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Lecture 47 - Boiling, Evaporation and Evaporators
WEBVTT Kind: captions Language: en
00:00:00.160 [Music] 00:00:14.72000:00:14.730 this would be the last class on 00:00:17.72000:00:17.730 convective heat transfer what we have 00:00:20.73000:00:20.740 covered in convective heat transfer so 00:00:22.50000:00:22.510 far or when we have flow outside of a 00:00:26.34000:00:26.350 plate or or a tube which is external 00:00:29.19000:00:29.200 flow internal flow the concept of fully 00:00:33.21000:00:33.220 developed flow the relations and 00:00:35.43000:00:35.440 correlations which are applicable both 00:00:37.50000:00:37.510 in laminar flow and in turbulent flow 00:00:40.08000:00:40.090 for flow outside as well as flow inside 00:00:43.93900:00:43.949 and we have seen what is how to design a 00:00:47.97000:00:47.980 heat exchanger based on the log mean 00:00:51.50900:00:51.519 temperature difference method and if 00:00:53.70000:00:53.710 both the outlet temperatures are not 00:00:56.54900:00:56.559 known then a better method known as 00:00:59.13000:00:59.140 epsilon NTU method or effectiveness if 00:01:02.49000:01:02.500 NTU method can be adopted and we have 00:01:05.94000:01:05.950 solved a number of problems and you'd be 00:01:09.63000:01:09.640 it be given more problems in the 00:01:11.85000:01:11.860 tutorial sheet which you try should try 00:01:13.95000:01:13.960 to solve and if there's any difficulty 00:01:15.95900:01:15.969 we will discuss about it but we will 00:01:17.96900:01:17.979 close the convective heat transfer today 00:01:20.37000:01:20.380 with something that we haven't discussed 00:01:22.49900:01:22.509 that much in our previous studies this 00:01:26.19000:01:26.200 is about evaporation boil and boiling 00:01:29.16000:01:29.170 heat transferred the utility the 00:01:33.35900:01:33.369 importance of evaporation and boiling 00:01:35.37000:01:35.380 heat transfer would be more apparent in 00:01:38.24900:01:38.259 the next half of this class when we talk 00:01:41.45900:01:41.469 about the evaporators as you are 00:01:44.24900:01:44.259 probably aware of evaporators is one of 00:01:47.06900:01:47.079 the major equipment major unit in many 00:01:50.78900:01:50.799 heat transfer many chemical plants where 00:01:54.14900:01:54.159 a solution a dilute solution is going to 00:01:56.45900:01:56.469 be concentrated by evaporating part of 00:02:00.51000:02:00.520 the solvent they are making it more 00:02:02.66900:02:02.679 concentrate so the applications of 00:02:05.58000:02:05.590 evaporators the types of evaporators and 00:02:09.48000:02:09.490 the reason for concentrating 00:02:12.74000:02:12.750 concentrating a specific product in a 00:02:15.33000:02:15.340 dilute solution you discuss about it so 00:02:19.11000:02:19.120 the first starting point is to know a 00:02:20.85000:02:20.860 little bit about evaporation and boiling 00:02:23.46000:02:23.470 and we are I'm also 00:02:25.86000:02:25.870 going to refer to a specific experiment 00:02:28.11000:02:28.120 which is a very famous experiment where 00:02:30.17000:02:30.180 we would see how the boiling curve that 00:02:34.22900:02:34.239 is what the heat flux removed from where 00:02:38.61000:02:38.620 which is submerged in water how does it 00:02:42.39000:02:42.400 vary when you change the temperature 00:02:45.47900:02:45.489 difference between the word which can be 00:02:48.36000:02:48.370 heated electrically and that of the 00:02:50.85000:02:50.860 surrounding water which is assumed to be 00:02:53.58000:02:53.590 saturated so at any given point of time 00:02:56.25000:02:56.260 you can control the amount of heat which 00:02:58.80000:02:58.810 passes through the which is generated 00:03:02.00900:03:02.019 due to the passage of current through 00:03:04.38000:03:04.390 the wire and at steady state the amount 00:03:08.13000:03:08.140 of heat which which is provided to the 00:03:11.22000:03:11.230 to the thin where most of the cases it's 00:03:14.39900:03:14.409 a nichrome where of high resistance so 00:03:17.22000:03:17.230 the Joule heating is we would see how 00:03:20.52000:03:20.530 the Joule heating had how the 00:03:22.11000:03:22.120 dissipation of heat from the were 00:03:24.99000:03:25.000 follows a specific train with delta T 00:03:28.55900:03:28.569 which is the temperature difference 00:03:30.05900:03:30.069 between the where and the surrounding 00:03:32.22000:03:32.230 liquid that has been imposed on the 00:03:35.09900:03:35.109 system and we will get some interesting 00:03:37.37900:03:37.389 or interesting idea out of that boiling 00:03:39.96000:03:39.970 curve the evaporation is also known as 00:03:44.43000:03:44.440 quite boiling in which case most let's 00:03:48.53900:03:48.549 say you take a beaker of water and you 00:03:51.00000:03:51.010 place it on a hot plate so near the near 00:03:55.19900:03:55.209 the interface with the hot plate the 00:03:58.92000:03:58.930 temperature is going to be almost close 00:04:00.86900:04:00.879 to that of the hot plate but this 00:04:02.93900:04:02.949 temperature would change drastically if 00:04:06.05900:04:06.069 you just move up a little bit so a large 00:04:09.36000:04:09.370 change in T can be observed in a region 00:04:13.37900:04:13.389 very close to that of the heater beyond 00:04:16.80000:04:16.810 that point the temperature more or less 00:04:18.99000:04:19.000 remains constant and when you come close 00:04:22.43900:04:22.449 to the interface it is assumed that the 00:04:25.58900:04:25.599 interface is at equilibrium with its 00:04:28.62000:04:28.630 vapor so the interface temperature is 00:04:31.46000:04:31.470 for the case of evaporation the 00:04:34.05000:04:34.060 interface temperature most of the