EXHAUSTING MACHINERY
497
Fig. 306.—Threb-stage Waller Exhauster.
common axis but in separate cells. By means of suitable passages the gas is passed from cell to cell in series until it is discharged from the machine at the requisite pressure, this pressure being the sum of the amounts by which it is increased at each stage. The intensity of this increase of pressure per stage is dependent upon the diameter and speed of the impellers, and also upon the specific gravity of the gas at the time of its being passed through, the machine. Constructional difficulties inherent to this type of exhauster restricted its use at one time to works with a large output. So far it lias been found impracticable to build them in sizes which will deal with tlie requirements of small or medium-sized works with the efficiency and reliability which. characterize the ordinary rotary type.
As miglit be expected the several designs now to be liad differ considerably, but many are based on the principles of Professor Rateau, who has devoted con-siderable time to the study of the subject. Figs. 305 and 306 show the Waller type of turbo-exhauster, the two-stage machine being driven by steam turbine, and the three-stage by electrical motor. The impellers of these machines are built up from steel plates bolted to a cast-iron centre, whilst tlie spindles are of steel, running at tlie driving end in white metal bearings with oil-ring lubrication, the outer euds running in ball bearings. A ball bearing is also provided at this end to take up the axial thrust of the spindle due to the pressure of the gas on the impellers. The exhauster and prime mover are mounted on a cast-iron box section. The machina shown in Fig. 305 is driven by a de Laval turbine running at 4,000 revolutions per minute, and will pass 50,000 cubic feet of gas per hour against a pressure of 30 Inches. The larger machine (Fig. 306) is rated for a capacity of 170,000 cubic feet
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