Evaporator performance metric: The number of kilograms of solvent vaporized per kilogram of steam supplied to an evaporator is called the ________ of the evaporator.

Introduction / Context:
Two key metrics describe evaporator performance: capacity and economy. Capacity is the quantity vaporized per unit time, while economy indicates how efficiently supplied steam is used to evaporate solvent (often water) in the process.

Given Data / Assumptions:

• Single- or multiple-effect evaporators handling aqueous solutions.
• Steam as the heating medium.
• Steady operation.

Concept / Approach:
Economy is defined as kg of solvent vaporized per kg of steam fed. In multiple-effect systems, economy exceeds 1 because steam is reused across effects; in single-effect units, economy is typically near 1 (minus losses). Capacity, by contrast, is expressed as kg/h evaporated, independent of steam flow.

Step-by-Step Solution:
Identify the requested ratio: kg vaporized per kg steam.Map to standard terminology: this ratio is 'economy'.Distinguish from capacity (kg/h) and generic 'rate' wording.Select 'Economy'.

Verification / Alternative check:
Texts on multiple-effect design emphasize economy improvement with additional effects, vapour recompression, or heat integration, all targeting this ratio.

Why Other Options Are Wrong:
Capacity and rate options refer to throughput per time, not normalized to steam usage.

Common Pitfalls:

• Confusing economy with thermal efficiency; economy focuses on steam reuse effectiveness.
• Ignoring steam leaks and noncondensables that reduce practical economy.

Final Answer:
Economy