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Multiple effect evaporator.

 

Theory:

  • In a single effect evaporator steam is supplied for heating the liquor. The total heat is not transferred from the steam. So the rest of the heat is wasted. 

  • To use that heat efficiently, connections are made so that the vapor from one effect serves as the heating medium for the next effect.

  • The dilute feed (liquor) enters the first effect, where it is partly concentrated; it flows to the second effect for additional concentration and then to the third effect for final concentration. This liquor is pumped out of the third effect.

  • In the first effect raw steam is fed in which the vapor pressure in the evaporator is the highest, p1. 

  • The second effect has the intermediate vapor pressure; i.e. p1>p2>p3. This pressure gradient is maintained by drawing the vapor through a vacuum pump and condensing after the final effect.

  • Depending on the lowering of vapor pressure boiling point of the liquids of 2nd and 3rd effect will also be lowered; i.e. T1 > T2 > T3.

  • In the 2nd effect vapor from the 1st effect (T1 ) is heating the liquor (having temperature T2). So there is a temperature gradient (T1 – T2); consequently the liquor will be heated.

  • Similar heating will be there in the 3rd effect also.

Methods of feeding:

  1. Forward feed.

  1. Backward Feed.

  1. Mixed Feed.

  1. Parallel Feed.

ECONOMY OF MULTIPLE EFFECT EVAPORATOR

  • Assumptions:

    • (a) Feed is at boiling point and 

    • (b) Loss of heat is negligible

  • In effect-1

    • 1 Kg of steam transfers its heat to feed. Since feed is at boiling point so the total amount of heat is used as latent heat of vaporization. Therefore, 1 kg steam will produce 1 kg vapor.

  • In effect – 2

    • 1 Kg vapor from effect-1 will transfer heat to the liquor of effect -2. Here also 1 kg vapor produces 1 kg vapor from the liquor.

  • In effect – 3

    • 1 Kg vapor from effect-II will produce 1 kg vapor in effect-3.

  • Therefore, 1 kg steam will produce 3 kg vapor.
    Now, 

  • economy of a single effect evaporator =


  • And economy of a triple effect evaporator =


    So for N number of effects the economy will be N times that of a single effect evaporator.

Commonly Asked Questions.

  1. Define evaporation and write a short note on “Multiple Effect Evaporators”.

  2. What is a multiple effect evaporator? Write a short note on the economy of “Multiple Effect Evaporator”.
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