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  • SECTION 5COMMERCIAL REFRIGERATION UNIT 23COMPRESSORS

  • UNIT OBJECTIVES After studying this unit, the reader should be able to Explain the function of the compressor Discuss the concept of compression ratio List common compressors found in refrigeration systems Describe four different methods of compression Describe the component parts of reciprocating compressors

  • FUNCTION OF THE COMPRESSORConsidered the heart of the refrigeration systemsCompressors are vapor pumpsResponsible for lowering the pressure on the suction side of the systemResponsible for increasing the pressure on the discharge side of the systemSuction gas from the evaporator enters the compressorRefrigerant is discharged to the condenser

  • COMPRESSION RATIOCompares pumping conditions for compressorsDefined as the high side pressure (psia) divided by the low side pressure (psia)High compression ratio can lead to overheated compressor oilHigh compression ratio leads to reduced refrigerant flow through the systemReduced refrigerant flow reduces system capacity

  • COMPRESSION RATIO EXAMPLESR-12 compressor169 psig high side, 2 psig low side183.7 psia high side, 16.7 psia low side183.7 psia 16.7 psia = 11:1 compression ratioR-134a compressor184.6 psig high side, 0.7 in. Hg. vacuum low side199.3 psia high side, 14.35 psia low side199.3 psia 14.35 psia = 13.89:1 compression ratio

  • TWO-STAGE COMPRESSIONLowers the compression ratioUtilizes two compressorsOne compressor discharges into suction of the otherAlso referred to as compound compressionOften used when the compression ratio of a single compressor system exceeds 10:1Often used in low-temperature commercial and industrial storage applications

  • TWO-STAGE COMPRESSIONFIRST STAGE SECOND STAGE21 psig 100 psig 169 psigSuctionDischargeDischargeSuction

  • TYPES OF COMPRESSORSReciprocatingFully welded, hermetic compressorsSemi-hermetic compressorsOpen-drive compressorsBelt-driven and direct-drive compressorsScrew compressorsRotary compressorsScroll compressorsCentrifugal compressors

  • WELDED HERMETIC RECIPROCATING COMPRESSORSMotor and compressor contained in a welded shellCannot be field servicedTypically a throw-away compressorConsidered to be a low-side componentCooled by suction gas from the evaporatorLubricated by the splash method

  • SEMI-HERMETIC COMPRESSORSBolted together, can be field servicedHousing is made of cast ironHas a horizontal crankshaftSmaller compressors are splash lubricatedLarger compressors use pressure lubrication systemsOften air cooledPiston heads are located at the top of the compressor

  • OPEN DRIVE COMPRESSORSCan be direct drive or belt-driven compressorsMust have a shaft seal to prevent leakageBolted together, can be field servicedBelt-driven compressors have the compressor and motor shafts parallel to each otherBelt-driven compressors use belts and pulleysDirect drive compressors have the compressor and motor shafts connected end to end

  • OTHER COMPRESSOR TYPESScrew compressor Used in large commercial/industrial applicationsUses two matching, tapered gears, and open motor designRotary compressorUsed in residential and light commercial applications primarily in domestic refrigeratorsScroll compressorUses a matched set or scrolls to achieve compressionBecoming more popular for their ability to handle liquid refrigerant without compressor damageCentrifugal compressorsUsed extensively for air conditioning in large structures

  • RECIPROCATING COMPRESSOR COMPONENTSCrankshaftTransfers motor motion to the pistonCreates the back and forth motion of the pistonConnecting rodsConnects the crankshaft to the pistonsPistonsSlide up and down in the cylinderUsed to compress and expand the refrigerant

  • RECIPROCATING COMPRESSOR COMPONENTS (contd)Refrigerant cylinder valves (suction)Durable, flexible steelLocated on the bottom of the valve plateOpen when refrigerant is introduced to the pumpRefrigerant cylinder valves (discharge)Durable, flexible steelOpen when refrigerant is discharged from the pumpLocated on the top of the valve plate

  • Suction lineDischarge lineValve plateHeadDischarge valve Suction valvePiston Rings CrankshaftConnecting Rod

  • RECIPROCATING COMPRESSOR COMPONENTS (contd)Compressor headHolds the top of the cylinder and its components togetherContains both high and low pressure refrigerantMufflersDesigned to reduce compressor noiseCompressor housingEncases the compressor and sometimes the motor

  • BELT-DRIVE MECHANISMSMotor pulley is called the drive pulleyCompressor pulley is called the driven pulleyPulleys can be adjusted to change compressor speedDrive size x Drive rpm = Driven size x Driven rpmShafts must be properly alignedPulleys with multiple grooves must used matched sets of belts

