From Mechanical to Electronic Control on …

1 Energy Technology & Control LtdFrom Mechanical toElectronic Control onindustrial burnersUnderstand how using electroniccontrols can help significantlyreduce a cost you thought you'dalready optimisedEvery Plant Manager knows the costof materials and labour but how manyknow the proportion of energy coststhat relate directly to production?Energy costs are considerable inmany of the industries that employprocess heating but energy is treatedas a utility, an overhead. So, younegotiate a good price for your fueland electricity and you keep yourenergy costs under Control but areyou missing something?Similarities between car enginesand industrial burnersCar engines and industrial burnersboth convert fuel to energy; bothemploy combustion techniques andboth emit pollutants but the similaritiesdo not stop there: the car engine hasbecome a high performance engine,more efficient and less , coils and distributors havebeen supplemented by enginemanagement systems.

2 Carburettorshave given way to fuelinjectionsystems. Lambda Control hasimproved economy and reducedpollution while exhaust gas re-circulation has further for industrial burners havealso changed: Mechanical camfuel:air ratio Control , the equivalent tothe carburettor, has been replaced byself checking fuel:air ratio electroniccontrol,. Oxygen trim, the burner sequivalent to lambda Control has beenadded and flue gas re-circulationreduces emissions as it does in thecar. Performance has been improvedby increasing turn down ratios throughmore flexible Control strategies andfan speed Control and by bettermatching of load and demand fromembedded 3 term PID options such as boiler lead /lag Control improve boiler / burnerutilisation and communicationssoftware be greatly enhancesinformation of the improvements to burnercontrols have been introduced in thelast few years.

3 These controls can bespecified on new burners or retrofittedto existing plant. It is now possibletoreduce energy costs, to loweremissions and to obtain trendinginformation for managementdecisions. Self checking controls withautomatic logging and dial-out canrelease boiler house manpower forother process cams and linkagesConsider a typical industrial boilerwith Mechanical cam Control and letus assume that the burner hasrecently been serviced. At high fire,the oxygen level in the flue gas wouldhave been set at a safe level- slightlyabove optimum- to allow for changesin conditions that affect , say the ambient temperatureswing is say 10 to 20oC in a 24 hourperiod and accept that as the airtemperature increases, air expandsand becomes less dense, resulting inless oxygen per cubic metre beingdelivered to the burner .

4 If moisture is Energy Technology & Control Ltdpresent in the air it will causedisplacement again reducing theoxygen being delivered to the burning oil, viscosity, calorificvalue and filter condition will all causevariations in combustion and whenfiring on gas the gas supply pressureand calorific value of the gas will bothcause variations in burner engineer will makeallowances for any backlash in thelinkages associated with the cam andtaking the above variables intoaccount, will set the cam to ensurethat the oxygen cannot fall too all, if the oxygen level in the fluegas is too low the risk of explosion isincreased. In addition the risk ofemissions containing unburned fuelwill also be increased, and fuel couldbe wasted.

5 When oxygen is low theflame from the burner lengthenswhich can cause damage to the tubesin packaged fire-tube , if the engineer sets theoxygen level too high, there will be anincrease in excess air and heat will belost up the stack. So, the engineersets the cam s high fire point at a safeposition above optimum and wastesheat up the engineer s next aim is to achievea maximum turn-down ratio for theburner. (Turndown is the ratio of highfire to low fire. For example a turn-down of say 7:1 would mean that thelow fire fuel flow would be 1/7th of thehigh fire fuel flow.) With mechanicalcam controls the ignition pointdetermines and is the same as thelow fire point. For essential safetyreasons,each time the burner startsup the boiler is purged with ambientair which cools the turn down on a gas only burnermight be 7:1 but on dual fuel burnersthe turn down is often only 3 that on off cycles are morefrequent.

6 If a boiler cycles on and offevery 10 minutes then the number ofpurge cycles per day is 6 x 24 = purge typically takes 3 minutes,then we could have 3 x 144 = 432minutes of purge every day. ie 432minutes of cold air being blownthrough the maximising the turn-down on/off cycles is not taken from thevessel and wasted up boiler can more readilyrespond to increases in cycling,which increases boiler down-time,is low fire, fuel flow is at its minimumand efficiency is less important than athigh fire, so the engineerconcentrates on acceptablecombustion consistent with achievingmaximum turn-down ratio and areliable start-up. Again, he sets theoxygen level other points on the cams are setto give a smooth curve between lowand high fire with emphasis onachieving best practical efficiencies atmid fire and cams and linkages havebeen around for so long that theirlimitations have been forgotten andeveryone, including the plantmanager, believes that they have theirburners set and operating atmaximum efficiency.

7 However, itwould be more accurate to say that aburner has been set to give the bestpossible result within the scope of thecontrols available. Energy Technology & Control LtdElectronic controls can reduce theenergy an Electronic fuel:air ratiocontrol is retro-fitted the existing PIDcontrol, modulation motor and thecams and linkages are removed andservo motors (actuators) are fitted toair dampers and fuel , when Electronic Control isspecified on a new burner the burnerarrives with its new Control and itsservo motors factory Electronic controls,plantpersonnel can expect burners have no a frequently modulating burnerenergy savings of up to 1% an Electronic Control , the low firepoint can be set lower than theignition point which means that theturn-down ratio can be increased.

8 Inaddition, burner on/off cycles and theirassociated cold air purges also canbe reduced, which will result savings from reduced on/offcycles will vary with boiler utilisation,savings of 5% have been reported ona burner that prior to conversion hadan on/off frequency of approximatelyonce every 10 second PID Electronic fuel:air ratio controlshave two internal PID modulationcircuits. If a plant does not runcontinuously then this second PIDcontrol s setpoint can be used toswitch the boiler to a lower steampressure or hot water temperatureduring periods of reduced manufacturer employing thisapproach is LandRover in company uses hot water for paintdrying in their paint shop but theprocess is held on stand-by at a second boiler setpointprovides energy savings ofapproximately 10% speed Mechanical cam Control and withbasic Electronic fuel:air ratio controlsprocessors sacrifice combustionefficiency at low fire to achieve animprovement in burner air dampers leak and evenwhen fully closed the air flow can besignificant.

9 In effect, processors canreduce the fuel valve setting butcannot reduce the air to efficiency can beimproved at low fire if the fan speed speed Control is an easy to addoption on some Electronic adding fan speed Control burnerturn-down can be increased withoutcompromising efficiency, andadditional fuel savings can beachieved. The benefits of variablespeed Control do not end here: Whenan inverter is used to slow the speedof an ac electric motor, electricalenergy savings result. For example,when a fan motor is slowed to 25Hz ieto half speed an 80% electrical energysaving is trim is a closed loop systemavailable as an option on someelectronic fuel:air ratio trim was employed onindustrial boilers / burners 15 yearsago- about the same time thatLambda Control was first used oncars.

10 But, the oxygen probes onsome early trim systems suffered from Energy Technology & Control Ltdshort operating lifetimes. More robustprobes now are trim automatically andcontinuously compensates for thevariables that affect efficientcombustion. You will recall that onmechanical cam controls the engineerset the oxygen level high to retain amargin of safety. When oxygen trimis included the oxygen levels can beset at their optimum level. In addition,if the trim Control is adaptive then itwill contribute energy savings ofapproximately 2% to 3%.Boiler sequencing (lead/lag) Control and Electronic fuel:air ratio controlsincorporate boiler sequencing andcommunications software. Boilersequencing Control enables the plantoperator to achieve better utilisationand additional energy savings arepossible.