1

®

SCROLL COMPRESSORS

HIGH EFFICIENCY COMPRESSION FORCOMMERCIAL AND INDUSTRIAL APPLICATIONS

CarCarCarCarCarrier Corrier Corrier Corrier Corrier CorporporporporporaaaaationtiontiontiontionSyrSyrSyrSyrSyracuseacuseacuseacuseacuse, Ne, Ne, Ne, Ne, New Yw Yw Yw Yw Yorororororkkkkk

October October October October October 20042004200420042004

2

TABLE OF CONTENTS

INTRODUCTION ............................................... 2Scroll Compressor History and Development 2

OPERATING PRINCIPLES ................................ 3Geometry of a Scroll ....................................... 3The Scroll Set ................................................. 3Compression Process ...................................... 4Compliance ..................................................... 4Sealing Techniques ......................................... 6

CONSTRUCTION AND OPERATION.............. 7Shell ................................................................ 7Motor and Crankshaft ..................................... 8

INTRODUCTION

What is a scroll compressor? According toASHRAE’s HVAC Systems and Equipment Hand-book (ASHRAE 2004, Chapter 34), “Scroll com-pressors are orbital motion, positive-displacementmachines that compress with two interfitting,spiral-shaped scroll members.” A reader under-stands ASHRAE’s brief, written description morereadily when he or she understands the conceptualsimplicity of how a scroll compressor really oper-ates. But, understanding how a scroll compressorworks always inspires the thought: “It’s so simple!I wish I had thought of that.” This paper’s purposeis to explain how scroll compressors work, theiroperating principles and their applications. It alsodiscusses scroll compressor performance, effi-ciency, and reliability.

ASHRAE classifies scroll compressors as orbitingpositive-displacement compressors. For practicalpurposes, however, scroll compressors are fre-quently considered rotary machines, a class thatincludes: twin-screw, single-screw, moving vaneand rolling piston compressors. ASHRAE’s

classification may be technically more exact, butthe distinction is not compelling in the HVACmarketplace. Orbiting scroll compressors androtary twin-screw compressors are both viable,positive-displacement technologies in commercialapplications such as air-cooled water chillers. Therotary-like movement of their compressing ele-ments distinguishes them both from the linearmovement of pistons in reciprocating compressors,and the spinning action of centrifugal compressors.

Scroll Compressor History and Development

Although the idea of a scroll compressor is notnew, scroll compressors themselves are a relativelynew technology. The first scroll compressor patentdates back to 1905. Léon Creux, a French engi-neer, developed the first scroll compressor designthat was literally ahead of its time. Not until theearly 1970s had precision machining technologyadvanced sufficiently to make a working prototypepossible. Development continued, primarily inJapan and the United States, and widespreadintroduction to HVAC and refrigeration applica-tions began in the mid-1980s. Today, scroll com-

Scrolls ............................................................. 9Seals .............................................................. 10Compressor Protection .................................. 10

EFFICIENCY AND PERFORMANCE ............ 11

APPLICATIONS ............................................... 13

RELIABILITY ................................................... 13

SUMMARY ....................................................... 14

REFERENCES .................................................. 15

3

pressors are found in many commercial and resi-dential applications.

Screw compressors, in contrast, have a muchlonger history, but their actual adaptation to HVACapplications is not as old as might initially bethought. The first patent awarded for a screwcompressor is dated 1878, but the first moderntwin-screw compressor did not appear until 1935.Screw compressors found their first uses in indus-trial applications and did not move into the HVACarena until late in the 20th Century.

OPERATING PRINCIPLES

Geometry of a Scroll

By definition, a volution is a turn or twist about acenter. A volute is a spiral. A spiral is a circularcurve: the locus of a point moving with an everincreasing radius about a fixed center. A spiralmay have one or more volutions, as shown inFigure 1. There are many different kinds of spi-rals, each defined by variations of a basic math-ematical equation. The fundamental spiral form isthe Spiral of Archimedes, which is defined by thesimple equation: r = aθ, where r is the radius froma fixed center, a is a constant and θ is the angle (inpolar coordinates). Other variants include thehyperbolic spiral, parabolic spiral, logarithmic

spiral and involute spiral.

