flat at the peak, which allows optimal efficiency to be maintained over a wide range of velocity ratios. Ideal diagram efficiency is achieved with a velocity ratio that is twice that of Rateau blading and four times that of Curtis blading. For a 50% reaction design, optimum efficiency occurs at an ideal velocity ratio of 0.707. Therefore, for stages operating at the same diameter and stage pressure drop, the peak efficiency for a reaction stage would occur at a blade velocity, or wheel speed, considerably higher than for an impulse stage.

Enthalpy differentials for each stage of a reaction turbine are usually lower than those for impulse stages. Thus, reaction turbines require more stages. Reaction stages are generally more efficient than impulse stages, with reduced friction losses due to lower flow velocities. However, reaction turbines are more complicated and fragile due to tighter clearances. They are used in constant speed applications when efficiency is of critical importance. The greater number of stages allows for less space per stage and generally less rugged construction.

In practice, stages may be constructed with varying degrees of reaction and impulse blading and turbines may include various combinations of reaction, Curtis and Rateau stages. Turbine blades may be classified according to their degree of reaction or the ratio of the enthalpy drop in the moving blades to the enthalpy drop in the entire stage. Zero reaction would be the case with a pure impulse stage. When there is an equal enthalpy drop in the stationary and moving rows, the blades are 50% reaction.

One design used to balance capital cost and efficiency in larger turbines combines the impulse and reaction principles by employing a Curtis or Rateau stage as the first stage and using reaction stages for the remainder of the expansion. The Curtis stages reduce the pressure of the steam to a moderate level and produce a high proportion of output work, while the more efficient Rateau or reaction stages absorb the balance of the energy available.

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