250+ TOP MCQs on Capacity and Economy of Multiple Effect Evaporators

Heat Transfer Operations Questions and Answers for Campus interviews on “Capacity and Economy of Multiple Effect Evaporators”.

1. We have a triple effect evaporator which has steam supply of 5kg/hr, the setup is a forward pass setup, feed enters the 1st setup at 10kg/hr of 50% concentration and leaves the final effect at 75% concentration. What is the evaporate discharged if the total evaporate from 1st and 2nd effect are 2kg/hr and 1kg/hr respectively?
a) 6.67 kg/hr
b) 3.33 kg/hr
c) 0.36 kg/hr
d) 0.33 kg/hr

Answer: d
Clarification: Applying mass balance, Total feed F = E + P₃, where E = E₁+E₂+E₃, P₃ = X_FF/X_P = 0.5×10/0.75 = 6.67kg/hr. Hence, we have E = 10-6.67 = 3.33kg/hr. Thus as the total is 3.33kg/hr and E₁ = 2kg/hr and E₂ = 1kg/hr then E₃ = 0.33kg/hr.

2. We have a triple effect evaporator which has steam supply of 5kg/hr, the setup is a forward pass setup, feed enters the 1st setup at 10kg/hr of 50% concentration and leaves the final effect at 75% concentration. What is the product discharged from this setup?
a) 6.67kg/hr
b) 3.33kg/hr
c) 10kg/hr
d) 11kg/hr

Answer: a
Clarification: Applying mass balance, Total feed F = E + P₃, where E = E₁+E₂+E₃, P₃ = X_FF/X_P = 0.5×10/0.75 = 6.67kg/hr.

3. The formula for the economy of the evaporator does not depend on the type of feeding.
a) True
b) False

Answer: a
Clarification: Yes, the type of feeding depends on the product P and feed F, but the equation of economy is = capacity/consumption which is independent of the product or feed rate.

4. Which type of feeding is supposed to give maximum economy to a particular consumption?
a) Forward feed
b) Backward feed
c) Parallel feed
d) Mixed feed

Answer: b
Clarification: The backward feed does have more economy than others because it pumps backwards and makes sure that the highest concentration product receives the maximum heat (directly from steam) which increases the evaporate yield considerably.

5. We have a triple effect evaporator which has steam supply of 5kg/hr, the setup is a forward pass setup, the feed enters the 1st setup at 10kg/hr of 50% concentration and leaves the final effect at 75% concentration. What is the economy of this setup?
a) 65%
b) 66%
c) 68%
d) 33%

Answer: b
Clarification: Applying mass balance, Total feed F = E + P₃, where E = E₁+E₂+E₃, P₃ = X_FF/X_P = 0.5×10/0.75 = 6.67kg/hr. Hence, we have E = 10 – 6.67 = 3.33kg/hr. Economy of an evaporator is defined as, E = Capacity/Consumption = (E₁+E₂+E₃)/S = 3.33/5 = 0.66 = 66%.

6. What is the Evaporator economy of the 1st effect if we evaporate 10kg/hr of water from a feed of 50kg/hr at a temperature of 25^oC to a final temperature of 100^oC in an hour in a triple effect forward feed evaporator which has steam supply of 25kg/hr? Given the concentration changes from a value of 0.3 to 0.55 in the 1st effect of the setup.
Latent heat of vaporization of water = 2,260 kJ/kg
Specific heat capacity = 6 KJ/Kg K
a) 95%
b) 91%
c) 68%
d) 33%

Answer: b
Clarification: We know that F = 50kg/hr, as the concentration change is given we can calculate the product P₁ = X_FF/X_P = 0.3×50/0.55 = 27.27kg/hr, hence E₁ = 22.72kg/hr. Hence economy = Capacity/Consumption = 22.72/25 = 91%.