sistema com três compressore em paralelos

7/31/2019 sistema com trs compressore em paralelos

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Parallel Application 8RECIPROCATING COMPRESSORS

general this means that doublesuction risers with U-traps areimperative. See figure 3.2.

compressors running. It isimportant to ensure sufficientoil return under any load. In

3.1 Suction Line

The gas velocity in the commonsuction line between evaporatorsand the suction header must beminimum 4 m/s in horizontallines and minimum 8 m/s invertical lines (recommendation8 to 12 m/s). Gas velocities

greater than 12 m/s will createhigh noise levels and high suctionline pressure drop which willdecrease system capacity.The suction line diametershould be calculated accordingto the minimum refrigerantflow (at minimum evaporatingtemperature and maximumcondensing temperature).

The common suction line

when horizontal shall be slopingdown toward the suction header(0.5% slope, 5mm rise for every1m of run). If the suction linemust rise over 6 meters, multiplesuction risers must be createdevery 4 meters as shownin fig.3.1.

This is necessary to increasethe velocity of returning gas andtherefore insure proper oil return.

To avoid large quantities of oilaccumulation, create U-trap(s)as short as possible.

It is important to note thatstandard size suction rotolockvalves ordered through DanfossManeurop are designed forgeneral-purpose applications. Itis important to select rotolockvalve sizes according to the gasflow and not only according to

pipe size. Gas velocity in suctionlines of parallel systems may varyconsiderably due to fluctuationsin load and the number of

Figure 3.2 Double suction riser with U-traps

3. SUCTION AND DISCHARGE LINES

Figure 3.1 Suction line design

max. 4 m

max. 4 m8 to 12 m/s

U- trap,as short as possible

0.5 % slope4 m/S or more

0.5 % slope4 m/S or more

To pack


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Parallel Application9RECIPROCATING COMPRESSORS

Figure 3.4 Detail of suction line insidethe suction header

Figure 3.5 Construction of suction header above the compressor suctionconnections

3.2 Suction Header

The suction header must beplaced as close as possible tothe compressors.

The lines between the suctionheader and the compressorsuction connection must beflexible, i.e. equipped withvibration absorbers.

The suction lines fromthe header towards each

individual compressor must be

fitted into the suction header

and the end tube cut at an angleof 60 (see figures 3.4 & 3.5).This configuration will result inhigher gas velocity at pick uptube inlet and proper oil returnwhen the oil level in the suctionheader rises.The compressorsuction lines must always enterthe suction header on the topside.A recommended suction headerdesign is shown in figure 3.3.

To insure ideal pressure

equalization with an oil equalization

line system, the header must besymmetrical and the lines fromthe suction header to eachcompressor must be short &identical.These precautions arenot as critical when an oil levelregulator is used.

The following recommendationsare necessary for secureinstallation: The suction line "A" and

the suction header "B" must

A

B

Figure 3.3 Suction header configuration with header below the compressor suction connections.

Suction

60

2-3 mm 60 60


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be horizontal to avoid

unbalanced oil distribution. The gas velocity in the suction

header must be a maximumof 4 m/s.

The suction line and the suctionheader must be insulated tolimit the suction gas superheat.

Note: An alternative tothe construction of a suctionheader is to purchase a pre-made

multiple compressor suctionaccumulators.

3.3 Discharge Header

A properly constructeddischarge header will not allowliquid flood back or migration ofthe oil back to the compressor.Furthermore it must restrict hotdischarge gas from returning toswitch off compressors where it

will condense and cause liquidhammering at compressorstart up.

Therefore the following

construction details are ofimportance: Whenever possible, mount

the discharge header belowthe compressor dischargeconnections (see figure 3.6).The compressor discharge linesare below the compressorsand enter the discharge headerfrom above.This designeliminates the risk of liquidflood back. Check valves(or non return valves), suchas Danfoss NRV, locatedbetween the compressordischarge connection andthe discharge header mustbe installed to prevent liquidcondensation

In non-running compressors.

If it is impossible to mountthe discharge header below

the discharge connectionsa special construction isrequired.The key is to ensure

the discharge lines enter

the discharge header fromthe top. (See figure 3.7).

