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Commercial Compressors

RECIPROCATING COMPRESSORS

Parallel Application Guidelines

2Parallel ApplicationRECIPROCATING COMPRESSORS

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . 3

2. OIL EQUALIZATION . . . . . . . . . . . . . . . . . . . 3

2.1 Oil equalization line system . . . . . . . 42.1.1 Individual oil separator . . . . . . . . . . 52.1.2 Common oil separator . . . . . . . . . . 52.2 Oil level regulator system . . . . . . . . 62.2.1 Individual oil level regulators

with individual oil separators . . . . . . 62.2.2 Individual oil level regulators

with common oil separator . . . . . . . 7

3. SUCTION AND DISCHARGE LINES . . . . . 8

3.1 Suction line . . . . . . . . . . . . . . . . . . . . 83.2 Suction header . . . . . . . . . . . . . . . . . 93.3 Discharge header . . . . . . . . . . . . . . 10

4 COMPRESSOR CONTROL SEQUENCE . 11

5. SYSTEM COMPONENTS . . . . . . . . . . . . . . 12

5.1 Suction line accumulator . . . . . . . . 125.2 Suction filter . . . . . . . . . . . . . . . . . . 125.3 Crankcase heater . . . . . . . . . . . . . . 125.4 Thermostatic expansion valve . . . . 125.5 Condensing pressure control . . . . 12

6. INSTALLATION AND SERVICE . . . . . . . . . 13

6.1 Compressor mounting . . . . . . . . . . 136.2 Rotation direction . . . . . . . . . . . . . 136.3 Total capacity and

application envelope . . . . . . . . . . . . 136.4 Pressure switch settings . . . . . . . . . 136.5 Failure analysis . . . . . . . . . . . . . . . . 13

7. MANEUROP COMPRESSORCHARACTERISTICS . . . . . . . . . . . . . . . . . . .14

7.1 Oil level regulator and oil sight glass . . . . . . . . . . . . . . 14

7.2 Oil equalization connection . . . . . . 14

Oil equalization is one of the most critical aspects inparallel installations. Oil returningfrom the system must becontrolled otherwise improperoil amounts will flow to anddamage compressors.Therefore,to achieve equal oil distributionbetween all compressors, the twomost popular solutions are:• Oil equalization line system.

This economical systemfor oil equalization connects the compressors crankcases’via an oil line.The oilequalization line system shouldbe used for installations of nomore than 3 compressors ofthe same size / capacity.

• Oil level regulator system.This more complex systemprovides stable, predictable oil

management under nearly allcircumstances.An oil levelregulator is mounted on eachindividual compressor and is fed oil via an oil separator in the discharge gas line.The oillevel regulator system shouldautomatically be used forinstallations of 4 or morecompressors.

3 Parallel ApplicationRECIPROCATING COMPRESSORS

A parallel compressor installationrefers to a system of compressorsinterconnected and workingtogether.Though several benefitsexist for parallel installation,the primary reason is reducedoperating cost through greatercontrol of capacity and powerconsumption.This is achieved bystaggering compressor switch-onsequences that allow the parallelsystem to match its power with the capacity needed.The following sections willprovide detailed information for installation and operation ofparallel compressor installations:

• 2: Oil Equalization systemsand their particularities.

• 3: Suction and Discharge Linesof parallel compressors.

• 4: Compressor SwitchSequence of compressors in parallel.

• 5: Additional informationabout System Components.

• 6: Installation and Servicerequirements.

• 7: Maneurop CompressorCharacteristics.

The number of compressors in a parallel system along withspecific operational needs variesgreatly from operation tooperation.

That is why the informationcontained in this leaflet providesa generalized overview only. Formore specific details pertainingto individual needs please contactyour local Danfoss applicationengineering support team.

Note:

To ensure proper parallelinstallation and runningconditions, the following

recommendations must befollowed:

• It is essential to respect allinstructions given in thisdocument, the installationinstruction leaflet deliveredwith every compressor, andthe compressor’s Selection &Application Guidelines.

• This application guidelineconcerns single stage parallelsystems for reciprocatingcompressors only. It is notvalid for two-stage or cascadesystems (or so-called boostersystems) which require morespecific installationprecautions.

• For additional systemcomponents (i.e. oil levelregulator, oil separator, etc.),the supplier recommendationsmust always be respected.

1. INTRODUCTION

2. OIL EQUALIZATION

Parallel Application 4RECIPROCATING COMPRESSORS

2.1 Oil equalization linesystem

This system insures oil supplyand oil equalization in allcompressor crankcases throughthe oil equalization connections.

For this system it is importantthat the crankcase pressure(suction pressure) in allcompressors is equal. If this is not the case uneven oildistribution will occur. If noprecautions are taken,the internal suction pressure of

is used, cold suction gas isallowed to circulate just abovethe oil level.This cold suctiongas will condense inside idlecompressors and in timedangerous amounts of liquidrefrigerant will build up.

• The oil sight glass must neverbe used in place of the oilequalization connection.

