Switching compressors

Question: I have a question regarding high, medium and low temperature compressors.

Is it true that a medium temp. or low temp. compressor can be used as a substitute to replace a high temp. compressor; but a high temp. or medium temp. compressor cannot be used as a replacement for a low temp. compressor. If this is the case, why is it so?

The reason this question arose was due to some yard cranes using a ½ hp CFC-12 high temp. for the cabs. The CFC-12 units were to be replaced with ½ hp HCFC-22 units and the only compatible HCFC-22 units were medium temp. units.

-Brian Hall, Lewiston NY

Answer: Medium and low temperature compressors should not be used to replace high temperature compressors because of compressor displacement (cubic inch per revolution). Listed are examples of what could be expected:

•  A medium temperature compressor rated the same horsepower as a high temperature compressor will have 10% to 25% more displacement (depending on compressor manufacturer). This will cause higher discharge pressure/lower suction pressure. The motor will run hotter and could trip the motor protector.

•  A low temperature compressor rated the same horsepower as a high temperature compressor will have 20% to 50% more displacement (depending on compressor manufacturer). Discharge pressure will be high and suction pressure low. The motor will run hot and will most likely trip the motor protector.

When a high temperature compressor replaces a low or medium temperature compressor, the reverse happens. The replacement compressor will have less displacement causing the unit to run all the time without satisfying the demand.

Some low temperature compressors have different valve plates than medium and high temperature compressors. The reason for this is to increase efficiency and reduce the re-expansion volume in the cylinders.

The examples I gave were for refrigeration CFC-12 compressors. The statement you made about changing some yard cranes from CFC-12 high temperature to HCFC-22 medium temperature units is another issue. I suggest you call the Applications Engineering Department of the HCFC-22 unit you purchased to confirm the application.

-J.C. Powers, Bristol