Tips and advice on increasing boiler efficiency
May 1st 2006

Information is readily available on the wide range of energy saving 'add-on' control equipment for steam boilers. There is no doubt that for example variable speed drives will increase efficiency, however retrofitting on old boiler plant may give significantly less efficiency gains than a completely new boiler

A spokesman for Future Energy Solutions has advised that replacement should be considered for all boilers over 15 years old, as efficiency drops over the life span of even a well maintained boiler. Also, new manufacturing techniques and insulation materials ensure that today's boilers are more efficient than those available 15-20 years ago. To find out how much fuel you could save, the Carbon Trust will carry out a free energy survey, see www.thecarbontrust.co.uk for details.

Ian Roberts (BEng,Cert IBO, CEng, MIMechE), who is currently working on a joint KTP project with Leeds University and Byworth Boilers, is this year presenting a series of talks at CEA seminars and various exhibitions giving tips and advice on increasing boiler efficiency. His main points are:

Lifetime cost - Always base financial decisions on the lifetime running costs of your boiler plant, not purely on purchase price. For example a 10 tonne boiler running on gas on even a light load for 20 years would use almost £6million pounds worth of fuel. The boiler would only cost in the region of £70 000, so a few extra thousand pounds invested in a well designed, more efficient boiler is a very wise move.

Boiler type - For most industrial applications the three pass wetback fire design is the boiler of choice, a robust design suited to long arduous workloads.

For lighter loads a reverse flame boiler involves less capital investment but is still efficient to run. For very light loads a vertical boiler offers a small footprint but this would compromise efficiency.

Boiler Size - To supply applications where a sudden high demand for steam occurs, a steam accumulator may reduce the size of boiler required. Steam is stored in the accumulator vessel to even out the peaks and troughs of demand. Your boiler manufacturer will be able to offer advice.

F&A - Boilers are rated from and at 100°C (abbreviated to F&A100°C). This is a measure of energy, not steam output. For example a boiler rated at 5000kgs/hr F&A100°C can produce enough energy to change the state of 5000kgs of liquid water at 100 oC to 5000kgs of steam in one hour. However in operation boilers take water at 80°C, therefore requiring more energy. Consequently the boiler rated at 5000kgs/hr F&A 100°C in operation at 10 barg will actually produce 4616 kgs/hr.

This is important to bear in mind when calculating the size of boiler required to supply the demand of the process plant.

107% efficient? - Beware of taking published boiler efficiency figures at face value. The most common method is to measure losses from the heat content of the fuel, but there are several variables in this calculation which will result in a wide range of efficiency percentages. Firstly ascertain the standard to which it has been calculated. In the UK, BS or DIN could be encountered, which do not directly compare, so ask for DIN figures to be recalculated in the BS standard. Next, ascertain whether nett or gross calorific values have been used, as using a mixture of these values can produce bogus results such as 107% boiler efficiency!

Steam System and Demand Management - There is little point fitting out a new boiler house if the steam system it's attached to is inefficient or poorly maintained, you will literally be burning money. The Carbon Trust's free energy surveys are an excellent way of determining if anything needs to be improved. Similarly, managing you steam demand can generate significant savings and may even result in you needing a smaller boiler than you first thought.

Design features - Both shell and furnace diameters should be generous. The shell should be adequately proportioned to produce good quality dry steam. Too small a steam space increases the risk of priming and "carry over" of water into the steam system. This will lead to wet steam and possibly dissolved solids in your process, scaling and an increased risk of water hammer.

The furnace length to diameter ratio is critical to minimise NOx formation and is ideally 3.5:1. Likewise, a lower volumetric heat release rate would form less NOx as the flame temperature is cooler.

Combustion should be complete by the end of the furnace, however some manufacturers include reversal chamber dimensions in the calculation resulting in misleadingly low heat release rates.

Recently many advancements have been made in materials for insulation.

Ceramic fibre has exceptional insulating properties and is often used on smokebox doors with good results. Shell insulation can be compared by the 'U' value which is a measure of energy released per m2, therefore the lower the value the better.

This is a better comparison than the material thickness.

Energy saving features - On gas only systems a flue gas economiser can increase efficiency by up to 5%. Heat in the flue gas, which normally would just be lost to atmosphere, is utilised to pre-heat feedwater. 5% may not sound much but considering the 10 tonne boiler example used above, over 20 years fuel savings could be up to £600 000 for a heavily loaded boiler , and emissions of C02 would be down by, unbelievably, several million tonnes. Economisers are integral if purchased with a new boiler, or alternatively can be retro-fitted.

More articles from Byworth Boilers: