what do you know about refrigeration Compressors-00-1279
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by: Eng.Mohamed Fawzy
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A compressor is the common important and often the most expensive component of any vapor compression refrigeration system (V.C.R.S.).
The purpose of a compressor in a V.C.R.S. is to transfer the refrigerant vapor which come from the evaporator pressure (low pressure) to the condenser pressure (high pressure)
types of compressors:
Compressors used in refrigeration systems can be divided in many ways:
a) Based on the working principle:
i. Positive displacement type
ii. Rotor-dynamic type
In positive displacement type compressors, compression is provided by trapping a refrigerant vapor into an enclosed space and then reducing its volume.Since a fixed amount of refrigerant is trapped each time, its pressure rises as its volume is reduced.
When the pressure rises to a level that is slightly higher than the condensing pressure, then it is expelled from the enclosed space and a fresh charge of low-pressure refrigerant is drawn in and the cycle continues. Since the flow of refrigerant to the compressor is not steady, the positive displacement type compressor is a pulsating flow device.
However, since the operating speeds are normally very high the flow appears to be almost steady on macroscopic time scale. Since the flow is pulsating on a microscopic time scale, positive displacement type compressors are prone to high wear, vibration
and noise level. Depending upon the construction, positive displacement type
compressors used in refrigeration and air conditioning can be classified into:
i. Reciprocating type
ii. Rotary type with sliding vanes
iii. Rotary screw type
iv. Orbital compressors
v. Acoustic compressors
In rotor-dynamic compressors, the pressure rise of refrigerant is provided by increasing kinetic energy to a steadily flowing stream of refrigerant by a rotating mechanical element and then converting into pressure as the refrigerant flows through a diverging passage.
The rotor-dynamic type compressors are steady flow component, hence are subjected to less vibration. Depending upon the construction, rotor-dynamic type compressors can be divided into:
i. Radial flow type, or
ii. Axial flow type
These compressors are widely used in huge capacity refrigeration and air conditioning systems. Axial flow compressors are normally used in gas liquefaction applications.
b) Based on arrangement of compressor motor or external drive:
i. Open type
ii. Hermetic (or sealed) type
iii. Semi-hermetic (or semi-sealed) type
In open type compressors the shaft of the compressor extends through a seal in the crankcase for an external drive. The external drive may be an electrical motor or an engine.
The compressor may be belt driven or gear driven. Open type compressors are normally used in intermediate to huge capacity refrigeration system for all refrigerants and for ammonia (due to its incompatibility with hermetic motor materials).
Open type compressors are characterized by high efficiency, flexibility, better compressor cooling and service ability. However, since the shaft has to extend through the seal, refrigerant leakage from the system cannot be eliminated completely.
Hence refrigeration systems using open type compressors require a refrigerant reservoir to take care of the refrigerant leakage for some time, and then regular maintenance for charging the system with refrigerant, changing of seals, gaskets etc.
In hermetic compressors, the motor and the compressor are enclosed in the same housing to prevent refrigerant leakage. The housing has weld connections for refrigerant inlet and outlet and for power input socket.As a result of this, there is virtually no possibility of refrigerant leakage from the compressor.
All motors reject a part of the power supplied to it due to eddy currents and friction, that is, inefficiencies. Similarly the compressor also gets heated-up due to friction and also due to temperature rise of the vapor during compression.
In Open type, both the compressor and the motor normally reject heat to the surrounding air for efficient operation. In hermetic compressors heat cannot be rejected to the surrounding air since both are enclosed in a shell. Hence, the cold suction gas is made to flow over the motor and the compressor before entering the compressor.
This keeps the motor cool. The motor winding is in direct contact
with the refrigerant hence only those refrigerants, which have high dielectric strength, can be used in hermetic compressors.
The cooling rate depends upon the flow rate of the refrigerant, its temperature and the thermal properties of the refrigerant. If flow rate is not sufficient and/or if the temperature is not low enough the insulation on the winding of the motor can burn out and short-circuiting may occur. Hence, hermetically sealed compressors give satisfactory and safe performance over a very narrow range of design temperature and should not be used for off-design conditions.
The COP of the hermetic compressor based systems is lower than that of the
open compressor based systems since a part of the refrigeration effect is lost in cooling the motor and the compressor. However, hermetic compressors are almost universally used in small systems such as domestic refrigerators, water coolers, air conditioners etc, where efficiency is not as important as customer convenience (due to absence of continuous maintenance).
In addition to this, the use of hermetic compressors is ideal in systems, which use capillary tubes as expansion devices and are critically charged systems. Hermetic compressors are normally not serviceable. They are not very flexible as it is difficult to vary their speed to control the cooling capacity.
In some (usually larger) hermetic units, the cylinder head is usually removable so that the valves and the piston can be serviced. This type of unit is called a semi-hermetic (or semi-sealed) compressor.
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