480 MODERN GASWORKS PRACTICE
Fig. 289.— Gwynnb-Bbai.e Exhauster.
large amount of friction pro-duced by the guiding segments running at- a high speed, whilst the compound slide required frequent renewal. The machine was therefore soon displaced by an improved form, also due to Beale, in which the guiding segments were eliminated. The blades in the new type (Fig. 290) were guided from the centre of the outer casing by the use of a block running on a central sliaft fixed in one of the endplates. The two slides were then cast in one piece, and the frictional losses so reduced as to lessen the driv-
ing power required to about two-thirds of that of the original machine. A feature of the machine is that the outer casing is not bored to a true cylinder, but is oval in shape, this being neces-sary owing to the slightly eccentric movement of the blades which slide on. the centre block.
Multiple Blade Exhausters
The primary objection. to exhausters of the two-blade type is the irregularity of the vacuum they produce on the inlet side, particularly when running at slower speeds. In order to avoid such oscillation the exhauster with more than two blades was mtroduced about the year 1879 ; and owing to the many advantages offered
Fifl. 290.—Improved Beale Exhauster.