times 00:04:37.05000:04:37.060 is approximated as the saturation 00:04:40.11000:04:40.120 temperature of water of water or the 00:04:42.99000:04:43.000 liquid that we are using or for the 00:04:45.45000:04:45.460 given conditions 00:04:46.62000:04:46.630 so the temperature profile if you could 00:04:49.40900:04:49.419 draw it as a function of the distance 00:04:51.68900:04:51.699 from the heater in the beaker then it's 00:04:54.60000:04:54.610 going to be very short near near the 00:04:57.57000:04:57.580 region of the hot plate then it more or 00:05:00.15000:05:00.160 less follows a constant pattern and 00:05:02.37000:05:02.380 there is a small temperature jump which 00:05:05.15900:05:05.169 most of the kill most of the times it's 00:05:07.05000:05:07.060 neglected so tea set is going to be 00:05:09.62900:05:09.639 equal tea set is going to be equal to t 00:05:12.27000:05:12.280 naught that is the temperature of the on 00:05:14.27900:05:14.289 the liquid side of the interface so this 00:05:17.70000:05:17.710 is how the evaporation is perceived in 00:05:20.58000:05:20.590 evaporation is as slow as you can 00:05:22.86000:05:22.870 understand evaporation is a slow process 00:05:24.80000:05:24.810 it's totally different from that of the 00:05:27.15000:05:27.160 boiling where in the case of boiling you 00:05:29.46000:05:29.470 have phase change taking place 00:05:31.58000:05:31.590 everywhere in the liquid column that we 00:05:34.23000:05:34.240 were just discussing but in the case of 00:05:36.60000:05:36.610 evaporation phase change is taking place 00:05:38.34000:05:38.350 only from the top of the liquid and 00:05:40.89000:05:40.900 there are numerous relations and 00:05:42.93000:05:42.940 correlations which connect the heat 00:05:45.27000:05:45.280 which is dissipated from the heater as a 00:05:47.25000:05:47.260 function of delta T so these relations 00:05:50.15900:05:50.169 and correlations are available in the 00:05:51.99000:05:52.000 text that is the book fundamentals of 00:05:54.36000:05:54.370 fundamentals of heat and mass transfer 00:05:56.19000:05:56.200 by incra para and do it and also on any 00:05:59.27900:05:59.289 other book or textbook on heat transfer 00:06:01.82000:06:01.830 so you can take a look at the relations 00:06:04.95000:06:04.960 and whenever you have to find the heat 00:06:08.10000:06:08.110 transfer coefficient for such processes 00:06:10.80000:06:10.810 go to the go to the reference find out 00:06:13.52900:06:13.539 which ref which which which which 00:06:16.58900:06:16.599 relation or correlation is appropriate 00:06:19.02000:06:19.030 for the specific situation and use it 00:06:21.62900:06:21.639 but we are going to now talk about the 00:06:24.18000:06:24.190 case of boiling curve where where which 00:06:28.11000:06:28.120 is submerged in water the current is is 00:06:31.05000:06:31.060 being being passed through the wire 00:06:33.87000:06:33.880 which hits up and you can measure what 00:06:37.14000:06:37.150 is the you can find out how much of heat 00:06:39.75000:06:39.760 is being lost by the by the by the thin 00:06:44.07000:06:44.080 wire to the surrounding fluid and you 00:06:46.29000:06:46.300 can calculate you can you can measure 00:06:48.39000:06:48.400 the variation of this Q as a function of 00:06:51.30000:06:51.310 delta T and we would like to 00:06:53.91000:06:53.920 see how it changes and this has given us 00:06:56.73000:06:56.740 the boiling curve of water of boiling 00:07:00.69000:07:00.700 car for the case of water so near the 00:07:02.85000:07:02.860 vicinity of the word if this is the 00:07:04.65000:07:04.660 world then near the vicinity of the 00:07:06.63000:07:06.640 world what are the different steps that 00:07:09.51000:07:09.520 we can encounter that we will encounter 00:07:12.03000:07:12.040 as the temperature of the wear is 00:07:14.13000:07:14.140 progressively increased this is what we 00:07:16.59000:07:16.600 is going to give rise to the boiling 00:07:18.39000:07:18.400 curve so let us look at take a quick 00:07:20.13000:07:20.140 look at the boiling curve for saturated 00:07:23.01000:07:23.020 water at atmospheric pressure so you can 00:07:25.68000:07:25.690 see on the left-hand side I have plotted 00:07:28.40000:07:28.410 the heat flux which is Q double prime is 00:07:31.29000:07:31.300 in watt per meter square and delta T is 00:07:33.87000:07:33.880 the x-axis the independent variable 00:07:36.09000:07:36.100 where delta T is defined as TS minus T 00:07:39.63000:07:39.640 set so temperature temperature 00:07:41.67000:07:41.680 temperature of the surface minus T set 00:07:43.74000:07:43.750 so initially up to a delta T of about 5 00:07:47.58000:07:47.590 degree the heat flux gradually increases 00:07:51.87000:07:51.880 slowly increases with delta T so this 00:07:56.19000:07:56.200 this kind of relaxed increase of heat 00:07:59.13000:07:59.140 flux with delta T this region is mostly 00:08:02.07000:08:02.080 governed by free convection so up to 00:08:05.34000:08:05.350 about five five degree centigrade 00:08:06.90000:08:06.910 temperature difference it's the free 00:08:09.21000:08:09.220 convection which which is which is which 00:08:12.03000:08:12.040 which takes which which is responsible 00:08:14.76000:08:14.770 for heat transfer so you have the set up 00:08:18.54000:08:18.550 could be like this it's just a schematic 00:08:20.91000:08:20.920 of this setup so you have a specific 00:08:24.38000:08:24.390 wear of high resistivity high resistance 00:08:30.17000:08:30.180 which is connected to a power source and 00:08:34.52000:08:34.530 this is submerged in water so you was a 00:08:42.59000:08:42.600 current through it and from the voltage 00:08:45.42000:08:45.430 and the current V times a is going to 