THE DRY PURIFICATION OF COAL GAS 591
time No. 1 box is thrown out, revivified and remade, and is then ready for action once more as the final box when required. Thus we get'—
First series^ 1 -> 2 -> 3, 4 off.
Second „ ■> 2-> 3-> 4, 1 off.
Third „ 3 -> 4 -> 1, 2 off.
Fourtli „ 4 -> 1 -» 2. 3 off.
In this way each purifier comes in for an “ ofl ” period, so that it may be dealt with when it has become “ fouled.” The system of working as described applies to purifiers arranged either in series or around a centre valve. It should be mentioned that in the majority of works, with the exception of those of small dimensions, a
Main Inlet (Crude)
Fig. 358.—Rotation System with Catch-Boxes.
further safeguard against the possibility of sulphuretted hydrogen getting through to the district mains is pro vided for by the insertion of two “ catch-boxes,” as shown in Fig. 358.
Within recent years a new and effective method of arranging the series of purifiers has come into operation, and is known as “ backward rotation.” It dillers essen-tially from the ordinary system in that when a box operating first in the series becomes fouled the clean box is placed first in position instead of last. Thus if four purifiers are operating in series, we shall have—
First series -> 1 -> Second ,, -» 4 -> Third „ -> 3 ■>
2-> 3-> 4
2-> 3
4 -> 1 -> 2, and so on.
By the adoption of this method it is claimed that where “ catch-boxes ” have previously been in use they may be dispensed with, althougli such a procedure would seem to be attended by some risk. The author finds that where a works has outgrown its purifying capacity the method is particularly suitable, and instances are known where the cubical capacity available is so low as 0-25 cubic feet per 1,000 cubic feet of gas per diem, and yet no trouble is experienced.
Another method which has found some favour is that known as “ forward rotation.” That is to say, when the first “ taker ” becomes fouled it is placed at the end of the series, thus—
First series -* 1 -> 2 -> 3 -> 4
Second ,, etc.