Refrigeration & Air-Conditioning

Module 3. Refrigeration plant components

Lesson 10
RECIPROCATION COMPRESSOR- CONSTRUCTION, WORKING AND MAINTENANCE

10.1 Introduction

The compressor is referred as the heart of the vapour compression refrigeration system. The function of compressor is to suck the refrigerant gas from low pressure side and to compress it to discharge pressure so that condensation of gas can be done either by water or air at ordinary room temperature. The compressor also keeps low pressure in the evaporator for efficient evaporation of the refrigerant. There are three main groups of compressors.

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

Reciprocating compressors consist of one or more cylinders with suitable valves for suction and discharge of the refrigerant gas. The compression of suction gas is achieved by reciprocating pistons. The design of the cylinders may vary depending upon the number of cylinders, arrangement of cylinders and acting (single acting or double acting). Reciprocating compressors are widely used in dairy plants and these compressors are driven by the electric motor. Reciprocating compressors are classified as under.

1. Open type
2. Semi hermetic compressors
3. Hermetic compressors

10.3 Open Type Reciprocating Compressor

An open type reciprocating compressor consists of cylinders in which piston moves back and forth for suction and discharge of the refrigerant vapour. The main parts are piston, cylinder, connecting rod, crankshaft, cylinder head, crank case, suction and discharge valve etc. In open type of reciprocating compressor, power is received from an external source with one end of the crank shaft extending through the crank case to either a direct motor drive or a V-belt, gear or chain drive. With such a design, it is necessary to seal the crank case against the leakage of refrigerant.

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.

Small compressors often have fins cast integral with the cylinders and cylinder head to increase cylinder cooling, whereas large compressors contain water jacket for this purpose.

10.5 Piston

Pistons employed in refrigeration compressors are of two types (a) automotive and (b) double trunk.
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

Generally two types of valves are used in compressors.

1. Non flexing ring plate type valves

2. Flexible or Reed type valves.

All valves operate upon pressure differential, and there are many modification of each class. These valves are designed to open easily and close quickly.

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.

The suction valve opens on the down stroke of the piston because the cylinder pressure is less than the vapour pressure in the suction line. On the up stroke of the piston the suction valve closes and the pressure within the cylinder causes the discharge valve to open. Valves are manufacture from specially heat-treated alloys. The noise level during operation can be reduced by putting a plastic material installed in the valve.

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

Open compressors are made with the crankshaft extending through the crank case for connecting with motor, v-belt, gear or chain drive. A shaft seal is required to prevent refrigerant leakage. The leakage may take place under both static and moving conditions at the point at which the crank shaft passes through the housing.

10.9 Crank Shaft and Bearing

Crank shaft employed in large compressors are usually constructed of forged steel. The eccentric type shaft which consists of a cast iron eccentric mounted on a straight steel shaft is often used in smaller compressors. It is fastened for the shaft by a key and lock screw arrangement. Bearing may be of sleeve type or antifriction type and the crank shaft is made up of drop forged steel.

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.

10.13 Factors Affecting the Volumetric Efficiency of Reciprocating Compressor

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.