00:08:48.87000:08:48.880 give you the amount of heat that is 00:08:51.09000:08:51.100 being dissipated by the wear in the 00:08:54.03000:08:54.040 water and the tea set is the saturation 00:08:57.78000:08:57.790 temperature and in the Q this divided by 00:09:02.28000:09:02.290 area would give you you can also find 00:09:04.47000:09:04.480 out what is Q double prime that 00:09:06.61000:09:06.620 which has been plotted over here so when 00:09:09.76000:09:09.770 the temperature difference is about five 00:09:11.80000:09:11.810 degree it's free convection but as the 00:09:14.53000:09:14.540 temperature difference is increased the 00:09:16.54000:09:16.550 first bubbles will start to appear on 00:09:19.48000:09:19.490 the wire and it is going to be the onset 00:09:23.53000:09:23.540 of the nuclear boiling nucleate boiling 00:09:26.43000:09:26.440 so over here what you are going to get 00:09:29.05000:09:29.060 is onset of nucleate boiling so this 00:09:33.31000:09:33.320 point is where the first bubbles will 00:09:35.95000:09:35.960 appear but the bubbles are going to be 00:09:39.00000:09:39.010 detached from one another so bubbles are 00:09:41.95000:09:41.960 going to form and the bubbles will get 00:09:44.68000:09:44.690 released and they will try to move 00:09:46.66000:09:46.670 towards the top because of buoyancy but 00:09:49.78000:09:49.790 since the heat superheat content of the 00:09:52.39000:09:52.400 bubbles are not that great especially at 00:09:55.15000:09:55.160 low values of of delta T they will 00:09:57.91000:09:57.920 decent this they will discing 00:09:59.89000:09:59.900 disintegrate and collapse back to become 00:10:03.82000:10:03.830 the liquid once again so this formation 00:10:06.94000:10:06.950 of bubbles and the movement of the 00:10:10.09000:10:10.100 bubbles will ensure that your cue the 00:10:12.91000:10:12.920 slope of the Q double prime is versus 00:10:15.04000:10:15.050 delta T increases more rapidly with 00:10:18.10000:10:18.110 delta T as compared to the free 00:10:20.23000:10:20.240 convection so as mu as you increase it 00:10:23.14000:10:23.150 beyond that as you increase the current 00:10:25.63000:10:25.640 through this even more then what you are 00:10:27.94000:10:27.950 going to have is bubbles which are going 00:10:29.98000:10:29.990 to form and the bubbles are going to get 00:10:33.07000:10:33.080 detached and there's going to be a lot 00:10:36.28000:10:36.290 of mixing in over here so disturbances 00:10:39.67000:10:39.680 near the interface should increase and 00:10:42.04000:10:42.050 as the disturbance is mixing and the 00:10:45.70000:10:45.710 interference between the bubbles 00:10:47.23000:10:47.240 increase at this point so you are going 00:10:50.17000:10:50.180 to get higher and higher values of heat 00:10:52.15000:10:52.160 flux as a function of delta T so it's 00:10:55.21000:10:55.220 going to increase rapidly but as it's 00:10:58.60000:10:58.610 doing so a point would come where the 00:11:02.94000:11:02.950 the where will see will be covered by 00:11:06.81000:11:06.820 almost like a blanket will will start to 00:11:10.54000:11:10.550 get covered with a blanket of the vapor 00:11:12.67000:11:12.680 so this is your where which is going to 00:11:15.04000:11:15.050 be covered going to start to getting 00:11:17.02000:11:17.030 covered with a blanket of the vapor so 00:11:19.60000:11:19.610 therefore they 00:11:20.55000:11:20.560 the problem of the liquid reaching the 00:11:23.31000:11:23.320 surface changing its phase and forming 00:11:25.76900:11:25.779 the bubble so the additional resistance 00:11:27.96000:11:27.970 provided by the low thermal conductivity 00:11:30.48000:11:30.490 vapor bubbles clinging to the surface 00:11:33.80000:11:33.810 accumulating on the surface so there are 00:11:37.29000:11:37.300 two competing mechanisms one is the 00:11:39.60000:11:39.610 bubbles are taking the the vapor bubbles 00:11:41.76000:11:41.770 are changing I mean its liquid is 00:11:43.29000:11:43.300 changing phase phase creating the liquid 00:11:46.47000:11:46.480 bubble the liquid bubbles will take the 00:11:48.72000:11:48.730 latent heat and will rise in the liquid 00:11:50.76000:11:50.770 thereby creating the keeping the surface 00:11:53.43000:11:53.440 cool the other competing mechanism would 00:11:56.85000:11:56.860 be the river the computing mechanism 00:11:58.80000:11:58.810 will be the bubbles forming an 00:12:00.26000:12:00.270 additional layer of resistance only 00:12:02.76000:12:02.770 where therefore these two are acting at 00:12:07.04000:12:07.050 cross-purposes formation and release of 00:12:10.65000:12:10.660 the bubbles and formation of a blanket 00:12:13.68000:12:13.690 of the vapor over the solid which 00:12:16.91000:12:16.920 increases the heat transfer these two 00:12:19.71000:12:19.720 are the competing mechanism and they 00:12:21.93000:12:21.940 will balance out resulting at some point 00:12:24.87000:12:24.880 in the maximum heat flux which you can 00:12:29.40000:12:29.410 obtain for such cases and the point 00:12:32.22000:12:32.230 where this happens is known as the 00:12:34.77000:12:34.780 critical heat flux so in critical heat 00:12:39.99000:12:40.000 flux is the which takes place when these 00:12:43.92000:12:43.930 two more or less cancel out each other 00:12:46.68000:12:46.690 and this is what is known as the as the 00:12:50.16000:12:50.170 critical critical heat flux and from the 00:12:53.85000:12:53.860 nuclear boiling you start so this is the 00:12:56.82000:12:56.830 onset of nucleate boiling and over here 00:12:59.49000:12:59.500 in critical heat flux you are going to 00:13:01.59000:13:01.600 go from nucleate boiling nucleate 00:13:07.49000:13:07.500 towards film boiling so critical is the 00:13:12.77000:13:12.780 