Draft Tubes free notes for Diploma / BTech.

1. What is a Draft Tube?

A draft tube is a gradually expanding tube fitted at the outlet of a reaction turbine (such as Francis or Kaplan turbine). It connects the turbine runner to the tailrace (discharge channel).

In simple words:
A draft tube helps in recovering kinetic energy of water leaving the turbine and improves efficiency.

2. Function of Draft Tube

The main functions are:

• Converts kinetic energy into pressure energy
• Increases overall efficiency of the turbine
• Allows turbine to be installed above tailrace level
• Reduces loss of energy at outlet

3. Working Principle

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A draft tube works on the principle of a diffuser:

• Water leaves the turbine runner with high velocity
• The cross-sectional area of the draft tube gradually increases
• Due to this, velocity decreases
• According to Bernoulli’s principle, pressure increases
• Thus, kinetic energy is converted into pressure energy

4. Types of Draft Tubes

(a) Straight Conical Draft Tube

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• Simple straight expanding tube
• High efficiency
• Requires more space

(b) Simple Elbow Draft Tube

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• Bent shape (90° turn)
• Suitable where space is limited
• Moderate efficiency

(c) Elbow Draft Tube with Circular Inlet and Rectangular Outlet

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• Most commonly used
• Reduces losses
• Compact design

(d) Moody’s Spreading Draft Tube

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• Special design for better flow distribution
• High efficiency
• Complex construction

5. Efficiency of Draft Tube

Draft tube efficiency is defined as:$$\eta_d = \frac{\text{Gain in pressure head}}{\text{Velocity head at inlet of draft tube}}$$ηd​=Velocity head at inlet of draft tubeGain in pressure head​

👉 Higher efficiency means better recovery of kinetic energy.

6. Advantages of Draft Tube

• Improves turbine efficiency significantly
• Reduces energy loss
• Allows flexible turbine installation
• Enhances power output

7. Disadvantages

• Cavitation may occur if pressure falls too low
• Requires careful design
• Large size may increase cost

8. Applications

• Used in reaction turbines:
  • Francis turbine
  • Kaplan turbine
• Hydroelectric power plants

9. Important Terms

• Tailrace: Channel where water is discharged
• Velocity Head: Energy due to velocity
• Pressure Head: Energy due to pressure
• Cavitation: Formation of vapor bubbles due to low pressure

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