As the name indicates, it is a machine used for compressing the refrigerant. It sucks the low-pressure vapours coming from evaporator, decrease their volume and thus increase the pressure. When pressure increases, the temperature will also increase naturally. So the compressor discharges the high pressure and high temperature refrigerant to the condenser.
There are different types of compressors depending on their shape and methods of compression used in vapour compression refrigeration system and are classified in many ways as follows:
(i) According to the method of compression
There are different types of compressors depending on their shape and methods of compression used in vapour compression refrigeration system and are classified in many ways as follows:
(i) According to the method of compression
Reciprocating compressors (piston reciprocating in the cylinder).
Rotary compressor (cylindrical housing and eccentric roller arrangement)
Centrifugal compressor (valve casing and impeller arrangement)
(ii) According to the number of stages
(ii) According to the number of stages
Single stage compressor.
Multi-stage compressor
(iii) According to the location of the Prime Mover
(iii) According to the location of the Prime Mover
Open-type compressor
Hermetic compressor
The most commonly used compressors in the cold storage of a dairy plant are reciprocating type.
i. Reciprocating Compressor
Construction: The single stage, single acting reciprocating compressor. The main body of compressor is a metallic cylinder, on one end of which is attached to the cylinder head and the other end is attached with the crank casing. In the crank casing, crankshaft is fitted on bearings. On the crank bearing surface, the big end of connecting rod is fitted. The other end of connecting rod is joined with the piston through a pin called gudgeon pin. The piston is of round shape and of the diameter matching with inner diameter of cylinder. When the crankshaft is rotated by the electric motor, crank rotates and pushes and pulls the piston in the cylinder with the help of connecting rod. In this way, the piston moves back and forth i.e. reciprocate in the cylinder
The cylinder head on the other side consists of two hollow sections as suction and discharge manifolds. The suction and discharge manifold are connected to the cylinder head separately through suction and discharge valves. On the other side,these are connected to suction and discharge lines. Thus the main parts of a reciprocating compressor are as follows:
(a) Cylinder Block; (b) Cylinder Head; (c) Suction and Discharge Manifolds; (d) Suction and Discharge Valves; (e) Piston; (f)Connecting Rod; (g) Crank Shaft; (h)Crank Case; and (i) Main Bearings.
All the moving parts, i.e., piston in cylinder, connecting rod, gudgeon pin, crank,crank shaft, bearings, etc. are lubricated by some pressure lubrication system. The lubricating oil is filled in the crank casing up to the required level. While running and compressing the refrigerant, heat is also produced due to compression and friction in moving parts. Thus the compressor becomes hot and needs to be cooled.
The most commonly used compressors in the cold storage of a dairy plant are reciprocating type.
i. Reciprocating Compressor
Construction: The single stage, single acting reciprocating compressor. The main body of compressor is a metallic cylinder, on one end of which is attached to the cylinder head and the other end is attached with the crank casing. In the crank casing, crankshaft is fitted on bearings. On the crank bearing surface, the big end of connecting rod is fitted. The other end of connecting rod is joined with the piston through a pin called gudgeon pin. The piston is of round shape and of the diameter matching with inner diameter of cylinder. When the crankshaft is rotated by the electric motor, crank rotates and pushes and pulls the piston in the cylinder with the help of connecting rod. In this way, the piston moves back and forth i.e. reciprocate in the cylinder
The cylinder head on the other side consists of two hollow sections as suction and discharge manifolds. The suction and discharge manifold are connected to the cylinder head separately through suction and discharge valves. On the other side,these are connected to suction and discharge lines. Thus the main parts of a reciprocating compressor are as follows:
(a) Cylinder Block; (b) Cylinder Head; (c) Suction and Discharge Manifolds; (d) Suction and Discharge Valves; (e) Piston; (f)Connecting Rod; (g) Crank Shaft; (h)Crank Case; and (i) Main Bearings.
All the moving parts, i.e., piston in cylinder, connecting rod, gudgeon pin, crank,crank shaft, bearings, etc. are lubricated by some pressure lubrication system. The lubricating oil is filled in the crank casing up to the required level. While running and compressing the refrigerant, heat is also produced due to compression and friction in moving parts. Thus the compressor becomes hot and needs to be cooled.