transition between the nucleate and the 00:13:15.87000:13:15.880 film boiling if you go beyond that the 00:13:19.74000:13:19.750 way this region is marked as dotted 00:13:21.99000:13:22.000 because in most of the cases it's very 00:13:24.39000:13:24.400 difficult to get this region which is an 00:13:27.15000:13:27.160 unstable region in there as the 00:13:30.54000:13:30.550 temperature difference increases the 00:13:32.16000:13:32.170 thickness of this 00:13:33.90000:13:33.910 vapor layer will simply be more it will 00:13:37.74000:13:37.750 keep on increasing and the resistance 00:13:40.77000:13:40.780 due to the the entire wear is going to 00:13:43.77000:13:43.780 get blanketed by a layer of the vapor 00:13:47.25000:13:47.260 with very low thermal conductivity so 00:13:50.16000:13:50.170 with increase in delta T more bubbles 00:13:52.59000:13:52.600 are formed and the heat flux keeps on 00:13:54.87000:13:54.880 decreasing so the point where you are 00:13:57.27000:13:57.280 going to get the minimum heat flux for 00:14:00.87000:14:00.880 such a situation is known as the 00:14:03.32000:14:03.330 Leidenfrost point so laden frost point 00:14:10.82000:14:10.830 which simply says that this Q double 00:14:13.98000:14:13.990 prime is going to be minimum once you 00:14:19.77000:14:19.780 cross this the mode of heat transfer is 00:14:22.98000:14:22.990 going to be going to be I mean the 00:14:25.26000:14:25.270 radiation would start to play a role so 00:14:28.53000:14:28.540 the increase in delta T the the the heat 00:14:32.37000:14:32.380 flux will start to rise again because 00:14:34.98000:14:34.990 your T is delta T is more so therefore 00:14:38.01000:14:38.020 the change in delta T the increase in 00:14:40.23000:14:40.240 delta T will start to compensate for the 00:14:43.56000:14:43.570 lower lower heat transfer coefficient 00:14:46.71000:14:46.720 resulted because of the formation of 00:14:49.11000:14:49.120 this so it will start to increase and 00:14:52.41000:14:52.420 beyond certain point the radiation is 00:14:55.02000:14:55.030 going to take place take over and it's 00:14:58.17000:14:58.180 going to be very very steep curve from 00:15:01.32000:15:01.330 there so somewhere around this point 00:15:04.61000:15:04.620 somewhere around this point you are 00:15:07.32000:15:07.330 going to get the burnout of the nichrome 00:15:14.94000:15:14.950 where 00:15:20.03000:15:20.040 which is this one they were over here so 00:15:24.50000:15:24.510 when that happens this is roughly a 00:15:27.71000:15:27.720 measure of what is the heat flux so this 00:15:31.37000:15:31.380 is connected to a voltmeter to a power 00:15:34.34000:15:34.350 source so you slowly increase the power 00:15:37.01000:15:37.020 and you see how how this this is 00:15:39.74000:15:39.750 changing and when at certain point the 00:15:43.19000:15:43.200 wire breaks that's the point which is 00:15:46.49000:15:46.500 known as the critical which is known as 00:15:49.01000:15:49.020 the critical heat flux so the knowledge 00:15:53.54000:15:53.550 of critical heat flux is extremely 00:15:56.51000:15:56.520 important in the design of equipments 00:15:59.33000:15:59.340 which involve boiling so whenever you 00:16:02.15000:16:02.160 have a boiling situation for a for in in 00:16:05.39000:16:05.400 in in a heat exchange equipment for 00:16:07.52000:16:07.530 example in an evaporator you need to 00:16:09.74000:16:09.750 know what is the maximum heat flux that 00:16:11.72000:16:11.730 this surface can handle without going to 00:16:15.77000:16:15.780 hell going to a point where we change it 00:16:18.68000:16:18.690 with increase in delta T your heat flux 00:16:21.32000:16:21.330 will decrease so you're going to be on 00:16:23.27000:16:23.280 the other side of the boiling curve 00:16:24.83000:16:24.840 rather than with change in Delta T heat 00:16:27.80000:16:27.810 flux is increasing if you are at or near 00:16:30.68000:16:30.690 the critical heat flux as small a change 00:16:33.41000:16:33.420 in delta T can be counterproductive 00:16:36.20000:16:36.210 instead of getting more heat flux you 00:16:39.02000:16:39.030 are going to get less heat flux so the 00:16:42.40000:16:42.410 limitation of your boiling equipment in 00:16:45.14000:16:45.150 order to in order to evaluate the point 00:16:48.41000:16:48.420 up to which your system your equipment 00:16:51.89000:16:51.900 your designed equipment evaporator can 00:16:54.17000:16:54.180 work you really need to know what is the 00:16:56.45000:16:56.460 critical heat flux for such as such as 00:16:58.28000:16:58.290 such a point so the important points 00:17:00.95000:17:00.960 here are that through this experiment 00:17:02.89000:17:02.900 you have an idea of the different modes 00:17:06.02000:17:06.030 of heat transfer starting from natural 00:17:08.78000:17:08.790 convection to force convection and in 00:17:11.54000:17:11.550 forced convection you are going to get 00:17:13.10000:17:13.110 the onset of nucleate boiling where the 00:17:15.98000:17:15.990 bubbles are vapor bubbles are going to 00:17:17.84000:17:17.850 form detach from the surface rise 00:17:19.73000:17:19.740 towards the top but will probably 00:17:22.09000:17:22.100 assimilate once again by a phase change 00:17:24.80000:17:24.810 process with water so more number of 00:17:28.40000:17:28.410 bubbles means more heat transfer so with 00:17:31.43000:17:31.440 the change in delta T in the 00:17:33.25000:17:33.260 nucleate boiling region your temperature 00:17:35.71000:17:35.720 your your heat flux keeps on increasing 00:17:37.66000:17:37.670 but as that happens there's going to be 00:17:40.81000:17:40.820 formation slowly that's going to be 00:17:42.73000:17:42.740 formation of a vapor blanket on the 00:17:45.40000:17:45.410 