00:00:00.650 all right so we're moving into this 00:00:02.81000:00:02.820 chapter chapter 11 dealing with heat 00:00:04.82000:00:04.830 exchangers this is a very practical 00:00:06.73000:00:06.740 useful chapter I would say I get the 00:00:09.95000:00:09.960 students that will come back after they 00:00:11.57000:00:11.580 passed this class if they have any 00:00:13.00900:00:13.019 questions for me about where they're 00:00:15.44000:00:15.450 working for the summer or what they saw 00:00:17.99000:00:18.000 out there and practice a lot of it deals 00:00:19.97000:00:19.980 with heat exchangers and then they want 00:00:24.38000:00:24.390 to understand why this was happening or 00:00:28.00900:00:28.019 what lighted when we did this in the 00:00:30.58900:00:30.599 factory or in the plant this happened 00:00:32.47900:00:32.489 this way it relates back to heat 00:00:34.34000:00:34.350 exchangers so we're gonna talk about a 00:00:37.19000:00:37.200 number of topics let's just jump into it 00:00:39.26000:00:39.270 so first of all concentric tube heat 00:00:42.26000:00:42.270 exchanger it is better or easier to 00:00:46.19000:00:46.200 analyze concentric tube heat exchangers 00:00:48.92000:00:48.930 you do find a few of them in practice 00:00:51.02000:00:51.030 but they're not that many in practice 00:00:52.85000:00:52.860 but we start with things that we can 00:00:54.86000:00:54.870 analyze and then move to the more 00:00:56.51000:00:56.520 complex which are more difficult maybe 00:00:59.20900:00:59.219 we have to use empirical results for the 00:01:02.38900:01:02.399 more complex heat exchangers so a 00:01:05.39000:01:05.400 concentric tube is just like what it 00:01:07.13000:01:07.140 says you have a tube and then you have 00:01:10.07000:01:10.080 another tube and they're aligned one is 00:01:13.28000:01:13.290 on the outside one is in the inside so 00:01:15.62000:01:15.630 you may have fluid flowing 00:01:17.17900:01:17.189 maybe it's hot fluid flowing in the 00:01:19.94000:01:19.950 inner tube and in the shell region you 00:01:22.88000:01:22.890 have fluid flowing either in the same 00:01:24.98000:01:24.990 direction or in the opposite direction 00:01:26.78000:01:26.790 in that annulus region I like to draw 00:01:30.35000:01:30.360 them like this and I'll talk about the 00:01:34.60900:01:34.619 hot side and the cold side and we could 00:01:38.81000:01:38.820 have the fluid coming in the temperature 00:01:40.85000:01:40.860 hot coming in and we could have the 00:01:43.67000:01:43.680 temperature hot going out there's a 00:01:46.39900:01:46.409 bunch of a questions I can ask today but 00:01:48.92000:01:48.930 this is kind of basic stuff in general 00:01:52.76000:01:52.770 if I have a heat exchanger I have a hot 00:01:54.71000:01:54.720 fluid in a cold fluid passing through it 00:01:56.66000:01:56.670 would I expect the temperature hot in to 00:01:59.53900:01:59.549 be greater than the temperature hot out 00:02:01.76000:02:01.770 or less than or equal to I mean I'm not 00:02:05.45000:02:05.460 going to do this as a clicker question 00:02:07.19000:02:07.200 but this is a type of question that we 00:02:09.61900:02:09.629 want to start answering in your own mind 00:02:11.80000:02:11.810 which one is the higher 00:02:13.85000:02:13.860 picture a hot in because if there's a 00:02:18.53000:02:18.540 heat transfer in the heat exchanger it's 00:02:20.90000:02:20.910 gonna go from the hot to the cold fluid 00:02:23.51000:02:23.520 and when it goes from the hot to the 00:02:25.76000:02:25.770 cold fluid the temperature of the hot 00:02:27.53000:02:27.540 fluid goes down unless there's a phase 00:02:29.60000:02:29.610 change we'll get to phase change but for 00:02:31.91000:02:31.920 now majority of this lecture if not all 00:02:34.76000:02:34.770 of this lecture we're just talking about 00:02:36.80000:02:36.810 something without phase change it's just 00:02:39.08000:02:39.090 gonna stay a liquid or stay a gas all 00:02:41.69000:02:41.700 right what about the cold the 00:02:44.09000:02:44.100 temperature the cold in vs. the 00:02:47.48000:02:47.490 temperature of the cold out which one is 00:02:50.21000:02:50.220 higher the cold out yeah it gained heat 00:02:55.67000:02:55.680 in the heat exchanger okay there's other 00:02:58.76000:02:58.770 types of heat exchangers maybe you have 00:03:00.92000:03:00.930 tube maybe I'll show it like this and 00:03:05.33000:03:05.340 another tube short like that and another 00:03:09.62000:03:09.630 tube but there's just a bunch of tubes 00:03:11.44900:03:11.459 and there's array of tubes and then you 00:03:14.03000:03:14.040 have fluid flowing over and across 00:03:17.17000:03:17.180 perpendicular to that axis of the tube 00:03:19.67000:03:19.680 so you have maybe the hot fluid flowing 00:03:22.13000:03:22.140 through the tubes and the cold fluid 00:03:25.13000:03:25.140 flowing across the tubes that would be a 00:03:27.28900:03:27.299 cross flow heat exchanger you put fins 00:03:30.92000:03:30.930 on the tubes to connect them and looks 00:03:33.14000:03:33.150 like an automobile radiator and we 00:03:35.78000:03:35.790 studied fin conduction before we had the 00:03:39.08000:03:39.090 first project on fin conduction Assoc 00:03:41.72000:03:41.730 with a lot of radiators okay we're not 00:03:46.25000:03:46.260 going to cover all the heat exchangers 00:03:48.11000:03:48.120 we're gonna analyze especially that 00:03:50.68000:03:50.690 concentric tube heat exchanger but we 00:03:54.25900:03:54.269 also want to cover a shell and tube heat 00:03:56.18000:03:56.190 exchanger so let me kind of say that you 00:03:59.75000:03:59.760 would have a big shell and you could 00:04:03.38000:04:03.390 have different configurations on this 00:04:05.15000:04:05.160 you could have a shell with a cap on the 00:04:08.78000:04:08.790 end and the cap on the end has a port in 00:04:13.94000:04:13.950 which fluid can enter and maybe it has a 00:04:18.50000:04:18.510 plate right here and another plate down 00:04:21.94900:04:21.959 here and another end down here and maybe 00:04:26.08900:04:26.099 the 00:04:27.29000:04:27.300 it's a bunch of tubes that are that just 00:04:30.77000:04:30.780 go through such that the fluid that 00:04:35.65900:04:35.669 enters here can enter one of the tubes 00:04:38.51000:04:38.520 let me see I'll show it like this and go 00:04:43.21900:04:43.229 and pass through the tube it would be a 00:04:45.20000:04:45.210 straight run gets to the end dumps out 00:04:48.24900:04:48.259 maybe it's a manifold or something like 