Book - School of Science and Technology

274 Boilers and firing equipmentFigure 10.11 Packaged hot water boilerbeen for service to high temperature hot water plants for very large industrial sites andsteam generation in power stations. Since fire-tube boilers are now available in larger sizesthan was the earlier practice, for absolute pressures up to about 1.5 MPa (15 bar), atendency now exists to use these for such systems.Low water content boilersSmall duty, low water content gas boilers have been used for many years in the domesticand modular boiler markets. The heat exchangers come in a variety of patterns andconfigurations to maximise surface area. Common materials used for the heat exchangersare aluminium, copper and stainless steel. Both natural and forced draught types of boilerare available. Forced draught types often pre-mix the gas and air before entry to theburner, thereby maximising efficiency within a small combustion chamber space.Good water circulation is essential to the operation of low water content boilers and ameans of flow sensing is usually required, interlocked with the burner.The main attributes of low water content boilers are rapid heat-up, high efficiency,compact size, and lightweight. They are particularly useful where plant space is at apremium. The main disadvantage with low water content boilers is the limited lifeexpectancy, which may be less than half that of a robust cast iron sectional or steel boiler.ElectrodeFor use with off-peak current and the large-scale thermal storage cylinders described inChapter 5, a heating source may be banks of immersion heaters but is more likely to beone or more electrode boilers. These may be connected to either a medium voltage supply