Refrigeration Scroll For Parallel Applications

Application G

uideline

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Scroll Compressors in Parallel for Refrigeration Applications

1. Introduction

The concept of parallel operation for refrigeration scroll compressors brings several benefits:Efficient Capacity Control: If the load or ambient conditions change, compressors can be switched on and off to match capacityrequirements. This is more efficient than running larger semi-hermetic compressors at part load andalso will generally give a better power factor. Use of unequal compressors allows many capacitypossibilities.Redundancy: If one compressor cannot run for any reason, the load can often be met by the remainingcompressors.Ease of Service: If a compressor needs to be changed for any reason, the small, lightweight nature of the scrollmakes this a much less costly operation and causes much less disruption. This is particularly truefor the 3 - 6 hp models.Matching Several Evaporating Conditions: If there are two or more temperatures to be controlled, individual compressor(s) can be matched toeach evaporator while working on a single condenser circuit, and this will give power savingscompared with an alternative system working at one (the lowest) suction pressure.Parallel operation of compressors is not new, but scroll compressors have no positive displacementoil pump, and therefore no oil pressure switch. The high performance Teflon bearings provideprotection for short periods in case of oil losses, but sustained running at high-pressure differenceswithout lubricant will fail the bearings. In order to ensure adequate protection it is necessary toobserve the following guidelines.

2. Definitions

Oil Equalisation Line - Tubing connecting compressor sumps at or below oil level.Regulator - A device to meter oil into a compressor sump, and control the oil level within limits.Oil Charge Supplied - Oil quantity supplied by Copeland when compressor is shipped.Maximum Oil Quantity - Maximum amount of oil recommended. Above this amount the oil level will be ab-ove the sight glass and could be too high affecting power consumption.Minimum Oil Quantity - Minimum amount of oil recommended. Below this amount lubrication may be in-sufficient and so the life of the compressor may be shortened.Oil Recharge Quantity - Amount of oil to refill compressor after drain the oil.

3. Model Range

Model Name Oil Charge Supplied (l) Oil Recharge Quantity

ZF 06/ ZS 15 1,0 1,0

ZF 08/ ZS 19/ZB 19 (2 HP) 1,0 1,0

ZF 09/ ZS 21/ZB 21 (3 HP) 1,24 1,12

ZF 11/ ZS 26/ZB 26 (3,5 HP) 1,24 1,12

ZF 13/ ZS 30/ZB 30 (4HP) 1,95 1,83

ZF 15/ ZS 38/ZB 38 (5 HP) 1,95 1,83

ZF 18/ ZS 45/ZB 45 (6 HP) 1,77 1,66

ZF 24/ ZS 56 (7,5 HP) 4,14 4,05

ZF 33/ ZS 75 (10 HP) 4,14 4,05

ZF 40/ ZS 92 (13HP) 4,14 4,05

ZF 48/ ZS 11 M (15 HP) 4,14 4,05Note: Air conditioning type applications may use ZR models in parallel. Due to operating envelope limitations ZRmodels are not normally suitable in Refrigeration applications.

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4. Parallel Application Considerations

If compressors are required to operate in parallel it is normally necessary to stop and startthem independently for capacity control purposes. There are three major considerations inthis type of application:

Oil control.

An adequate oil level at all times is necessary to maintain lubrication of the bearings. Toomuch oil can result in inefficient operation and excessive oil carry over to the system.

Tubing Stress.

If compressors are mounted close together, care must be taken to ensure that there issufficient flexibility in the tubing. If this is not done the starting pulse may result in excessstress at the mounting positions and eventually give rise to a leak. Also tube resonanceshould be avoided.

Running sequence.

A certain logical sequence control is recommended so that each compressor runs for thesame amount of time.

5. Oil Control using RegulatorsIndividual oil control for each compressor is the safest method. Each compressor can be fittedwith a regulator which supplies oil only when required. Oil carried over to the system isreplaced via the regulator when insufficient oil is being returned from the system. A reservoirof oil is provided, and hence variations in oil quantity in the system can be accommodated.There are many refrigeration scroll compressors operating today

with regulators and this method is recommended by Copeland provided that certain designpoints are observed. Because the scroll compressor has no positive displacement oil pumpand no oil pressure switch, it is recommended that the oil control system incorporatesprotection as detailed below. It will be necessary to add oil to the system and the Copelandapproved lubricants are:

HFC-refrigerants: Mobil EAL Arctic 22 CC

ICI Emkarate RL 32 CF

HCFC-refrigerants: Suniso 3GS

5.1 Low Pressure Oil ReservoirThis is the traditional method used also in many piston compressor systems. The oil reservoir ismaintained at a pressure slightly above the compressor sump pressure and this limits the amount ofrefrigerant dissolved in the oil in the reservoir. The pressure drop is low when the oil enters thecompressor and the amount of flash gas formed in the sump is small.

