 So, we talked about various gases. Do you know the boiling points of these gases? For example, nitrogen. What is the boiling point of nitrogen? Atmospheric pressure. So, this is something which you ought to know. Boiling point of nitrogen is 77 Kelvin. Boiling point of oxygen is 90 Kelvin. Alright, what is the boiling point of liquefied natural gas? LNG. Right now, LNG is used everywhere. It's around 110 Kelvin. So, this is LNG containers, LNG transport. All these are very, very important things which also comes under cryogenic gases and liquefaction of gases. What is the boiling point of helium? Somebody talked about helium. 4 Kelvin. 4.2 Kelvin. 20 Kelvin is the boiling point of hydrogen. Boiling point of argon. So, why don't you write down yourself? These boiling points are very important. Write from oxygen if I talk about. Maybe I can just write on the board so that you can possibly have the data with you. So, the boiling point of oxygen is 90 Kelvin. Boiling point of nitrogen is 77 Kelvin. Actually, it's 77.4 but I'm just writing right now like that. Boiling point of argon is 87 Kelvin. Boiling point of hydrogen or let's say neon is 27 Kelvin. Boiling point of hydrogen is 20 Kelvin. And boiling point of helium is 4.2 Kelvin. Now, you understand the importance. You can see oxygen gas everywhere in the hospital. Where does it come from? It comes from air. So, we separate out all the gases from air from where you get oxygen, nitrogen, argon, hydrogen, helium. Also, there are various sources from which, from where different gases can be obtained. But one of the most important usages of cryogenic engineering is to get these gases. So, we liquefy air first at 78 Kelvin and separate all these gases by fractional distribution, distillation. All right. So, this is very important application of cryogenics that otherwise and everything started with oxygen therefore, because we had so many wars and everybody wanted oxygen. And therefore, in order to have oxygen gas in the hospitals, cryogenic actually came into existence around late 1800. And I'll not go into details of that because this is what I teach in the cryo one course which you possibly can take next semester. Now, every gas actually can be demonstrated on or can be shown or represented on a temperature entropy diagram. So, this is the temperature and this is the entropy. I'm not sure whether you have seen but you might have seen pressure enthalpy diagram for example, but temperature entropy diagram is what normally we deal with. So, imagine you are here 300 Kelvin, what we normally call as room temperature, 27 degree centigrade and 1 atmosphere. In order to liquefy any of the gases, I have to come in this dome and you understand this dome, right? Because you have studied water and you know the dome and you know the latent heat and all that thing. So, this is a constant pressure line on this and from here you have to come inside the dome and this could be much below what you call as cryogenic temperature. This could be 77 Kelvin. So, point E is saturated vapor and this is saturated liquid. What will EF length correspond to? It corresponds to the latent heat of that particular fluid, right? So, we are talking about bringing temperature down from this to this, from 300 Kelvin to this and what do we do normally in order to do all these things? We have to compress the gas and expand the gas. That is what normally we do. So, you have to deal with such gases and every gas has got such independent data. What is this point called as? Critical point, all right? It's very important to have a critical point. Every gas will have its critical point, critical pressure and all those things. Every gas will have the latent heat for a given pressure. So, we have to deal with all these things. This is called normal boiling point and a normal boiling point let's say nitrogen is 77.4 Kelvin and this therefore point will be 77.4 Kelvin and every gas has therefore to be represented in this, all right? So, this is the diagram which every cryogenic engine has to use and this is the data which is available to everybody. On this diagram, you have got density, enthalpy, plot and everything will be given on this. Now, in order to liquefy the gases, I will have to come inside this dome. This is what we call as a dome and unless you come inside this, you will not get a liquefaction of gas. You understand this? Everything above the critical point normally we call as gas. Everything below the critical point we call as vapor. Anything is vapor condition or gaseous condition it will be here and what exists between the dome is liquid plus vapor which is a two-phase and two-phase heat exchanger, two-phase heat transfer is very common in cryogenic engine. We have to deal with that every time. I am sure in the next lectures you will deal with all the two-phase heat transfer, etcetera. I will just give a brief of summary, different summary. So, one of the major applications of cryogenics is cryogenic engine which is a space, all right? Which is space related. We are working very closely with ISRO on rocket propulsion also on cooling of IR detector, infrared sensor. We are also working on cryo-surgery, cell preservation, food preservation, one of the PADs working on cryo-surgery. You actually freeze the cells, cancerous cells and you can get rid of those cells, especially for the breast cancer we are working in our laboratory. Then all mechanical related manufacturing, heat treatment, recycling, you