THE BEARINGSTraditional centrifugal compressors use roller

bearings and hydrodynamic bearings, both ofwhich consume power and require oil and a lubrica-

tion system. Recently, ceramicroller bearings, which avoid issuesrelated to oil and reduce power con-sumption, were introduced to theHVAC industry. The lubrication of

these bearings is provided by the refrigerant itself.1

A new compressor technology intro-duced during the 2003 InternationalAir-Conditioning, Heating, Refriger-

ating Exposition (AHR Expo), held last January in Chicago, may have a significanteffect on the future of mid-rangechillers and rooftop applications in water-cooled, evaporativelycooled, and air-cooled chilled-wa-ter and direct-expansion (DX) systems. Designed and optimized to take full advantage of magnetic-bearing technology, the compressor was awarded the firstAHR Expo Innovation Award in theenergy category, as well as Canada’s Energy Efficiency Award for its poten-tial to reduce utility-generated green-house-gas emissions. The compressoris key to a new water-cooled centrifu-gal-chiller design, with Air-Condi-tioning and Refrigeration Institute(ARI) tests indicating integratedpart-load values (IPLVs) not normallyseen with conventional chillers in thistonnage range.

This article describes this new com-pressor technology and its first use inan ARI-certified chiller design.

New compressor makes chillers cleaner,

quieter, and more energy-efficient

CompressorF r i c t i o n l e s s -

Technology

By HUGH CROWTHER, PE, and

EUGENE SMITHART, PE

Inlet-guide-vaneassembly

Touchdownbearings

Compressorcooling

Magnetic bearingsand bearing sensors

Permanent-magnetsynchronous motor

Shaft andimpellers

The TT300 compressor’s onboard digital electronics manage operationwhile providing external control and Web-enabled access to a full array ofperformance and reliability information.

Hugh Crowther, PE, is the global director of applications for McQuay International in Minneapolis. With morethan 15 years of experience in large-HVAC-system design, he has written numerous articles and applicationguides related to HVAC design. Contact him at hugh.crowther@mcquay.com. Eugene (“Smitty”) Smithart,PE, is vice president of sales and marketing for Danfoss Turbocor in La Crosse, Wis. With nearly 30 years of expe-rience, he is well-known in the HVACR industry, having published numerous articles and been involved ina number of industry-changing initiatives. A recipient of the U.S. Environmental Protection Agency’s ClimateProtection Award, he can be contacted at smitty@turbocor.com.

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Magnetic-bearing technology is signifi-cantly different. A digitally controlledmagnetic-bearing system, consisting ofboth permanent magnets and electro-magnets, replaces conventional lubricatedbearings. The frictionless compressorshaft is the compressor’s only movingcomponent. It rotates on a levitated magnetic cushion (Figure 1). Magneticbearings—two radial and one axial—hold the shaft in position (Figure 2).

When the magnetic bearings are ener-gized, the motor and impellers, which are keyed directly to the magnetic shaft,levitate. Permanent-magnetic bearings do the primary work, while digitally controlled electromagnets provide thefine positioning. Four positioning signalsper bearing hold the levitated assembly toa tolerance of 0.00002 in. As the levitatedassembly moves from the center point,the electromagnets’ intensity is adjustedto correct the position. These adjustmentsoccur 6 million times a minute. The soft-ware has been designed to automaticallycompensate for any out-of-balance condi-tion in the levitated assembly.

SHUTDOWNS AND POWER FAILURESWhen the compressor is not running,

the shaft assembly rests on graphite-lined,radially located touchdown bearings. Themagnetic bearings normally position therotor in the proper location, preventingcontact between the rotor and other metallic surfaces. If the magnetic bearings

fail, the touchdown bearings (also knownas backup bearings) are used to prevent acompressor failure.

The compressor uses capacitors tosmooth ripples in the DC link in the motor drive. Instantaneously after a power failure, the motor becomes a“generator,” using its angular momentumto create electricity (sometimes known asback EMF) and keeping the capacitorscharged during the brief coastdown period. The capacitors, in turn, provideenough power to maintain levitation during coastdown, allowing the motorrotor to stop and delevitate. This featureallows the compressor to see a power outage as a normal shutdown.

OIL-FREE DESIGNOil management, particularly as it

pertains to the lubrication of compressorbearings, is a critical issue in refrigeration-system design. But with magnetic bear-ings, this issue is avoided. Only a verysmall amount of oil is required to lubricateother system components, such as sealsand valves; often, however, experienceshows that even this small amount of oil isnot needed. Avoiding oil-managementsystems means avoiding the capital cost ofoil pumps, sumps, heaters, coolers, and oilseparators, as well as the labor and time re-quired to perform oil-related services. Re-ports indicate that for many installations,compressor-maintenance costs have beencut by more than 50 percent.

