Modern Gasworks Practice

166 MODERN GAS WORKS PRACTICE this process the flow of gas from the top of the retorts is interrupted during the latter part of the period of carbonization by means of a valve in the offtake pipe, and superheated steam is introduced into the top of the retorts. The steam, being admitted into the open space above the charge, fills the entire area, and moves downwards at slow speed, coming at once into contact witli the horizontal section of the charge. The water-gas thus produced passes by way of a common collecting main to the bottom of a freshly charged retort, sweeping the rieh gases forward along with it, and protecting them against decomposition. An arrangement of valves permits variation as to the position and number of tlie retorts subjected either to “ down ” or “ up ” current. The blue water-gas, whicli is highly heated, dilutes the rieh gases formed in the newly charged retort, and greatly accelerates the rate of travel of the gases towards the gas outlet, resulting in a reduction of the secondary action, due both to the protective influence of di lution and to exposure for a skorter time to a decom-posing atmosphere. The temperature of the gases in the offtake pipes is remark-ably low, rarely reaching 60° C.; thus deposition of ammonia salts in the hydraulic and foul main, is prevented. The valves for steam supply and those in the offtake pipes are opened and closed from the charging stage by suitable hånd gear. Each. retort is provided with a separate offtake pipe of ample area, coimected to the hydraulic main by a dip pipe. The hydraulic mains are of special design, fitted with self-sealing inspection doors, so as to permit the pitch paus to be withdrawn during gasmaking, a separate main being provided for each setting. Each hydraulic main is also provided with a separate tar tower, arranged for sealing and unsealing the'dips and for flushing the main, which is done each time the retorts are charged. The uptakes from hydraulic mains enter the foul mains on the side, and are fitted with valves, so that tliey do not become blocked with deposit when the settings are let down. THE ELLAND INTERMITTENT SYSTEM The intermittent system (Fig. 87) differs from the continuously operated type in that the retorts are of .considerably less total length. A special feature is the method employed for ensuring a partly porous charge so that the gases as tliey are evolved are provided with a ready exit. Unscreened coal is charged in a thin stream against one side of the retort, this causing the nuts to accumulate on. the one side and the slack along that side at which charging takes place. In addition to ensuring a porous charge, it is claimed that a less dense coke is produced ; whilst the coal is permitted to sh rink freely from the walls of the retort, and in this way facilitates discharge. On charging, the flow of coal from the hopper ceases automatically when the required level is reached. The shoot is then withdrawn, and its contents drop into the retort. The self-sealing lids are of special construction, a toggie system of levers being used instead of eccentrics ; and a single pull by the attendant suffices to raise and seal the lids. The regenerators are of the tubulär type, and the setting is subdivided in a männer to give a maximum control of the heats. The retorts