Charging Systems that Use Zeotropic Refrigerant Blends

Most refrigerants such as HCFC-22 are made from a single pure chemical compound. Others are made by mixing (blending) two or more different refrigerants. Refrigerant blends can be HCFC based, HFC based, or a combination of both. A zeotropic blends are made when two different liquid refrigerants are mixed to form a new refrigerant with its own properties. Azeotropic blends act like a compound. A t a constant pressure, they do not change volumetric composition or saturation temperature as they evaporate or condense. The boiling temperature of the blend is independent of the boiling temperatures of the individual refrigerants used in the blend. A n example of an azeotropic blend refrigerant is AZ-50™, which is a mixture of HFC-125 and HFC-143a. CFC-500 and CFC-507 are some other examples of azeotropic blend refrigerants.

Zeotropic blends are made when two or more different liquid re frig e ran ts are mixed to form a new re frigerant that retains the individual properties of its constituent refrigerants. Unlike pure compounds or azeotropic blends, zeotropic blends change volumetric and saturation temperatures as they evaporate or condense. For a given system pressure, the zeotropic refrigerant blend will have one temperature at which it evaporates and a different one at which it condenses. This characteristic is called temperature glide. The exact amount of glide is determined by the system design and the refrigerants used in the blend.

Zeotropic blend refrigerants are commonly called ternary blends because most zeotropes are made from a blend of three refrigerants. An example of a zeotrope refrigerant is MP-39 (R-401 A) made from a blend of HCFC-22, HFC-152a, and HCFC-124- Zeotropic blend refrigerants are usually identified by 400-series R-numbers: e.g., R-407C.

The methods used to charge a system with a zeotrope refrigerant are somewhat different than those used with pure compounds or azeotropes. This is because each zeotropic refrigerant has a different temperature glide. For this reason, always follow the manufacturer’s instructions when charging a system that uses a zeotropic blend refrigerant.

Some important points to remember when charging a system that uses a zeotropic refrigerant are:
• If the refrigerant is in good condition (clean, dry, and no evidence of leakage prior to servicing), it can be charged back into the unit from which it was removed.
• If recharging a system after the repair of a leak, always recharge with new refrigerant. This is necessary because the leak can cause fractionation to occur in the zeotrope refrigerant remaining in the system. When there is a vapor leak, the refrigerant component of the blend that has the highest pressure will leak out o f the system at a faster rate than the lower pressure refrigerant components. This changes the chemical composition of the refrigerant remaining in the system. Operation of the system using this altered refrigerant can cause poor system performance and possible system damage. Some zeotropic refrigerants such as R-410Ahave such a small glide that they can effectively be treated as a pure refrigerant, with no regard for glide. If in doubt* always follow the manufacturer’s charging recommendations.
• Zeotropic refrigerants must be “ liquid charged” into the high side of the system, so the components in the blend do not separate. Charging by weight is the preferred method for admitting the correct charge.
• If it is necessary to charge refrigerant into the low side of an operating system, always make sure that all the liquid refrigerant taken from the liquid port of the cylinder is passed through a throttling (metering) valve so that the refrigerant enters the low side of the system in vapor form. This is necessary to avoid compressor damage.


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