McQ (Italy) - ПЕРЕЧЕНЬ ДОКУМЕНТАЦИИГЛАВНОЕ МЕНЮMcQuayAPPLICATIONSAs the cost for large horsepowerVariable Frequency Drives(VFDs) comes down, theirapplication on centrifugal chillershas become economically viable.For the right conditions VFDsoffer outstanding part loadperformance opportunities forcentrifugal chillers. Sincecentrifugal chillers spend most oftheir operating time at part load,the operating savings can offsetthe higher cost of a VFD in theright application.First, A CentrifugalCompressor Refresher…Centrifugal compressors worklike fans; (Except the refrigerantgas is compressed. Fans aremore like pumps in that the fluidis assumed to beincompressible.) they convertvelocity pressure to staticpressure. The higher the lift(higher required static pressure)the faster the refrigerant mustgo. Figure 1 shows the requiredlift conditions at ARI conditions.Centrifugal ChillersWith VFDsAPPLICATIONSBULLETINEdition No. 1 September 2000PRACTICAL APPLICATION INFORMATION FOR THE MCQUAY SALES FORCEWelcome to our first issue ofApplications Bulletin – practicalapplication information for theMcQuay sales force. This is aquarterly publication that willcontain information and ideas onhow to use McQuay products, aswell as how other manufacturersare applying their products. It isintended for internal use byMcQuay Sales Representativesand Distributors.The following are some otherapplication tools and their usethat are also available fromMcQuay:Sales and Engineering DataSheets (SED’s) are intended forinternal use by McQuay SalesRepresentatives. They areissued by product managers andcover new material issuedbetween catalog reprints,competitive analyses and specialinformation not necessarily foundin the general catalogs. SED’sare published as required. Engineering System Solutions isa quarterly publication intendedfor external distribution by SalesRepresentatives to consultingengineers. It provides a venuefor new products andapplications.If there is a topic you would liketo see covered or something thatshould be shared with the rest ofthe sales force, please let meknow.Hugh Crowther P.E.Manager of ApplicationsMcQuay [email protected] on next pageFigure 1For HFC-134a, the condenserpressure at 97°F is 118.3 psig. Theevaporator pressure at 42°F is 36.6psig. The compressor must provide alift of 81.7 psig. Imagine a ball on the end of a stringswung around your head, the fasterthe ball spins, the further it will gowhen released. The same is true forthe centrifugal compressor. Thefaster the tip speed, the more lift itwill generate. If the tip speed is notsufficient to meet the liftrequirements, the chiller will stall orsurge.Centrifugal Capacity ControlThe traditional way to modulate acentrifugal chiller is with inlet guidevanes. The vanes are located at theinlet to the impeller and look likeinlet guide vanes on a fan. As thevanes first start closing, they impartpre-swirl to the refrigerant gas. Thisefficiently lowers the capacity of thecompressor. As the vanes close offfurther, they "throttle" the refrigerantflow through the impeller by imposingmore pressure drop and less pre-swirl.VFDs modulate the chiller by slowingdown the impeller. Chillers with VFDsstill have inlet guide vanes. TheVFDs only slow the impeller down toapproximately 60%, then inlet guidevanes have to be used. At certainoperating conditions, the VFD maynot be appropriate even at speedsabove 60%, and inlet guide vaneswill be used in lieu of the VFD. Thechiller controller processes theoperating conditions and evaluateswhether to use inlet guide vanes orthe VFD. This is true for all chillermanufacturers.Reduced Lift Mandatory for VFD OperationIf the VFD slows the impeller down,then the lift capability of thecompressor is reduced. There mustbe a reduction in compressor lift forthe VFD to operate. This is not aMcQuay issue, this is physics. If thelift is not reduced, the Microtech unitcontroller will only use the inlet guidevanes and the VFD will become avery expensive starter.Lift can be lowered either by raisingthe supply water temperature orlowering the condenser watertemperature. The latter is morecommon. In short, condenser waterrelief must be present to takeadvantage of the VFD. Chillers thatoperate during the winter intemperate climates are excellentcandidates for VFDs because ofwinter hours with lower condenserwater temperature. Chillers thatoperate at full load a majority of thetime or applications in climates thatdo not have a wide range in wetbulbtemperature, are a poor choice forVFDs.ARI Condenser Water Relief and NPLVARI Standard 550/590-98 providesthe guidelines for testing chillers. TheStandard includes two issues thatare critical to applying chillers andVFD chillers in particular.The first issue is ARI condenserwater relief. ARI allows chillers to berated with reduced condenser watertemperature at part loads. There aremany applications where thecondenser water temperature willdrop off as the chiller load drops.However there also applications(such as process applications) wherethe load profile is independent ofcondenser water temperature. Figure 2 shows both the old curveand the new 1998 curve. The newcurve is much more favorable forVFD chillers. The McQuaySelectTOOLs Chiller programautomatically takes into accountcondenser water relief as do othermanufacturers. Note, when a ratingis performed, the programautomatically calculates the reducedcondenser water temperature. Thecondenser water temperature can beoverridden if more accurateinformation on condenser water isavailable.The second issue is Non-StandardPart Load Value (NPLV). This is aweighted average of how a chillerperforms from 25% to 100%. It isbased on condenser water relief andis given in kW/ton format. Here is theweighting;100% Load 1%75% Load 42%50% Load 45%25% Load 12%90807060501030507090Percent Chiller CapacityCEWT (F)ARI 550-92ARI 550/590-98Figure 2continued on next pageAlthough the NPLV load profile maynot match the load profile of yourproject, it does allow an "apples toapples" comparison of equipmentsince all manufacturers use it perARI Standard 550/590-98. Theseweightings are favorable for VFDchillers. For more information, seethe McQuay SED 20002 on ARIStandard 550/590-98.Chillers with VFDsVFDs replace the starters on achiller. They are approximately thesame size. Depending on the size ofchiller, the option to factory mountand wire the VFD is available. VFDshave some power losses which
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