498
MODERN GASWORKS PRACTICE
Fig. 307—British Thomsox-Hototon Tubbo-Exhaustbb.
per hour against a pressure of 24 inches. Owing to the high speed at which turbo-exhausters have to run, they are particu-larly suited for being driven direct by electric motor or steam tubine without the em-ployment of intermediate gearing. Another type of fan exliauster is shown in Fig. 307. This is the British Thomson-Houston machine driven by a Curtis impulse-type steam turbine through a flexible coupling. The ex-hauster is of the two-stage type, and is capable of deal-
ing with 500,000 cubic feet of gas per hour against a pressure of 40 inches. In this case the impellers are placed back to back in order to eliminate end-thrust. Fig. 308 shows one of the impellers, which are solid steel Castings, the blades and hub being in one piece, with the exception of the inlet ends which, being twisted to allow free entry of the gas, are mounted on a small extension. hub. On leaving the impeller the gas passes through a series of exit guide blades, which convert the velocity head into pressure head. As regards initial cost this is, on an average, about 50 per cent, less than that of an ordinary rotary installation of the same capacity. Owing to the high rate of speed, pulsation with this type of exliauster
is almost entirely eliminated.
The construction of the Sturtevant rotary blowers and exhausters is ilhistrated
in Figs. 309 and 310. The exterior cast-iron casing is composed of two hollow cylinders partially intersecting, on either side of which are intake and discharge openings. Inside the casing are two rotors, one of which, the impeller, is made up of three diamond-shaped bars, joined together by a central web, the whole being keyed to a central driving shaft. As the impeller revolves, three separate pockets are formed in the space between the casing and the stationary core, thiis carrying the gas from the inlet to the discharge. The idler rotor does no actual work, but successively provides spaces to receive and pass the impeller blades to
Fig. 308.— Exhauster Impei,leb.