Refrigeration Compressor
Posted by AdminFire on September 30th, 2008
Refrigeration compressors
Author: mohamed Fawzy Elsayed
A compressor is the most important and often the most expensive component (typically 30 to 40 per cent of total cost) of any vapor compression cooling system (VCR).
The function of a compressor in a VCRS is to maintain the refrigerant vapor out of the evaporator, so that a low pressure and low temperature can be maintained in the evaporator where the coolant can boil the extraction of heat from refrigeration space.
The compressor has to raise the pressure coolant to a level that can be condensed 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 principle of operation:
i. Positive displacement type
II. Rotor-dynamic type
In positive displacement compressors, the compression is achieved by trapping a refrigerant vapor in an enclosed space and then reducing its volume.Since a fixed amount of refrigerant is trapped at a time, the pressure
increases as its volume is small.
When the pressure rises to a level that is slightly higher than the condensing pressure is released from the enclosed space and a new refrigerant charge low pressure and is drawn into the cycle continues. As the flow of refrigerant to the compressor is not stable, the type of positive displacement compressor is a pulsatile flow device.
However, since operating speeds are usually very high flow appears to be nearly constant in the macroscopic time scale. Since the flow is pulsatile on a microscopic scale in time, compressors, positive displacement type are prone to wear, vibrations
and the noise level. Depending on construction, positive displacement type
compressors used in refrigeration and air conditioning are classified as:
i. Piston type
II. Rotary Vane type
III. Rotary Screw Type
IV. Compressors Orbital
v. compressor noise
The compressor rotor dynamics, the increased pressure of the refrigerant is achieved by imparting the kinetic energy of a stream flowing steadily from refrigerant in mechanical rotation and then converted into pressure as a coolant flowing through a divergent passage.
Unlike positive displacement type, the Roto-compressor dynamic type are constant current devices, therefore, are subject to less wear and vibration. Depending on the construction, the dynamics of rotor-type compressor can be classified into:
i. Radial flow rate, or
II. Axial flow type
Centrifugal compressors (also known as turbo-compressors) are the radial flow type, broken dynamic compressors. These compressors are widely used in High Capacity cooling systems air conditioning. Axial flow compressors are
normally used in gas liquefaction applications.
b) On the basis of the provision compressor motor or external drive:
i. Open Type
II. Waterproof (or sealed) Type
III. Semi-hermetic (or semi-closed) Type
In open-type compressor, the compressor rotational axis extends through a seal in the crankcase of an external drive. The external drive can be an Electric Motor or motor (eg diesel engines).
The compressor may be driven by belt or gear driven. Open-type compressors are commonly used in medium to large capacity of the cooling system for all refrigerants and ammonia (due to their incompatibility with the hermetic motor materials).
Compressors open type is characterized by high efficiency, flexibility, better refrigeration compressor and service. However, since the shaft must extend through the seal, leakage of refrigerant system can not be eliminated entirely.
Therefore refrigeration systems using open-type compressors require a deposit refrigerant to care for the coolant leak for some time, and then regular maintenance to load the system with the refrigerant, changing seals, gaskets, etc.
In hermetic compressors, the motor and compressor are enclosed in the same housing to prevent leakage of refrigerant. The housing has solder connections for input and the condenser and the power input socket.As result, there is virtually no possibility of leakage of refrigerant from the compressor.
Everyone engines reject a part of the power delivered to the same due to eddy currents and
friction, ie inefficiencies. Also, the compressor also heated-up due to the
friction and also due to higher steam temperature during compression.
In the open type, both the compressor and engine heat normally rejected to the
surrounding air for efficient operation. In the heat can not be hermetic
rejected the surrounding air, since both are enclosed in a shell. Therefore, the cold
suction gas is passed through the engine and compressor before entering
the compressor.
This keeps the engine cool. The motor winding is in direct contact
with refrigerant therefore only refrigerants, which have high dielectric
strength, can be used in hermetic compressors.
The cooling rate depends on the coolant flow rate, temperature and thermal properties of the refrigerant. If the flow is not sufficient and / or the temperature is low enough isolation of the motor windings and shorted can burn may occur. Hence performance, hermetically sealed compressors provide satisfactory and safe in a very narrow temperature range of design and should not be used to outside conditions design.
The Conference of the Parties hermetic systems of the base is less than that of the
Open systems based on the compressor and a part of the cooling effect is lost in
cooling the engine and compressor. However, hermetic compressors are
almost universally used in small systems, such as domestic refrigerators, water
refrigerators, air conditioners, etc., where efficiency is not as important as customer
convenience (due to lack of ongoing maintenance).
Besides, the use of hermetic compressors is ideal for systems that use capillary tubes as expansion devices and systems critically charged. Hermetic compressors usually not serviceable. They are not very flexible, since it is difficult to vary theirspeed to control the cooling capacity.
In some (usually larger) sealed units, the butt is usually removable
so that the piston valve can be repaired. This type of unit is called
semi-hermetic (or semi-closed) of the compressor.
For more information about Visit us at HVAC-Tools site
About the author:
Mohamed Fawzy is a mechanical engineer who has good experience in air conditioning, Fire Protection and plumbing systems design. Has Bachelor Degree in mechanical power engineering from the University of Mansura, Year 2005. He currently works as a design engineer for the allied company consultant since 2005 until now.He enjoys helping others succeed and teaches his strategies in the design through their lessons
Article Source: ArticlesBase.com – Refrigeration compressors
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