00:00:00.140 --> 00:00:18.820 [noise] Welcome back. Today we are going to start 00:00:18.820 --> 00:00:27.390 a new topic and we will be dealing with this topic for ah quite a few lectures. This is 00:00:27.390 --> 00:00:33.920 a very important ah topic as far as heat exchangers are concerned. And basically we are going 00:00:33.920 --> 00:00:41.280 to look into some typical heat exchanger . Again there are many variations, but we have classified 00:00:41.280 --> 00:00:48.989 them into ah or we have we have ah assembled them ah for the sake of understanding in one 00:00:48.989 --> 00:00:57.440 group and which I am calling as which I am calling as heat exchangers involving phase 00:00:57.440 --> 00:01:05.010 change. So, in some heat exchanger ah, we will see that ah there is a change of phase. 00:01:05.010 --> 00:01:09.660 So, those kind of heat exchangers we are calling heat heat exchanger involving phase change 00:01:09.660 --> 00:01:17.110 or Phase change heat exchanger. So, now let us go to the next slide. So, phase 00:01:17.110 --> 00:01:23.030 change heat exchangers. One of the fluids will change its phase at least one of the 00:01:23.030 --> 00:01:30.690 fluid that has to change its phase in some rare case both the fluids will change its 00:01:30.690 --> 00:01:34.840 will change phase. So, basically ah u can understand that we 00:01:34.840 --> 00:01:40.030 are considering two fluid heat exchanger. Of course, there is no harm having multi fluid 00:01:40.030 --> 00:01:45.780 heat exchanger, sometimes again in a very special case one can have multi fluid or multi 00:01:45.780 --> 00:01:56.070 stream heat exchanger . And then the requirement of its qualification as a phase change heat 00:01:56.070 --> 00:02:04.250 exchanger is there at least one of the fluid will change its phase. And change of phase 00:02:04.250 --> 00:02:11.510 could be many like from solid to liquid liquid to solid ah liquid to vapour vapour to liquid, 00:02:11.510 --> 00:02:16.330 but liquid vapour phase change or vapour liquid phase change is the concept. 00:02:16.330 --> 00:02:24.310 So, either ; that means, the change between liquid to vapour or wise versa is the concern. 00:02:24.310 --> 00:02:30.650 Basically the mechanism is either boiling or condensation . In some heat heat exchanger 00:02:30.650 --> 00:02:36.470 evaporation may also be present ah like there are falling film evaporator etcetera. So, 00:02:36.470 --> 00:02:43.310 evaporator may also be present in some cases. But mainly we will see that ah phase change 00:02:43.310 --> 00:02:50.550 is taking place either by condensation or by boiling . Condensation is also present 00:02:50.550 --> 00:02:56.680 in case of air being cooled and dehumidified. Very common experience that ah during summer 00:02:56.680 --> 00:03:02.730 particularly in a country like ah India, ah whenever air conditioner is there and lot 00:03:02.730 --> 00:03:09.440 of water drips from the air conditioner because the hot and humid air comes in the contact 00:03:09.440 --> 00:03:16.549 of the cooling coil. It gets dehumidified and the air gets ah dehumidified and the ah 00:03:16.549 --> 00:03:24.940 water vapour present in the air condenses. So, then this dehumidifier coil or or the 00:03:24.940 --> 00:03:33.099 cooling and dehumidifier coil of the air conditioner is also taking part in condensation. But those 00:03:33.099 --> 00:03:39.319 kind of heat exchanger we will not discuss here. There are many direct contact heat exchanger 00:03:39.319 --> 00:03:45.950 where heat exchange takes place. So, ah not very elaborated description of those heat 00:03:45.950 --> 00:03:52.450 exchanger will be possible with the minimum amount of time ah we are having ah by this 00:03:52.450 --> 00:03:57.802 time probably you have appreciated that heat exchanger is a vast topic. 00:03:57.802 --> 00:04:02.240 So, ah we will try to give some general idea of phase change heat exchanger and some of 00:04:02.240 --> 00:04:09.159 the important phase change heat exchangers, we will try to deal in a bit details. So, 00:04:09.159 --> 00:04:20.300 ah with this ah let us go to the next slide. Now here, I am showing how the temperature 00:04:20.300 --> 00:04:28.190 changes in a purely phase change heat exchanger. What is purely phase change heat exchanger 00:04:28.190 --> 00:04:36.150 ? One fluid in the heat exchanger will be ah changing its phase from the inlet of the 00:04:36.150 --> 00:04:43.270 heat exchanger till the exit of the heat changer. Always if we call this mode of heat transfer 00:04:43.270 --> 00:04:51.140 ah or this ah condition as a two phase condition, so one of the flowing fluid will always be 00:04:51.140 --> 00:04:55.800 in two phase condition from the beginning of the heat exchanger till the end of the 00:04:55.800 --> 00:05:02.080 heat exchanger. So, this is a purely phase change heat exchanger or let us define it 00:05:02.080 --> 00:05:09.450 to be a purely phase change heat exchanger. So, as I have told that boiling and condensation 00:05:09.450 --> 00:05:17.590 are the two mechanisms of phase change which we will ah consider in our discussion. So, 00:05:17.590 --> 00:05:23.640 either we are dealing with a condenser or we are dealing with a boiler. So, let us say 00:05:23.640 --> 00:05:29.780 that we are dealing with a condenser . So, the ah the condenser ah temperature change 00:05:29.780 --> 00:05:35.500 is shown here as it is ah although the heat exchanger in the two phase condition. So, 00:05:35.500 --> 00:05:43.050 it is maintaining that phase change temperature T FC it is maintaining throughout the length 00:05:43.050 --> 00:05:50.510 . And then ah as it is um it has to be cooled or heat has to be extracted from here, there 00:05:50.510 --> 00:05:54.840 is a cooling stream ah in case of ah power plant condenser. 