The GATE Grind

GATE 2025 CH – Question 30

Heat Transfer · Heat exchangers and evaporators: types and design · 1 mark · Multiple select

Which of the following statements regarding multiple effect evaporators is/are TRUE?

  1. The pressure of the effect with fresh steam is the lowest for both forward feed and backward feed.
  2. Backward feed is preferred over forward feed for cold feed.
  3. Backward feed is preferred over forward feed for highly viscous concentrated product.
  4. The temperature of the effect with fresh steam is the lowest for both forward feed and backward feed.

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (B) Backward feed is preferred over forward feed for cold feed.; (C) Backward feed is preferred over forward feed for highly viscous concentrated product.

Explanation

Fresh steam always goes to the first effect, which is at the highest pressure and temperature in both feed arrangements, so statements A and D are false. In backward feed the cold feed enters the last effect, which is at the lowest temperature, so little high temperature steam is wasted on heating the feed, which is why it is preferred for cold feed. The concentrated, viscous product leaves from the effect that is hottest in backward feed, where the viscosity is lower and the heat transfer coefficient is better. So B and C are true.