Modern Gasworks Practice

524 MODERN GASWORKS PRACTICE such, cases it may be advisable to construct a domed top having a rise equal to one-sixth of its diameter and made from steel. The function of the scrubber is to bring the partly purified gas in contact with large areas of wetted surface, and to this end the tower is packed. with. various materials wh.ich are laid on the grids at the base of each tier of plates. The type of packing required and its disposition have no little influence on. the efficiency of the scrubber, hence considerable care should. be given to this point. The chief points to be kept in mind when deciding upon the method of packing are the following:— (1) A large area of exposed surfaces is required. (2) The surfaces should be rough. (3) Any tendency for the gas to “ slip ” must be avoided, and the stream should be broken up as far as possible. (4) Initial cost of the packing material. (5) The life of the material. (6) Ease of cleaning out. Brushwood and fascines were largely used as a packing in bygone days, but have now been. superseded by layers of coke, wooden blocks, drain-pipes, grids, and boards on edge. As regards the relative areas exposed to the gas by these materials, Livesey has given the following figures:— Coke . . . . 8| square feet per cubic foot of scrubber capacity. Drain-pipes, 3 inch diameter 17 „ » „ >> » „ 2 inch „ 21 „ „ „ „ „ Boards on edge . . 31 „ „ „ » „ Dr. Carpenter has obtained extremely high efficiency by packing serubbers with metal bundles som.ewh.at similar to those employed in. Kirkham, Hulett and Chandler’s horizontal washer-serubber (Fig. 320). There is little doubt, l i o wever, that, considering efficiency in conj un etion with expense, it is difficu.lt to improve on a packing composed of boards placed on. edge. These should. be rough. sawn and. arranged in ch.equ.er fashion, so as to give a sieve-like construction. The size of boards conforms to no hard and fast rule. The following may, however, be taken as affording satisfactory results:— 6-incIi boards, | inch. thick, laid f inch apart. 11-inch. boards, | inch thick, laid | inch. to f inch apart. A typical tower scrubber complete with. board filling is shown in Fig. 329. In this case the bottom compartment forms a washer for the purpose of tar removal. Board or grid filling for serubbers is somewhat costly in the first instance, and where expense is of material importance tiers of coke may be substituted. Approxi-mately, board filling will cost six times more than coke tiers, but it may be made use of over and over again. Whatever type of filling is employed it is essential that the upper surface of each. layer should not be in. contact with. the underside of the grid supporting the layer above it, otherwise irregulär distribution of gas and liquor, also inereased back-pressure, will result. When the distance between th.e