Most air-cooled products (includingchillers, rooftop units, and condensingunits) use DX evaporators. Most DX systems allow oil to travel through the refrigeration circuit and back to the compressor oil sump. Great care must betaken during design to provide oil return,particularly at part load, when refrigerantflow rates are reduced.

Water-cooled chillers often use floodedevaporators. In a flooded evaporator, evensmall amounts of oil can coat evaporatortubes and significantly diminish chillerperformance. This can lead to an elabo-rate oil-recovery system. Magnetic bear-ings eliminate the need for these systemsand oil management in general. In fact,the only required regular maintenance ofthe compressor is the quarterly tighteningof the terminal screws, the annual blow-ing off of dust and cleaning of the boards,and the changing of the capacitors everyfive years. Complete service agreementsand extended maintenance contracts canbe provided by the manufacturer.

THE MOTORMost hermetic compressors use in-

duction motors cooled by either liquidor suction-gas refrigerant. Inductionmotors have copper windings that,when alternating current is run throughthem, create the magnetic fields thatcause the motor to turn. These copperwindings are bulky, adding size andweight to the compressor.

F R I C T I O N L E S S C O M P R E S S O R S

Target sleeve

Y-axispositionsensor

Y-axispositionsensor

Channels0-1

Channels2-3

Channel4

Axial bearingRearradial

bearing

Sensorring

Sensorring

MotorFrontradial

bearing

Z-axispositionsensor

Impellers

X-axisposition sensor

X-axisposition sensor

FIGURE 2. A digitally controlled magnetic-bearing system consisting of two radial andone axial bearing levitate the compressor’s rotor shaft and impellers during rotation.

FIGURE 1. Electromagnetic cushionscontinually change in field strength to keepthe rotor shaft centrally positioned.

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tronic components to be refrigerant-cooled. The VSD also acts as a soft starter;as a result, the compressor has an ex-tremely low startup in-rush current: lessthan 2 amps, compared with 500 to 600amps for a traditional 75-ton, 460-v screwcompressor with a cross-the-line starter.

With the integration of the motor,VSD, and magnetic-bearing system, thecapacitors required for the motor anddrive can be used as a backup powersource for the bearings in the event of apower outage or emergency shutdown.

CAPACITY AND EFFICIENCYAmong the key parameters affecting

performance are capacity (tons) and effi-ciency (kilowatts perton). The compres-sor’s capacity rangesfrom 60 to 90 tons,depending on theoperating conditions.Plans call for thatrange to be extendedto 150 tons water-cooled and 115 tonsair-cooled by the endof 2004 with the useof R-134a refriger-

ant. An R-22 version is planned for retro-fit applications.

Efficiency improvements stem from acombination of the centrifugal compres-sor, permanent-magnet motor, and mag-netic bearings. Within the compressor,efficiency is affected by the compressorisentropic efficiency (the efficiency of thewheels), the motor, and the bearings.Traditional induction motors of this sizetypically are in the 92-percent efficiencyrange. This compressor’s permanent-magnet motor has an efficiency of 96 to97 percent.

Efficiency is further enhanced withthe use of magnetic bearings, whichavoid the friction of rubbing parts associ-ated with traditional oiled bearings.Conventional bearings can use as muchas 10,000 w, while magnetic bearings re-quire only 180 w. That amounts to 500times less friction loss. Current develop-

ment projects are expanding the rangeand duty of the compressor wheels andpromise to offer even greater efficiencyfor water-cooled and air-cooled dutiesand different capacities.

CONTROLSThe new compressor effectively is

a computer. It provides diagnostic andperformance information through Mod-bus to the refrigeration system, whichthen communicates to the building automation system through Modbus,LonWorks, or BACnet.

CHILLER APPLICATIONThe compressor manufacturer and a

major chiller manufacturer teamed up to develop a line of ARI-certified water-cooled chillers, which were expected to beintroduced in January 2004. The combi-nation of flooded-evaporator technologyand an oil-free system has allowed veryclose approaches and, subsequently, enhanced performance. The integratedVFD allows excellent part-load perform-ance as power consumption drops off, depending on the head relief, near thecube root of the shaft speed.

The compressor includes wheels tunedfor water-cooled duty in the dual-com-pressor format, which further enhancespart-load performance. Tested in accor-dance with ARI Standard 550/590-98,Water Chilling Packages Using the VaporCompression Cycle, a 150-ton (nominal)chiller has a full-load performance of0.629 KW per ton (5.6 COP) and anIPLV of 0.375 KW per ton (9.4 COP).

All IPLVs are weighted for standardoperating conditions and the time spentat those conditions. Specific operatingpoints for a 150-ton nominal-capacitychiller are shown in Figure 3.

SOUND AND VIBRATIONBecause the rotating assembly levitates,

there essentially is no structure-borne vibration. The magnetic bearings createan air buffer that prevents the only majormoving part—the motor rotor—fromtransmitting vibration to the structure.

