Generally, hydro turbines are classified as impulse turbines and reaction turbines, having some differences between them. A major one is how the energy is exchanged between the fluid and the turbine. Their similarities will be how hydro turbines convert potential energy and kinetic energy of water flow into mechanical. I’ll explain this further.
Stick with me. In this reading, we’ll explore the difference between an impulse turbine and a reaction turbine. These differences will also be presented in tabular form.
Let’s get started!
Impulse Turbine
An impulse turbine basically works under the principle of Newton’s 2nd law. Instead of blades on the rotor hub, a number of the elliptical half-sized buckets are fitted. So, when water strikes the buckets at high speed, the rotor starts rotating, this means, the kinetic energy of water gets converted into rotational mechanical energy.
Therefore, electricity is generated when one end of the turbine shaft is connected to the generator. Examples of an impulse turbine include Pelton, Turgo, and Cross-flow.
The Pelton and Turgo turbines are similar in construction. However, the cross-flow turbine is a modification of an impulse turbine that is classified as one this is because of the rotation of the runner at atmospheric pressure and not as a submerged turbine.
Related: What is Turbine? its Diagram and How it Works
Reaction Turbine
In a reaction turbine, the sum of potential energy and kinetic energy of water due to the pressure and velocity, respectively causes the turbine blades to rotate.
The entire body of this turbine is immersed in water and changes in water pressure along with the kinetic energy of the water cause power exchange. Applications of this turbine are usually at lower heads and higher flow rates than impulse type.
The turbine blades or impeller blades are designed to be able to generate a force on one side when water flows through it just like an airfoil. In an airplane, the force produced by an airfoil is responsible for its lifting. Similarly, here, the force causes the blades to rotate.
Different types of reaction turbines have their own ideal operating conditions. For examples,
- Pelton turbines are preferred where a low discharge rate can be obtained & a high head(80-1600m) is available.
- Kaplan turbines require a high discharge rate along with low or medium heads (2-70m).
- Francis turbine work on medium flow rate & medium head. Francis turbine is a combination of impulse & reaction turbines.
Francis turbines are the most widely used turbines because they offer the highest efficiency & could also work in a wide range of operating conditions.
Related: What is Kaplan Turbine? its Diagram and How it Works
Difference between Impulse Turbine and Reaction Turbine
Below is the difference between impulse turbine and reaction turbine in tabular form:
Reaction Turbine |
Impulse Turbine |
| More maintenance is required for it. | It requires less maintenance. |
| Only some quantity of the hydraulic energy is transformed into K.E. | The total amount of hydraulic energy is transformed into K.E. |
| Water flow is an axial and radial direction to the turbine wheel. | The direction of the flow of water is tangential to the turbine wheel. |
| Its degree of reaction is among ‘0’ to ‘1.’ | Its degree of reaction is zero. |
| It requires high and medium water discharge. | It requires low water discharge. |
| Reaction turbine works at low and medium water heads. | It works at the high head. |
| The reaction turbine has comparatively high hydraulic efficiency. | Impulse turbine has comparatively less efficiency. |
| Francis and Kaplan’s turbine are its example. | Pelton Wheel turbine is its example. |
| Water enters around the impeller. | Water is admitted only in the form of jets. |
| The runner must be closed in a water-tight casing. | In these turbines, casings are not compulsory. Casing works as a safeguard. |
| Velocity and pressure vary as the fluid passes
by the impeller. The pressure at the suction point is much more than the discharge point. |
The velocity of the jet varies the pressure through the remaining atmospheric. |
| The flow control takes place via the guide vane. Other important parts are scroll casing, stay ring, runner, and the draft tube. | The flow control takes place via a needle valve fitted into the Nozzle. |
| Water fills at the passage between the buckets and, while flowing between inlet and outlet sections, do work on the blades. | The turbine does not run entirely, and air has free access to the buckets. |
| The reaction turbine has not symmetrical blades. | Impulse turbine has symmetrical blades. |
| The pressure of water is decreased during its flow. | The pressure of water remains constant during its flow. |
| It has high working speed than an impulse turbine. | It has low working speed than a reaction turbine. |
| The efficiency of buckets is high. | The efficiency of buckets is low. |
| These turbines require less space. | It requires high space as compare to the reaction turbine. |
| Newton’s 3rd law defines the energy transfer from reaction turbines. | Newton’s 2nd law defines the energy transfer from impulse turbines. |