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Discuss Exergetic Efficiency Heat Exchanger No Mixing

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Language: en

00:00:00.030
this first one is without mixing so
00:00:03.020 00:00:03.030 typically you have a hot stream on one
00:00:04.789 00:00:04.799 side and a cold stream on the other and
00:00:06.440 00:00:06.450 the heat is being transferred through
00:00:07.970 00:00:07.980 the metal that separates them
00:00:09.980 00:00:09.990 maybe that metals copper tubes so it is
00:00:13.009 00:00:13.019 good at conducting heat through okay
00:00:16.760 00:00:16.770 maybe they have ribs or fins or make it
00:00:19.640 00:00:19.650 you know but this is a simple concept to
00:00:22.730 00:00:22.740 promote the heat transfer you come in
00:00:25.759 00:00:25.769 with the hot fluid mhm I wonder why they
00:00:28.040 00:00:28.050 picked the subscript H for the hot food
00:00:30.560 00:00:30.570 I wonder why they picked a subscript C
00:00:33.229 00:00:33.239 for the cold fluid makes sense right
00:00:35.620 00:00:35.630 comes in it one goes out at two you can
00:00:39.170 00:00:39.180 talk about all your other properties but
00:00:41.060 00:00:41.070 the key property is you have the flow
00:00:43.160 00:00:43.170 exergy one coming in and the flow exergy
00:00:45.560 00:00:45.570 two going out likewise for the cold
00:00:48.410 00:00:48.420 fluid it comes in at three and out at
00:00:50.810 00:00:50.820 four so we had the flow exergy three and
00:00:53.750 00:00:53.760 the flow exergy four couple questions
00:00:55.490 00:00:55.500 before we get too bogged down can you
00:00:58.369 00:00:58.379 compare EF 2 and EF 1 which one's
00:01:01.700 00:01:01.710 greater are they the same
00:01:05.299 00:01:05.309 no they're not the same come on what
00:01:08.800 00:01:08.810 what is alright it really help you if
00:01:15.679 00:01:15.689 you do the temperature one temperature
00:01:17.960 00:01:17.970 to this temperature one greater than or
00:01:20.120 00:01:20.130 less than the temperature to it's
00:01:23.510 00:01:23.520 greater that's exactly right and so now
00:01:26.830 00:01:26.840 this flow at 2 is going to be less than
00:01:30.410 00:01:30.420 the float one or EF one is be greater
00:01:32.300 00:01:32.310 than two true now go to the cold fluid
00:01:36.230 00:01:36.240 and do the same thing talk about the
00:01:38.690 00:01:38.700 temperature you know temperature 3
00:01:40.609 00:01:40.619 compared to temperature fours
00:01:42.080 00:01:42.090 temperature 3 greater than or less than
00:01:44.210 00:01:44.220 temperature for temperature four is
00:01:49.160 00:01:49.170 greater that's true so temperature 3 is
00:01:53.600 00:01:53.610 less than 4 thumbs up I know I'm moving
00:01:57.260 00:01:57.270 probably pretty fast on this but you
00:01:59.030 00:01:59.040 need to really think about it okay and
00:02:01.219 00:02:01.229 then how about the flow exergy ef3 and
00:02:05.530 00:02:05.540 ef4 oops kind of right over which one
00:02:10.490 00:02:10.500 four is greater three is less makes
00:02:14.120 00:02:14.130 sense so now we go to an excerpt G
00:02:17.870 00:02:17.880 balance for the control volume
00:02:19.310 00:02:19.320 surrounding the entire system okay so on
00:02:25.640 00:02:25.650 their exergy balance we have steady
00:02:28.040 00:02:28.050 state nothing with the heat transfer
00:02:30.740 00:02:30.750 what do you mean nothing with the eve
00:02:32.030 00:02:32.040 transfer it's a heat exchanger there's a
00:02:33.680 00:02:33.690 lot of heat transfer but it's not to the
00:02:37.370 00:02:37.380 surroundings it they say something like
00:02:40.040 00:02:40.050 it's an insulated heat exchanger that
00:02:42.980 00:02:42.990 sounds like a contradiction right
00:02:45.470 00:02:45.480 well it's insulated with respect to the
00:02:47.960 00:02:47.970 surroundings
00:02:48.920 00:02:48.930 so there's no heat transfer out okay
00:02:52.990 00:02:53.000 alright so but within the heat exchanger
00:02:56.360 00:02:56.370 there's hopefully a lot and then you
00:02:58.040 00:02:58.050 have no shaft in or out and then what
00:03:02.390 00:03:02.400 you're going to have the mass flow rate
00:03:03.740 00:03:03.750 of the hot and we have e F 1 minus e F 2
00:03:08.420 00:03:08.430 whoops EF 2 plus the mass flow rate of
00:03:14.810 00:03:14.820 the cold e f3 minus e F 4 plus or no not
