A Draft Tube is an essential component of a Reaction Turbine because it
- AIncreases the pressure at the runner exit above atmospheric pressure
- BAllows the turbine to be set above the tailrace level without losing head
- CConverts the kinetic energy rejected at the runner exit into useful pressure energy
- DBoth (B) and (C)Correct
Solution
Concept
Draft Tube in Reaction Turbines
Formula
ηd = [(V₁² - V₂²) / 2g - hf] / (V₁² / 2g)
Step-by-step solution
1. Identify the main functions of a draft tube in a reaction turbine. 2. Recall that a draft tube permits setting the turbine above the tailrace level without losing head and converts kinetic energy at the runner exit into pressure energy. 3. Evaluate the options and select Option (d) as both (B) and (C) are correct.
Explanation
A draft tube is a pipe fitted at the exit of a reaction turbine that serves two primary purposes: it allows the turbine to be placed above the tailrace level without losing net head, and it converts the remaining kinetic energy of the water leaving the runner into useful pressure energy to boost overall efficiency. Thus, both statements (B) and (C) are correct.
Shortcut
To remember the main functions of a Draft Tube, use the phrase "Height & Pressure": - Height: Allows setting the turbine above the tailrace level without losing head. - Pressure: Converts kinetic energy rejected at the runner exit into useful pressure energy.
Common mistake
A common mistake is selecting Option B or Option C individually without reading all options to realize that both statements describe essential functions of a draft tube, making Option D the correct choice. Another frequent error is selecting Option A by incorrectly assuming that pressure at the runner exit is above atmospheric pressure, whereas it is actually sub-atmospheric.
Correct answer: D) Both (B) and (C)