must be knowing about soon, the Large Hadron Collider is completely working at 1.8 Kelvin. They are all dipped in 4.2 Kelvin of helium and then you remove the vapor pressure over there and you come down to 1.8 Kelvin. So, the very important application from where actually cryogen is originated. And the next important project that the entire world is looking at is called ITER project. Have you heard of ITER project? If you just Google it, you can find. This is a fusion reactor now. So, bringing sun to Earth. This is for maybe your next generation project that if we had to consume all the power available with coal, etcetera, we will not have anything for you. This is the next generation answer for ITER for power generation. The biggest application is gas industry where liquefaction, separation and storage is very, very important. If you go out, you can see on Bombay Pune Road big trucks taking Linde, have you seen a company called Linde? Have you heard about this company? They make gases. Inox makes gases again, Indian company. And then superconductivity is the major application where all the applications related to cryogen is will result in a superconducting magnet which is normally used in our MRI and NMR applications. This is what brief outline about cryogenics. One of the most important components in cryogenics is heat exchanger which is what our major objective of this lecture is. And I don't want to really go through all this, but you know that heat exchanger is a device in which the cooling effect from cold fridges transfer to pre-cool the hot fluid. Now, it's a very important thing that you may not have considered. You can have two, three, four fluids simultaneously flowing. And this is one of the very important aspects related to cryogenics. That I can have three different gases, three different phases. You can have one gas, one liquid, one two phase and you have to analyze such heat exchangers which is very critical. So you can have a two fluid heat exchanger, you can have a three fluid heat exchanger also. And the process of heat exchanger occurs at constant pressure and whatever if you call it isobaric but in actual cases as you know there's a pressure drop and consideration of pressure drop is a very important thing. I'm not sure whether you can see this but heat exchangers can be classified based on various things according to heat transfer process according to surface compactness. Please see the surface compactness. Then classification according to construction, tubular plate type, extended fin, regenerative heat exchanger or flow arrangement which you know parallel flow, counter flow and cross flow. And number of fluids, two fluid, three fluid, four fluids. Classification according to heat transfer process, single phase, two phase, combined, convention, convection, radiation whatever. We'll not go through this. The use of heat exchangers in cryogenics major will look at two applications. As I said there are several applications but for your class I will choose two applications. Almost every equipment has heat exchanger in cryogenics because you see you are taking, you are putting in lot of power to go down to lower in temperature and this lower temperature or enthalpy at low temperature cannot be wasted. It has to be recuperated. It has to be taken when the gas gets warmed up we want to have that cold enthalpy to be preserved, to be used for pre-cooling something. The big application that I am going to talk about is when is the gas liquefaction or air liquefaction is a major industry, 80% industry works on air liquefaction. And the second is cryocoolers which is a refrigerator working at very low temperature. You got a domestic refrigerator at home which gives a temperature of minus 5 degrees centigrade or minus 10 degrees centigrade. Let's say lowest temperature. But we have made cryocoolers which can actually come down to 4 Kelvin and that can be used for various applications. And they work in a closed cycle. The way you got a domestic refrigerator working in your house for years together, isn't it? Nowadays they work for 5 years or 10 years you don't have to bother about it. Similarly, you got cryogenic refrigerator or cryocoolers which actually bring down the temperature to very, very low temperature. You can have cryocoolers working at 1 Kelvin also. And I work for 5 years working in a factory where the temperatures were less than 1 Kelvin, 0.5 Kelvin, 10 mili Kelvin and you have to pay equivalent for that. So one cryocooler in such a case would cost around 3 to 4 crores also. Okay. So let's say thermodynamically ideal system that is what normally we did. The first thing is ideal, second thing is going to be practical system or actual system. How do I liquefy air? First part, as I said, is liquefaction. If I say ideal system that means whatever is compressed is liquefied. So you can see here you got a compressor and you got an expander and because we want everywhere the gas to be liquefied that means we want to increase its pressure first and decrease its pressure because of which the temperature will come down. So every ideal system will have everything which is getting compressed, getting expanded and will get liquefied. So you can see here compressor, expander and liquefaction. And if I were to represent this on a TS diagram, because whatever has got compressed has got expanded, has got liquefied. On a TS diagram, this is isentropic expansion and therefore you can see vertical