Draft Tube, functions of Draft Tube and different types of draft tube

Draft Tube:

Draft tube: Draft Tube is used to increase the pressure at the end of the turbine and this decreases the exit velocity. Draft Tube is the main component of reaction turbines. It is the type of tube which discharge the water from outlet of reaction turbine to the tail race. A draft tube pipe is gradually increases in area. Draft tube improves the output and efficiency of overall turbine.

In it one end of the draft tube is immersed below the water level in the tail race and the other end is connected to the outlet of the runner of a reaction turbine. in this the rejected kinetic energy at out let is used to any other useful work by it.

Types of Draft Tubes:

1. Moody Spreading Tube
2. Simple Elbow tube
3. Conical diffuse tube/ divergent tube
4. Elbow with varying cross section

1. moody spreading tube: This is also called as hydraucone, this has bell mouth shape. this is more efficient in energy recovering at Swirl motion and linear motion of water. this is more efficient than elbow draft tube.it has one inlet and two outlets. it's efficiency is about 88%.

2. Simple elbow tube: In this type of tube the direction of flow is changed by using this, it has less efficiency about 60%.

3. Conical diffuse tube/ divergent tube: This is mostly used in francis turbine, it is used for low specific speed. the efficiency is about 90%.

4. Elbow with varying cross section: This is developed from simple elbow,It has a rectangular outlet but the inlet is circular. this cross-section is used to increase like a conical type.it is mostly used in Kaplan turbine. it's efficiency is about 70%.