00:04:50.83900:04:50.849 that it's where it can mix and then it's 00:04:53.96000:04:53.970 a subject to another bunch of tubes and 00:04:58.08900:04:58.099 they can re-enter another tube and go 00:05:02.60000:05:02.610 out maybe you have a plate that splits 00:05:05.02900:05:05.039 here prevents a short-circuiting and 00:05:08.45000:05:08.460 another outlet as so the fluid 00:05:11.36000:05:11.370 eventually goes in and leaves on the 00:05:13.85000:05:13.860 same end pretty convenient 00:05:16.15900:05:16.169 you could unbolt this and maybe put in a 00:05:19.99900:05:20.009 tube cleaner maybe every couple years 00:05:22.79000:05:22.800 clean that heat exchanger because 00:05:24.46900:05:24.479 there'll be some fouling some deposit of 00:05:26.74900:05:26.759 unwanted material inside the tubes let's 00:05:36.02000:05:36.030 get to that later let me just kind of 00:05:38.30000:05:38.310 get through some introductory concepts 00:05:40.36000:05:40.370 we'll get the practical issues in a 00:05:42.74000:05:42.750 minute well this was maybe the hot fluid 00:05:45.01900:05:45.029 the temperature hot in and the 00:05:47.02900:05:47.039 temperature hot out the cold you can 00:05:49.37000:05:49.380 have a bunch of different configurations 00:05:50.80000:05:50.810 maybe the cold is going to come on the 00:05:53.30000:05:53.310 shell side come in here maybe the 00:05:56.74900:05:56.759 temperature cooled in and maybe for 00:05:58.61000:05:58.620 convenience in the plant you want the 00:06:00.49900:06:00.509 cold to come out so kind of all the 00:06:02.45000:06:02.460 inlet and outlet pipes are on one end 00:06:04.51000:06:04.520 but this one's feeding the shell side 00:06:07.33900:06:07.349 and in your design what you want to do 00:06:09.92000:06:09.930 is force the flow to come in go across 00:06:13.58000:06:13.590 the tubes go across the tubes across the 00:06:17.05900:06:17.069 tubes across the tubes across the tubes 00:06:19.21900:06:19.229 cross tubes cross tubes cross tubes 00:06:21.62000:06:21.630 cross tubes cross tubes cross tubes you 00:06:24.32000:06:24.330 can kind of see the pattern here 00:06:28.54000:06:28.550 how would you accomplish that well baby 00:06:31.12900:06:31.139 inter duct introduce something that 00:06:33.11000:06:33.120 blocks the flow from short-circuiting 00:06:35.26900:06:35.279 there it didn't introduce things that 00:06:38.89900:06:38.909 would 00:06:40.76000:06:40.770 the flow across the tubes true you know 00:06:48.14000:06:48.150 and this would be a very common type of 00:06:53.11000:06:53.120 configuration for a shell and tube heat 00:06:55.90900:06:55.919 exchanger I saw on the shell side you're 00:06:58.27900:06:58.289 forcing that flow across the tubes now 00:07:00.55900:07:00.569 I've shown what two tubes you could put 00:07:02.74900:07:02.759 50 tubes in there you could put I don't 00:07:05.96000:07:05.970 know a pile of tubes in there I mean 00:07:07.87900:07:07.889 you're trying to draw it in 2d but 00:07:09.32000:07:09.330 there's a lot of tubes you can put in 00:07:11.08900:07:11.099 there and so you have vision the pathway 00:07:15.35000:07:15.360 in that and this is these are baffles as 00:07:18.40900:07:18.419 you might expect things to force the 00:07:20.30000:07:20.310 flow to go where you want it all right 00:07:22.99900:07:23.009 so shell in tube well here you're going 00:07:25.52000:07:25.530 to reply and rely more on empirical 00:07:28.39900:07:28.409 results for something as complicated as 00:07:30.86000:07:30.870 a shell and tube heat exchanger where 00:07:33.08000:07:33.090 for the concentric tube we can analyze 00:07:35.36000:07:35.370 it more analytically okay so the overall 00:07:39.68000:07:39.690 heat transfer coefficient a new 00:07:41.42000:07:41.430 parameter but if you have a wall that 00:07:44.08900:07:44.099 separates the two materials on one side 00:07:47.18000:07:47.190 maybe it's the hot side and on the other 00:07:49.45900:07:49.469 side it's the cold side and I want to 00:07:51.74000:07:51.750 analyze heat transfer at that location 00:07:53.77900:07:53.789 through the wall from the hot to the 00:07:55.79000:07:55.800 cold fluid first I have to get it out of 00:07:58.15900:07:58.169 the hot fluid and to the wall on the hot 00:08:00.70900:08:00.719 side then I have to get it through that 00:08:03.08000:08:03.090 wall then I have to get it back into the 00:08:06.11000:08:06.120 cold food so there's three resistances 00:08:08.68900:08:08.699 to the heat transfer to a convective one 00:08:11.99000:08:12.000 is conductive so I'll have a 1 over H 00:08:15.80000:08:15.810 and a both of these are our inside I 00:08:19.37000:08:19.380 have the thickness L of the wall the 00:08:23.33000:08:23.340 thermal conductivity of the wall and 00:08:24.92000:08:24.930 then the area of the wall L over K a 00:08:27.29000:08:27.300 conduction through that wall and then 1 00:08:29.62900:08:29.639 over H on the cold side maybe I should 00:08:32.83900:08:32.849 have put it outside or cold side area 00:08:35.08900:08:35.099 outside those three resistors are in 00:08:38.00000:08:38.010 series or parallel series so if I want 00:08:42.40900:08:42.419 the overall resistance do I just sum up 00:08:44.42000:08:44.430 the three values yeah so if I said the 00:08:48.01900:08:48.029 some of those ours is the equivalent 00:08:50.51000:08:50.520 resistance now sometimes those three 00:08:54.07900:08:54.089 areas 00:08:54.65000:08:54.660 exactly the same sometimes they're not 00:08:56.84000:08:56.850 because there's curvature and whatever 00:08:59.05900:08:59.069 but but just leave it with the A's like 00:09:01.28000:09:01.290 this well then somebody says what I'd 00:09:03.19900:09:03.209 like to do is just have one simple model 00:09:05.26900:09:05.279 which is one over you a what is u U is 00:09:09.94900:09:09.959 the overall heat transfer coefficient 00:09:12.55900:09:12.569 what's it defined s by this equation 00:09:14.30000:09:14.310 which is 1 over H a on the inside or bot 00:09:18.59000:09:18.600 side or whatever side that is plus the L 00:09:21.35000:09:21.360 over ka for the wall plus one over H a 00:09:24.88900:09:24.899 for the other side outside the cold side 00:09:27.35000:09:27.360 something now you can manipulate this 00:09:30.53000:09:30.540 equation more but that's probably good 00:09:33.35000:09:33.360 enough for the introduction what would 00:09:35.80900:09:35.819 it how is this defined U is defined such 00:09:38.60000:09:38.610 that it accounts for the to convective 00:09:42.31900:09:42.329 plus the