THE STEAM TURBINE CHAPTER I. GENERAL REMARKS ON TURBINES. A TURBINE is a machine in which a rotary motion is obtained by the gradual change of momentum of a fluid. B 1 B B Fig. 1 shows a turbine diagrammatically. The partitions B between the passages A are called vanes, or blades, or buckets. Now, it is obvious that, if a fluid enters the space between two vanes in the direction shown by the arrow 1, and leaves in the direction shown by the arrow 2, the component of its velocity perpendicular to the radius will gradually change in its passage. The component might not change during the whole of the passage of the fluid owing to the vanes themselves having a velocity; but it will have a gradual change during at least some part of this passage. The fluid, therefore, has its momentum gradually changed, and it is this change of momentum which causes the vanes to rotate. The turbine wheel in the figure would rotate in the direction of the arrow 3. The action of the fluid on the FIG. 1.-Diagrammatic Illustration of Turbine. B turbine will be discussed more fully later on; it is only desired at present to give a general idea of a turbine. Turbines may be classified in several ways. Firstly, they may be classified according to the actuating fluid. The fluids most commonly used are water and steam, and the turbines actuated thereby are called respectively hydraulic turbines and steam turbines. Turbines may be classified according to the direction of flow of the fluid into three classes: (1) In radial-flow turbines the fluid travels from the centre to the circumference of the wheel, or from the circumference to the centre. This class is subdivided into outward-flow and inward-flow turbines, according as the fluid passes from the centre to the circumference, or from the circumference towards the centre. (2) In parallelflow or axial-flow turbines the direction of the flow of the fluid is parallel to the axis of the wheel, or in a spiral co-axial with the wheel. (3) In mixed-flow turbines the fluid flows both as in a radial-flow and as in a parallel-flow turbine. Turbines are classified in other ways besides these; but as the other ways are not of importance, or do not hold good with steam turbines, we shall not refer to them. Fig. 2 illustrates the principle of a parallel-flow De Laval steam turbine. The steam reaches the wheel by way of the divergent nozzles, where it expands and attains a great velocity. With this velocity it impinges on the vanes of the wheel, and causes the latter to rotate at a high speed. The wheel is enclosed loosely in a box or case, from which the steam escapes to the atmosphere or to a condenser. A section of one of the nozzles is shown at Fig. 2A drawn to an enlarged scale. In this figure the dotted line indicates the axis of rotation of the Figs. 3, 4, 5, 6, 7, and 8 illustrate parts of a Parsons parallel-flow steam turbine. In this turbine the steam acts successively on a number of rings of blades. Part of one of these is shown in perspective view in Fig. 3, in elevation in FF FIG. 3.-Blades and Shrouds of Parsons Parallel-flow Steam Turbine. Fig. 4, and in plan in Fig. 5. Each ring of blades in this example is formed of blades, c, gripped in suitable recesses in shrouds, A and B. The rings thus formed are fixed alternately to the inside of the fixed cylindrical casing of the turbine, and to a revolving drum mounted inside the casing. Figs. 6 and 7 show parts of the casing and drum, the casing being lettered I and the drum H. Fig. 6 is a section taken through Blades and Shrouds of a Parsons Parallel-flow Steam Turbine. the axis of the casing, while Fig. 7 is a cross-section on the Power is obtained from the spindle G, line CD of Fig. 6. on which the drum H is keyed. It will be seen that the larger shroud A of each ring is secured to the casing or drum, while the smaller shroud B is free. The steam passing in the direction of the arrows in Fig. 6 acts on the moving blades so as to rotate them, and with them the drum and spindle. The fixed blades serve as guides to cause the steam after leaving one ring of moving blades to impinge in the right direction on the next ring of moving blades. The action of the steam on the blades can be clearly seen in Fig. 8, where the vertical arrows show the direction of motion of the moving blades and the horizontal arrows the direction of flow of the steam. This figure, which is drawn about full size, also shows the size, shape, and arrangement of the blades. These particulars, however, vary somewhat. Fig. 9 is a partial axial section through a Parsons radial FIG. 9.-Parsons Radial-flow Steam flow turbine, and Fig. 10 illustrates a ring of blades for the same drawn to an enlarged scale. The blades c, both fixed and moving, |