Heat exchanger 1

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Kind: captions
Language: en

00:00:00.650
all right so we're moving into this
00:00:02.810 00:00:02.820 chapter chapter 11 dealing with heat
00:00:04.820 00:00:04.830 exchangers this is a very practical
00:00:06.730 00:00:06.740 useful chapter I would say I get the
00:00:09.950 00:00:09.960 students that will come back after they
00:00:11.570 00:00:11.580 passed this class if they have any
00:00:13.009 00:00:13.019 questions for me about where they're
00:00:15.440 00:00:15.450 working for the summer or what they saw
00:00:17.990 00:00:18.000 out there and practice a lot of it deals
00:00:19.970 00:00:19.980 with heat exchangers and then they want
00:00:24.380 00:00:24.390 to understand why this was happening or
00:00:28.009 00:00:28.019 what lighted when we did this in the
00:00:30.589 00:00:30.599 factory or in the plant this happened
00:00:32.479 00:00:32.489 this way it relates back to heat
00:00:34.340 00:00:34.350 exchangers so we're gonna talk about a
00:00:37.190 00:00:37.200 number of topics let's just jump into it
00:00:39.260 00:00:39.270 so first of all concentric tube heat
00:00:42.260 00:00:42.270 exchanger it is better or easier to
00:00:46.190 00:00:46.200 analyze concentric tube heat exchangers
00:00:48.920 00:00:48.930 you do find a few of them in practice
00:00:51.020 00:00:51.030 but they're not that many in practice
00:00:52.850 00:00:52.860 but we start with things that we can
00:00:54.860 00:00:54.870 analyze and then move to the more
00:00:56.510 00:00:56.520 complex which are more difficult maybe
00:00:59.209 00:00:59.219 we have to use empirical results for the
00:01:02.389 00:01:02.399 more complex heat exchangers so a
00:01:05.390 00:01:05.400 concentric tube is just like what it
00:01:07.130 00:01:07.140 says you have a tube and then you have
00:01:10.070 00:01:10.080 another tube and they're aligned one is
00:01:13.280 00:01:13.290 on the outside one is in the inside so
00:01:15.620 00:01:15.630 you may have fluid flowing
00:01:17.179 00:01:17.189 maybe it's hot fluid flowing in the
00:01:19.940 00:01:19.950 inner tube and in the shell region you
00:01:22.880 00:01:22.890 have fluid flowing either in the same
00:01:24.980 00:01:24.990 direction or in the opposite direction
00:01:26.780 00:01:26.790 in that annulus region I like to draw
00:01:30.350 00:01:30.360 them like this and I'll talk about the
00:01:34.609 00:01:34.619 hot side and the cold side and we could
00:01:38.810 00:01:38.820 have the fluid coming in the temperature
00:01:40.850 00:01:40.860 hot coming in and we could have the
00:01:43.670 00:01:43.680 temperature hot going out there's a
00:01:46.399 00:01:46.409 bunch of a questions I can ask today but
00:01:48.920 00:01:48.930 this is kind of basic stuff in general
00:01:52.760 00:01:52.770 if I have a heat exchanger I have a hot
00:01:54.710 00:01:54.720 fluid in a cold fluid passing through it
00:01:56.660 00:01:56.670 would I expect the temperature hot in to
00:01:59.539 00:01:59.549 be greater than the temperature hot out
00:02:01.760 00:02:01.770 or less than or equal to I mean I'm not
00:02:05.450 00:02:05.460 going to do this as a clicker question
00:02:07.190 00:02:07.200 but this is a type of question that we
00:02:09.619 00:02:09.629 want to start answering in your own mind
00:02:11.800 00:02:11.810 which one is the higher
00:02:13.850 00:02:13.860 picture a hot in because if there's a
00:02:18.530 00:02:18.540 heat transfer in the heat exchanger it's
00:02:20.900 00:02:20.910 gonna go from the hot to the cold fluid
00:02:23.510 00:02:23.520 and when it goes from the hot to the
00:02:25.760 00:02:25.770 cold fluid the temperature of the hot
00:02:27.530 00:02:27.540 fluid goes down unless there's a phase
00:02:29.600 00:02:29.610 change we'll get to phase change but for
00:02:31.910 00:02:31.920 now majority of this lecture if not all
00:02:34.760 00:02:34.770 of this lecture we're just talking about
00:02:36.800 00:02:36.810 something without phase change it's just
00:02:39.080 00:02:39.090 gonna stay a liquid or stay a gas all
00:02:41.690 00:02:41.700 right what about the cold the
00:02:44.090 00:02:44.100 temperature the cold in vs. the
00:02:47.480 00:02:47.490 temperature of the cold out which one is
00:02:50.210 00:02:50.220 higher the cold out yeah it gained heat
00:02:55.670 00:02:55.680 in the heat exchanger okay there's other
00:02:58.760 00:02:58.770 types of heat exchangers maybe you have
00:03:00.920 00:03:00.930 tube maybe I'll show it like this and
00:03:05.330 00:03:05.340 another tube short like that and another
00:03:09.620 00:03:09.630 tube but there's just a bunch of tubes
00:03:11.449 00:03:11.459 and there's array of tubes and then you
00:03:14.030 00:03:14.040 have fluid flowing over and across
00:03:17.170 00:03:17.180 perpendicular to that axis of the tube
00:03:19.670 00:03:19.680 so you have maybe the hot fluid flowing
00:03:22.130 00:03:22.140 through the tubes and the cold fluid
00:03:25.130 00:03:25.140 flowing across the tubes that would be a
00:03:27.289 00:03:27.299 cross flow heat exchanger you put fins
00:03:30.920 00:03:30.930 on the tubes to connect them and looks
00:03:33.140 00:03:33.150 like an automobile radiator and we
00:03:35.780 00:03:35.790 studied fin conduction before we had the
00:03:39.080 00:03:39.090 first project on fin conduction Assoc
00:03:41.720 00:03:41.730 with a lot of radiators okay we're not
00:03:46.250 00:03:46.260 going to cover all the heat exchangers
00:03:48.110 00:03:48.120 we're gonna analyze especially that
00:03:50.680 00:03:50.690 concentric tube heat exchanger but we
00:03:54.259 00:03:54.269 also want to cover a shell and tube heat
00:03:56.180 00:03:56.190 exchanger so let me kind of say that you
00:03:59.750 00:03:59.760 would have a big shell and you could
00:04:03.380 00:04:03.390 have different configurations on this
00:04:05.150 00:04:05.160 you could have a shell with a cap on the
00:04:08.780 00:04:08.790 end and the cap on the end has a port in
00:04:13.940 00:04:13.950 which fluid can enter and maybe it has a
00:04:18.500 00:04:18.510 plate right here and another plate down
00:04:21.949 00:04:21.959 here and another end down here and maybe
00:04:26.089 00:04:26.099 the
00:04:27.290 00:04:27.300 it's a bunch of tubes that are that just
00:04:30.770 00:04:30.780 go through such that the fluid that
00:04:35.659 00:04:35.669 enters here can enter one of the tubes
00:04:38.510 00:04:38.520 let me see I'll show it like this and go
00:04:43.219 00:04:43.229 and pass through the tube it would be a
00:04:45.200 00:04:45.210 straight run gets to the end dumps out
