Modern Gasworks Practice

752 MODERN GASWORKS PRACTICE intö the revolving kiln for the finishing-off process, and it is also findmg its way into the brickmaking industry. STEAM GENERATION ON GASWORKS The large extent to which water-gas manufacture has Leen adopted in modern gasworks has placed gas authorities amongst the chief users of steam for manufacturing purposes. The importance of this section of the works equipment warrants a more detailed study than it is possible to include in the present work, and it is proposed only to consider broadly the more important factors which usually govern the selection, lay-out and operation of steam generating plant. Selection The principal factors which. usually determine the type of boiler selected are unit evaporative capacity for a given floor space, where this is an important con-sideration ; but in a lay-out for a new works more importance would be attached to convenience of handling fuel and ash, arrangement of main flues, and provision for spare plant and .extensions. Steam generating plant is usually designed and rated on the assumption that some kind of natural coal fuel will be the staple fuel used, and the influence of coke or breeze (which may be taken as the staple solid steam-raising fuel of tlie gasworks) on the steaming capacity of the boilers has to be borne in mind in making a selection. Types of Generators Broadly, there are in present use two types of boilers, each having their special features, namely, the water-tube or externally fired boiler, typified in the Babcock and Wilcox pattern, and the fire-tube or internally fired Comish and Lancashire types. When. capacity and limited floor space are the chief factors, the range of selection is limited to the various patterns of the former type, which have usually a ratio of lieating surface to grate area of 60 or 60 to 1, as compared with. about 25 to 1 available in the Lancashire type. For rapid'steaming and quick response to sudden heavy demands, the water-tube boiler is now recognized as the more sensitive, but the greater steam and water reserve capacity of the Lancashire type boiler renders it the more suitable for the intermittent heavy pulis of the water-gas plant. To realize the advantages of both types a combination battery working in parallel has been found to be most effective in responding to the severest conditions imposed. In the matter of thermal efficiency there would appear to be little, if any, in favour of either type. Grate Area The economic rate of combustion of fuel and the area of the boiler fire-grates determine the normal evaporative capacity. With coke as fuel, 22 Ib. to 25 Ib. per square foot of grate area per liour represent the economic average maxima for hand-fired fixed grates. Higher rates may, of course, be attained, but only at the expense of efficiency. Probably 18 Ib. to 20 Ib. would represent the best normal working