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

1. In the given setup, what is the capacity of this forward feed evaporator?

a) E_{1} + E_{2} + E_{3}

b) E_{3}

c) E_{1}

d) E_{1} + E_{2}

View Answer

Explanation: The capacity of an evaporator is defined as the amount of water removed from a particular feed during the course of operation of the evaporator, in the given diagram, the amount of water evaporated is equal to the sum of water removed from each effect, i.e. C = E

_{1}+ E

_{2}+ E

_{3}.

2. In the given setup, what is the capacity of the 2^{nd} effect of this parallel feed evaporator?

a) E_{1} + E_{2} + E_{3}

b) E_{3}

c) E_{2}

d) E_{1} + E_{2}

View Answer

Explanation: The capacity of an evaporator is defined as the amount of water removed from a particular feed during the course of operation of the evaporator, in the given diagram, the amount of water evaporated is equal to the water removed from that effect, i.e. C = E

_{2}.

3. What is the economy of the given Forward feed evaporator?

a) ∈ = S /(E_{1}+E_{2}+E_{3})

b) ∈ = E_{1}/(E_{1}+E_{2}+E_{3})

c) ∈ = E_{3}/E_{2}

d) ∈ = E_{2}/E_{1}

View Answer

Explanation: Economy of an evaporator is defined as, E = Capacity/Consumption = E

_{2}/E

_{1}

As the capacity = total water evaporated = E

_{2}and the consumption = E

_{1}.

4. What is the economy of the 3rd effect in the following mixed feed evaporator?

a) ∈ = S/(E_{1}+E_{2}+E_{3})

b) ∈ = E_{1}/(E_{1}+E_{2}+E_{3})

c) ∈ = E_{3}/E_{2}

d) ∈ = E_{2}/E_{1}

View Answer

Explanation: Economy of an evaporator is defined as, E = Capacity/Consumption = E

_{3}/E

_{2}

As the capacity = total water evaporated = E

_{3}and the consumption = E

_{2}.

5. 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

View Answer

Explanation: Applying mass balance, Total feed F = E + P

_{3}, where E = E

_{1}+E

_{2}+E

_{3}, P

_{3}= X

_{F}F/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 E

_{1}= 2kg/hr and E

_{2}= 1kg/hr then E

_{3}= 0.33kg/hr.

6. 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

View Answer

Explanation: Applying mass balance, Total feed F = E + P

_{3}, where E = E

_{1}+E

_{2}+E

_{3}, P

_{3}= X

_{F}F/X

_{P}= 0.5×10/0.75 = 6.67kg/hr.

7. Carry out the mass balance on the 1st effect of the following setup.

a) P_{3} = P_{1}+E_{1}

b) P_{3} = P_{1}+S_{1}

c) P_{1} = P_{3}+E_{1}

d) P_{} = P_{1}+E_{1}

View Answer

Explanation: The feed that enters the 1

^{st}effect in this setup is the product from the 3

^{rd}effect = P

_{3}

And hence the outgoing mass is Product P

_{1}and evaporate E

_{1}.

8. The formula for the economy of the evaporator does not depend on the type of feeding.

a) True

b) False

View Answer

Explanation: 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.

9. 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

View Answer

Explanation: 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.

10. 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%

View Answer

Explanation: Applying mass balance, Total feed F = E + P

_{3}, where E = E

_{1}+E

_{2}+E

_{3}, P

_{3}= X

_{F}F/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 = (E

_{1}+E

_{2}+E

_{3})/S = 3.33/5 = 0.66 = 66%.

11. 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^{o}C to a final temperature of 100^{o}C 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%

View Answer

Explanation: We know that F = 50kg/hr, as the concentration change is given we can calculate the product P

_{1}= X

_{F}F/X

_{P}= 0.3×50/0.55 = 27.27kg/hr, hence E

_{1}= 22.72kg/hr. Hence economy = Capacity/Consumption = 22.72/25 = 91%.

12. What is the mass balance in complete setup in this backward feed evaporator?

a) F=P_{1}+E_{1}

b) P_{3}=P_{1}+S_{1}

c) F=P_{3}+E_{1}

d) F=P_{1}+E_{3}

View Answer

Explanation: If we make an enclosing surface for the total setup, we observe only the incoming and outgoing masses, we see Feed F entering, and product P1 and evaporate E

_{3}leaving. Hence the mass balance is F=P

_{1}+E

_{3}.

**Sanfoundry Global Education & Learning Series – Heat Transfer Operations.**

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