure has on each of the individual refrigerants contained in the blend. From a service point o f view, fractionation means that some precautions must be taken to make sure that the composition of the blend remains unchanged while being charged into a system. Typically this is done by removing the refrigerant blend from the charging cylinder as liquid only, applying it through a metering valve or other expansion device to ensure that the liquid is converted into vapor, then charging the vapor into the low side of the system. To avoid damage to the compressor, care must be taken to make sure that all the liquid refrigerant is converted to vapor prior to entering the system. Another effect of fractionation is that a leaking system cannot just be topped-off after the leak is repaired. The system charge must be removed and the system recharged.
Zeotropic blends typically have identifiers in the 400 series. Refrigerant blends R -401A (SUVA™ and Gentron™ MP-39) and R -4 0 2A (SU V A™ MP -80) and R -4 0 4A (SU V A™ MP-62) are examples of some zeotropic blends in use.
New, environmentally-safe alternative refrigerants are continuously being developed to either replace the harmful CFC and HCFC refrigerants in existing equipment or for use in new equipment. SUVA™ refrigerant MP-39 and Gentron™ AZ-50 are examples of replacement refrigerants. MP-39 is a zeotropic blend made to replace CFC-12; AZ-50 is an azeotropic blend made to replace CFC-502. SUVA™ blend MP-33 is a refrigerant being made for use in new refrigeration equipment. SUVA™ AC9000 is an HFC-based alternative refrigerant ternary blend developed to replace R-22 in new air conditioning equipment.
It is important to remember that most replacement refrigerants are not direct replacements for the CFC-type refrigerants. System modifications are usually necessary for the replacement refrigerant to be compatible with existing equipment. Never substitute a refrigerant in a system ivithotit first getting the manufacturer’s approval. Even though “replacement” refrigerants claim to be an equal substitute for various refrigerants, none is identical to the refrigerant it replaces.
The kind of refrigerant used in a system can be identified using one or more of the following methods:
• Check the equipment nameplate.
• On equipment that uses a thermostatic expansion valve, the refrigerant type is usually marked on the valve.
• When no printed data can be found, take a pressure and saturation temperature reading for the refrigerant when the machine is off and ambient temperatures have been reached. Compare the saturation temperature/pressure relationship with refrigerant cards or charts to identify the refrigerant.