line coming down here. The gas gets compressed and expanded and that's it. This red line has no meaning. Can you understand this? I am showing it on two pressure lines. This pressure, this is isothermal compression. Ideal process. So from 1 to 2 I am compressing the gas and from 2 to F I am expanding the gas isentropically and resulting in 100% liquid. So whatever gas was compressed, has expanded, has reached 100% liquid at this point. This is what we call as ideal cycle. Is this an open cycle or a closed cycle? Is this an open system or a closed system? Close system? Is this an open system or a closed system? Is the gas going back here? No, I am getting the next gas in. The mass is transferring, isn't it? Every time the fresh gas is coming, gas gets liquefied, that's it. All the gas is getting liquefied. So this is an open system. This gas is not going back. I have just connected it. But this gas comes here and gets expanded and that's it. It becomes liquid. But such a system is not possible. This is an ideal system and this system is not possible. Why? This system is not possible because the pressure at point 2 is going to be of the order of 70,000 bar. In order that all this gas gets liquefied, the pressure at point 2 is going to be 70,000 bar. On a TES diagram, you can see for nitrogen, for example, such a pressure cannot be generated by any compressor. Otherwise, my compressor will be as big as this building and we'll never get that kind of pressure. So we never can have all the gas that is coming down here getting liquefied. And therefore, we do it to a pressure which is possible by us. If we were to do that, all that thing, then I have to go to now actual system. The ideal system of compressing all the gas up to this point is impossible and therefore now I've got something called Linday-Hamsen system. Or Linday. Linday is a person's name after whom the company has come out. All right? And this system would look like this. What is the difference between the first and the second system? The first system, what was the difference? What is the difference between the first system and the second system? The ideal system and the Linday-Hamsen cycle? Yeah, this is a kind of a close system. Actually, it's not exactly close, but the gas goes back because some gas gets liquefied and it will come out. All right? But what you see here is a presence of heat exchanger. In the first system, there was no heat exchanger. Whatever gas got compressed, expanded and we got liquid. Now, whatever gas got compressed, will get come down here and the part of the gas will get liquefied and the remaining gas will go back because the remaining gas has cooled down but it has not got liquefied. All right? So, some of the gas that got compressed here got expanded, came down here. It liquefied a part of it and the remaining gas which was at low temperature, it goes back to the compressor again, like a domestic actually start imagining like always like a domestic refrigerator. In the domestic gas, however, we don't get liquefied component. It is just to produce refrigeration. But here I want liquid gas. All right? I want this gas to liquefied. So, I will take this out. But then the remaining gas at low temperature is going back and I don't want to let it go like that. I would like to recollect all the enthalpy that it has at low temperature and this is done by this heat exchanger. And now, if I were to represent the entire cycle on the TS diagram, this will look like this. All right? So, now this is the heat exchanger process. Heat exchanger is between 2 to 3 and G to 1. Now, you can see that the gas M dot is compressed from 1 to 2. From 2 to 3, the heat exchanger the gas will get cooled and then it gets isenthalpically expanded. I am sure, you know the Joule Thompson expansion or isenthalpic expansion or throttling process. The remaining gas at G will go back and while it is going back, how much gas is going back? Because out of M dot, Mf dot has got actually liquefied. The return gas will have M minus Mf. All right? So, this M minus Mf is going back and again it gets compressed and whatever gas was liquefied is added at the top. So, this is what is called a make-up gas. It is what is called a make-up gas. So, this is the way the cycle works. Out of M dot, Mf dot is getting liquefied and M dot get added and this is this is the heat exchanger working therefore between 2 to 3 and G to 1. If I were to represent this on a TS diagram you can see here 1 to 2. So, 2 to 3 is isobaric cooling process and then expansion. Now, because it has fallen into JT, you can see that the part of the gas got liquefied remaining gas goes back. Now, the heat exchanger process happens between 2, 3 and G1. Can you understand this? The heat exchanger is happening between 2, 3 and G1 and this is the most important thing now and I will now spend more time on this. So, I can actually put a first law across this and develop a relation as to how much gas has got liquefied. I can do energy balance here. The energy balance will show that what is entering is M minus Mf at 0.1 here and Mf at 0.f. So, I can do the energy balance and calculate a ratio of how much gas got liquefied out of what was compressed. So, if I do the energy balance, I can say M1 H1 is the enthalpy that is coming here. Compressor work, QR which is the heat of compression, if at all work done and this is my expression. M dot f which is liquefied is fraction gas liquefied is H1 minus H2 divided by