Using variable-speed drives to reduce combustion costs

Published:  18 January, 2005

A variable-speed drive on a burner motor rapidly reduces its running costs as the burner output is reduced. This graph is for a 5.5 kW motor and electricity costing 5 p/kWh.

SHARON KULIGOWSKI reviews the benefits of fitting inverters to burners and their place in the specifier’s energy-efficiency toolkit.

Saving energy and reducing electricity costs are the main benefits gained from investing in variable-speed drives, or inverters as they are commonly known.

Apart from small variations under load, standard burner motors are essentially constant-speed drives when operated directly from the mains supply. An electronic inverter provides a variable-voltage and frequency supply, which enables the motor speed to be precisely controlled. Standard inverters, using simple open-loop controls, are used in most applications — such as pumps and fans. Traditionally, throttling has been used to regulate air or fluid flows. But while throttling reduces flow, the motor still runs at full speed and consumes full electrical power. By reducing the speed of the motor, the inverter ensures that no more energy than is necessary is used to achieve the required flow and reduce noise. Typically, payback on investment on an inverter is less than nine months.

The table shows the benefits of using an inverter compared with a fixed speed motor on a 5.5 kW burner.

Cost savings calculation:

A motor generally runs at 2800 rpm. Based on an average electricity cost of 5 p/kWh, a 5.5 kW motor would cost £6.60 per 24 hour operational day in running costs. An inverter reduces speed without loss of efficiency, so that when the burner is operating at a low fire of (typically) 18% of output, the motor running costs will reduce to only 79 p per 24 hour operational day.

Savings across a full year continuous operation will be in the region of £ 2117 per burner motor.

Inverters are one of three key ‘tools’ in the specifier’s efficiency toolbox.

Inverters fitted to burner motors save electricity. But electrical consumption is only 2% of the cost fuel used for combustion. To optimise life-cycle costs of burner/boiler plant, it is vital for specifiers to link the use of inverters to the two other key performance enhancers — axial-airflow burner design and digital burner modulation.

The axial-airflow burner design ensures perfect air distribution to the burner head at all levels of firing (including, in particular, low fire). Low-fire is often overlooked when airflow distribution is being measured. Some burner manufacturers only state efficiency at full firing rate. In fact, most burner/boiler installations tend to over-provide capacity and, over a full year cycle, the most common mode will be medium-to-low fire.

When used with correctly dimensioned boilers, axial-airflow burners are also inherently low emitters of NOx. This eliminates the need for investment in costly, high-maintenance, flue-gas recirculation systems.

The third energy-efficiency tool is burner modulation control to manage turndown ratios.

Old style 2-stage ‘high-low’ burners are now rapidly being replaced in all but the most basic ‘steady state’ incineration, furnace or process operations because the high cost of fuel used in the purge and re-fire cycle that is required after every switch off and which is prohibitively expensive.

With mechanical modulation systems prone to poor mixing between firing modes as well as hysteresis, the specifier’s choice is now realistically between electro-pneumatic modulation systems (which only provide a turndown on gas between 4:1 and 5:1) and full digital modulation. The advantage of full digital modulation is the consistent delivery of precise fuel–air mixing over the full turndown range of a burner. Assuming that the boiler design can handle low flue-gas temperature, digital turndown — supported by an inverter — will be at least 10:1 for gas and 4:1 with oil (oil turndown can be increased to 10:1 with atomisation).

The burner is the brain in the boiler house

Although a burner is only a fraction of the size and about half the capital cost of a boiler shell, it has by far the greater impact on the cost of fuel over the whole life of an installation.

The burner is the brain in the boiler house. There is no energy-management technology in a boiler. So when specifying new heat or steam plant, the specifier’s focus should be on the burner, which will have the major impact on energy efficiency, lifetime fuel costs and emissions. An axial-airflow burner fitted with an inverter and a Ratiotronic digital modulation system will typically provide payback on system investment in a year. It will deliver significant lifetime energy-cost savings — as well as both quieter and cleaner operation.

Sharon Kuligowski is managing director of Dunphy Combustion Ltd, Queensway, Rochdale, Lancs OL11 2SL

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