496 MODERN GASWORKS PRACTICE
foul main, almost immediately atter the hydraulic main, others make use of it after the condensers, but at Gloucester it was used just prior to the dry purification. plant. Some advocates of the steam-jet exhauster have said that it gives rise to a clirect increase in illuminating power equal to as much as three-quarters of a candle. It is difficult, however, to see on wliat grounds such a gain in quality could take place.
Turbo-Exhausters
The chief drawback to the ordinary rotary exhauster is the necessity for a considerable amount of Tübbing contact and the large frictional area brought into play, with. a consequent reduction in efficiency. In recent years t.hese difficulties have been overcome by the introduction of the high-speed machine working on the fan principle and providing large clearances. A further advantage offered by the centrifugal fan exhauster is its greater Volumetrie capacity in proportion to the floor-space occupied.
In Order to deal with pressures commonly met with on gasworks, it is generally necessary to build these machines with two or more impellers which rotate on a
Fig. 305.—Two-stage Waller Tukbo-Exhauster.