Edition No 1 September 2000 McQuay APPLICATIONS B U L L E T I N PRACTICAL APPLICATION INFORMATION FOR THE MCQUAY SALES FORCE Welcome to our first issue of Applications Bulletin practical application information for the McQuay sales force This is a quarterly publication that will contain information and ideas on how to use McQuay products as well as how other manufacturers are applying their products It is intended for internal use by McQuay Sales Representatives and Distributors The following are some other application tools and their use that are also available from McQuay Sales and Engineering Data Sheets SED s are intended for internal use by McQuay Sales Representatives They are issued by product managers and cover new material issued between catalog reprints competitive analyses and special information not necessarily found in the general catalogs SED s are published as required Engineering System Solutions is a quarterly publication intended for external distribution by Sales Representatives to consulting engineers It provides a venue for new products and applications If there is a topic you would like to see covered or something that should be shared with the rest of the sales force please let me know Hugh Crowther P E Manager of Applications McQuay International hugh crowther mcquay com Centrifugal Chillers With VFDs As the cost for large horsepower Variable Frequency Drives VFDs comes down their application on centrifugal chillers has become economically viable For the right conditions VFDs offer outstanding part load performance opportunities for centrifugal chillers Since centrifugal chillers spend most of their operating time at part load the operating savings can offset the higher cost of a VFD in the right application First A Centrifugal Compressor Refresher Centrifugal compressors work like fans Except the refrigerant gas is compressed Fans are more like pumps in that the fluid is assumed to be incompressible they convert velocity pressure to static pressure The higher the lift higher required static pressure the faster the refrigerant must go Figure 1 shows the required lift conditions at ARI conditions continued on next page Figure 1 VFDs modulate the chiller by slowing down the impeller Chillers with VFDs still have inlet guide vanes The VFDs only slow the impeller down to approximately 60 then inlet guide vanes have to be used At certain operating conditions the VFD may not be appropriate even at speeds above 60 and inlet guide vanes will be used in lieu of the VFD The chiller controller processes the operating conditions and evaluates whether to use inlet guide vanes or the VFD This is true for all chiller manufacturers Reduced Lift Mandatory for VFD Operation If the VFD slows the impeller down then the lift capability of the compressor is reduced There must 80 ARI 550 92 ARI 550 590 98 70 60 90 70 50 50 The traditional way to modulate a centrifugal chiller is with inlet guide vanes The vanes are located at the inlet to the impeller and look like inlet guide vanes on a fan As the vanes first start closing they impart pre swirl to the refrigerant gas This efficiently lowers the capacity of the compressor As the vanes close off further they throttle the refrigerant flow through the impeller by imposing more pressure drop and less preswirl Figure 2 30 Centrifugal Capacity Control many applications where the condenser water temperature will drop off as the chiller load drops However there also applications such as process applications where the load profile is independent of condenser water temperature 90 10 Imagine a ball on the end of a string swung around your head the faster the ball spins the further it will go when released The same is true for the centrifugal compressor The faster the tip speed the more lift it will generate If the tip speed is not sufficient to meet the lift requirements the chiller will stall or surge be a reduction in compressor lift for the VFD to operate This is not a McQuay issue this is physics If the lift is not reduced the Microtech unit controller will only use the inlet guide vanes and the VFD will become a very expensive starter CEWT F For HFC 134a the condenser pressure at 97 F is 118 3 psig The evaporator pressure at 42 F is 36 6 psig The compressor must provide a lift of 81 7 psig Percent Chiller Capacity Lift can be lowered either by raising the supply water temperature or lowering the condenser water temperature The latter is more common In short condenser water relief must be present to take advantage of the VFD Chillers that operate during the winter in temperate climates are excellent candidates for VFDs because of winter hours with lower condenser water temperature Chillers that operate at full load a majority of the time or applications in climates that do not have a wide range in wetbulb temperature are a poor choice for VFDs ARI Condenser Water Relief and NPLV ARI Standard 550 590 98 provides the guidelines for testing chillers The Standard includes two issues that are critical to applying chillers and VFD chillers in particular The first issue is ARI condenser water relief ARI allows chillers to be rated with reduced condenser water temperature at part loads There are Figure 2 shows both the old curve and the new 1998 curve The new curve is much more favorable for VFD chillers The McQuay SelectTOOLs Chiller program automatically takes into account condenser water relief as do other manufacturers Note when a rating is performed the program automatically calculates the reduced condenser water temperature The condenser water temperature can be overridden if more accurate information on condenser water is available The second issue is Non Standard Part Load Value NPLV This is a weighted average of how a chiller performs from 25 to 100 It is based on condenser water relief and is given in kW ton format Here is the weighting 100 75 50 25 Load Load Load Load 1 42 45 12 continued on next page Although the NPLV load profile may not match the load profile of your project it does allow an apples to apples comparison of equipment since all manufacturers use it per ARI Standard 550 590 98 These weightings are favorable for VFD chillers For more information see the McQuay SED 20002 on ARI Standard 550 590 98 Chillers with VFDs VFDs replace the starters on a chiller They are approximately the same size Depending on the size of chiller the option to factory mount and wire the VFD is available VFDs have some power losses which
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