• Only ball valves are allowed in the oil equalization lines to

provide compressor isolationcapability.The use of othervalves, such as rotolock valves,may create undesirablepressure drops.

In parallel installations an oilseparator is stronglyrecommended. In parallelinstallations with either longpipelines, multiple condensersor multiple evaporators an oilseparator is required. In

an installation having multiple oil traps, an oil separator, or the possibility of slow oil returnan oil reservoir is often required.

The two most common methodsfor oil separation, which will be explained in detail, are the following:• Individual oil separator• Common oil separator

parallel compressors may vary.These variations, though small,can create considerabledifferences in oil levels. Example,a pressure difference of 0.001bar causes an oil level differenceof 1.1 cm.

The following points areguidelines for proper installationof oil line equalization systems.Please refer to figure 2.1:

• The oil equalization line mustbe installed in the horizontalplane.

• The oil equalization lines shouldnever rise above the horizontalplane of the oil equalizationconnections.

• Oil equalization lines betweenthe compressors must beflexible.The use of vibrationabsorbers may be required insome cases.

• The oil equalization linediameter must be 3/8’’. If a smaller oil equalization line isused, oil flow will be restricted.If a larger oil equalization line

Figure 2.1 Example of correctly mounted oil equalization line system.Note all compressors are of equal size/capacity and mounted on the same level.

Parallel Application5RECIPROCATING COMPRESSORS

Figure 2.2 Individual oil separators.

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Oil separator

NRV Shut offvalve Sight glass Vibration absorber

2.1.1 Individual oil separator

The individual oil separatormethod allows for eachcompressor to have its own oil separator. These separatorsreturn oil directly to eachcompressor’s oil equalisationport (see figure 2.2).

Note: The size of the dischargeline between compressor andoil separator must never besmaller than the rotolockdischarge port.

Remark: If an oil separatormalfunction occurs, oil

distribution and balanced oillevels will be strongly affected.

2.1.2 Common oil separator

The common oil separatormethod allows for one oilseparator to be used by the entire system.The oil is fed into the compressors via the suction line, though it shouldbe noted that the oil returntube must intercept the suctionline at least one meter beforethe suction header (see figure 2.3page 6). If oil separator floatingvalve leakage occurs, a hot gas

bypass can be avoided by theuse of an oil injection sequence.This injection sequence, througha normally closed solenoid valvelocated in the oil return line, isrecommended 5 seconds forevery 10 minutes of run time.

In either the individual oil separatormethod or the common oilseparator method, be sure toadd an additional amount of oilfor reliable operation.The amountof extra oil needed is indicatedin the oil separator’s instructions.It is also good practice to fit anoil filter in the oil return line.

Parallel Application 6RECIPROCATING COMPRESSORS

2.2 Oil level regulatorsystem

If a parallel system consists of 4 or more compressors,compressors of differentsize/capacity, or if the quantity of oil return from the system isin question, oil level regulatorsin combination with an oilseparator must be used. Underthese conditions the crankcasepressure difference will notinfluence the oil equalizationlevel.

The oil level regulator must bemounted on the compressor

oil sight glass connection. Oil istrapped in the oil separator.From the oil separator it flowsto the oil reservoir. From the oilreservoir it flows to the oilregulator before entering thecompressor. For proper flow,the oil reservoir must maintain a higher pressure than the suctionpressure (standard difference is1.4 bar).

The two most commonmethods of oil level regulators,which will be explained in detail,are the following:• Individual oil level regulators

with individual oil separators• Individual oil level regulators

with common oil separator

2.2.1 Individual oil levelregulators withindividual oilseparators

This method introduces one oilseparator and one oil levelregulator for every compressor.The oil separator feeds the oillevel regulator that in turn feedsthe compressor. Refer to §2.1.1for information regardingindividual oil separators.

Figure 2.3 Common oil separator▼ ▼ ▼

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To the suction header

Suction tube

From the oil separator

Suction tube

To the compressor

Discharge gas

Oil separator

Oil return

NRV Vibration absorber

Parallel Application7RECIPROCATING COMPRESSORS

Figure 2.4 Individual oil level regulators with common oil separator

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Oil receiver

To suction header

Pressure differential valve

From oilseparator

Shut off valve Sight glass Oil level regulator Filter

2.2.2 Individual oil levelregulators withcommon oil separator

This method introduces one oilseparator for the system andone oil level regulator for eachcompressor.The common oilseparator feeds the individual oillevel regulators that in turn feedthe compressors. Oil separatorselection must be made

regarding the capacity range ofthe parallel system to ensure a proper oil separation at bothpart load and full load. See figure2.4 for a detailed drawing of a correctly assembled exampleof this system.

Always be sure your selectedmethod is in accordance withthe manufacturerrecommendations.

When the parallel system isinstalled in cold ambient the oilseparator must be insulated toavoid efficiency losses andrefrigerant liquid condensationduring off cycle periods.A checkvalve (Danfoss NRV) located inthe discharge line after the oilseparator can reduce such a risk.In some situations, separatorheating may be required.