inner tube E, and the middle portions of the segments touch the outer tube A. Oil is supplied in this case also to the

spaces between the tubes and sleeve, but the fluid friction is aided by the elasticity of the segments F, G, H. In both cases suitable means, such as projections D, are provided to prevent rotation of the sleeve K. The tubes are often perforated.

Any end-thrust of the spindle, due to want of perfect

j k l m

FIG. 66.-Thrust-block of Parsons Turbine.

FIG. 67. Slotted Ring for Thrustblock.

balance of the steam pressure, is taken up by the thrustblock L (Fig. 60), which is made in halves and provided with flanges and recesses to engage with recesses and flanges on the spindle. Sometimes the construction shown in Fig. 66 is

adopted where rings, i, j, k, l, m, are used, which are separate from both block and spindle, and are of sufficient diameter and thickness to possess the requisite elasticity. The elasticity may be increased by providing slots in the rings as shown in Fig. 67; or spring washers may be inserted between the rings and the recesses for them in the block L.

All these devices for taking up end-thrust and damping vibration have been patented by Parsons.

Radial-flow turbines were constructed by Messrs. C. A. Parsons and Co. chiefly between 1889 and 1891. Parsons turbines are now usually constructed of the parallel-flow type.

Fig. 68 shows in vertical longitudinal section a modern Parsons parallel-flow turbine. Steam passes through the equilibrium valve H and enters the annular space J, from which it proceeds through the fixed and moving blades in the highpressure cylinder, or part A; then through those in the intermediate cylinder, or part B; and then through those in the low-pressure cylinder, or part C. The arrangement and construction of the rings of blades will be best understood by referring back to Figs. 3-8. In passing through these rings of blades the steam is expanded in small steps from the initial pressure right down to the pressure in the exhaust pipe, which in a condensing turbine ought to be practically the pressure in the condenser.

In order to balance the axial thrust of the steam on the moving blades, balance pistons, D, E, F, are provided. The spaces between these balance pistons are connected by ducts, U, V, with different parts of the turbine casing, while the space beyond the largest balance piston is connected to the exhaust end of the turbine by the pipe W.

Another duct of

the same nature as U and V sometimes takes the place of the pipe W.

Parallel-flow turbines are not now made double, as shown in Figs. 54 and 57, as this necessitates the employment of a much longer shaft. Besides, as this construction really means two smaller turbines instead of one larger one, the cost is greater and the efficiency less.

Instead of making the turbine cylinder of increasing

diameter, the fixed rings of blades at the high-pressure end. may contain only a few blades, the spaces where blades are not placed being occupied by solid or hollow segments. The number of blades on the fixed rings will then increase progressively from the high-pressure end to the low-pressure end of the turbine. All the moving rings, however, are provided with blades round their whole circumferences. A section. of the fixed and moving blades is shown drawn to a large scale in Fig. 69, where 10 represents the fixed blades, 11 the

E

spaces between them for the passage of steam, 12 the segments occupying the remaining space of the fixed rings, and 13 the rotating blades.

When the turbine is increased in diameter by steps as shown in Fig. 68, it is desirable that the area of section for the passage of the steam shall increase continuously. Messrs. C. A. Parsons and Co. therefore construct the fixed blades at the high-pressure end of each cylinder or part of the turbine with narrower exit openings than at the low-pressure end of the same cylinder or part. Thus the blades at the highpressure end, M, of the cylindrical part B of the turbine shown in Fig. 68 are arranged with wider escape openings than at the low pressure end, N, of the same part B.

The angular velocity of a Parsons turbine depends on the initial and terminal pressures, and on the number and diameter of the rings of blades. An idea of the usual speeds of rotation is given by Table I., which has been compiled from turbines now running :

TABLE I.

SPEEDS OF ROTATION OF PARSONS TURBINES (CONDENSING).

By providing more rings of blades the number of revolutions per minute can be diminished when required. A turbine can also be arranged to suit the steam pressure without altering