If we add a third dimension to a simple spiral, theresult is a coiled plane similar in appearance to arolled strip of paper. Ancient scribes rolled writtenparchments onto wooden spools for storage andsafekeeping, which is where we derive the termscroll.

Of particular interest to scroll compressor design isthe involute spiral, shown in Figure 2. An involutespiral is a spiral with a continuously variableradius measured from the circumference of a basecircle centered on a fixed axis. The curve can bevisualized as the end point of a tightly pulled cordunwinding from a cylinder.

The shape of an involute permits opposing ma-chine elements to mesh so that the bearing facesroll against one another rather than slide (e.g., gearteeth). This reduces friction and wear, and pro-duces a constant angular-velocity ratio duringmeshing. The involute geometry of identicalmeshing scroll vanes creates a rolling action attangential points and minimizes sliding.

The Scroll Set

The scroll is the fundamental compressing elementin a scroll compressor. Conceptually, it is a free-standing strip of metal machined into the form of

X

Y

Fig. 1. A simple spiralFig. 2. The radius of an involute spiral is measured

from the circumference of a base circle

X

Y

VARIABLERADIUS (r)

BASECIRCLE

INVOLUTE

4

an involute spiral, and bound on one edge by asolid flat base. A scroll set uses two scrolls withidentical geometry. One scroll is inverted, rotated180 degrees, and inserted into the gaps of thesecond scroll as shown in Figure 3.

In most scroll compressors, the unit frame holdsthe upper scroll stationary. An eccentric motorshaft moves the lower scroll in an orbital pattern.A specially designed coupling, called an Oldhamcoupling, holds the lower scroll at a fixed angularposition, preventing rotation and allowing radialmovement in an orbital path.

Compression Process

When assembled, the flanks of the upper and lowerscroll vanes form crescent-shaped pockets. As thelower scroll orbits, the sealing points (tangentpoints) on the vane flanks migrate inward, pushingcrescent-shaped pockets toward the involutecenter. As the pockets move, they decrease in

volume and consequently compress the trappedgas. Figure 4 (on page 5) shows a sequence oforbits, and the movement and variation of thetrapped gas pockets.

In Figure 4, the first orbit begins with the ends ofboth scrolls fully open, allowing the interstitialspace to fill with low-pressure refrigerant gas(position A). The lower scroll’s orbit eventuallycloses the first pockets of refrigerant gas (PositionC). As the first orbit ends, the first pair of cres-cent-shaped pockets have migrated inward to amiddle position, and the scroll’s outer ends beginopening again to admit more low-pressure refriger-ant gas (Position D). The second orbit pushes thefirst gas pockets toward the center of the scroll set,continually decreasing the gas volume and increas-ing the gas pressure. The third orbit begins withthe crescent-shaped pockets just outside of thescroll set center. As the third orbit continues, theinner ends of the vanes break contact (Position J),admitting the compressed gas to the center dis-charge port. The third orbit continues the com-pression cycle, discharging high-pressure refriger-ant gas (Position L).

It is important to note the symmetry of the scrolland the crescent-shaped pockets. The shape andposition of both pockets described in the aboveparagraph are symmetrical and diametricallyopposed to each other through the complete com-pression cycle (e.g., 3 orbits). The natural symme-try in the scroll set balances radial gas forcesagainst the vanes, providing a smooth compressioncycle. Moreover, each orbit begins the compres-sion cycle anew so that at any given time there arethree pairs of symmetrical crescent-shaped pocketsat low-, medium-, and high-pressure conditions, asshown in Figure 5 (on page 6). Between PositionsA and L on Figure 4, compression is a smooth andcontinuous process without vibration or strongpulsations as in reciprocating compressors.

