Book - School of Science and Technology

Types of boiler 269Water waysFluecleaningdoorBurnerFigure 10.6 Cast iron sectional boiler (Hamworthy)integral nodules or studs cast on to them to provide additional surface for heat transfer.Sizes range from 20 kW to 1 MW per boiler unit and combustion efficiency may be about80%. Other small, modular boilers may have sections built up vertically.The normal range of cast iron boilers is designed for operation under water pressures ofup to about 400 kPa (40 m head of water) which covers most building sites except in highrise city centre developments. Certain manufacturers, however, do produce higher ratedboilers using a spheroidal graphite grade of cast iron which will withstand water pressuresof up to 1 MPa (100 m head of water), although 600 kPa (60 m of water) working pressureis the typical maximum.To supply low pressure steam, cast iron sectional boilers were very commonly usedduring the era when this heating medium was popular. The earlier types had a limitedsteam space made available actually within the sections but later patterns mounted ahorizontal steam drum above them. Ratings available were up to 750 kW for working atnot much more than 200 kPa absolute pressure (2 bar).Cast aluminiumCast sectional aluminium heat exchangers are available offering compact lightweightconstruction. Figure 10.8 illustrates a high efficiency, low NO x and condensing unit,available from one manufacturer with an output range 80±160 kW non-condensing and86±170 kW in condensing mode.