Float switches or other devices can be used as regulators. It is recommended that the regulator has anelectrical output which can be wired into the control circuit and stop the compressor if the oil level fallsbelow the minimum level and remains below the minimum level for a period of time (maximum 2minutes). This protects against failure of the oil supply to the individual compressor. A level switch in thereservoir will only protect against insufficient oil in the receiver, but not against loss of oil supply to anindividual compressor or against a fault on an individual regulator.

Regulators currently on the market, which meet these requirements, include AC&R S9040 and the ALCOOMA Traxoil.

When using a regulator care must be taken to set the oil level in the upper half of the sight glass. If anadapter is used to connect the regulator this may have a smaller internal diameter than the glass and thiscould give rise to a false oil level reading.

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ALCO OMA Traxoil

When commissioning these systems it is important to allow sufficient time for equilibriumrunning conditions to be attained. Because the oil carry over rate from the scroll compressorsis low it may take some time for stable oil quantities to build up in the coolers. Until this hashappened the total oil requirement of the system cannot be determined.

5.2 High Pressure Oil ReservoirThe need for a separate oil receiver may be avoidein this case the oil will be stored at discharge pressdisturbance and foaming when it enters the comprethe quantity of oil entering the sump when the valvesuitable for this type of application and has been prpressure oil supply.

6. Oil Equalisation LinesMuch simpler systems, which link the compressocontrol are obviously attractive. They are quite crefrigeration additional considerations apply. Thecompressors working at a common suction cond

If there are large variations in oil quantity in the sthis may result in too much or too little oil in the cchecking oil level is via he sight glass connectionis not possible to determine if the maximum oil qin the sight glass, there is a danger of operation

A tube connecting the oil level adjustment valvesbecause when a compressor is stopped, the prethe running compressor(s). The adjustment valvtherefore the stationary compressor will show notogether at all times, there will be small pressure

Alternative methods using the sight glass connebeen shown to work on specific systems. Notes but because of wide variations in system designcannot be given general approval by Copeland. tem type.

6.1 Gas Pressure Equalisation with ConneGenerously sized suction lines will minimise the floeliminate it. Use of a large diameter oil header tend

Model S1Oil Level Control

Functions: Oil fill Alarm Compressor Lockout

YesYesYes

Set point ½ Sight glassSwitch/Relay release time 120 secReset mode/Time delay Auto/ 13 secOil connection ¼“ flareMOPD psi/bar 300/ 20,7Max. working pressurepsi/bar

400/ 27,6

Contact ratings 0,4 A, 125 V0,2 A, 250 V

Solenoid Coil type/Watts MCK-1/10W

d if a combined separator/receiver is used, buture. It will therefore cause much moressor sump. For this reason it is desirable to limit opens. The ALCO OMA Traxoil (see picture) isoven to operate satisfactorily with a high

r sumps via tubes together but provide noommon on air conditioning applications, but iny can, of course, only be considered forition.

ystem due to changing conditions or defrostompressors. Usually the only means of. Once the oil level is above the sight glass ituantity is reached. Likewise if no oil is visiblebelow the minimum quantity.

on the compressors is not adequatessure in the shell rises and oil will transfer toe is located below sight glass level and oil level. Even if all compressors run differences which will cause the same effect.

ctions for oil equalisation connections haveare given below on some possible methods, and operating conditions, these methodsThe user has to verify operation in each sys-

cting Tube at Sight Glass Positionw along the oil connecting tube but will not to reduce oil transfer. Certain gas pressure

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equalisation designs have been shown to be satisfactory but each one has to be tested andproven.

6.2 Refrigeration Scroll Paralleling Applications w/o Active Oil Control

The reduction of costs as a major concern for thedesign engineers led Copeland to perform some testson parallel refrigeration equipment not utilizing activeoil controls.To determine, which is an acceptable tubingconfiguration for proper oil management enabling safecompressor operation over the entire envelope was thechallenge for these tests. Only equal combinations(same size, same family) of two and threecompressors in parallel were chosen and areconsidered here. Refrigerants R404A and R507 wereused. Other configurations than those as tested inCopeland’s laboratories might work satisfactory as well

but consultation with the Application Engineering Department is recommended to get an approvalfor different arrangements than those described following.

Test Model Range

6.3 Suction Line ConfigurationUse an adequately sized suction header providing equal distribution of returning refrigerantand oil to each individual compressor. It was found out that the non-symmetrical design asshown below is acceptable and does not create oil level problems between the compressors.

Comp 1 Comp 2 Comp 3

Suction Header

From Evaporator

Oil Return from Separator

6.4 Oil Equalization Line

An oil equalization line between the two or three compressors could be fitted to balance theoil levels between the compressors. Adaptors (Ident-No. 8538307) using the sight glassfittings plus a tube size of at least 7/8” (22 mm- 19 mm inner diameter) built the components.