enthalpy at 0.1 and enthalpy at 0.f and this is what we say that the fraction of gas that got liquefied is going to be M dot f upon M dot is equal to H1 minus H2 upon Mf H1 minus Hf. So, it depends on enthalpy difference across the compressor divided by H1 minus Hf. The fraction of gas liquefied what we call as Y which is equal to this. So, Y depends on the initial because H1 I can show you that here. H1 minus H2. So, the moment your pressure gets fixed your point f also is fixed because that is the latent heat associated with one bar pressure or whatever to start with. Now, important is to understand what is the role of heat exchanger here because entirely you can see that the return gas is going back with lot of cold enthalpy which we want to actually capture. So, the heat exchanger role is this. I think you know this heat exchanger works across the hot fluid temperature versus length diagram like that. You have been taught about this right and therefore you have got the heat exchanger definition. This is the hot side this is the cold side and I got a definition of epsilon which is Q actual upon Q maximum. Actual heat transfer divided by maximum possible heat transfer. And this epsilon value the heat exchanger effectiveness will lie between 0 and 1. So, this is my heat exchanger and the temperature versus length diagram and you know mCpB into delta T of that particular fluid or mA fluid A or fluid B and it is respective delta T across it and the Q actual or Q maximum is going to be mCp minimum of these two into maximum temperature difference that you have all right. So, Tb in minus Ta in and this effectiveness therefore will be Q actual upon Q maximum. Now, can I apply this heat exchanger effectiveness definition like the and here we talked about process of compression from 1 to 2 then we talked about 3 to 4 as expansion process and then we talk about heat exchanger process if we are having 100 percent efficient heat exchanger this 2 plus 2 to 3 process is having heat exchange with G21 process. So, there are two streams going on when as high pressure and when at low pressure the high pressure stream comes at 2 enters the heat exchanger at 0.2 and 3 and this gas is getting pre cooled by the gas which is going back what is the gas going back at this point will be how much gas is there m minus mf and what is the gas coming here m dot you understand this m dot is coming out of this m dot m dot f has actually got liquefied and the gas which is returning is going to be m dot minus mf dot. So, this stream has m minus mf this stream effectiveness we have to consider if it were 100% you have got 2, 2, 3 and G21 but in no case none of the heat exchanger are 100% efficient alright in actual the heat exchanger is not perfect and hence these processes will become now instead of coming from 2, 2, 3 it will come up from 2, 2, 3 dash alright instead of coming from 2, 2, 3 it will come from 3 dash what is the effect start from here and you will get this point here and instead of going from G21 it will go to G21 dash because of effectiveness not being 100% is this clear just because the heat exchanger effectiveness not 100% instead of going from G21 it would go from G21 dash instead of coming from 2, 2, 3 it will come from 2, 2, 3 it will come from 2, 2, 3 dash and because of which what has happened now the gas will get compressed from 1 dash to 2 and the expansion will happen from 3 dash to 4 dash this is very important that the expansion instead of from 3 to 4 now it will happen from this to this point and therefore you will get liquid only corresponding to this length if it happens here you will not possibly will come down here and if it happens up to here you will come down here and you will not enter the dome do you understand that if this point 3 dash goes up and up because of non-effectiveness or because of effectiveness less than 100% of the heat exchanger at a particular point the liquefaction will not even happen the gas after isenthalpic expansion will not come in the dome meaning which it will not get liquefied very important and now it is clear that the actual heat exchange therefore start from 1 dash to 2 and because the gas is not coming from G21 on the other hand or this warm side or the hot side or the pressure on the other side the heat exchange will happen from 2 to 3 dash and not from 2 to 3 these are the most important thing and that has happened because of the ineffectiveness of heat exchangers in an ideal system changing enthalpy on these two isobaric lines so in an ideal system whatever enthalpy change happened here similar enthalpy change should happen here whatever heat is given is taken that is what we assume in an ideal system here and therefore we can write that the definition of epsilon now which will be divided by H1 by G assuming that both side heat capacity is the same we are talking about enthalpy terms since do not temperature terms so H1 dash upon G which is the actual heat transfer divided by maximum possible heat transfer what was the maximum possible heat transfer going from G21 let us look at only at the cold stream right now which is the which is this stream so actual heat transfer happened from G21 dash while ideally the definition of heat transfer so actually here we define heat exchanger effectiveness in different terms I can define heat exchanger based on the low pressure side or based on the high pressure side this is on a low pressure side actual heat transfer happened which