00:05:54.840 --> 00:06:00.460 Let us say the steam is condensing it is maintaining a constant temperature and water is cooling 00:06:00.460 --> 00:06:05.620 it. So, water is getting heated up. So, this is the cooling stream which is getting heated 00:06:05.620 --> 00:06:14.360 up. What is important to see that let us say the first arrangement the top curve here T 00:06:14.360 --> 00:06:18.480 FC that is T F exchange and this is the cooling curve. 00:06:18.480 --> 00:06:24.660 So, typically this is a parallel flow heat exchanger. Now if we change the direction 00:06:24.660 --> 00:06:32.040 of the condensing fluid. So, then , it becomes some sort of a counter current flow heat exchanger, 00:06:32.040 --> 00:06:38.851 but the performance will not change; the performance of the heat exchanger will not change. If 00:06:38.851 --> 00:06:45.750 we keep the heat exchanger designed constant, then the inlet and outlet temperatures of 00:06:45.750 --> 00:06:54.260 the fluid that will not change. So, it is independent of orientation. This is one thing 00:06:54.260 --> 00:06:59.810 we have keep it in mind that in phase change heat transfer when there is purely phase change 00:06:59.810 --> 00:07:06.350 for a particular fluid, there is it is independent of orientation. So, same thing for a boiler 00:07:06.350 --> 00:07:14.600 it has been shown that here the fluid is ah evaporating throughout the length of the heat 00:07:14.600 --> 00:07:20.860 exchanger and probably it is getting its ah um thermal energy needed for ah let it heat 00:07:20.860 --> 00:07:26.660 of vaporization ah from the from the from a hot fluid. So, the hot fluid is getting 00:07:26.660 --> 00:07:33.900 cold ah cooled. So, here also the ah it will not matter whether we are going for a parallel 00:07:33.900 --> 00:07:39.660 flow or a counter flow. So, LMTD is given by ah these expressions 00:07:39.660 --> 00:07:47.270 and this LMTD is independent of orientation. And if we go for effectiveness NTU, ah all 00:07:47.270 --> 00:07:53.720 the heat exchanger with phase change will have this relationship; very easy to remember. 00:07:53.720 --> 00:08:00.820 So, this is the relationship between NTU and effectiveness ah this is effectiveness. And 00:08:00.820 --> 00:08:04.120 this is N T U number of transfer unit and this is the relationship. 00:08:04.120 --> 00:08:12.280 So, for a simple phase change heat exchanger this kind of relationship we will get and 00:08:12.280 --> 00:08:18.280 I have shown two arrangement parallel and counter flow. It does not matter which way 00:08:18.280 --> 00:08:25.240 you are arranging it . Now, this is a cross flow heat exchanger. 00:08:25.240 --> 00:08:30.830 Let us say our phase change heat exchanger is a cross flow heat exchanger. If you remember 00:08:30.830 --> 00:08:36.680 in fin tube heat exchanger, we have taken some example where the end and we have ah 00:08:36.680 --> 00:08:44.110 basically going to solve the problem of finned tube condenser. So, that is that was a cross 00:08:44.110 --> 00:08:48.390 flow condenser. So, here also this is a cross flow heat exchanger 00:08:48.390 --> 00:08:58.390 . So, for cross flow heat exchanger; let us say this is the ah condensing fluid and this 00:08:58.390 --> 00:09:03.510 is the condensing fluid. It is inside, this will be the temperature distribution and this 00:09:03.510 --> 00:09:10.211 will be the temperature distribution at the condensing fluid ah exit. And as it is in 00:09:10.211 --> 00:09:14.620 the phase change condition, there will not be any change in the temperature from inlet 00:09:14.620 --> 00:09:21.460 to outlet. And there will not be any variation of temperature also along the inlet ah plain 00:09:21.460 --> 00:09:30.250 or along the ah exit plain. This is the ah coolant which is gaining heat. So, when at 00:09:30.250 --> 00:09:35.820 the inlet it will have uniform temperature low temperature. At the outlet there will 00:09:35.820 --> 00:09:44.140 not be any variation of temperature variation of temperature at the outlet is ah ah is characteristic 00:09:44.140 --> 00:09:52.140 of cross flow heat exchanger, but here we will not get it. So, here we will have this 00:09:52.140 --> 00:10:01.970 kind of temperature distribution and . So; obviously, you see that there could be 00:10:01.970 --> 00:10:10.530 other reasons for selecting ah ah the arrangement ah whether we will go for parallel flow ah, 00:10:10.530 --> 00:10:18.270 counter flow or cross flow for a phase change heat exchanger, but thermally all of them 00:10:18.270 --> 00:10:23.870 are equivalent. There could be other mechanical reason, there could be that we have to accommodate 00:10:23.870 --> 00:10:30.250 more amount of ah tubes etcetera. So, other reasons planned related reason could be there, 00:10:30.250 --> 00:10:37.190 but thermally they are identical . Cooling fluid temperature is uniform at the outlets. 