Mechanical Engineering Department

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De La Salle University

Mechanical Engineering Department

Exercise 4-1 and 4-2

Types of Compressors and Operation of the Compressor

REMARKS :  _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________

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Exercise 4-1

1. Exercise Objective

• Completing the exercise will enable the performers to identify the three types of compressors.

1. Discussion

There are several types ol reciprocating compressors used for different applications. Factors which determine the type of compressor used are the type of refrigerant, the size of the refrigeration system, and the pressure drop required across the system. The types of compressors discussed in this exercise are:

• Hermetic
• Semi-hermetic

• Open or external type

These three types of compressors are all classified as reciprocating compressors. The classification of compressors is determined by the action of the mechanical operation of the compressor. A reciprocating compressor contains a piston moving back and forth (reciprocating) in a cylinder.

Hermetic Reciprocating Compressors

Hermetic compressors, as shown in Figure 4-1, are totally encased in a sealed housing. The sealed housing contains both the electric motor and the compressor

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With this type of design, the compressor operates at the speed of the motor, which is driven directly by the motor, and requires no shaft seal. The motor may be mounted either on top of or the bottom of the compressor. The motor is cooled by the transfer of heat from the stator to the case and by passing the returning gas around

the motor windings before the gas is compressed. This method, however, reduces the efficiency of the compressor since the returning gas is warmed. Hermetic compressors are extremely difficult to repair since the compressor casing is welded closed.

Figure 4-2 illustrates the cross-sectional view of a hermetic compressor with the motor at the bottom and the compressor at the top. Note that the compressor has only one piston and the motor is connected directly to the crankshaft. The rotation of the motor and the crankshaft, provides the action for the piston to travel back and forth within the cylinder. The unit is usually spring mounted inside the hermetic dome in order to prevent compressor vibration from being transferred outside the dome. The exhaust and suction lines inside the dome are flexible to prevent damage to the lines from the continuous vibration of the motor and compressor. The electrical connections to the motor pass through the dome by means of an insulated leak proof seal.

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Semi-Hermetic Compressor

The semi-hermetic compressor, as shown in Figure 4-3,is also called a serviceable hermetic compressor. This compressor also contains both electric motor and compressor in one unit. This type of unit is bolted rather than sealed, and may be taken apart and reassembled.

The compressor operates at motor speed, since it is driven directly by the motor and requires no shaft seal.

The motor is cooled by the transfer of heat from the stator to the casing and bypassing the returning gas around the motor windings before the gas is compressed. This reduces the compressor efficiency since it warms the returning gas.

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Figure 4-4 illustrates the cross-sectional view of a semi-hermetic compressor.

Semi-hermetic compressors are larger in size than hermetic compressors since they usually contain two or more pistons. The larger capacity of a semi-hermetic compressor allows it to be used on larger refrigeration systems than herm compressors.

The sturdy construction and precise balancing of the two piston crankshaft in this type of compressor eliminates the need for suspension springs inside the casing too reduce vibrations.

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Open Type Compressor

The open type compressor, as shown in Figure 4-5, is also called the external drive compressor. The drive motor and compressor on this unit are separate. The crank shaft of the compressor extends through the crankcase and is driven by a pulley and belt, or can be driven directly by an electric motor. A crankshaft seal is required where the crankshaft comes through the crankcase.

The external drive compressor is usually driven at less than the motor speed. This requires that the motor belt pulley be smaller than the compressor pulley. The size of the drive pulley on the motor and the pulley on the compressor govern the speed of the compressor.

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