May 1992

BEST PRACTICE PROGRAMME

AUTOMATICconditions. The duty, of the condenser fanoperating under 'head pressure control also

PURG NG ON

COLD STORE

REFRIGERATION

PLANT

Host OrganisationExel Logistics Temperature ControlledServices LId350 Renfrew RoadShieldhall

GlasgowG51 4SP

Monitoring ContractorMarch Consulting GroupMarch House13 Park StreetWindsorBerkshireSL41LUTel: 0753869346Mr J R Graham

Equipment SupplierHansen Technologies Corp98 Manor Road

Barton-le-ClayBedfordshireMK45 4NSTel: 0582881281Mr S P Willison

Case Study ObjectiveTo demonstrate the energy savings and otherbenefits of using an automatic air purgingsystem compared with purging manually on a

weekly basis.

Potential UsersOperators of refrigeration systems withcompressors drawing more than 100 kW.

Investment Cost

£8,980 (1989 prices)

Savings Achieved630 GJ/year valued at £8,800. Saving in

maintenance costs of £2,400/year (1991

prices).

Payback Period10 months.

Case Study SummaryPurging non-condensable gases from a

refrigeration system's condenser and receiverreduces system head pressure. This has thesimultaneous effect of reducing compressor

power consumption and increasingrefrigeration capacity. High pressure cut-out

problems are avoided.In September 1989, Exel Logistics installed a

five-point automatic purger to the evaporativecondenser and receiver of their ammoniarefrigeration system. The plant was previouslymanually purged on a weekly basis. Test workhas shown that when the automatic purger isswitched off., the head pressure increaseduring the course of one week causes an 18%increase in compressor shaft powerrequirements from 60 kW to 71 kW under test

"RAt''\ /'~ ;.z. .. -'".., ... . '"= -0 '"

~ ~IJflR~\\.

Energy Efficiency OfficeDO"RTMONT DF TNO ON.,RDNMONT

Air Purger

T/1,d

Small High-Stage Compressor

manually purged their condenser on a weeklybasis. This was found to be a time consumingprocess and some refrigerant loss was alwaysexperienced. Also, it was never certain .that allnon-condensables were removed. Air ingresscaused head pressure to increase betweenmanual purges and so the weekly averagehead pressure was always above the minimumattainable.

The Refrigeration SystemRefrigeration to 16 cold store chambersranging in volume from 190 m3 to 2;800 m3 isprovided by pumped liquid ammonia from atwo-stage plant. Room temperatures varyaccording to clients' requirements betweenapproximately +4°C and -30°C. Systempressure levels are varied according to storetemperatures, with the low side typically beingunder sfight vacuum.Five high-stage reciprocating compressors oftwo different sizes are installed; typically, onelarge and one small compressor operate,discharging to a siQgle evaporative condenser.Ammonia drains from the condenser to ahorizontal liquid rece!ver.

Purging BackgroundThe presence of non-condensable gas in thecondenser and receiver increases systemhead pressure in two ways:

. the partial pressure of non-co~dehsablegases adds to the partial pressure'ofrefrigerant to increase the total system head

pressure;. non-condensable' gases impede heat transfer

making a high condensing 1emperature (andhence pressure) necessary for a given

condensing duty.The incre..sed system head pressure causesexcessive compressor power consumption andreduces system capacity, Coefficient ofperformance (COP) is reduced.To avoid these performance penalties, Exel

Automatic Purger Installation andCommissioningTo overcome the problems associated withmanual purging, Exel installed an automaticpurging system containing the purger itselftogether with all controls necessary to governthe operation of up to eight purge points.Purging connections were taken from fivepoints:. each end of the liquid receiver;. the upper part of each of two condenseroutlets;. the hot gas line.

A high-pressure liquid refrigerant supply wastaken from the ammonia receiver. The purgersuction connection was made to the low-stagecompressor discharge line.Exel installed the purger themselves in twodays and found installation straightforward.The particular model of purger was chosenbecause it is a completely packaged unit.Purger Maintenance and ReliabilityRoutine maintenance is limited to six-monthlyinspection and cleaning of seven strainersused to protect solenoid valves. These valvesserve the five purge points and the high-pressure liquid and foul gas inlets to thepurger.One purger failure has occurred due to a faultin a solenoid coil. Replacement was simpleand cost only £15

Energy SavingsThe purger was operated one week on, oneweek off, between October 1990 and January1991. During weeks in which the purger wasswitched off, the system head pressureincreased. As a result, power consumptionalso increased proportionately. The curve onthe graph shows the mean monitored trendwith which compressor power consumptionincreased during those weeks when the purgerwas switched off. ffi fact, consumptionincreased from 60 kW to 89 kW; an average of71 kW for the period.For the conditions under which the refrigerationplant was operating at that time, the operationof the purger effectively reduced the week!y

Air Purger (Top Left) and Receiver (Foreground)

Evaporative Condenser

payback of the installation is just over 12months in energy terms. When maintenancecost savings are included the payback falls to10 months.

average compressor shaft power consumptionfrom 71 kW to 60 kW, a saving of 15%.Allowing for losses in the compressor motorand drive. and for changing load conditions

throughout the year, it is estimated that thepurger has reduced compressor energy

consumption by 120,000 kWh/year, worth£6,OOO/year to Exel at 1991 electricity prices.The condenser fan operation is governed by ahead pressure control system, cutting in at10.5 bar(g) and out at 6.5 bar(g). When the

purger operates, the fan cycles as headpressure fluctuates. When the purger is off,thehead pressure never falls enough to turn off thefan. The purging system has reduced theannual energy consumption of the condenserfan by around 56,000 kWh, worth £2,800/yearto Exel at 1991 electricity prices. Hence, th~total energy saving achieved by the purger is176,000 kWh, worth £8,800/year.

