Question

A multiple effect evaporator is one in which
A. Vapour from one effect is used directly as the heating medium in the next.
B. The pressure of vapour is increased and the resulting high pressure steam is re‐used as a heating medium
C. Vapour is used to heat the incoming feed liquor or condensed vapour is used to raise steam in a boiler
D. Vapours from one effect is not utilized in heating
E. Vapours are condensed and condensed liquid is further heated
Choose the correct answer from the options given below:

• A. A only
• B. A and B only
• C. B and C only
• D. D and E only

Hint:

In a multiple-effect evaporator, the steam economy (kg of water evaporated per kg of steam supplied) increases with the number of effects:
- 1 Effect \(\approx 0.8\) to \(0.9\)
- 2 Effects \(\approx 1.5\) to \(1.8\)
- 3 Effects \(\approx 2.2\) to \(2.4\)

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Evaporation is an energy-intensive process used to concentrate liquid foods by boiling off water.
A multiple-effect evaporator is a system that connects several individual evaporators (effects) in series to reuse thermal energy and improve steam economy.

Step 2: Detailed Explanation:
Let us analyze the statements in relation to multiple-effect evaporators:
- Statement A: "Vapour from one effect is used directly as the heating medium in the next."
This is the defining principle of a multiple-effect evaporator.
Fresh steam is fed only to the first effect to boil the liquid feed. The vapour generated from this boiling liquid is then directed into the heating chest of the second effect, which is operated at a lower pressure (and lower boiling point). This allows the vapour from the first effect to act as the heating steam for the second, significantly reducing the amount of fresh steam required.
- Statement B: "The pressure of vapour is increased and the resulting high pressure steam is re-used..."
This describes thermal or mechanical vapour recompression (TVR or MVR), which are separate energy-saving technologies, not the defining feature of a standard multiple-effect evaporator.
- Statement C: This describes feed preheating and heat recovery, which can be integrated into many types of single or multiple-effect systems.
- Statement D and Statement E are incorrect because they contradict the core principle of heat reuse in multiple-effect systems.
Therefore, only Statement A is correct.

Step 3: Final Answer:
The correct option is (A).