Simple Pharmanotes: Multiple effect evaporator.

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.

ECONOMY OF MULTIPLE EFFECT EVAPORATOR

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.

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.

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.

Define evaporation and write a short note on “Multiple Effect Evaporators”.
What is a multiple effect evaporator? Write a short note on the economy of “Multiple Effect Evaporator”.