where therefore these two competing 00:17:48.07000:17:48.080 mechanisms would try to make we try to 00:17:51.55000:17:51.560 come to come to a balance and the point 00:17:54.16000:17:54.170 where the the maximum occurs is known as 00:17:57.10000:17:57.110 the critical heat flux beyond that with 00:17:59.95000:17:59.960 change in delta T the heat flux will 00:18:02.32000:18:02.330 keep on decreasing or at the most will 00:18:06.22000:18:06.230 remain more or less a constant so you 00:18:08.11000:18:08.120 are not utilizing anything getting 00:18:10.75000:18:10.760 anything new out of your equipment by 00:18:13.39000:18:13.400 having it operating near at or near the 00:18:15.97000:18:15.980 critical heat flux region so designing 00:18:18.52000:18:18.530 of heat exchange equipment especially 00:18:20.17000:18:20.180 with those which involve boiling 00:18:22.39000:18:22.400 would require that you have a fair 00:18:24.79000:18:24.800 knowledge accurate idea almost what is 00:18:27.70000:18:27.710 going to be the critical heat flux for 00:18:29.20000:18:29.210 such situations and again there are 00:18:31.57000:18:31.580 relations and there are correlations 00:18:33.13000:18:33.140 available which would give you the the 00:18:35.92000:18:35.930 the value of the critical heat flux for 00:18:37.96000:18:37.970 some of the substrates liquid 00:18:40.72000:18:40.730 combinations and you can look at your 00:18:42.91000:18:42.920 text to find out what they are I am NOT 00:18:44.95000:18:44.960 going to write all of them down what I'm 00:18:46.84000:18:46.850 going to do next is to show you some 00:18:48.88000:18:48.890 quickly a little bit about the 00:18:50.71000:18:50.720 evaporators their designs and so on so 00:18:53.14000:18:53.150 let's move on to evaporators and we will 00:18:57.91000:18:57.920 risk deal with chemical evaporators 00:19:09.03000:19:09.040 and in chemical evaporators are of two 00:19:13.06000:19:13.070 types one is natural circulation it can 00:19:22.57000:19:22.580 be a single or a multiple effort I will 00:19:32.05000:19:32.060 talk about what single then multiple 00:19:33.88000:19:33.890 effects are in the second obviously is 00:19:38.29000:19:38.300 going to be forced circulation where you 00:19:44.28000:19:44.290 require higher throughput higher values 00:19:47.17000:19:47.180 of heat transfer you are going to get 00:19:49.60000:19:49.610 you're going to use this so the natural 00:19:52.99000:19:53.000 convection type one if we if we start 00:19:56.73000:19:56.740 working with natural convictions some of 00:20:00.13000:20:00.140 the possible exchanges are going to be 00:20:04.41000:20:04.420 this is the simplest one where you have 00:20:13.00000:20:13.010 a liquid up to this point and then there 00:20:18.64000:20:18.650 are steam which is coming in and you 00:20:26.05000:20:26.060 have pipes or tubes like this so steam 00:20:31.06000:20:31.070 is going to pass through these tubes 00:20:32.98000:20:32.990 come to the come to the other side and 00:20:35.77000:20:35.780 whatever is room whatever is remaining 00:20:38.22000:20:38.230 it goes out of the vent whatever 00:20:41.32000:20:41.330 condenses the condensed its steam is 00:20:44.35000:20:44.360 going to come out as drips the feed 00:20:49.93000:20:49.940 comes in here at the top and the 00:20:54.61000:20:54.620 concentrated product which is known as 00:20:58.12000:20:58.130 the thick liquor come can be collected 00:21:02.20000:21:02.210 at the end so this this this is this is 00:21:05.80000:21:05.810 a horizontal tube this is a horizontal 00:21:10.48000:21:10.490 tube heaters for a turn it can also have 00:21:14.41000:21:14.420 as the name suggests you can also have 00:21:22.47000:21:22.480 tubes like this where steam is going to 00:21:28.21000:21:28.220 be go in and out and so the same thing 00:21:37.63000:21:37.640 is on this side so the this is filled 00:21:44.17000:21:44.180 with liquid it is for the vapor to come 00:21:51.40000:21:51.410 out of this so you have steam which is 00:21:55.84000:21:55.850 going so I'll write this the steam will 00:21:59.77000:21:59.780 is going to pass through them and you 00:22:01.78000:22:01.790 have the same thing on the right hand 00:22:03.46000:22:03.470 side and the liquid is going to come 00:22:07.36000:22:07.370 down rise due to natural convection 00:22:10.26000:22:10.270 reach over here change its direction and 00:22:13.69000:22:13.700 come back again so this kind of 00:22:15.55000:22:15.560 circulation will go on in the 00:22:18.07000:22:18.080 intervening space between the tubes and 00:22:21.37000:22:21.380 therefore you have the feed which is 00:22:23.89000:22:23.900 coming in here and the thick liquor 00:22:29.58000:22:29.590 would be collected from the bottle and 00:22:34.26000:22:34.270 you have steam which is which is in here 00:22:38.74000:22:38.750 so steam comes in here and this is 00:22:43.36000:22:43.370 obviously going to be the drips in any 00:22:47.52000:22:47.530 remaining vapor can come out of this so 00:22:52.00000:22:52.010 these these are two examples of natural 00:22:54.19000:22:54.200 circulation evaporators and in many of 00:22:57.91000:22:57.920 the cases the vapor which comes out from 00:23:00.67000:23:00.680 these streams these vapors can then be 00:23:04.51000:23:04.520 used in another evaporator which is in 00:23:08.11000:23:08.120 series which is after this one so 00:23:10.66000:23:10.670 instead of instead of having just one 00:23:13.65000:23:13.660 evaporator which is known also known as 00:23:16.63000:23:16.640 the single effect evaporator you can 00:23:19.99000:23:20.000 have multiple effect evaporator but more 00:23:22.54000:23:22.550 number of evaporators 2 3 4 are going to 00:23:25.84000:23:25.850 be in series and