The Francis turbine is a hydraulic turbine used in hydraulic power installations that operate at a considerable height.
James Bicheno Francis developed this turbine. The function of the Francis turbine is mainly to generate electricity with the help of a generator.
The Francis turbine is the most common type of turbine installed in power generation plants that operate based on the mass flow of water through a production plant.
In contrast to the Pelton wheels, the Francis turbine runs at its best, filled with water at all times.
Francis Turbine, Advantages, and Disadvantages
The main benefits it offers are:
It is straightforward to control even with variable heights.
The variation of its efficiency is very low over time.
The corridor size is small compared to other types of turbines.
It has a lower maintenance cost compared to other types of turbines.
The main disadvantages are:
Francis turbines are designed for a specific flow. It is not the best option to use in front of significant flow variations since the performance lowers.
It presents problems when working with heights greater than 800 m.
It has a cavitation problem.
The entry of dirty water with particles can cause significant problems to the turbine.
Francis Turbine Is Usually Used For
Large Francis turbines are individually designed to operate with the water supply and water head, given the highest possible efficiency.
In addition to the production of electrical energy, this turbine can be installed in pumped-storage hydroelectric plants. In this type of hydroelectric power station, there are two reservoirs at different heights. When there is energy surplus, the upper reservoir is filled with water pumped by the turbine that performs the pump function.
The electric generator acts as a large electric motor to pump water during periods of low power demand. When energy demand is high, the generator is reversed and used to generate electricity.
Francis Turbine Working
The working principle of a Francis turbine is as a type of reaction turbine. It uses the kinetic energy of running water as pressure energy to turn a water wheel.
The turbine is located between the intake port where the high pressure is applied and the low-pressure discharge port.
The inlet part of the turbine is a spiral-shaped casing (spiral case), and several guide vanes make the water flow tangentially into the corridor. We can adjust the opening degree of the guide fin for efficient operation according to the amount of water used. Decreasing the cross-sectional area of the casing ensures that the flow enters the central part of the turbine with uniform velocity throughout the perimeter.
The water that passes through the corridor also acts on it when directed towards the axial center. Both impulse force and lift force will make the runner rotate.
Finally, the water comes out with the kinetic energy, and with the potential energy minimized, and the vortex also disappears. The water is deflected in the axial direction to exit a draft tube to the tailrace.
The turbine's exit tube is shaped to help decelerate the water flow and recover the pressure.
What Are the Parts of a Francis Turbine?
The turbine designed by James B. Francis is composed of:
Spiral chamber: it has the function of distributing the fluid at the entrance of the impeller. The spiral or snail shape allows the average speed of the liquid to remain constant.
Predistributor: It is composed of stay vanes that have a structural function. The hydrodynamic design allows hydraulic losses to be minimized.
Distributor: made up of moving turbine blades to direct the water towards the impeller blades (fixed) and regulate the admitted flow. The distributor allows running the fluid to improve performance and adjust the power to the load variations of the electrical network. Stay and guide vanes guide the water to the runner blades. The upper part of the blades uses the reaction force of water flowing through it.
Rotor or impeller: it is responsible for the exchange of kinetic energy, potential energy, and pressure energy into mechanical energy of rotation in the rotation of the impeller. The impeller, in turn, transmits this energy to an electric generator.
Suction tube: the suction tube is the outlet of the turbine.
Governing system: Governing system is an essential device that controls the speed or the power output of a turbine.
What Is the Performance of a Francis Turbine?
Francis turbines have high hydraulic efficiency, more than 90%. In addition, this type of hydraulic turbine can be designed with a varied range of heads and flows.
They have a wide range of activities compared to the height of the fluid flowing through the turbine.
When it works with water, this turbine achieves optimum performance in a drop from 20 meters to 700 meters. The power output varies from a couple of kilowatts to 750 MW. The diameter of the rotor can be from 1 to 10 m, and the speed of rotation is from 83 to 1000 r.p.m.