Compliance

Some scroll compressor manufacturers haveadopted the term compliance to describe the orbital

Fig. 3. A scroll set is made up of two identicalscrolls—one scroll is inverted and rotated180 degrees to intermesh with the oppositescroll

STATIONARYSCROLL

ORBITINGSCROLL

BASE

5

ORBITDIRECTION

1ST ORBIT 2ND ORBIT 3RD ORBIT

STATIONARYSCROLL (WHITE)

ORBITINGSCROLL (GRAY)

A

B

C

D

I

J

K

L

E

F

G

H

Fig. 4. The complete compression cycle requires several orbits to move refrigerant gas from the low-pressure suction condition (at Position A) to the high-pressure discharge condition (at Position L)

6

path between the upper and lower scrolls in acompressor. A radially compliant compressorallows the orbiting scroll to follow a flexible paththat is defined by its contact with the stationaryscroll (much the same as a cam and follower). An“unloader” bushing installed between the orbitingscroll and the motor shaft absorbs variations inorbit radius created by machining and assemblydiscrepancies. Axial compliance refers to theability of the orbiting and stationary scrolls toseparate axially. In a non-compliant compressor,

the orbiting scroll follows a fixed path where theorbiting and the fixed scrolls never touch. Carrierand Danfoss refer to this as a controlled orbitdesign. The geometric relationship between thescrolls in a controlled orbit compressor is constantunder all operating conditions.

The decision to make a compressor with scrollsthat contact each other or scrolls in a contact-freecontrolled orbit follows the method used to seal thescrolls.

Sealing Techniques

Compressor performance is directly related tointernal leakage and mechanical losses. Eachcrescent-shaped pocket of refrigerant gas trappedbetween the scroll vanes naturally tries to find aplace of equilibrium. If the gas on one side of avane is at a higher pressure than the gas on theother side, the high-pressure gas will seek a path tothe low-pressure side. In a scroll compressor, thereare only two leakage paths: radial and axial.1

Figure 6 shows that radial leakage occurs betweenthe flanks of the scroll vanes where an advancinghigh-pressure crescent-shaped pocket attempts toleak back into the following pocket of lowerpressure gas. Axial leakage occurs between thescroll vane tip (the free involute scroll edge) andthe baseplate of the opposite scroll. Axial leakageis generally considered more critical than radialleakage (ASHRAE 2004).

Leakage increases power consumption, reducescompressor capacity, and diminishes efficiency.

Radial Sealing

Compliant compressors use contact between theorbiting and the fixed scrolls as the sealing mecha-nism. However, radially compliant compressorsmay not have uniformly effective sealing at all

AxialLeakage

RadialLeakage

Fig. 6. Radial leakage occurs between adjoiningflanks of the scroll vanes, and axial leakageoccurs between the vane tips and the baseof the opposite scroll

Fig. 5. Because the compression process iscontinuous, at any given time the scrollvanes contain pockets of low-, medium-,and high-pressure refrigerant gas

STATIONARYSCROLL (WHITE)

ORBITINGSCROLL (GRAY)

LOW-PRESSUREREFRIGERANT

HIGH-PRESSUREREFRIGERANT

MEDIUM-PRESSUREREFRIGERANT

HIGH-PRESSUREREFRIGERANT

DISCHARGE

1 As a comparison, rotary twin-screw compressors have atleast three leakage paths: a) axially between mesh of the maleand female lobes, b) radially between the lobe edges and thehousing, and c) between the rotor ends and the housing.

7

contact points when new. These designs require a“wear-in” period to equalize contact on all sur-faces. Contact makes compliance mandatory.

Controlled orbit compressors, in contrast, rely onan ultra-precise scroll profile to ensure scroll flanktightness. Computer-controlled machine toolscreate precise surface geometry, maintainingtolerances measured in microns (one micron is1x10-6 meters, or 0.000039 inches). The vaneflanks never touch. Tolerances are so precise that athin oil film seals the gap and provides a lubricat-ing surface for the orbiting scroll to pass over withno friction or wear. Since the controlled orbitconcept never allows mechanical contact betweenthe flanks, compliance is unnecessary and thecompressor maintains a fixed geometry over thelife of the scroll set.