Small capacity compressors are air-cooled and large capacity compressors are water-cooled. In air-cooled compressor, fins are provided on the outside of cylinder to increase the area of heat transfer and hence to increase the cooling of hot compressor. And in water-cooled compressor, water jackets are provided over the outer surface of cylinder. The crankshaft is connected to the electric motor shaft via a coupling. In this way, electric motor rotates the crankshaft and runs the compressor.
Working: When the compressor is run by an electrical motor or a diesel engine,its crank shaft rotates. With the rotation of crankshaft, piston continuously reciprocates in the cylinder. When the piston reaches to top end position in the cylinder, there is already some refrigerant vapour present at higher discharge pressure. As the piston moves downward, volume in the cylinder increases and pressure of refrigerant gas starts decreasing. On decrease of pressure, discharge valve is automatically closed. As the pressure further decreases below the level of suction pressure of vapours, suction valve opens and the refrigerant vapours from the evaporator starts flowing in the cylinder until the piston reaches to bottom end position. This stroke of piston is called suction stroke. After the suction stroke, piston moves upward,suction valve closes due to spring action and the volume of vapours sucked in suction stroke decreases and thus pressure increases. When pressure becomes greater than the discharge pressure on the other side of discharge valve, it opens automatically and the vapour refrigerant is discharged into the condenser through discharge line until the piston again comes to the top end position. This stroke is called discharge stroke. In this way the cycle of suction and discharge strokes is repeated continuously at high speed and compressor continuously sucks the low pressure vapours from evaporator and discharge it at high pressure in the condenser.
In single stage compressor, the refrigerant vapours are directly compressed from evaporator pressure to condenser pressure in one stage.In multi-stage compressors, there is more than one cylinder. The first stage cylinder discharge is given to the suction of next stage cylinder and so on. The pressure is raised from evaporator pressure to condenser pressure in two or more stages.Multistage compressor is generally used when the difference between the suction and discharge pressure is large.
ii. Centrifugal Compressor
The type of compressor as discussed before is positive displacement type compressor because it sucks and compresses the refrigerant vapours in the same way as a simple cycle pump does. But the centrifugal compressor does not act on this principle. What it does, it simply gives a rotary motion to the refrigerant vapour.Due to rotary motion, centrifugal force develops which push the refrigerant vapours outward and compresses. The detailed construction and working is given below. Its working is same as that of a water tullu pump used in our houses for lifting the water.
Construction: There is a volute casing made of iron/steel.In the casing an impeller is fixed on the shaft rotating in casing. The shaft comes out at the center of casing and coupled to the shaft of electric motor via a coupling.The construction of impeller is a circular plate on which vanes are fixed. On the other side, the center part of casing is connected to suction line from where suction vapours come to casing.
Working: When the impeller is rotated at high speed by the motor, the vapours entrapped between vanes of impeller also rotates and directed outwards due to development of centrifugal force. In this way, refrigerant vapour rotates in the volute casing and also being pulled outwards with high velocity. As they get the discharge opening in the casing, move out of the casing and the high velocity
converts into high pressure. Thus with continuous running of impeller, the vapour continuously enters the center part of impeller and move out from casing with high pressure.
ii. Hermetically Sealed Compressors
The reciprocating and centrifugal compressors as discussed above are also called as open type compressors because the compressor and motor are separate and joined by the rotating shaft. In open type compressor, there are always chances of leakage of refrigerant through any annual space between the shaft and stuffing box of compressor. To avoid this leakage and also to get some more benefit, the hermetically sealed compressors are designed. In this compressor, both the motor and compressor are closely fitted in an outer casing called hermetically sealed casing. It has cylindrical shaped compact body and only inlet and outlet pipes of refrigerant are welded to it. In this way, chances of leakage of refrigerant become nil. This type of compressor is also very less noisy and requires less space. The lubrication of moving parts of compressor is also very simple and effective because there are no chances of loss of lubricating oil due to leakage, fuming, etc. Due to all these advantages, hermetically sealed compressors are preferred and used in domestic refrigerator, room air conditioners (ACs), water coolers and other small capacity refrigerating units. The maintenance of this compressor is very less but once any maintenance is required, it cannot be done at the site itself and the system is first evacuated from refrigerant and then compressor is removed and sent to the company repair shop.