00:03:21.740 00:03:21.750 it's not plus minus exergy destruction
00:03:25.699 00:03:25.709 now I'm gonna pause and I'm gonna ask
00:03:28.340 00:03:28.350 you to look at that equation and tell me
00:03:30.949 00:03:30.959 if I have an error in it if not give me
00:03:34.220 00:03:34.230 a thumbs up if you agree with that
00:03:35.720 00:03:35.730 equation
00:03:39.610 00:03:39.620 well not either I have an error or I
00:03:42.740 00:03:42.750 don't have an error I have okay but
00:03:49.729 00:03:49.739 that's term was struck to zero I have a
00:03:56.840 00:03:56.850 negative here and a negative there if 3
00:04:02.780 00:04:02.790 3 is in and one isn't in and - is it out
00:04:08.570 00:04:08.580 and 4 is an out true so I'm looking for
00:04:16.280 00:04:16.290 thumbs up if you like the equation or
00:04:18.250 00:04:18.260 tell me where my error is thumbs up
00:04:22.159 00:04:22.169 thumbs up thumbs up
00:04:24.080 00:04:24.090 thumbs up thumbs up sorry
00:04:29.810 00:04:29.820 some people are not wanting to
00:04:33.260 00:04:33.270 participate
00:04:33.950 00:04:33.960 I like 100% participation how's that I
00:04:36.909 00:04:36.919 can call you out you got it you two
00:04:39.530 00:04:39.540 thumbs up back there up good did you
00:04:43.430 00:04:43.440 give me a thumbs up you're still
00:04:46.400 00:04:46.410 thinking we can wait these are really
00:04:53.960 00:04:53.970 tricky aren't they because what you're
00:04:56.120 00:04:56.130 doing probably in your mind is saying
00:04:57.440 00:04:57.450 hold hold hold it hold it this this is
00:05:01.430 00:05:01.440 gonna be a positive quantity isn't it
00:05:03.770 00:05:03.780 but this is gonna be a negative quantity
00:05:05.960 00:05:05.970 isn't it don't sweat it we'll take care
00:05:08.300 00:05:08.310 of that minute but is it right all right
00:05:11.840 00:05:11.850 uncle he's called uncle already so now
00:05:15.980 00:05:15.990 let's rewrite this equation because m
00:05:18.230 00:05:18.240 dot h times e f1 minus f2 is what comes
00:05:23.290 00:05:23.300 into the system supplied by the hot
00:05:26.750 00:05:26.760 fluid and where does it go it either
00:05:31.040 00:05:31.050 goes to increasing the flow exergy and
00:05:34.310 00:05:34.320 this is where I'm gonna switch that sign
00:05:36.790 00:05:36.800 of the cold fluid or it goes into being
00:05:41.570 00:05:41.580 destroyed now I'm gonna stop because I
00:05:44.620 00:05:44.630 switched ef4 and ef5 energy and I put a
00:05:50.029 00:05:50.039 poss sign in front of that we just did I
00:05:52.760 00:05:52.770 do the algebra correct now we can read
00:05:55.940 00:05:55.950 this equation can't leave its what came
00:05:59.000 00:05:59.010 into the system bought from the hot
00:06:03.589 00:06:03.599 fluid and it goes into one of two places
00:06:09.560 00:06:09.570 it goes into the cold fluid or destroyed
00:06:17.980 00:06:17.990 but that's our exergy accounting it has
00:06:22.490 00:06:22.500 to be taken into account exergy doesn't
00:06:24.560 00:06:24.570 vanish it can be destroyed but we have
00:06:27.980 00:06:27.990 an accounting true what does each one of
00:06:30.920 00:06:30.930 these terms in units of kilo joules
00:06:34.010 00:06:34.020 kilowatts
00:06:37.420 00:06:37.430 kilowatts kilowatts kilowatts well as a
00:06:40.850 00:06:40.860 good engineer we want as much to go to
00:06:44.240 00:06:44.250 here and a little to go whoops didn't
00:06:48.230 00:06:48.240 get the arrow all the way across as
00:06:50.240 00:06:50.250 little to go there why destruction
00:06:55.010 00:06:55.020 doesn't help anybody true so here is an
00:06:59.180 00:06:59.190 efficiency exegetical fishin see second
00:07:03.110 00:07:03.120 law efficiency for this heat exchanger
00:07:05.680 00:07:05.690 without mixing it's a counter flow heat
00:07:07.970 00:07:07.980 exchanger and what do we want its we
00:07:12.470 00:07:12.480 wanted the flue an extra G to go
00:07:14.180 00:07:14.190 somewhere and it came at the expense of
00:07:16.820 00:07:16.830 someplace we wanted it to go m dot c EF
00:07:22.010 00:07:22.020 for minus e f3 true m dot h EF 1 minus e
00:07:29.030 00:07:29.040 f 2 does that appeal to your logic do
00:07:34.940 00:07:34.950 you like that flow i mean that extra
00:07:37.520 00:07:37.530 genetic efficiency for our non mixing
00:07:40.520 00:07:40.530 heat exchanger

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