THE HORIZONTAL RETORT BENCH
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that of allo wing square foot of grate area for each. lineal foot of retort in the setting, and for effective working this should be looked upon as the irreducible minimum. For a through setting of ten retorts the rule gives a grate area of 20 square feet. There is little doubt that the producer now used in conjunction with the con.tmu.ous vertical retort owes its economy in part to the increased grate area allotted. In the Woodall-Duckliam system this amounts to as much as 28 square feet for each producer, or rather more than j square foot per lineal foot of retort in the beuch. Mr. Thos. Glover has stated that with. certain coals a grate area of f square foot per 10 feet 6 inch. run of retort is sufficient, but with. other coals, contaming different proportions of lime and iron in the ash, the clinker is difficult to remove. Ånothcr rule is that of allowing 1 square foot of area for each 8 Ib. of coke consumed by the fumace per horn. This basis, however, is not altogether satisfactory, as the fu.el consumption is arrived at by guesswork, and necessarily varies with the type of coke in use.
Different rules must be applied when the sloping forms of grates, as described below, are employed.
Inclined Grates
Various methods have been introduced to prevent the formation of hard masses of clinker by maintaining conditions that will ensure the inert constituents of the fuel being thrown down as a powdered ash only. With this end partly in view, the “ step ” grate has been substituted to some extent for the more common firebar method of supporting the fuel-bed. In. a typical step-grate (Fig. 34) the fire rests on th.e base of the fumace, and is supported by a series of water-cooled cast-iron plates arranged in progression. This system is certainly an advance over the more common method; but it has the disadvantage of requiring attention at compara-tively frequent intervals. In order to preclude the primary air from cutting in under the soffit of the furnace arch. and thus causing local heating of the fuel-bed