the vapour from the 00:23:28.30000:23:28.310 evaporator 1 is going to be the is going 00:23:32.02000:23:32.030 to going to be used to heat 00:23:34.08000:23:34.090 that of that of two and so on which I 00:23:37.20000:23:37.210 will draw when I'll show you 00:23:40.74000:23:40.750 so the forced circulation when we look 00:23:44.88000:23:44.890 at the forced circulation one you simply 00:23:48.41900:23:48.429 have the design construction or probably 00:23:51.65900:23:51.669 would approximately remain the same 00:25:10.25000:25:10.260 so in a forced circulation type of 00:25:14.33000:25:14.340 evaporator you still have the steam in 00:25:17.79900:25:17.809 the liquid is forced by a pump which 00:25:21.40900:25:21.419 goes through the space between them goes 00:25:26.06000:25:26.070 over to over here to another another 00:25:29.15000:25:29.160 chamber where the vapor is collected so 00:25:32.81000:25:32.820 you have a you have the liquid over here 00:25:35.33000:25:35.340 then you keep on circulating it keep on 00:25:38.57000:25:38.580 circulating it keep on circulating this 00:25:41.90000:25:41.910 and it gets more and more heated the 00:25:44.90000:25:44.910 vapors are going to be collected this is 00:25:47.09000:25:47.100 the drips from the feed and you can give 00:25:50.24000:25:50.250 some you can provide the feed from over 00:25:53.18000:25:53.190 here and these kind of systems are used 00:25:56.65900:25:56.669 where the concentration problem involves 00:26:00.26000:26:00.270 a solution with poor flow and it may 00:26:03.32000:26:03.330 have it may form scales it may have 00:26:07.07000:26:07.080 thermal undesirable thermal 00:26:08.96000:26:08.970 characteristics or it is highly viscous 00:26:11.87000:26:11.880 so you really need to force the liquid 00:26:15.20000:26:15.210 through through that through that 00:26:17.06000:26:17.070 through the through the spacing where 00:26:20.33000:26:20.340 it's in connect in contact with the 00:26:22.61000:26:22.620 steam through this through these tubes 00:26:24.95000:26:24.960 so for highly viscous fluids this this 00:26:29.51000:26:29.520 kind of forced circulation would be an 00:26:31.52000:26:31.530 ideal evaporator where you force the 00:26:35.63000:26:35.640 feed to go through this collect the 00:26:37.76000:26:37.770 vapor and you can you can continuously 00:26:40.52000:26:40.530 it reduce the solvent content of the 00:26:44.41900:26:44.429 material and therefore you can you can 00:26:47.03000:26:47.040 have a system in which it will 00:26:49.07000:26:49.080 automatically start reducing producing 00:26:51.95000:26:51.960 the water content of this so as I said 00:26:54.98000:26:54.990 you can have a multiple effective 00:26:57.02000:26:57.030 operators as well so one reactor one 00:26:59.65900:26:59.669 evaporator after the other so you get a 00:27:02.29900:27:02.309 feed and the feed comes in contact with 00:27:05.03000:27:05.040 steam the feed temperature has also 00:27:07.52000:27:07.530 increased the vapor which is generated 00:27:09.98000:27:09.990 from the feed contains some significant 00:27:12.56000:27:12.570 amount of energy so vapor from the feed 00:27:15.11000:27:15.120 is then going to go as the heating 00:27:18.64900:27:18.659 material into the into another 00:27:21.14000:27:21.150 evaporator which follows the first one 00:27:23.24000:27:23.250 so 00:27:23.86900:27:23.879 where the freed from the weather vapor 00:27:26.23900:27:26.249 from the first evaporator is going to 00:27:28.48900:27:28.499 come and heat the material and so on so 00:27:31.87900:27:31.889 in that way you can utilize more and 00:27:34.36900:27:34.379 more heat the heat efficiency of the 00:27:36.85900:27:36.869 entire system will it will enhance and 00:27:40.99900:27:41.009 you can have a system in which both the 00:27:43.57900:27:43.589 feed and the steam will will come in the 00:27:47.02900:27:47.039 same direction will travel in the same 00:27:48.64900:27:48.659 direction or you can have a system of 00:27:50.94900:27:50.959 evaporators a series of you operators 00:27:53.65900:27:53.669 where the feed is going to come from 00:27:55.51900:27:55.529 wandering the father feed is going to 00:27:57.64900:27:57.659 come to the first evaporator where the 00:27:59.47900:27:59.489 steam is going to come to the last 00:28:00.97900:28:00.989 evaporator so you can have a forward 00:28:03.31900:28:03.329 feed or a backward feed so I'll quickly 00:28:06.10900:28:06.119 draw the pictures of the figures of 00:28:09.01900:28:09.029 these two forward feed evaporator 00:28:11.37900:28:11.389 backward freedom operators and talk 00:28:14.47900:28:14.489 about their merits and demerits and once 00:28:16.45900:28:16.469 I draw the pictures I think the concept 00:28:19.06900:28:19.079 would be very clear to you what is known 00:28:20.89900:28:20.909 as what is a multiple divet evaporator 00:28:23.35900:28:23.369 and what is going to be forward feed and 00:28:25.60900:28:25.619 what's going to be the backward free so 00:28:27.68000:28:27.690 let's quickly draw these pictures and 00:28:31.18900:28:31.199 try to see what we get out of this 00:31:32.70900:31:32.719 so this one is as you can see it's the 00:31:37.51900:31:37.529 forward feed and this is the backward 00:31:41.38900:31:41.399 feed so let's see what happens over here 00:31:44.04900:31:44.059 the feed and the steam both come in 00:31:48.44000:31:48.450 evaporator one the concentrated product 00:31:52.96900:31:52.979 which is slightly heated because since 00:31:55.66900:31:55.679 it has come in contact with the steam 00:31:57.49900:31:57.509 goes to the next evaporated the vapor 00:32:02.08900:32:02.099 generated out of this contains lot of 00:32:04.78900:32:04.799 energy so it goes to heat up