To avoid operational noisethe check valve dimensionmust always be selectedaccording to the compressorcapacity and not the pipe size.Contact your check valvesupplier for correct selection.

The discharge header crosssection must be the sum of eachdischarge tube cross section.In an effort to avoid oil trapsnever oversize the dischargeheader.

Figure 3.6 Discharge header below compressor discharge connections.

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w NRV Vibration absorber


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By starting only one compressorat a time the peak load on thepower supply will be reduced.

Parallel installations also offermany possibilities for efficientcapacity control.

For example, three compressorsof different capacity mounted inparallel offer 7 different ratedcapacities.The control systemshould be designed to matchthe cooling demand of the systemby staging compressors on andoff, thus reducing the amountof wasted energy in a system.

A secondary function ofthe control system is to ensurethe compressors have an equalnumber of running hours and

reduce the risk of refrigerantmigration in compressors stoppedover long periods of time.

One example of a compressorcontrol sequence:

First start: Start compressorsin order 1,2, 3 and also stop inorder 1, 2, 3

Second start: Startcompressors in order 2, 3, 1and also stop in order 2, 3, 1

Third start: Start compressorsin order 3,1, 2 and also stop inorder 3, 1, 2

Repeat sequence to balance eachcompressors number of runninghours. Recommendations forproper control:

There shall be no morethan 12 starts per hour percompressor.A higher numberreduces the service life ofthe compressor. It is necessary

to use an anti-short cycle timerin the control circuit to limitthe number of starts.

If a risk of long minimum loadoperation exists, it is recommendto install a timer which will forcerun the entire system at 100%capacity for 5 minutes every5 hours.This will insure properoil return.

4. COMPRESSOR CONTROLSEQUENCE

Figure 3.7 Discharge header above compressor discharge connections.

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NRV Vibration absorber


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Typical system requirements andrecommendations for parallelinstallations are listed below:

5.1 Suction lineaccumulator

For parallel installations, it isrecommended to always usea suction line accumulator. Be sure

to select the proper size suctionline accumulator according tomanufacturer recommendations.In systems with an oil levelregulator it is best to use eitherone suction line accumulatorper compressor or a multi-lineaccumulator having a connectionfor each compressor.

5.2 Suction filter

It is strongly recommendedto install a large suction filterjust before the suction headeror suction accumulator.This willfilter all foreign particles outof the installation to protectthe compressors. Danfoss DCRfilters with interchangeable strainertype 48-F are recommended.These filter shells provide the

possibility to install filter driercartridges (type 48-DN), or burnout cartridges (type 48-DA),while allowing easy after salesservice.

5.3 Crankcase heater

Compressors must be installed

with continuously energizedself-regulating crankcase heaters(PTC elements).These heatersare to be installed with heatconductive paste.The use ofan additional belt crankcase heatermay be necessary in some cases.That belt crankcase heatersmust be switched on only whenthe compressor is not running.

The compressors should be

installed in a room of moderatetemperature to avoid refrigerantcondensation in non-runningcompressors.

References of crankcase heatersare given in the selection andApplication Guidelines"for MT/MTZ or LTZcompressors. Contactyour local Danfoss Maneurop

organization for furtherinformation.

5.4 ThermostaticExpansion Valve

When the parallel installationis serving a single evaporatorsystem the dimensioning ofthe thermostatic expansion valve

(TEV) becomes critical and mustbe made in relation to bothminimum and maximum capacity.This will ensure correct superheatcontrol in all situations.

5.5 Condensing pressurecontrol

A condensing pressure controlshould be installed to maintainthe minimum condensing pressure

according to the applicationrange published for eachcompressor / refrigerantcombination.This condensingpressure control will also allowfor an ideal liquid distribution inthe system (expansion valveoperation).

5. SYSTEM COMPONENTS


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Installation and serviceprocedures for a parallel systemare similar to basic systeminstallations.The selection ofadditional system componentsfor parallel installations followsthe basic system common rules.Therefore refer to the "Selectionand Application Guidelines" of

the compressor for detailedinstallation and serviceprocedures.

