VARIABLE SPEEDSCREW COMPRESSOR

RAISING THE BAR FOR VARIABLE SPEED PERFORMANCE

Carrier CorporationSyracuse, New York

October 2005

INTRODUCTION

Today’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 COMPRESSORS

Centrifugal 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.)

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Fig. 1. Centrifugal compressor impeller and diffuser.

Centrifugal compressor performance can bemodeled by the following ideal fan laws.

Ideal Fan Laws

Law 1

Law 2

Law 3

Lift

Lift 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 Temperature

ARI conditions: (2 F approach) Lift = 97 F - 42 FWith 65 F entering condenser water, Lift = 77 F - 42 F

WHY INLET GUIDE VANES ARE USED INVARIABLE SPEED APPLICATIONS

The 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.

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Fig. 2. Pressure enthalpy chart.

Lift LiftRPM

RPM2 12

1

2

∝ ×

Flow Rate FlowRateRPM

RPM2 12

1

∝ ×

Power PowerRPM

RPM2 12

1

3

∝ ×

Lift LiftRPM

RPM2 12

1

2

∝ ×

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Recalling 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 COMPRESSORS

Heinreich 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 modern screw compres-sor for gas and steam turbine applications. Screwcompressors have been used in HVAC applicationsfor nearly three decades. (Figure 3).

The screw compressor is classified a positive dis-placement compressor, which simply means that avolume of gas is trapped with an enclosed spacewhose volume is then reduced. Conventional rotaryscrew compressors are composed of two parallelrotors with external helical profiles fit into a casing.(Figure 3.) One of the rotors is coupled to the motor (drive rotor) and as it turns it moves the other rotor(driven rotor), similar to a common gear set. Thegeometric profile of the rotating rotors is difficult to

visualize. It is easier to relate the compressionprocess to a reciprocating compressor, if you con-sider the drive rotor as the piston and the drivenrotors as the cylinder. As the drive rotors and drivenrotors unmesh, an empty cylinder is created, draw-ing in suction gas through the synchronized openingon the rotor suction face. As rotation continues, thesuction and discharge rotor faces are sealed off,trapping the gas in the cylinder. When this happens,the meshing point moves toward the discharge endof the rotors and drives the gas ahead of it. The dis-charge port provided for the gas escape is relativelysmall, compared to the suction port, resulting in pos-itive displacement compression.

Rotary screw compressors are well known for theirrobustness, simplicity, and reliability. They aredesigned for long periods of continuous operation,needing very little maintenance. Screw compressorscan overcome high lift when speed is reduced,allowing energy savings without the possibility ofsurge as the compressor unloads.

VARIABLE SPEED SCREW COMPRESSORS

For positive displacement compressors, speed isindependent of lift, or worded another way, thecompressor can develop the same amount of lift atany speed. Therefore, mechanical loaders can bereplaced entirely by speed control. As discussedearlier, centrifugal compressors may require speedcontrol coupled with some closure of the inlet guidevanes. The variable speed screw compressor neverhas to temper speed control with a guide vane orslide valve, and therefore captures the maximum

Fig. 3. Conventional twin screw compressor.

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energy reduction available at a given operatingcondition. Even small changes in speed createsignificant changes in energy consumption.Combine this incremental advantage with superiorcompression efficiencies, and a clear picture of theenergy savings potential of this technology emerges.

Variable speed drives used with screw compressorsare new to the HVAC industry, but they have beenused successfully in various air compression andrefrigeration applications. In these applications,screw compressors coupled with VFDs have beenutilized to ensure efficient compression at partialand full capacities. Variable speed screw compres-sors allow a wide range of capacity control whilemaximizing efficiency.

WHY COMPROMISE?

The variable speed screw chiller combines the bestfeatures of positive displacement screw chillers andvariable speed technology. The result is a superiorchiller with no compromises. A screw compressorwith a tri-rotor provides simplicity and reliability.(Figure 4.)

The direct-drive, tri-rotor design allows for shorterrotors and is about 5 to 10% more efficient than thetwin-rotor design. The tri-rotor also balances thrust,which reduces bearing loads.

Fig. 4. - Tri-rotor screw compressor.

A VFD CHILLER FOR HUMID CLIMATES

Applications where high lift requirements remaineven as cooling loads decrease may favor variablespeed screw chillers more significantly. An exampleof this application is a building located in a hot andhumid climate where cooling loads can vary whileoutdoor-air temperatures (dry bulb/wet bulb) remainhigh.

A variable speed screw compressor chiller is approx-imately 10 to 20% more efficient than all variablespeed centrifugal compressors with constant 85 Fentering condenser water.

CHILLER SELECTION CRITERIA

System part load performance is a crucial factor inchiller selection, since chillers rarely operate at fullload design conditions. Consulting engineers andowners should understand the importance of usingsystem part load performance for their specificapplication as the selection criterion. Simply, theunit with the lowest system part load performancewill provide the greatest energy savings across itsentire operational range. The system part load,when multiplied by total annual ton-hours of cool-ing, provides an estimate of the total annual kilo-watt-hour consumption. A variable speed screwcompressor chiller saves energy in all operatingconditions.

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NOTE: It is the responsibility of the user to evaluatethe accuracy, completeness or usefulness of any con-tent in this paper. Neither Carrier nor its affiliatesmake any representations or warranties regardingthe content contained in this paper. Neither Carriernor its affiliates will be liable to any user or anyoneelse for any inaccuracy, error or omission, regardlessof cause, or for any damages resulting from any use,reliance or reference to the content in this paper.

REFERENCE

Reindl, Douglas T. and Todd B. Jekel. “Screw Compressors: Selection Considerations for Efficient Operation.” Paper presented at the 21st IIR (International Institute of Refrigeration) International Congress of Refrigeration, Washington, D.C., 17-22 August 2003.

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Copyright 2005 Carrier Corporation www.carrier.com Printed in U.S.A 10-05