Two-pole, 60-Hz induction motorsoperate at approximately 3,600 rpm. Ahigher number of revolutions per minutecan be obtained by increasing the fre-quency. Compressors that require highershaft speeds tend to use gears. While gearsare a proven technology, they create noiseand vibration, consume power, and re-quire lubrication.

The magnet-bearing compressor fea-tures a synchronous permanent-magnetbrushless DC motor with a completely integrated variable-frequency drive(VFD). The stator windings found onconventional induction motors are replaced with a permanent-magnet rotor.Alternating current from the inverter en-

ergizes the armature windings. The stator(excitation) and rotor (armature) changeplaces. No commutator brushes are re-quired. The motor and key electroniccomponents are internally refrigerant-cooled, so no special cooling is requiredfor the VFD or the motor.

The use of permanent magnets insteadof rotor windings makes the motorsmaller and lighter than induction mo-tors. Using magnetic-bearing technology,a 75-ton compressor weighs 265 lb—about one-fifth the weight of a conven-tional compressor.

A variable-speed drive (VSD) is re-quired for the motor to operate. The VSDvaries the frequency between 300 and 800Hz, which provides a compressor-speedrange from 18,000 to 48,000 rpm. Thisavoids a gear set. The VSD is integrated into the compressor housing,avoiding long leads and allowing key elec-

Magnetic-bearing compressors offereconomic,energy,and environmentalbenefits, including increased energyefficiency, the elimination of oil, and

considerably less noise and vibration.

F R I C T I O N L E S S C O M P R E S S O R S

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Similarly, sound levels are extremelylow, primarily because of refrigerant-gasmovement through the compressor andthe rest of the refrigeration system.There are no tonal issues, such as thosefound with some screw compressors,and the noise occurs in the higher octavebands, where it is easier to attenuate.When two magnetic-bearing compres-sors were integrated into a chiller, thesound pressure was 77 dBA at 3.3 ft un-der ARI Standard 575-94, Method ofMeasuring Machinery Sound Within anEquipment Space.

MODELED ENERGY SAVINGSChiller applications were modeled

for Phoenix; Chicago; Tampa, Fla.; andNew York to estimate operating costs andpayback times. The program comparedan hourly analysis of a 150-ton friction-less chiller with that of a water-cooled reciprocating chiller (Phoenix, Chicago,and Tampa) and a water-cooled centrifu-gal chiller (New York). Each city showedan annual energy savings of more than$4,500 and a two- to three-year payback(Table 1).

SUMMARYAfter 10 years of development,

magnetic-bearing compressors offereconomic, energy, and environmentalbenefits. Chief among them are increasedenergy efficiency, the elimination of oil and oil management, and consider-ably less weight, noise, and vibration.This initial mid-range package offers centrifugal-compression efficienciespreviously reserved for large-tonnage systems only.

REFERENCE1) Ivanovich, M.G. (2002, June). The

market for chillers: drives, controls, simplicity. HPAC Engineering, pp. 11,12, 15, 17.

For HPAC Engineering feature articlesdating back to January 1992, visitwww.hpac.com.

Copyright © 2004 by Penton Media, Inc.

F R I C T I O N L E S S C O M P R E S S O R S

0

2

4

6

8

10

12

Coef

ficie

nt o

f per

form

ance

Percent load25 50 75 100

11.110.5

7.8

5.6

FIGURE 3. According to ARI testing, a 150-ton frictionless chiller has a full-loadperformance of 0.629 KW per ton (5.6 COP) and an IPLV of 0.375 KW per ton (9.4 COP).

Reciprocating vsmagnetic-bearing

Reciprocating vsmagnetic-bearing

Reciprocating vsmagnetic-bearing

Centrifugal vs.magnetic-bearing

Location Phoenix Chicago Tampa, Fla. New York

Machines

Three-story office Three-story office Three-story office Three-story officeBuilding type

58,200 69,000 67,800 69,600Square footage

150 149 150 150Design coolingload (tons)

241,121 116,256 312,305 119,521Annual coolington-hours

6.0 cents per KWH 5.0 cents per KWH 6.4 cents per KWH 10.9 cents per KWHOn-peak charge

6.0 cents per KWH 2.1 cents per KWH 4.4 cents per KWH 10.9 cents per KWHOff-peak charge

$1.75 per KW $16.41 per KW $8.12 per KW $20 per KWSummer demand

$1.75 per KW $12.85 per KW $8.12 per KW $20 per KWWinter demand

$18,000 $18,000 $18,000 $12,000Capital-costdifference

6 percent 6 percent 6 percent 6 percentInterest rate

$5,197 $5,035 $9,919 $4,587Energy savings

3.46 years 3.57 years 1.81 years 2.62 yearsSimple payback

$96,278 $92,872 $195,914 $87,880Net present value

36.72 percent 35.8 percent 63.3 percent 46.27 percentInternal rateof return

TABLE 1. Estimated operating costs and payback times.

2F-6000(800) 432-1342

www.mcquay.com

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