Multiple-effect evaporators — Why is backward-feed arrangement preferred over forward feed for concentrating a cold feed?

Introduction:
In multiple-effect evaporation, the arrangement of effects (forward, backward, mixed) influences temperature driving forces, pumping requirements, economy, and capacity. For cold feeds, backward feed often offers thermodynamic advantages despite added mechanical complexity.

Given Data / Assumptions:

• Cold feed entering the last (lowest-temperature) effect in backward feed.
• Vapor from a higher-temperature effect heats the next lower-temperature effect.
• Comparable total heating surface and operating pressures for both schemes.

Concept / Approach:

Backward feed exposes the most concentrated, viscous liquor to the highest temperature difference (first effect), improving heat transfer under difficult conditions and enhancing steam economy for cold feeds. However, because pressure increases from later to earlier effects, liquor must be pumped from low to high pressure, increasing hydraulic work and potentially reducing throughput (capacity) for a given pump set compared with gravity-aided forward feed.

Step-by-Step Solution:

Verification / Alternative check:

Design guides list backward feed as favorable for cold feeds and viscous products; forward feed is simpler and often higher in capacity for hot feeds.

Why Other Options Are Wrong:

A alone ignores the capacity penalty; B alone ignores the economy benefit; D is opposite; E is false because pumps are typically needed in backward feed.

Common Pitfalls:

Assuming one arrangement dominates in all metrics; selection depends on feed temperature, viscosity profile, and utility costs.

Final Answer:

Both (a) and (b).