00:00:03.330 --> 00:00:09.950 In this screencast I am going to explain how a triple-effect evaporator works. And 00:00:09.950 --> 00:00:16.770 its objective is to concentrate a dilute solution, usually some solute in water, where we use 00:00:16.770 --> 00:00:22.400 significantly less energy than we would use in a single-effect evaporator. So there is 00:00:22.400 --> 00:00:26.779 a separate screencast on the single-effect evaporator that it would be worth watching 00:00:26.779 --> 00:00:33.720 first that describes the behavior for one of these evaporators in the triple-effect. 00:00:33.720 --> 00:00:39.830 The advantage of the triple-effect evaporator is that we can use the vapor from this first 00:00:39.830 --> 00:00:45.760 evaporator as the source of the heat for evaporating the liquid in the second evaporator, and then 00:00:45.760 --> 00:00:52.119 we can use the vapor from the second evaporator as the source of heat for evaporating the 00:00:52.119 --> 00:00:57.430 liquid in the third effect. And I will go through in detail to show you what we are 00:00:57.430 --> 00:01:01.769 referring to here. So what we are going to have in the triple effect in the first effect, 00:01:01.769 --> 00:01:08.340 we are going to have a feed coming in. A liquid at low concentration of solute. We are going 00:01:08.340 --> 00:01:14.330 to vaporize some fraction of this and then we are going to create a more concentrated 00:01:14.330 --> 00:01:23.740 solution of liquids. We carry out this evaporation by having steam being fed to these heat transfer 00:01:23.740 --> 00:01:32.420 coils. And then we have liquid steam leaving so we're assuming that the feed is at saturation 00:01:32.420 --> 00:01:40.000 pressure. We condense and drip out the liquid, also at saturation temperature and pressure. 00:01:40.000 --> 00:01:46.200 So we have the feed coming at a feed temperature T1, would be the temperature in the evaporator 00:01:46.200 --> 00:01:53.260 which also means it is the temperature of the liquid and the steam is coming in at a 00:01:53.260 --> 00:02:00.630 temperature TS where TS must be greater than the evaporator temperature for evaporation 00:02:00.630 --> 00:02:08.819 to occur. So what we do now is feed this liquid leaving the first evaporator into 00:02:08.819 --> 00:02:16.990 the second evaporator and we create the evaporation by using the vapor, from the first evaporator 00:02:16.990 --> 00:02:24.400 and we feed that into the heating coils and again, we get liquid at saturation conditions 00:02:24.400 --> 00:02:34.120 leaving. And so we are evaporating steam in the vapor phase V2 and the liquid, more concentrated 00:02:34.120 --> 00:02:41.860 from the second effect. And then we are going to take this vapor and feed it into the third 00:02:41.860 --> 00:02:49.250 heat transfer coil. Again what is leaving is condensed steam so liquid at saturation 00:02:49.250 --> 00:02:57.340 conditions. The concentrated liquid from the second evaporator is then fed into the third 00:02:57.340 --> 00:03:06.739 evaporator where we get a more concentrated solution, L3. And we have a final vapor stream 00:03:06.739 --> 00:03:14.190 coming off. So we can get a significant improvement in energy efficiency by using the vapor from 00:03:14.190 --> 00:03:20.640 previous effects to vaporize to a liquid in the next effect. So the temperature in this 00:03:20.640 --> 00:03:29.200 first effect is T1. Temperature here is T2, and here is T3. And when I say T3, that means 00:03:29.200 --> 00:03:36.020 both the liquid and the vapor are at T3. Then T1 must be greater than T2 and must be greater 00:03:36.020 --> 00:03:44.680 than T2 and must be greater than T3 because we are using the temperature T1, the steam 00:03:44.680 --> 00:03:53.140 here at T1, to vaporize the liquid in the second effect so T1 must be greater than T2 00:03:53.140 --> 00:04:00.660 for heat transfer. So if we ignore the boiling point rise due to the solute in the liquid 00:04:00.660 --> 00:04:08.690 then we can use steam tables to also determine the pressures. So if the first effect is at 00:04:08.690 --> 00:04:16.239 atmospheric pressure or 1 bar, then the pressure must be greater than the pressure in the second 00:04:16.239 --> 00:04:21.769 must be greater than the pressure in the third which would mean we would be running these 00:04:21.769 --> 00:04:30.339 effects under vacuum in order to create this temperature difference. And the concentrations 00:04:30.339 --> 00:04:37.740 we would feed a low concentration of solute, there would be no solute in the vapor phase, 00:04:37.740 --> 00:04:46.430 we will have concentrated that solute which is now fed to the second phase, because more 00:04:46.430 --> 00:04:52.409 concentrated and then more concentrated, since that is the objective of triple effect evaporators. 00:04:52.409 --> 00:05:01.889 And if we assume that the feed coming in is at T1 or very close to T1 then our heat transfer, 00:05:01.889 --> 00:05:07.659 all the energy goes to vaporize the liquid and we can write the heat transfer in the 00:05:07.659 --> 00:05:13.659 first effect. Heat transfer coefficient, area for heat transfer, temperature of the steam 00:05:13.659 --> 00:05:21.300 minus temperature T1. But this should be approximately equal to the heat transfer in the second effect 00:05:21.300 --> 00:05:30.080 because now we are going to condense back all that vapor that we formed. So q2 which is U2 00:05:30.080 --> 00:05:36.369 area for heat transfer. Temperature in the second effect, temperature in the first effect, 00:05:36.369 --> 00:05:41.509 which is now the heating for the second. And then the same argument means this should be 00:05:41.509 --> 00:05:49.879 equal to q3 which then is the temperature we are heating and the temperature in the 00:05:49.879 --> 00:05:57.900 third effect. And typically the heat transfer areas are the same, these are identical evaporators 00:05:57.900 --> 00:06:06.909 and the heat transfer coefficient is approximately the same also. And so TS minus T1 then is 00:06:06.909 --> 00:06:15.869 approximately T1 minus T2, approximately T2 minus T3 and these temperatures will then 00:06:15.869 --> 00:06:22.939 determine which pressures we have to operate the effect at to get a steady-state operation. 00:06:22.939 --> 00:06:34.770 The total amount of heat transferred is then is q1+q2+q3 which is going to be equal to the 00:06:34.770 --> 00:06:43.930 UA, T of the steam minus T3, this total temperature difference. And so to a first approximation, 00:06:43.930 --> 00:06:54.800 also 1 kilogram of steam fed in here, will vaporize three kilogram of our original liquid. And the triple 00:06:54.800 --> 00:07:02.259 effect evaporator of this type certainly is best used if the feed is hot coming in. So 00:07:02.259 --> 00:07:08.279 we don't have to use much of our steam energy to heat the feed. And if this liquid here 00:07:08.279 --> 00:07:16.099 leaving our product, the concentrated solution, has some temperature sensitivity as we increase 00:07:16.099 --> 00:07:20.610 its concentration and we would like to be operating that at a lower temperature. The 00:07:20.610 --> 00:07:26.710 actual calculations are really very similar to those for a single effect evaporator.
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