is advisable to put a baffle-board in front of the gas outlet to reduce the carrying along of water in the conduits.
Fig. 128.—Fichet-Heurtey scrubber.
Fig. 129.—Scrubber-doors.
The tower of the washer should be provided with three openings having air-tight closures, easily fastened by screws (Fig. 129). One of the openings is located in the lower compartment, slightly above the water level, to allow the deposits to be removed and to permit the cleaning of the orifice of the gas-supply tube, which is particularly liable to be obstructed. The second opening is placed above the grating which supports the filtering material. The third opening is provided on the top of the apparatus to permit the examination and cleaning of the water feed device and the gas outlet without the necessity of taking the lid of the washer apart, the joint of which is kept tight with difficulty. The two openings last mentioned also serve for introducing and removing the filtering material.
Purifying Apparatus.—In some cases, where it is necessary to have very clean gas or where coal is employed which is softer than anthracite coal, and which therefore produces an appreciable amount of tar, supplementary purifying means must be employed. The apparatus for this purpose may, like the washers, be based upon a physical action or upon a chemical action. The physical action has for its purpose chiefly to retain the pitch and the dust which may have passed through the washer.
This is accomplished by means of sawdust or wood shavings arranged in a thin layer and capable of filtering
the gas without opposing too great a resistance to its passage. These materials are spread on one or more shelves superposed to form successive compartments in a box closed in an air-tight manner by an ordinary lid or a water seal cover (Pintsch, Fig. 130; Fichet and Heurtey, Fig. 131). It may be well to point out that the presence of the water carried along will, in the end, destroy the efficiency of the precipitated materials, because they swell up and cease to be permeable to the gas. These materials must therefore be renewed rather frequently. To obviate this drawback, vegetable moss may be employed, which is much less affected by moisture than most filters and keeps its spongy condition for a long time.
