Another approach to reducing the energy that goes up the flue

Published:  02 September, 2007

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Even with the most efficient boiler plant, fuel use can be reduced considerably with a chimney-automation system to ensure the correct ratio of fuel to air.

Peter Roege discusses a new approach to improving boiler fuel efficiency by 30%

Boiler combustion control has improved dramatically as fuel costs rise, environmental pressures grow and technologies improve. In the UK alone, there is an installed base of more than 50 000 boilers of 2 to 20 MW, and some 29 000 boilers of 50 to 600 kW are installed every year.

The boiler house in industrial premises, hospitals or hotels is often the largest user of energy on such sites, yet while you hear a lot about cutting boiler costs by improving or fine tuning combustion, you rarely hear about the major contribution to boiler efficiency that can be made by electronic control of the chimney draught.

Peak efficiency

To fire any boiler at peak efficiency, the ratio of fuel and air supplied to the burner must be right for the prevailing circumstances — but the optimum ratio varies with the load on the boiler.

With an atmospheric burner, air is introduced at the bottom of the burner propelled only by natural draught, and the fuel/air ratio is determined simply by regulating the fuel supply.

In a fully modulating forced-draught burner, the air is supplied by a fan and controlled by one or more dampers, and the air/fuel ratio is managed by electronic control of the fuel supply. As heat demand increases, the burner demands more fuel and air, but the fuel and air must be in the correct proportions and get to the burner at precisely the right time.

Gas analysers are often used as part of automatic trim systems to measure and control either the oxygen supplied to the burner or the mix of combustion gases above the flame. Accurate analysis of the gases resulting from combustion is a precise way of determining combustion efficiency, and feedback from the analyser is used to control automatically the fuel/air ratio. However, something as simple as a poorly specified fan supplying air to the burner, or the fan not operating correctly, can seriously affect combustion efficiency.

Chimney efficiency

You next need to consider the efficiency (and safety) of the chimney. The efficiency with which the chimney vents the combustion gases generated by fossil fuels directly affects the efficiency of the entire boiler system, particularly when several boilers are vented into the same chimney system and when the chimney is exposed to adverse weather. The answer is a properly designed chimney automation system that takes account of the variable heat outputs of modulating boiler systems and the variable chimney flow caused by lengthy horizontal ducting or bends in the vertical ducting to the flue made necessary by the structure of the boiler house.

Such an approach is made possible by my company’s chimney-automation system, which deals with all these issues and also provides control of the chimney draught, even when adverse weather conditions may threaten the flow of combustion products in the flue.

Designed for installation with multiple and/or modulating boiler systems or water heaters, the chimney-automation system modulates the speed of a purpose-designed chimney fan to maintain the optimum chimney draught for any type of boiler installation. The system is activated by an electronic chimney-automation controller when there is a call for heat and monitors the draught constantly, providing feedback to the chimney fan to ensure that values set at commissioning of the system are maintained to provide optimum efficiency.

An integrated safety system shuts the boiler system down should the chimney draught fall more than 40% below the pre-commissioned set point for more than 20 seconds, and shut-off prevents danger to people in the building by eliminating the risk of carbon monoxide contamination.

As energy efficiency becomes increasingly important, it is important to make all boiler users aware that a constant controlled optimum draught in the flue system is vital to maintaining optimum efficiency of the boiler system. The Exhausto chimney automation system has been shown in trials conducted in the USA to cut boiler fuel costs by as much as 30%.

There are other advantages. A chimney-automation system can improve the appearance of a building by reducing chimney height— the system needs only terminations at roof level. Removing the need for a tall chimney dramatically cuts building costs. The diameter of the flue system can be reduced, saving space in the boiler house. In many cases, these savings pay for the additional investment in improved energy efficiency. Perhaps most important of all, a chimney-automation system increases safety by eliminating the risk of carbon monoxide spillage.

Controlled trials

Exhausto is currently looking for organisations with two identical multiple boiler installations on the same site for controlled efficiency trials that could cut the boiler fuel costs of participating companies’ dramatically. Companies interested in taking part in the controlled efficiency trial should contact the author.

Peter Roege is with Exhausto. por@exhausto.co.uk.



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