A turbine is a machine in which a rotary motion is obtained by transference of the momentum of a fluid or gas. In general the fluid is guided by fixed blades, attached to a casing, and, impinging on other blades mounted on a drum or shaft, causing the latter to revolve.
Turbines are classed in various ways as: 1, radial flow, when the steam enters near the center and escapes toward the circumference; and 2, parallel flow, when the steam travels axially or parallel to the length of the turning body.
Turbines are commonly, yet erroneously classed as:
1. Impulse;
2. Reaction.
Ques. What is the distinction between these two types?
Ans. In the so called impulse type, steam enters and leaves the passages between the vanes at the same pressure. In the so called reaction type, the pressure is less on the exit side of the vanes than on the entrance side.
Fig. 2,750.—Sectional view of Parsons-Westinghouse turbine, showing rotor and governor.
[Fig. 2,750] is a sectional view of the Parsons-Westinghouse parallel flow turbine. Steam from the boiler enters first a receiver in which are the governor controlled admission valves. These valves are actuated by a centrifugal governor.
Steam does not enter the turbine in a continuous blast, but intermittently, or in puffs. The speed regulation is therefore accomplished by proportioning the duration of these puffs to the load of the engine, this being effected by the governor, [fig. 2,752].