whatever is 00:32:08.35900:32:08.369 coming at this point and it goes into 00:32:10.77900:32:10.789 number two the the same way the product 00:32:17.41900:32:17.429 is going to go as the feed to the third 00:32:21.46900:32:21.479 evaporator and the vapor which is 00:32:24.37900:32:24.389 generated in two goes as the as the 00:32:29.52900:32:29.539 hitting stream into three so finally 00:32:33.88900:32:33.899 what you get out of three at the bottom 00:32:36.91900:32:36.929 is the final product and any vapor which 00:32:40.94000:32:40.950 leaves three is going to be cooled using 00:32:44.89900:32:44.909 cooling water and then it can be 00:32:48.79900:32:48.809 brighter it can be brought back to the 00:32:51.28900:32:51.299 product or it can be stored elsewhere so 00:32:54.91900:32:54.929 once again in forward feed both the in 00:32:58.94000:32:58.950 what in in forward feed both of them 00:33:02.38900:33:02.399 travel in the same direction and the 00:33:07.21900:33:07.229 liquid feed flows in the same direction 00:33:08.98900:33:08.999 as that of the vapor it's a forward feed 00:33:11.95900:33:11.969 so the vapor and the liquid the vapor 00:33:14.59900:33:14.609 and the liquid are traveling in the same 00:33:17.93000:33:17.940 direction so it's it's known as the 00:33:19.57900:33:19.589 backward fit in in the forward field in 00:33:22.51900:33:22.529 a in the backward feed the steam comes 00:33:26.59900:33:26.609 at one the feed comes at three and there 00:33:31.12900:33:31.139 can be more such more such evaporators 00:33:34.63900:33:34.649 in one after the other so when the feed 00:33:37.90900:33:37.919 comes it's cold its first going to get 00:33:42.22900:33:42.239 its it it's first going to encounter the 00:33:44.86900:33:44.879 vapor which is coming 00:33:46.37000:33:46.380 from so this is the vapor which is 00:33:47.87000:33:47.880 coming from - so the feed gets slightly 00:33:50.89000:33:50.900 concentrated you are going to it's going 00:33:54.32000:33:54.330 to lose some amount of some amount of 00:33:56.53900:33:56.549 vapor which can be condensed and 00:33:58.90900:33:58.919 collected so the feed then slightly 00:34:01.97000:34:01.980 heated now it it is pumped through - - 00:34:07.27900:34:07.289 as the new feed the heating one comes 00:34:12.10900:34:12.119 from the vapor of one so into the 00:34:16.99000:34:17.000 concentrated part from three is the feed 00:34:21.37900:34:21.389 and the vapor from one is the heating 00:34:25.34000:34:25.350 heating heating medium you get even more 00:34:29.38900:34:29.399 concentration being done at - and then 00:34:32.69000:34:32.700 it is pumped to one so when it goes into 00:34:36.29000:34:36.300 one the new feed it has the maximum 00:34:39.40900:34:39.419 concentration and therein it comes in 00:34:42.59000:34:42.600 contact with live steam it gets further 00:34:45.56000:34:45.570 concentrated and it is one this is going 00:34:48.40900:34:48.419 to be the final product okay so let's 00:34:52.09900:34:52.109 see the relative advantages and 00:34:54.34900:34:54.359 disadvantages first backward feed the 00:34:57.26000:34:57.270 feed which is coming is the feed which 00:35:00.68000:35:00.690 is coming is let's talk about the 00:35:03.23000:35:03.240 forward feet first the forward feed a 00:35:05.51000:35:05.520 vacuum is maintained in the last effect 00:35:08.63000:35:08.640 this effect and the liquid liquid flows 00:35:11.66000:35:11.670 itself from 1 to 2 2 to 3 and so on in 00:35:16.84900:35:16.859 backward feed you need a pump to make 00:35:21.20000:35:21.210 the liquid flow towards from 3 to 1 okay 00:35:27.50000:35:27.510 and in order to remove this feed at the 00:35:31.19000:35:31.200 final at the product you probably need a 00:35:34.40000:35:34.410 pump at this point whereas you required 00:35:37.40000:35:37.410 pumps after every effect in the case of 00:35:40.46000:35:40.470 backward reactor if the if the feed is 00:35:46.34000:35:46.350 at a higher temperature is higher than 00:35:48.74000:35:48.750 the if the feed liquid itself is higher 00:35:50.87000:35:50.880 than the saturation temperature then 00:35:53.06000:35:53.070 some evaporation some flashing will take 00:35:56.24000:35:56.250 place automatically as in in one and 00:36:00.41000:36:00.420 this flashing will take place in every 00:36:02.93000:36:02.940 effect there by reducing the steam 00:36:05.18000:36:05.190 requirement so if your feed is at a 00:36:07.46000:36:07.470 higher temperature than the saturation 00:36:09.20000:36:09.210 temperature the moment it enters the 00:36:11.93000:36:11.940 first effect it's going to flash and 00:36:14.42000:36:14.430 there's going to be part of the vapor 00:36:15.98000:36:15.990 which is going to evaporate and then the 00:36:19.46000:36:19.470 rest is going to condense and the same 00:36:21.92000:36:21.930 thing will happen in two and three and 00:36:23.84000:36:23.850 since the feed is at a higher 00:36:25.49000:36:25.500 temperature than that of that saturation 00:36:28.22000:36:28.230 temperature your steam requirement would 00:36:30.68000:36:30.690 be would be would be less the problem 00:36:34.67000:36:34.680 that we throw that's advantageous the 00:36:36.44000:36:36.450 problem that you see in followed fit is 00:36:38.54000:36:38.550 as the product from one becomes the feed 00:36:42.38000:36:42.390 of to the product of two becomes the 00:36:46.37000:36:46.380 feed of three and then ultimately you 00:36:48.62000:36:48.630 are going to get the final product the 00:36:51.67000:36:51.680 concentration keeps on increasing 00:36:53.75000:36:53.760 between one two and three so the so the 00:36:58.16000:36:58.170 liquid starts to become more and more 00:36:59.99000:37:00.000 viscous therefore it's it's it's 