Modern Gasworks Practice

306 MODERN GASWORKS PRACTICE during the “ firing draw ” than is tlie case when the coke is merely pushed out and taken to the stock lieap. By installing the poker the necessity for extra labour is averted, and the work can be more quickly and effectively carried out. Tlie hydraulic type of machine is shown in Fig. 224. As will be seen, it is customary to attacli the poker to the side of the framework of one of the “ pushers,” so that no separate travelling carriage is necessary. The poker itself consists of a rod 9 feet long by 3 inches wide by | inch in thickness, and is provided with a cast-iron fisK-tailed head. It is actuated by two rams each about 4 feet in lengtli and inches in diameter, attaclied by means of chains to a slipper fixed at the back end of the rod. One ram is used for the forward stroke and one for the backward stroke, the length of the stroke being 6 feet. The time taken to charge a producer with about 9 cwts. of coke is approximately 25 seconds. GENERAL POINTS The cost of operating a retort house depends very naturally upon the magnitude of the installation, the proportion of the work which. is carried out by mechanical means, and whether or not the men are worked up to their full capacity. A retort house may be fitted with the most economical type of charging and discharging machinery and yet be served in a more or less primitive männer as regards the intake of coal and the disposal of coke. The methods of conducting the retort-house work may be classified under three headings :— (1) Manual labour entirely, for all works up to about 20 million cubic feet per anmim. (2) Manual machines, for works between 20 and 120 million cubic feet per annum. (3) Power machines, for works making more than 120 million, cubic feet per annum. It shoulcl be noted that there are a limited number of works making less than this quantity in which power machinery is installed, but in general this limit may be adhered to unless exceptional considerations come into account. THE USE AND CONSTRUCTION OF GRABS Little more than a decade ago the methods employed for discharging coal from ships and conveying it into the works or stockyard were largely dependent upon manual labour. At that time the grab bad not come into general use ; the coal was discharged by being shovelled into tanks raised by hydraulic cranes and into stationary hoppers, from which the coal was taken, often some considerable distance to the retort house in trucks propelled by men. The skips, however, gave way to grabs, and the original type of the latter, which could only be discharged at a pre-scribed lieight, is now being supplanted by the self-acting grab, which effects discharge at any point immediately on dumping. This latter form is particularly suitable where coal may have to be discharged at about the same level as the filling —i.e. when working from ship into bärge—for no man is required to release the catch, and economy of botli time and labour is effected. In general, modern grabs may be classified into two types, namely, those