Other BenefitsThe automatic air purging system has brought

substantfal non-energy benefits. Routinemanual purging is now no longer necessary.System purging after compressor maintenanceis taken care of by the automatic purger;

previously it mi~ht have been necessary topurge the system manually several times.As air purging car;tnot now be overlooked, it isnot possible for air accumulation to reach alevel which causes compressor high pressurecut-off. This used to occur three or four times ayear, usually when cold weather was suddenlyfollowed by a hot day.The increase in system capacity and COP hasenabled high-stage compressor running timeto be reduced by 6% with consequent savingsin maintenance costs of £2,400/year:Loss of refrigerant associated with purging isall but eliminated.

Economic AnalysisThe total installed cost of the purger was£8,980 in September 1989. Total annual

energy savings are £8,800, hence the simple

Future PotentialThe savings which an automatic purger willachieve are site specific. They depend,principally, on plant size and on the extra non-condensables which the ~utomat.1c purgingsystem can remove compared with the existingpurging method.Large quantities of air were drawn into the Exel

refrigeration low-stage system which wascontinuously under vacuum. Many plants whichdo not operate continuously under vacuumnevertheless experience vacuum from time totime - when defrosting or if a low pressure cut-

out system is used to control operation, for

example. Non-condens~bles can alsoaccumulate in refrigeration systems for reasonsother than leakage into evacuated pipeworksystems. Air may be introduced during plantmaintenance or charging, refrigerant can becontaminated, and refrigerant and oils candissociate at high temperatures.The less frequently a system is purged thehigher will be the savings achieved byautomatic purging. This Case Study shows,however, that automatic purging can beeconomic even when a refrigeration plant is

purged regularly.The typical installed cost of a multipoint

purging systehm is likely 10 be £10,000. Toachieve a two-year simp~e payback, andassuming 15% savings, implies a typicalannual compressor running cost of £33;000.For continuous operation, this means a

compressor power consumption of around100 kW.

Exel LogisticsExel Logistics, an NFC company, provides acontract cold storage and refrigerateddistribution service to a variety of commercialorganisations. The company has 22 cold stores

nationwide.

Comments from Exel LogisticsThe air purger fitted at this store has now beenin ou~rati.on.t°.ralmost two ye~rs. During th!speriod,lheplar1ned,maintenance on the unithascorysi~~d of ~six.monthly examiMtion ofstrainers and fifters,with a purger down-time ofapproximately six hourS on each occasion. Inthe two years of operation, one breakdownocburred due to a faulty solenoid coil. Themaintenance and fault diagnosis on these unitsis relatively simple.Plant efficiency has improved and substantialenergy savings are evident. Compressorrunning hours have been reduced by 6%,therefore maintenance costs have alsodecreased by £2,400/year. With the absence ofair or non-condensables in the system, the fullplant design capacity is available and theminimum power costs are achieved.In an ammonia plant, manual purging can behaphazard end time consuming and a certainamount of refrigerant escaJpes to theatmosphere. If an excessive amount of air ispresent in the system, increased wear oncompressor parts will occur and chemicalbreakdown of the. lubricating oil is alsopossible.Generally, the amount of energy andmaintenance savings will depend on theefficiency of the plant prior to being fitted with apurger. Most companies employ a minimum ofmaintenance staff and, in some cases, propercontrol of non-condensables is impractical. Inthese circumstances the payback period couldbe a few months only.

W M McCreeryChief EngineerExel Logistics

The installation described here was selected as an e~~mple of Good Practice, whichjs one element of the Energy Efficiency Office's (EEQ) Bestpractice programme, an initiative aimed at advancing and promoting ways of improvinQ the efficiency with which energy is used in the UK.' "

For further information on this or other industrial projects, please contact the Energy Efficiency Enquiries Bureau, the Energy TechnologySupport Unit (ETSU), Building 156 Harwell Laboratory, axon aX11 ORA. Tel No: 0235436747. Fax No: 0235432923. Telex No: 83135.

For information on buildings-related projects, please contact the Building Research Energy Conservation Support Unit (BRECSU), BuildingResearch Establishment, Garston, Watford WD2 7JR. Tel No: 0923664258. Fax No: 0923664097. Telex No: 923220.

Information on participation in the Best Practice programme and on energy efficiency generally is also available from your Regional EnergyEfficiency Office.