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July, 1914 permits the use of nozzle control. The use of the impulse wheel, fur- thermore, materially shortens the ahead and astern turbines, thus mak- ing a more compact and lighter in- stallation, and at the same time elimi- nating the short high pressure re- action blades so that only a compara- tively few rows of large substantial blades are required. The use of the combination impulse reaction turbine has the advantage that complete ahead and astern tur- bines of high economy can be con- tained in a common cylinder, thus obviating, except under some unusual! condition, the necessity of employing THE MARINE REVIEW bine, since, as previously mentioned, it obviates the short, frail blades of the high pressure section of the pure reaction turbine and obviates the troublesome diaphragms and_ inter- stage packings of the pure impulse turbine. Though the impulse wheel is not as efficient as reaction blading, the advantages which result from its use are so important where high blade speeds can be employed that its use is amply justified. It is possible to so design an im- pulse wheel with two rows of blades that its efficiency will not fall off se- riously when the steam velocity is or HIGS..6 AND 7 the cross-compound arrangement of turbines. Each turbine, whether two Or more turbines are used to» drive a Common gear or independent gears, is a complete unit in itself and may be operated independently of other turbines. In twin or triple screw vessels, each engine room is thus complete in itself without any cross- Connections and the attendant com- Plication required to permit operating fither high or low pressure. turbine When either one or the other is out of commission. _ The combination turbine using the tmpulse and reaction elements, has a decided advantage over the pure re- action or pure impulse type of tur- - only two to two and a half times the blade speed, and will remain nearly constant until the velocity of the steam reaches four to four and one- half times the blade speed. Thus by the use of nozzle control, the efficiency of the high pressure portion of the turbine can be kept nearly constant over a considerable range of power and speed. If we assume an initial condition of 200 Ibs. steam pressure and 550 ft. per second blade speed for the im- pulse wheel, the steam velocity at full power may be as low as 1,200 ft. per second, corresponding to. a heat drop of 32 b. t. u. and expansion from 200 lbs. absolute to 135 lbs. absolute. ae If we consider the power to vary as the cube of the revolutions per minute, then at 37.5 per cent of full power, the revolutions would be 72 per cent of those corresponding to: full power, and if we further assume that it requires 40 per cent of full power steam to develop 37.5 per cent of full power at the reduced revolu- tions, the absolute pressure at the in- let to the reaction blading (which is directly proportional to the flow of steam), would be 40 per cent of 135, or 54 lbs. absolute. In expanding from 200 lbs. absolute to 54 lbs. absolute, 92 b. t. u. would become available, which, allowing for the nozzle efficiency, would corre- spond to a steam velocity of approxi- mately 2,140 feet per second, and since this is but five and four-tenths times the reduced blade speed, the falling off in economy of the impulse wheel would be but very slight. It will be evident, however, that if the impulse blade speed is low, as in the case of some. direct turbine installations, since the b. t. u. drop which can be efficiently provided for in the impulse wheel is very small (generally less than one-fourth of - those used in the above illustration), a comparatively small decrease in power, and consequent increase in heat drop in the impulse wheel, very seriously decreases the efficiency. Special Cruising Turbine Unnecessary With the impulse. wheel, blade speeds possible in a Westinghouse geared installation, such a large range of heat drop can be efficiently em- ployed, that economies only ap- proached at full power in the past, are possible at cruising speeds and special cruising turbines are entirely unnecessary. A difficulty, which arises in the pure impulse marine turbine when designed for high speed in connection with re- duction gearing, is that in order to avoid excessive length it is difficult to obtain hubs of sufficient length be- tween the wheels of the _ various stages to entirely eliminate the danger of distortion of the discs and hubs by gyroscopic action of the discs at high speed. In connection with direct connected turbines, this has frequent- ly been a subject of discussion, but owing to the fact that the bending moment set up in the shaft by the gyroscopic action of one disc neutral- izes that set up by the adjacent discs, this has been considered of little im- portance, but the possibility of side swiping of the blades due to stretch- ing of the hubs from this cause, has been entirely overlooked. From the following illustrations and connected -