00:04:48.249 00:04:48.259 maybe it's a manifold or something like
00:04:50.839 00:04:50.849 that it's where it can mix and then it's
00:04:53.960 00:04:53.970 a subject to another bunch of tubes and
00:04:58.089 00:04:58.099 they can re-enter another tube and go
00:05:02.600 00:05:02.610 out maybe you have a plate that splits
00:05:05.029 00:05:05.039 here prevents a short-circuiting and
00:05:08.450 00:05:08.460 another outlet as so the fluid
00:05:11.360 00:05:11.370 eventually goes in and leaves on the
00:05:13.850 00:05:13.860 same end pretty convenient
00:05:16.159 00:05:16.169 you could unbolt this and maybe put in a
00:05:19.999 00:05:20.009 tube cleaner maybe every couple years
00:05:22.790 00:05:22.800 clean that heat exchanger because
00:05:24.469 00:05:24.479 there'll be some fouling some deposit of
00:05:26.749 00:05:26.759 unwanted material inside the tubes let's
00:05:36.020 00:05:36.030 get to that later let me just kind of
00:05:38.300 00:05:38.310 get through some introductory concepts
00:05:40.360 00:05:40.370 we'll get the practical issues in a
00:05:42.740 00:05:42.750 minute well this was maybe the hot fluid
00:05:45.019 00:05:45.029 the temperature hot in and the
00:05:47.029 00:05:47.039 temperature hot out the cold you can
00:05:49.370 00:05:49.380 have a bunch of different configurations
00:05:50.800 00:05:50.810 maybe the cold is going to come on the
00:05:53.300 00:05:53.310 shell side come in here maybe the
00:05:56.749 00:05:56.759 temperature cooled in and maybe for
00:05:58.610 00:05:58.620 convenience in the plant you want the
00:06:00.499 00:06:00.509 cold to come out so kind of all the
00:06:02.450 00:06:02.460 inlet and outlet pipes are on one end
00:06:04.510 00:06:04.520 but this one's feeding the shell side
00:06:07.339 00:06:07.349 and in your design what you want to do
00:06:09.920 00:06:09.930 is force the flow to come in go across
00:06:13.580 00:06:13.590 the tubes go across the tubes across the
00:06:17.059 00:06:17.069 tubes across the tubes across the tubes
00:06:19.219 00:06:19.229 cross tubes cross tubes cross tubes
00:06:21.620 00:06:21.630 cross tubes cross tubes cross tubes you
00:06:24.320 00:06:24.330 can kind of see the pattern here
00:06:28.540 00:06:28.550 how would you accomplish that well baby
00:06:31.129 00:06:31.139 inter duct introduce something that
00:06:33.110 00:06:33.120 blocks the flow from short-circuiting
00:06:35.269 00:06:35.279 there it didn't introduce things that
00:06:38.899 00:06:38.909 would
00:06:40.760 00:06:40.770 the flow across the tubes true you know
00:06:48.140 00:06:48.150 and this would be a very common type of
00:06:53.110 00:06:53.120 configuration for a shell and tube heat
00:06:55.909 00:06:55.919 exchanger I saw on the shell side you're
00:06:58.279 00:06:58.289 forcing that flow across the tubes now
00:07:00.559 00:07:00.569 I've shown what two tubes you could put
00:07:02.749 00:07:02.759 50 tubes in there you could put I don't
00:07:05.960 00:07:05.970 know a pile of tubes in there I mean
00:07:07.879 00:07:07.889 you're trying to draw it in 2d but
00:07:09.320 00:07:09.330 there's a lot of tubes you can put in
00:07:11.089 00:07:11.099 there and so you have vision the pathway
00:07:15.350 00:07:15.360 in that and this is these are baffles as
00:07:18.409 00:07:18.419 you might expect things to force the
00:07:20.300 00:07:20.310 flow to go where you want it all right
00:07:22.999 00:07:23.009 so shell in tube well here you're going
00:07:25.520 00:07:25.530 to reply and rely more on empirical
00:07:28.399 00:07:28.409 results for something as complicated as
00:07:30.860 00:07:30.870 a shell and tube heat exchanger where
00:07:33.080 00:07:33.090 for the concentric tube we can analyze
00:07:35.360 00:07:35.370 it more analytically okay so the overall
00:07:39.680 00:07:39.690 heat transfer coefficient a new
00:07:41.420 00:07:41.430 parameter but if you have a wall that
00:07:44.089 00:07:44.099 separates the two materials on one side
00:07:47.180 00:07:47.190 maybe it's the hot side and on the other
00:07:49.459 00:07:49.469 side it's the cold side and I want to
00:07:51.740 00:07:51.750 analyze heat transfer at that location
00:07:53.779 00:07:53.789 through the wall from the hot to the
00:07:55.790 00:07:55.800 cold fluid first I have to get it out of
00:07:58.159 00:07:58.169 the hot fluid and to the wall on the hot
00:08:00.709 00:08:00.719 side then I have to get it through that
00:08:03.080 00:08:03.090 wall then I have to get it back into the
00:08:06.110 00:08:06.120 cold food so there's three resistances
00:08:08.689 00:08:08.699 to the heat transfer to a convective one
00:08:11.990 00:08:12.000 is conductive so I'll have a 1 over H
00:08:15.800 00:08:15.810 and a both of these are our inside I
00:08:19.370 00:08:19.380 have the thickness L of the wall the
00:08:23.330 00:08:23.340 thermal conductivity of the wall and
00:08:24.920 00:08:24.930 then the area of the wall L over K a
00:08:27.290 00:08:27.300 conduction through that wall and then 1
00:08:29.629 00:08:29.639 over H on the cold side maybe I should
00:08:32.839 00:08:32.849 have put it outside or cold side area
00:08:35.089 00:08:35.099 outside those three resistors are in
00:08:38.000 00:08:38.010 series or parallel series so if I want
00:08:42.409 00:08:42.419 the overall resistance do I just sum up
00:08:44.420 00:08:44.430 the three values yeah so if I said the
00:08:48.019 00:08:48.029 some of those ours is the equivalent
00:08:50.510 00:08:50.520 resistance now sometimes those three
00:08:54.079 00:08:54.089 areas
00:08:54.650 00:08:54.660 exactly the same sometimes they're not
00:08:56.840 00:08:56.850 because there's curvature and whatever
00:08:59.059 00:08:59.069 but but just leave it with the A's like
00:09:01.280 00:09:01.290 this well then somebody says what I'd
00:09:03.199 00:09:03.209 like to do is just have one simple model
00:09:05.269 00:09:05.279 which is one over you a what is u U is
00:09:09.949 00:09:09.959 the overall heat transfer coefficient
00:09:12.559 00:09:12.569 what's it defined s by this equation
00:09:14.300 00:09:14.310 which is 1 over H a on the inside or bot
00:09:18.590 00:09:18.600 side or whatever side that is plus the L
00:09:21.350 00:09:21.360 over ka for the wall plus one over H a
00:09:24.889 00:09:24.899 for the other side outside the cold side
00:09:27.350 00:09:27.360 something now you can manipulate this
00:09:30.530 00:09:30.540 equation more but that's probably good
00:09:33.350 00:09:33.360 enough for the introduction what would
00:09:35.809 00:09:35.819 it how is this defined U is defined such
00:09:38.600 00:09:38.610 that it accounts for the to convective