  • DIRECT-DRIVE COMPRESSOR CHARACTERISTICSDirect drive compressors turn at the same speed as the motor usedMotor shaft and compressor shaft must be perfectly aligned end to end Motor shaft and compressor shafts are joined with a flexible coupling

  • RECIPROCATING COMPRESSOR EFFICIENCYDetermined by initial compressor designFour processes take place during the compression processExpansion (re-expansion)Suction (Intake)CompressionDischarge

  • COMPRESSION PROCESS - EXPANSIONPiston is the highest point in the cylinderReferred to as top dead centerBoth the suction and discharge valves are closedCylinder pressure is equal to discharge pressureAs the crankshaft continues to turn, the piston moves down in the cylinderThe volume in the cylinder increasesThe pressure of the refrigerant decreases

  • Suction valve closedDischarge valve closedPiston moving downward in the cylinderRefrigerant trapped in the cylinderPressure of the refrigerant in the cylinder is equal to the discharge pressure

  • COMPRESSION PROCESS SUCTIONAs the piston moves down, the pressure decreasesWhen the cylinder pressure falls below suction pressure, the suction valve opensThe discharge valve remains in the closed positionAs the piston continues downward, vapor from the suction line is pulled into the cylinderSuction continues until the piston reaches the lowest position in the cylinder (bottom dead center)At the bottom of the stroke, suction valves close

  • Suction valve openDischarge valve closedPiston moving downward in the cylinderPressure of the refrigerant in the cylinder is equal to the suction pressureSuction gas pulled into the compression cylinderMost of the energy that entering the compressor in the suction cylinder is latent heat.

  • COMPRESSION PROCESS - COMPRESSIONPiston starts to move upwards in the cylinderThe suction valve closes and the discharge valve remains closedAs the piston moves upwards, the volume in the cylinder decreasesThe pressure of the refrigerant increasesCompression continues until the pressure in the cylinder rises just above discharge pressure

  • Suction valve closedDischarge valve closedPiston moving up in the cylinderPressure of the refrigerant in the cylinder is equal to the suction pressureVolume is decreasing, compressing the refrigerant

  • COMPRESSION PROCESS - DISCHARGEWhen the cylinder pressure rises above discharge pressure, the discharge valve opens and the suction valve remains closedAs the piston continues to move upwards, the refrigerant is discharged from the compressorDischarge continues until the piston reaches top dead center

  • Suction valve openDischarge valve closedPiston moving up in the cylinderPressure of the refrigerant in the cylinder is equal to the discharge pressureDischarge gas pushed from the compression cylinderWhat do you think may happen if both of the valves remain closed on an upstroke?The gas leaving the compressor is very warm.

  • Clearance volume of the space between the bottom of the valve plate and the top of the piston at top dead center?

  • LIQUID IN THE COMPRESSION CYLINDERIf liquid enters the cylinder, damage will occurLiquids cannot be compressedLiquid slugging can cause immediate damage to the compressor componentsCommon causes of liquid slugging include an overfeeding metering device, poor evaporator air circulation, low heat load, defective evaporator fan motor and a frosted evaporator coil

  • Page 439 Discus Valve design

  • SYSTEM MAINTENANCE AND COMPRESSOR EFFICIENCYHigh suction pressures and low discharge pressures keep the compression ratio lowDirty evaporators cause suction pressure to dropLow suction reduces compressor pumping capacityDirty condensers increase head pressureCompression ratio is increased by dirty or blocked condenser and evaporator coils

  • UNIT SUMMARY - 1The compressor is responsible for pumping refrigerant through the refrigeration systemThe compressor lowers the pressure on the low side of the system and increases the pressure on the high side of the systemThe compression ratio compares pumping conditions for compressorsComp. Ratio = High side (psia) Low side (psia)

  • UNIT SUMMARY - 2Two-stage compression uses two compressors where one compressor discharges into the suction of the second compressorUsed when the compression ratio for single-stage compression is higher than 10:1Common compressor types include the rotary, the reciprocating, the scroll, the screw and the centrifugal

  • UNIT SUMMARY - 3Hermetic compressors are factory welded and not field serviceableSemi-hermetic compressors are bolted together and can be serviced in the fieldOpen drive compressors have the motor separate from the compressorOpen drive compressors can be direct drive or belt-driven

  • UNIT SUMMARY - 4Reciprocating compressors are equipped with suction and discharge valves