In this screencast I am going to explain how a triple-effect evaporator works. And
its objective is to concentrate a dilute solution, usually some solute in water, where we use
significantly less energy than we would use in a single-effect evaporator. So there is
a separate screencast on the single-effect evaporator that it would be worth watching
first that describes the behavior for one of these evaporators in the triple-effect.
The advantage of the triple-effect evaporator is that we can use the vapor from this first
evaporator as the source of the heat for evaporating the liquid in the second evaporator, and then
we can use the vapor from the second evaporator as the source of heat for evaporating the
liquid in the third effect. And I will go through in detail to show you what we are
referring to here. So what we are going to have in the triple effect in the first effect,
we are going to have a feed coming in. A liquid at low concentration of solute. We are going
to vaporize some fraction of this and then we are going to create a more concentrated
solution of liquids. We carry out this evaporation by having steam being fed to these heat transfer
coils. And then we have liquid steam leaving so we're assuming that the feed is at saturation
pressure. We condense and drip out the liquid, also at saturation temperature and pressure.
So we have the feed coming at a feed temperature T1, would be the temperature in the evaporator
which also means it is the temperature of the liquid and the steam is coming in at a
temperature TS where TS must be greater than the evaporator temperature for evaporation
to occur. So what we do now is feed this liquid leaving the first evaporator into
the second evaporator and we create the evaporation by using the vapor, from the first evaporator
and we feed that into the heating coils and again, we get liquid at saturation conditions
leaving. And so we are evaporating steam in the vapor phase V2 and the liquid, more concentrated
from the second effect. And then we are going to take this vapor and feed it into the third
heat transfer coil. Again what is leaving is condensed steam so liquid at saturation
conditions. The concentrated liquid from the second evaporator is then fed into the third
evaporator where we get a more concentrated solution, L3. And we have a final vapor stream
coming off. So we can get a significant improvement in energy efficiency by using the vapor from
previous effects to vaporize to a liquid in the next effect. So the temperature in this
first effect is T1. Temperature here is T2, and here is T3. And when I say T3, that means
both the liquid and the vapor are at T3. Then T1 must be greater than T2 and must be greater
than T2 and must be greater than T3 because we are using the temperature T1, the steam
here at T1, to vaporize the liquid in the second effect so T1 must be greater than T2
for heat transfer. So if we ignore the boiling point rise due to the solute in the liquid
then we can use steam tables to also determine the pressures. So if the first effect is at
atmospheric pressure or 1 bar, then the pressure must be greater than the pressure in the second
must be greater than the pressure in the third which would mean we would be running these
effects under vacuum in order to create this temperature difference. And the concentrations
we would feed a low concentration of solute, there would be no solute in the vapor phase,
we will have concentrated that solute which is now fed to the second phase, because more
concentrated and then more concentrated, since that is the objective of triple effect evaporators.
And if we assume that the feed coming in is at T1 or very close to T1 then our heat transfer,
all the energy goes to vaporize the liquid and we can write the heat transfer in the
first effect. Heat transfer coefficient, area for heat transfer, temperature of the steam
minus temperature T1. But this should be approximately equal to the heat transfer in the second effect
because now we are going to condense back all that vapor that we formed. So q2 which is U2
area for heat transfer. Temperature in the second effect, temperature in the first effect,
which is now the heating for the second. And then the same argument means this should be
equal to q3 which then is the temperature we are heating and the temperature in the
third effect. And typically the heat transfer areas are the same, these are identical evaporators
and the heat transfer coefficient is approximately the same also. And so TS minus T1 then is
approximately T1 minus T2, approximately T2 minus T3 and these temperatures will then
determine which pressures we have to operate the effect at to get a steady-state operation.
The total amount of heat transferred is then is q1+q2+q3 which is going to be equal to the
UA, T of the steam minus T3, this total temperature difference. And so to a first approximation,
also 1 kilogram of steam fed in here, will vaporize three kilogram of our original liquid. And the triple
effect evaporator of this type certainly is best used if the feed is hot coming in. So
we don't have to use much of our steam energy to heat the feed. And if this liquid here
leaving our product, the concentrated solution, has some temperature sensitivity as we increase
its concentration and we would like to be operating that at a lower temperature. The
actual calculations are really very similar to those for a single effect evaporator.