Refrigeration Compressors
A compressor is the most important and often the costliest component (typically 30 to 40 percent of total cost) of any vapour compression refrigeration system (VCRS).
The function of a compressor in a VCRS is to continuously draw the refrigerant vapour from the evaporator, so that a low pressure and low temperature can be maintained in the evaporator at which the refrigerant can boil extracting heat from the refrigerated space.
The compressor then has to raise the pressure of the refrigerant to a level at which it can condense by rejecting heat to the cooling medium in the condenser.
Classification of compressors:
Compressors used in refrigeration systems can be classified in several ways:
a) Based on the working principle:
i. Positive displacement type
ii. Rotor-dynamic type
In positive displacement type compressors, compression is achieved by trapping a refrigerant vapour 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 achieved by imparting 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.
Unlike positive displacement type, the roto-dynamic type compressors are steady flow devices, hence are subjected to less wear and vibration. Depending upon the construction, rotor-dynamic type compressors can be classified into:
i. Radial flow type, or
ii. Axial flow type
Centrifugal compressors (also known as turbo-compressors) are radial flow type, roto-dynamic compressors. These compressors are widely used in large 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 rotating 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 (e.g. diesel engine).
The compressor may be belt driven or gear driven. Open type compressors are normally used in medium to large 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 serviceability. 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 theirspeed 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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Mohamed Fawzy is a mechanical engineer who has a good experience in air conditioning, fire protection and plumbing systems design. He has B.Sc. Degree in mechanical power engineering From Mansoura University, Year 2005. He is currently working as a design engineer for allied consultant company since 2005 till now.He enjoys helping others succeed and he teaches his strategies in the design via his lessons
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