Choosing the number of effects in evaporation In a multiple-effect evaporator, the optimum number of effects is primarily decided on the basis of what criterion?

Introduction / Context:
Multiple-effect evaporation improves steam economy by reusing vapor as heating medium in downstream effects. However, each additional effect increases capital cost and complexity. Engineers must therefore select an optimum number of effects balancing capital and operating costs.

Given Data / Assumptions:

• Conventional forward or backward feed arrangement.
• Steady operation; no exotic heat recovery beyond the effects.
• Goal is “optimum,” not merely feasible or maximum possible.

Concept / Approach:
Adding effects increases steam economy but with diminishing returns. The optimum number minimizes total annual cost = annualized capital charges + operating (steam, power, maintenance) costs. While temperature driving force (terminal parameters) sets a practical upper bound and capacity requirements size equipment, the choice of how many effects to install is an economic (cost–benefit) decision.

Step-by-Step Solution:

Verification / Alternative check:
Design handbooks present plots of annual cost vs number of effects showing a shallow minimum at the economic optimum.

Why Other Options Are Wrong:

• Floor area availability: A constraint, not the economic decision basis.
• Terminal parameters: Determine feasibility and temperature profiles but do not alone set the optimum.
• Capacity required: Influences equipment size, not the number of effects per se.

Common Pitfalls:
Equating the “maximum feasible effects” with the “economic optimum”; ignoring the annualization of capital when comparing steam savings.

Final Answer:
Cost benefit analysis