Industrial burner controls

How using electronic burner controls can significantly reduce costs

Energy costs in many of the industries that employ process heating are considerable. Negotiating a good fuel and electricity price can help to keep energy costs under control. But is your burner controls set up working as hard as it should be? Our industrial burner control experts examine the money saving solutions available in the UK.

Control is key

Controls for industrial burners have changed significantly in recent years; mechanical cam fuel:air ratio control has been replaced by self-checking fuel:air ratio electronic control. Oxygen trim has been added with flue gas re-circulation working to reduce emissions. Performance has been improved by increasing turn down ratios through more flexible control strategies and fan speed control, as well as by improved matching of load and demand from embedded 3 term PID systems.

Software options such as boiler lead/lag control can improve burner utilisation and communications software can greatly enhance information flow.

Most of the improvements to industrial 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.

If you haven’t reviewed your industrial burner controls in the past five years, make 2019 the year that you take action.

Mechanical cams and linkages – is your boiler really as efficient as you think?

Consider a typical industrial boiler with mechanical cam control and let us 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.

When burning oil, viscosity, calorific value and filter condition will all cause variations in combustion and when firing on gas the supply pressure and calorific value of the gas will both cause variations in combustion. The burner engineer will make allowances for any backlash in the linkages associated with the cam and will set the cam to ensure that the oxygen cannot fall too low.

After all, if the oxygen level in the flue gas is too low the risk of explosion is increased. In addition the risk of emissions containing unburnt fuel will also be increased, potentially resulting in fuel being wasted.

When oxygen is low, the flame from the burner lengthens which can cause damage to the tubes in packaged fire-tube boilers. However, if the oxygen level is set 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 above optimum and wastes heat up the stack.

The burner engineer’s next aim is to achieve a maximum turn-down ratio for the burner. Turn-down is the ratio of high fire to low fire. A turndown of 7:1, for example, would mean that the low fire fuel flow would be 1/7th of the high fire fuel flow. With mechanical cam controls the ignition point determines, and is the same as, the low fire point. For essential safety reasons, each time the burner starts up the boiler is purged with ambient air which cools the boiler.

The turn-down on a gas only burner might be 7:1 but on dual fuel burners the turn down is often only 3:1, meaning that on – off cycles are more frequent. If a boiler cycles on and off every 10 minutes then the number of purge cycles per day is 6 x 24 = 144. If purge typically takes 3 minutes, then there could be 3 x 144 = 432 minutes of purge every day.

By maximising the turn-down ratio:

Burner on/off cycles is minimised.
Heat is not taken from the vessel and wasted up the stack.
The boiler can more readily respond to increases in load.
Expansion/contraction cycling, which increases boiler downtime, is minimised.
At low fire, fuel flow is at its minimum and efficiency is less important than at high fire, so the burner engineer concentrates on acceptable combustion consistent with achieving maximum turn-down ratio and a reliable start-up. As before, he sets the oxygen level high. The other points on the cams are set to give a smooth curve between low and high fire with emphasis on achieving the best practical efficiencies at mid fire and above.

Mechanical cams and linkages have been around for so long that their limitations have been forgotten and everyone, including the plant manager, believes that they have their burners set and operating at maximum efficiency. However, it would be more accurate to say that a burner has been set to give the best possible result within the scope of the controls available.

How reviewing your electronic burner controls can reduce your energy bill

When an electronic fuel:air ratio control is retro-fitted the existing PID control, modulation motor and the cams and linkages are removed and servo motors (actuators) are fitted to air dampers and fuel valves. Likewise, when electronic control is specified on a new burner the burner arrives with its new control and its servo motors factory fitted. When using electronic burner controls, plant personnel can expect several benefits.

No backlash
Linkageless burners have no backlash. On a frequently modulating burner, energy savings of up to 1% are common!

Increased turndown
On an electronic burner control, the low fire point can be set lower than the ignition point which means that the turn-down ratio can be increased. In addition, burner on/off cycles and their associated cold air purges also can be reduced, resulting in energy savings.

While savings from reduced on/off cycles will vary with boiler utilisation, savings of 5% 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
Some electronic fuel:air ratio controls have two internal PID modulation circuits. If a plant does not run continuously, then this second PID control’s set point 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 Land Rover in Solihull. The company uses hot water for paint drying in their paint shop but the process is held on stand-by at night.

Using a second boiler set point provides energy savings of approximately 10% pa.

Fan speed control
With mechanical cam control and with basic electronic fuel:air ratio controls, burner engineers can sacrifice combustion efficiency at low fire to achieve an improvement in burner turn-down. Some air dampers leak and even when fully closed the air flow can be significant. In effect, engineers can reduce the fuel valve setting but cannot reduce the air to match. Combustion efficiency can be improved at low fire if the fan speed is reduced.

Fan speed control is an easy to add option on some electronic burner controls. By adding fan speed control, burner turn-down can be increased without compromising efficiency, and additional fuel savings can be achieved.

The benefits of variable speed control do not end here: 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% electrical energy saving is achieved.

Oxygen trim
Oxygen trim is a closed loop system available as an option on some electronic fuel:air ratio controls. Oxygen trim has been available for application to industrial boilers/burners for more than 35. Oxygen trim automatically and continuously compensates for the variables that affect efficient combustion. You will recall that on mechanical cam controls the engineer set the oxygen level high to retain a margin of safety. When oxygen trim is included the oxygen levels can be set at their optimum level. In addition, if the trim control is adaptive then it will contribute energy savings of approximately 2% to 3%.

Boiler sequencing (lead/lag) control and Communication Software
Modern electronic fuel:air ratio controls incorporate boiler sequencing and communications software.

Boiler sequencing control enables the plant operator to achieve better utilisation and additional energy savings are possible.

Communications software provides vastly improved information gathering and boiler operation data. Typical protocols are ModBus, ProfiBus and BACNET which allow plant operators to retrieve data for integration into master display systems and logging systems for analysis.

Flue gas temperature trend data can indicate when boiler servicing is due and oxygen trim trend data can indicate when burner servicing is due.

By reviewing your industrial burner controls strategy it is possible to achieve significant energy savings and substantially reduce outgoings. If you would like to discuss how our electronic burner controls could help your business please do get in touch.