00:09:42.319 00:09:42.329 plus the conductive resistance to heat
00:09:45.019 00:09:45.029 transfer all right every now and then
00:09:49.160 00:09:49.170 you'll see that well the coefficient is
00:09:51.679 00:09:51.689 based on the outer area oh no the
00:09:54.110 00:09:54.120 coefficients based on the inner area
00:09:56.119 00:09:56.129 when the area is different sometimes you
00:09:58.519 00:09:58.529 have to read carefully what is the U
00:10:00.230 00:10:00.240 with what is the area associated with
00:10:02.929 00:10:02.939 that hue the overall is the coefficient
00:10:06.679 00:10:06.689 but I have to pick is it the area on the
00:10:08.569 00:10:08.579 inside or the area on the outside as if
00:10:11.179 00:10:11.189 it's just a plain wall they have the
00:10:12.590 00:10:12.600 same area all right well not only can
00:10:19.970 00:10:19.980 you have the fluids flowing through and
00:10:23.509 00:10:23.519 you have the convective resistance if
00:10:25.129 00:10:25.139 you wait long enough some of those
00:10:27.230 00:10:27.240 fluids will actually deposit some stuff
00:10:29.870 00:10:29.880 on the sides which will inhibit or
00:10:31.819 00:10:31.829 degrade the heat transfer it'll be an
00:10:33.920 00:10:33.930 additional resistance it's called a
00:10:35.870 00:10:35.880 fowling fouling of that heat exchanger
00:10:38.540 00:10:38.550 so you would have the convective then
00:10:41.900 00:10:41.910 you'd have some fouling then you'd have
00:10:43.939 00:10:43.949 some conductive then maybe another
00:10:45.949 00:10:45.959 fouling and then convective out to the
00:10:48.920 00:10:48.930 fluid again T infinity or T infinity
00:10:51.679 00:10:51.689 here tea hot tea cold and so this was a
00:10:55.369 00:10:55.379 1 over H a this is a 1 over H a this is
00:10:59.809 00:10:59.819 an L over ka we just added these and so
00:11:03.769 00:11:03.779 you say how do they model it well you
00:11:06.170 00:11:06.180 could have modeled it 1 over
00:11:08.360 00:11:08.370 sort of fouling convection coefficient
00:11:10.910 00:11:10.920 area no not in this book they introduced
00:11:16.400 00:11:16.410 big R double Prime
00:11:18.940 00:11:18.950 okay so here's my question for you
00:11:23.590 00:11:23.600 instead of putting it right here that's
00:11:26.180 00:11:26.190 not the answer I need to know what is my
00:11:29.180 00:11:29.190 resistance due to that fouling is it
00:11:31.760 00:11:31.770 just R double Prime
00:11:33.290 00:11:33.300 answer a clicker question or is it R
00:11:36.769 00:11:36.779 double prime times my area for that
00:11:40.940 00:11:40.950 inside or outside that's answer B or is
00:11:44.090 00:11:44.100 it R double prime divided by a answer C
00:11:47.360 00:11:47.370 do you see the three choices let's go
00:11:50.060 00:11:50.070 ahead and start that alright let's go
00:11:58.250 00:11:58.260 ahead and stop okay for those that did
00:12:01.760 00:12:01.770 it correctly and C is the correct answer
00:12:07.730 00:12:07.740 how did you know that C was the right
00:12:09.860 00:12:09.870 answer how did you check your units so
00:12:14.720 00:12:14.730 what do we know about the units of one
00:12:18.650 00:12:18.660 over H a what are the SI units for the
00:12:21.320 00:12:21.330 resistance associated with one over H a
00:12:24.230 00:12:24.240 well if you didn't work on it for a
00:12:26.180 00:12:26.190 while the H is watts per meter squared
00:12:29.990 00:12:30.000 temperature change and then the area is
00:12:33.440 00:12:33.450 meter squared so oh yeah now I remember
00:12:36.650 00:12:36.660 all of these R's have units degree C or
00:12:39.680 00:12:39.690 degree K it's a temperature change
00:12:41.530 00:12:41.540 divided by watts of transferred through
00:12:43.970 00:12:43.980 it so I know that this R right here has
00:12:47.360 00:12:47.370 to have units degrees C or Kelvin per
00:12:53.030 00:12:53.040 watt true does that what went through
00:12:55.190 00:12:55.200 your mind yeah and so you look up here
00:12:58.190 00:12:58.200 and you say well this is gonna be our
00:13:00.380 00:13:00.390 double prime they show me the units up
00:13:02.210 00:13:02.220 here its meter squared Kelvin per watt
00:13:05.260 00:13:05.270 let's see do I multiply by meter squared
00:13:08.630 00:13:08.640 multiplied by area no do nothing no
00:13:13.280 00:13:13.290 divided by meters squared
00:13:15.170 00:13:15.180 yeah and so that's how C comes to be the
00:13:20.630 00:13:20.640 right answer
00:13:21.690 00:13:21.700 and again to me it's just like this
00:13:24.210 00:13:24.220 double prime when we talked about
00:13:26.960 00:13:26.970 contact resistance and that to me is
00:13:31.500 00:13:31.510 confusing I wish they didn't have the
00:13:33.300 00:13:33.310 double Prime up there on that are I kind
00:13:35.700 00:13:35.710 of wish they would have just reported it
00:13:38.070 00:13:38.080 as one over H fouling or something like
00:13:42.600 00:13:42.610 that
00:13:43.200 00:13:43.210 it would be easier I think to my mind
00:13:46.100 00:13:46.110 anyway you have both the fouling here
00:13:48.840 00:13:48.850 and you could have different fouling on
00:13:50.370 00:13:50.380 other sides when you see data like this
00:13:53.130 00:13:53.140 how well is this data known how many
00:13:57.560 00:13:57.570 significant digits is this number
00:14:00.330 00:14:00.340 reported to clicker question is it
00:14:03.090 00:14:03.100 reported to one two three four or five
00:14:08.090 00:14:08.100 significant digits answer a B C D or E
00:14:22.730 00:14:22.740 what can I say
00:14:24.530 00:14:24.540 have I been trying to hide information
00:14:27.090 00:14:27.100 and knowledge from you what part of the
00:14:30.180 00:14:30.190 equation am i letting you down with too
00:14:37.050 00:14:37.060 many options huh that was it
00:14:39.530 00:14:39.540 there was a way that you could pin it
00:14:41.540 00:14:41.550 back on me and that's okay all right I
00:14:46.009 00:14:46.019 need to move on so we have already added
00:14:50.810 00:14:50.820 an adjusted inning change to you based
00:14:54.379 00:14:54.389 on adding the following factor what was
00:14:56.509 00:14:56.519 the equation again one over you a is
00:14:59.420 00:14:59.430 equal to one over the H a maybe this is
00:15:02.960 00:15:02.970 on one side we have the one over H a on
00:15:05.540 00:15:05.550 the outside we have this one over I
00:15:09.170 00:15:09.180 can't remember is this R double prime
00:15:12.079 00:15:12.089 divided by a is that what we concluded
00:15:14.389 00:15:14.399 that should be yeah did we had 2l over K