conductive resistance to heat 00:09:45.01900:09:45.029 transfer all right every now and then 00:09:49.16000:09:49.170 you'll see that well the coefficient is 00:09:51.67900:09:51.689 based on the outer area oh no the 00:09:54.11000:09:54.120 coefficients based on the inner area 00:09:56.11900:09:56.129 when the area is different sometimes you 00:09:58.51900:09:58.529 have to read carefully what is the U 00:10:00.23000:10:00.240 with what is the area associated with 00:10:02.92900:10:02.939 that hue the overall is the coefficient 00:10:06.67900:10:06.689 but I have to pick is it the area on the 00:10:08.56900:10:08.579 inside or the area on the outside as if 00:10:11.17900:10:11.189 it's just a plain wall they have the 00:10:12.59000:10:12.600 same area all right well not only can 00:10:19.97000:10:19.980 you have the fluids flowing through and 00:10:23.50900:10:23.519 you have the convective resistance if 00:10:25.12900:10:25.139 you wait long enough some of those 00:10:27.23000:10:27.240 fluids will actually deposit some stuff 00:10:29.87000:10:29.880 on the sides which will inhibit or 00:10:31.81900:10:31.829 degrade the heat transfer it'll be an 00:10:33.92000:10:33.930 additional resistance it's called a 00:10:35.87000:10:35.880 fowling fouling of that heat exchanger 00:10:38.54000:10:38.550 so you would have the convective then 00:10:41.90000:10:41.910 you'd have some fouling then you'd have 00:10:43.93900:10:43.949 some conductive then maybe another 00:10:45.94900:10:45.959 fouling and then convective out to the 00:10:48.92000:10:48.930 fluid again T infinity or T infinity 00:10:51.67900:10:51.689 here tea hot tea cold and so this was a 00:10:55.36900:10:55.379 1 over H a this is a 1 over H a this is 00:10:59.80900:10:59.819 an L over ka we just added these and so 00:11:03.76900:11:03.779 you say how do they model it well you 00:11:06.17000:11:06.180 could have modeled it 1 over 00:11:08.36000:11:08.370 sort of fouling convection coefficient 00:11:10.91000:11:10.920 area no not in this book they introduced 00:11:16.40000:11:16.410 big R double Prime 00:11:18.94000:11:18.950 okay so here's my question for you 00:11:23.59000:11:23.600 instead of putting it right here that's 00:11:26.18000:11:26.190 not the answer I need to know what is my 00:11:29.18000:11:29.190 resistance due to that fouling is it 00:11:31.76000:11:31.770 just R double Prime 00:11:33.29000:11:33.300 answer a clicker question or is it R 00:11:36.76900:11:36.779 double prime times my area for that 00:11:40.94000:11:40.950 inside or outside that's answer B or is 00:11:44.09000:11:44.100 it R double prime divided by a answer C 00:11:47.36000:11:47.370 do you see the three choices let's go 00:11:50.06000:11:50.070 ahead and start that alright let's go 00:11:58.25000:11:58.260 ahead and stop okay for those that did 00:12:01.76000:12:01.770 it correctly and C is the correct answer 00:12:07.73000:12:07.740 how did you know that C was the right 00:12:09.86000:12:09.870 answer how did you check your units so 00:12:14.72000:12:14.730 what do we know about the units of one 00:12:18.65000:12:18.660 over H a what are the SI units for the 00:12:21.32000:12:21.330 resistance associated with one over H a 00:12:24.23000:12:24.240 well if you didn't work on it for a 00:12:26.18000:12:26.190 while the H is watts per meter squared 00:12:29.99000:12:30.000 temperature change and then the area is 00:12:33.44000:12:33.450 meter squared so oh yeah now I remember 00:12:36.65000:12:36.660 all of these R's have units degree C or 00:12:39.68000:12:39.690 degree K it's a temperature change 00:12:41.53000:12:41.540 divided by watts of transferred through 00:12:43.97000:12:43.980 it so I know that this R right here has 00:12:47.36000:12:47.370 to have units degrees C or Kelvin per 00:12:53.03000:12:53.040 watt true does that what went through 00:12:55.19000:12:55.200 your mind yeah and so you look up here 00:12:58.19000:12:58.200 and you say well this is gonna be our 00:13:00.38000:13:00.390 double prime they show me the units up 00:13:02.21000:13:02.220 here its meter squared Kelvin per watt 00:13:05.26000:13:05.270 let's see do I multiply by meter squared 00:13:08.63000:13:08.640 multiplied by area no do nothing no 00:13:13.28000:13:13.290 divided by meters squared 00:13:15.17000:13:15.180 yeah and so that's how C comes to be the 00:13:20.63000:13:20.640 right answer 00:13:21.69000:13:21.700 and again to me it's just like this 00:13:24.21000:13:24.220 double prime when we talked about 00:13:26.96000:13:26.970 contact resistance and that to me is 00:13:31.50000:13:31.510 confusing I wish they didn't have the 00:13:33.30000:13:33.310 double Prime up there on that are I kind 00:13:35.70000:13:35.710 of wish they would have just reported it 00:13:38.07000:13:38.080 as one over H fouling or something like 00:13:42.60000:13:42.610 that 00:13:43.20000:13:43.210 it would be easier I think to my mind 00:13:46.10000:13:46.110 anyway you have both the fouling here 00:13:48.84000:13:48.850 and you could have different fouling on 00:13:50.37000:13:50.380 other sides when you see data like this 00:13:53.13000:13:53.140 how well is this data known how many 00:13:57.56000:13:57.570 significant digits is this number 00:14:00.33000:14:00.340 reported to clicker question is it 00:14:03.09000:14:03.100 reported to one two three four or five 00:14:08.09000:14:08.100 significant digits answer a B C D or E 00:14:22.73000:14:22.740 what can I say 00:14:24.53000:14:24.540 have I been trying to hide information 00:14:27.09000:14:27.100 and knowledge from you what part of the 00:14:30.18000:14:30.190 equation am i letting you down with too 00:14:37.05000:14:37.060 many options huh that was it 00:14:39.53000:14:39.540 there was a way that you could pin it 00:14:41.54000:14:41.550 back on me and that's okay all right I 00:14:46.00900:14:46.019 need to move on so we have already added 00:14:50.81000:14:50.820 an adjusted inning change to you based 00:14:54.37900:14:54.389 on adding the following factor what was 00:14:56.50900:14:56.519 the equation again one over you a is 00:14:59.42000:14:59.430 equal to one over the H a maybe this is 00:15:02.96000:15:02.970 on one side we have the one over H a on 00:15:05.54000:15:05.550 the outside we have this one over I 00:15:09.17000:15:09.180 can't remember is this R double prime 00:15:12.07900:15:12.089 divided by a is that what we concluded 00:15:14.38900:15:14.399 that should be yeah did we had 2l over K 00:15:17.90000:15:17.910 a and then we had the R double prime 00:15:20.05900:15:20.069 