Compressors
Compressor is most important component out of the four-component viz. evaporator, condenser, compressor, and expansion valve. This is the only component, which is dynamic and therefore it is called the heart of the refrigeration system. If we had infinite amount of a refrigerant available, there would be no need for a compressor. We can go on using the refrigerant and letting it off into the atmosphere without recapturing. The compressor is used to suck the vapours from the evaporator and raise the pressure to a suitable level (depending on type of condenser used whether it water-cooled or air-cooled) so that the heat picked up the evaporator and heat of compression can be easily rejected to atmosphere (water or air), where it is not objectionable. Vapours are condensed and liquefied and the same refrigerant can be used again and again in a closed cycle.
Classification of Compressors :
Hermetically Sealed Compressor :
● In ordinary compressor, the crankshaft extends through the compressor housing and is connected to the driving motor.
● A sealed must be provided at the place where the shaft comes out through the compressor housing. This is necessary to prevent the leakage of refrigerant outside or leakage of air inside for this to avoid the complete leakage of the refrigerant.
● It is a unit in which the compressor and motor operates on the same shaft and are enclosed in a common single housing which is known as Hermetically sealed compressor. These compressors may operate on either reciprocating or rotary principal and may be mounted with the shaft in either the vertical or horizontal position.
● Service valves are used for suction, discharge refrigerant to compressor.The suction and discharge line come out from housing and a service line is also provided for charging of refrigerant and lubricating oil.
● An anti-slug device consists basically of two assemblies, one is the centrifugal, and pressure fitting on the crank shaft rotates at the speed of compressor.
● The refrigerant is drawn in through the tube in the top. Any liquid or oil is expelled through the slots on the side by centrifugal force and gases being lighten in drawn through the slots the hub. The second assembly collects the gas and directs it into the cylinder head. a muffler is used to reduce the noise that is created by the suction process. It is fitted between the suction connector and the suction side of the pump unit.
● The winding of motor in the housing is exposed to the suction vapour refrigerant which helps for cooling also.
● The power is supplied to three terminals of the starting and running the motor through the relay. The relay cut of starting winding after start up leaving running winding in circuit. The relay acts as an overload protector also.
● The dehydration of the unit before charging is essential as the moisture in the system may damage the motor. These hermetic units used excessively in domestic refrigerants.
● A 230 volts power supply is given to the stator through relay. The rotor which has crank shaft generally rotates. The A crank shaft through eccentric shaft rotates. The refrigerant is sucked and discharge through suction head and flapper valve plate.
Advantages of hermetically sealed compressor:
(1) It is less noisy than ordinary system.
(2) The leakage of refrigerant is completely avoided.
(3) Being more compact unit require less space.
(4) The lubrication is simple as the motor and compressor operate in a sealed space with the lubricating oil.
Disadvantages of hermetically sealed compressor:
(1) The maintenance is not easy because the moving parts are inaccessible.
(2) Separate pump is required for evacuation and charging of refrigerant.
Applications of hermetically sealed compressor :
In Refrigerator, water cooler, deep freezer, window/ split air conditioner and eve package air conditioner.
Open Type Reciprocating Compressor :
The working cycle of reciprocating compressor consists of two stroke i.e. suction stroke and delivery stroke.
Suction stroke:
● During this stroke the piston start moving from Top Dead Centre (TDC) to Bottom Dead (BDC).
Compression stroke:
● When the piston start moving from BDC to TDC. The compression of vapour refrigerant start. At this time both valve closed. As the piston reaches at near to TDC, the delivery valve open and air is discharged to the condenser. During compression, the pressure and temperature of vapour refrigerant increases.
● The cylinder and cylinder head of high capacity compressors are cooled by means of water jackets, and low capacity compressor are air cooled.
● These are classified as:
(1) Single acting vertical reciprocating compressor
(2) Double acting horizontal compressor
(1) Single Acting Reciprocating Compressor:
● Single acting reciprocating compressor are always vertical.