is H1 dash upon HG divided by maximum possible heat transfer which can happen from H1 to HG this if effectiveness definition is actual heat transfer happened from 2 to 3 dash but ideally it should have occurred from 2 to 3 and therefore H3 dash minus H2 divided by H3 minus H2 is the epsilon definition on high pressure side so many times this is also the definition of heat exchanger effectiveness that will occur in this place now because of this effectiveness what will happen to the yield we talked about y which is m dot F upon m dot I will use this effectiveness definition in the definition I had reproduced so this is my actual system and this gas after the heat exchanger is coming at 1 dash and then this get compressed in the compressor and so instead of now 2 to 3 it has come from 2 to 3 dash and instead of going up to 1 it has come from G to 1 dash this is what has happened because of the ineffectiveness of heat exchangers now I will redo the entire calculation the way I had done for this H1 dash and G HG dash etc and now my heat exchanger or my y definition will come out therefore like this so instead of now y which was equal to H1 minus H2 I have got H1 dash minus H2 divided by H1 dash minus HF earlier it was H1 minus H2 but because of ineffectiveness of heat exchanger it is not coming out at 0.1 but H1 dash alright so because of the ineffectiveness of heat exchanger the gas which is getting compressed is getting compressed from H1 dash alright and this is my new y or yield definition that has come and my effectiveness definition has been H1 dash minus HG upon H1 minus HG which is what we have seen can I now put the value of H1 dash here and write H1 dash in terms of epsilon if I want to integrate the definition of heat exchanger effectiveness I will put the value of H1 dash HG and now I will put this term of H1 dash here in the y alright and therefore I will get y in terms of now please look at this expression this is my expression of y which considers the value of epsilon which is the effectiveness of heat exchanger so what has become y as now H1 minus H2 minus 1 minus epsilon into this divided by H1 minus HF minus 1 minus 100% what would have happened if the value of epsilon is 1 then this will get cancelled out and this is what my 100% value of y would have been H1 minus H2 divided by H1 minus HF do you understand this now if the epsilon value get reduced you can see what will happen to y alright so this expression now considers the effectiveness of heat exchanger if this epsilon is 100% y is original definition of H1 minus H2 upon H1 minus HF but if this epsilon is finite 0.9 0.8 0.7 or 0.5 whatever then you can see that this y value will come down and now to demonstrate this I have taken a small tutorial and we can look at that so this is my new definition I will just get this tutorial and I will do some calculation for you to understand the role of epsilon the role of effectiveness on the definition of y what happens here so this is the problem please read the problem I am solving this problem for you I am not asking you to solve but we can see the effectiveness in atmosphere 1 bar and 200 bar at 3 kill the effectiveness I am using is 100%, 95%, 90% and 85% and as a comment on the results alright so this is my problem and let us try to see what happens if the heat exchanger effectiveness decreases from 100% to 95% to 90% to 85% what is happening corresponding to the value of y or the yield and g1 this is the definition of y this gives me the definition of y as a function of epsilon what is epsilon the effectiveness alright now I will put the value of epsilon to be equal to 1 first what will happen as soon as I put epsilon to be equal to 1 what will happen same value right so if I these are all my values which I are given these values from the 8.5% yield what do you mean by 8.5% yield whatever gas is compressed only 8.5% of that has got liquefied remaining gas actually goes back 91.5% goes back that means you can see how small liquefaction is m dot f is only 8.5% of m dot m dot is the gas which was compressed okay so that even if the heat exchanger is 100% right even if the heat exchanger is 100% only 8.5% gas has got liquefied remaining gas is going back so y1 is 8.5 or 0.085 now I will go for second value of effectiveness which is equal to 0.95 which is 95% effectiveness what will happen is 0.95 here and my effectiveness will increase or decrease y2 will increase or decrease decrease it has become 6% so the moment I come from 100% to 95% the yield got decreased from 8.5% to 6% and therefore y2 is 0.06 now let us take the next value of epsilon 3 which is 90% alright this is coming out to be 3.4% so 8.5% the 6% 3.4% here and let us do one more calculation of 85% effectiveness is 85% is very high in normal circumstances alright in normal circumstances even 85% effectiveness is very very high but if I put that value here now in this expression you understand that how much is this 0.6% so now you can imagine do you remember the diagram you will not come in the dome now because you will not get in liquefaction so only 0.6% of the gas which was compressed could get converted to liquid because the heat exchanger effectiveness came down from 100% to 85% and if I plot all these things now y against epsilon what is y the yield alright depends on how much percentage of gas got liquefied what is epsilon here effectiveness now you can see that from 0.1 here let's say 100% effectiveness and these are my