How much heat goes up your chimney?
November 1st 2006 As the UK becomes a net importer of both gas and oil, the budgets, if not the consciences, of UK businesses will enforce renewed efforts towards energy efficiency. In many industries, steam boilers are responsible for a sizeable proportion of fuel usage, therefore it is imperative to utilise heat produced by the boiler and minimise waste The typical temperature of flue gases flowing from the final pass of tubes from a 3 pass wetback 150 psi steam boiler would be 240OC. This heat is then lost up the chimney to atmosphere but some of it could be recovered. A flue gas economiser is a heat exchanger which utilises excess heat in the boiler flue gases to heat the feed water. This pressure vessel can be included as an integral part of the boiler, if factory fitted, situated above the rear smokebox. Indirect heat transfer takes place between the hot gases flowing around banks of tubes and the feed water passing through them. The tubes are designed with spiralled surfaces to achieve maximum surface area and enhanced heat transfer. Because the economiser is pressurised, greater water temperatures are achievable than in the feed water tank. With the water in the boiler at a higher temperature less fuel is therefore required to raise steam. Typical efficiency gains are between 4 and 6% and capital payback period can often be as little as 12 months. For example a 10,000kgs/hr gas fired boiler on even a light load could cost almost £6m in fuel over it's 20 year lifetime. A 5% fuel saving would equate to £15k per year. An economiser would cost in the region of £10k. At first consideration, economisers are a simple and effective means of utilising waste energy. However there are some further complications to bear in mind. Standard economisers should only be used with sulphur free, clean burning fuels such as natural gas. If it is desired to use an economiser with fuels containing sulphur, specialist advice must be taken at the order stage to minimise fouling and corrosion issues (sulphuric acid will condensate out of the flue gases at around 138°C). In order to prevent steam forming in the economiser, there must be a guaranteed constant flow of water through the economiser at all times that the boiler is running. This means that the feed pump must be permanently running, using more energy. Inverter driven pumps often halve the energy required, and should be considered whether fitting an economiser or not. Modulating water level controls must be used rather than on-off, and are linked to a control valve located in the water circuit between the economiser and the boiler shell. To protect the pump, the water circuit must have a spillback to the feed water tank and the economiser should be fitted with a safety valve. Due to the restriction of physical size, internal economisers are normally limited to six rows of tubes and result in a back end temperature of 135°C. The economiser must be positioned above the water level of the boiler, usually above the rear smokebox, in order to prevent heat transfer directly between the shell and the economiser. External economisers are usually retrofitted in existing boilerhouses as an integral unit is often impractical. These do not suffer from size restrictions and eight row units are normally specified which will return a lower temperature and therefore be more efficient. If considering a retro-fit economiser, the age and condition of the boiler should be taken into account: there would be little point fitting a new economiser to a boiler with only a couple of years life left. More articles from Byworth Boilers: | 
