THE DRY PURIFICATION OF COAL GAS
579
tiers of about 30 inches. The last named, however, frequently give rise to excessive back-pressure, and are not to be advised in the ordinary way. A space of not less than. 2 inches should always be allowed between the top surface of one layer and the underneath surface of the grid above it. Oxide as it becomes fouled increases in bulk—hence this precaution.
Partial revivification in situ is almost always arranged for nowadays, the small quantity of air admitted during the process considerably adding to the working life of the material. Care has, of course, to be exercised with regard to the quantity of air introduced. The usual allowance is 2| per cent, for every 1 per cent, of sul-phuretted hydrogen present; and, as the H2S amounts in normal cases to about | per cent., the theoretical quantity of air required is about 1 per cent. Some engineers arrange for 15 cubic feet of air per 1,000 cubic feet of gas. The admission of air may be arranged for in two ways. It may be sucked in from the atmosphere through a pipe on the inlet or vacuum side of the exhauster (in which case the air should be drawn through a meter so that its volume may be measured), or it may be injected direct into the mains leading to the purifiers by means of an ordinary steam injéctor. An efEective means of arranging an injéctor system is shown in Fig. 350. On the larger works an injéctor may be fitted to the inlet of each purifier, whilst in small works a run of two or three tours’ duration every day should answer the purpose. When air is sucked through from the inlet side of the exhauster it is advisable to introduce some safeguard in. case sudden pressure might arise from any cause. The safety seal shown in Fig. 351 is simple and convenient. It consists of a cast-iron box about 12 inches square by 4 inches deep, with a flat cover securely jointed.
Injéctor
Steam Inlet
Fig. 350.—Aib Injéctor for Dby Pubtfiubs, showing Method of Gonnbcting Up.
Water should be poured in at B until it overflows from the bib-cock C. The outlet of the meter is connected to A, and the service D leads to the gas main. The base of the seal-pipe A, which is screwed, is so adj usted that it dips below the surface of the water to the depth of about an inch. In the event of back-pressure the water