6.1 Compressor mounting

Individual compressors mustalways be mounted usingthe provided mountinggrommets. If this is not donethe system will transmitvibration and in turn reduce

compressor life.A common baseframe, rigid enough to supportthe weight of the compressors,must be used for installation.The common frame may bemounted on grommets to reducetransmission of vibration tothe floor. It is recommended toinstall all control and safetydevices on an independentframe.These devices shouldbe connected to the commonframe using flexible tubing.

Suction and discharge lines musthave adequate 3 dimensionalflexibility. For parallel systemsthe simplest means of acquiringthis is by the use of vibrationabsorbers.

The connecting suction anddischarge header pipes must beas short as possible.Therefore it

is a benefit to mount compressorsas close together as possible.Allcompressors must be mountedon the same level.

6.2 Rotation direction

Compressors should run withthe same rotation direction.This can be achieved by havingthe same phase sequence oneach compressor motor

terminal (L1-T1, L2-T2, L3-T3).

6.3 Total capacity andapplication envelope

The total cooling capacity ofa parallel installation is slightlyless that the sum of the coolingcapacities of the individualcompressors.This is due tothe extra pressure drop in suctionand discharge headers. In generalthis loss is about 1% of the total

capacity.The application envelopeof a parallel installation isthe same as for the individualcompressors.

6.4 Pressure switch settings

The pump down pressure switch(for example Danfoss type KP1)must have a set point slightly

higher than the lowestcompressor safety pressureswitch set point.The high-pressure safety switch shallstop all compressors.

6.5 Failure analysis

When one compressor ina parallel system fails the chanceof foreign particles entering othercompressors is greatly increased.

Therefore a failure analysis mustbe done quickly to insure furtherproper running conditions forthe overall installation (i.e.: oilanalysis).

6. INSTALLATION & SERVICE


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Figure 7.2 Oil equalization connection, 3/8" flare connection Figure 7.3 Former rotolockwith adaptor accessory.

Figure 7.1 Correct oil levelin sight glass

compressors (until 1988) had a1"1/4 rotolock connection forthis purpose (see figure 7.3).

For compressor replacement ina parallel system withcompressors fitted with thisrotolock connection, an adapteris available through DanfossManeurop (ref. 7754010).

The position of the 3/8 flare oilequalization fitting has recentlybeen modified so that equalizationonly occurs when the oil level inthe compressor reaches themaximum.

Further, to prevent oil drainagefrom the non-running compressorsduring part load operation,

the flare connector protrudesinto the compressor shell(see figure 7.2).

FSA adapter (023 U 8014) isavailable to convert the flareconnection to a brazedconnection design.

All Maneurop reciprocatingcompressors MT, MTZ, LT &LTZ are available in VE version,specially designed for parallelinstallation.

These compressors are providedwith an oil equalization connectionand a threaded oil sight glass(see figure 7.1).

The standard compressors arenot suitable for parallel mounting.

7.1 Oil level regulator andoil sight glass

An oil level regulator can bemounted on the compressor oilsight glass connection

7.2 Oil equalizationconnection

A 3/8" oil equalization line canbe connected to the 3/8 flareconnection (see figure 7.2).

Recommended torque is 30Nm. Note that older Maneurop

7. MANEUROP COMPRESSOR

CHARACTERISTICS

maxi

mini

15.9 0.1

Compressorshell

Oilsump

45

7.9

13.5

5/8-18UNF


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GA

045-

10/00-

GB

LawrencevilleGeorgia - USA

TrvouxFrance

The Danfoss Maneuropfacilities

AnseFrance

Danfoss ManeuropCommercial Compressors

BP 331 F- 01603 Trvoux FranceTl. 04 74 00 28 29 - (33) 4 74 00 28 29Fax 04 74 00 52 44 - (33) 4 74 00 52 44

Every effort has been made to provide accurate descriptions and data.However, due to the co ntinuing process of product improvement and evolvement, all information in this brochure is subject to change without notification.Maneurop is a registered trademark of Danfoss.

sistema com três compressore em paralelos

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