00:10:37.190 --> 00:10:47.120 So, this is ah one very important observation. Now, ah [noise] let us take up a problem. 00:10:47.120 --> 00:10:54.990 A condenser is required for an organic vapour having a latent heat of 250 kilo joule per 00:10:54.990 --> 00:11:02.350 kg flowing at 5 kg per second. The vapour is pure and may be assumed condensate at its 00:11:02.350 --> 00:11:11.850 saturation temperature of 80 degree Celsius. The condenser is cooled ah by by site cooling 00:11:11.850 --> 00:11:18.510 water specific heat has been given with an inlet temperature of 20 degree Celsius and 00:11:18.510 --> 00:11:25.360 outlet temperature of 35 degree Celsius. The overall ah heat transfer coefficient that 00:11:25.360 --> 00:11:35.650 has been given calculate the cooling water flow rate the value of delta T and the surface 00:11:35.650 --> 00:11:45.600 area. ah What is the f value for this case. So, ah water flow rate we have got. So, from 00:11:45.600 --> 00:11:53.130 that M dot ah c p e cool and we can find find out and this is equal to this is the total 00:11:53.130 --> 00:11:59.279 heat gained by heat exchange sorry gained through heat transfer by the cooling water 00:11:59.279 --> 00:12:05.520 stream. So, this is the heat which is rejected by steam also. So, heat rejected by steam 00:12:05.520 --> 00:12:13.720 that will be equal to ah the steam flow rate and the latent heat because it is in the two 00:12:13.720 --> 00:12:19.529 phase condition. So, right hand side everything is known and from the left hand side, hm it 00:12:19.529 --> 00:12:28.240 has been told that the water ah ah ah inlet temperature of the water is 20 degree celsius 00:12:28.240 --> 00:12:33.400 and outlet temperature is 35 degree celsius. So, from there, we will get the mass flow 00:12:33.400 --> 00:12:41.300 of water that is 19.84 kg per second. So, mass flow of water is known at this point. 00:12:41.300 --> 00:12:50.680 Next ah what we can do? We can calculate the log mean temperature. So, log mean temperature 00:12:50.680 --> 00:12:57.300 we have calculated ah. One fluid will ah be at a constant temperature that is your 80 00:12:57.300 --> 00:13:05.850 and other fluid, there will be inlet temperature of ah 20 and outlet temperature of 35. So, 00:13:05.850 --> 00:13:14.460 hm other fluid also we can get ah the temperature. And from there, we can calculate the log mean 00:13:14.460 --> 00:13:21.880 temperature which is 52.1degree Celsius. Now in this case, as I have already told that 00:13:21.880 --> 00:13:29.760 whether we are having cross flow, counter flow ah hm parallel flow everywhere ah , one 00:13:29.760 --> 00:13:36.560 fluid is at a constant temperature. So, the flow distribution really does not affect the 00:13:36.560 --> 00:13:43.500 thermal performance . So, we will get F is equal to 1 for any arrangement. 00:13:43.500 --> 00:13:48.930 So, that is what has been written in this case F is equal to 1. Effectively the condensing 00:13:48.930 --> 00:13:57.220 fluid behave as a fluid at infinite specific heat capacity and the heat exchange is always 00:13:57.220 --> 00:14:04.770 equivalent to that of a pure countercurrent flow. Thus we have got this kind of relationship. 00:14:04.770 --> 00:14:16.180 This is very important and you can note this. So, this is one important point and this is 00:14:16.180 --> 00:14:25.660 also important to note for a purely phase change heat exchanger. Already I have explained 00:14:25.660 --> 00:14:33.500 what is purely phase change heat exchanger. So, this is very important to note ok. 00:14:33.500 --> 00:14:41.170 So, please try to correlate the problem which we have done at the end of ah our discussion 00:14:41.170 --> 00:14:46.500 on fin tube heat exchanger. That was much more elaborate because the fin side heat transfer 00:14:46.500 --> 00:14:51.600 coefficient pressure drop etcetera, we wanted to take care off. But that was also a cross 00:14:51.600 --> 00:14:58.510 flow heat exchanger, there also we have calculated LMTD. So, please ah ah look into that problem 00:14:58.510 --> 00:15:03.870 go back to your ah to my earlier lecture and please look into that problem. But whatever 00:15:03.870 --> 00:15:14.430 i have underlined these points are very important and this is how we will get the delta T . 00:15:14.430 --> 00:15:24.080 Now let us go to the next page [noise]. So, surface area we can calculate from here. So, 00:15:24.080 --> 00:15:30.290 this is our end of the problem ah very simple problem, but it gives us an idea how to calculate 00:15:30.290 --> 00:15:36.560 the basic calculation of a phase change heat heat exchanger. 00:15:36.560 --> 00:15:43.320 Now, you see I have coined some sort of a word or terminology a purely phase change 00:15:43.320 --> 00:15:50.080 heat exchanger. Now most of the practical heat exchangers where phase change of a fluid 00:15:50.080 --> 00:15:57.620 stream takes place is not like this . So, ah in phase change heat exchanger, we will 00:15:57.620 --> 00:16:04.860 have change of phase, but before that and after that there could be single phase heating 00:16:04.860 --> 00:16:09.339 or cooling. So, let us say here we can see condenser with 00:16:09.339 --> 00:16:14.210 a super heated vapour at the entry. So, let us say this is the condensing fluid which 00:16:14.210 --> 00:16:21.620 we have to condense , but the fluid is not exactly at the saturated condition when it 00:16:21.620 --> 00:16:30.510 is entering the heat exchanger. It is it is ah the hotter fluid is entering at super heated 00:16:30.510 --> 00:16:36.910 state. So, if it is entering at a super heated state over here, so then it has to be brought 00:16:36.910 --> 00:16:43.950 to this saturation condition and this is single phase cooling for the fluid, then there will 00:16:43.950 --> 00:16:49.850 be phase change heat transfer and then again there could be sub cooling and this is again 00:16:49.850 --> 00:16:54.800 single phase cooling. And this is how the coolant temperature coolant is a single phase 00:16:54.800 --> 00:17:00.740 fluid. So, coolant temperature will continuously increase . I have shown linear, but this curve 00:17:00.740 --> 00:17:06.309 this curve can be non-linear, this can be non-linear and this can be non-linear in general. 00:17:06.309 --> 00:17:11.300 I have shown them linear drawn them linear, but this curves could be non-linear. 00:17:11.300 --> 00:17:19.709 So, in most of the practical cases, we will get a heat exchanger like this . Let us consider 00:17:19.709 --> 00:17:30.660 the ah let us consider the hm ah the condenser of a refrigeration unit. So, in a refrigeration 00:17:30.660 --> 00:17:38.620 unit after the compressor, hot ah vapour comes out and that will be in super heated state. 00:17:38.620 --> 00:17:45.830 And then in the condenser, it condenses either it will be partially condensed ah also, but; 00:17:45.830 --> 00:17:53.750 obviously, it will enter with a super heated state in the condenser. So, you see that ah 00:17:53.750 --> 00:17:59.770 the curve will be something like this and this either it can go to this range or it 00:17:59.770 --> 00:18:05.650 can come out out of the condenser like this. So, sub cooling could be there, sub cooling 00:18:05.650 --> 00:18:11.520 may not be there, but; obviously, there will be super heated vapour which is to be cooled 00:18:11.520 --> 00:18:17.720 first and then it has to be condensed. Next if you see what is happening in case 00:18:17.720 --> 00:18:23.340 of a boiler. Let us say it is a steam power boiler ah steam power plan boiler. So, here 00:18:23.340 --> 00:18:31.650 sub cooled water goes to the boiler and then there will be vaporization and then there 00:18:31.650 --> 00:18:37.451 is super heated liquid. And let us say this is the flue gas . ah Only thing is that in 00:18:37.451 --> 00:18:44.309 a power plant boiler this may not be a single heat exchanger the super cool sub cool liquid 00:18:44.309 --> 00:18:50.630 and the vaporizer and the super heated liquid or super heated vapour should be super heated 00:18:50.630 --> 00:18:55.880 vapour. So, they are not in the same heat exchanger. So, initially there could be some 00:18:55.880 --> 00:19:02.410 sort of economizer, then there could be ah boiler drum and the raiser tubes in which 00:19:02.410 --> 00:19:07.170 vaporization will take place. And then there could be a super heater. 00:19:07.170 --> 00:19:16.470 So, but one can think of some sort of an evaporator ah where there could be hm sub cool liquid 00:19:16.470 --> 00:19:22.110 to ah ah super heated vapour coming from the evaporator and all the three parts will be 00:19:22.110 --> 00:19:28.510 present . Why I am ah presenting this kind of pictures you know, because you have to 00:19:28.510 --> 00:19:34.360 appreciate one thing that real phase change heat exchanger are complex in nature. The 00:19:34.360 --> 00:19:40.640 problem which I have done at the beginning of this class may give a misconception ok, 00:19:40.640 --> 00:19:47.780 things are easy because one side the fluid temperature is not changing. One may think 00:19:47.780 --> 00:19:53.270 that ok one side the fluid temperature is not changing, so, things are little bit easy. 00:19:53.270 --> 00:20:00.210 But things are not easy because ah rather the things are more complex because we have 00:20:00.210 --> 00:20:07.970 got this kind of abrupt change in the temperature curve. If there is abrupt change in the temperature 00:20:07.970 --> 00:20:19.970 curve, then what happens ? Basically we are having three heat exchangers. So, we can think 00:20:19.970 --> 00:20:30.000 that this is one heat exchanger [noise], this is a [noise] purely phase change heat exchanger 00:20:30.000 --> 00:20:57.480 and this is another heat exchanger. So, let me write [noise] heat exchanger 1 [noise] 00:20:57.480 --> 00:21:03.850 heat exchanger 2 [noise] and we are having [noise] heat exchanger [noise] 3 [noise]. 00:21:03.850 --> 00:21:09.820 So, 3 heat exchanger. So, when I have to do the analysis I have to do the analysis for 00:21:09.820 --> 00:21:16.210 3 heat exchanger. Then there is another thing let me tell you, probably I will take up this 00:21:16.210 --> 00:21:23.900 point later on also. Suppose I am a normal two fluid heat exchanger 00:21:23.900 --> 00:21:36.860 and both the fluids are are [noise]. So, let us say this is a counter current 2 fluid heat 00:21:36.860 --> 00:21:43.540 exchanger and both the fluids are in single phase. So, this side it is temperature and 00:21:43.540 --> 00:21:50.630 this side it is length. So, here you see the ah counter current heat exchanger and how 00:21:50.630 --> 00:21:57.730 I will decide the length of the heat exchanger. One consideration is that the minimum temperature 00:21:57.730 --> 00:22:04.830 difference which can be allowed for heat transfer [noise] to take place delta T minimum [noise]. 