efficiency () can be defined as the ratio of the work done on the blades to kinetic energy supplied to the fluid, and is given by Stage efficiency A stage of an impulse turbine consists of a nozzle set and a moving wheel. The stage efficiency defines a relationship between enthalpy drop in the nozzle and work done in the stage. Where is the specific enthalpy drop of steam in the nozzle. By the first law of thermodynamics: Assuming that is appreciably less than , we get . Furthermore, stage efficiency is the product of blade efficiency and nozzle efficiency, or . Nozzle efficiency is given by , where the enthalpy (in J/Kg) of steam at the entrance of the nozzle is and the enthalpy of steam at the exit of the nozzle is . The ratio of the cosines of the blade angles at the outlet and inlet can be taken and denoted . The ratio of steam velocities relative to the rotor speed at the outlet to the inlet of the blade is defined by the friction coefficient . and depicts the loss in the relative velocity due to friction as the steam flows around the blades ( for smooth blades). The ratio of the blade speed to the absolute steam velocity at the inlet is termed the blade speed ratio . is maximum when or, . That implies and therefore . Now (for a single stage impulse turbine). Therefore, the maximum value of stage efficiency is obtained by putting the value of in the expression of . We get: . For equiangular blades, , therefore , and we get . If the friction due to the blade surface is neglected then . Conclusions on maximum efficiency For a given steam velocity work done per kg of steam would be maximum when or . As increases, the work done on the blades reduces, but at the same time surface area of the blade reduces, therefore there are less frictional losses. Reaction turbines In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam