When choosing a refrigeration compressor for a particular job, there are several specifications a service technician must consider in order to choose the right one. Some of these specifications include refrigerant used, evaporating and condensing temperatures, application (high, medium, or low temperature), starting torque, and the system’s required capacity.

Many times when ordering a compressor, a technician states its capacity in horsepower (hp). However, this is not the best way. The capacity should be stated in Btuh as this is a much more accurate way to discuss the compressor’s capacity.

Compressor specification sheets always rate compressors in Btuh. Stating the horsepower of a compressor does not give any useful information. It is important to remember that the listed Btuh capacity of a compressor is based on a specific refrigerant, evaporating temperature, and condensing temperature. Be sure to choose a compressor with the right capacity at the system’s design condition.

Capacities And Options

The Btu capacity of the compressor is rated over a specific period of time. Normally it is stated as per hour, such 10,000 Btuh — which is 10,000 Btu per hour. When listing the capacity, many times the “per hour” symbol (h) is omitted. If the per hour symbol is omitted, it is assumed to be per hour.

If the compressor’s capacity was rated over a different time period, such as minutes or 24 hours, then the capacity must be stated as such. For example, if the capacity of the compressor were 400 Btu per minute, then it should be written as 400 Btu/min.

Making the correct starting torque selections is necessary in order to ensure the compressor will start normally each time the system calls for cooling. Starting torque requirements are based on how the suction and discharge pressures equalize during the off cycle. If the suction and discharge pressures were to equalize during the off cycle, such as when the system uses a capillary tube as its metering device, then a lower starting torque compressor can be used.

If the suction and discharge pressures do not equalize during the off cycle, such as when a standard thermostatic expansion valve is used, a higher starting torque compressor must be used.

A technician must also make sure that the compressor chosen is designed for the refrigerant used in the system. With all the types of refrigerants on the market today, it is easy to overlook this important specification. Sometimes compressors are rated for more than one type of refrigerant, offering a little more flexibility when choosing a compressor.

The refrigerant oil should also be looked at to make sure it is acceptable for the refrigerant used and the application.

Choosing the right compressor for the job is not a terribly difficult task, but it is still terribly important. It simply requires some understanding of the system’s design and the different specifications that need to be matched accordingly.

Joe Marchese is owner of Coldtronics of Pittsburgh. He can be reached at 412-734-4433, www.coldtronics.com, or joe@coldtronics.com.

Publication date: 07/07/2003