divided by a okay without any fouling no 00:15:23.93000:15:23.940 fouling those go away 00:15:25.60000:15:25.610 if the convection coefficient on one of 00:15:30.35000:15:30.360 the sides goes up increases and 00:15:34.36000:15:34.370 everything else stays the same the 00:15:36.53000:15:36.540 thickness the conductivity of the metal 00:15:38.54000:15:38.550 the convection coefficient on the other 00:15:40.40000:15:40.410 side what happens how does you change 00:15:43.73000:15:43.740 does you go up as well stay about the 00:15:46.79000:15:46.800 same no change or go down answer a B or 00:15:50.54000:15:50.550 C and this is the new one I D K let's go 00:16:02.80900:16:02.819 ahead and stop this so if the H goes up 00:16:08.09000:16:08.100 what happens to this group of terms 00:16:10.10000:16:10.110 right here let's say that's the H that 00:16:11.90000:16:11.910 changes if it goes up what happens to 00:16:13.69900:16:13.709 that group of terms it goes down the 00:16:17.24000:16:17.250 rest these constants stay the same but 00:16:19.75900:16:19.769 because that one term went down and the 00:16:21.76900:16:21.779 rest of them stayed constant does this 00:16:23.56900:16:23.579 term have to go down or up to match has 00:16:26.84000:16:26.850 to go down has to go down and we're not 00:16:29.75000:16:29.760 changing area to make the one over you a 00:16:33.11000:16:33.120 go down what does you have to do has to 00:16:36.29000:16:36.300 go up what's the right answer 00:16:37.90000:16:37.910 hey let's grade it hey that's pretty 00:16:41.62900:16:41.639 good I need to ask are there questions 00:16:43.90000:16:43.910 seriously this previous question was not 00:16:48.05000:16:48.060 even a t-ball question right I mean it 00:16:50.56900:16:50.579 was so easy 00:16:53.36000:16:53.370 too many options that's right I gave you 00:16:55.10000:16:55.110 too many options that's why you didn't 00:16:56.69000:16:56.700 do well now let's move to a harder one 00:16:59.00000:16:59.010 the same game what happens if our double 00:17:02.42000:17:02.430 Prime starts to come in if there's no 00:17:05.03000:17:05.040 fouling you can think of our double 00:17:06.62000:17:06.630 prime is zero true but let's say we 00:17:09.14000:17:09.150 start to get more and more fouling more 00:17:12.17000:17:12.180 and more fouling what does that do to 00:17:14.29900:17:14.30900:17:16.27000:17:16.280 does it increase it answer a no change 00:17:19.54900:17:19.559 answer beat or does it decrease it 00:17:21.94900:17:21.959 answer C or I don't know answer D 00:17:32.93000:17:32.940 let's go ahead and stop this and let's 00:17:36.68000:17:36.690 jump into it 00:17:37.61000:17:37.620 so if the resistance goes up the overall 00:17:43.00900:17:43.019 heat transfer is degraded if you if you 00:17:47.06000:17:47.070 want a good heat treat exchanger you 00:17:49.15900:17:49.169 want to promote heat transfer inside a 00:17:51.13900:17:51.149 heat exchanger usually heat exchangers 00:17:53.26900:17:53.279 are not introduced to insulate things 00:17:55.97000:17:55.980 but to promote the transfer not to 00:17:58.73000:17:58.740 degrade the transfer of heat so fouling 00:18:01.99900:18:02.009 is bad and often they have to stop take 00:18:05.84000:18:05.850 it out of service and clean it to get 00:18:07.78900:18:07.799 the fouling eradicated and get it back 00:18:10.36900:18:10.379 in the service so what's good a high U 00:18:13.31000:18:13.320 is typically considered good a high 00:18:16.33000:18:16.340 overall convective heat transfer 00:18:18.35000:18:18.360 coefficient is good to promote heat 00:18:21.83000:18:21.840 transfer make sense let's process press 00:18:25.43000:18:25.440 forward so we have a temperature 00:18:29.62900:18:29.639 distribution and a concentric tube heat 00:18:31.94000:18:31.950 exchanger with parallel flow so the way 00:18:34.39900:18:34.409 I sketched the concentric tube is just 00:18:38.65900:18:38.669 to separate the hot fluid from the cold 00:18:41.45000:18:41.460 fluid and so we're gonna have parallel 00:18:45.59000:18:45.600 flow you could have either parallel or 00:18:48.36900:18:48.379 counter flow in the concentric tube heat 00:18:52.00900:18:52.019 exchanger if you have parallel flow do 00:18:54.68000:18:54.690 you think that both of them flow in the 00:18:56.69000:18:56.700 same direction you think that's it and 00:19:00.01900:19:00.029 then counter flow what's gonna happen 00:19:02.48000:19:02.490 one of them is gonna go in the opposite 00:19:04.85000:19:04.860 direction so this would be the 00:19:06.98000:19:06.990 temperature hot in this would be the 00:19:09.56000:19:09.570 temperature hot out this would be the 00:19:11.62900:19:11.639 temperature cold in this would be the 00:19:13.34000:19:13.350 temperature cold out true now let's say 00:19:16.66900:19:16.679 we start the heat exchanger at X equal 00:19:19.58000:19:19.590 to zero and we ended at X equal to L and 00:19:22.90900:19:22.919 this is the direction of X you have a 00:19:25.97000:19:25.980 longer heat exchanger larger L and we're 00:19:30.20000:19:30.210 gonna plot as a function of X going from 00:19:33.32000:19:33.330 0 to L temperatures so let's go ahead 00:19:38.02900:19:38.039 and put the highest temperature that we 00:19:39.91900:19:39.929 see out there it's T hot in what is the 00:19:43.54900:19:43.559 lowest of the four temperatures what's 00:19:45.91900:19:45.929 the lowest 00:19:46.76000:19:46.770 of these four temperatures the cold in 00:19:49.91000:19:49.920 that'll be the lowest of the low the 00:19:52.13000:19:52.140 temperature cold in now if you look at 00:19:57.08000:19:57.090 it we're gonna do the differential 00:19:58.49000:19:58.500 equations and get a mathematical 00:20:00.79900:20:00.809 rigorous treatment but I'm just building 00:20:02.66000:20:02.670 up to that having conceptual 00:20:04.61000:20:04.620 introduction heat exchangers so when you 00:20:07.58000:20:07.590 first look at that first couple inches 00:20:09.50000:20:09.510 in there heat exchanger you have a large 00:20:13.01000:20:13.020 delta T to promote a more rapid or our 00:20:18.04900:20:18.059 larger Q a higher Q so what's gonna 00:20:22.66900:20:22.679 happen is is you're gonna cool off the 00:20:25.49000:20:25.500 hot fluid pretty rapidly and heat up the 00:20:28.10000:20:28.110 cold fluid pretty rapidly 00:20:29.75000:20:29.760 but then the delta T drops doesn't it 00:20:32.51000:20:32.520 and so the rate at which the hot fluid 00:20:35.24000:20:35.250 heats up and the cold fluid 00:20:37.10000:20:37.110 