Question Bank • QB_EXP_APE_EVP_003 MSQ • 2 Marks

Section 6: Agricultural Process Engineering

Evaporation & Liquid Food Concentration

Subtopic: Boiling point elevation and Duhring rule

Question Statement:

Which of the following statements regarding evaporator thermal design is/are TRUE?

Options:

  • (A) Boiling Point Elevation (BPE) reduces the effective temperature driving force ($\Delta T$) available for heat transfer in an evaporator
  • (B) Dühring's rule states that the boiling point of a solution is a linear function of the boiling point of pure water at the same pressure
  • (C) In forward feed multiple-effect evaporators, liquid moves without pumps because pressures decrease in successive effects
  • (D) Steam economy of a single-effect evaporator can exceed $3.0$ without vapor recompression

Answer Key:

A, B, C

Step-by-Step Solution & Explanation

1. BPE increases boiling temperature, shrinking $\Delta T = T_{steam} - (T_{boil} + BPE)$ (A is true). 2. Dühring lines are straight lines plotting solution boiling point against pure water boiling point (B is true). 3. Successive effects operate at deeper vacuum, driving forward liquid flow by pressure gradient (C is true). 4. In a single effect without mechanical/thermal vapor recompression, $1\text{ kg}$ steam can evaporate at most $\approx 0.85-0.95\text{ kg}$ vapor (economy
lt; 1.0$) due to heat losses and sensible heating (D is false).
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