Methods for Compressor Cooling

December 4, 2006

To prevent overheating of a hermetic compressor’s internal motor, some type of cooling method must be employed. Compressor manufacturers will use one of three methods to provide the necessary cooling for their motors. One method is to use the returning suction vapor to cool the motor windings. This is used on all fully-hermetic compressors, as well as many semi-hermetic compressors. This type of compressor is generally referred to as a refrigerant-cooled compressor.

Another method used is to pass a sufficient amount of air across the body of the compressor. This design is used on many semi-hermetic compressors and the compressor is generally referred to as an air-cooled compressor.

The third method used is to wrap a water jacket around the body of a semi-hermetic compressor and use the water leading to a water-cooled condenser to cool the motor’s windings. These compressors are generally referred to as water-cooled compressors.

Table 1.

Since refrigerant-cooled compressors rely on the returning suction vapor to cool the motor windings, there is generally a maximum return gas temperature associated with a compressor’s design. Always check with the manufacturer to determine the maximum return gas temperature for a particular compressor. Generally speaking, vapor temperatures no higher than 65°F under low temperature loads and less than 90° for high temperature loads are acceptable.

Refrigerant-cooled compressors are usually adequately cooled with the returning suction gas down to an evaporating temperature of 0°. Below 0° additional cooling by means of airflow is necessary. Many manufacturers will employ a head-cooling fan to provide the additional cooling for these compressors. It is important for a service technician to realize the importance of these fans. During a service or maintenance inspection, these fans can be overlooked leading to compressor overheating and future service problems.

Air-cooled compressors will generally use the discharge air from an air-cooled condenser to provide the required cooling for a compressor. If the compressor and air-cooled condenser are not located next to each other (as in the case of a remote condenser application), a fan must be provided to pass air over the body of the compressor.

Another concern with regard to compressor cooling is when compressors or condensing units are located in a machine room, adequate ventilation air must be provided to avoid excessive temperature rise in the room. During peak summer temperatures, the ambient temperature should not rise more than 10°.

The cfm requirement for ventilation of a machine room can be determined by using the formula:

The total heat of rejection must be factored into the equation. This includes the compressor’s refrigerating capacity at its design operating condition as well as the heat equivalent of all the motors used with the compressor and the system.

Here is an example: Suppose we have an air-cooled condensing unit with a refrigerating capacity of 18,000 Btuh with a 5,400-Watt input to the compressor motor, and a ¾-horsepower condenser fan motor located in a machine room and we need to determine the ventilation requirements for this room. (See Table 1.)

Publication date: 12/04/2006