00:15:17.900 00:15:17.910 a and then we had the R double prime
00:15:20.059 00:15:20.069 divided by a okay without any fouling no
00:15:23.930 00:15:23.940 fouling those go away
00:15:25.600 00:15:25.610 if the convection coefficient on one of
00:15:30.350 00:15:30.360 the sides goes up increases and
00:15:34.360 00:15:34.370 everything else stays the same the
00:15:36.530 00:15:36.540 thickness the conductivity of the metal
00:15:38.540 00:15:38.550 the convection coefficient on the other
00:15:40.400 00:15:40.410 side what happens how does you change
00:15:43.730 00:15:43.740 does you go up as well stay about the
00:15:46.790 00:15:46.800 same no change or go down answer a B or
00:15:50.540 00:15:50.550 C and this is the new one I D K let's go
00:16:02.809 00:16:02.819 ahead and stop this so if the H goes up
00:16:08.090 00:16:08.100 what happens to this group of terms
00:16:10.100 00:16:10.110 right here let's say that's the H that
00:16:11.900 00:16:11.910 changes if it goes up what happens to
00:16:13.699 00:16:13.709 that group of terms it goes down the
00:16:17.240 00:16:17.250 rest these constants stay the same but
00:16:19.759 00:16:19.769 because that one term went down and the
00:16:21.769 00:16:21.779 rest of them stayed constant does this
00:16:23.569 00:16:23.579 term have to go down or up to match has
00:16:26.840 00:16:26.850 to go down has to go down and we're not
00:16:29.750 00:16:29.760 changing area to make the one over you a
00:16:33.110 00:16:33.120 go down what does you have to do has to
00:16:36.290 00:16:36.300 go up what's the right answer
00:16:37.900 00:16:37.910 hey let's grade it hey that's pretty
00:16:41.629 00:16:41.639 good I need to ask are there questions
00:16:43.900 00:16:43.910 seriously this previous question was not
00:16:48.050 00:16:48.060 even a t-ball question right I mean it
00:16:50.569 00:16:50.579 was so easy
00:16:53.360 00:16:53.370 too many options that's right I gave you
00:16:55.100 00:16:55.110 too many options that's why you didn't
00:16:56.690 00:16:56.700 do well now let's move to a harder one
00:16:59.000 00:16:59.010 the same game what happens if our double
00:17:02.420 00:17:02.430 Prime starts to come in if there's no
00:17:05.030 00:17:05.040 fouling you can think of our double
00:17:06.620 00:17:06.630 prime is zero true but let's say we
00:17:09.140 00:17:09.150 start to get more and more fouling more
00:17:12.170 00:17:12.180 and more fouling what does that do to
00:17:14.299 00:17:14.309 00:17:16.270 00:17:16.280 does it increase it answer a no change
00:17:19.549 00:17:19.559 answer beat or does it decrease it
00:17:21.949 00:17:21.959 answer C or I don't know answer D
00:17:32.930 00:17:32.940 let's go ahead and stop this and let's
00:17:36.680 00:17:36.690 jump into it
00:17:37.610 00:17:37.620 so if the resistance goes up the overall
00:17:43.009 00:17:43.019 heat transfer is degraded if you if you
00:17:47.060 00:17:47.070 want a good heat treat exchanger you
00:17:49.159 00:17:49.169 want to promote heat transfer inside a
00:17:51.139 00:17:51.149 heat exchanger usually heat exchangers
00:17:53.269 00:17:53.279 are not introduced to insulate things
00:17:55.970 00:17:55.980 but to promote the transfer not to
00:17:58.730 00:17:58.740 degrade the transfer of heat so fouling
00:18:01.999 00:18:02.009 is bad and often they have to stop take
00:18:05.840 00:18:05.850 it out of service and clean it to get
00:18:07.789 00:18:07.799 the fouling eradicated and get it back
00:18:10.369 00:18:10.379 in the service so what's good a high U
00:18:13.310 00:18:13.320 is typically considered good a high
00:18:16.330 00:18:16.340 overall convective heat transfer
00:18:18.350 00:18:18.360 coefficient is good to promote heat
00:18:21.830 00:18:21.840 transfer make sense let's process press
00:18:25.430 00:18:25.440 forward so we have a temperature
00:18:29.629 00:18:29.639 distribution and a concentric tube heat
00:18:31.940 00:18:31.950 exchanger with parallel flow so the way
00:18:34.399 00:18:34.409 I sketched the concentric tube is just
00:18:38.659 00:18:38.669 to separate the hot fluid from the cold
00:18:41.450 00:18:41.460 fluid and so we're gonna have parallel
00:18:45.590 00:18:45.600 flow you could have either parallel or
00:18:48.369 00:18:48.379 counter flow in the concentric tube heat
00:18:52.009 00:18:52.019 exchanger if you have parallel flow do
00:18:54.680 00:18:54.690 you think that both of them flow in the
00:18:56.690 00:18:56.700 same direction you think that's it and
00:19:00.019 00:19:00.029 then counter flow what's gonna happen
00:19:02.480 00:19:02.490 one of them is gonna go in the opposite
00:19:04.850 00:19:04.860 direction so this would be the
00:19:06.980 00:19:06.990 temperature hot in this would be the
00:19:09.560 00:19:09.570 temperature hot out this would be the
00:19:11.629 00:19:11.639 temperature cold in this would be the
00:19:13.340 00:19:13.350 temperature cold out true now let's say
00:19:16.669 00:19:16.679 we start the heat exchanger at X equal
00:19:19.580 00:19:19.590 to zero and we ended at X equal to L and
00:19:22.909 00:19:22.919 this is the direction of X you have a
00:19:25.970 00:19:25.980 longer heat exchanger larger L and we're
00:19:30.200 00:19:30.210 gonna plot as a function of X going from
00:19:33.320 00:19:33.330 0 to L temperatures so let's go ahead
00:19:38.029 00:19:38.039 and put the highest temperature that we
00:19:39.919 00:19:39.929 see out there it's T hot in what is the
00:19:43.549 00:19:43.559 lowest of the four temperatures what's
00:19:45.919 00:19:45.929 the lowest
00:19:46.760 00:19:46.770 of these four temperatures the cold in
00:19:49.910 00:19:49.920 that'll be the lowest of the low the
00:19:52.130 00:19:52.140 temperature cold in now if you look at
00:19:57.080 00:19:57.090 it we're gonna do the differential
00:19:58.490 00:19:58.500 equations and get a mathematical
00:20:00.799 00:20:00.809 rigorous treatment but I'm just building
00:20:02.660 00:20:02.670 up to that having conceptual
00:20:04.610 00:20:04.620 introduction heat exchangers so when you
00:20:07.580 00:20:07.590 first look at that first couple inches
00:20:09.500 00:20:09.510 in there heat exchanger you have a large
00:20:13.010 00:20:13.020 delta T to promote a more rapid or our
00:20:18.049 00:20:18.059 larger Q a higher Q so what's gonna
00:20:22.669 00:20:22.679 happen is is you're gonna cool off the
00:20:25.490 00:20:25.500 hot fluid pretty rapidly and heat up the
00:20:28.100 00:20:28.110 cold fluid pretty rapidly
00:20:29.750 00:20:29.760 but then the delta T drops doesn't it
00:20:32.510 00:20:32.520 and so the rate at which the hot fluid
