Modern Gasworks Practice

EXHAUSTING MACHINERY 485 for running at a very low speed. Electric motors of normal construction, however, are built to run at high speeds, consequently the special type required is very costly, and also very much heavier and larger tlian the normal type. The question of governing, too, is one which must be given due consideration, for although the “throttle” method as applied with the steam-driven machine may be employed (by arranging for speed variation by means of a shunt regulator), it is probably better, in Order to avoid anytliing in the nature of complications, to maintain a constant speed and to effect governing by the ordinary by-pass method. Exhausters—more especially those of the turbo pattern—driven by steam turbine have been more in evidence lately, and their development has probably been the result of the successful operation of water-gas blowers by this means. The chief merit of this type of exhauster is its ability to maintain an absolutely constant suction whatever the volume of gas deEvered. In Order to main-tain this uniformity the speed of the turbine is varied within wide limits by means of an automatic governor. The turbine, of course, produces a much steadier turning moment than the reciprocating engine, and, in general, collateral advantages are to be found in the economy of fuel consumption and the low charges for maintenance. Running Speed It is not generally realized that when. regulating an exhauster the speed to be considered is not the actual number of revolutions performed by the drum per minute, but the peripheral speed of the tips of the blades. For a given number of revolutions per minute the peripheral speeds of (say) a 6-inch drum and a 4-feet diameter drum would be widely different, the blade tips travelling at a greatly increased linear velocity in the latter case. In both cases, however, the peripheral speeds should. be approximately equal, and for exhausters of all sizes there is very nearly a constant peripheral speed, which should vary only between the limits of '650 and 850 feet per minute. Thus a 9-inch exhauster running at 275 revolutions per minute gives a peripheral speed of 650 feet per minute, and a 3-foot machine running at 70 revolutions per minute has also a peripheral speed of about 650 feet per minute. Governing the Capacity There are two standard methods of governing exhausters :—■ (a) By inserting an automatically controlled by-pass between inlet and outlet. (b) By automatically throttling the steam supply, or Controlling the electric current. When a gas-engine drive is employed the first method only is practicable, as the speed of the engine cannot be varied to any great extent by the ordinary means of governing employed. Scheines for effecting this have certainly been suggested; but, at the most, the result is only partial, and the most convenient arrangement is to work the engine in conjunction with. a countershaft to which a stepped pulley is fitted. The ordinary arrangement of .governing with a by-pass valve is shown in Fig.