Evaporation performance: steam economy is evaporation per unit steam used; capacity is total evaporation per hour. What is the effect of using multiple effect evaporation?

Introduction / Context:
Evaporators concentrate solutions by vaporizing solvent, usually water. Two key performance metrics are capacity (kg/h evaporated) and steam economy (kg evaporation per kg of live steam). Multiple-effect arrangements reuse vapor from one effect as the heating medium in the next, improving energy utilization.

Given Data / Assumptions:

• Comparable total heat-transfer area when comparing single vs multiple effect.
• Standard forward-feed or backward-feed multiple-effect configuration.
• No change in overall equipment size beyond distributing area across effects.

Concept / Approach:
Multiple effects primarily improve steam economy because latent heat is cascaded. Capacity depends primarily on total area and available temperature driving force. With fixed total area, capacity does not inherently increase just by splitting into effects; the main benefit is economy.

Step-by-Step Solution:
Define economy: kg evaporation per kg steam.Using vapor from an upstream effect as heating medium increases total evaporation per unit live steam.Capacity: For a fixed total area, the hourly evaporation capacity remains roughly unchanged.Therefore, economy increases while capacity is not inherently affected.

Verification / Alternative check:
Textbook comparisons show economy roughly proportional to the number of effects (idealized), while capacity scaling requires more area or higher temperature driving force, not merely more effects.

Why Other Options Are Wrong:

• Increases capacity: Not a direct consequence without more area or driving force.
• Does not affect capacity (alone): True, but ignores the key improvement in economy.
• Reduces economy: Opposite of the thermodynamic advantage of reusing vapor.

Common Pitfalls:
Assuming more effects always mean more capacity; conflating economy with capacity; overlooking the role of total heat-transfer area and temperature profile across effects.

Final Answer:
Both (b) and (c)