00:20:35.240 00:20:35.250 heats up and the cold fluid
00:20:37.100 00:20:37.110 I'm sorry cold food hot fluid cools down
00:20:40.130 00:20:40.140 and cold fluid heats up decreases that
00:20:42.620 00:20:42.630 rate and so there's a sequence of curves
00:20:45.320 00:20:45.330 for the hot fluid and a sequence occurs
00:20:49.790 00:20:49.800 for the cold fluid does that look
00:20:52.370 00:20:52.380 reasonable that look reasonable alright
00:20:57.110 00:20:57.120 somebody asked well if you make this
00:21:00.530 00:21:00.540 heat exchanger a little longer do you
00:21:03.590 00:21:03.600 ever think that you could get the cold
00:21:06.049 00:21:06.059 fluid and the hot fluid to sort of swap
00:21:11.710 00:21:11.720 you know why not look well what out here
00:21:15.020 00:21:15.030 what would happen you'd have your delta
00:21:17.180 00:21:17.190 T and you would drive the heat transfer
00:21:19.270 00:21:19.280 but what started out to be the cold
00:21:21.980 00:21:21.990 foods now the hot fluid it's not gonna
00:21:24.350 00:21:24.360 work they're gonna exponentially get
00:21:26.270 00:21:26.280 closer if you make it longer well
00:21:28.640 00:21:28.650 they'll just sort of slow down and and
00:21:31.010 00:21:31.020 get a little little closer together true
00:21:34.220 00:21:34.230 that's what we would expect somebody
00:21:37.940 00:21:37.950 asks what is Q then for this heat
00:21:41.150 00:21:41.160 exchanger Q would be how many watts is
00:21:45.049 00:21:45.059 transferred out of the hot and into the
00:21:46.760 00:21:46.770 cold how could I calculate Q especially
00:21:51.470 00:21:51.480 how could I relate it to the temperature
00:21:53.720 00:21:53.730 hot n minus the temperature hot out
00:21:58.990 00:21:59.000 I want to pause and I want you to finish
00:22:01.330 00:22:01.340 this equation where I put an underline
00:22:03.880 00:22:03.890 put in there what should be there what
00:22:07.840 00:22:07.850 is from the perspective of the hot fluid
00:22:10.540 00:22:10.550 finish this equation and you can then
00:22:13.720 00:22:13.730 write Q is equal to something times T
00:22:17.320 00:22:17.330 cold out minus T cold in it won't be the
00:22:21.490 00:22:21.500 same thing that's missing this is what's
00:22:24.370 00:22:24.380 missing that's what's missing they're
00:22:25.900 00:22:25.910 not the same thing they're slightly
00:22:27.130 00:22:27.140 they're related that they're similar but
00:22:29.860 00:22:29.870 they're different I want you to fill in
00:22:32.020 00:22:32.030 those two equations while I walk around
00:22:33.910 00:22:33.920 and see if you're with me so a lot of
00:22:44.170 00:22:44.180 people wanted to put in something like
00:22:45.640 00:22:45.650 UA not gonna work is it did some people
00:22:50.290 00:22:50.300 put in all the mass flow-rate specific
00:22:52.870 00:22:52.880 heat of the hot fluid and now those that
00:22:56.290 00:22:56.300 put in the mass flow-rate specific heat
00:22:58.060 00:22:58.070 of the hot fluid why why did you do that
00:23:02.320 00:23:02.330 why did why did you know that's the
00:23:04.120 00:23:04.130 00:23:06.510 00:23:06.520 it's thermo one it's an energy balance
00:23:09.220 00:23:09.230 right EB have I ever emphasized EB for
00:23:13.240 00:23:13.250 something
00:23:13.750 00:23:13.760 don't forget energy is conserved you can
00:23:16.300 00:23:16.310 do an energy balance first law of thermo
00:23:18.400 00:23:18.410 kind of had that experience for a while
00:23:20.680 00:23:20.690 here now right but you're saying hey
00:23:22.510 00:23:22.520 what happened to the enthalpy it's the
00:23:24.910 00:23:24.920 mass flow rate times the change in
00:23:26.800 00:23:26.810 enthalpy that's true that's why we put a
00:23:29.830 00:23:29.840 CP delta T when we talk talking about
00:23:33.250 00:23:33.260 liquids or gases not changing phase we
00:23:36.010 00:23:36.020 could talk more about enthalpies later
00:23:38.140 00:23:38.150 when we get the phase change all right
00:23:41.140 00:23:41.150 but m dot c Sapir
00:23:43.750 00:23:43.760 m dot CP of the cold fluid if I leave
00:23:48.190 00:23:48.200 off any subscript on the specific heat
00:23:51.010 00:23:51.020 it's typically assumed at constant
00:23:53.470 00:23:53.480 pressure but for a liquid or an
00:23:55.900 00:23:55.910 incompressible substance there's no
00:23:57.730 00:23:57.740 difference CP is equal to C B is equal
00:24:01.300 00:24:01.310 to C
00:24:15.640 00:24:15.650 you're talking about if I kind of cut
00:24:18.110 00:24:18.120 right here and that took a look I would
00:24:20.090 00:24:20.100 have a I would have this and I'm
00:24:22.520 00:24:22.530 assuming that the hot fluid I'm talking
00:24:24.860 00:24:24.870 about the bulk fluid temperature the
00:24:26.750 00:24:26.760 mean fluid temperature of that flowing
00:24:30.020 00:24:30.030 in that section and the mean fluid
00:24:32.720 00:24:32.730 temperature flowing in that section yeah
00:24:37.550 00:24:37.560 instead of drawing a cold on top again I
00:24:39.710 00:24:39.720 just simplify it
00:24:40.700 00:24:40.710 just thinking about heat transfer for
00:24:42.680 00:24:42.690 one side to the other from the inside to
00:24:44.660 00:24:44.670 the outside that helped okay now
00:24:49.540 00:24:49.550 somebody says I'm gonna leave the mass
00:24:52.550 00:24:52.560 flow rate of the cold fluid and then I'm
00:24:54.620 00:24:54.630 gonna not change the fluids so the
00:24:56.930 00:24:56.940 specific heats are not gonna change and
00:24:58.820 00:24:58.830 I'm not gonna change the mass flow rate
00:25:00.410 00:25:00.420 of the cold fluid but I'm gonna crank up
00:25:04.220 00:25:04.230 the mass flow rate of the hot fluid how
00:25:07.580 00:25:07.590 is that gonna change my temperature
00:25:09.080 00:25:09.090 distribution let's say the mass flow
00:25:11.810 00:25:11.820 rate of the hot fluid goes up it gets
00:25:14.000 00:25:14.010 doubled tripled times four I mean you're
00:25:17.180 00:25:17.190 starting to really put the pumps to it
00:25:19.130 00:25:19.140 and really whip it through does the red
00:25:22.760 00:25:22.770 line change how would it change if it
00:25:27.050 00:25:27.060 did change would the blue line change
00:25:28.970 00:25:28.980 how would it check let's say you turn
00:25:31.160 00:25:31.170 it's now 10 times oh there's no limit to
00:25:35.000 00:25:35.010 this 100 times the flow rate 100 times
00:25:40.010 00:25:40.020 the flow rate got it you're really
00:25:42.620 00:25:42.630 whipping a lot of fluid hot fluid
00:25:44.360 00:25:44.370 through there what happens to the red
00:25:46.580 00:25:46.590 line it's good it's gonna creep up and
