Book - School of Science and Technology

282 Boilers and firing equipmentFuel hopperBoilerBoilerFuel bunkerFigure 10.19 Hopper and bunker supply to underfeed stokerThus, forced draught is provided and a very high combustion rate is possible, so high infact that no grate is necessary. The fuel is burnt as it passes over the edge of the fire-potand all the ash is reduced to clinker in the process. Any accumulation of clinker, as animpediment to combustion, is prevented as the fresh coal brought in by the worm pushesit to one side from which position it is removed at intervals.An alternative method of feeding the fuel is direct from a fuel bunker. In this application,either the firing worm or a subsidiary extends into the main fuel store and the hopperis thus eliminated. Both hopper and bunker feed methods are illustrated in Figure 10.19.Boiler firing ± oil fuelAchievement of clean and efficient burning with oil fuel rests almost entirely with thematter of atomisation, that is to say the intimate mixing on a molecular scale of thecarbon in the fuel with the oxygen in the air supply. All manner of methods have beenused over the years in attempts to produce an ideal solution but, for the size and type ofboilers which are chosen to serve heating plants, it is necessary to consider only thefollowing:VaporisationThis may be compared with the principle used in a blow-lamp or a primus stove and isapplied only to very light oils, such as kerosene. On start-up, the oil is preheated, oftenelectrically, to form a vapour which is then ignited: subsequent vaporisation is producedby heat from the flame. Pot-type burners, now probably obsolete, which used thisprinciple were notoriously unstable.