energy conservation

IntroductionThe current literature has based cost savings on looselyestimated mean costs of electricity and only touches thesurface of the problem of auditing equipment. Work thathas been carried out at Crabtree Electrical Industries(CEI), Walsall plant, a medium size, vertically integrated,light engineering company, has produced a relativelyaccurate model of the compressed air system byconsidering its operating characteristics and integratingthe appropriate tariffs into the model. The model has alsoallowed for the further development of cost savings to beconsidered and analysed and payback periods readilycalculated. The model does have limitations, mainlyowing to the several necessary assumptions, but hasprovided accurate results. The methodology that hasdeveloped is readily applicable to similar companies. Theneed for complex modelling has been avoided through theanalysis of the total system and the medium size sub-systems, rather than calculating each machinesrequirements and using statistical analysis of machineusage. This would have taken too long and have beenexpensive to complete.

The work also involved electricity tariff analysis, anintegral part of the model. A brief discussion of thecompetitive electricity market is also given whichidentifies the necessary requirements that will enablecompetitive bids to be received from the qualifiedsuppliers of electricity.

The air compressors have been identified from the initialaudit as the single largest consumer of electricity at thesite. The systems analysis showed that the total energyconsumption was significantly larger than the first audithas shown. This identified that the total system wasinefficient and could be significantly improved by

replacement of equipment which would have the effect ofreducing the total energy consumption of the system.

The direct running cost auditTaking into account Figure 1, the approach that has beentaken to auditing the energy users has been a step by stepapproach which identifies system costs, rather thanindividual users, in order to determine the largestachievable savings. The audit has determined the largestcost users of energy rather than largest energyconsumers.

OverviewThe audit involved the following stages:

l meter identification;

l working schedules/production patterns;

l rated input;

l actual usage of energy consumer;

l energy consumer running characteristics.This methodology has been used to enable resources tobe maximized so that time and effort is concentrated onthe areas which would benefit the most. It has beenassumed that the systems which incur the greatestcost have the greatest cost-effective potential forimprovement.

Meter identificationThe historical readings of the utilities have beenobtained from recorded meter readings. Where no datawere readily available, the historical bills have beenaudited to give aggregate meter readings over theduration of the bill. In cases where there has been nointernal recording of the meters, or no meters in place, ametering system should be set up which records theconsumption of each meter at least monthly. However, itis demonstrated later in the article that understandingof the load profile over a day rather than a month couldprovide useful information, enabling the optimal tariff

9ENERGY CONSERVATION

Energy conservation:the application of standard energy conservationtechniques at Crabtree Electrical IndustriesT.J. Hughes and G.A. Bohoris

How standard energy conservation techniques can reduce operating costs and energy consumption

Industrial Management and Data Systems, Vol. 95 No. 9, 1995, pp. 9-16 MCB University Press Limited, 0263-5577

to be chosen. At CEI the site has one main electricitymeter that has been used for billing purposes by theMEB, and a number of gas meters which differentiatebetween production and non-production use. At othercompanies there may be oil, steam, and other types ofmeters. The bills should be audited in order to deter-mine what is paid for and to gain an understanding of

when the utility, and therefore the aggregate of all theequipment, is used.

Working schedules/production patternsA detailed audit of the working schedules has shownhow the plant operates in relation to the weekly,monthly and annual production patterns. Where datawere not available, an understanding of how workingpatterns may differ according to the product has beenassumed. At CEI, the shift patterns varied according tomachine grouping and not physical location. Anaccurate understanding therefore of the plantsmanufacturing schedule and machine use has beengained by auditing according to these groups. Table Ishows the variance in working schedules at the plant.The standard schedules are shown, but shifts may varyaccording to the production load, in particular Saturdaymorning and the twilight shift when usage variesaccording to demand.

The demand at the site may change significantlythroughout the year, depending on the type of productmade or seasonality of sales. This may significantly alterthe annual demand at the site of the measured utility.Where this is the case an adjustment factor based onmonthly sales should be introduced to minimize this

10 INDUSTRIAL MANAGEMENT & DATA SYSTEMS 95,9

Table I. Standard working shifts

MondayThursday Friday Saturday SundayArea Start Finish Start Finish Start Finish Start Finish

Press shopToolroom 07:00 17:40 07:00 17:40 07:00 12:00Opposite toolroom 06:00 14:00 14:00 22:00 06:00 12:00 14:00 22:00 12:00 16:00Press assembly Hare 08:00 16:30 17:00 21:00 08:00 14:45 17:00 21:00Power press 08:00 16:30 08:00 14:45 08:00 12:00Colour shop 06:00 14:00 14:00 22:00 06:00 12:00 14:00 22:00 06:00 12:00Opposite colour shop 08:00 16:30 08:00 16:30 08:00 12:00

Special purposeNext to office 06:00 14:00 14:00 22:00 06:00 14:00 14:00 22:00

(Old toolroom)Opposite office A 08:00 16:30 08:00 14:45Opposite office B 08:00 16:30 08:00 14:45

Autocapstan 08:00 16:30 08:00 14:45Cold heading 06:00 14:00 14:00 22:00Sheet metal

Pullmax 06:00 14:00 14:00 22:00 06:00 14:00 14:00 22:00Main 08:00 16:30

TrimmingTrimming 08:00 16:30 08:00 14:45Wheelabrator 06:00 14:30 06:00 12:30

ToolroomOpposite I/M 07:00 18:00 07:00 18:00 07:00 14:30Hilts 08:00 16:30 08:00 14:45

Energy consumer 1

Energy consumer N

Water

Gas

Electricity

Compressed air

Steam

Hydraulic power

Secondary energyEnergy consumer Bill

Indirect costDirect cost

Figure 1. The interaction between energy consumers, energygenerators and the bills

characteristic. These data should be available fromproduction or sales figures, although using sales figures alead time allowance may be appropriate to cater for theperiod which the items are kept as stock. This willhighlight seasonality in production and distinguishwhere the current production is in terms of maximumcapacity. At CEI, the load is greatest during the springand summer months, with production cut back during thewinter when the holiday periods are taken intoconsideration.

Rated inputThis audit has identified the rated motor input of theelectric motors and/or the rated gas input for gasappliances. Several machines use electricity to heat tools;where this has been the case two values have beenrequired, the initial mean kWh while heating the machinefrom cold and the typical mean running kWh while themachine is kept at the required temperature. Thesevalues have been ordered by the size of kW input.

The appliances (shown in Table II) in some cases usemore than one type of energy input. In these cases themajor user has been audited at this stage.

Actual usage of energy consumerThe use of the machine has been calculated using theworking shifts and a multiplying factor, where appro-priate, based on the number of times the machine hasbeen actually used during the working period. As anexample, there are three compressors of which only twoare used. Of these one is run for 24 hours and the other for8.5 hours a day.

Energy consumer running characteristicsThe characteristics of motors or heaters affect the totalenergy consumption. Some motors may run on and offload, particularly reciprocating compressors and injectionmoulding machines. Surprisingly when motors are offload they still consume a relatively large amount of

electricity, although significantly less than during on loadperiods[1]. The audit needs to measure both the on and offload time period where relevant. The approximate totalelectricity consumption is therefore given by:

(Percentage of time on load kW on load)+ (Percentage of time off load kW off load).

An equation which included the characteristics of themotor under different loads would have been appropriateif the motor had been measured against a more efficientmotor. However, a mean kW on load figure shouldprovide enough accuracy.

The load profile changes during the day. The machineshave therefore been measured at different half hourperiods throughout the day, and randomly over severalweeks. This has provided an indication as to the meandemand during the morning, midday, afternoon andnight working periods. A total kWh value is obtained bylocalized electricity meters adding up the half hour totalelectricity consumption which would dispense with theabove equation.

Tariff analysisElectricityBackground the competitive marketThe work carried out by OFFER, the electricity regulator,has enabled companies whose maximum demandexceeds 100kW to purchase electricity from a second tiersupplier or their host REC. A second tier supplier is one of61 companies who have been granted a licence to sellelectricity and the Host REC is the local electricitycompany. The regulator has left the purchasing managerwith a number of deadlines and requirements in order toreceive electricity from suppliers other than the HostREC (Figure 2).