Axial Sealing

Compliant designs depend on contact between thevane tips and the opposite baseplate. Axial flex-ibility is necessary to provide allowances forthermal growth and wear. Some manufacturers usegas pressure to load the stationary scroll against theorbiting scroll.

Controlled orbit compressors maintain dynamiccontact between the orbiting vane tips and thestationary baseplate with floating seals. Groovesmachined into the vane tips hold seal elements that“float” between the vane and the opposite base-plate as shown in Figure 7. Refrigerant gas pres-sure loads the seals against baseplate for properdynamic contact during operation. Contact forcesare very small, which, combined with reducedcontact surface area, substantially reduces frictionlosses and increases efficiency.

CONSTRUCTION AND OPERATION

Scroll compressors are fully hermetic. The scrollset, coupling, counter weights, motor and bearingsare assembled in a cylindrical, welded steel shell.Most scroll compressors for refrigeration and

HVAC service have a vertical orientation with thescroll sets mounted on the upper end of the motorshaft as shown in Figure 8 (on page 8).

Although there are variations in constructionbetween manufacturers, the fundamental featuresare similar. The descriptions below highlightfeatures of the Performer® controlled orbit scrollcompressor used in Carrier’s AquaSnap™ air-cooled chillers.

Shell

The Performer scroll compressor shell is a cylin-drical vessel, oriented vertically, and divided into alow-pressure and a high-pressure end. The largestvolume of the shell operates at the refrigerantsuction pressure and contains the motor, oil pump,and the moving components of the scroll set. Arelatively small high-pressure area lies above thecompressor’s stationary scroll and acts as a dis-charge muffler to reduce gas pulsation sound andvibration.

Cool refrigerant suction gas enters large suctionshell via the lower connection. Gas velocity dropssubstantially in the shell, allowing lubricant andany small amounts of liquid refrigerant to separatefrom the gas. In a Performer scroll compressor, allof the suction gas passes upward through the motoron its way to the scroll set. The small amount of

Fig. 7. Scroll compressors with controlled orbitdesigns use floating seals installed inmachined grooves in the vane tips

VANE TIPCLEARANCE

FLOATINGTIP SEAL

P1 P2

VANE

BASEPLATE OFOPPOSING SCROLL

HIGH- PRESSUREGAS LOADS SEAL

OIL FILM PREVENTSGAS LEAKAGEAROUND TIP SEAL

8

oil carried to the compressor as a mist entrained inthe refrigerant gas provides the necessary lubrica-tion for sealing the scroll vanes. Compressed gasdischarges through a check valve into the high-pressure dome and then exits the compressor shellthrough a discharge connection.

The lower portion of the shell serves as an oil andliquid reservoir. The Performer compressor’s highcapacity sump enables operation in systems withlong pipe runs and large refrigerant charges.Performer scroll compressors also use a centrifugalforce-driven oil pump that distributes lubricant tothe bearings and drive coupling through a diagonal

channel drilled in the motor shaft, as shown inFigure 9.

Motor and Crankshaft

A fully hermetic, 100% suction gas-cooled, squir-rel cage induction motor drives the compressor.Suction gas cooling prolongs motor life by ensur-ing the motor stays at a stable, low temperature.The motor also serves as a barrier between anyliquid refrigerant that may migrate through thesuction line to the compressor at shut down.Liquid refrigerant stays in the compressor shell.Upon startup, liquid refrigerant must pass throughthe motor before reaching the compression area of

Discharge check valve

Fixed scroll

Discharge port

Oldham coupling

Upper bearing, shaftand counterweight

Shielded motor 100%suction gas cooled

Lower bearing andcentrifugal oil pump

Discharge connection

Orbiting scroll

Multiple knock outconnecting box

Motor protection

Oil sight glass

Rigid base plate andvibration absorbingmounting assemblies

Suction connection

Reverse rotationprotection

Fig. 8. A fully-hermetic scroll compressor

9

the scroll set. The presence of liquid raises therefrigerant gas pressure drop through the motor.That, combined with heat from the motor, causesliquid refrigerant to evaporate before reaching thescrolls. Suction gas cooling also eliminates anyneed for external cooling.

