che article - upgrading a chiller system

8/4/2019 ChE Article - Upgrading a Chiller System

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. a-,er

~ ~cenariOverage loadtor all users

Lood380 kW (Two chil-Iers, each at 550;.of desjan dut y)

Peok lood withone group ofusers, plus the restot overoQe lood

635 kW (Twochillers eachI running at 90%of desion dutv)Aggarwal and S. Singh

Eastern Pte. Ltd AI! users runningat peak capacity900 kW (Threechillers eachrunning at 87%of design dut y)

LEGENDFC Flow controllerFl Flow indicator

FV Flow control valveHGBV Hot gas-bypass valve. It serves

the dual hJnction of compres-sor protection and capacityadjustment during low-duty sit-uations

Lag Compressor to be switched onnext after Lead, and to shutdown prior to Lead

Lead Compressor to be switched onfirst and to shut down lastOverride Function to skip prepro-

grammed sequencesPC Pressure controllerPI Pressure indicator

PlC Programmabie logic controllerPV Pressure control valve

Stondby Compressor to be switched on Iast and to shut down first.Normolly remains offTC Temperoture controllerTI Temperature indicatorTV Temperature control valve

XCV On-off type valveElectrical or software signal

into the warm glycol tank wheneverprocess usage decreases. This config-uration holds the supply temperatureto users constant. Even better, theflow through the chillers is also beingheld constant.

pgrades of existing chillerfacilities often turn out to betrickier than expected.Adding incremental capaci-eventually forcing prolonged

out the prob-usually tracesa rushed modification. Some-has made a bad choice o build im-analyze later. This arti-of a glycol chiller system,how adding a third chillera set of two could cause instability.describes how to control the

Pharmaceutical facilities undergoly toincrease manufac-capacities. Production is inwhere several similarbatches may be handled

to be amply flexible to facilitate aof processing schemes -in-some hat were not even envis-during the inception stage. Suchconditions with chang-support utility facilities.

Expansion of such manufacturingtypically occurs by increasing theoftrains operating in parallel.increases in support-systems

The "quick" proposalOne meeting yields a "quick" proposal.It looks like expansion should be easy:just add a third chiller in parallel toPre-expansion setup the existing two chillers (Figure 2),Before expansion, the chiller system and add a third 100-m3/h warm-glycolcomprises two glycol-chiller packages, pump. Also, the plant must tap a newnormally operating in parallel (Figure header from the cold-pumps discharge1). The warm-glycol temperature manifold to run to the new user facil-serves as input to a PLC (programma- ity. Engineering's task is to imple-ble-logic controller, dedicated to the ment this idea.chiller compressor) o turn the chillers The users of the system will beon or off under various situations. The needing an increased flow of cold gly-PLC decides if one or both chillers col. Clearly, the system requires ad-should be online, and has a pro- ditional pumping capacities to catergrammed imposition ofmanufacturer's I to an increased warm-glycol flow tosafety criteria: minimum run times, the chillers, due to an additionaltrip scenarios and stoppingsequences. chiller in parallel.The warm and cold tanks are bal- The warm-glycol flow to chillersanced, to allow passage of cold glycol needs to be increased by 50 m3/h, so

CHEMICALNGINEERIN

tion of equipment in parallel appears .to be a simple affair, and doesn't toooften invite as much attention as it de-serves. Actually, extreme cautionneeds to be exercised for such capacityenhancements. Expansions desired ina hurry onsite are generally compro-mises -not the way one would expectnew plants to be.Our aim in this article is to illustratethe pitfalls using an ostensibly simpleproblem. The objective is to upgrade anexisting chiller system's capacity byone-third. Glycol chillers provide thecooling to diverse batch users. Due tothe batch production, the number andduty of customers fluctuate consider-ably, causing cooling demand to varysubstantially all the time.

UpraderequirementsDue to the plant expansion, the totalcooling demand will increase. The ex-pected usage pattem will become asshown in Table 1. This calls for instal-lation of one additional chiller to caterto the simultaneous peak demand ofall the users.


