Controls improve efficiency

6th June 2014

  

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Combustion controls, used in boilers, can boost thermal efficiency and reduce excess air curves, resulting in improved combustion reactions within the boiler.

Boiler and environmental solutions company John Thompson’s series of Enviropac boilers is fitted with combustion sensor and system company Lamtec’s combustion controls to monitor thermal efficiency and adjust air and fuel ratios to reduce excess air curves.

The combustion controls measure the amount of flue gas gen- erated in the combustion process of a boiler by determining the amount of fuel burned and the volume of combustion-air consumed during the combustion process.

The purpose of the burner combustion control system is to ensure that the right amount of oxygen is available for complete combustion for any given load. For combustion, oxygen is usually supplied in the form of atmospheric air containing 21% oxygen.

Using the method of measuring the oxygen content in the flue gas leaving the boiler makes it possible to determine the amount of air used in the combustion process, and, subsequently, it is possible to adjust the fuel-to-air ratio to create the ideal situation for combustion to take place.

Burners are commissioned by setting up the flue gas oxygen levels according to the specifications of the burner manufacturer. Levels of oxygen in the flue gas are typically in the range 2% to 4.5%, the company states.

When gas firing, a reduction of 1% in the flue gas oxygen content will increase the thermal effici- ency by 0.3%, provided combustion is still complete, whereby no significant carbon monoxide (CO) is formed or unburnt hydrocarbons are present.

Control of Combustion Air
The bulk of modern burner control systems use a microprocessor and servomotors to vary the air and fuel inputs to the burner. These combustion controllers incorporate some form of excess air curve or look-up table to ensure the correct air: fuel ratio is maintained.

John Thompson states that, to cater for variations in ambient air temperature and fuel composition, the flue gas oxygen levels are set slightly higher than ideal, as it is better to have too much oxygen than too little under any firing condition.

Insufficient oxygen will result in unreleased chemical energy of partially completed chemical reactions like CO formation. Accurate, repeatable and safe operation over a wide turndown is possible with such controllers and servomotors.

Enhanced Control with Emission Measurement
An oxygen sensor in the flue gas can be added to the combustion controller to achieve a sligh- tly lower preset excess air curve. This system will account for variations in fan performance owing to changes in ambient air temperature and pressure, as well as variations in fuel composition. Efficiency improvements of 0.5% to 1% can be expected with such systems.

By adding a CO sensor, the combustion controller can self-learn the ideal shape of the burner excess air curve. This self-learning is achieved by the controller constantly testing the minimum air: fuel ratio by automatically cutting back on air flow in small increments and checking for the presence of CO, an indication that the minimum possible setting has been reached for the prevailing conditions. Levels of oxygen below 2% are possible with a CO control.

Gas-fired boiler thermal efficiency is mostly dependent on the waste heat that leaves the boiler through the stack. The amount of heat lost in the exhaust gas is governed by the temperature of the gas and the amount of flue gas going up the stack.

Improvements in thermal efficiency can be achieved by reducing the flue gas temperature and enhancing the amount of combustion air.

Other than keeping the boiler clean, there is little an operator can do to reduce the flue gas temperature of a boiler; how- ever, when gas-firing, a reduction of the flue gas temperature of 25 °C will improve the thermal efficiency by 1%.

The flue gas temperature is determined by the boiler des- igner, a function of the installed heating surface and how effectively this area has been arranged by the designer using heat transfer enhancing techniques.

Boiler Specialist
Being a division of electromech- anical equipment company Actom, John Thompson provides a vast range of services and products associated with boilers.

The company’s offering includes consultation and advisory ser- vices; the manufacture and sale of package boilers; the remanufacture and sale of used boilers; repair and refurbishment; turnkey installations; training; and coal/oil firing conversions.

The range of package boilers that John Thompson offers is the result of experienced engineers with many years of experience in the steam industry. The business is based on internationally competitive technology, price and delivery.

Firetube Boilers
Firetube boiler designs include coal-fired boilers with chaingrate stokers, wood-fired boilers with fixed grates, oil/gas-fired boilers and custom-designed waste heat boilers.

John Thompson’s firetube boilers are designed to the EN12953 standard with a design pressure of 1 100 kPa to 2 000 kPa.

With the exception of John Thompson’s Redipac range of boilers, all other boilers use a three-pass conventional fire- tube wetback design with spiral tubes in both tube passes.

Flanged end-plates are features on selected boilers, eliminating tee-butt weld joints, reducing the susceptibility to corrosion fatigue and extending boiler life span.

John Thompson’s spiral tube technology is used throughout its boiler range, offering heat transfer up to 100% greater-than- normal plain tubes, thereby reducing the gas outlet temperature to 15 °C above steam saturation temperature. This results in a thermal efficiency of 85%.

Electrode Boilers
The advantages of using an electrode boiler include having full steam pressure in ten minutes from start-up, full automation of the system, zero pollution or fire risk, the nonrequirement of a boiler house and chimney and no fuel-store requirement.

Electrode boilers from John Thompson range from outputs of 30 kg/h to 3 000 kg/h, with the system’s design incorporating a feed water tank, controls and all the necessary valves and fittings.

The full range of support services offered by John Thompson includes turnkey boiler house supply, installation and project management, international commissioning and maintenance services and full spares availability.


For improved boiler plant effi- ciency, through increased steam output or a reduction in fuel use, John Thompson offers clients on-site advice and upgrading programmes.

Further, new spiral tubes, new stokers improved grit arresters, new fans and other boiler performance systems can be installed on site.

John Thompson boilers are designed to facilitate fuel con- versions of oil, gas or a mixture of these.


John Thompson has established the Boiler Development and Training Centre at its Bellville works, in Cape Town. The centre offers courses in the skills required for the efficient operation of modern boiler plants.

The centre’s two-week training course is aimed at boiler house supervisors and standby or emergency personnel.

The centre’s one-week course aims to further the knowledge of boiler house supervisors, fitters, electricians and instrument mechanics, maintenance super- visors and engineers.

The centre’s four-day inspection course is aimed at maintenance engineers, maintenance foremen or experienced boiler house fitters, and the refresher course combines two or three of the elements of the course.

Edited by Megan van Wyngaardt
Creamer Media Contributing Editor Online

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