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CHAPTER VII

DE LAVAL STEAM TURBINE

De Laval steam turbine-High velocity—The De Laval divergent

nozzle-Adiabatic expansion of steam within nozzle-Conversion of static energy into kinetic-Form of De Laval wheelSpeed of buckets-Speed of turbine shaft, and how it is reduced-Construction of the wheel-Number of buckets required—Number of nozzles-Gear and flexible shaftDescription of governor-Vacuum valve-Operation of governor-Efficiency tests—Steam consumption-Cross section of wheel showing correct design-Table of sizes, giving speed and weight. The De Laval steam turbine, the invention of Carl De Laval of Sweden, is noted for the simplicity of its construction and the high speed of the wheel -10,000 to 30,000 R. P. M. The difficulties attending such high velocities are, however, overcome by the long, flexible shaft and the ball and socket type of bearings, which allow of a slight flexure of the shaft in order that the wheel may revolve about its center of gravity, rather than the geometrical center or center of position. All high speed parts of the machine are made of forged nickel steel of great tensile strength. But one of the most striking features of this turbine is the diverging nozzle, also the invention of De Laval.

It is well known that in a correctly designed nozzle the adiabatic expansion of the steam from maximum :o 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 issuing froin 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, how

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ever, which for obvious reasons has never yet been attained in practice with the reciprocating engine. But with the divergent nozzle the conditions are different.

Referring to Fig. 142, a continuous volume of steam

at maximum pressure is entering the nozzle at E, and, passing through it, expands to minimum pressure at F, the temperature of the nozzle being at the same time constant and equal to the temperature of the passing steam. The principles of the De Laval expanding nozzle are in fact more or less prominent in all steam

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FIGURE 143. THE DE LAVAL TURBINE WHEEL AND NOZZLES.

turbines. The facilities for converting heat into work are increased by its use, and the losses by radiation and cooling influences are greatly lessened.

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

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guide blades, the angular position of the nozzles giving direction to the jet. Fig. 143 shows the form of wheel

and the nozzles. The nozzles are placed at an angle of 20° to the plane of motion of the buckets, and the course of the steam is shown by the illustration.

The heat energy in the steam is practically devoted to the production of velocity in the expanding on divergent nozzle, and the velocity thus attained by the issuing jet of steam is about 4,000 ft. 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 ft. 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 ft. per second.

Fig. 144 shows a De Laval steam turbine motor of 300 H. P., which is the largest size built up to the present time, its use having been confined chiefly to light work.

The turbine illustrated in Fig. 144 is shown directly connected to a 200 K. W. two-phase alternator. The steam and exhaust connections are plainly shown, as also the nozzle valves projecting from the turbine casing. The speed of the turbine wheel and shaft is entirely too high for most practical purposes, and it is reduced by a pair of very perfectly cut spiral gears, usually made 10 to 1. These gear wheels are made of solid cast steel, or of cast iron with steel rims pressed

The teeth in two rows are set at an angle of 90° to each other. This arrangement insures smooth running and at the same time checks any tendency of the shaft towards end thrust, thus dispensing with a thrust bearing

The working parts of the machine are clearly illus

on.

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