Save energy through cunning control
Understand how using electronic controls can help significantly reduce a cost you thought you’d already optimised. Terry West, sales and service manager at Energy Technology & Control Ltd, shows how energy can be saved by changing from mechanical to electronic control on industrial burners.
Every Plant Manager knows the cost of materials and labour but how many know the proportion of energy costs that relate directly to production? Energy costs are considerable in many of the industries that employ process heating but energy is treated as a utility, an overhead. So, you negotiate a good price for your fuel and electricity and you keep your energy costs under control but are you missing something?
Relays, coils and distributors have been supplemented by engine management systems. Carburettors have given way to fuel injection systems. Lambda control has improved economy and reduced pollution while exhaust gas re-circulation has further reduced emissions.
Controls for industrial burners have also changed: mechanical cam fuel:air ratio control, the equivalent to the carburettor, has been replaced by self checking fuel:air ratio electronic control. Oxygen trim, the burner’s equivalent to lambda control has been added and flue gas re-circulation reduces emissions as it does in the car.
Turn down ratios
Performance has been improved by increasing turn down ratios through more flexible control strategies and fan speed control and by better matching of load and demand from embedded three-term PID circuits. Software options such as boiler lead / lag control improve boiler / burner utilisation and communications software be greatly enhances information flow.
Most of the improvements to burner controls have been introduced in the last few years. These controls can be specified on new burners or retrofitted to existing plant. It is now possible to reduce energy costs, to lower emissions and to obtain trending information for management decisions. Self-checking controls with automatic logging and dial-out can release boiler house manpower for other process applications.
Consider an industrial boiler with mechanical cam control and assume that the burner has recently been serviced. At high fire, the oxygen level in the flue gas would have been set at a safe level – slightly above optimum – to allow for changes in conditions that affect combustion.
Then, say the ambient temperature swing is 10 to 20ºC in a 24-hour period and accept that as the air temperature increases, air expands and becomes less dense, resulting in less oxygen per cubic metre being delivered to the burner. If moisture is present in the air, it will cause displacement again reducing oxygen delivered to the burner.
When burning oil, viscosity, calorific value and filter condition will cause variations in combustion and when firing gas the supply pressure and calorific value will also cause variations in combustion.
The engineer will make allowance for any backlash in the linkages associated with the cam and taking the above variables into account, will set the cam to ensure that oxygen cannot fall too low avoiding a hazard and emissions containing unburned fuel.
When oxygen is low the flame from the burner lengthens which can cause damage to the boiler. If the engineer sets the oxygen level too high, there will be an increase in excess air and heat will be lost up the stack. So, the engineer sets the cam’s high fire point at a safe position slightly above optimum and wastes some heat up the stack.
The engineer’s next aim is to achieve maximum turn-down ratio for the burner. Turndown is the ratio of fuel flows at high fire and low fire. With mechanical cams the ignition point determines and is the same as the low fire point. For essential safety reasons, each time the burner starts the boiler is purged with ambient air which cools the boiler.
By maximising the turn-down ratio:
- burner on/off cycles are minimised,
- heat from the vessel is not wasted up the stack,
- the boiler can more readily respond to load increases,
- expansion/contraction, which contributes to boiler down-time, is minimised.
At low fire, fuel flow is at its minimum and efficiency is less important, so the engineer concentrates on a reliable start-up consistent with acceptable combustion and maximum turn-down ratio.
Plant personnel can expect several benefits when changing from mechanical to electronic controls.
- no backlash – on a frequently modulating burner energy savings of up to one per cent are common.
- increased turndown – on an electronic control, the low fire point can be set lower than the ignition point which means that the turn-down ratio can be increased and burner on/off cycles and their associated cold air purges reduced. Savings of five per cent have been reported on a burner that prior to conversion had an on/off frequency of approximately once every 10 minutes.
- a second PID control – if a plant does not run continuously, then a second PID control’s setpoint can be used to switch the boiler to a lower steam pressure or hot water temperature during periods of reduced activity. One manufacturer employing this approach is LandRover in Solihull. The company uses hot water for paint drying but the process is held on stand-by at night. Using a second boiler setpoint provides energy savings of approximately 10 per cent per annum.
Oxygen trim automatically and continuously compensates for the variables that affect efficient combustion. When oxygen trim is included, oxygen levels can be trimmed to their optimum level and if the trim control is adaptive then it will contribute energy savings of three per cent.
Fan speed control
Some air dampers leak and even when fully closed the air flow can be significant. Combustion efficiency can be improved at low fire if the fan speed is reduced.
By adding fan speed control burner turn-down can be increased without compromising efficiency, and additional fuel savings can be achieved. Also, when an inverter is used to slow the speed of an ac electric motor, electrical energy savings result. For example, when a fan motor is slowed to 25Hz, i.e. to half speed an 80 per cent electrical energy saving is achieved.
Boiler sequencing control enables the plant operator to achieve better utilisation through matching of boiler output to demand.
As you can see, it is possible to reduce unit cost by reviewing burner control strategy.
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