REFRIGERATING COMPRESSORS

THE REFRIGERATION SYSTEM CONSISTS OF A COMPRESSOR , CONDENSER,EXPANSION VALVE, EVAPORATOR IN IT AND WHOLE SYSTEM IS USED BOTH FOR REFRIGERATING FOOD STUFFS AND ALSO FOR THE AIR CONDITIONING  SYSTEM. TODAY I WILL DISCUSS ABOUT COMPRESSORS.

COMPRESSORS ARE USED IN THE COMPRESSION OF LOW PRESSUE VAPOUR INTO HIGH PRESSURE VAPOUR. COMPRESSORS ARE OF 4 TYPES RECIPROCATING , SCREW AND CENTRIFUGAL TYPE AND ROTARY TYPE. ALL ARE USED DEPENDING ON THE REQUIREMENTS OF THE SYSTEM  AND USAGE.

RECIPROCATING TYPE:

THE RECIPROCATING COMPRESSORS ARE OF VERTICAL INLINE TYPE. THEY ARE ARRANGED IN EITHER 'V' OR 'W' CONFIGURATION.

CONSTRUCTION:

EACH CRANK CARRIES THE BOTTOM ENDS OF PISTONS IN CYLINDERS. 

PISTONS OF MODERN COMPRESSORS ARE ALUMINIUM ALLOY PISTONS. PISTON RINGS ARE OF CAST IRON BUT SPECIAL RINGS HAVE PHOSPHOR BRONZE INSERTS AS THEY ARE HELPFUL WHEN RUNNING IN. EACH THROW CARRIES FOUR BOTTEM ENDS. MAIN BEARINGS ARE WHITE METAL LINED STEEL SHELLS. THE SUCTION AND DISCHARGE VALVES MAY BE SPRING LOADED PLATE TYPE OR REED VALVES DEPENDING ON THE SIZE OF THE COMPRESSOR. THE SPRING LOADED SAFETY HEADS TO PREVENT DAMAGE IN THE EVENT OF SLUGS OF INCOMPRESSIBLE LIQUID REFRIGERANT OR OIL ENTERING THE CYLINDERS.

AMMONIA(R717) AND R22 COMPRESSORS ARE SOMETIMES FITTED WITH WATER COOLED JACKETS AROUND THE UPPER PARTS OF THE CYLINDERS TO PREVENT EXCESSIVELY HIGH DISCHARGE TEMPERATURES AND ALSO TO INCREASE THE EFFECIENCY OF THE COMPRESSOR.

UNLOADING MECHANISM:

LARGE COMPRESSORS ARE FITTED WITH UNLOADING MECHANISM WHICH ENABLES THE COMPRESSOR TO START EASILY WITH NI VAPOUR PRESSURE LOAD IN THE CYLINDER, AND PERMITTING THE USE OF ELECTRIC MOTORS WITH LOW STARTING TORQUES. UNLOADING IS ALSO DONE WHEN THE REQUIRED TEMPERATURE IS GOING TO BE OBTAINED AND ONE BY ONE THE UNLOADING OCCURS. THIS IS EFFECTED BY HOLDING THE SUCTION VALVES OPEN, OR BY OPENING A BYPASS VALVE BETWEEN SUCTION AND DISCHARGE SIDE. IT IS ACTUATED BY SOLENOID VALVE.

WORKING:

GAS FROM THE EVAPORATOR PASSES THROUGH THE SUCTION STRAINER HOUSED IN THE SUCTION CONNECTION AND IT PASSES THROUGH THE SUCTION VALVES. THE GAS GETS COMPRESSED AND AT A HIGHER PRESSURE IS BEING DISCHARGED THROUGH THE DISCHARGE VALVE AND GOES TO THE CONDENSER. 

ADVANTAGES:

1. GIVES A HIGHER REFRIGERATING EFFECT

2.GIVES GOOD AND EFFECIENT OPERATION

DISADVANTAGES:

1. OCCUPIES A LARGER SPACE AREA.

2. MORE NOISE AND VIBRATION.

3.MAINTENANCE IS DIFFICULT.

4.COSTLY

2 STAGE RECIPROCATING COMPRESSORS:

THE COMPRESSION CAN BE DONE IN STAGES TO INCREASE THE EFFECIENCY AND TO HAVE THE FOLLOWING ADVANTAGES:

A. LOWER CYLINDER PRESSURES, HENCE LOWER BEARING LOADS AND WEAR

B. LOWER DISCHARGE GAS TEMPERATURES, PREVENTING OIL DECOMPOSITION AND VALVE DAMAGE

C. INCREASED VALVE EFFECIENCY

WORKING:

IN CASE OF A TWO STAGE COMPRESSOR THE GAS AFTER BEING COMPRESSED IN FIRST STAGE IS PASSED THROUGH THE INTERCOOLER AND ITS TEMPERATURE IS REDUCED TO ATM TEMPERATURE AND THEN PASSED THROUGH THE SECOND STAGE AND IS BEING DICHARGED AT HIGHER PRESSURE AND BETTER EFFECIENCY.

2.SCREW COMPRESSORS:

THE SCREW COMPRESSORS ARE 2ND TYPE USED FOR REFRIGERATION PURPOSE. 

