Modern Gasworks Practice

606 MODERN GASWORKS PRACTICE purifier is the flat sieve, made in wood, and built up with eitlier open or butt-encls, as shown in Figs. 366 and 367. Within recent years tliere has been some tendency to employ grids of the hurdle type, either by themselves or in conjunction with the ordinary flat sieves. The chief advantage attached to such grids is that they may, in some cases, be responsible for a reduction in. back-pressure, but it seems that their advantages are sometimes outweighed by their defects. Amongst the latter may be mentioned— (a) The tendency of the gas to short-circuit through the material. (b) The wear and tear on the grids is greater. (c) A common method of overcoming back-pressure in an ordinary purifier is by drawing a slide and thus by-passing a tier of the material. With the hurdle grid this cannot be done. (<Z ) The operation of discharging is more costly, and occupies a greater length of time. It must not be supposed, however, that the hurdle grids have not their good points, and many engineers adhere entirely to their use. THE COST OF PURIFIERS No definite figure for costs based on the capacity per 1,000 cubic feet of gas dealt with can be given, owing to the expenditure per unit volume undergoing considerable reduction as the capacity increases. For instance, a set of four purifiers complete with connec -tions, lifting crane, etc., to deal with 100,000 cubic feet of gas per diem, will cost about £1,750, i.e. £17 10s. per 1,000 cubic feet; whilst a similarset capable of taking two million cubic feet per diem will nm into rather more than £15,750, i.e. about £8 per 1,000 cubic feet. These figures refer to dry-luted ground-level boxes, and do not in.clu.de the building. If water-luted boxes are under consideration, 15 to 20 per cent, may be added. The diagram shown in Fig. 368 gives an indication of the männer in whicli the cost varies in accordance with the capacity. The figures include Connections, lifting gear, valves and foundations. The following table shows the männer in which. the outlay is distributed over the various portions of the installation in normal cases for medium-sized plant:— 3000 2800 20,000 Diem Z600 2400 — 17,500 2200 <ü D. 2000 - ü Q 1800 15,000 O 1600 Cubi 1400 — — 12,500 1Z00 CD 1000 10,000 •c 800 CO (D O — 7,500 _c 600 =T) 400 5,000 CÜ Q. 200 ro — 2,500 100 -— f CD 01 .S' »ai Fig. 368.—Diagram showing ÅPPROXIMATE COST OF A ÖET OF lass. Four Purifiers. Purifying boxes . Covers (dry lute) . Lifting apparatus . Grids (ordinary flat type) 6 to 10 50 per cent, of total expenditure. 1 • 99 9} 4t0 6 „ „ Connections and valves 20 to 30