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FIGURE 1. Cold glycolleavesthe chillers under temperaturecontrol and collects in the coldtank. From there it is supplied tothe users through cold pumps.The cold-glycol temperature con-trolIer varies the respectivechiller's compressor capacity,through adjustment of the com-pressor's slide valve. Processusers use glycol to controlprocess temperature by adjust-ing the control valve on the gly-col return. The warm glycol re-turns to a warm tank and ispumped back to chillers, thuscompleting the loop.

FIGURE 2. The"quick" system ex-pansion added anew chiller with TCand FC, one cold-glycol pump, andassociated piping.The warm-glycolpumps were re-Dlaced with a largersize. But, becauseof problems withunbalanced pump-Ing and controllog-jcs, it was neces-sary to replace theobsolete PLC anddo some control re-visions (depicted in-side the yellow rec-tangle)

we have a little margin. The addi-tional pump of 100 m3/h in parallelchanges the total pumping capacity to200 m3/h. Rut now all pumps will op-erate at below rated flow, therebycausing power waste. Rather, replacethe existing warm pumps with new,higher capacity, 150-m3/h pumps.The cold-glycol pumps are not con-trolled. Their flow fluctuates depend-ing upon the number of users online.Glycol flow ta online users is modu-lated by valves to control the processtemperature. So, it is all right ta addanother cold-glycol pump similar ta theexisting ones, so that two run in paral-lel with one in standby mode. Here,higher pumping capacity than nor-mally required is actually desirable,since it enables these pumps to delivera higher head at required lower flow.

.@":l ~

~lr ---@--eJ New 50m3/h,Dr---"

Warm#Ji-.,ia.i ew 50m3/h, tandbyIlvcol pumps ~

I = ~~~, per oplion-1 I.1 = ~:i~lingicif/~

New105 m3/h, operati"g

Gold-glycol pumps

.The existing common PLC is obso-lete. Newer models of PLCs do notallow a download from this oldmodel, but can be reprogrammedwith the same logic.There is some spare capacityin theexisting PLC, but apparently onlyenough to just do last on, first offcontrol of the new compressor

.The new chiller control system'ssoftware is different from that of ex-isting chillers. So, the new chillercompressor's control panel may notbe able to commun~cate directlywith the existing PLC. However,some new PLC models can commu-nicate directly with the new and ex-isting compressor control panels.

Design review of controlsLooking into the finer details of the"quick" proposal raises a multitude ofcontrol questions:1. How will the new chiller be con-trolled alongside existing chillers?2. Will the existing PLC control it?3. Does the existing PLC have anyspare capacity?4. How many chillers should normallyrun?5. How will the upgraded system ad-just to varying loads?6. Can the new chiller integrate withthe functioning of existing chillers?7. Are the existing and new compres-sor-control software compatible? (Acompressor is the heart of a chiller .)8. Can the existing chiller's controlsystem allow interference from athird chiller?9. For that matter, can existing chillersfunction as standalone units?10. As pumping capacities in the coldand warm glycolloopsare designeddif-ferently, is there a cause or concern?11. What is the impact on supply tem-perature and flow to existing users?12. Is the increased volumetric holdupamong users a cause of concern?13. Have the existing warm and coldstorage tanks sufficient capacities? Back to the drawing boardIt thus became clear that more engi-neering work was required. The objec-tive became o minimize change in theexisting setup and achieve the desiredfunctionality. We came up with fiveoptions as listed in a box, p. 88. We

losophy," on p. 87, for readers who likedetails. Reading it helps you to under-stand operation of the loop while fullyanalyzing the upgrade.The existing chillers were identifiedto have several control constraints:.The existing chillers get switched onand off in response to glycol- returntemperature via a common PLC.Existing compressor-control panelscommunicate with the common PLCfor capacity determination, and sodetermine whether one or both ex-isting chillers should run. Based oncomputed operating capacity, thisPLC controls the operation of a hot-gas-bypass valve (HGBV), and alsohas a part to play in operation ofevaporator expansion valve. TheHGBV also switches on for a prede-termined time every time the com-pressor switches onAnalyze the upgrade .Existing chillers had several trip sce-We have written exact details of the narios for machine safety; some acti-control logic and operation for the vated via the PLC, and others throughchillers in the box, "Chiller Control Phi- individual compressorcontrol panels