The motor shaft, referred to as the crankshaft,transmits the rotary motion of the motor to theorbital motion of the lower scroll. The crankshaftalso carries the counterweights necessary to bal-ance the compressor mechanism. Two oil-lubri-cated sleeve bearings align the crankshaft—onebelow and one above the motor. The lower bear-ing is lightly loaded and the upper bearing carriesthe bulk of the compression load.

Scrolls

Scrolls are individually machined from carbonsteel blanks that are cast into the basic scroll form.High-speed, computer-controlled milling machines

produce the precise surface geometry necessary tocreate identical scrolls. Modern metalworkingequipment, using no-lag digital controls, producesthe necessary contour accuracy with a high qualitysurface finish (residual roughness Ra < 0.7 µm).

A hydrodynamic thrust bearing supports theorbiting scroll and resists the axial forces imposedby compressed gas between the scrolls. Properbearing design and lubricant selection are impor-tant elements in obtaining the best possible com-pressor efficiency.

The scroll design, including the involute heightand diameter proportions, is optimized for eachdifferent refrigerant. Geometric proportions of thescroll set increase uniformly for larger capacitycompressors. Currently, the practical capacitylimit for a single scroll compressor is 25 tons,although manufacturers are conducting research todevelop larger units.

Lower bearing

Suctionconnection

Centrifugal oilpump

Motor shaft

Scroll set

Upper bearing

Lower bearing

Upper bearing

Centrifugal forcedrives oil up

diagonal channelto bearings and

scroll set

Oil inlet

Motor shaft

Oil sump

Dischargeconnection

Mobile scrollbearing

To mobilescroll bearing

Fig. 9. Using a vertical orientation, the Performer® scroll compressor shell serves as alarge oil sump that gives it excellent liquid handling capability

10

Seals

Controlled orbit scroll compressors have seals atthe vane tips. There are two types of seals com-monly used. The first design uses multiple, nar-row, metallic strips installed with the flats laidside-to-side (like a laminate) as shown in Figure10. The second design uses a single graphiteelement set into the tip groove. In both cases, theseal element floats in the tip groove enabling sealeffectiveness to remain uniformly high undercontinuously changing pressure and temperature.

Compressor Protection

Scroll compressor protection is conceptuallystraightforward and not unlike that necessary forany other type of compressor. Fundamentally,scroll compressors must be protected against: over-pressurization, overheating, reverse rotation andslugging. Carrier combines the scroll compressorprotection features into a Scroll Protection Modulethat mounts in a housing on the side of the com-pressor shell.

High-Pressure Switch

High-pressure switches must be standardequipment for compressors meeting ULrequirements. In a system with multiplerefrigeration circuits, such as that in anair-cooled chiller, each circuit must havea separate high-pressure switch toprotect against over-pressurization. Theswitches must be wired in series so thata high-pressure incident stops the entiresystem.

High Temperature Limit

An internal temperature sensor protectsthe compressor from potentially damag-ing high temperatures. In Performerscroll compressors, the sensor mountsinternally so that it will be influenced byboth the motor temperature and thedischarge gas temperature. A rise in

temperature to 220°F (104°C) causes the high-limitswitch to initiate a compressor shutdown.

Discharge Check Valve

A check valve mounted on the compressor dis-charge connection (near the top of the compressorshell) prevents reverse rotation in the compressorat shut down. An external check valve has slightlylower pressure drop than an internal check valve,and provides improved protection againstbackflow.

When the compressor stops, high-pressure gastrapped in the crescent-shaped pockets betweenscrolls will leak back to the suction side. This letsthe compressor start unloaded, which reducesstarting current, torque and mechanical stresses.

Crankcase Heater

Refrigerant liquid may accumulate in the oil sumpof hermetic scroll compressors. To prevent slug-ging liquid refrigerant into the compressor duringstartup, manufacturers have traditionally used anelectric heater that energizes when the compressorstops. The heater warms the oil sump sufficiently

Fig. 10. Vane tip seals used in controlled orbit scrollcompressors have less contact area and less frictionthan compliant compressors where the vane tip and thebase of the opposite scroll are in contact

VANE TIPCLEARANCE

SINGLE ELEMENTGRAPHITE TIP SEAL

VANE (TYP.)