00:37:03.74000:37:03.750 difficult to make it flow so the last 00:37:07.67000:37:07.680 one since it has the highest viscosity 00:37:10.04000:37:10.050 liquid the it will have the least 00:37:14.15000:37:14.160 overall heat coefficient C since the 00:37:17.78000:37:17.790 liquid is going to be very concentrated 00:37:20.30000:37:20.310 at this point on the other hand in 00:37:24.23000:37:24.240 backward feed the dilute lake liquor 00:37:27.02000:37:27.030 enters at the last and the coldest the 00:37:30.05000:37:30.060 last end and leaves concentrated from 00:37:33.41000:37:33.420 the first which is the hottest one which 00:37:37.25000:37:37.260 is at the highest temperature so here 00:37:39.86000:37:39.870 the liquid must be heated in each effort 00:37:42.74000:37:42.750 and you would require more more more 00:37:46.97000:37:46.980 more more steam in in backward feed but 00:37:51.14000:37:51.150 if the feed is in it initially hot 00:37:53.00000:37:53.010 therefore if the fluid is initially hot 00:37:56.11000:37:56.120 then it's there's no point in letting it 00:37:59.63000:37:59.640 enter into three in backward backward to 00:38:03.50000:38:03.510 feed where backward feed so if the feed 00:38:06.35000:38:06.360 is initially hot use forward feed if the 00:38:10.97000:38:10.980 feed is initially cold and or if the 00:38:14.06000:38:14.070 free 00:38:14.39000:38:14.400 discuss introduce it here so as the 00:38:17.69000:38:17.700 liquid becomes more and more 00:38:19.01000:38:19.020 concentrated the temperature is also 00:38:21.98000:38:21.990 increasing 00:38:22.88000:38:22.890 so therefore the effect of increase in 00:38:26.99000:38:27.000 viscosity the detrimental effect of 00:38:29.33000:38:29.340 increase in viscosity is compensated in 00:38:32.09000:38:32.100 backward flow as compared to that in 00:38:35.96000:38:35.970 forward flow so steam cost will be less 00:38:40.13000:38:40.140 for backward feed if the feed is cold 00:38:43.22000:38:43.230 and less for forward feed if the feed 00:38:47.03000:38:47.040 liquor is approximately at the operating 00:38:49.22000:38:49.230 temperature of the first effect or 00:38:50.87000:38:50.880 higher so depending on the condition of 00:38:53.30000:38:53.310 the feed you have to choose whether you 00:38:56.36000:38:56.370 are going to use the backward feed or a 00:38:57.89000:38:57.900 forward feed so if your feed is hot at a 00:39:01.43000:39:01.440 high temperature relatively high 00:39:02.99000:39:03.000 temperature then by all means use 00:39:05.84000:39:05.850 forward feed 00:39:06.80000:39:06.810 we're flashing is going to take place 00:39:08.87000:39:08.880 inside the evaporator one and that vapor 00:39:11.93000:39:11.940 is going to be utilized in two and so on 00:39:15.08000:39:15.090 therefore the steam requirement that 00:39:16.82000:39:16.830 each of these effects will be lowered 00:39:18.71000:39:18.720 because of vapor flashing which can take 00:39:21.71000:39:21.720 place only when the liquid feed liquid 00:39:24.74000:39:24.750 is at a high temperature but as the in 00:39:27.68000:39:27.690 forward feed as you move from one to two 00:39:30.17000:39:30.180 two to three it is becoming more and 00:39:32.15000:39:32.160 more concentrated its viscosity 00:39:35.09000:39:35.100 increases and therefore your heat it's 00:39:37.40000:39:37.410 hit term heat transfer performance will 00:39:39.23000:39:39.240 keep on decreasing so if you are dealing 00:39:42.44000:39:42.450 with this drilling with a liquid which 00:39:45.14000:39:45.150 when it is concentrated when it is part 00:39:47.99000:39:48.000 when the solvent is evaporated it 00:39:49.79000:39:49.800 becomes highly viscous do not use 00:39:52.40000:39:52.410 forward feed in that case if it's if it 00:39:55.13000:39:55.140 you use if a backward feed would be 00:39:57.52000:39:57.530 justified because as it goes from one to 00:40:00.44000:40:00.450 two to three in the reverse direction 00:40:02.26000:40:02.270 the temperature keeps on increasing and 00:40:04.97000:40:04.980 therefore and the concentration keeps on 00:40:07.49000:40:07.500 increasing as well so the increase of 00:40:09.92000:40:09.930 viscosity due to concentration increase 00:40:12.83000:40:12.840 will be offset by an increase in 00:40:15.62000:40:15.630 temperature so use of a backward feed 00:40:18.41000:40:18.420 would be justified in such a case so 00:40:21.20000:40:21.210 this is in a very nurse race in a 00:40:24.08000:40:24.090 nutshell is about evaporators we 00:40:27.19000:40:27.200 spoke about boiling we spoke about 00:40:28.83900:40:28.849 evaporators the type of operations the 00:40:31.90000:40:31.910 natural convection they the evaporators 00:40:34.45000:40:34.460 which are which depend on natural flow 00:40:36.76000:40:36.770 natural convective flow and in some 00:40:39.97000:40:39.980 cases you have to have forced force 00:40:41.89000:40:41.900 convection effects force convection 00:40:43.72000:40:43.730 convective evaporators and the 00:40:46.69000:40:46.700 arrangement of multiple evaporators 00:40:49.18000:40:49.190 either in forward flow or in backward 00:40:52.48000:40:52.490 flow with their advantages and 00:40:54.40000:40:54.410 disadvantages the relations and 00:40:56.71000:40:56.720 correlations are available in the text 00:40:58.99000:40:59.000 which I am NOT reproducing over here so 00:41:01.51000:41:01.520 you can simply see these those in your 00:41:03.57900:41:03.589 text and use them whenever appropriate 00:41:05.80000:41:05.810 so this concludes our discussion on 00:41:10.23000:41:10.240 convection and from next class we will 00:41:13.00000:41:13.010 move over move over to radiative heat 00:41:16.15000:41:16.160 transfer
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