Dowson Gas-Producers.—The first pressure-generators were introduced by Dowson of London and necessitated installations of quite a complicated nature. Later improvements made by the designers contributed much to the general employment of their system. Many installations varying from 50 to 100 horsepower and more may be found in the United Kingdom, all of them made by Dowson. Indeed, for a long time the name of Dowson was coupled with producer-gas itself. The Dowson system necessitates the utilization of anthracite or of comparatively hard coal, such as that mined in Wales and Pennsylvania. Owing to the necessity of employing this special quality of coal the Dowson system and the systems that sprang from it were burdened with cooling, washing, and purifying apparatus, which complicated the installations to such an extent that they resembled gas works. The generator that took the place of the retort was fed with air and steam, blown in under pressure, necessitating the employment of a boiler. Furthermore, the production
of the gas under pressure necessitated the use of a gasometer for its collection before it was supplied to the engine-cylinder. Such Installations were evidently costly, and were, moreover, difficult to maintain in proper working order. Nevertheless, there are many cases in which they must be industrially employed.
Fig. 77.—A complete Dowson producer-gas plant.
Fig. 78.—A Simplex producer-gas plant.
Among these may be cited works in which producer-gas is employed as a furnace fuel or as a soldering or roasting medium. Still other cases are those in which the producer-gas must be piped to some distance from a central generating installation to various engines, in the manner rendered familiar in gas-lighting practice.
Most pressure gas-generators have been copied from the original type invented by Dowson. These include a generator in which the gas is produced; an injector fed by a boiler; a fan or a compressor by means of which a mixture of steam and air is blown under the generator-furnace; washing apparatus termed "scrubbers"; gas-purifying apparatus; and a gas-holder (Fig. 77).
Generators.—The generator consists of a retort made of refractory clay, vertically mounted, and cylindrical or conical in form. This retort is protected on its exterior by a metal jacket with an intermediate layer of sand which serves to reduce the heat lost by radiation. The fuel is charged through the top of the retort, which is provided with a double closure in order to prevent the entrance of air during the charging operation. The generator rests on a grid arranged at the base of the retort, upon which grid the ashes fall. The outlet of the injector-pipe opens into the ash-pit, and this injector constantly supplies a mixture of steam and air. The mixture is generally superheated by passing it through a coil arranged in the fire-box of the boiler, in the generator, or in the outlet for burnt gases. Sometimes the