Francis turbines are the most frequently used hydro turbines including 60% of the global hydropower capacity in the globe. This is a large rotary mechanical device, used to change both the energies like potential and kinetic into hydroelectricity. These types of turbines have been used for over 100 years for the generation of industrial power & at present used for the generation of hydropower energy. These turbines work efficiently in a broad range of operating conditions. This article discusses an overview of a Francis turbine, construction with working & its applications.
What is Francis Turbine?
Francis turbine definition is the combination of impulse turbine and reaction turbine. It is a mixed flow turbine where the water flows radially toward the impeller blades and exits axially. These turbines are mostly used in medium or large hydropower plants. These turbines are used for heads that are 2 meters low & 300 meters high.
The main benefit of these turbines is that they work the same whether they are arranged horizontally or vertically. These turbines are most frequently used in hydroelectric plants. All the blades within the turbine turn through both the reaction & impulse water force supplying throughout the blades. So that efficiency can be increased for the turbine and generates more electricity within the hydroelectric plant.
Francis Turbine Construction
A Francis turbine construction can be done by using different components. So Francis turbine components mainly include Spiral Casing, Guide Vanes, Stay vanes, Draft Tube, Runner Blades, Penstock & Impeller or Runner.
- Spiral Casing
- Guide Vanes
- Stay vanes
- Draft Tube
- Runner Blades
- Penstock
Spiral Casing
The spiral casing is the inlet medium for water supply to the turbine. This pipe allows passing the water flow from the dam or reservoir with high pressure. The turbine blades are arranged circularly, which means the water hitting the blades of the turbine must flow within the circular axis for efficient hitting. This is the reason to use spiral casing but because of the circular water movement, it loses its force. To keep the same force, the diameter of the casing will be gradually reduced.
Guide Vanes
These vanes are not immobile, but they transform their angle based on the requirement to control the hitting angle of water toward turbine blades to enhance efficiency. They also control the water flow rate into the runner blades so the power output is controlled for a turbine based on the load of the turbine.
Stay Vanes
Stay vane’s function is to guide the water flow toward the runner blades. These vanes stay motionless at their place & decrease the swirling of water because of radial flow. Once it enters the runner blades, then the turbine will become very efficient.
Draft Tube
The force at the outlet of the runner in the turbine is usually low as compared to atmospheric force. The water supply at the outlet cannot be discharged directly to the tailrace. So, a pipe or tube is used for water discharging from the outlet of the turbine toward the tailrace. This tube is known as Draft Tube where one finish of the tube is directly connected to the exit of the runner whereas another end is immersed under the water level within the tail-race.
Runner Blades
Runner blades are very essential components in this turbine. These are arranged at the center of the turbine where the water hits & the tangential power of the impact causes the shaft to turn for generating torque. These blades mainly include two parts where the lower half is designed in a small bucket shape to revolve the turbine with the help of the impulse action of the water supply. The upper half of the turbine blades utilize the reaction power of water supplied through it. So the runner turns through these two forces.
Penstock
Penstock plays a key role in transferring the water from the dam to the Francis turbine and it is made with cement or cast steel.
Francis Turbine Working
The water from the penstock enters into the spiral casing of the turbine, after that it flows through the guide vanes & stay vanes. The spiral case in the turbine is kept in decreasing diameter to maintain the pressure of water flow.
The stay vanes are motionless at their position to remove the spins from the water supply, which are produced because of the spiral casing & try to make the water flow more linear to be turned aside through changeable guide vanes.
The angle of guide vanes will decide the angle of the strike of water supply at the runner blades. The runner blades in the turbine are motionless & cannot change their position so the guide vanes control the turbine’s power output.
The turbine efficiency & performance mainly depends on the runner blades’ design. In this turbine, runner blades are mainly separated into two parts like the upper part and the lower part.
The lower part of the turbine is made in a small bucket shape so that it can use the impulse action of the water supply to revolve the turbine. The upper or higher part of the turbine uses the reaction pressure of the water supply through it. So runner blades use both the pressure & kinetic energies of the water supply to turn the runner in the most efficient mode.
Once the water exit from the runner blade then it will lack both the energies of kinetic & pressure, so we utilize the draft tube to improve the force when it moves forward to the tailrace. However, we still cannot improve the force to that level that we can prevent air to go into the runner housing thus causing cavitation.
Difference b/w Francis Turbine and Kaplan Turbine
The difference between Francis and Kaplan turbines include the following.
Francis Turbine | Kaplan Turbine |
| The flow of water in this turbine enters radially into the runner vanes & exists axially. | In this turbine, water supplies into the runner and exists in the axial direction. |
| Its efficiency is low. | Its efficiency is high. |
| High friction losses. | Low friction losses. |
| Large size | Compact size |
| The runner in this turbine contains 16 to 24 runners. | The runner in this turbine contains 4 to 8 runners. |
| The shaft’s direction is vertical or horizontal based on the requirement. | The shaft’s direction is vertical always |
| It requires a medium size of head that ranges from 100 to 600 meters. | It requires a very low head like 100 meters. |
| Its water flow rate is medium. | Its water flow rate is high. |
| Its specific speed ranges from 60 to 300. | Its specific speed ranges from 600 to 1000. |
| Its governing mechanism is simple. | Its governing mechanism is not simple or complicated |
| The runner vanes in this turbine are fixed. | The runner vanes in this turbine are adjustable. |
| The energy loss because of the friction of the blades is higher due to a high number of blades. | The energy loss because of the friction of the blades is low because of the less number of blades. |
| It uses a single servomotor for the regulation of vanes by rotating runner blades. | It uses different types of servomotors for the regulation of the vanes by rotating runner blades. |
| It is applicable in water pumping sets of irrigation to push water from the earth for irrigation. | It is used in mini hydro-power plants for generating electricity. |
Francis Turbine Characteristics
Francis turbine characteristics include the following.
- The main characteristic of a Francis turbine is that water flow changes direction when it supplies throughout the turbine.
- The flow of water entering into the turbine is in the radial direction, flowing in the direction of its axis, however after hitting & cooperating with the blades of the turbine, it outlets along the axis direction.
Advantages
The advantages of the Francis turbine include the following.
- Its runner size is small
- Simple control even with changeable heads.
- Efficiency change with time is very little.
- No head breakdown happens still at the less water discharge.
- The variation within the operating head can be simply controlled.
- The maximum & minimum least operating head ratio is even two within these turbines.
- The head range covered by this turbine unit mainly ranges from 20 to 700 M & their o/p changes from kilowatts to megawatts
- These turbines are mainly designed for an extensive range of flows & heads.
Disadvantages
The disadvantages of the Francis turbine include the following.
- The water contains contaminants which may cause problems to the turbine.
- It is very expensive.
- Its design is very complex.
- The number of moving parts is considerable within this turbine.
- The runner is not obtainable usually because it has a typical spiral casing.
- Its maintenance is complicated and costly
- It faces the risk of cavitation.
- Current losses within the turbine are expected.
Applications
The applications of the Francis turbine include the following.
- These turbines are the most efficient hydro-turbines.
- These turbines are used widely to produce electricity.
- This turbine is particularly designed for the site to work at the highest efficiency like above 90%.
- These turbines are mainly designed for a wide range of heads & flows.
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