values so first point is this which is y is equal to 8.5% somewhere here the second point the third point and the fourth point and if I join this what do you come what do you understand from this the effectiveness comes down to 85% you are just getting y is something some finite value which was 0.6% and if I extrapolated that you can see that at if the effectiveness comes out to be 84% your y is almost equal to 0 which is what tells how important it is to have a heat exchanger effectiveness let's say more than 85% in fact 85% gets just 0.6% so it has to be actually more than 90% and preferably more than 95% now can you have a heat exchanger which is 90% and 95% well above the fact that I need such high effectiveness a simple challenge to be heat exchanger is never using cryogenics because this effectiveness is normally 85% or 80% or sometimes 75% also so such heat exchangers are never using cryogenics but then I have to use very special heat exchangers which are called high effectiveness heat exchangers and they got all other requirements but the important fact here is how important how significant it is to have very high effectiveness of heat exchangers in order to get liquid yield in order to get liquefaction of gases which is one of the most important applications of cryogenic engineering so all these points actually it is clear that as the effectiveness decreases the y decreases drastically furthermore the effectiveness should be more than 85% in order to have a liquid yield but in order to have significant yield you have to have significant means what even if 100% was there we just get 8.5% yield alright so I just want 6% or 7% therefore and this all needs to be optimized we have taken the pressure to be around 200 bar isn't it so I have to actually worry about those process parameters also can I have 150 bar can I have 250 bar can I have 300 bar the pressure of 0.2 after compression because I am going to do lot of work there in compressing the gas so compressing the gas to what pressure what will be the effectiveness of all that thing and correspondingly what is the liquid yield is all the optimization optimization statement for me having done that then I have to really actually do design of heat exchangers in order to get 95% effectiveness of heat exchanger so this is the requirement or significance of high effectiveness requirements for us the second problem now I will talk about is I need to go lower in temperature now very very low in temperature and this is the diagram this is called calling cycle calling is a professor was a professor from MIT who invented how to go down to temperature to liquefy helium can you see now there are six heat exchangers so I am the same system in linde hampton you had only one heat exchanger but here I can see one two three a train of heat exchangers and turbo expanders let's say this is the way the gas helium will get liquefied and you have to worry about the effectiveness of every heat exchanger now and every heat exchanger need not be the same it can be different depending on the temperature that we are talking about all right so very important that I have to design each and every heat exchanger and you can see that some of the gas is taken here to expand and it joins the return stream here also there are expanders to lower the temperature and at the end you have got isenthalpic expansion now these are isentropic expansions and there are isenthalpic expansions and we will talk about that maybe if you do the course but that is a lot of mechanical engineering in that but you can see how many heat exchangers are there now and there all can be different types and they all need to be designed they all need to be having more than 90% effectiveness then only I can reach down to temperatures of 4 Kelvin all right very significant and this is a very important task on this yeah so it came in 1946 and so Collins lab is very famous in MIT lot of people are still working in this so this is just I wanted to highlight regarding all these things all right now this is what it looks like on a TS diagram there are two vertical lines actually gives you two isentropic expansions and one isenthalpic expansion at the end so I just talked about all the heat exchangers now let us look at the characteristic of these heat exchangers what is the requirement first high effectiveness and what we have understood right now is only high effectiveness of heat exchangers minimum pressure drop very important we are assuming isobaric line right now isent it but as you come down the heat exchanger the pressure drop will be there and it can be significant and the moment you have got pressure drop I have to worry about the COP of your system or efficiency of your system also so minimum pressure drop what we want is a very compact heat exchanger now when I say compact heat exchanger that means I want small volume small mass that means I want very high heat transfer area in a given small volume why in a cryogenic application we always look at very compact heat exchangers and you know the definition of compact heat exchanger the compact heat exchanger heat exchanger should have heat transfer area right nowadays we call it more than 1000 meter square per meter cube so I want small volume small mass but very high heat transfer area and there are very special kind of heat exchangers that are coming with the additive manufacturing now such heat exchangers are possible to be made in a simple way also so one