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Liquid Injection Takes the Heat Off Compressors
2017/09/06
September 4, 2017
A large percentage of compressor failures in low-temperature vapor-compression refrigeration system applications originate from overheating. Typical compressor capacity rating data is based on an industry standard of a 65°F return vapor temperature. While a 65° return vapor temperature may be satisfactory for high-temperature applications and, perhaps, some medium-temperature applications, unsatisfactorily high-discharge temperatures are virtually unavoidable on low-temperature applications. This is due, in large part, to the thermodynamic properties of the refrigerant.

Refrigeration oils have been highly refined in an effort to elevate the temperature at which chemical decomposition occurs, and experience has shown mineral oil decomposition begins when compressor internal temperatures reach 350°. Compressor manufacturers typically indicate that discharge gas temperatures are 50° to 75° higher at the discharge valve inside the compressor than out on the discharge line, 6-8 inches from the service valve. Therefore, dangerously high temperatures already exist inside the compressor when discharge line temperatures are well below 350°. This lubricant decomposition is accelerated by residual contaminants that are present in a typical system. Studies have shown the rate of chemical reaction doubles with every 18° temperature increase. For example, a chemical reaction that takes 10 years to complete at 100° will only take five years to complete at 118°. At 136°, it would be complete in two-and-a-half years.
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