As an example of a partial admission radial flow impulse turbine, a 100 h.p. turbine at Immenstadt may be taken. The fall varies from 538 to 570 ft. The external diameter of the wheel is 41⁄2 ft., and its internal diameter 3 ft. 10 in. Normal speed 400 revs. per minute. Water is discharged into the wheel by a single nozzle, shown in fig. 202 with its regulating apparatus and some of the vanes. The water enters the wheel at an angle of 22° with the direction of motion, and the final angle of the wheel vanes is 20°. The efficiency on trial was from 75 to 78%.
§ 199. Theory of the Impulse Turbine.—The theory of the impulse turbine does not essentially differ from that of the reaction turbine, except that there is no pressure in the wheel opposing the discharge from the guide-blades. Hence the velocity with which the water enters the wheel is simply
vi = 0.96 √2g (H − ɧ),
where ɧ is the height of the top of the wheel above the tail water. If the hydropneumatic system is used, then ɧ = 0. Let Qm be the maximum supply of water, r1, r2 the internal and external radii of the wheel at the inlet surface; then
ui = Qm / {π(r22 − r12)}.
The value of ui may be about 0.45 √2g (H − ɧ), whence r1, r2 can be determined.
The guide-blade angle is then given by the equation
sin γ = ui / vi = 0.45 / 0.94 = .48;
γ = 29°.
The value of ui should, however, be corrected for the space occupied by the guide-blades.