00:25:55.160 00:25:55.170 up it up as the mass flow rate of the
00:25:58.460 00:25:58.470 hot fluid goes up true you're gonna be
00:26:02.390 00:26:02.400 pumping so much hot fluid through it
00:26:03.980 00:26:03.990 that oh you'll have a small decrease in
00:26:06.230 00:26:06.240 the hot fluid temperature but it's not a
00:26:11.030 00:26:11.040 lot the temperature change because the
00:26:13.940 00:26:13.950 mass flow rate specific heat is so high
00:26:15.950 00:26:15.960 all right what would happen to the cold
00:26:18.800 00:26:18.810 fluid as you pump
00:26:19.850 00:26:19.860 that would it stay the same outlet
00:26:21.320 00:26:21.330 temperature no wouldn't it do that yeah
00:26:29.330 00:26:29.340 right so that's there's a conceptual
00:26:33.110 00:26:33.120 question you could turn this around
00:26:34.190 00:26:34.200 leave the hot fluid flow rate alone and
00:26:37.100 00:26:37.110 just change the cold fluid flow rate as
00:26:40.100 00:26:40.110 I originally shown it where they both
00:26:42.740 00:26:42.750 approach to kind of come together in the
00:26:44.900 00:26:44.910 middle guess what's about the same is
00:26:49.390 00:26:49.400 the mass flow rate of the hot about the
00:26:52.460 00:26:52.470 same as the mass flow to cold or is it
00:26:54.650 00:26:54.660 the mass flow rate of the hot times a
00:26:56.299 00:26:56.309 specific heat of the hot about the mass
00:26:58.159 00:26:58.169 flow rate of the cold times the specific
00:26:59.990 00:27:00.000 heat of the cold answer A or B I say if
00:27:07.909 00:27:07.919 if they're both approaching about the
00:27:10.250 00:27:10.260 same middle temperature on the outlet
00:27:13.480 00:27:13.490 does that mean the mass flow rates are
00:27:15.710 00:27:15.720 about the same or the mass flow rate
00:27:17.419 00:27:17.429 specific heats are about the same let me
00:27:27.440 00:27:27.450 do this could I have a water flow for
00:27:32.360 00:27:32.370 the hot fluid and could I have air flow
00:27:35.750 00:27:35.760 for the cold fluid could I sure there's
00:27:39.500 00:27:39.510 nothing in the heat exchanger says I'll
00:27:41.900 00:27:41.910 know if you have water on one side you
00:27:43.340 00:27:43.350 have to have water on the other it's
00:27:44.630 00:27:44.640 very common to have a liquid on one side
00:27:46.580 00:27:46.590 and the gas on the other alright
00:27:48.470 00:27:48.480 somebody say give me the specific heat
00:27:50.630 00:27:50.640 of water from memory you already gave me
00:27:55.010 00:27:55.020 the specific heat of air from memory one
00:27:58.060 00:27:58.070 thousand joules per kilogram Kelvin how
00:28:05.090 00:28:05.100 about for the water 4,200 joules per
00:28:10.280 00:28:10.290 kilogram Kelvin or did I botch it do you
00:28:14.360 00:28:14.370 agree or not let me do this let me see
00:28:17.390 00:28:17.400 how many I don't know how to ask a
00:28:19.669 00:28:19.679 clicker question on this give me a
00:28:22.400 00:28:22.410 thumbs up if you agree look reasonable
00:28:27.100 00:28:27.110 maybe you remember one point zero five
00:28:30.710 00:28:30.720 kilojoules per kilogram the
00:28:33.520 00:28:33.530 Griese is there any difference for air
00:28:35.380 00:28:35.390 you know 1.0 of kilo joule I just put it
00:28:39.280 00:28:39.290 as a thousand joules per kilogram Kelvin
00:28:42.430 00:28:42.440 all right so this is about a factor of I
00:28:45.580 00:28:45.590 don't know
00:28:46.210 00:28:46.220 here's a clicker question a factor of 1
00:28:48.190 00:28:48.200 a factor of 4 a factor of 10 or a factor
00:28:52.960 00:28:52.970 of 40 this is a confidence rebuilder you
00:28:55.630 00:28:55.640 answer a B C or D this is a confidence
00:29:02.260 00:29:02.270 rebuilder here so what I what I have
00:29:11.440 00:29:11.450 done is I proven that there are some
00:29:13.540 00:29:13.550 people who don't call me friend there's
00:29:17.350 00:29:17.360 not they're not gonna call me a friend
00:29:19.270 00:29:19.280 that's it
00:29:20.110 00:29:20.120 that's all it's a factor of four people
00:29:23.400 00:29:23.410 1 to 4.2 or 1000 to 4200 right so what
00:29:30.640 00:29:30.650 did I do to talk to people off I don't
00:29:32.680 00:29:32.690 know but anyway there you go we could
00:29:37.810 00:29:37.820 have where the specific heats are a
00:29:40.270 00:29:40.280 factor of four different easy just have
00:29:42.520 00:29:42.530 water and air does that mean that if I
00:29:46.690 00:29:46.700 put in the mass flow rate of 10
00:29:49.990 00:29:50.000 kilograms per second in the mass flow
00:29:52.900 00:29:52.910 rate of 10 kilograms per second that
00:29:55.720 00:29:55.730 they would all kind of go to the middle
00:29:57.370 00:29:57.380 temperature nope if I put in a mass flow
00:30:06.940 00:30:06.950 rate of 10 kilograms per second for the
00:30:10.540 00:30:10.550 air the cold fluid what would I have to
00:30:13.240 00:30:13.250 put in for the mass flow rate of the hot
00:30:15.010 00:30:15.020 fluid in order for it to kind of go to
00:30:17.380 00:30:17.390 the middle how about I work in that you
00:30:20.110 00:30:20.120 work it out I'm gonna walk around
00:30:28.810 00:30:28.820 all right so a lot of people are coming
00:30:31.299 00:30:31.309 in with what four point what they put a
00:30:34.629 00:30:34.639 ten and they divided by 4.2 what does
00:30:37.720 00:30:37.730 that come in at two point I can't
00:30:40.960 00:30:40.970 remember what did it come in at two
00:30:43.720 00:30:43.730 point for us two point four so if I put
00:30:46.720 00:30:46.730 in a flow rate of two point four
00:30:48.370 00:30:48.380 kilograms per second for the water with
00:30:51.070 00:30:51.080 its specific heat and I put it at ten
00:30:53.529 00:30:53.539 kilograms per second for the air with
00:30:55.690 00:30:55.700 this specific heat I anticipate the
00:30:58.480 00:30:58.490 curves be pretty symmetric and me you
00:31:01.480 00:31:01.490 know they're depending how long the heat
00:31:03.669 00:31:03.679 exchanger is they could still be quite a
00:31:06.070 00:31:06.080 far apart before when they hit the end
00:31:09.310 00:31:09.320 of the heat exchanger could be a short
00:31:10.810 00:31:10.820 heat exchanger or very long so the best
00:31:14.320 00:31:14.330 answer for this original question was
00:31:17.200 00:31:17.210 beep I already graded it didn't I
00:31:21.330 00:31:21.340 that's what this one I needed the grade
00:31:26.169 00:31:26.179 be there right very good
00:31:28.950 00:31:28.960 all right so guess what they have a name
00:31:31.779 00:31:31.789 for the product of m dot time C because
00:31:36.879 00:31:36.889 often we you compared them not just the
