Turbine Water

The draft tube also, if well designed, serves a further purpose in that it enables a large proportion of the kinetic energy of dis charge from the runner to be converted into pressure head, and so to be utilized. For this the tube must be designed with a gradu ally increasing diameter, so that the velocity is gradually reduced from vi to v2 before discharge (fig. 4). The velocity of discharge from the draft tube should not exceed about 4 f.s.

Hydraulics of the Reaction Turbine.

In the following dis cussion let w = angular velocity of the runner in radians per second (co= 2 in N÷.6o where N = revolutions per minute) ; u = coy =velocity of wheel at point indicated by a suffix; v = ab solute velocity of water; w= tangential component of v; f = radial component of v; v= relative velocity of water and vane; a = guide vane angle ; fl =wheel vane angle at entrance; 7 =wheel vane angle at exit; Q = flow in c.f.s., W = weight of i cu.ft. of water; suffix (2) refer to inlet to wheel vanes; suffix (3) refer to exit from wheel vanes.

For entry without shock, the direction of the relative velocity is increased the value of 12 and hence the volume of water passing a wheel of given size diminishes, so that to obtain the same output the size of the wheel must be increased. If, as is usually the case in low-head plants, a high rotative speed is required, the inlet area is increased by increasing the depth of the runner. Such a turbine has a comparatively large ratio of inlet area to discharge area, and the velocities of discharge are relatively high. For high heads 13 may be between 6o° and 9o°, and, for medium and low heads, between 9o° and 135°. Similarly, while the hydraulic efficiency decreases as a increases, the volume of flow increases with a, and the maximum output is obtained when the product of Q and i is a maximum. For high efficiency a should be as small as mechan ical considerations permit, generally between 12° and i8°.

In one modern turbine, the Kaplan, the wheel vanes are not fixed but are pivoted on a central drum, and their leading angle can be adjusted while running so as to suit any variations in the working head or discharge. In this way a very high efficiency may be maintained at all loads.

Specific Speed of a Turbine.

In order to afford a basis of comparison of turbines of different diameters and proportions operating under different heads, the term known as "specific speed" has been introduced. This may be defined as the speed at which a runner would operate if reduced geometrically to such a size that it would develop r h.p. under unit working head. The figures for specific speed given below refer to a unit head of r foot. If the metre be adopted as the unit, these figures require to be multiplied by 4.45.

The specific speed of a reaction turbine may be varied by vary ing the diameter of the runner, the angle of the guide vanes and the angle of the wheel vanes. By modifying the design as indi cated in the sketches of fig. 3 it is possible, while maintaining high efficiencies at full load, to increase the specific speed from about 15, its minimum value with the type shown in fig. 3A, to about 125 with the type shown in fig. 3D. Specific speeds as high as 150 are possible with some sacrifice in efficiency, and it is probable that further developments will see the value increased still farther. These high specific speeds are extremely valuable for low-head installations since they enable the size and cost of the turbine and of the generator to be greatly reduced. In fact, many existing low-head installations would have been commercially impracticable but for the development during recent years of the high-speed turbine.

High specific speeds are, however, attended by some disadvan tages. The part-gate efficiency in general falls off as the specific speed increases. Also if the speed is unduly high it becomes very difficult to avoid very high local velocities and centres of low pressure in the runner, which invariably give rise to severe corro sion. At the present stage of design, the maximum specific speeds to be used under normal circumstances with various heads are ap proximately as follows: The reaction turbine may be built either as a horizontal or vertical shaft machine. The latter type, having the weight of the rotating parts supported by a thrust bearing of the Michell or Kingsbury type, has been gaining in popularity of recent years.