Book - School of Science and Technology

280 Boilers and firing equipmentIt must be emphasised that no single type of automatic stoker is suited to all types andsizes of coal. It is fundamental to selection of the appropriate type of machine that thecharacteristics of the fuel available are examined, in consultation with the local solid fuelagency. Table 11.2 lists published coal Rank numbers and other details.The principal design features of the various generic types of stoker may be summarisedas follows:Underfeed (Figure 10.16(a))This has always been the most popular type of stoker for application to boilers servingheating systems since it is simple in construction and application; tolerant of a low level ofintelligence for maintenance and is relatively inexpensive. Requirements as to fuel are thatcoal be weakly caking, low in ash content and of even size at not much over 20 mm (coalRank 600±900). More details of this type are set out in later paragraphs.Sprinkler (Figure 10.16(b))The original arrangement for this type was conceived early in the nineteenth century toimitate the spreading motion achieved by hand-firing using a shovel. Several basicpatterns of equipment have since been developed. One, as illustrated, spreads fuel via arotary distributor mounted at the mouth of an overhead hopper. A second type sprayspneumatically conveyed fuel from a pipe terminal fitted to the furnace front plate. Yet athird, which is integral to the boiler design, also sprays fuel on the fire-bed but in this casefrom a vertical manifold penetrating the pressure shell as shown in Figure 10.17. Sprinklerswill burn most types of small washed coal except anthracite: a low ash content ispreferred (coal Rank 600±700).Worm or pneumaticcoal feed to boilerFigure 10.17 Overhead fuel feed (Vekos worm feed type)