I'm sorry cold food hot fluid cools down 00:20:40.13000:20:40.140 and cold fluid heats up decreases that 00:20:42.62000:20:42.630 rate and so there's a sequence of curves 00:20:45.32000:20:45.330 for the hot fluid and a sequence occurs 00:20:49.79000:20:49.800 for the cold fluid does that look 00:20:52.37000:20:52.380 reasonable that look reasonable alright 00:20:57.11000:20:57.120 somebody asked well if you make this 00:21:00.53000:21:00.540 heat exchanger a little longer do you 00:21:03.59000:21:03.600 ever think that you could get the cold 00:21:06.04900:21:06.059 fluid and the hot fluid to sort of swap 00:21:11.71000:21:11.720 you know why not look well what out here 00:21:15.02000:21:15.030 what would happen you'd have your delta 00:21:17.18000:21:17.190 T and you would drive the heat transfer 00:21:19.27000:21:19.280 but what started out to be the cold 00:21:21.98000:21:21.990 foods now the hot fluid it's not gonna 00:21:24.35000:21:24.360 work they're gonna exponentially get 00:21:26.27000:21:26.280 closer if you make it longer well 00:21:28.64000:21:28.650 they'll just sort of slow down and and 00:21:31.01000:21:31.020 get a little little closer together true 00:21:34.22000:21:34.230 that's what we would expect somebody 00:21:37.94000:21:37.950 asks what is Q then for this heat 00:21:41.15000:21:41.160 exchanger Q would be how many watts is 00:21:45.04900:21:45.059 transferred out of the hot and into the 00:21:46.76000:21:46.770 cold how could I calculate Q especially 00:21:51.47000:21:51.480 how could I relate it to the temperature 00:21:53.72000:21:53.730 hot n minus the temperature hot out 00:21:58.99000:21:59.000 I want to pause and I want you to finish 00:22:01.33000:22:01.340 this equation where I put an underline 00:22:03.88000:22:03.890 put in there what should be there what 00:22:07.84000:22:07.850 is from the perspective of the hot fluid 00:22:10.54000:22:10.550 finish this equation and you can then 00:22:13.72000:22:13.730 write Q is equal to something times T 00:22:17.32000:22:17.330 cold out minus T cold in it won't be the 00:22:21.49000:22:21.500 same thing that's missing this is what's 00:22:24.37000:22:24.380 missing that's what's missing they're 00:22:25.90000:22:25.910 not the same thing they're slightly 00:22:27.13000:22:27.140 they're related that they're similar but 00:22:29.86000:22:29.870 they're different I want you to fill in 00:22:32.02000:22:32.030 those two equations while I walk around 00:22:33.91000:22:33.920 and see if you're with me so a lot of 00:22:44.17000:22:44.180 people wanted to put in something like 00:22:45.64000:22:45.650 UA not gonna work is it did some people 00:22:50.29000:22:50.300 put in all the mass flow-rate specific 00:22:52.87000:22:52.880 heat of the hot fluid and now those that 00:22:56.29000:22:56.300 put in the mass flow-rate specific heat 00:22:58.06000:22:58.070 of the hot fluid why why did you do that 00:23:02.32000:23:02.330 why did why did you know that's the 00:23:04.12000:23:04.13000:23:06.51000:23:06.520 it's thermo one it's an energy balance 00:23:09.22000:23:09.230 right EB have I ever emphasized EB for 00:23:13.24000:23:13.250 something 00:23:13.75000:23:13.760 don't forget energy is conserved you can 00:23:16.30000:23:16.310 do an energy balance first law of thermo 00:23:18.40000:23:18.410 kind of had that experience for a while 00:23:20.68000:23:20.690 here now right but you're saying hey 00:23:22.51000:23:22.520 what happened to the enthalpy it's the 00:23:24.91000:23:24.920 mass flow rate times the change in 00:23:26.80000:23:26.810 enthalpy that's true that's why we put a 00:23:29.83000:23:29.840 CP delta T when we talk talking about 00:23:33.25000:23:33.260 liquids or gases not changing phase we 00:23:36.01000:23:36.020 could talk more about enthalpies later 00:23:38.14000:23:38.150 when we get the phase change all right 00:23:41.14000:23:41.150 but m dot c Sapir 00:23:43.75000:23:43.760 m dot CP of the cold fluid if I leave 00:23:48.19000:23:48.200 off any subscript on the specific heat 00:23:51.01000:23:51.020 it's typically assumed at constant 00:23:53.47000:23:53.480 pressure but for a liquid or an 00:23:55.90000:23:55.910 incompressible substance there's no 00:23:57.73000:23:57.740 difference CP is equal to C B is equal 00:24:01.30000:24:01.310 to C 00:24:15.64000:24:15.650 you're talking about if I kind of cut 00:24:18.11000:24:18.120 right here and that took a look I would 00:24:20.09000:24:20.100 have a I would have this and I'm 00:24:22.52000:24:22.530 assuming that the hot fluid I'm talking 00:24:24.86000:24:24.870 about the bulk fluid temperature the 00:24:26.75000:24:26.760 mean fluid temperature of that flowing 00:24:30.02000:24:30.030 in that section and the mean fluid 00:24:32.72000:24:32.730 temperature flowing in that section yeah 00:24:37.55000:24:37.560 instead of drawing a cold on top again I 00:24:39.71000:24:39.720 just simplify it 00:24:40.70000:24:40.710 just thinking about heat transfer for 00:24:42.68000:24:42.690 one side to the other from the inside to 00:24:44.66000:24:44.670 the outside that helped okay now 00:24:49.54000:24:49.550 somebody says I'm gonna leave the mass 00:24:52.55000:24:52.560 flow rate of the cold fluid and then I'm 00:24:54.62000:24:54.630 gonna not change the fluids so the 00:24:56.93000:24:56.940 specific heats are not gonna change and 00:24:58.82000:24:58.830 I'm not gonna change the mass flow rate 00:25:00.41000:25:00.420 of the cold fluid but I'm gonna crank up 00:25:04.22000:25:04.230 the mass flow rate of the hot fluid how 00:25:07.58000:25:07.590 is that gonna change my temperature 00:25:09.08000:25:09.090 distribution let's say the mass flow 00:25:11.81000:25:11.820 rate of the hot fluid goes up it gets 00:25:14.00000:25:14.010 doubled tripled times four I mean you're 00:25:17.18000:25:17.190 starting to really put the pumps to it 00:25:19.13000:25:19.140 and really whip it through does the red 00:25:22.76000:25:22.770 line change how would it change if it 00:25:27.05000:25:27.060 did change would the blue line change 00:25:28.97000:25:28.980 how would it check let's say you turn 00:25:31.16000:25:31.170 it's now 10 times oh there's no limit to 00:25:35.00000:25:35.010 this 100 times the flow rate 100 times 00:25:40.01000:25:40.020 the flow rate got it you're really 00:25:42.62000:25:42.630 whipping a lot of fluid hot fluid 00:25:44.36000:25:44.370 through there what happens to the red 00:25:46.58000:25:46.590 line it's good it's gonna creep up and 00:25:55.16000:25:55.170 up it up as the mass