ACTUAL NOZZLE ANGLE
We must now modify this result to account for the geometry restrictions of a real turbine. In our derivation, the acute angle between V1 and the tangential direction, called the nozzle angle, is zero. In an actual turbine, because of physical constraints, the nozzle angle must be greater than zero but not so great as to cause an appreciable loss in efficiency. Nor should the angle be so small as to cause an excessively long nozzle that would increase friction and decrease efficiency. "The values used in practice range from 10 to 30 deg., 12 to 20 deg. being common. The larger angles are used only when necessary and usually at the low-pressure end of large turbines." Equation (1), corrected for a finite nozzle angle, a, becomes: