is the available head for working the turbine, and on this the
calculations for the turbine should be based. In some cases it is
necessary to place the turbine above the tail-water level, and there
is then a fall [h] from the centre of the outlet surface of
the turbine to the tail-water level which is wasted, but which is
properly one of the losses belonging to the turbine itself. In that
case the velocities of the water in the turbine should be calculated
for a head H - [h], but the efficiency of the turbine for the
head H.
§ 186. _Gross Efficiency and Hydraulic Efficiency of a Turbine._--Let
T_d be the useful work done by the turbine, in foot-pounds per second,
T_t the work expended in friction of the turbine shaft, gearing, &c.,
a quantity which varies with the local conditions in which the turbine
is placed. Then the effective work done by the water in the turbine is
T = T_d + T_t.
The gross efficiency of the whole arrangement of turbine, races, and
transmissive machinery is
[eta]_t = T_d/CQH_t. (6)