00:31:39.279 00:31:39.289 specific heat not just the flow rate the
00:31:41.169 00:31:41.179 product of them they call that cap C
00:31:44.080 00:31:44.090 they give it a name I didn't name it
00:31:46.029 00:31:46.039 it's called the heat capacity rate the
00:31:49.840 00:31:49.850 heat capacity rate and so you'll see oh
00:31:57.399 00:31:57.409 the heat capacity rate of the cold food
00:31:59.259 00:31:59.269 oh the heat capacity rate of the hot
00:32:00.669 00:32:00.679 fluid cap see there's a heat capacity
00:32:04.149 00:32:04.159 rate
00:32:05.820 00:32:05.830 okay so heat capacity rate was defined
00:32:08.980 00:32:08.990 as something oh man he just told me it
00:32:11.289 00:32:11.299 was on the previous slide but I forgot
00:32:13.029 00:32:13.039 now he needs to answer this question
00:32:16.289 00:32:16.299 what is the SI units for the heat
00:32:20.049 00:32:20.059 capacity rate is it kilojoules
00:32:22.419 00:32:22.429 kilojoules per Kelvin kilowatts
00:32:25.210 00:32:25.220 kilowatts per Kelvin or some other units
00:32:27.940 00:32:27.950 si will give you enough time for you to
00:32:30.220 00:32:30.230 individually work this one out
00:32:38.330 00:32:38.340 all right so let's pick it up here it
00:32:40.799 00:32:40.809 was the product of the mass flow rate
00:32:42.750 00:32:42.760 times the specific heat and we just went
00:32:46.169 00:32:46.179 through those units kilogram per second
00:32:48.600 00:32:48.610 till joules per kilogram degree C you
00:32:52.769 00:32:52.779 cancel those you're left with kilowatts
00:32:56.100 00:32:56.110 per degree C but you know this is a
00:32:58.320 00:32:58.330 temperature change so it's just as well
00:33:00.419 00:33:00.429 kilowatts per Kelvin and the best answer
00:33:04.019 00:33:04.029 is D everybody on board all right so now
00:33:14.549 00:33:14.559 let's talk about saying concentric tube
00:33:16.830 00:33:16.840 I know it looks like this but instead of
00:33:19.289 00:33:19.299 trying to draw it very complicated I'll
00:33:21.480 00:33:21.490 just put the wall that separates the hot
00:33:24.990 00:33:25.000 stream from the cold stream like this
00:33:27.389 00:33:27.399 hot and then cold now that's how very
00:33:31.490 00:33:31.500 general description of concentric tube
00:33:34.560 00:33:34.570 but here we're going to go from 0 to L
00:33:38.070 00:33:38.080 in the XO that's finite length
00:33:41.419 00:33:41.429 concentric tube heat exchanger but this
00:33:44.250 00:33:44.260 time we're going to have parallel flow
00:33:47.299 00:33:47.309 we already did parallel flow didn't we
00:33:51.320 00:33:51.330 well parallel flow ok we're going to
00:33:54.120 00:33:54.130 change that to consent to counter flow
00:33:57.769 00:33:57.779 counter flow if it's counter flow this
00:34:01.919 00:34:01.929 is the hot in that's still the hot out
00:34:05.250 00:34:05.260 but over here let me change it to
00:34:07.860 00:34:07.870 different color is the cold in and the
00:34:12.480 00:34:12.490 cold out and what's neat about this one
00:34:16.829 00:34:16.839 is that you can get the hot fluid if you
00:34:20.820 00:34:20.830 have it really really long to really
00:34:23.310 00:34:23.320 have a significant drop in temperature
00:34:25.379 00:34:25.389 and you can get the cold fluid to come
00:34:28.889 00:34:28.899 up in temperature and then if you
00:34:31.409 00:34:31.419 compared the temperature cold out with
00:34:34.770 00:34:34.780 the temperature hot out you can get a
00:34:38.159 00:34:38.169 flip where before if it was parallel
00:34:41.849 00:34:41.859 flow there was no way there was no way
00:34:44.190 00:34:44.200 that they could have but you can have
00:34:47.399 00:34:47.409 for here the temperature
00:34:49.470 00:34:49.480 cold out can be greater than the
00:34:54.180 00:34:54.190 temperature hot out it doesn't have to
00:34:56.790 00:34:56.800 be I mean the actual heat exchanger may
00:34:59.579 00:34:59.589 look like this with the hot fluid doing
00:35:02.609 00:35:02.619 this and the hot out is still greater
00:35:05.460 00:35:05.470 than the cold out but if it's especially
00:35:08.430 00:35:08.440 if it's long enough the cold out can be
00:35:11.940 00:35:11.950 warmer than the hot out so there's a lot
00:35:14.910 00:35:14.920 of heat exchangers which are counterflow
00:35:17.599 00:35:17.609 okay this clicker question is still
00:35:20.130 00:35:20.140 valid I've written four equations right
00:35:22.859 00:35:22.869 here ABC and D one of them is incorrect
00:35:27.210 00:35:27.220 one of them is wrong which one is wrong
00:35:39.200 00:35:39.210 alright so let's go ahead and stop what
00:35:42.060 00:35:42.070 we found was that this one gives us a
00:35:45.060 00:35:45.070 negative Q the others are all positive
00:35:48.359 00:35:48.369 and why is it negative is it because
00:35:50.760 00:35:50.770 mass flow-rate specific Heat are
00:35:52.349 00:35:52.359 negative no it's because the temperature
00:35:55.440 00:35:55.450 hot out is greater than the temperature
00:35:57.569 00:35:57.579 I'm sorry cold out is greater than the
00:36:00.059 00:36:00.069 cold in isn't it so this one is the
00:36:03.300 00:36:03.310 wrong one and there it is okay so we
00:36:10.620 00:36:10.630 already just talked about the counter
00:36:12.240 00:36:12.250 flow heat exchanger get that we quickly
00:36:14.579 00:36:14.589 sketch it for the temperature profile
00:36:16.920 00:36:16.930 you can have the hot coming this way you
00:36:20.069 00:36:20.079 could have the cold doing this they can
00:36:22.950 00:36:22.960 be different slopes they can have a
00:36:25.950 00:36:25.960 bunch of different cases and you can
00:36:29.040 00:36:29.050 start to play a lot of games with this
00:36:31.620 00:36:31.630 and there's almost no end to the number
00:36:33.720 00:36:33.730 of games so you say for the counter flow
00:36:36.569 00:36:36.579 concentric tube heat exchanger if either
00:36:38.970 00:36:38.980 I say L changes Oh
00:36:41.130 00:36:41.140 let L increase decrease whatever but
00:36:44.339 00:36:44.349 let's just say L increases no no let you
00:36:47.099 00:36:47.109 increase no let the mass flow rate the
00:36:49.140 00:36:49.150 hot increase see what I can do I can ask
00:36:51.180 00:36:51.190 so many questions you can get really
00:36:54.079 00:36:54.089 confused no no you learn a lot how does
00:36:58.020 00:36:58.030 the temperature hot out change or how
00:37:00.510 00:37:00.520 does the temperature cold out change or
00:37:02.849 00:37:02.859 how
00:37:03.630 00:37:03.640 the cube amount of heat transferred
00:37:07.170 00:37:07.180 within this device the rate of heat
