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friction disc by means of its sleeve and bell crank lever. This revolving disc is keyed to a solid shaft which is driven by a coupling from a hollow shaft. This hollow shaft is driven by the turbine shaft through the medium of a worm gear. The solid shaft, with the continuously revolving friction disc, can be slightly shifted by the governor sleeve so that the two friction discs come into contact when the sleeve moves, that is, when the angular velocity changes. If this change is relatively great, the sleeve will draw the periodically revolving friction disc far from the center of the always revolving one, and this disc will quickly drive the stem of the regulating valve and the flow of steam will thus be regulated. As soon as the angular velocity falls below a certain percentage of the normal speed, the driving friction disc is drawn back by the governor, the regulating valve remains open and the whole regulating mechanism rests or stops, although the shaft is still running.

Should the angular velocity of the shaft reach a point 2.5 per cent higher than normal, the governor will shut down the turbine. If an accident should happen to the governor, due to imperfect material or breaking or weakening of the springs, the result would be a shutdown of the turbine.

In order to change the speed of the turbine while running, which might be necessary in order to run the machine parallel with another prime mover, a spring balance is provided, attached to the bell crank lever of the regulating mechanism. The hand wheel of this spring balance is outside of the pedestal for regulating mechanism and near the floor-stand and hand wheel. With this spring balance the speed of the turbine may be changed 5 per cent either way from normal.

All the bearings of the unit are thoroughly lubricated with oil forced under pressure by the oil pump driven by means of worm gearing by the turbine itself. After flowing through the bearings the oil is passed through a filter and from thence to the oil tank located within the bed plate, from whence it is taken by the oil pump. All revolving parts are enclosed, and the principal part of the regulating mechanism operates in a bath of oil.

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

B

of steam 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, 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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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

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