Evaporator hydraulics — Increasing the liquor level (higher submergence) in an evaporator primarily leads to which operational change?

Introduction:
Evaporator performance depends on available temperature driving force and overall heat-transfer coefficient. The liquor level (submergence) affects hydrostatic head at the heating surface, which in turn alters the local boiling temperature and effective temperature difference. Understanding this relation helps optimize capacity and economy.

Given Data / Assumptions:

• Single-effect or multiple-effect evaporator with submerged heating surface.
• Boiling under a given shell-side steam pressure.
• Physical properties change modestly with level.

Concept / Approach:

A higher liquor column increases hydrostatic head at the heating surface. The saturation temperature at depth rises with pressure head, so the liquid boils at a higher temperature there. This reduces the effective temperature difference between the condensing steam and the boiling liquor at the surface, lowering the driving force for heat transfer and hence the evaporation rate (capacity). Any modest increase in wetted area or film coefficient typically cannot offset the loss in driving force.

Step-by-Step Solution:

Verification / Alternative check:

Design charts show capacity dropping with increased submergence for vertical-tube and rising-film units beyond an optimum wetting level.

Why Other Options Are Wrong:

B: Film coefficient may change but the dominant effect is reduced ΔT. C: Hydrostatic head increases, not decreases. D: True temperature drop decreases, not increases. E: No practical condition yields infinite U; fouling and resistances remain.

Common Pitfalls:

Confusing increased wetting with improved performance; driving force reduction is usually controlling.

Final Answer:

decreased capacity