If a turbine of 5 m runner diameter and 361 m head is running at 150 rpm, the speed of a geometrically similar turbine of 1 m runner diameter working under 81 m head, would be
- A355 rpmCorrect
- B295 rpm
- C375 rpm
- D389 rpm
Solution
Concept
Unit speed
Formula
N D/√H = constant
Step-by-step solution
1. N₂ = N₁ (D₁/D₂) √(H₂/H₁). 2. N₂ = 150 × (5/1) × √(81/361) = 150 × 5 × 9/19 = 355 rpm.
Explanation
Unit speed is constant for geometrically similar turbines.
Why the other options are wrong
Other values do not follow from the ratio.
Shortcut
N ∝ √H/D.
Correct answer: A) 355 rpm