Many steam users are running old boilers, still reliable, but how efficient are they? At what point would it become more economical to replace them with new?
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. Combine a new boiler with the latest burner and control technology, and the chances are that you will see a marked reduction in fuel bills. Find out how much you could save by an Energy Survey.
Boilers are rated from and at 100 ºC (abbreviated to F&A100 ºC). This is a measure of energy, not steam output.
For example a boiler rated at 5000kgs/hr F&A100 ºC 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 ºC, therefore requiring more energy.
Consequently the boiler rated at 5000kgs/hr F&A 100 ºC 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. 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.
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 £16million pounds worth of fuel. The boiler would only cost in the region of £150,000, so a few extra thousand pounds invested in a well designed, more efficient boiler is a very wise move.
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.
Options on add-on equipment such as variable speed drives on burners and feed pumps are definitely going to save fuel when purchased with new boiler plant. However do be careful when investing in new energy saving equipment to fit on old boiler plant, as the efficiency gains may significantly less than for a completely new boiler. The boiler design, age, condition and maintenance regime are the prime aspects of energy efficiency.
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!
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.
Furnaces expand at a different rate to the shell. Bowling hoops are incorporated by Byworth in furnaces over 3 mtrs long to absorb the expansion. Claims have been made that corrugated furnaces improve heat transfer, but as most heat is transferred by radiation or convection the insignificant improvement doesn’t warrant the extra investment.