00:22:04.830 --> 00:22:09.480 One can call it terminal temperature difference. So, what is the minimum terminal temperature 00:22:09.480 --> 00:22:14.880 difference we can allow? That is how i have to this is one kind of a decision the design 00:22:14.880 --> 00:22:21.350 engineer has to take and this takes place at one end of the heat exchanger; that means, 00:22:21.350 --> 00:22:28.420 this terminal temperature difference is taking place at one end of the heat exchanger. 00:22:28.420 --> 00:22:34.620 In case of this phase change heat exchanger where there are sub cooling and super heating, 00:22:34.620 --> 00:22:44.059 the minimum temperature takes place here which is called pinch [noise]. So, this gives one 00:22:44.059 --> 00:22:49.210 restriction to the heat exchanger design and this is happening inside the heat exchanger. 00:22:49.210 --> 00:22:55.240 So, this is very important and we have to maintain a pinch temperature probably I will 00:22:55.240 --> 00:23:03.730 elaborate this as we proceed on and that is how another complexity is added to your phase 00:23:03.730 --> 00:23:09.120 change heat exchanger. We will ah elaborate this thing as we will proceed. 00:23:09.120 --> 00:23:17.520 So, ah for phase change heat exchanger we have seen one hm ah unique feature that the 00:23:17.520 --> 00:23:23.309 minimum temperature difference between the two fluids that can take place somewhere in 00:23:23.309 --> 00:23:31.780 between the heat exchanger. Particularly when sub cooling or super heating or both are involved 00:23:31.780 --> 00:23:39.790 in a phase change heat exchanger . So, once again I ah like to draw your attention to 00:23:39.790 --> 00:23:46.780 this particular figure. So, ah I have taken a typical case of a boiler where the flue 00:23:46.780 --> 00:23:55.090 gas is heating the ah liquid ah. So, that from the sub cool state the liquid goes to 00:23:55.090 --> 00:24:00.730 the saturated condition, then vaporization takes place and then we are having super heated 00:24:00.730 --> 00:24:04.560 vapour. So, super heated liquid it has been written, 00:24:04.560 --> 00:24:09.460 but it it is in the super heated condition. So, it is super heated vapour . So, probably 00:24:09.460 --> 00:24:15.510 in a boiler, we will find this kind of an ah phenomena or in a condenser also, we can 00:24:15.510 --> 00:24:24.000 find out the sub cool region then ah the phase change region and then ah super heated region 00:24:24.000 --> 00:24:30.590 in the reverse order. That means, in case of a condenser it will start from hm super 00:24:30.590 --> 00:24:37.000 heated vapour, it will go to the phase change zone and then it will be in the sub cool condition 00:24:37.000 --> 00:24:43.980 depending on the process requirement . Now the point which I liked to make is that that 00:24:43.980 --> 00:24:51.350 initially let us say we are considering a condenser. So, initially the ah ah fluid is 00:24:51.350 --> 00:24:59.429 at vapour state, its density is low and then it is in the 2 phase region the ah quantity 00:24:59.429 --> 00:25:05.090 of vapour reduces and the quantity of liquid increases and then it comes out totally in 00:25:05.090 --> 00:25:09.460 the sub cool region. So, you can imagine that from the beginning 00:25:09.460 --> 00:25:17.150 of the from the start of the ah heat exchanger to its end, there is a large change in ah 00:25:17.150 --> 00:25:25.410 the density of the fluid or fluid mixture ah. So, volume flow rate is changing . So, 00:25:25.410 --> 00:25:31.990 ah it is a challenge to the designer that how we can accommodate it in a single heat 00:25:31.990 --> 00:25:38.060 exchanger. Let be elaborate this let us go to the next 00:25:38.060 --> 00:25:47.190 slide. In the next slide I have shown the schematic diagram and cutaway view of a ah 00:25:47.190 --> 00:25:55.740 ah of an automotive condenser. You know in cars or automobile, there will be for comfort 00:25:55.740 --> 00:26:00.470 condition to ensure comfort condition. There will be a refrigeration system and in there 00:26:00.470 --> 00:26:06.350 there will be a condenser ah. Vapour compression refrigeration system; so, there will be a 00:26:06.350 --> 00:26:13.790 compressor up ah after the compressor the compressed vapour; that means, which is at 00:26:13.790 --> 00:26:19.990 superheated state and high pressure that will enter the condenser. So, in vapour state it 00:26:19.990 --> 00:26:26.910 will enter, then it will ah become liquid and in many cases it will come out of the 00:26:26.910 --> 00:26:33.660 condenser as sub cool liquid . So, you see from super heated vapour to sub 00:26:33.660 --> 00:26:38.811 cool liquid, there is lot of change in ah density lot of change in flow rate. How do 00:26:38.811 --> 00:26:45.970 we take care of it ? So, we take care of it very ah interestingly. We take care of it 00:26:45.970 --> 00:26:51.890 in this particular heat exchanger by providing different pass. So, you see there are different 00:26:51.890 --> 00:26:59.770 passes, this is the inlet this is the first pass and ah then this is the second pass pass 00:26:59.770 --> 00:27:07.429 2 and then this is pass 3 and then this pass 4. Well the passes are not ah very uncommon 00:27:07.429 --> 00:27:10.280 in heat exchanger. In heat exchanger, there could be number of 00:27:10.280 --> 00:27:18.030 passes and here also we are having, but what is to be noted in the first pass where the 00:27:18.030 --> 00:27:26.130 super heated vapour is flowing, there are ah 14 tubes . All the tubes are of same dimension. 00:27:26.130 --> 00:27:32.110 So, 14 tubes means large cross sectional area as the density is low for vapour we have to 00:27:32.110 --> 00:27:37.809 provide large cross sectional area . Then in the second pass the vapour is now being 00:27:37.809 --> 00:27:44.170 cooled and it will go to the ah two phase region in the second pass there are 7 tubes. 00:27:44.170 --> 00:27:51.841 So, drastic reduction in the number of tubes and in the third pass there are 6 tubes and 00:27:51.841 --> 00:27:57.410 fourth pass the fluid will be in the sub cooled liquid condition, then there will be only 00:27:57.410 --> 00:28:04.510 4 tubes; pass 4, there will be 4 tubes . So, you see this is one opportunity to see 00:28:04.510 --> 00:28:10.480 a cutaway view of the heat exchanger . This is a cross flow kind of heat exchanger which 00:28:10.480 --> 00:28:18.410 is very common in ah refrigeration application, refrigeration condenser . So, tube side refrigerant 00:28:18.410 --> 00:28:24.190 flows and the tubes are also very unique. You can see the tubes this are flatten tubes 00:28:24.190 --> 00:28:30.250 and in one tube, there are seven passages . So, basically what we are saying a cross 00:28:30.250 --> 00:28:39.150 flow two face heat exchanger using ah micro tubes because this ah passages are of small 00:28:39.150 --> 00:28:45.840 dimension. This is also a compact heat exchanger because, the area density area of heat transfer 00:28:45.840 --> 00:28:52.760 area density per unit volume is very large . And so, this is cross flow refrigerant is 00:28:52.760 --> 00:29:00.400 flowing in this direction or in this direction depending on the ah which pass it is and then, 00:29:00.400 --> 00:29:06.059 it is air cooled or rather the heat transfer is ah to air. 00:29:06.059 --> 00:29:12.080 So, ah air passage is provided by this fins. So, these are the different types of fins 00:29:12.080 --> 00:29:17.750 ah sorry these are the fins they are attach to the flatten tube. So, on through the fin 00:29:17.750 --> 00:29:23.170 side through the fin side air passes. So, this is very unique heat exchanger it has 00:29:23.170 --> 00:29:29.250 got. So, many speciality. Let me point out ah all of them one by one . First thing it 00:29:29.250 --> 00:29:34.970 is a phase change heat exchanger , second thing in this phase change heat exchanger 00:29:34.970 --> 00:29:41.010 both ah three I mean, all the three regions are there; super heated vapour region, then 00:29:41.010 --> 00:29:46.660 two phase region, then the sub cooled region . Third it is a cross flow heat exchanger. 00:29:46.660 --> 00:29:53.490 It is a compact type heat exchanger ah flatten tube are used and ah in this tubes there are 00:29:53.490 --> 00:30:02.370 7 passages in each tube and the air side is ah made by ah fins. And then ah there are 00:30:02.370 --> 00:30:10.179 4 passes and ah the number of tube per pass is not uniform to take care of the change 00:30:10.179 --> 00:30:16.789 in volume. Alright. So, ah one thing I like to mention that we 00:30:16.789 --> 00:30:23.240 have started our discussion on phase change heat exchanger. Most of the cases I will deal 00:30:23.240 --> 00:30:30.910 with condenser ah ; that means, liquid is condensing. The reverse is the evaporation 00:30:30.910 --> 00:30:38.390 or boiling of liquid; though we call evaporation, but the exactly reverse of it will be boiling 00:30:38.390 --> 00:30:49.059 of liquid and boiling of liquid is needed in many places . But where we are ah ah boiling 00:30:49.059 --> 00:30:56.580 the liquid with the direct application of heat like ah firing cool or nuclear power 00:30:56.580 --> 00:31:05.880 or hm ah or ah some sort of coil or in some cases electric ah heating in small ah sized 00:31:05.880 --> 00:31:13.669 unit. So, we call it a boiler. So, where particularly where there is firing, 00:31:13.669 --> 00:31:20.690 we call it a boiler , but we can have boiling in other places also like in a refrigeration 00:31:20.690 --> 00:31:28.090 system the refrigerant will boil ah, but that we call as evaporator . So, what I will do 00:31:28.090 --> 00:31:35.679 as mostly I have discussed condenser. Evaporators are not very different from them. So, I think 00:31:35.679 --> 00:31:44.290 one can pickup, but boiler I have not discussed. So, I like to briefly discuss boiler ah hm 00:31:44.290 --> 00:31:47.520 before I end this lecture. So, let us go to the next slide . 