PISTON COMPRESSOR TYPES

As previously stated, open type and hermetic compressors are the main differentiation between helical rotary compressors. The term open type refers to the compressors that are driven by belt and pulley drive or directly connected motor. In these compressors, there is a shaft extending to the compressor’s crankcase coupled with the seal for tightness. The hermetic compressors are, however, characterized by the motor and compressor coupled to each other in the same casing. Thanks to the hermetic tightness, there is no shaft between the driving and driven part of the compressors, without any sealing element. Both types have their advantages over other.

Belt and pulley compressors are extremely flexible. The same compressor might be modified to make use in the units at two or three different sizes. In most cases, the same compressor can be used for low, medium or high temperature applications, let alone with different size motors, just by changing the size of pulley and compressor valve gap. This feature of open type compressor is most apparent advantage over hermetic compressors.

These compressors can also be used under irregular voltage and frequency conditions, where hermetic compressors are not suitable for such adverse conditions. This property has been an important advantage in overseas markets where the conditions are not as in United States of America. For example, where direct current under 50 Hz frequency are the only power source, hermetic or semi-hermetic compressors are not suitable, since they cannot be operated with direct current.
Open type compressors can be provided with in-situ maintenance. This is not always the case for hermetic compressors. To replace the broken motor is easy in open type compressor than in hermetic compressors where the motor is in contact with the refrigerant. For hermetic compressors, complete disassembly is necessary in case of a motor failure, usually requiring service in the workshop or at the manufacturer premises. Besides, the system should be evacuated and cleaned throughout in order to eliminate the possibility of circulation of the acid in the system that may give rise to defective insulation.

On the other hand, there are several advantages of the hermetic compressors over open type compressors. This is probably the use of the connecting shaft without a sealing element. The sealing element is vulnerable to the dirt in the system, short interruption in the lubrication, an abrasive foreign element in the system (like chips), physical damages and severe use of compressor, etc. Contemporary shaft sealing elements are far advanced than those used 15 to 20 years ago, however, they still prone to fail in case of the vacuum generated at the low pressure side and fluctuations in the system temperature. Any failure in the sealing element gives rise to ingress of the air or steam into the cooling system. This results the system failure.

One another advantage of hermetic compressors is their compact size, more resistant to the vibration, proper cooling of the motor and good lubrication. Besides, they do not have the belts that should frequently need to be replaced.

The open type compressors are continuously cooled by the air around the motor enclosure. If the ambient air temperature is high or suitable ventilation is not provided, it might often be impossible to provide appropriate cooling. In hermetic compressors, the motors are cooled by the gas refrigerant passing through or around the motor windings and then the refrigerant are conveyed to compression chamber where the heat is dissipated. Another advantage of such operation principle is that, the refrigerant is superheated to make it easy for the gas to enter into the suction opening. The operation performed at low temperature also incorporates the drying of the gas.

In fact, open type compressors share quite small portion in the market and generally used in industrial cooling applications where the refrigerant temperatures and ambient conditions are not suitable for comfort air conditioners. There are still open type compressors operating and it is probable for the technician to encounter open type compressor that need to be repaired or maintained and therefore, it is important to be familiar with its basic structure.