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Lesson 10. Reciprocation compressor- construction, working and maintenance
Module 3. Refrigeration plant components
Lesson 10
RECIPROCATION COMPRESSOR- CONSTRUCTION, WORKING AND MAINTENANCE
10.1 Introduction
1. Reciprocating compressor
2. Rotary compressor
3. Centrifugal compressor
The reciprocating compressor consists of a piston moving back and forth in the cylinder with suction and discharge valve arranged to allow pumping to take place.
The rotary and centrifugal compressors have rotating members but the rotary compressor has a positive displacement where as a centrifugal compressors draws the vapour and discharges it at high pressure by centrifugal force.
10.2 Reciprocation Compressor
2. Semi hermetic compressors
3. Hermetic compressors
On the down stroke of the piston, the low pressure is created between the top of the piston cylinder head and the suction side of the evaporator. This causes the refrigerant vapour to run in to the cylinder. On the discharge stroke of the piston, the gas is compressed and discharged to the out let of the compressor. The valves in the cylinder head are so designed that, depending on the position of the stroke, one is open while the other is closed.
10.4 Compressor Cylinders
The number of cylinders varies from one to as many as sixteen. In multi cylinder compressors, the cylinders may be arranged in line, radically or at an angle to each other to form a V or W pattern. Compressor cylinders are usually constructed of close-grained cast iron which is easily machined. For small compressors, the cylinders and crank case housing are often cast in one piece while for large compressors the cylinders and crank case housing are usually cast separately which is flanged and bolted together. The cylinders of large compressors are usually equipped with replacement liners or sleeves.
10.5 Piston
Automotive type pistons are used when the suction gas enters the cylinder through suction valves located in the cylinder head. Double trunk pistons are used when the suction gas enters through ports in the cylinder wall and in the side of the piston and passes into the cylinder through suction valves located in the top of the piston. These pistons are provided with piston rings. The pistons are manufactured from close-grained cast iron.
10.6 Design of Values
1. Non flexing ring plate type valves
2. Flexible or Reed type valves.
10.6.1 Ring plate type valves
The ring plate is a thin ring which is held closed in the top of the cylinder by spring. The valve is held closed by small springs. When the refrigerant vapour pressure inside the cylinder is greater than the spring tension, the valve opens on the up stroke of the piston to allow vapour to pass through the large discharge ports to the discharge outlet.
10.6.2 Flexing type valves
Small modern refrigeration compressors use high grade steel reed or disc valves. These valves are quiet, simple and long lasting. For these reasons, they are especially adopted to high speed compressors.
10.7 Cooling of Compressor Head
Temperature rise of compressor head is controlled by cooling the upper part of the cylinder walls and cylinder head to prevent the over heating of cylinder head. Cooling of compressor head prevents the damage to head, piston and rings. The cooling is done in ammonia compressor by jacketing a cylinder wall and head through which water is circulated. In case of R-12 or R-22 refrigerant, the discharge temperature will be lower than ammonia compressor and hence generally fines are provided which facilitates the transfer of heat to the surrounding air.
10.8 Shaft Seal
10.9 Crank Shaft and Bearing
10.10 Lubrication
Proper lubrication is essential for the compressors. It is essential to know the interval of lubrication, type of lubricant, quantity of lubricant etc. In some compressors force feed system is used for the circulation of oil. The oil pump takes suction from the crank case through filters and discharge oil vertically in the passage in the compressor. It is also necessary to know the manufactures instructions for the lubrication requirement.
Two systems, splash lubrication and forced lubrication are common at present. In splash lubrication with each rotation of the shaft the crank and connecting rod dip into the crank case oil reservoir and thereby splash the lubricant in to openings. As a general rule, small vertical compressors (up to 10 kW) are splash lubricated and above this size, most compressors employ some type of forced feed lubrication.
10.11 Hermetically Sealed Compressor
The hermetic type of compressor may be either reciprocating or rotary and is a direct drive unit with both motor and the compressor hermetically sealed with in the housing. It is used in domestic refrigerator, air conditioner and other small capacity systems. This arrangement eliminates the necessity of many shaft seals. Some advantages of hermetically sealed compressors are listed below.
1. It prevents the leakage of refrigerant.
2. It also reduces the operating noise considerably.
3. It eliminates external drive
4. Lubrication is greatly simplified
5. Motor operates in an ideal atmosphere
6. Cooling of motor is done by suction gas
10.12 Volumetric Efficiency of Reciprocating Compressor
The volumetric efficiency of reciprocating compressor is defined as the ratio of actual volume of refrigerant gas delivered on each stokes to the piston displacement of the compressor. Higher volumetric efficiency of the compressor is desirable to get higher actual capacity of the refrigeration system. The actual volume of gas handled by the compressor is normally less than the piston displacement of the compressor due to several factors.
a) Clearance volume
The volume of the cylinder between the top of the piston and the delivery valve plate when the piston is at top dead center is known as clearance volume. Clearance volume is necessary, though it should be minimum, to prevent damage to the valves. The refrigerant gas present in the clearance volume will expands on the down ward movement of the piston. There will not be any intake of suction gas until the pressure of the cylinder gas drops below the suction pressure of the refrigeration system. The clearance volume is expressed as % of the total volume. It is about 2-4 % of the cylinder volume. Higher cylinder volume reduces the volumetric efficiency of the reciprocating compressor.
b) Compressor ratio
Higher compression ratio reduces the volumetric efficiency of the compressor. Higher compression ratio either due to higher discharge pressure or lower suction pressure. This requires more piston displacement in order to expand the refrigerant gas present in the clearance volume for the intake of suction gas. Therefore, lower compression ratio during operation of the refrigeration system is desirable to achieve higher volumetric efficiency of the compressor.
c) Leakage through valves
Any leakage of gas through suction valve or delivery valve will reduce the volumetric efficiency of the compressor. It is obvious that the gas filled in the cylinder leaks back either on suction side or from delivery side to the cylinder will reduce the volumetric efficiency of the compressor.
d) Cylinder heating effect
Higher temperature of cylinder wall increases the volume of the gas which intern reduces the intake of suction gas. Therefore, effective head cooling of the compressor is important to avoid reduction in volumetric efficiency of the compressor due to cylinder heating effect.
e) Wire drawing effect
The restriction of area for the flow of refrigerant which causes pressure drop of the refrigerant is known as wire drawing effect. When refrigerant gas enters through suction valve of the compressor, it experiences wire drawing effect. The extent of pressure drop depends on the velocity of the refrigerant, type of refrigerant and valve design. Wire drawing effects adversely the volumetric efficiency of the compressor.
10.14 Maintenance of Reciprocating Compressors
It is very important to carry out maintenance (preventive and break down maintenance) of compressor by skilled personals. The following points must be considered for effective maintenance of the compressors.
1. Regular lubrication programme considering the recommendations of the manufacturers.
2. Replacement of piston rings, piston, liner, shaft seal, valves etc. if required.
3. Regular checking of bolts, nuts, foundation, etc.