00:37:09.359 00:37:09.369 transfer how does that change so will it
00:37:13.380 00:37:13.390 go up will it negligible negligible
00:37:16.739 00:37:16.749 change or will it go down see all these
00:37:20.210 00:37:20.220 so let's do the let's do maybe an easy
00:37:24.120 00:37:24.130 one if the mass flow rate of the hot
00:37:26.579 00:37:26.589 fluid I'm gonna increase that boom
00:37:29.700 00:37:29.710 everything else remains the same the
00:37:31.410 00:37:31.420 length the same the you the same the
00:37:34.380 00:37:34.390 mass flow rate is cold
00:37:36.180 00:37:36.190 how does the temperature of the hot out
00:37:39.170 00:37:39.180 change give it a minute it'll come back
00:37:51.829 00:37:51.839 how many minutes do you want to give it
00:37:53.910 00:37:53.920 professor I don't know be patient and
00:37:59.779 00:37:59.789 the clicker sessions huh we've lost
00:38:07.019 00:38:07.029 connection well I'm only going to give
00:38:11.309 00:38:11.319 it a few more seconds and we're gonna
00:38:12.660 00:38:12.670 have to stop and Bandhan the clickers
00:38:17.989 00:38:17.999 it'll come back up you think please
00:38:23.640 00:38:23.650 check okay we're done with that sorry
00:38:29.239 00:38:29.249 you're trying to get a point all right
00:38:31.979 00:38:31.989 so now we have to continue this because
00:38:34.440 00:38:34.450 of Technology failure here so we'll just
00:38:37.620 00:38:37.630 do it with the show of hands temperature
00:38:40.499 00:38:40.509 hot out will it go up will the hot out
00:38:44.099 00:38:44.109 which is over here will it go up a
00:38:46.109 00:38:46.119 little bit and maybe the new temperature
00:38:48.299 00:38:48.309 hot profile look like that if you
00:38:50.759 00:38:50.769 increase the mass flow rate of the hot
00:38:52.920 00:38:52.930 fluid yes all correct that's perfect
00:38:56.999 00:38:57.009 I do is nobody didn't have their hand on
00:38:58.979 00:38:58.989 every hand was up we are now hundred
00:39:02.069 00:39:02.079 percent correct
00:39:03.210 00:39:03.220 now we're gonna stay there with the mass
00:39:07.170 00:39:07.180 flurry of the hot is going to be
00:39:08.849 00:39:08.859 increased what happens to the
00:39:10.890 00:39:10.900 temperature of the cold out where is the
00:39:14.130 00:39:14.140 temperature that cold out right here
00:39:15.809 00:39:15.819 temperature the cold out
00:39:17.160 00:39:17.170 remember it's counter flow what happens
00:39:20.250 00:39:20.260 to the temperature the cold out who
00:39:29.309 00:39:29.319 would like to give me their answer
00:39:30.450 00:39:30.460 verbally go ahead it would increase a
00:39:35.010 00:39:35.020 little bit and why would it increase a
00:39:37.289 00:39:37.299 little bit you didn't sign up for that
00:39:43.319 00:39:43.329 you just want the oven answer for that
00:39:49.470 00:39:49.480 winter should I give it to somebody else
00:39:50.789 00:39:50.799 somebody else
00:39:52.200 00:39:52.210 he's willing to share the glory here who
00:39:55.260 00:39:55.270 would like to say why it would go up a
00:39:57.539 00:39:57.549 little bit yes sir yeah you're looking
00:40:03.329 00:40:03.339 at the average delta T throughout the
00:40:05.430 00:40:05.440 heat exchanger and by pushing the hot up
00:40:08.370 00:40:08.380 you've created a larger overall Delta T
00:40:11.760 00:40:11.770 for the whole heat exchanger hence
00:40:13.200 00:40:13.210 there's a larger Q and so it's going to
00:40:16.530 00:40:16.540 bring up basically you've you had to
00:40:20.309 00:40:20.319 answer this question first
00:40:21.900 00:40:21.910 I think the Q is going to go up because
00:40:24.210 00:40:24.220 of the increase and now that it's gone
00:40:26.970 00:40:26.980 up then the cold flow rate didn't change
00:40:31.109 00:40:31.119 hence the cold outlet temperature has to
00:40:33.420 00:40:33.430 change right there's three equations
00:40:37.049 00:40:37.059 that you're going to work with in this
00:40:38.640 00:40:38.650 chapter q is equal to cap C hot
00:40:42.890 00:40:42.900 temperature change of the hot Q is equal
00:40:46.710 00:40:46.720 to cap C cold temperature change of the
00:40:49.890 00:40:49.900 cold and the one that we haven't gotten
00:40:52.200 00:40:52.210 to Q is equal to you a some appropriate
00:40:57.859 00:40:57.869 best average delta T throughout the heat
00:41:02.130 00:41:02.140 exchanger that appropriate delta T is
00:41:05.730 00:41:05.740 called the LM delta T it's the log mean
00:41:10.589 00:41:10.599 temperature difference through that heat
00:41:12.660 00:41:12.670 exchanger intuitively if the temperature
00:41:16.170 00:41:16.180 difference throughout is large
00:41:18.870 00:41:18.880 throughout the heat exchanger delta T
00:41:20.789 00:41:20.799 log mean is large okay okay
00:41:25.589 00:41:25.599 so kind of this is your logic that you
00:41:28.380 00:41:28.390 know kind of this average delta T
00:41:30.720 00:41:30.730 throughout
00:41:31.140 00:41:31.150 the heat exchangers increased these are
00:41:33.600 00:41:33.610 you know maybe change to you a little
00:41:35.340 00:41:35.350 bit because you increase the H on the
00:41:38.310 00:41:38.320 hot side probably improved the
00:41:40.170 00:41:40.180 convection coefficient on the hot side
00:41:41.880 00:41:41.890 maybe improved you a little bit but the
00:41:44.880 00:41:44.890 the you're gonna lead to a larger cube
00:41:47.820 00:41:47.830 because of a larger q a larger delta t/2
00:41:50.780 00:41:50.790 of these are energy balance equations
00:41:54.480 00:41:54.490 conservation of energy this is why we
00:41:57.900 00:41:57.910 take a whole class and heat transfer it
00:42:00.630 00:42:00.640 is our rate equation there was a rate
00:42:05.250 00:42:05.260 equation for heat conduction what was
00:42:08.160 00:42:08.170 the name of the rate equation for heat
00:42:09.660 00:42:09.670 conduction for Hayes law there was a
00:42:12.420 00:42:12.430 rate equation for convection heat
00:42:14.340 00:42:14.350 transfer what was the name Newton's law
00:42:17.820 00:42:17.830 of cooling
00:42:18.450 00:42:18.460 there was a rate equation for radiative
00:42:21.450 00:42:21.460 heat transfer Stefan Boltzmann law and
00:42:25.950 00:42:25.960 this is just a rate equation like a
00:42:28.980 00:42:28.990 modified convection equation for the
00:42:34.440 00:42:34.450 rate of heat transfer and heat
00:42:36.000 00:42:36.010 exchangers all right
00:42:38.610 00:42:38.620 I'm kind of getting tired and we have to
00:42:40.950 00:42:40.960 do our quiz so we'll stop here and pick
00:42:43.590 00:42:43.600 up there next time
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