The first stage has been to ensure that the company has ameter operator appointed, then the purchaser must


Table II. Rated input audit

Approximate Normally in use TotalEquipment rated input (kW) Gas/electric Quantity (Standard hours) (kW)

Warm air heating 3,400 (Total) Gas 91 Dependent on 3,400weather

Hot water heating 1,000 (Total) Gas 7 Dependent on 1,000weather

Steam boiler 855 Gas 1 1 855Compressors 178 Electric 3 2 356Hydraulic pump motors 33 Electric 3 2 66Injection mould presses 22 Electric 31 15 330Automould bipal presses 4 (2) Electric 56 28 112 (56)

accept a tariff four working weeks before the contractstarts, so that if the supplier is a new company, the meteroperator may be informed of the change and install theappropriate communications hardware at the site. Therehave been problems in the past with the UKDCS (UnitedKingdom Data Collection Service) and bills have had to bepredicted but as the market matures these problemsshould be reduced.

The market is divided up into several categories each ofwhich, dependent on the maximum demand, requires ameter of the appropriate code of practice. The categoriesare over 10MW, between 1MW and 10MW, and between100kW and 1MW. The Code of Practice meters whichapply are 2, 3 and 5 respectively. This may createdifficulties where a site is in the process of change and thenew demand is unknown, or where the site demand mayjump between two categories.

The purchase of electricityElectricity is a good where electricity suppliersdiscriminate against the smaller users, i.e. the greater the

demand the lower the cost per kWh. It may beadvantageous, if there are several sites spanning theboundaries of the markets, to negotiate all the companyssites with one supplier, or alternatively pick and mix thebest deals on offer. Analysing which deal is the most cost-effective is a necessity and a stronger position ismaintained by requesting quotations for all sites at thesame time. The provision of a years historical kWh datais the most practical way of providing each potentialsupplier with the same information which may bechecked. It is worthwhile checking the load profile(electricity consumption over half-hourly periodsthroughout the year) since the least expensive tariff afterchecking has been found to be more expensive. Selectionof comparable tariffs should then be optimized for the onewhich is the simplest to reduce the total cost. This doesnot mean reducing the kWh consumption by the most.With some tariffs, the cost of electricity increasessignificantly during specified times of the day. Duringthese times reducing the electricity consumption by asmall amount will lead to significant cost savings.Therefore, the analysis of the kWh consumption by shifttimes, which is matched with a tariff that changes its cost


Appoint meteroperator (COP5)

Host REC

Lease purchase meter,install and maintain,appropriate meter,

excludingcommunications

Provide, install andmaintain, appropriate

meter, excludingcommunications

Purchase meter

Install meter

Meter operator/maintenance

Contractnegotiations

Remain with hostREC Second tier

One month noticeprior to second tier

supply

No communicationslink required

Install communicationslink

Responsibility ofUKDCS

Crabtree Electrical Industries are charged, irrespective of supplier, 200 per annumfor communications by UKDCS. This forms part of the National Settlements Charge

Note:

Figure 2. Appointment of the host REC as meter operator

during the start or finish of shifts, has enabled potentiallylarge savings to be made with simple electricityreductions.

Pool has been expressed as the tariff which is mostappropriate for companies who are able to load managethe electricity. There is however more to the argumentthan just load management. Pool is a risk whichelectricity companies have gone some way towardsreducing with the provision of software programmessuch as Wigwam, which will predict the cost ofelectricity for the next day based on the bid prices at 10:00a.m. by the generating companies the previous day. Thisprediction software is useful but there is a limit to theamount of load management available by any company.When prices have reached extremes, as has happenedrecently, the pool will be expensive.

Systems analysis indirect running costs auditThe Direct Running Cost Audit has enabled the largestenergy consumers to be identified singularly. This audithas been designed to link individual machines intosystems and model their use as one dynamic system. Thishas produced an accurate estimate of the real runningcosts throughout the day at any particular time of year.The latter is important as it enables cost reducingmeasures to be analysed by putting the new data into themodel and payback periods to be calculated to areasonable accuracy.

The compressed air system has been modelled in enoughdetail to provide an accurate analysis of the demand onthe system in cubic feet per minute (cfm)[2], at a constantpressure, with electricity being purchased on the pooltariff. The survey data are specific to compressed air, butthe methodology could quite easily be applied to othersystems with different data collection.

