3.suction and discharge lines -...

9
Parallel Application 8 RECIPROCATING COMPRESSORS general this means that double suction risers with U-traps are imperative. See figure 3.2. compressors running. It is important to ensure sufficient oil return under any load. In 3.1 Suction Line The gas velocity in the common suction line between evaporators and the suction header must be minimum 4 m/s in horizontal lines and minimum 8 m/s in vertical lines (recommendation 8 to 12 m/s). Gas velocities greater than 12 m/s will create high noise levels and high suction line pressure drop which will decrease system capacity. The suction line diameter should be calculated according to the minimum refrigerant flow (at minimum evaporating temperature and maximum condensing temperature). The common suction line when horizontal shall be sloping down toward the suction header (0.5% slope, 5mm rise for every 1m of run). If the suction line must rise over 6 meters, multiple suction risers must be created every 4 meters as shown in fig.3.1. This is necessary to increase the velocity of returning gas and therefore insure proper oil return. To avoid large quantities of oil accumulation, create U-trap(s) as short as possible. It is important to note that standard size suction rotolock valves ordered through Danfoss Maneurop are designed for general-purpose applications. It is important to select rotolock valve sizes according to the gas flow and not only according to pipe size. Gas velocity in suction lines of parallel systems may vary considerably due to fluctuations in 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 m 8 to 12 m/s U- trap, as short as possible 0.5 % slope 4 m/S or more 0.5 % slope 4 m/S or more To pack

Upload: ngothuan

Post on 30-Jan-2018

243 views

Category:

Documents


4 download

TRANSCRIPT

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 velocitiesgreater 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 refrigerant flow (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 shown in fig.3.1.

This is necessary to increase the 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 topipe 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

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 from the header towards eachindividual compressor must be

fitted into the suction headerand 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 equalizationline 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 °

Parallel Application 10RECIPROCATING COMPRESSORS

be horizontal to avoidunbalanced oil distribution.

• The gas velocity in the suctionheader must be a maximum of 4 m/s.

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

Note: An alternative to the construction of a suctionheader is to purchase a pre-mademultiple 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 itwill condense and cause liquidhammering at compressor start up.

Therefore the followingconstruction 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), such as Danfoss’ NRV, locatedbetween the compressordischarge connection and the discharge header must be installed to prevent liquidcondensation

In non-running compressors.

• If it is impossible to mount the discharge header belowthe discharge connections a special construction isrequired.The key is to ensure

the discharge lines enter the discharge header from the top. (See figure 3.7).

• To avoid operational noise the 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.

▼ ▼ ▼

▼ NRV Vibration absorber

Parallel Application11RECIPROCATING COMPRESSORS

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 match the cooling demand of the systemby staging compressors on andoff, thus reducing the amount of wasted energy in a system.

A secondary function of the 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 more than 12 starts per hour percompressor.A higher numberreduces the service life of the compressor. It is necessaryto 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 every 5 hours.This will insure properoil return.

4. COMPRESSOR CONTROL SEQUENCE

Figure 3.7 Discharge header above compressor discharge connections.

▼▼▼

NRV Vibration absorber

Parallel Application 12RECIPROCATING COMPRESSORS

Typical system requirements andrecommendations for parallelinstallations are listed below:

5.1 Suction lineaccumulator

For parallel installations, it isrecommended to always use a suction line accumulator. Be sureto select the proper size suctionline accumulator according tomanufacturer recommendations.In systems with an oil levelregulator it is best to use eitherone suction line accumulator per compressor or a multi-lineaccumulator having a connectionfor each compressor.

5.2 Suction filter

It is strongly recommended to install a large suction filterjust before the suction headeror suction accumulator.This willfilter all foreign particles out of the installation to protect the 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 installedwith continuously energizedself-regulating crankcase heaters(PTC elements).These heatersare to be installed with heatconductive paste.The use of an 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 beinstalled 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. Contact your local Danfoss Maneurop

organization for furtherinformation.

5.4 Thermostatic Expansion Valve

When the parallel installation is serving a single evaporatorsystem the dimensioning of the 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 pressureaccording to the applicationrange published for eachcompressor / refrigerantcombination.This condensingpressure control will also allowfor an ideal liquid distribution inthe system (expansion valveoperation).

5. SYSTEM COMPONENTS

Parallel Application13RECIPROCATING COMPRESSORS

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" ofthe compressor for detailedinstallation and serviceprocedures.

6.1 Compressor mounting

Individual compressors mustalways be mounted using the provided mountinggrommets. If this is not done the system will transmitvibration and in turn reducecompressor 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 to the floor. It is recommended toinstall all control and safetydevices on an independentframe.These devices should be 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 itis 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 motorterminal (L1-T1, L2-T2, L3-T3).

6.3 Total capacity andapplication envelope

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

capacity.The application envelopeof a parallel installation is the 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 slightlyhigher than the lowestcompressor safety pressureswitch set point.The high-pressure safety switch shall stop all compressors.

6.5 Failure analysis

When one compressor in a 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

Parallel Application 14RECIPROCATING COMPRESSORS

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

Figure 7.1 Correct oil level in 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 °

Ø 7.9

Ø 13.5

Ø 5/8 - 18U

NF

GA

045

- 1

0/00

- G

B

LawrencevilleGeorgia - USA

TrévouxFrance

The Danfoss Maneuropfacilities

AnseFrance

Danfoss ManeuropCommercial Compressors

BP 331 F-01603 Trévoux FranceTél. 04 74 00 28 29 - (33) 4 74 00 28 29Fax 04 74 00 52 44 - (33) 4 74 00 52 44Every effort has been made to provide accurate descriptions and data.However, due to the continuing process of product improvement and evolvement, all information in this brochure is subject to change without notification.Maneurop® is a registered trademark of Danfoss.