● The piston is directly driven by connecting rod and crankshaft enclosed in a crankcase which is high pressure tight.
● There is only one delivery for high pressure refrigerant for every revolution of crankshaft.
● It requires less floor space for their installation.
Double Acting Reciprocating Compressor:
● Double acting compressor are always horizontal.
● Two deliveries from both sides of the piston for each revolution of crankshaft.
● This compressor require more space as compared to single acting vertical compressor.
● As both the side of the piston are exposed to rapid change in temperature and pressure, there is a chance of leakage. Considerably large stuffing box is used to prevent the leakage. The discharge on piston rod side is slightly less than other side because of the volume occupied by piston rod.
● Application of Reciprocating Compressor :
(1) Reciprocating compressor are used for refrigeration plant ranging in size from 0.25 tonnes to 1000 tonnes capacity
(2) These compressors are built in sizes ranging from a fraction of horse power to several hundred of HP.
Difference between Open Type and Hermetically Sealed Compressor :
Open type compressor → Hermetically sealed compressor
1. Used for large capacity plant. → Used for low capacity plant.
2. Noise is the problem. → No noise.
3. Leakage of refrigerant occur so large stuffing box required. →No leakage of refrigerant
4. More space required. → Less space required or compact size.
5. Required lubrication for motor and compressor separately. → Lubrication is simple as motor and compressor are operate in single sealed space with lubricating oil.
6. Maintenance is easy → Maintenance is difficult.
7. No need of evacuation pump. → Required evacuation pump.
Rotary Compressor
Types of Rotary Compressor
(1) Roller type rotary compressor
(2) Rotating vane type rotary compressor
(3) Screw compressor
Centrifugal Compressor :
The use of centrifugal compressor for refrigeration application was made by Dr. Willis Carrier in 1920.
● It mainly consist of impeller and diffuser. The impeller consist of an impeller disc and impeller vanes, attached on the impeller disc radially forming radial diverging passage.
● The impeller rotates with the shaft at high speed and refrigerant vapour is drawn into the impeller eye in an axial direction and flows radially outwards through the impeller passages due to centrifugal force, and kinetic energy is imparted to the refrigerant vapour with some static pressure rise.
● The remaining pressure rise is obtained in the diffuser. The diffuser which is stationary, consist of a number of fixed diverging passages. The refrigerant vapour leaves the impeller tip with high velocity and enters the diffuser.
● The diffuser in which reduces the high velocity thus by diffusion process or deceleration of refrigerant vapour in the diffuser, kinetic energy is converted into pressure energy.
● The flow from the diffuser is collected in a spiral passage from which it is discharged from the compressor.
● Single stage compressor can develop a small pressure ratio with a single stage reciprocating compressor therefore for high compression ratio multi-stage centrifugal compressor is required.
1) The baffles must be provided in the suction path of refrigerant to prevent the flow of liquid refrigerant droplet with vapour, which damage the impeller.
2) The power requirement of the compressor drastically (gradually) increase with increase in the pressure in the system. The pressure increases may be due to the leakage of air or non-condensable gases into the system.
Advantages of Centrifugal Compressor Over Reciprocating Compressor:
1) Since Centrifugal Compressor has no valve, piston cylinder therefore working life is more.
2) No vibration as there is no unbalance masses.
3) The efficiency of the compressor is high.
4) Less floor space required than reciprocating compressor.
5) Because of high speed, these can handle large volume of vapour refrigerant than reciprocating compressor.
Disadvantages:
1) The main disadvantage is surging. It occurs when the refrigerant load decrease to below 35% of the rated capacity and cause several stress condition in the compressor.
2) The increase pressure per stage is less.
3) The refrigerant used with these compressors should have high specific volume.
Application :
Used extensively for air conditioning in large structures. Becoming more popular for their ability to handle liquid refrigerant without compressor damage Centrifugal compressors
Advantages of Rotary Compressor Over Reciprocating Compressor :
(1) Rotary compressors are more silent in operation and free from vibration.
(2) These compressor are satisfactory used with refrigerant having specific volume at low suction pressure.