flow rate of the 00:25:58.46000:25:58.470 hot fluid goes up true you're gonna be 00:26:02.39000:26:02.400 pumping so much hot fluid through it 00:26:03.98000:26:03.990 that oh you'll have a small decrease in 00:26:06.23000:26:06.240 the hot fluid temperature but it's not a 00:26:11.03000:26:11.040 lot the temperature change because the 00:26:13.94000:26:13.950 mass flow rate specific heat is so high 00:26:15.95000:26:15.960 all right what would happen to the cold 00:26:18.80000:26:18.810 fluid as you pump 00:26:19.85000:26:19.860 that would it stay the same outlet 00:26:21.32000:26:21.330 temperature no wouldn't it do that yeah 00:26:29.33000:26:29.340 right so that's there's a conceptual 00:26:33.11000:26:33.120 question you could turn this around 00:26:34.19000:26:34.200 leave the hot fluid flow rate alone and 00:26:37.10000:26:37.110 just change the cold fluid flow rate as 00:26:40.10000:26:40.110 I originally shown it where they both 00:26:42.74000:26:42.750 approach to kind of come together in the 00:26:44.90000:26:44.910 middle guess what's about the same is 00:26:49.39000:26:49.400 the mass flow rate of the hot about the 00:26:52.46000:26:52.470 same as the mass flow to cold or is it 00:26:54.65000:26:54.660 the mass flow rate of the hot times a 00:26:56.29900:26:56.309 specific heat of the hot about the mass 00:26:58.15900:26:58.169 flow rate of the cold times the specific 00:26:59.99000:27:00.000 heat of the cold answer A or B I say if 00:27:07.90900:27:07.919 if they're both approaching about the 00:27:10.25000:27:10.260 same middle temperature on the outlet 00:27:13.48000:27:13.490 does that mean the mass flow rates are 00:27:15.71000:27:15.720 about the same or the mass flow rate 00:27:17.41900:27:17.429 specific heats are about the same let me 00:27:27.44000:27:27.450 do this could I have a water flow for 00:27:32.36000:27:32.370 the hot fluid and could I have air flow 00:27:35.75000:27:35.760 for the cold fluid could I sure there's 00:27:39.50000:27:39.510 nothing in the heat exchanger says I'll 00:27:41.90000:27:41.910 know if you have water on one side you 00:27:43.34000:27:43.350 have to have water on the other it's 00:27:44.63000:27:44.640 very common to have a liquid on one side 00:27:46.58000:27:46.590 and the gas on the other alright 00:27:48.47000:27:48.480 somebody say give me the specific heat 00:27:50.63000:27:50.640 of water from memory you already gave me 00:27:55.01000:27:55.020 the specific heat of air from memory one 00:27:58.06000:27:58.070 thousand joules per kilogram Kelvin how 00:28:05.09000:28:05.100 about for the water 4,200 joules per 00:28:10.28000:28:10.290 kilogram Kelvin or did I botch it do you 00:28:14.36000:28:14.370 agree or not let me do this let me see 00:28:17.39000:28:17.400 how many I don't know how to ask a 00:28:19.66900:28:19.679 clicker question on this give me a 00:28:22.40000:28:22.410 thumbs up if you agree look reasonable 00:28:27.10000:28:27.110 maybe you remember one point zero five 00:28:30.71000:28:30.720 kilojoules per kilogram the 00:28:33.52000:28:33.530 Griese is there any difference for air 00:28:35.38000:28:35.390 you know 1.0 of kilo joule I just put it 00:28:39.28000:28:39.290 as a thousand joules per kilogram Kelvin 00:28:42.43000:28:42.440 all right so this is about a factor of I 00:28:45.58000:28:45.590 don't know 00:28:46.21000:28:46.220 here's a clicker question a factor of 1 00:28:48.19000:28:48.200 a factor of 4 a factor of 10 or a factor 00:28:52.96000:28:52.970 of 40 this is a confidence rebuilder you 00:28:55.63000:28:55.640 answer a B C or D this is a confidence 00:29:02.26000:29:02.270 rebuilder here so what I what I have 00:29:11.44000:29:11.450 done is I proven that there are some 00:29:13.54000:29:13.550 people who don't call me friend there's 00:29:17.35000:29:17.360 not they're not gonna call me a friend 00:29:19.27000:29:19.280 that's it 00:29:20.11000:29:20.120 that's all it's a factor of four people 00:29:23.40000:29:23.410 1 to 4.2 or 1000 to 4200 right so what 00:29:30.64000:29:30.650 did I do to talk to people off I don't 00:29:32.68000:29:32.690 know but anyway there you go we could 00:29:37.81000:29:37.820 have where the specific heats are a 00:29:40.27000:29:40.280 factor of four different easy just have 00:29:42.52000:29:42.530 water and air does that mean that if I 00:29:46.69000:29:46.700 put in the mass flow rate of 10 00:29:49.99000:29:50.000 kilograms per second in the mass flow 00:29:52.90000:29:52.910 rate of 10 kilograms per second that 00:29:55.72000:29:55.730 they would all kind of go to the middle 00:29:57.37000:29:57.380 temperature nope if I put in a mass flow 00:30:06.94000:30:06.950 rate of 10 kilograms per second for the 00:30:10.54000:30:10.550 air the cold fluid what would I have to 00:30:13.24000:30:13.250 put in for the mass flow rate of the hot 00:30:15.01000:30:15.020 fluid in order for it to kind of go to 00:30:17.38000:30:17.390 the middle how about I work in that you 00:30:20.11000:30:20.120 work it out I'm gonna walk around 00:30:28.81000:30:28.820 all right so a lot of people are coming 00:30:31.29900:30:31.309 in with what four point what they put a 00:30:34.62900:30:34.639 ten and they divided by 4.2 what does 00:30:37.72000:30:37.730 that come in at two point I can't 00:30:40.96000:30:40.970 remember what did it come in at two 00:30:43.72000:30:43.730 point for us two point four so if I put 00:30:46.72000:30:46.730 in a flow rate of two point four 00:30:48.37000:30:48.380 kilograms per second for the water with 00:30:51.07000:30:51.080 its specific heat and I put it at ten 00:30:53.52900:30:53.539 kilograms per second for the air with 00:30:55.69000:30:55.700 this specific heat I anticipate the 00:30:58.48000:30:58.490 curves be pretty symmetric and me you 00:31:01.48000:31:01.490 know they're depending how long the heat 00:31:03.66900:31:03.679 exchanger is they could still be quite a 00:31:06.07000:31:06.080 far apart before when they hit the end 00:31:09.31000:31:09.320 of the heat exchanger could be a short 00:31:10.81000:31:10.820 heat exchanger or very long so the best 00:31:14.32000:31:14.330 answer for this original question was 00:31:17.20000:31:17.210 beep I already graded it didn't I 00:31:21.33000:31:21.340 that's what this one I needed the grade 00:31:26.16900:31:26.179 be there right very good 00:31:28.95000:31:28.960 all right so guess what they have a name 00:31:31.77900:31:31.789 for the product of m dot time C because 00:31:36.87900:31:36.889 often we you compared them not just the 