BASEPLATE OFOPPOSING SCROLL

PRESSURE LOADEDSEAL FLOATS INGROOVE TO ASSURECONTINUOUS DYNAMICCONTACT WITHMINIMAL FRICTION

SINGLE ELEMENTDESIGN

MULTIPLEELEMENT DESIGN

MULTI-STRIPTIP SEAL

11

to boil liquid refrigerant (not the oil) so that onlyrefrigerant gas is present during startup. OnPerformer scroll compressors, the heater mountsexternally on the shell bottom.

Other Means of Compressor Protection

Compressor protection is not always achievedinternally to the compressor. Carrier appliesadditional, external safeguards to assure safe,reliable compressor operation. For instance, inAquaSnap air-cooled chillers, the Scroll ProtectionModule (SPM) houses a “communicating” circuitboard that continually exchanges operating infor-mation with the chiller’s main control panel.Software elements provide the following compres-sor protection features:

• Single-Phase Protection. The chiller maincontrol panel continuously examines the three-phase power source entering the unit. Upon theloss of any one phase, the chiller controlsystem interrupts the power source to the unit.Loss of a phase in a three-phase system causesreverse rotation in the compressors.

• Startup Protection. Using information fromthe SPM(s), the chiller control system monitorscompressor suction and discharge pressure inthe first few seconds of startup. If the com-pressor fails to achieve a differential pressure(e.g., discharge pressure increases less than 10psig), or if the discharge pressure decreases,the compressor stops.

• Excessive Starts. The chiller cycles compres-sors on and off to achieve a defined leavingchilled water temperature set point. TheAquaSnap chiller control software uses anadaptive deadband to automatically increase ordecrease the deadband around the set point.This ensures that a compressor never startsmore than 12 times in any given hour. Exces-sive starts can cause overheating in the motor.

• Compressor Operating Parameters. Everyscroll compressor has defined operating param-eters, that is, acceptable combinations of

suction and discharge pressure (suction anddischarge temperature) in which it was de-signed to perform continuously and reliably.Operation outside the acceptable parameterscan damage the compressor. The AquaSnapchiller control system has the parameters,referred to as the operating map, programmedinto the unit software. The chiller controlsystem uses the operating map to stage com-pressors on and off within acceptable param-eters, and to continuously monitor overallrefrigerant circuit performance.

EFFICIENCY AND PERFORMANCE

There are two ways to think of compressor effi-ciency: a) the individual, thermodynamic effi-ciency of a compressor alone, or b) the efficiencyof the compressor as it performs in a system. Thefirst method is a useful measure for compressordesigners; the second has more meaning for build-ing owners and HVAC designers.

Compressor designers customize scroll compres-sors for each different refrigerant, isolating on thecompressor’s individual performance. Variationsin scroll geometry, shell design, oil selection andother features optimize the compressor-refrigerantcombination.

Chiller designers focus on the efficiency of theoverall system. Carrier’s Model 30RB AquaSnapair-cooled chiller operates with R-410A and hasfull-load EER values of 9.6 to 9.9 Btu/hr·W (1.25to 1.21 kW/ton), and IPLV (Integrated Part LoadValue—ARI 1998) of about 13.5 to 14.0 Btu/hr·W(0.89 to 0.86 kW/ton).

By comparison, similar chillers with screw com-pressors have somewhat better full load efficiency,but do not achieve part-load efficiency availablewith scroll compressors. Table A (on page 12)compares typical efficiencies for similarly sizedchillers with different compressor types operatingat the same conditions.

12

A share of the indicated efficiency difference isdue to the nature of part-load control with mul-tiple, smaller scroll compressors versus single,large screw compressors with a slide valve or liftvalves.