86 CHEMICALNGINEERINGWW.CHE.COMUNE 002


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+@ TI! I !I ! j i l r, , , ,: I! .glycol .:~.I~ ~t i -HGBV, .-1 ~

CHILLER CONTROL PHILOSOPHYE ach glycol chiller package has its own refrigeration system capa-

bie of cooling glycol down to -20C (Figure 3). Refrigerant liquidenters the evaparator colder than -20C, and by exchange with

the glycol exits as a superheated gas. Controller C -1 ensures super-heat by reading the temperature and pressure at the vapor outlet andadjusting the expansion valve V-1 accordingly. Superheat is neces-sary because the compressor must not have any liquid in its feed. Theother side of the evaparator converts warm glycol into cold glycol.

Compressor K-1 raises the pressure and temperature of the refriger-ant. lts warm gas is condensed in the condenser by heat exchangewith water. The bulk of the condensed refrigerant goes to the econo-mizer and then to expansion valve V-1 that in turn cools it downbelow -20C

Two small streams provide fine control. A smallline of refriger-ant is led through the economizer, where its vaporization coolsthe bulk of the refrigerant. This stream then enters into the com-pressor's sideport, thereby serving to increase compressor effi-ciency. Valve V-2 maintains a constant superheat byadjustingthe flowrate of vaporized refrigerant, depending upon the com-pressor's operating capacity. In a screw compressor, operatingcapacity is determined by adjusting the position of a slide valve(not shown), which is varied to control the temperature of the cold

~ CondenserC1V-2 "" t~y~2~ f

FIGURE 3. Operation of the compressor shown here is ex-plained in the text aboveglycol. The hot-gas-bypass line for the condenser through theHGBV is activated to protect the compressor, and adiust capacitywhen in a low-duty situation. D

FIVE WORKING OPTIONSOption 1: Discord the exist-ing PLC. Provide 0 new PLCdesigned for communicotingdirectly with 011 he chillercontrol ponels. Duplicote 011possible necessory controlsond safety trips from existingPLC into the new PLC. Ensurethot the new PLC octuollycommunicotes os designed.This option requires 0 lot ofwork, but is 0 good long-term solution. See CEEXTRAat www .che.com for 0 sam-ple operoting sequence cor-responding to this option.Option 2: Retoin the existingPLC. Provide on odditionolnew PLC for communicotingwith the new chiller's controlponeland the existing PLC.The new PLC would not com-municote with existing chillers

direclly. This option ensuresminimum impact on existingchillers, minimizing risk anddowntime. But more space isrequired. The very old existingPLCwill remain as the weaklink in the system, probablyforcing review after moreyears of operation.Option 3: ( 1) Ensure that ex -isting chillers can operatestandalone at normal duty(without the existing PLC).Chillers are already con-trolled by cold glycol tem-perature at respective chilleroullet. (2) Discard existingPLC. Provide a replacementPLC capable of doing the fol-lowing : (a) Switching on thechillers, based on the coldglycol temperature. When-ever 'set' temperature of

