VARIABLE SPEEDSCREW COMPRESSORRAISING THE BAR FOR VARIABLE SPEED PERFORMANCECarrier CorporationSyracuse, New YorkOctober 2005INTRODUCTIONToday’s building owners and managers requirewell-engineered solutions to keep long-term opera-tional costs under control. The ability to lowerheating and cooling costs is critical to this goal. TheAmerican Society of Heating, Refrigeration andAir-Conditioning Engineers (ASHRAE) estimatesthat 50% of all building energy is consumed byHVAC operation. VARIABLE FREQUENCY DRIVES Variable frequency drives (VFDs) prevent wastingenergy by precisely matching motor speed withcooling requirements, which results in dramaticreductions in power usage. Affordable and factory installed in most cases,VFDs are one of the most cost-effective ways tomaximize efficiency and reduce operating costs.According to ARI (Air Conditioning andRefrigeration Institute) Standard 550/590-2003,chillers typically run 99% of the time at part-load(off design conditions). Therefore, having yourchiller match your building’s load profile will pro-vide both efficiency and comfort. To date, variable speed centrifugal compressorshave been the best means to effectively reduce ener-gy consumption during the majority of the opera-tional hours. When variable speed is applied to ascrew compressor, the savings are increased, sincethe variable speed screw chiller always provides themaximum amount of speed reduction.In order to fully appreciate the benefits of variablespeed screw water-cooled chillers, an understandingof centrifugal water-cooled chillers is required. CENTRIFUGAL COMPRESSORSCentrifugal air compressors had been in use for75 years when, in 1916, Dr. Willis H. Carrier recog-nized their potential for air conditioning applica-tions. Carrier sold the first water-cooled centrifugalchiller in 1924 to the Onondaga Pottery Company inSyracuse, New York. The machine ran for 26 yearsand provided air conditioning throughout that peri-od. The compressor of that first machine was retiredto the Smithsonian Institute in Washington, D.C.,where it remains as one of the major technicaldevelopments in the history of the United States.Centrifugal compressors are dynamic compressiondevices that continuously exchange angularmomentum between a rotating impeller and steadilyflowing refrigerant. As refrigerant molecules areaccelerated outward by centrifugal force, new onesare drawn into the compressor to replace them. Theoverall effect is one of continuously compressing astream of refrigerant. (Figure 1.)2Fig. 1. Centrifugal compressor impeller and diffuser.Centrifugal compressor performance can bemodeled by the following ideal fan laws.Ideal Fan LawsLaw 1Law 2Law 3LiftLift is defined as the difference between the con-densing (discharge) pressure and the evaporating(suction) pressure. Therefore, lift, or the amount ofwork the compressor performs on the refrigerant, isdependent on the leaving chilled water temperatureand condenser water temperature. The compressoronly experiences full lift conditions when the wetbulb temperature is at design and refrigeration loadis 100%. As the wet bulb temperature decreases, thecooling tower provides colder condenser water tothe chiller, reducing the lift required of the com-pressor. (Figure 2.)In addition, reductions in load will reduce liftbecause lower saturated condensing pressure resultwhen less heat is rejected to the condenser. A vari-able speed chiller responds to changes in lift andrefrigeration load by adjusting speed.As demonstrated by the ideal fan laws, a reductionin speed of the centrifugal compressor will have anexponential (cubic) decrease in compressor powerconsumption. Given this fact, it is no surprise that todate, variable speed centrifugal compressors havebeen the best means to effectively reduce energyconsumption during the majority of the operationalhours.SST = Saturated Suction TemperatureSCT = Saturated Condensing TemperatureARI conditions: (2 F approach) Lift = 97 F - 42 FWith 65 F entering condenser water, Lift = 77 F - 42 FWHY INLET GUIDE VANES ARE USED INVARIABLE SPEED APPLICATIONSThe most common form of capacity control for con-stant speed centrifugal chillers is to modulate guidevanes at the impeller inlet (also called pre-rotationvanes). As load is decreased, the mass flow ofrefrigerant moving through the compressor must bereduced. On constant speed machines, the guidevanes are closed to match compressor capacity tothe load. When centrifugal machines are equippedwith VFDs, speed control can also be used to con-trol capacity. In this case, the impeller speed can bereduced to match the compressor capacity to theload. 3Fig. 2. Pressure enthalpy chart.Lift LiftRPMRPM21212∝×Flow Rate FlowRateRPMRPM2121∝×Power PowerRPMRPM21213∝×Lift LiftRPMRPM21212∝×4Recalling that the lift produced by a centrifugalcompressor is also reduced when speed is reduced,we can determine that speed adjustment alone can-not always be used to regulate the variable speedcentrifugal chiller. Under certain lift conditions, thespeed is reduced as much as lift requirements willallow and then guide vanes are used to complete theload reduction. Mechanical unloaders of any kindintroduce inefficiency. So while speed reduction isalmost always obtained with any reduction of lift orload requirements, the question becomes one ofmagnitude. The amount of capacity reduction per-formed by speed reduction, relative to the amountof capacity reduction performed by guide vanes isan indication of the centrifugal chillers ability tocapture all theoretical savings at a given operatingpoint. Conversely, the more the guide vanes areclosed, the higher the amount of inefficiency intro-duced into the system. Given the cubic relationship of speed and powereven a small amount of speed reduction yields a sig-nificant reduction in energy. However, the morespeed reduction possible, the greater the energysavings.UNDERSTANDING SCREW COMPRESSORSHeinreich Krigar of Germany developed the firstscrew compressor in 1878. In the early 1930’s, aSwedish engineer by the name of Alf Lysholmdeveloped the profile of the
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