00:31:39.27900:31:39.289 specific heat not just the flow rate the 00:31:41.16900:31:41.179 product of them they call that cap C 00:31:44.08000:31:44.090 they give it a name I didn't name it 00:31:46.02900:31:46.039 it's called the heat capacity rate the 00:31:49.84000:31:49.850 heat capacity rate and so you'll see oh 00:31:57.39900:31:57.409 the heat capacity rate of the cold food 00:31:59.25900:31:59.269 oh the heat capacity rate of the hot 00:32:00.66900:32:00.679 fluid cap see there's a heat capacity 00:32:04.14900:32:04.159 rate 00:32:05.82000:32:05.830 okay so heat capacity rate was defined 00:32:08.98000:32:08.990 as something oh man he just told me it 00:32:11.28900:32:11.299 was on the previous slide but I forgot 00:32:13.02900:32:13.039 now he needs to answer this question 00:32:16.28900:32:16.299 what is the SI units for the heat 00:32:20.04900:32:20.059 capacity rate is it kilojoules 00:32:22.41900:32:22.429 kilojoules per Kelvin kilowatts 00:32:25.21000:32:25.220 kilowatts per Kelvin or some other units 00:32:27.94000:32:27.950 si will give you enough time for you to 00:32:30.22000:32:30.230 individually work this one out 00:32:38.33000:32:38.340 all right so let's pick it up here it 00:32:40.79900:32:40.809 was the product of the mass flow rate 00:32:42.75000:32:42.760 times the specific heat and we just went 00:32:46.16900:32:46.179 through those units kilogram per second 00:32:48.60000:32:48.610 till joules per kilogram degree C you 00:32:52.76900:32:52.779 cancel those you're left with kilowatts 00:32:56.10000:32:56.110 per degree C but you know this is a 00:32:58.32000:32:58.330 temperature change so it's just as well 00:33:00.41900:33:00.429 kilowatts per Kelvin and the best answer 00:33:04.01900:33:04.029 is D everybody on board all right so now 00:33:14.54900:33:14.559 let's talk about saying concentric tube 00:33:16.83000:33:16.840 I know it looks like this but instead of 00:33:19.28900:33:19.299 trying to draw it very complicated I'll 00:33:21.48000:33:21.490 just put the wall that separates the hot 00:33:24.99000:33:25.000 stream from the cold stream like this 00:33:27.38900:33:27.399 hot and then cold now that's how very 00:33:31.49000:33:31.500 general description of concentric tube 00:33:34.56000:33:34.570 but here we're going to go from 0 to L 00:33:38.07000:33:38.080 in the XO that's finite length 00:33:41.41900:33:41.429 concentric tube heat exchanger but this 00:33:44.25000:33:44.260 time we're going to have parallel flow 00:33:47.29900:33:47.309 we already did parallel flow didn't we 00:33:51.32000:33:51.330 well parallel flow ok we're going to 00:33:54.12000:33:54.130 change that to consent to counter flow 00:33:57.76900:33:57.779 counter flow if it's counter flow this 00:34:01.91900:34:01.929 is the hot in that's still the hot out 00:34:05.25000:34:05.260 but over here let me change it to 00:34:07.86000:34:07.870 different color is the cold in and the 00:34:12.48000:34:12.490 cold out and what's neat about this one 00:34:16.82900:34:16.839 is that you can get the hot fluid if you 00:34:20.82000:34:20.830 have it really really long to really 00:34:23.31000:34:23.320 have a significant drop in temperature 00:34:25.37900:34:25.389 and you can get the cold fluid to come 00:34:28.88900:34:28.899 up in temperature and then if you 00:34:31.40900:34:31.419 compared the temperature cold out with 00:34:34.77000:34:34.780 the temperature hot out you can get a 00:34:38.15900:34:38.169 flip where before if it was parallel 00:34:41.84900:34:41.859 flow there was no way there was no way 00:34:44.19000:34:44.200 that they could have but you can have 00:34:47.39900:34:47.409 for here the temperature 00:34:49.47000:34:49.480 cold out can be greater than the 00:34:54.18000:34:54.190 temperature hot out it doesn't have to 00:34:56.79000:34:56.800 be I mean the actual heat exchanger may 00:34:59.57900:34:59.589 look like this with the hot fluid doing 00:35:02.60900:35:02.619 this and the hot out is still greater 00:35:05.46000:35:05.470 than the cold out but if it's especially 00:35:08.43000:35:08.440 if it's long enough the cold out can be 00:35:11.94000:35:11.950 warmer than the hot out so there's a lot 00:35:14.91000:35:14.920 of heat exchangers which are counterflow 00:35:17.59900:35:17.609 okay this clicker question is still 00:35:20.13000:35:20.140 valid I've written four equations right 00:35:22.85900:35:22.869 here ABC and D one of them is incorrect 00:35:27.21000:35:27.220 one of them is wrong which one is wrong 00:35:39.20000:35:39.210 alright so let's go ahead and stop what 00:35:42.06000:35:42.070 we found was that this one gives us a 00:35:45.06000:35:45.070 negative Q the others are all positive 00:35:48.35900:35:48.369 and why is it negative is it because 00:35:50.76000:35:50.770 mass flow-rate specific Heat are 00:35:52.34900:35:52.359 negative no it's because the temperature 00:35:55.44000:35:55.450 hot out is greater than the temperature 00:35:57.56900:35:57.579 I'm sorry cold out is greater than the 00:36:00.05900:36:00.069 cold in isn't it so this one is the 00:36:03.30000:36:03.310 wrong one and there it is okay so we 00:36:10.62000:36:10.630 already just talked about the counter 00:36:12.24000:36:12.250 flow heat exchanger get that we quickly 00:36:14.57900:36:14.589 sketch it for the temperature profile 00:36:16.92000:36:16.930 you can have the hot coming this way you 00:36:20.06900:36:20.079 could have the cold doing this they can 00:36:22.95000:36:22.960 be different slopes they can have a 00:36:25.95000:36:25.960 bunch of different cases and you can 00:36:29.04000:36:29.050 start to play a lot of games with this 00:36:31.62000:36:31.630 and there's almost no end to the number 00:36:33.72000:36:33.730 of games so you say for the counter flow 00:36:36.56900:36:36.579 concentric tube heat exchanger if either 00:36:38.97000:36:38.980 I say L changes Oh 00:36:41.13000:36:41.140 let L increase decrease whatever but 00:36:44.33900:36:44.349 let's just say L increases no no let you 00:36:47.09900:36:47.109 increase no let the mass flow rate the 00:36:49.14000:36:49.150 hot increase see what I can do I can ask 00:36:51.18000:36:51.190 so many questions you can get really 00:36:54.07900:36:54.089 confused no no you learn a lot how does 00:36:58.02000:36:58.030 the temperature hot out change or how 00:37:00.51000:37:00.520 does the temperature cold out change or 00:37:02.84900:37:02.859 how 00:37:03.63000:37:03.640 the cube amount of heat transferred 00:37:07.17000:37:07.180 within this device the rate of heat 