Power Plant Engineering

3. Maximum Efficiency. The maximum efficiency, the turbine can develop, depends upon the
type of the runner used.
In case of impulse turbine, low specific speed is not conducive to efficiency, since the diameter of
the wheel becomes relatively large in proportion to the power developed so that the bearing friction and
windage losses tend to become too large in percentage value. The value of N
S
for highest efficiency is
nearly 20.
The low specific speed of reaction turbine is also not conducive to efficiency. The large dimen-
sions of the wheel at low specific speed contribute disc friction losses. In addition to this, the leakage
loss is more as the leakage area through the clearance spaces becomes greater and the hydraulic friction
through small bracket passages is larger. These factors tend to reduce the efficiency as small values of
specific speed are approached.
The high specific speed reaction turbines are associated with large discharge losses (V
c
2
/2g) as
mentioned earlier. The friction and leakage losses are reduced with an increase in specific speed but the
discharged losses increase rapidly and the net effect of increase in specific speed is to decrease the
efficiency total loss (friction, leakage and discharge) is minimum at medium specific speed. Therefore,
it is always preferable to select the reaction turbines of medium specific speed if they operate at constant
load conditions. Me effect of specific speed on the maximum efficiency is shown in Fig. 11.30.
Higher efficiencies have been attained
with reaction turbines than with Pelton wheels.
The maximum recorded efficiency till now for
reaction turbine is 93.7% but quite a large units
have shown efficiencies over 901-W the highest
recorded value of efficiency for impulse Turbine
is 89% but usual maximum is 82%.
The efficiency of the Pelton wheel is not
dependent on its size like reaction turbine. Hence
the Pelton wheel may have higher maximum
efficiency than the reaction turbine for smaller
powers.