Compressed airThe compressed air system required several stages to becarried out to provide a comprehensive model of howmuch air is required around the plant. Further study isbeing currently carried out into the pressurerequirements of the system as more savings may beeasily obtained through the reduction of pressure atvarious times of the day.

The systemThe system is as follows:

l Generation how compressed air is generated andits associated costs.

l Layout location of compressor, air dryer andcooling towers; pipe network; location and size ofvalves and air receivers.

l Use volume, location, pressure and duration thatair is used; volume, location, pressure and durationthat air is actually required.

l Leakage quantification.

GenerationThe current system comprises three approximately 1,000cfm compressors, two BroomWade 1,000 reciprocatingcompressors and an Alley compressor. The compressorsfeed air into air dryers and this air is then stored in airreceivers. Water is used as the coolant for the aircompressors and after coolers, which is pumped fromcooling towers around a water circuit. The water iscontinuously drawn from South Staffordshires watermain since there is evaporation at an estimated 1 per centof the total volume of the cooling towers. The water alsorequires chemicals to maintain standards set byparliament to minimize the risk of legionellas.

LayoutThe compressors are situated at two sites, either end ofthe main L pipework (5in). Feeds supply the differentfactory areas at either 2in, 2.5 or 3in which receive air intoair receivers before use at machines. No analysis into theoptimal size of pipework has been carried out since thereis no significant pressure loss due to distribution.

UseAir is used at the site for three major applications:

(1) to balance clutches on presses;(2) at low pressure, less than 4 bar inside machines;(3) to blow off or clean mould tools.

The major pressure has historically been set at a mean 90psi; however only a few machines, which run between06:00 and 22:00, use air at 80 psi or more. The nextpressure level is at 60 psi which is adequate for the vastmajority of uses. Significant saving due to a reduction ingeneration cost and volume of air lost through leaks ispossible by generating the maximum pressure requiredby the machines throughout the day.

Air leak quantificationAir leak quantification will enable a value to be put on thevolume of air wasted around the plant. Since the cost ofair is a large proportion of the electricity cost in a lightmanufacturing plant, reducing the leaks will reduce thecost of air per unit of production, and hence the electricitybill, considerably. The layout of compressed air pipesshowing the location of valves has been used to assist indeveloping a map of air leaks, which may be turnedoff by the insertion of automatic valves.

Running the compressors with the valves open and thenselectively closing off areas will put a volume of air perminute on to the areas. Then, either these areas can beclosed off when not in use to reduce the cost ofcompressed air, or further analysis can be carried out into


the way in which they are used, to try to determine waysof reducing costs. It is important, however, to understandthat the total cost of a leak is not simply the volume of airmeasured during the half-hour test period. Instead it isrelative to the volume of air used throughout the year andthe cost is related to the tariff that the electricity bill ischarged. This highlights the importance of a systemsapproach to cost reduction.

The modelThe aim of the model has been to calculate the currentcost attributable to the generation of compressed air andthe ideal system which meets the criteria of the objectivefunction, the optimal size, and results, in the short term,in a three-to-four-year payback.

Objective function (specification)There are five objective functions these are:

(1) Provide a reliable compressed air supply to theWalsall plant.

(2) Minimize the running cost of compressed airsystem.

(3) Reduce the maintenance cost of the compressed airsystem.

(4) Remove the water towers and associatedequipment.

(5) Maintain enough capacity to cover the failure ofone compressor.

The cost of running the complete system should becalculated. This will include the electricity costs of thewater towers, air dryers and air compressors; the waterusage cost of the water towers; the maintenance cost of theair dryers and compressors; and the chemical cost ofmaintaining the water to current regulations. Indirect costssuch as stoppage times could not be quantified owing to alack of information, although the compressors were knownto be unreliable and cause periods of lost production.

A rough and fairly accurate approximation to how muchair is used at the plant has been determined through theuse of calculations based on the volume of air generatedby the compressors at any time. The use of air may beroughly determined by equation 1. A survey of the on/offtimes of the compressors will then produce the loadcharacteristic of the plant throughout the day. This isimportant as it will give a good indication of the volumeof air used by the different shifts at the plant. The data,shown in Table III, were the result of a survey carried outover only a few days; since it is known that this is thebusiest period as production increased to allow for theholiday layoff, this is sufficient to be used to find theoptimum size of the new compressors. The survey is alsobacked up by the results of a survey, carried out byAirmech using air meters, which was able to show thepeak system demand of 1,080 cfm which could not beaccurately calculated by equation 1.