(3) It is preferred for low temperature application.
Screw Compressor :
● In screw compressor, the suction and discharge valve is replace by port in the housing and a piston by helical screw rotor.
● It consists of two helical grooved rotors which are mesh with each other in a housing. An electric motor drives a male rotor shaft and female follows it but in some design the metallic cylindrical rotor. Male (lobes) and female (flutes or gullies or anti lobe).
Compression process:
Compression process is obtained by direct volume reduction with pure rotary motion. For clarity the following description of the three compression phase is limited to one make rotor lobe and one female rotor interlobe space.
(a) Suction :
● As the rotors begin to unmesh, a void is created on both male side and female side and refrigerant vapour is drawn in through the inlet port.
● As rotor continue to tum interlobe space increase in size, and refrigerant vapour flow continuously into the compress just prior to the point at which the interior space leaves the inlet port, the entire length of the interior completely filled with refrigerant vapour.
(b) Compression
● Further rotation start the meshing of another male lobe with another female interlobe space on the suction end progressively compresses the air in axial direction of the discharge port.
● Thus occupied volume of the trapped refrigerant vapour within the interlobe space is decreased and the refrigerant vapour pressure consequently increased.
(c) Discharge
At a point determined by the designed built in volume ratio, the discharge port is uncovered and the compressed refrigerant vapour is discharged by further meshing of the lobe and interlobe space.
It gives smooth and continuous flow of refrigerant vapour due to intake and discharge cycle overlap.
Advantages of Screw Compressor :
(1) Less maintenance due to few moving part.
(2) Smaller size, lighter weight.
(3) Steeples capacity control
(4) Less starting torque.
(5) No need of balancing, less wear and tear and less vibration.
Disadvantage of Screw Compressor:
Performance can be affected by the presence moisture in suction refrigerant vapour.
Scroll Compressor:
A scroll compressor is also known as scroll pump. Scroll compressors are orbital motion, positive displacement machine uses two interleaved spiral like vanes to pump to compressor (gases) vapour.
The vane geometry may be involute, Archimedean spiral or hybrid curves. They are recently used in residential commercial air conditioning, refrigeration and heat pump application as well as in automotive air conditioning.
Scroll Gas Flow
1. Compression in the scroll is created by the interaction of an orbiting spiral and a stationary spiral. Gas enters an outer opening as one of the an outer opening as the one of the spirals orbits
2.The open passage is sealed off as gas is drawn into the spiral
3. As the spiral continues to orbit, the gas is compressed into an increasingly smaller pocket.
4. By the time the gas arives at the center port, discharge pressure has been reached.
5. Actually, during operation, all six gas passages are in various stages of compression at all times, resulting in nearly continuous suction and discharge.Working of Scroll Compressor:
The scroll compressor is designed around two identical spirals or scrolls that, when inserted together from crescent shaped packets. During a compression cycle, one scroll remains stationary while the other orbits round the first. As this motion occurs, gas is drawn into the scrolls and moved in increasingly smaller pocket toward the center. At this point, the gas, now compressed to a high pressure is discharged from a part in the center of the fixed scroll. It is constant volume ratio compressor. It has no valve. This aspect and continuous flow gives high volumetric efficiency.
During each orbit, several pockets of gas are compressed simultaneously creating smooth nearly continuous compression.
Advantages of Scroll Compressor :
(1) Operates more smoothly, quietly and reliable than conventional compressor.
(2) Continuous discharge of gas.
(3) Minimum vibration, low noise.
(4) Fewer moving parts, less maintenance.
Application of Scroll Compressor:
Primarily in domestic refrigerators Scroll compressor. Manufactured in a variety of sizes up to 25 tons and found into a variety of refrigeration and HVAC applications. It have been successfully used for bulk milk cooling, truck transportation, marine containers and grocery display cases. Heavy commercial HVAC applications in unitary (roof top) systems, heat pumps, water chillers for process and building cooling, large split system condensing units and also used in transportation industry.
