Reader Mail: 08/11/2003

Refrigerant Characteristics

Flammability: He correctly points out that a blend of refrigerants that contain a flammable component is not a safety hazard. Any refrigerant with an ASHRAE classification of A1 is not only nonflammable and nontoxic as formulated, but also cannot become flammable or toxic under any fractionation scenario. This means that if part of a refrigerant blend with a 400 series number, (e.g., 402A, 409A, 414B, 416A ) leaks out, the residual refrigerant, which may no longer meet the specification, will still be safe.

Oil compatibility: Mr. Hale is also correct when he discusses oil compatibility. A chemical reaction between an oil and refrigerant is almost impossible. The real issue is whether adequate oil return to the compressor can be maintained under all conditions since a compressor without oil is doomed to a short life. Compatibility is not the real issue; a property called miscibility, which refers to how well the oil and liquid refrigerant dissolve in one another, is really the major issue here. None of the alternatives have as good miscibility as CFC-12, and therefore a synthetic lubricant is recommended when converting. The reason that manufacturers add the hydrocarbon, like butane or propane, is to improve miscibility. A small amount of hydrocarbon can have an appreciable effect on miscibility while the blend still remains nonflammable.

Higher pressure vs. lower pressure: This comparison begs for more discussion. Mr. Hale fails to address the effects that a higher pressure refrigerant has on an operating system or to accurately describe the operating characteristics of the refrigerant alternatives.

The laws of physics dictate that a higher pressure refrigerant will have higher capacity and higher power consumption than a lower pressure refrigerant. The percentage in-crease in capacity and power consumption is almost exactly the same as the percent increase in pressure. At first glance, higher capacity would seem to be a benefit, but it is important to consider the effects on the system. The higher capacity gained by the higher pressure refrigerant causes the evaporating temperatures to de-crease and the condensing temperatures to rise. Higher condensing temperatures create a higher load on the compressor motor, which translates into increased amp draw and lower overall system efficiency. The increase in the compressor load makes the compressor work harder and will generally shorten the compressor life.

Conversely, a lower pressure refrigerant will cause the evaporating temperature to rise slightly and the condensing temperature to fall. This reduces the compression ratio and tends to unload the compressor slightly and consume less power. Reduced load on the compressor equates to longer compressor life. The reduction in capacity is actually less than would be predicted from the refrigerant properties because of the increase in evaporating temperature. This has been described as a “rebound effect.”

There are a few cases where the use of a lower pressure alternative is not the best choice. In those rare cases where the original capacity of the CFC-12 system was marginal and capacity is the most important issue, a higher pressure blend may be a better selection. Another area where the higher pressure refrigerant should be superior is in low temperature applications. However, the vast majority of systems will benefit from using a refrigerant blend with condensing pressures more similar to the R-12 that it is replacing.

Temperature glide: Mr. Hale failed to address the issue of refrigerant glide in his letter. Refrigerant glide is one of the most important criteria when considering a refrigerant blend. All 400 series refrigerants are zeotropic blends, which all have a tendency to fractionate and will have some degree of temperature glide. The greater the fractionation, the higher the refrigerant glide. Higher pressure R-22-based blends have higher glide characteristics than lower pressure R-134a-based blends. For example, at a 30 degrees F evaporator temperature, R-414B has a glide of about 14 degrees F, which would be characterized as a high glide, while R-401A has a glide of 9 degrees F, which would be characterized as a medium glide, and R-416A has a glide of less than 3 degrees F, which would be characterized as a low glide. Generally speaking, low-glide refrigerants have advantages in terms of superior heat transfer and handling, and they perform more consistently in the event of a gradual refrigerant leak.

Mr. Hale’s final statement, “Look beyond the headlines, and be sure to ask lots of questions,” sums it up very well.

H. Michael Hughes, Engineering Consultant, Lavonia, GA

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Publication date: 08/11/2003