Modern Gasworks Practice

484 MODERN GASWORKS PRACTICE engines. Many devices have been introduced with. the object of ensuring efficiency and economy. With the larger exhausters it is always profitable to instal a compound engine, owing to the considerable saving it effects in steam consumption. On the larger works a condensing engine still further increases the efficiency of steam con-sumption. When, however, a single cylinder engine is installed some attention to the valve setting is necessary. All exhauster engines may be operated with a comparatively early “ cut-ofi,” and the admission of steam for the whole length of the stroke must be looked upon as unjustifiable extravagance. In the majority of cases the “ cut-off ” may be arranged. at half-stroke, tliough many engines will be found working at three-quarters or seven-eighths. The later “ cut-off ” probably emanates from the faet that some engineers are of the opinion tliat closure at any earlier period is conducive to unsteady running ; but if the fly-wheel is of sufficient proportions this will not be found to be the case. Cases are known where engines are running at a quarter “ cut-off,” and with no undesirable efiect on the “ draw.” When a “ cut-off ” is used, however, a rather larger engine is necessary; and for high-pressure steam it is certainly advisable to fit an expansion gear, so that a sharper and earlier efiect may be obtained. With very low-pressure steam it is essential to work with admission during the whole stroke. Electrical Driving Electrically operated gasworks exhausters liave as yet been adopted only in isolated instances in this country. Speaking in general, there is frequently a good case for the employment of electrical power in almost any other department of the works, but with the usual rather primitive generating plant so frequently found on a gasworks it will usually be better policy to drive the exhausters by other means. In the first place, with. the usual electrical plant considerable fluctuations of pressure are common. whereas the maintenance of a uniform pressure is essential if the exhauster is to be operated effectively. It has been said that in Order to ensure an even pressure it is essential to employ a battery of accumulators in conjunction with the generating plant on account of the steadying effect produced. Every en-gineer who is familiär with electrical stoking machinery knows that at some period or other his plant is temporarily put out of action by the biowing of a fuse or some such, common mishap, and while this is of comparatively little importance in the case of intermittently worked plant, it is a matter of considerable importance where the continuously running exhauster is concerned. Certainly, continuity of supply may be ensured by a proper arrangement of the switch gear ; but, for all that, an additional responsibility is introduced from which the more common methods may be said to be free. Thus, from the point of view of reliability, tendency to breakdown, etc., the electric drive is not to be compared with the single-cylinder steam engine or gas engine. From the standpoint of economy and efficiency, however, there is little question that the electric motor is superior for the purpose, particularly when compared with the average form of non-condensing steam engine employed. A difficulty with the electric motor which is frequently overlooked is that if it iS direct coupled to the exhauster it must (unless a turbo-exhauster is used) be bullt