Working: When the compressor is run by an electrical motor or a diesel engine,its crank shaft rotates. With the rotation of crankshaft, piston continuously reciprocates in the cylinder. When the piston reaches to top end position in the cylinder, there is already some refrigerant vapour present at higher discharge pressure. As the piston moves downward, volume in the cylinder increases and pressure of refrigerant gas starts decreasing. On decrease of pressure, discharge valve is automatically closed. As the pressure further decreases below the level of suction pressure of vapours, suction valve opens and the refrigerant vapours from the evaporator starts flowing in the cylinder until the piston reaches to bottom end position. This stroke of piston is called suction stroke. After the suction stroke, piston moves upward,suction valve closes due to spring action and the volume of vapours sucked in suction stroke decreases and thus pressure increases. When pressure becomes greater than the discharge pressure on the other side of discharge valve, it opens automatically and the vapour refrigerant is discharged into the condenser through discharge line until the piston again comes to the top end position. This stroke is called discharge stroke. In this way the cycle of suction and discharge strokes is repeated continuously at high speed and compressor continuously sucks the low pressure vapours from evaporator and discharge it at high pressure in the condenser.
In single stage compressor, the refrigerant vapours are directly compressed from evaporator pressure to condenser pressure in one stage.In multi-stage compressors, there is more than one cylinder. The first stage cylinder discharge is given to the suction of next stage cylinder and so on. The pressure is raised from evaporator pressure to condenser pressure in two or more stages.Multistage compressor is generally used when the difference between the suction and discharge pressure is large.
ii. Centrifugal Compressor
The type of compressor as discussed before is positive displacement type compressor because it sucks and compresses the refrigerant vapours in the same way as a simple cycle pump does. But the centrifugal compressor does not act on this principle. What it does, it simply gives a rotary motion to the refrigerant vapour.Due to rotary motion, centrifugal force develops which push the refrigerant vapours outward and compresses. The detailed construction and working is given below. Its working is same as that of a water tullu pump used in our houses for lifting the water.
Construction: There is a volute casing made of iron/steel.In the casing an impeller is fixed on the shaft rotating in casing. The shaft comes out at the center of casing and coupled to the shaft of electric motor via a coupling.The construction of impeller is a circular plate on which vanes are fixed. On the other side, the center part of casing is connected to suction line from where suction vapours come to casing.
Working: When the impeller is rotated at high speed by the motor, the vapours entrapped between vanes of impeller also rotates and directed outwards due to development of centrifugal force. In this way, refrigerant vapour rotates in the volute casing and also being pulled outwards with high velocity. As they get the discharge opening in the casing, move out of the casing and the high velocity
converts into high pressure. Thus with continuous running of impeller, the vapour continuously enters the center part of impeller and move out from casing with high pressure.
ii. Hermetically Sealed Compressors
The reciprocating and centrifugal compressors as discussed above are also called as open type compressors because the compressor and motor are separate and joined by the rotating shaft. In open type compressor, there are always chances of leakage of refrigerant through any annual space between the shaft and stuffing box of compressor. To avoid this leakage and also to get some more benefit, the hermetically sealed compressors are designed. In this compressor, both the motor and compressor are closely fitted in an outer casing called hermetically sealed casing. It has cylindrical shaped compact body and only inlet and outlet pipes of refrigerant are welded to it. In this way, chances of leakage of refrigerant become nil. This type of compressor is also very less noisy and requires less space. The lubrication of moving parts of compressor is also very simple and effective because there are no chances of loss of lubricating oil due to leakage, fuming, etc. Due to all these advantages, hermetically sealed compressors are preferred and used in domestic refrigerator, room air conditioners (ACs), water coolers and other small capacity refrigerating units. The maintenance of this compressor is very less but once any maintenance is required, it cannot be done at the site itself and the system is first evacuated from refrigerant and then compressor is removed and sent to the company repair shop.
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