ZS15K4E ZB15KCE ZF06K4EZS19K4E ZB19KCE ZF08K4EZS21K4E ZB21KCE ZF09K4EZS26K4E ZB26KCE ZF11K4EZS30K4E ZB30KCE ZF13K4EZS38K4E ZB38KCE ZF15K4EZS45K4E ZB45KCE ZF18K4E

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From the System

Suction Lines to Compressors From the System

Suction Lines to Compressors

Recommended !

From the System

Suction Lines to Compressors

Not Recommended !

This compressor would pick up too much oil !

Adaptor for Oil Sight Glass Connection

Multiple tests have shown that smaller tube diameters do not match proper oil balancing. Forthe overall system reliability an oil separator should be used and the oil return line from theseparator should be connected to the oil equalization line.

If no oil separator is installed the oil returns into the suction header and is picked up therefrom the individual compressor suction lines. However, during the tests no advantages werefound of the system with oil separator versus suction header arrangement.

7. Oil Return to Running CompressorsIf oil returns to the compressor at approximately the same rate as it leaves, then the oil levelwill be maintained. It is always preferable to design the suction manifold in such a way that oilreturning with the suction gas can only enter the running compressor(s). This can be done ina number of ways. Perhaps the most common is to use a suction header with vertical pick upsto each compressor which induce sufficient velocity to lift the oil. Again, certain designs havebeen shown to be satisfactory but each one has to be tested proven. With some systemdesigns this is sufficient to ensure adequate oil level at all times, but there is no fail safepressure switch if one compressor loses oil.

3/4“ - 14 NPTF1 1/4“ –12 UNF

7/8“

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8. Tubing and Mounting Considerations8.1 Starting PulseThe standard mount for the scroll compressor is a soft rubber mount. This is designed to transmitthe lowest possible disturbance to the mounting frame. Because of the flexibility of this mount, it isessential that tubing to each compressor is designed to accommodate the movement which willoccur, particularly on start up.

The three phase motors used in the refrigeration scroll compressors exhibit a high starting torquecharacteristic. The

reaction of the stator is directly on the shell because there are no internal spring mounts. Thisreaction causes easily observable movement of the shell when the standard mounts are used andthis is normal. However, if the suction and discharge tubing of the compressor is closely clamped tothe frame or linked to another compressor, the tubing will restrain the movement - the motorreaction force will be taken by the tubing and may cause excessive stress and premature failure oftubing joints.

Care should be taken to avoid this situation in parallel arrangements of compressors where it isdesired to minimise the space between them (see also 8.3.).

8.2 ResonanceThe discharge pulse can in some configurations give rise to resonance of tubing. It isdesirable to avoid a natural frequency of between 45 and 55 Hz for the section of tubingbetween the compressor and the first clamp. Often it is not possible to determine this inadvance, but in the unlikely event of a resonance problem arising, a change in tubeconfiguration will usually resolve it.

8.3. Recommendations for Tubing and MountingAlways allow some flexibility in

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ANW.5.04.01

ANW.5.04.00

Mounting Parts for Multiple Operation

ANW.5.02.01

ANW.5.04.00

7,5-15 HP3-6 HP

Hard Rubber Spacer Steel Spacer

the tubing to the suction anddischarge connections. If thestandard mounts are used thereshould be at least two bends anda vertical section before the tubejoins a header. Test the finishedassembly by rocking thecompressor on its mounts.Vibration eliminators may beused but are not essential if thetubing has sufficient flexibility.Vibration eliminators should befitted in the vertical plane

An alternative harder mount asdetailed on the left is recommen-ded. This will allow more loads tobe transmitted via the feet, andthe very small additional vibrationtransmission is not usually aproblem in refrigerationapplications. It is still desirable toincorporate a vertical section oftube between the compressorand the first rigid mount. This willgive flexibility and minimise therisk of resonance.

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9. Additional Design PointsThe topics described in the Application Guidelines are applicable. The following additionalpoints are for parallel installations.

9.1. Check valves and Pump DownPump down is not normally used or necessary for parallel Scroll installations. All Scroll com-pressors are fitted with an internal check valve, and ZF/ZS models have a dynamic dischargevalve also. Therefore external check valves are not needed.

9.2. Heat Exchangers for Economised Scrolls (Vapour Injection)A single heat exchanger may be used in multiple compressor installations. A plate heat ex-changer with a single circuit on the liquid side, and multiple circuits on the vapour side is agood solution (see below). It is essential that solenoid valves are fitted to ensure that refriger-ant is not fed to the injection port of a stationary compressor.

From Condenser

Ifacoth

From Condenser

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Filter

Heat Exchanger

Solenoid ValvesTo Evaporator

TEV

Compressor 1 Compressor 2Injection Port

Filter

Heat Exchanger

Solenoid Valves

To Evaporator

Cap Tube Cap Tube

Compressor 1

Compressor 2Injection Port

a single circuit heat exchanger is used, the solenoid valves must be fitted in the vapour liness shown in the next diagram. A TEV can be used to regulate the injection. Depending on thenfiguration, the operational envelope may be restricted due to higher vapour temperaturesan with the capillary tube. Discharge temperature thermostats should always be fitted.