Evaporators – Actual temperature drop across the heating surface The actual temperature drop across the heating surface of an evaporator is influenced by several practical factors. Which combination best represents the dependencies?

Introduction / Context:
Design correlations often assume an available temperature difference based on steam and vapor pressures. In real evaporators, the effective temperature drop across the heating surface also depends on hydrodynamics, hydrostatic head, and operating pressures, which together alter boiling temperatures and driving forces.

Given Data / Assumptions:

• Boiling occurs on the liquor side; condensing steam supplies heat.
• Feed condition can vary (cold, preheated, viscous).
• Liquid depth and pressure profiles are not negligible.

Concept / Approach:
The liquor boiling temperature rises with hydrostatic head (deeper liquid layers). Feed condition affects sensible heating requirement and local boiling point. The pressure difference between the steam chest and vapor space sets saturation temperatures on each side, thus directly changing ΔT available. Consequently, all listed factors contribute to the actual temperature drop across the heating surface.

Step-by-Step Solution:

Verification / Alternative check:
Operating data show ΔT varies along the calandria; shallow pools and reduced pressure drop increase effective driving force.

Why Other Options Are Wrong:
Each single-factor choice omits other equally important influences, giving an incomplete picture of ΔT determination.

Common Pitfalls:
Using nominal steam–vapor temperature differences without correcting for BPE and hydrostatic head can overpredict capacity.

Final Answer:
all (a), (b) and (c).