The capacity of a screw compressor depends on thecompressor displacement and the relative locationsof the discharge and suction ports. If the ports areat the extreme opposite ends of the screws (with nointermediate openings), the compressor operates atits full displacement capacity. Adjusting compres-sor displacement is the most commonly employedmethod of capacity control.

Lift valves adjust displacement in a finite numberof incremental steps. Lift valves are pluggedopenings at defined locations along one or bothrotor bores. When all the valves are closed, refrig-erant gas follows the normal line of compressionfrom the fixed suction port to the fixed discharge

port, and the compressor operates at full capacity.Opening the first lift valve (closest to the suctionport) shortens the effective lobe length and delaysthe start of compression until a point downstreamof the open valve. Compression cannot start untilrefrigerant gas is trapped between the meshedlobes and the compressor housing. Compressorcapacity depends on which lift valves (at whatlocations) and how many lift valves are open.

There are two types of slide valves used for screwcompressor capacity control. The first, used insmaller compressors, opens and closes fixed portswith essentially the same effect as lift valves. Thesecond, which is frequently used in large ma-chines, adjusts both the displacement, and thedischarge port size and location through a theoreti-cally infinite modulating range. In both cases,when the slide valve adjusts, it creates a radialopening that reduces compressor displacement.

Slide valves are slightly more efficient than liftvalves; however, both types of capacity controlinvolve inefficiencies. In contrast, multiple, stagedscroll compressors operating in a system have nocapacity control-related inefficiencies. When ascroll compressor is operating, the system capacityincreases by an incremental step. When the com-pressor stops, the system capacity drops by anincremental step and the energy flow associatedwith that compressor stops. Table B compares thepercent load and percent energy consumption ofsimilar systems with three different compressor

types. The first uses three scroll com-pressors with staged capacity control.The second and third systems use single,twin-screw compressors, one equippedwith lift valves and the other with a slidevalve for capacity control. The scrollcompressor advantage is apparent. Attwo-thirds load, the screw compressorsuse 9% to 27% more energy than thescroll machine, and at one-third load, thescrew machines consume 61% to 97%more than the scroll system.

Table A Comparison of Typical Chiller Efficiencies

Chiller Compressor Type

Full Load EER, Btu/hr·W

Part Load IPLV, Btu/hr·W

Scroll 9.6 – 9.9 13.5 – 14.0

Standard Rotary Twin-Screw 9.6 – 9.8 12.5 – 13.3

High-Efficiency Rotary Twin-Screw 10.0 – 10.5 12.7 – 13.8

Note: Efficiencies shown are typical of chillers that are available as of October 2004.

Table B Comparison of Typical Compressor Performance

Three Scroll Compressors

Single, Twin-Screw Compressor with

Lift Valves

Single, Twin-Screw Compressor with

Slide Valve Nominal Control

Step % Load %

Energy % Load % Energy % Load %

Energy

Full 100 100 100 100 100 100

2/3 67 67 70 85 67 73

1/3 33 33 45 65 33 53

Note: Performance information is at the same operating conditions and is typical for compressor that are available as of October 2004.

13

Fig. 11. Carrier Model 30RA AquaSnap air-cooledchiller, 9 to 55 tons

APPLICATIONS

Manufactured in a variety of sizes up to 25 tons,scroll compressors have found their way into avariety of refrigeration and HVAC applications. Inthe “refrigeration” category, scroll compressorshave been successfully used for: bulk milk cooling,truck transportation, marine containers and grocerydisplay cases. The residential and light commer-cial air-conditioning segment, a huge success storyfor scroll compressors, was one of the first HVACapplications to employ scroll compressors. Heavycommercial HVAC applications quickly followedsuit and employed scroll compressors in: unitary(rooftop) systems, heat pumps, water chillers forprocess and building cooling, and large splitsystem condensing units. The transportationindustry has also joined the scroll movement andhas successfully applied efficient and reliablescroll compressors to automotive air-conditioning.Scroll compressors are also commonly used forcompressed air and oil-less compressed air service.

