Multiple-effect evaporators – Factors affecting steam economy For a given number of effects, which listed factor does not significantly influence the steam economy (kg water evaporated per kg of steam) of a multiple-effect evaporator?

Introduction / Context:
Steam economy, the hallmark performance metric for multiple-effect evaporators, measures how effectively each kilogram of steam drives evaporation across effects. Some parameters alter economy directly, while others mainly affect equipment size or area without much changing economy itself.

Given Data / Assumptions:

• Number of effects fixed; forward/backward feed arrangement chosen.
• BPE (boiling point elevation) reduces available temperature driving force per effect.
• Feed condition (cold vs hot) changes how much sensible heat must be supplied.

Concept / Approach:
Steam economy primarily depends on the number of effects and total available temperature drop after accounting for BPE and feed heating requirements. The overall rate of heat transfer (through UA) influences required area and duty distribution, but with enough area the same economy can be reached; it does not fundamentally change the kilograms evaporated per kilogram of steam for a fixed configuration and steady operation.

Step-by-Step Solution:

Verification / Alternative check:
Design texts separate thermodynamic economy (governed by effects and ΔT availability) from economic sizing (UA). Increasing area raises throughput but not kg water per kg steam fundamentally.

Why Other Options Are Wrong:

• BPE: Directly reduces ΔT per effect, lowering economy.
• Feed temperature: Cold feed increases steam use for sensible heating.
• Thin/thick liquor ratio (concentration factor): Influences sensible loads distribution and flashing, affecting economy.

Common Pitfalls:
Confusing capacity (throughput) with economy; adding area increases capacity but not the inherent economy for a set effect count and ΔT.

Final Answer:
rate of heat transfer