Equation 1: standard equation to determine the volumeof air used at time T

whereVT =Total volume of air (scfm).Vri =Rated output (cfm) of the ith compressor.ti(on) =Time that the ith compressor is on load.ti(off) =Time that the ith compressor is off load.

The resultant chart may look similar to Figure 3. (This isonly applicable to reciprocating compressors.)

This profile is not a perfect model of the demand, butshows the systems peaks that must be catered for. Thereadings were also taken during the maximumproduction period of the factory and should therefore beconsidered near the maximum cfm usage required. Thesystem during certain periods would increase airconsumption by approximately 200 cfm while deflashingmachines were used in the production process. Postanalysis of the demand has shown the demand at the siteto be larger than that predicted. This has been due in partto new machinery installed at the site and productionincreases, but also to inaccuracy of the night timemeasurements of the air demand.

The mean direct use kWh charges were also calculated,from a years historical pool price data, according torelated periods of the day that the compressors loadchanges, so that a more accurate running cost could becalculated (see Table IV).

The current tariff pool contains two other charges; kWhyearly maximum demand and kVA triad charge. Thetotal kWh used by the compressors during the time thatthe charge occurs, i.e. when the site historically used themaximum kWh, is multiplied by the charge to calculatethe cost attributable to the compressors. Likewise thesame method is used to calculate the cost attributable forthe kVA triad charge, but an estimate of the compressorspower factor (80 per cent) is used to convert kWh to kVA.

The air dryers and water towers have been modelledusing the same methodology, measuring the time thatthey are in use in order to calculate the yearly runningcosts. The water towers also consume approximately 1per cent of the total volume of the towers throughout theyear at a cost of 61p/m3 and require the addition ofchemicals throughout the year.

Comparison with air cooled screw compressorsThe major compressed air manufacturers machineshave been considered using the manufacturers ownpublished kilowatt consumption figures. The machineshave been compared against each other using the abovedata, to find which manufactures machines best suited

V Vt

t tT rii

i ii

n=

+

=

( )

( ) ( )

on

on off1


the plant characteristics. For the Walsall site themachine which gave the optimal purchase cost, runningcost and capacity performance trade-off was theIngersol Rand ML90 compressor. Several other factors

have also been considered such as the importance ofavailable parts and the availability of a 24-hourbreakdown service if the worst case scenario arose. Thelatter two factors may be specific to the Birminghamgeographical area.

In order to remove the requirement for water, the newsystem would require air cooling. The three com-pressors are cycled for equal use during the year, but arecurrently located at two areas in the factory. Thelogistics of the compressors should be changed so thatall the compressors would feed one air dryer, thusreducing the running and purchase costs. The newscheme would also allow for the automatic rotation ofcompressors and automatically turn the compressorson/off when required.


Table III. Compressed air audit

CompressorDate Time BroomWade VM1000 Alley BroomWade VM1000

On Off On Off Meter On Off

Maximum Minimum Maximum Minimum

Time 12/07/94 12:04 13 15 13 12

Time 12/07/94 14:32 13 12 12 13Pressure 100 85

Time 12/07/94 04:10 Off Off OffPressure

Time 13/07/94 09:00 FullPressure 100

Time 13/07/94 15:43 17 18 4 6Pressure 100 85 95 90

Time 14/07/94 10:20 28 32 4 12Pressure 100 85 97 90

Time 14/07/94 11:25 6 22 5 5Pressure 97 85 100 90

Note: Time is measured in seconds and pressure measured in psi

1,2001,000

800600400200

00 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1516 17 1819 20 21 22 23

Time

Scfm

Figure 3. Daily mean air consumption

Table IV. Compressed air model data

Monday to Friday Saturday

16:30 to 00:000:00 to 08:00 to 12:30 to 13:00 to excluding Fridays 00:00 to08:00 12:00 13:00 16:30 14:45 to 00:00 14:00

Air consumption (scfm) 352 901 658 1024 352a 352

Electricity cost(p/kWh)b 2.3746 3.6167 3.8887 3.2814 3.5393 2.9286

a This value may increase significantly, dependent on productionb This does not include the fossil fuel levy

Economic analysisThe historical cost of parts, labour and oil for the currentsystem has also been compared against the estimatedcost of parts for the new system. The direct savingscalculated are approximately 4,000 per annum and theindirect costs associated with stoppages should bereduced to zero, since there is the spare capacity to copewith one compressor breakdown.

