Vertical-tube evaporators and liquor level: In a vertical tube evaporator, how does increasing the liquor level generally affect the overall heat-transfer coefficient U?

Introduction / Context:
Evaporator performance depends on boiling regime, circulation, and film characteristics. Vertical-tube natural-circulation units rely on density differences to drive flow; liquor level influences hydrostatic head and boiling intensity, thereby impacting heat transfer.

Given Data / Assumptions:

• Vertical tube natural-circulation evaporator (not forced circulation).
• Comparable steam-side conditions.
• Liquor properties within typical ranges.

Concept / Approach:
Higher liquor levels increase static head, suppressing bubble growth and reducing circulation velocities through the tubes. This weakens convective boiling and can thicken liquid films, decreasing the overall heat-transfer coefficient U. Conversely, lower levels enhance circulation and boiling vigor, raising U (up to flooding or entrainment limits).

Step-by-Step Solution:
Recognize that U in boiling combines film coefficients and resistances (liquid-side dominates).Increasing level → higher hydrostatic head → reduced vapor lift and circulation.Reduced circulation → lower h_liquid → lower U overall.Hence select 'Decreases'.

Verification / Alternative check:
Operating data often show improved U at optimized, lower liquid heights due to stronger natural circulation, with U falling as level is raised above design.

Why Other Options Are Wrong:
'Increases' contradicts the usual natural-circulation behavior.

'Not affected' ignores hydrodynamics; 'may increase or decrease' is noncommittal and not representative for this standard case.

Common Pitfalls:

• Confusing vertical-tube natural circulation with falling-film evaporators, where film behavior differs.
• Overlooking foaming or entrainment at too-low levels, which has other drawbacks.

Final Answer:
Decreases