00:31:47.520 --> 00:31:57.160 So, this shows the schematic diagram of a boiler. So, first I have shown super critical 00:31:57.160 --> 00:32:03.380 boiler sorry sub critical boiler where the pressure of the ah working fluid is below 00:32:03.380 --> 00:32:08.011 the critical point and then I show the super critical boiler. Here the pressure of the 00:32:08.011 --> 00:32:17.159 hm working fluid that is low at the beginning, but this pressure raises and ultimately this 00:32:17.159 --> 00:32:26.470 pressure goes above the critical point. So, ah I have ah given a name to this slide 00:32:26.470 --> 00:32:31.290 boiler as a heat exchanger. So, these two heat exchanger; these are basically heat exchanger, 00:32:31.290 --> 00:32:37.700 they are slightly different. Because in the sub cool condition particularly if we are 00:32:37.700 --> 00:32:44.299 away from the critical point, there is a large difference between the density of liquid and 00:32:44.299 --> 00:32:52.520 vapour even at a particular saturation pressure or particular saturation condition . So, the 00:32:52.520 --> 00:32:58.690 same problem occurs here which I have discussed that I have to handle sub cool liquid in the 00:32:58.690 --> 00:33:06.049 economizer, then I have to handle a two phase mixture in the vaporizer portion of the boiler 00:33:06.049 --> 00:33:10.620 and then I have to handle super heated vapour in the super heater of the boiler. 00:33:10.620 --> 00:33:18.049 So, this is ah true for a subcritical boiler and generally what happens boiler is a very 00:33:18.049 --> 00:33:23.710 big ah unit. So, there these three though there is a connection, these three are three 00:33:23.710 --> 00:33:30.410 different kind of heat exchangers. Grossly if I call them heat exchangers, then these 00:33:30.410 --> 00:33:38.000 three are 3 different types of heat exchangers. So, this is your ah economizer and in economizer, 00:33:38.000 --> 00:33:42.750 we will find the type of heat exchanger we have studied it is not much different from 00:33:42.750 --> 00:33:50.309 that . Then ah evaporator which is not this section, but this loop kind of a section. 00:33:50.309 --> 00:33:55.080 Here of course, we are having a completely different kind of arrangement which I will 00:33:55.080 --> 00:33:59.909 explain little bit . Ah This is the main part of the boiler. We 00:33:59.909 --> 00:34:05.480 do not have much scope to discuss this in this particular course because boilers are 00:34:05.480 --> 00:34:11.149 considered as fired heat exchanger and in our course fired heat exchanger, we have excluded 00:34:11.149 --> 00:34:19.489 only a glimpse of it I will discuss . And then after that we will have ah the saturated 00:34:19.489 --> 00:34:24.629 vapour which will be taken to the super heated condition at the corresponding pressure in 00:34:24.629 --> 00:34:30.349 the super heater [noise]. Whereas, in the super critical boiler, it is sometimes also 00:34:30.349 --> 00:34:37.150 called ah it is called once through boiler because the fluid passes continuously may 00:34:37.150 --> 00:34:43.460 be through different ah sections of the tube and it enters as in the sub cool liquid in 00:34:43.460 --> 00:34:51.029 the economizer and comes out as super heated vapour at the end of the super heater . Whereas, 00:34:51.029 --> 00:34:56.319 if I see the subcritical boiler what we will find there is a loop kind of a thing, here 00:34:56.319 --> 00:35:03.979 we will have liquid circulation or liquid and vapour circulation which is ah I mean 00:35:03.979 --> 00:35:09.640 to some extent different from ah in concept ah from the other kind of heat exchangers. 00:35:09.640 --> 00:35:16.009 So, you will look it look at it in to little bit details. Let us go to the next slide [noise]. 00:35:16.009 --> 00:35:24.289 . So, in this slide , schematically a ah 300 megawatts subcritical power plant is show 00:35:24.289 --> 00:35:29.119 ah of course, not the entire power plant may mostly we have concentrated on the boiler 00:35:29.119 --> 00:35:35.789 side. So, this is the furnace. So, furnace is part and partial of the boiler. So, you 00:35:35.789 --> 00:35:45.839 see boiler ah hm income process um um many kind of ah hm operations like one is furnace 00:35:45.839 --> 00:35:51.099 where heat generation will be there and at the same time the heat exchanger part of it 00:35:51.099 --> 00:35:57.230 where ah the from the hot gas or from the hot source of combustion, the heat will be 00:35:57.230 --> 00:36:06.069 transferred to the fluid . So, the furnace ah here the coal or oil or ah hm some suitable 00:36:06.069 --> 00:36:11.799 fuel will be fired, it will be generated. So; obviously, the furnace is some sort of 00:36:11.799 --> 00:36:20.259 a enclosed body and at the wall of the furnace, we can have the tubes these are vertical tubes 00:36:20.259 --> 00:36:24.880 and through this vertical tubes ah liquid will pass. And when it is passing through 00:36:24.880 --> 00:36:28.920 vertical tube, some amount of vapour will be generated. 00:36:28.920 --> 00:36:37.349 . So, if we go to the ah hm previous slide once, ah what we can find that this is the 00:36:37.349 --> 00:36:45.999 furnace wall; here heat is given to the flowing water. So, it will get evaporated, but it 00:36:45.999 --> 00:36:51.009 will not get completely evaporated, partly it will get evaporated and it will come this 00:36:51.009 --> 00:36:58.000 ah hm come to this ah circular section which is called steam drum where the vapour and 00:36:58.000 --> 00:37:06.630 liquid will get separated . The vapour will ah go to the super heater and the liquid this 00:37:06.630 --> 00:37:11.269 is called down comer through the down comer it will come, there is a small drum at the 00:37:11.269 --> 00:37:18.329 bottom. And from this drum, again it will go up which is called raiser. So, in a boiler, 00:37:18.329 --> 00:37:24.759 you see there are two very important component; one is a raiser and one is a boil ah one is 00:37:24.759 --> 00:37:30.900 a down comer and liquid circulation is hm continuously ah through this raiser ah to 00:37:30.900 --> 00:37:39.680 the steam drum and to the down comer. Here there could be a pump ah, it could be ah assisted 00:37:39.680 --> 00:37:47.130 by pump or the circulation could be assisted by buoyancy. So, this is completely a different 00:37:47.130 --> 00:37:53.239 kind of heat exchanger though it is heat exchanger, but it is a completely different kind of heat 00:37:53.239 --> 00:38:00.749 exchanger what we find in case of boiler . Let us go back to the ah next slide. 