00:37:09.35900:37:09.369 transfer how does that change so will it 00:37:13.38000:37:13.390 go up will it negligible negligible 00:37:16.73900:37:16.749 change or will it go down see all these 00:37:20.21000:37:20.220 so let's do the let's do maybe an easy 00:37:24.12000:37:24.130 one if the mass flow rate of the hot 00:37:26.57900:37:26.589 fluid I'm gonna increase that boom 00:37:29.70000:37:29.710 everything else remains the same the 00:37:31.41000:37:31.420 length the same the you the same the 00:37:34.38000:37:34.390 mass flow rate is cold 00:37:36.18000:37:36.190 how does the temperature of the hot out 00:37:39.17000:37:39.180 change give it a minute it'll come back 00:37:51.82900:37:51.839 how many minutes do you want to give it 00:37:53.91000:37:53.920 professor I don't know be patient and 00:37:59.77900:37:59.789 the clicker sessions huh we've lost 00:38:07.01900:38:07.029 connection well I'm only going to give 00:38:11.30900:38:11.319 it a few more seconds and we're gonna 00:38:12.66000:38:12.670 have to stop and Bandhan the clickers 00:38:17.98900:38:17.999 it'll come back up you think please 00:38:23.64000:38:23.650 check okay we're done with that sorry 00:38:29.23900:38:29.249 you're trying to get a point all right 00:38:31.97900:38:31.989 so now we have to continue this because 00:38:34.44000:38:34.450 of Technology failure here so we'll just 00:38:37.62000:38:37.630 do it with the show of hands temperature 00:38:40.49900:38:40.509 hot out will it go up will the hot out 00:38:44.09900:38:44.109 which is over here will it go up a 00:38:46.10900:38:46.119 little bit and maybe the new temperature 00:38:48.29900:38:48.309 hot profile look like that if you 00:38:50.75900:38:50.769 increase the mass flow rate of the hot 00:38:52.92000:38:52.930 fluid yes all correct that's perfect 00:38:56.99900:38:57.009 I do is nobody didn't have their hand on 00:38:58.97900:38:58.989 every hand was up we are now hundred 00:39:02.06900:39:02.079 percent correct 00:39:03.21000:39:03.220 now we're gonna stay there with the mass 00:39:07.17000:39:07.180 flurry of the hot is going to be 00:39:08.84900:39:08.859 increased what happens to the 00:39:10.89000:39:10.900 temperature of the cold out where is the 00:39:14.13000:39:14.140 temperature that cold out right here 00:39:15.80900:39:15.819 temperature the cold out 00:39:17.16000:39:17.170 remember it's counter flow what happens 00:39:20.25000:39:20.260 to the temperature the cold out who 00:39:29.30900:39:29.319 would like to give me their answer 00:39:30.45000:39:30.460 verbally go ahead it would increase a 00:39:35.01000:39:35.020 little bit and why would it increase a 00:39:37.28900:39:37.299 little bit you didn't sign up for that 00:39:43.31900:39:43.329 you just want the oven answer for that 00:39:49.47000:39:49.480 winter should I give it to somebody else 00:39:50.78900:39:50.799 somebody else 00:39:52.20000:39:52.210 he's willing to share the glory here who 00:39:55.26000:39:55.270 would like to say why it would go up a 00:39:57.53900:39:57.549 little bit yes sir yeah you're looking 00:40:03.32900:40:03.339 at the average delta T throughout the 00:40:05.43000:40:05.440 heat exchanger and by pushing the hot up 00:40:08.37000:40:08.380 you've created a larger overall Delta T 00:40:11.76000:40:11.770 for the whole heat exchanger hence 00:40:13.20000:40:13.210 there's a larger Q and so it's going to 00:40:16.53000:40:16.540 bring up basically you've you had to 00:40:20.30900:40:20.319 answer this question first 00:40:21.90000:40:21.910 I think the Q is going to go up because 00:40:24.21000:40:24.220 of the increase and now that it's gone 00:40:26.97000:40:26.980 up then the cold flow rate didn't change 00:40:31.10900:40:31.119 hence the cold outlet temperature has to 00:40:33.42000:40:33.430 change right there's three equations 00:40:37.04900:40:37.059 that you're going to work with in this 00:40:38.64000:40:38.650 chapter q is equal to cap C hot 00:40:42.89000:40:42.900 temperature change of the hot Q is equal 00:40:46.71000:40:46.720 to cap C cold temperature change of the 00:40:49.89000:40:49.900 cold and the one that we haven't gotten 00:40:52.20000:40:52.210 to Q is equal to you a some appropriate 00:40:57.85900:40:57.869 best average delta T throughout the heat 00:41:02.13000:41:02.140 exchanger that appropriate delta T is 00:41:05.73000:41:05.740 called the LM delta T it's the log mean 00:41:10.58900:41:10.599 temperature difference through that heat 00:41:12.66000:41:12.670 exchanger intuitively if the temperature 00:41:16.17000:41:16.180 difference throughout is large 00:41:18.87000:41:18.880 throughout the heat exchanger delta T 00:41:20.78900:41:20.799 log mean is large okay okay 00:41:25.58900:41:25.599 so kind of this is your logic that you 00:41:28.38000:41:28.390 know kind of this average delta T 00:41:30.72000:41:30.730 throughout 00:41:31.14000:41:31.150 the heat exchangers increased these are 00:41:33.60000:41:33.610 you know maybe change to you a little 00:41:35.34000:41:35.350 bit because you increase the H on the 00:41:38.31000:41:38.320 hot side probably improved the 00:41:40.17000:41:40.180 convection coefficient on the hot side 00:41:41.88000:41:41.890 maybe improved you a little bit but the 00:41:44.88000:41:44.890 the you're gonna lead to a larger cube 00:41:47.82000:41:47.830 because of a larger q a larger delta t/2 00:41:50.78000:41:50.790 of these are energy balance equations 00:41:54.48000:41:54.490 conservation of energy this is why we 00:41:57.90000:41:57.910 take a whole class and heat transfer it 00:42:00.63000:42:00.640 is our rate equation there was a rate 00:42:05.25000:42:05.260 equation for heat conduction what was 00:42:08.16000:42:08.170 the name of the rate equation for heat 00:42:09.66000:42:09.670 conduction for Hayes law there was a 00:42:12.42000:42:12.430 rate equation for convection heat 00:42:14.34000:42:14.350 transfer what was the name Newton's law 00:42:17.82000:42:17.830 of cooling 00:42:18.45000:42:18.460 there was a rate equation for radiative 00:42:21.45000:42:21.460 heat transfer Stefan Boltzmann law and 00:42:25.95000:42:25.960 this is just a rate equation like a 00:42:28.98000:42:28.990 modified convection equation for the 00:42:34.44000:42:34.450 rate of heat transfer and heat 00:42:36.00000:42:36.010 exchangers all right 00:42:38.61000:42:38.620 I'm kind of getting tired and we have to 00:42:40.95000:42:40.960 do our quiz so we'll stop here and pick 00:42:43.59000:42:43.600 up there next time
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