Washing and Purifying.—In pressure producers the gas is generally washed and purified with much more care than in suction apparatus. Given a sufficient pressure, the gas can be driven through the different apparatus and the spaces between the material which they contain without any difficulty. The gases emerge from the generator highly heated, and this heat is used either to warm the injection water or to generate the steam fed to the furnace. The gases then enter the
washing apparatus, which most frequently consists of a succession of contrivances in which the gas is washed either by causing it to bubble up through the water, or by subjecting it to superficial friction against a sheet of water, or by systematically circulating it in a mass of continuously besprinkled inert material. The object of washing is to remove the dust contained in the gas and to precipitate it in the form of a slime which can be removed by flushing.
Fig. 84.—Moss or fiber purifier.
Physical purification thus begun is completed by passing the gas through a filtering bed consisting of fiber, sawdust, or moss (Figs. 83 and 84). Chemical purification if it is necessary, is effected by means of calcium hydrate, iron oxide, or, still better, by a mixture of lime and iron sulphate. This filtering material must necessarily be renewed after it is exhausted.
Fig. 85.—Combined gas-holder and washer.
Gas-Holder.—The gas-holder is composed essentially of a tank and a bell. Sometimes, for the purpose of simplifying the apparatus, the tank is so arranged as to take the place of a washer or scrubber (Fig. 85). The bell should be provided with mechanism which, when the bell is full, automatically diminishes or stops the generation of gas. It is advisable to provide the
bell with a blow or flap valve opening toward the interior. If, therefore, it should happen that the gas supply is cut off while the engine still continues to run, the suction of the engine will not draw the water from the tank of the gas-holder.