Water chillers using scroll compressors havetraditionally been relatively small units offered insizes less than 125 tons. Carrier introduced theModel 30RA AquaSnap air-cooled scroll chiller,shown in Figure 11, in 2001 and produces units insizes up to 55 tons.

Carrier’s introduction of the Model 30RBAquaSnap chiller, shown in Figure 12, extends theenvelope for air-cooled scroll chillers in capacitiesbetween 58 tons to 285 tons. The smallest size hasthree nominal 20-ton compressors on two refrig-eration circuits (one 20-ton and one 40-ton circuit).At the opposite end of the size spectrum, the 285-ton chiller has 12 nominal 25-ton compressors onthree equally sized refrigeration circuits. AllModel 30RB sizes come with at least two indepen-dent refrigeration circuits, and the maximumnumber of compressors per circuit is four. Theindividual compressor sizes on a given circuit canbe mixed (e.g., a 10-ton paired with a 12.5-toncompressor) similar to that shown in Figure 13.

Fig. 12. Carrier Model 30RB AquaSnap air-cooledchiller, 58 to 285 tons

Fig. 13. Scroll compressors piped together on acommon refrigerant circuit

RELIABILITY

Scroll compressors have a successful history inHVAC applications. Acceptance has been quick,creating a demand for millions of units over thepast 20 years. Scroll compressors proved their

14

TABLE C Comparison of Advantages and Disadvantages

Scroll Compressors Rotary, Twin-Screw Compressors

Adva

ntag

es

• Excellent individual full-load and part-load efficiency

• Chillers operating with multiple compressors on common refrigeration circuits provide better part-load efficiency (IPLV) than chillers with a single large screw compressor and capacity controls

• Very few moving parts (three) • Proven reliability • A single compressor failure in a chiller with

multiple refrigeration circuits results in loss of capacity, but the chiller can remain in service

• Very quiet operation • Very low vibration • Continuous compression process with almost no

pulsation or vibration • Precise machining permits sealing vane flanks with

a thin film of oil • Non-compliant designs (where there is no contact

between the scrolls) have very low friction, which improves efficiency

• Excellent individual full-load and part-load efficiency

• Very few moving parts (three or more depending on capacity control method)

• Proven reliability • Continuous compression process with almost no

pulsation or vibration • Modulating capacity control between minimum

and maximum load • Very low vibration

Dis

adva

ntag

es • Compressor cannot be disassembled in field for

maintenance • Incremental capacity control on systems with

multiple compressors

• Higher noise level than scroll compressors • Requires oil flooding to seal compressor lobes • Requires exhaust silencer and oil separator • A single compressor failure in a chiller with only

one compressor results in a complete loss of cooling

reliability in that time to be as good or better thanother technologies. Since their introduction,millions of scroll compressors have seen success-ful service worldwide in food and grocery refrig-eration, truck transportation, marine containers,and residential and light commercial air-condition-ing.

SUMMARY

Carrier has brought the scroll compressors to adefining moment in its history. Continued researchand development of scroll technology has made it

possible to manufacture single units with capaci-ties up to 25 tons. Compressor sets with two, threeand four compressors makes it possible to success-fully apply scroll compressors in chillers with totalcapacities approaching 300 tons.

Scroll compressors have many distinctly appealingqualities. They are efficient, quiet, and reliable.However, their features cannot be called advan-tages or disadvantages unless compared to acompeting technology. For that reason, Table Csummarizes advantages and disadvantages ofscroll compressors as compared with features ofrotary twin-screw compressors.

15

REFERENCES

ARI. 1998. Standard 550/590, Water ChillingPackages Using the Vapor Compression Cycle.Air-Conditioning and Refrigeration Institute,Arlington, VA.

ASHRAE. 2004. 2004 ASHRAE Handbook–Heating, Ventilating and Air-Conditioning Systemsand Equipment. American Society of Heating,Refrigerating and Air-Conditioning Engineers,Inc., Atlanta, GA.

16

®

CARRIER CORPORATION

SYRACUSE, NEW YORK

Copyright 2004 Carrier Corporation 811-20065 Printed in U.S.A. 10-04