-20C is exceeded for a pre-set time, switchon the subse-quent chiller. (b) Switching ol!one chiller whenever operat-ing capacity of chillers ap-proach a "minimum." Thisminimum should be set suchthat opening of the HGBVs stotally prevented, unless dic-tated by individual compres-sor control panels.Option 4: (I) New chillerhas its own separate con-trols. It always runs as eitherthe lead or the standby. Itcannot run as a lag item. Instandby mode, it's on-ol!switching is controlled byTC-03, based on on warmglycol. (2) Two existingchillers continue to be con-trolled by the existing PLC(3) Set points in existing PLC

are adjusted, such that ifone of them starts behavinglike lag, the other becomesstandby. Setpoints are ad-justed to prevent the exist-ing chiller's HGBV tromopening, unless dictated byindividual machine's trip-scenario tor machine protec-tion (and this is controlled bythe existing PLC). (4) Controlthe chillers using cold glycoltemperature at respectivechiller outlet.Option 5: Operate only ex-isting chillers as lead or lag,as in existing plant. The newchiller will be standby andswitched on and off by TIC-03 during extremes of glycoltemperature. No new PLC srequired, but operationalflexibility is not achieved.

standby ehiller starts up only whenthe return temperature (ofTIC-O3) in-creases beyond -12C despite twochillers running at funload.Each chiner is fitted with its ownHGBV. This valve anows the refriger-ation systems to be run on zero loadwhile keeping the compressor runningin a running mode, and not trippingon low-pressure cutout. The hot-gasbypass arrangement win be used onlyas a protection for individual chiners.A HGBV win not be relied upon to de-liver low process duties.Under normal conditions, a HGBVcan only open when its system (chiner)is selected as the lead system; in otherwords, more than one HGBV cannotopen at the same time. An exception to

went with Option 1. Unfamiliar termsare listed in the Legend, p.85.

The loading pattem in Table 1 sug-gests that the operation of thirdchiller will be required only during co-incident peak demands in all theunits. This won't be a frequent occur-ance, so all three chillers are not nor-mally operatingat the same time. as aresult, we contigured the controls tonormally operate two chillers on par-tial load, with the third chiller onstandby. This provides fast responseto increased dut y requirements, withmore uniform operation of chillers(that is, without frequent on-offs).

We can assume that the glycol flowto each evaporator is constant at 50m3/h throughout its operation. The in-

trol. An XCV on the supply line willswitch off the glycol supply to theoffline chiller .The temperature of the cold glycolexiting the evaporator is also con-trolled using TC-01/02/NEW at-20C. TC-01/02/NEW adjusts therespective screw compressor's oper-ating capacity. Hence, the compres-sor capacity would modulate in re-sponse to variations in the return ofhot glycol temperature to thechillers, resulting from changing theuser's load. The online chillers willthus automatically and equally mod-ulate their capacities, to respond tochanging user requirements.TIC-03 (temperature indicator-con-troller on warm-glycol feed to chillers)


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.Change in number of customers.Change in user flow requirements.The customers circulate glycol for tem-perature control. So, increased supplytemperature results in more glycol flow,probably causing the distributionpumps to operate lower on their curvesand thus function less efficiently. Morewarm flow will cause a cascading effectby further increasing the cold-glycoltemperature, aggravating the situation.Fix the disparityIt is fortuitous that a new PLC is part of Ithe upgrade because more controls areneeded to alleviate the problem. Thepreferred solution is to add a flow con-troller to force the discharge rate ofthecold pumps and the flowrate of thewarm pumps to match (Figure 2). Then,bypass will always be zero. Addition-ally, provide a pressure-controlled flowbypass or the cold pump, so hat excesscold glycol can be returned in case cus-tomers (under individual temperaturecontrol) are using lesser flow.

systems running with their 20-In such a case, t is pos-

We propose to modulate chiller ca-100% of design capacitychiller operation and to re-the frequency of chiller cycles.Every time a chiller switches off andthere is a flow through theline from a warm tank to a cold

that the users receive. By reduc-frequent on-offs, we assure atemperature suppJy to

