Heat engineering; a text book of applied thermodynamics for engineers and students in technical schools . i jS STEAM NOZZLES, INJECTORS, STEAM TURBINES 297 This is in reality the same proof as the former one since iffor a fixed wb and blade the triangle is isosceles giving wa,for a fixed wa and blade the isosceles triangle should be the mostefficient. The three cases have proven that where the angle a is fixedthe turbine blade must travel at one-half the velocity of whirl,whether there be friction or not, and that if the angles /3 and ftare fixed the best efficiency with either wa fixed or wb
Heat engineering; a text book of applied thermodynamics for engineers and students in technical schools . i jS STEAM NOZZLES, INJECTORS, STEAM TURBINES 297 This is in reality the same proof as the former one since iffor a fixed wb and blade the triangle is isosceles giving wa,for a fixed wa and blade the isosceles triangle should be the mostefficient. The three cases have proven that where the angle a is fixedthe turbine blade must travel at one-half the velocity of whirl,whether there be friction or not, and that if the angles /3 and ftare fixed the best efficiency with either wa fixed or wb fixed isobtained when the inflow triangle is isosceles. This holdswhether there be friction or not. In the first case without fric-tion, the outflow will be perpendicular to the movement of theblade while in all other cases the line may be inclined , however, the speed of the blade is desired and the angle a isfixed, then the highest efficiency is obtained if the velocity ofwhirl is made twice the speed of the blade. This fixes Fig. 145.—Velocity compounding. In the above figures it has been seen that when the steam isused on one blade the velocity of this blade has to be equal toone-half the velocity of whirl if a is fixed and /3 and 180 — ft arefixed. This means that the velocity of the wheel would be veryhigh. To reduce this, the velocity of whirl could be decreasedby decreasing the velocity from the nozzle. This is obtained by asmall drop in pressure in the nozzle. If steam is to be used through a large difference in pressurethis would have to be utilized in a series of nozzles and is called pressure compounding. Another method is touse a series of movable and fixed blades as shown in Fig. 145,utilizing the high kinetic energy of discharge from one blade insuccessive blades. This is known as velocity compounding. 298 HEAT ENGINEERING The velocity diagrams are combined in Fig. 146, in which0 = 180 - ff0i = 180 - /3\In this the work per pound i
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