ConclusionsThe analysis showed that the optimal solution would bethree, 590 cfm, compressors, two would be used from08:00 to 16:30, one used through the night, leaving onecompressor spare for emergencies or for use when and ifdemand increases. The third compressor also enables thecompressors to be rotated on a time basis so that over ayear they are all used equally.

Significant annual running cost savings of approximately20,000 were realized against a total cost of 61,000 forthe three compressors, one air dryer and one sequencer.

Further methods of cost reductionThe method in which air is used at the site enables furthercost reductions to be made. The greatest waste of air atthe site has been through air leakage and applicationsthat use air for blow off or cleaning.

Automatic solenoid valvesThe installation of solenoid valves goes some way, due tothe production and air pipe layout, towards reducing thecost of air. Each area is to be switched off whenproduction finishes with a solenoid valve and 24Vactuator and with the aid of the model the optimalnumber of valves may be found to reduce the leaks. Theinstallation of valves would enable the pressure to bereduced once certain areas are closed off.

NozzlesNozzles are a cost-effective method of further reducing airconsumption at the site. The test nozzles have reduced thedirect consumption of air and have been designed to useatmospheric air to magnify the compressed air that isused. The nozzles that are being considered will reduceair consumption of blow off nozzles by one third, and thecost savings are able to be calculated by using the modelthat has been developed.

ConclusionsThe article discussed the application of operationsresearch to produce accurate models of real systems. Themethodology developed has been used to predict how

standard energy conservation techniques, published by theEnergy Efficiency Office, would reduce operating costsand energy consumption. The cost of implementing thesetechniques has been analysed. The analysis has shownthat such systems are economically justified. Theapproach proposed in this article adds to the encourage-ment that industry has had from the British Standardscommittee for the adoption of the new environmentalstandard, which has been shown to be effective inachieving the reduction of energy usage and costs.

The article also considered the purchase of electricity intodays competitive environment and concluded that,when a sites demand profile has been identified,significant cost savings can be achieved.

Note and reference

1. Good Practice Case Study 137 Compressed Air CostsReduced by Automatic Control System, Energy EfficiencyOffice, West Midlands Region, Birmingham, 1994.

2. Scfm (standard cubic feet per minute) and cfm (cubic feetper minute) are the standard units used by compressedair system manufacturers and are not part of theInternational System of Units (SI).

Further reading

Best Practice Programme New Practice Final Profile 66,Energy Efficiency Office, West Midlands Region,Birmingham, 1994.

Coker, A.J., Electric Motors, Midlands Rewind Group,Heinemann, London.

Davidson, NC., Air Compressors Condensed Air Power Data,Ingersol Rand, 1988.

Fuel Efficiency Booklet 4 Compressed Air and Energy Use,Energy Efficiency Office, West Midlands Region,Birmingham, 1994.

Future Practice R&D Profile 50 Higher Efficiency InductionMotors, Energy Efficiency Office, West Midlands Region,Birmingham, 1994.

Good Practice Case Study 136 Cost and Energy SavingsAchieved by Improvements to a Compressed Air System,Energy Efficiency Office, West Midlands Region,Birmingham, 1994.

Good Practice Guide 14 Retrofitting AC Variable SpeedDrives, Energy Efficiency Office, West Midlands Region,Birmingham, 1994.

Good Practice Case Study 88 Variable Speed Drives on WaterPumps, Energy Efficiency Office, West Midlands Region,Birmingham, 1994.

Good Practice Case Study 89 Variable Speed Drives onCool ing Water Pumps, Energy Efficiency Office, WestMidlands Region, Birmingham, 1994.


T.J. Hughes is an Industrial Engineer at Crabtree Electrical Industries Ltd, Walsall, UK, and G.A. Bohoris is a Lecturer atthe University of Birmingham, Birmingham, UK.

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