In this article we will discuss ‘Draft Tube’ and its importance in reaction turbines. Later on, in the article we will learn different types of Draft tubes.
What is a Draft Tube?
A draft tube is a kind of tube which connects (or acts as an intermediately) between the water turbine exit and tail race.
Image source: NPTEL
Note: Tail race is the water channel that carries away water from the turbine exit.
A draft tube has an increasing cross-sectional area from the inlet to the out let (as shown in figure).
Why do we require draft tube in reaction turbines?
In case of Kaplan and Francis turbines (or reaction turbines) the head available at the turbine inlet is generally low, hence the turbine is placed much closer to the tail race to obtain maximum head.
Since most of the pressure energy of the water is converted into the Mechanical Energy of the turbine, hence the pressure at the exit of the turbine is below the atmospheric pressure.
We have already discussed in the Kaplan and Francis turbines, the turbine exit placed near to tale race. So, if the pressure of water at the exit of turbine will be less than atmospheric pressure than it will cause back flow of water from the tail race (which is at atmospheric pressure) to the turbine.
Note: fluid flows from high pressure zone to low pressure zone.
To avoid this problem of back flow a draft tube is used between turbine exit and tale race. It increases the pressure of exit water at the expense of its kinetic energy.
Now, the increased pressure of water becomes some what above to the atmospheric pressure. It solves our problem of back flow of water from the tail race to the exit of turbine.
Note: Back flow of water can cause serious damage to the turbine and can stop turbine from functioning.
Do Pelton (or Impulse) turbines needs Draft Tube?
No, in case of impulse turbines the head available to the turbine is very large which results in an above atmospheric pressure at the turbine exit. Since we have a above atmospheric pressure of water at the turbine exit then there is no problem of back flow of water.
Types of Draft Tubes
There are four major types of draft tubes which are available to us.
- Conical draft tube
- Simple elbow draft tube
- Moody spreading draft tube
- Draft tube with circular inlet and rectangular outlet
Moody spreading and Conical draft tubes are the most efficient draft tubes.
Draft tube with circular inlet and rectangular outlet, and Simple elbow draft tube requires less space than above two.
Conical tube: By Adhish24 – paintPreviously published: No where, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=26399997
Simple elbow tube: By Adhish24 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=26387933
Tube with circular inlet and rectangular outlet: By Adhish24 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=26387931