294 STEAM TURBINES
absolute velocity v₁ of steam leaving the first set of moving vanes would equal the absolute velocity V, in passing through the inter- mediate set of guide vanes; and relative velocities R, and r, would be equal. The dotted arcs indicate which velocities are to be drawn equal in this construction. Case Where There is Loss Through Friction.-If it is desired to take account of frictional loss this may be done as in Fig. 9, where relative velocity r, is made less than relative velocity R₁; absolute velocity V₂ less than v₁; and relative velocity r₂ less than R. The diminution of the velocities may be made either an arbitrary amount in each case or a certain percentage of the velocity, as desired.
It will be noted that in Figs. 8 and 9 the vane angles are differ- ent for the two wheels. If desired to make them the same for convenience in manufacture, a plan must be followed similar to that now to be described in connection with reaction blades.
The Vanes of Reaction Turbines.
In Fig. 10 is a diagram by which the action of the steam in reaction turbines may be studied. For convenience in manu- facture the guide and moving vanes in any one step or series of the turbine are usually made alike. The upper parallelogram shows the absolute velocity and direction of steam leaving the guide vanes, its velocity and direction R relative to the moving vancs at the point of entrance, and the velocity and direction w of the moving vanes.
The lower parallelogram shows the absolute velocity and direc- tion of the steam leaving the moving vanes, its velocity and direction r relative to the moving vanes at the point of exit, and the velocity and direction w of the moving vanes. Characteristics of the Reaction Diagram. The essential dif- ference between this diagram and those for impulse turbines is that the relative velocity r is greater than R. In the reaction tur- bine steam first expands and acquires a velocity in the guide pas- sages, as in the impulse turbine. Then, in flowing through the wheel passages it continues to expand and acquires a greater velocity relative to the moving vanes than it had at the entrance. The absolute velocity of the steam, of course, diminishes.