00:38:00.749 --> 00:38:06.900 So, here what we can see. So, this is the furnace wall. So, this wall are to be kept 00:38:06.900 --> 00:38:14.049 cool hm ah. So, what we do then ah we ah provide the ah water tube through this? So, that is 00:38:14.049 --> 00:38:20.029 why these are called water wall. So, it ah hm ah absorbs the heat some amount of steam 00:38:20.029 --> 00:38:25.329 is generated is goes to the drum and then through the down comer it comes . Now what 00:38:25.329 --> 00:38:31.109 is to be noted that in a boiler, there are number of heat exchanger and many of them 00:38:31.109 --> 00:38:38.690 are phase change heat exchanger . So, ah here ah hm of course, some of them are not phase 00:38:38.690 --> 00:38:43.630 change heat exchanger, but some of them or many of them are phase change heat exchanger 00:38:43.630 --> 00:38:50.489 ah. The way I have shown it this is ah to make this things simple ah, because ah hm 00:38:50.489 --> 00:38:58.049 ah it will be easy to ah understand, but actual boiler there will be ah lot of complexities 00:38:58.049 --> 00:39:04.930 and pipe layouts are quite complex.So, I will ah show another slide to give you some sort 00:39:04.930 --> 00:39:08.589 of an idea . So, what kind of heat exchanger what we can 00:39:08.589 --> 00:39:15.900 see here? Here we can see super heated hm ah super heater . So, this super heater will 00:39:15.900 --> 00:39:20.180 be a single phase change heat exchanger this boiler evaporators section that will be a 00:39:20.180 --> 00:39:26.069 two phase heat exchanger. Here we can have feed water heater which is a ah single phase 00:39:26.069 --> 00:39:31.749 heat exchanger. But here what we can have feed water heater. So, these are again two 00:39:31.749 --> 00:39:36.170 phase change heat exchanger . So, let us go back to some sort of a boiler 00:39:36.170 --> 00:39:42.079 ah. This is how it looks; you can see the vertical lines. These are all ah tubes and 00:39:42.079 --> 00:39:48.200 furnace wall is surrounded by this tubes. So, that the heat absorption is there by this 00:39:48.200 --> 00:39:56.109 ah by the ah water flowing through this tube and tube wall is also kept ah cooled ok. And 00:39:56.109 --> 00:40:02.170 here we can see through this nozzle ah the fuel is injected either in the form of pulverized 00:40:02.170 --> 00:40:06.650 coal or in the form of oil and ah combustion takes place. 00:40:06.650 --> 00:40:13.380 So, other heat exchangers if you closely look ah hm into this figure you will find other 00:40:13.380 --> 00:40:20.839 heat exchanger re heater super heater etcetera you will find . So, there is some air preheater 00:40:20.839 --> 00:40:23.779 also which is a gas to gas heat exchanger . 00:40:23.779 --> 00:40:29.130 Quickly let us go to the next slide which is the last slide of my hm this one ah of 00:40:29.130 --> 00:40:34.680 this lecture. So, here we can see one heat recovery steam generator. So, basically no 00:40:34.680 --> 00:40:42.789 combustion is there in this case or may be some auxiliary combustion is there with the 00:40:42.789 --> 00:40:46.559 help of burner. So, prior to this there could be some sort of a gas turbine plant and the 00:40:46.559 --> 00:40:51.660 exhaust gas of the gas turbine which is at high temperature it is coming . So, what I 00:40:51.660 --> 00:40:57.059 want to show that in ah boiler, there are three operation the sub cooled water is to 00:40:57.059 --> 00:41:05.589 be brought to the ah saturated condition . The saturated ah liquid is to be evaporated and 00:41:05.589 --> 00:41:12.510 then the saturated vapour is to be superheated. And though in small kind of heat exchanger 00:41:12.510 --> 00:41:18.349 we can do it in same heat exchanger in a boiler, there are three elaborate section. This is 00:41:18.349 --> 00:41:25.930 the economizer where the liquid temperature is ah ah increased and it comes to the saturated 00:41:25.930 --> 00:41:31.410 condition . Then this is the boiler where through circulation, we generate steam evaporator 00:41:31.410 --> 00:41:38.930 or vaporizer. And after this steam generated we have got the super heater where the saturated 00:41:38.930 --> 00:41:44.940 steam temperature is increased. So, this is how we are having a boiler plant. 00:41:44.940 --> 00:41:50.999 So, this gives in a nut cell some ah introduction to phase change heat exchanger. We will see 00:41:50.999 --> 00:41:56.440 more of it, we will solve problem in our ah next classes; next lectures. 00:41:56.440 --> 00:41:58.500 Thank you thank you for your attention.
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