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THE STEAM TURBINE-FUNDAMENTAL PRINCIPLES
Ques. 707.-What are the basic principles governing the action of steam turbines?
Ans.-There are wo fundamental principles upon which all steam turbines operate, viz., reaction and impulse. In some types of turbines the reaction principle alone is utilized, and in others the impulse, while in still others, and probably the most successful ones, both principles are combined.
Ques. 708.-In what general direction does the steam flow when used in a turbine?
Ans.--Parallel with the shaft or rotor, and also in a screw-like direction around it. This definition does not apply, however, to turbines of the purely impulse type, like the De Laval, for instance.
Ques. 709.-What causes the rotor to revolve?
Ans.—The action of the steam, coming, as it does, with tremendous velocity and great force against the small buckets or vanes with which the rotor is fitted, causes it to revolve, and as there is a continuous current of steam passing into the cylinder, the motion is continu
Ques. 710.—What law of turbo-mechanics governs the relation of bucket-speed, and fluid or steam speed?
Ans.-For purely impulse-wheels, bucket-speed equals one-half of jet-speed. For reaction wheels, bucket-speed equals jet-speed.
Ques. 711.—With what velocity would steam of 100 pounds pressure discharge into a vacuum of 28 inches?
Ans.—The theoretical velocity would be 3,860 feet per second.
Ques. 712.-What amount of energy would a cubic foot of steam under 100 pounds pressure exert if allowed to discharge into a vacuum of 28 inches?
Ans.--59,900 foot pounds.
Ques. 713.-Does the steam impinge against the first rows or sections of buckets at full pressure?
Ans. In turbines of the Parsons type, the initial pressure of the steam is practically boiler-pressure, but it gradually falls as it I us on through the cylinder, which becomes larger in diameter as the exhaust end is approached. In other types of turbines, the steam is admitted to and directed against the blades or buckets, through expanding nozzles, and by the time it strikes the first stage, or section of moving vanes, the pressure has fallen to one-third or less of the original boiler-pressure, but the velocity is very great.
Ques. 714.-In what particular respect does the steam turbine appear to possess an advantage over the reciprocating engine, in the use of steam?
Ans.-The turbine, if designed along correct lines, is capable of utilizing in the highest degree one of the most valuable properties of steam, viz., velocity.
Ques. 715.-Give an example of the great increase in the amount of work performed by an agent when velocity is one of the factors made use of.
Ans.-Suppose that a man is standing within arm's
length of a heavy plate-glass window and that he holds in his hand an iron ball weighing 10 pounds. Suppose the man should place the ball against the glass and press the same there with all the energy he is capable of exerting. He would make very little, if any, impression upon the glass. But suppose that he should walk away from the window a distance of 20 feet, and then exert the same amount of energy in throwing the ball against the glass, a different result would ensue. The velocity with which the ball would impinge against the surface of the glass would no doubt ruin the window. Now, notwithstanding the fact that weight, energy, and time involved were exactly the same in both insla..ces, yet a much larger amount of work was performed in the latter case, owing to the added force imparted to the ball by the velocity with which it impinged against the glass.
Ques. 716.-Describe the construction and action of the De Laval steam turbine.
Ans.—The De Laval steam turbine is termed by its builders a high-speed rotary steam-engine. It has but a single wheel, fitted with vanes or buckets of such curvature as has been found to be best adapted for receiving the impulse of the steam-jet. There are no stationary or guide-blades, the angular position of the nozzles giving direction to the jet. The nozzles are placed at an angle of 20 degrees to the plane of motion of the buckets. The heat energy in the steam is practically devoted to the production of velocity in the expanding or divergent nozzle, and the velocity thus attained by the issuing jet of steam is about 4,000 feet per second. To attain the
maximum of efficiency, the buckets attached to the periphery of the wheel against which this jet impinges should have a speed of about 1,900 feet per second, but, owing to the difficulty of producing a material for the wheel strong enough to withstand the strains induced by
such a high speed, it has been found necessary to limit the peripheral speed to 1,200 or 1,300 feet per second.
Ques. 717.—Describe the action of the steam in its passage through the De Laval diverging nozzle.
Ans. It is well known that in a correctly designed nozzle the adiabatic expansion of the steam from max
imum to minimum pressure will convert the entire static energy of the steam into kinetic. Theoretically this is what occurs in the De Laval nozzle. The expanding steam acquires great velocity, and the energy of the jet of steam
FIG. 189. DE LAVAL, NOZZLE, SECTION A Virw.
issuing from the nozzle is equal to the amount of energy that would be developed if an equal volume of steam were allowed to adiabatically expand behind the piston of a reciprocating engine, a condition, however, which for obvious reasons has never yet been attained in practice