MIBRAHIM

Single-jet Pelton wheel turbines are used to generate an electrical power of 27 MW for the available gross head of 280 meters in a hydro-electric power plant exploitation a natural water resource. Using the information provided below find the number of pelton wheels required for the task??? Relevant information: Overall Efficiency speed of the wheels, 88% Maximum Specific Speed of wheels, Ns = 40 Loss of head in pipes bringing water to nozzle, hf = 20 m Coefficient of discharge for nozzle, CD = 0.94 Coefficient of velocity for nozzle, CV = 0.97 Bucket speed / Jet speed ratio = 0.46 Rotational speed of wheels, N = 640 rev/min What I have done is find the Specific feed and then dividing that answer by the maximum specific speed of the wheels but I am getting 2.4 while my lecturer found 7 somehow as the final value. If someone can help it would be appreciated. I used: Specific Speed: Ns = NP0.5 / H5/4 = 640 x 27,0000.5 / 2805/4 = 91.82 / 40 = 2.3 ???/ i NEED TO GET 7. i PROBABLY AM USING THE WRONG METHOD so any help would be appreciated