VARIABLE SPEED SCREW COMPRESSOR RAISING THE BAR FOR VARIABLE SPEED PERFORMANCE Carrier Corporation Syracuse New York October 2005 INTRODUCTION CENTRIFUGAL COMPRESSORS Today s building owners and managers require well engineered solutions to keep long term operational costs under control The ability to lower heating and cooling costs is critical to this goal The American Society of Heating Refrigeration and Air Conditioning Engineers ASHRAE estimates that 50 of all building energy is consumed by HVAC operation Centrifugal air compressors had been in use for 75 years when in 1916 Dr Willis H Carrier recognized their potential for air conditioning applications Carrier sold the first water cooled centrifugal chiller in 1924 to the Onondaga Pottery Company in Syracuse New York The machine ran for 26 years and provided air conditioning throughout that period The compressor of that first machine was retired to the Smithsonian Institute in Washington D C where it remains as one of the major technical developments in the history of the United States VARIABLE FREQUENCY DRIVES Variable frequency drives VFDs prevent wasting energy by precisely matching motor speed with cooling requirements which results in dramatic reductions in power usage Affordable and factory installed in most cases VFDs are one of the most cost effective ways to maximize efficiency and reduce operating costs According to ARI Air Conditioning and Refrigeration Institute Standard 550 590 2003 chillers typically run 99 of the time at part load off design conditions Therefore having your chiller match your building s load profile will provide both efficiency and comfort Centrifugal compressors are dynamic compression devices that continuously exchange angular momentum between a rotating impeller and steadily flowing refrigerant As refrigerant molecules are accelerated outward by centrifugal force new ones are drawn into the compressor to replace them The overall effect is one of continuously compressing a stream of refrigerant Figure 1 To date variable speed centrifugal compressors have been the best means to effectively reduce energy consumption during the majority of the operational hours When variable speed is applied to a screw compressor the savings are increased since the variable speed screw chiller always provides the maximum amount of speed reduction Fig 1 Centrifugal compressor impeller and diffuser In order to fully appreciate the benefits of variable speed screw water cooled chillers an understanding of centrifugal water cooled chillers is required Centrifugal compressor performance can be modeled by the following ideal fan laws 2 Ideal Fan Laws Law 1 Flow Rate2 FlowRate1 RPM2 RPM1 Law 2 RPM2 Lift2 Lift1 RPM1 2 Law 3 RPM2 Power2 Power1 RPM1 3 SST Saturated Suction Temperature SCT Saturated Condensing Temperature Lift Lift is defined as the difference between the condensing discharge pressure and the evaporating suction pressure Therefore lift or the amount of work the compressor performs on the refrigerant is dependent on the leaving chilled water temperature and condenser water temperature The compressor only experiences full lift conditions when the wet bulb temperature is at design and refrigeration load is 100 As the wet bulb temperature decreases the cooling tower provides colder condenser water to the chiller reducing the lift required of the compressor Figure 2 In addition reductions in load will reduce lift because lower saturated condensing pressure result when less heat is rejected to the condenser A variable speed chiller responds to changes in lift and refrigeration load by adjusting speed As demonstrated by the ideal fan laws a reduction in speed of the centrifugal compressor will have an exponential cubic decrease in compressor power consumption Given this fact it is no surprise that to date variable speed centrifugal compressors have been the best means to effectively reduce energy consumption during the majority of the operational hours 3 ARI conditions 2 F approach Lift 97 F 42 F With 65 F entering condenser water Lift 77 F 42 F Fig 2 Pressure enthalpy chart WHY INLET GUIDE VANES ARE USED IN VARIABLE SPEED APPLICATIONS The most common form of capacity control for constant speed centrifugal chillers is to modulate guide vanes at the impeller inlet also called pre rotation vanes As load is decreased the mass flow of refrigerant moving through the compressor must be reduced On constant speed machines the guide vanes are closed to match compressor capacity to the load When centrifugal machines are equipped with VFDs speed control can also be used to control capacity In this case the impeller speed can be reduced to match the compressor capacity to the load RPM2 Lift2 Lift1 RPM1 2 Recalling that the lift produced by a centrifugal compressor is also reduced when speed is reduced we can determine that speed adjustment alone cannot always be used to regulate the variable speed centrifugal chiller Under certain lift conditions the speed is reduced as much as lift requirements will allow and then guide vanes are used to complete the load reduction Mechanical unloaders of any kind introduce inefficiency So while speed reduction is almost always obtained with any reduction of lift or load requirements the question becomes one of magnitude The amount of capacity reduction performed by speed reduction relative to the amount of capacity reduction performed by guide vanes is an indication of the centrifugal chillers ability to capture all theoretical savings at a given operating point Conversely the more the guide vanes are closed the higher the amount of inefficiency introduced into the system Given the cubic relationship of speed and power even a small amount of speed reduction yields a significant reduction in energy However the more speed reduction possible the greater the energy savings UNDERSTANDING SCREW COMPRESSORS Heinreich Krigar of Germany developed the first screw compressor in 1878 In the early 1930 s a Swedish engineer by the name of Alf Lysholm developed the profile of the modern screw compressor for gas and steam turbine applications Screw compressors have been used in HVAC applications for nearly three decades Figure 3 The screw compressor is classified a positive displacement compressor which simply means that a volume of gas is trapped with an enclosed space whose volume is then reduced Conventional rotary screw compressors are composed of two parallel rotors with external helical profiles
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