Refrigerant compressors Akshay Kr Mishra Sharda University 4 TH YR. MECHANICAL

What is a refrigerant compressor? A machine that is used to compress the vapor refrigerant from the evaporator and to raise its pressure so that the corresponding saturation temperature is higher than that of cooling medium.

Classification of compressors Acc. To method of compression Reciprocating compressors Rotary compressors Centrifugal compressors Acc. To no. of working strokes Single acting compressor Double acting compressor Acc. to the method of drive employed Direct drive compressor Belt drive compressor Acc. To the location of prime mover Hermetic compressors Semi hermetic compressor

Reciprocating Compressors A reciprocating compressor or piston compressor is a positive- displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure. Applications include oil refineries, gas pipelines, chemical plants, and refrigeration plants. Compressors 1-50 HP are reciprocating.

Rotary Compressors Rotary compressors are positive displacement compressors. They are commonly used to replace piston compressors where large volumes of high pressure air are needed, either for large industrial applications or to operate high-power air tools such as jackhammers. Rotary screw compressors use two meshing helical screws, known as rotors, to compress the gas. Gas enters at the suction side and moves through the threads as the screws rotate. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws.

Applications : Typically, they are used to supply compressed air for general industrial applications. Trailer mounted diesel powered units are often seen at construction sites, and are used to power air operated construction machinery.

Centrifugal Compressors It is a dynamic High Speed compressor which depends on transfer of energy from a rotating impeller to the air. They achieve a pressure rise by adding kinetic energy/velocity to a continuous flow of fluid through the rotor or impeller. This kinetic energy is then converted to an increase in potential energy/static pressure by slowing the flow through a diffuse These types of compressors are also designed for higher capacity because flow through the compressor is continuous.

Direct Drive Compressors A direct drive compressor is an compressor that can be hooked right to a product that needs air without having to fill an air tank first. They are also usually self lubricating which means that no oil is usually required. They tend to draw low amperage and can handle pressures around 110psi. provide compactness and minimum drive maintenance. Compressors can be flange mounted or direct-coupled to the power source..

Belt Drive compressors They are most commonly used with electric motors and internal combustion engines. V-Belt drives provide great flexibility in matching compressor load to power source load and speed at minimum cost. Belts must be properly shielded for safety.

Advantages & Disadvantages for Belt Drive Advantages: 1. Belt drives are easier to install. 2. Absorb minor misalignments. 3. Relatively cheaper than direct drives. Disadvantages: 1. Induces higher loads on compressor bearings and support system. Reduces life by approx. 6-8 times. 2. May need additional drive supports to handle bending loads. 3. Requires relatively more frequent checks and maintenance. 4. Has lower transmission efficiencies. P.S. For direct drives just flip the pros & cons for the belt drives. Realistically, belt drives are used for relatively smaller units and lower HPs, while direct drives are used for larger units and higher HPs.

Hermetic compressors A hermetic or sealed compressor is one in which both compressor and motor are confined in a single outer welded steel shell. The motor and compressor are directly coupled on the same shaft, with the motor inside the refrigeration circuit. Thus the need for a shaft seal with the consequent refrigerant leakage problem was eliminated. Hermetic compressors are ideal for small refrigeration systems, where continuous maintenance (replenishing refrigerant and oil charge etc) cannot be ensured. Hence they are widely used in domestic refrigerators, room air conditioners etc.

With reference to the above figure, One can see the cooper windings inside the outer shell and also the refrigerant connections (copper pipes). Since, the motor is in the refrigerant circuit, the efficiency of hermetic compressor based systems is lower as the heat dissipated by the motor and compressor becomes a part of the system load. Since the complete system is kept in a welded steel shell, the hermetic compressors are not meant for servicing. A variation of hermetic compressor is a semi-hermetic compressor, in which the bolted construction offers limited serviceability.

Advantages and Disadvantages Advantages: 1.Leakage of refrigerant removed 2.Less noisy and compact. 3.Lubrication is simple as the motor and compressor operate in a sealed space with the lubricating oil. Disadvantages: 1.Maintenance is not easy coz the moving parts are inaccessible. 2.A separate pump is reqd. for evacuation and charging of refrigerant.

Semi Hermetic compressors A semi hermetic compressor is a sealed mechanical device that is used to regulate gas pressure by varying the volume of the gas. The way a semi hermetic compressor works is that a motor is energized by current then in turn rotates a crankshaft, during the down stroke, a piston creates low pressure between the cylinder head and the piston and during the upstroke the compressed gas force a high pressure on the outside system.

Advantages Due to the semi-hermetic design such compressors are very compact and occupy less space compared to open type machines thus saving expensive building and plant room costs. The foot print and weight of a semi-hermetic machine is 70% of that of the corresponding open type machine. These compressors are hence easier to handle and do not require heavy foundations like open type machines. The common problem of refrigerant leakage through the shaft seal is not face with the semi-hermetic compressor because the motor is directly mounted on the compressor crankshaft. Semi-hermetic compressors do not require any pulleys and belts and have no alignment problems and no wear and tear problem of the d rive elements. Such compressors are also quieter and produce less vibration compared to the open type. This reason alone makes them the ideal choice for roof-top applications.

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