Francis turbine Applications

Application

Francis turbines can be designed for various heads and flows.This versatility and high efficiency make them the most widely used turbines in the world.Francis units have heads ranging from 40 to 600 meters (130 to 2,000 ft), and they are connected to generators with outputs ranging from a few kilowatts to 1,000 megawatts.Large Francis turbines are individually designed for each site to operate at the highest possible efficiency for a given water flow and head, typically over 90% (to 99%).Compared with Pelton turbines Francis turbines are always in the best state of being filled with water.Turbines and outlet channels can be placed below external lake or sea level, reducing cavitation.In addition to electricity production, they can also be used for pumped hydro storage, where reservoirs are filled by turbines (acting as pumps) and generators act as large electric motors during periods of low electricity demand, then reversed and used to generate electricity during periods of peak demand.These pumped storage reservoirs act as large-scale energy storage, storing "excess" electricity in the form of water in elevated reservoirs.This is one of the few methods that allows storage of temporary excess capacity for later use.

Draft tube 

The draft tube is a draft tube installed at the outlet of the turbine runner which is used to utilize the kinetic energy of the water at the outlet of the runner.A draft tube at the end of the turbine increases the pressure of the exiting fluid at the expense of its speed.This means the turbo can reduce pressure to a greater extent without worrying about tailstock backflow.In impulse turbines, the usable head is so high that the efficiency is not significantly affected if the turbine is placed a few meters above the tailstock.But with reaction turbines, if the net head is low and the turbine is mounted above the tailstock, there can be a significant loss of available head to power the turbine. Also, if the fluid pressure in the wake is higher than the pressure at the turbine outlet, backflow of fluid into the turbine can cause serious damage.By placing a guide tube (also known as a diffuser or draft tube) at the turbine outlet, the turbine head is increased by reducing the outlet velocity, thereby increasing the overall efficiency and output of the turbine.The working principle of the draft tube is to convert part of the kinetic energy at the outlet of the turbine runner into useful pressure energy.Using a draft tube also has the added benefit of allowing the turbine structure to be positioned above the tailstock, making any necessary inspections easier and reducing the amount of excavation required for construction.

Types of draft tube

1.Conical Diffuser or Straight Diverging Tube-This type of draft tube consists of a conical diffuser with a half angle usually 10° or less to prevent flow separation.Usually used for low specific speed, vertical shaft Francis turbines.These nozzles are 90% efficient.

2.Simple elbow draft tube-consists of an extended elbow draft tube.Typically used when the turbine must be placed close to the tailway.To help reduce excavation costs, the outlet diameter should be as large as possible to recover the kinetic energy at the runner outlet.This diversion tube is nearly 60% less efficient.

3.Variable cross-section elbow-similar to the elbow draft pipe, except that the curved part is variable cross-section and the outlet is rectangular.The horizontal portion of the draft tube is generally sloped upwards to prevent air from entering at the outlet end.