CONSTRUCTION:

THESE COMPRESSORS CONSIST OF 2 ROTORS ONE FEMALE AND ANOTHER MALE. THE MALE ROTOR CONSISTS OF 4 LOBES AND FEMALE ROTOR CONSISTS OF 6 LOBES. THE CONSTRUCTION IS SIMPLE AND IS ONE OF THE BEST AND EFFECIENT ONE.

WORKING:

THE PATH OF GAS FLOW STARTS AT INLET PORT. AS THE PASSAGE BETWEEN FEMALE ROTOR PASS THIS PORT GAS IS DRAWN IN. AS THE ROTOR CONTINUES TO TURN THE MALE LOBE FILLS UP THE SPACE AND GAS IS FORCED AXIALLY AND COMPRESSED IN THE SPACE UNTIL IT ESCAPES FROM THE OUTLET.

TO OBTAIN EFFECIENT COMPRESSION LEAKAGES ARE TO BE MINIMIZED BETWEEN THE LOBES BY INJECTING OIL AND MAINTAINING ITS CONTINUITY. OIL ALSO SERVES AS A COOLANT TO REDUCE THE OPERATING TEMPERATURE.

ADVANTAGES:

1. UNAFFECTED BY LOAD VARIATION

2.MAINTENANCE FREE

3.VIBRATION FREE

4.LESS SPACE

5. HIGH SERVICE LIFE

1.OIL CARRYOVER IS THE ONLY DEFECT.

ALL THE ABOVE HAVE BEEN COMPARED TO RECIPROCATING COMPRESSORS AND YOU WILL NOTICE THAT THE LATTER IS THE BEST.

CENTRIFUGAL COMPRESSORS:

FOR THE MARINE USE THESE COMPRESSORS ARE USED FOR ONLY AIRCONDITIONING DUTIES. THIS IS SIMILAR TO A CENTRIFUGAL PUMP.

CONSTRUCTION:

THIS COMPRESSOR CONSISTS OF AN IMPELLER WITH AN EYE AND IS BEING KEPT IN A CASING WHICH IS AIR TIGHT. IT ALSO CONSISTS OF A SUCTION AND DISCHARGE IN WHICH THE FORMER AREA IS BIGGER. A DIFFUSER IS ALSO PRESENT CONCENTRIC TO THE IMPELLER. 

WORKING

 THE GAS IS BEING TAKEN AS SUCTION BY THE EYE OF THE IMPELLER AND IMPARTS CENTRIFUGAL FORCE TO IT AND THEN IT WILL BE PASSED ON TO THE DIFFUSER WHERE THE KINETIC ENERGY IS CONVERTED INTO PRESSURE ENERGY. THIS GAS THEN PASSES THROUGH THE VOLUTE CASING AND  IS BEING RELEASED AT HIGH PRESSURE THROUGH THE OUTLET.

ADVANTAGES:
1. ELIMINATION OF RUBBING SURFACES,PISTON RINGS,VALVE GEARS ETC.
2. REDUCTION IN MAINTENANCE
3. NO INTERNAL LUBRICATION IS NECESSARY
4. OIL CARRYOVER IS NOT A PROBLEM.


DISADVANTAGES:
1.CANNOT BE DESIGNED TO OPEARATE OVER A WIDE RANGE OF COMPRESSION RATIOS.
2. NOT PREFERRED FOR CARGO DUTIES.
3. ELECTRIC DRIVES CAN CREATE LARGE PROBLEMS DUE TO HIGH STARTING CURRENTS.
4. SEPERATELY NEED A DRIVING EQUIPMENT LIKE MOTORS, STEAM TURBINES. 



ROTARY COMPRESSORS:

 IN A ROTARY COMPRESSOR THE REFRIGERANT IS COMPRESSED BY THE ROTATING ACTION OF A ROLLER INSIDE A CYLINDER. THE ROLLER ROTATES ECCENTRICALLY AROUND A SHAFT SO THAT PART OF THE ROLLER IS ALWAYS IN CONTACT WITH THE INSIDE WALL OS THE CYLINDER. A SPRING MOUNTED BLADE IS ALWAYS RUBBING AGAINST THE ROLLER. THE 2 POINTS OF CONTACT CREATE TWO SEALED AREAS OF CONTINUOUSLY VARIABLE VOLUME INSIDE THE CYLINDER. AT A CERTAIN POINT IN THE ROLLER ROTATION, THE INTAKE PORT IS EXPOSED AND A QUANTITY OF REFRIGERANT IS SUCKED INTO THE CYLINDER, FILLING ONE OF THE SEALED AREAS. AS THE ROLLER CONTINUES TO ROTATE THE VOLUME OF THE AREA THE REFRIGERANT OCUUPIES IS REDUCED AND THE REFRIGERANT IS COMPRESSED. WHEN THE EXHAUST VALAVE IS EXPOSED, THE HIGH PRESSURE REFRIGERANT FROM THE EXHAUST VALAVE IS OPEN AND THE REFRIGERANT IS RELEASED.

ADVANTAGES:

1.VERY EFFECIENT
2.LESS MAINTENANCE
3.LESS NOISE AND VIBRATION
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