WATER GAS
Ci
W
rate for average coke or breeze. The limiting factor in grate area in the Lancashire type boiler is the length of grate which a man may efficiently stoke and clinker, usually 6 feet 6 inches to 7 feet 6 inch.es, measured from the furnace front. This factor limits the total effective area to 36 or 38 square feet in the larger capacity boilers of this type. Now, assuming evaporative valnes for coke and breeze respectively of 91b. and 51b. of steam per 1b. of fuel, as fired, the steaming capacity at the various rates of firing indicated may be accurately arrived at. To the gas engineer, the relatively large proportion of grate area obtainable in the water-tube boiler is certainly an attractive feature, having in mind the characteristics of the staple fuel.
Draught
Except perhaps where an existing chimney shaft provides the necessary draught, some system of impelled draught maybe “ induced ” or “forced,” i.e., by means of an cxhaust fan located near and discharging into the chimney shaft; or a pressure
Fis. 457.—London Coke Committbb’s Apparates fob “ Impelled ” Draught.
fan applied to the under-side of the fire-grate. The induced system depends upon the maintenance of a partial vacuum over the fire, but possesses the disadvantage of increasing the cold air infiltration through the brickwork setting, etc., while it tends, by increasing the speed of the waste gases, to carry over and discharge at the chimney a portion of tlie grit and dust, which, with the “ forced ” system properly applied, remains on the grate or falls immediately beyond the bridge wall or first baffle. The various types of steam-jet forced draught furnaces, with. one or more short chimney shafts of large area, are among the most favoured systems now adopted. The heiglrt to which the boiler shaft is carried. and consequently the cost of con-
3C