Vivek Aggarwal is the chiefprocess engineer for FosterWheeler Eastern Pte Ltd (32Maxwell &ad #02-03, Singa-pare 069115; Phone: 65-68900671; Fax: 65-63242890;Email: [email protected]). Prior to this he workedfor the processdivision of Engi-neers India Limited. Ris workcomprises conceptual studies,front-end engineering design,revamp, capacity upgrades and debottlenecking.He has ten years of experience n basic processde-sign and engineering of refineries, pharmaceutical,crude oil and natura!-gas facilities. He has writtenan article on the recovery of natura! gas iquids .Hehas an honors degree n chemica! engineering fromBanaras Hindu University , India and two graduatecourses n finance and operations management.Satendra Singh is director ofengineering, process and tech-nologies with Foster WheelerPte Ltd(Phone: 65-68900671Fax: 65-63242890; Email:[email protected]).He is re-spansible for process designand engineering functions. Ear-lier experience was with Re-liance Industries Limited andEngineers India Limited. Hehas a B.S. and M.S. in chemica!engineering from the University of Roorkee.Pbildon R Stapleton is a se-nior project manager withGMS Engineering, Technol-ogy & Capital Management (1Pioneer Sector 1, Jurong,Sin-gapare 628413; Phone: 656-8608-310; Email: [email protected]).He is in charge ofactive-inl(redient pharmaceu-tical prooucts and support fa-cilities. Prior to joining GSK,he was with Bush Boake &Allen Ltd. He graduated from the University ofSurrey with an honours degree n mechanica! en-gineering and is a Chartered Engineer (UK) anda member of the Institute of Mechanical Engi-neering.

Discarded pumping optionsWe had a few options to solve thepump imbalance problem. For comple-ness we summarize the unused ones.1. Buy a warm pump with capacityof 210 m3/h, to operate the chillers at70 m3/h instead of 50 m3/11. his wouldensure that the cold- and warm-glycolsystems were ba1anced as in existingsetup. However it would cause wide-spread change in chi1lers, FCVs, pip-ing and instrumentation. Pressuredrop through the chi1lers would in-crease, LlT would drop, heat transfercoefficient would increase, and LMTDwould increase. Though the work in-

volved would be expensive, solutionreached would offer stable and pre-dictable operation. We did not havethe time to implement this. I2. Control the warm-glycol tempera-

pumping capacitiesps differ in size and, since thealways be open, we found an ad-problem. The return flowrateglycol to the warm tank must be esstotal feedrate to chiller units.we see glycol flowfrom the

pumps are bigger, warmpass over from the warm totank most of the time. As athe temperature of theglycol to all users creeps higherthe design value of -20C, andeven warmer until thecomes online. This will re-ult in an erratic temperature supplyo the users.We discovered this complication byonstruding the complete glycol circuit

as a spreadsheet and running dynamicsimulations. We ran a series of heatand material balances.Table 2 providesa sample of computed supply tempera-ures. We confirmed that the tempera-ure to users win gradually becomeeven hotter as the cold-glycol supplyemperature to users becomeshotter .What caused this?The problem was caused by one ormore of the following:.Changing flow between he warm andcold tanks (via the balancing ine).Change in nuffiber of online chillers,as cold- and warm-glycolloops willhave different flowrates90 CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2002

1 ture returning to the warm tank at-12C. Providing a temperature-controlvalve between the supply and returncould control this temperature return.That would mean that the bypass em-perature were always steady at -12C.As long as bypass flow remainedsteady, supply temperature to userswould remain constant, although atwarmer than the design value of -20C.However, this solution doesn't offertotal stability as it stillleads to distur-bance (undesirable unpredictable fluc-tuation) whenever bypass flowchanges. For example, a change in thenumber of online chillers wouldchange the bypass flow. Changed by-pass flow at -12C would lead tochange in the supply temperature tousers. Similarly, every time therewere a change in the number of on-line users, the bypass flow would alsochange -leading to a change in sup-ply temperature to the users. .Edited by Peter M. SilverbergAuthors

I lnereased volumetrie holdupThe upgraded system has substan-tially more glycol inventory .Holdupcapacity of the existing glycol tanksmay become limiting. During shut-down, the glycol inventory tends to set-tIe down to an equilibrium level. Gly-col in the piping and equipment dropsinto the glycol starage tanks. We en-sured adequate storage for shutdown.

che article - upgrading a chiller system

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