of the most important requirement in cryogenics normally is compact heat exchangers and I will show you the compactness this is the minimum mass to minimize start up time for example if I got a helium plant here we have helium plant in IIT the cool down time is so high that today if I start the plant after 24 hours helium gas will liquefy till that time the entire cycle is getting cool all the heat exchangers are getting cool all the expander are getting cool then only I can reach 4.2 Kelvin do you understand this that if I start it today at 9 o'clock next day 9 o'clock I will get the first drop of helium till that time the machine is cooling itself the heat exchangers are cooling themselves because there are 6 or 7 heat exchangers now they need to be cooled first then only the gases or the fluids will be cooled these are all the transient time calculations and modeling are very very important when you design such heat exchangers then multi channel capabilities can we have different fluids simultaneously flowing there could be fluid A, fluid B and fluid C so can they be fluid how will it design 3 different fluids are without mixing in each other they are actually flowing so these are very important design requirements for this and they should of course hold very high pressure that means they have to have pressure vessel design also ACME code and all those things come into picture so thermal design and mechanical designs are very important that they should be standing low temperatures and high pressure the material should stand low temperatures and high pressure at the same time they should be compact and have multi channel capabilities high reliability and all that thing so there are simply different types of heat exchangers the first one is simple tube and tube the second one is not a compact heat exchanger which is a simple heat exchanger so in inner tube you got high pressure gas outer tube you got a cold pressure and you can actually coil them and we have made several of this type normally we use in two phase heat exchangers we have made several of this and you can actually use them for a two phase fluid basically then instead of one tube in tube we can have three tubes in one big tube we have got one big tube and through which three different tubes are there and you can actually coil the entire thing together we have used in tube heat exchanger and we have made seven tubes in one big tube kind of heat exchanger here where again it works for a two phase flow basically alright so here we have used this sometimes they use a turbulent in between so that turbulence get increased and you can have better heat transfer in that case but the pressure drop increases see heat exchanger always has to fight two contradictory requirement you want to have maximum possible heat transfer at the same time you want minimum pressure drop and you can see every aspect if you want to have heat transfer to be increased you got to increase the pressure drop also and therefore optimization of heat exchanger design is a very important thing now these are all different types of heat exchanger that are used three tubes then wire spacer as we call it here and this is an important heat exchanger now this is what Collins had used in helium lipofile so you can have a fin tubes going across a mandrel so you can have inner mandrel here which is this multiple layers of fin tubes going like that so the one gas goes inside this tube and other gas flows over the fins and this is why we make it compact because the fin density is a very important aspect how many fins per millimeter, how many fins per inch that is the aspect which is actually normally dealt with to make it more and more compact alright this is Collins heat exchanger it works for several concentric tubes so you can see that you can have several layers which could be used over here and all these now heat exchanger need to be designed properly what kind of equation what kind of heat transfer coefficient could be used what kind of fin efficiency that we are going to talk about can you ensure that all the gas which goes over the fins actually touches the fins parts sometimes there is a flow mild distribution I will not talk and go into details of that the flow mild distribution is a very important term are you using all the heat transfer area that is available to you alright so these are very important design parameters what is the helix around that and all those things this is you can see the heat exchanger here it is going in a helix line and the gas one gas goes inside the tube the other gas comes over the fins this is what we call as coiled fin tube heat exchanger can you see here this is what we have made you can see the fins which are actually mechanically coupled fins but they can be brazed also at the bottom so you can see the fins and very high fin density there are around 24 fins that means one fin per millimeter alright these are one fin per millimeter and this is we are making in a college in amadabad right now to develop these things so you can see that on a small copper tube around 12 millimeter we are actually making and you can see the coiled heat exchanger now here and you can have one more layer outside this now also so you can have several layers like that so you can have and one gas is going inside this tube the other gas goes over these fins and the heat exchange between the fluid A and fluid B very important class amongst these is called plate fin heat exchangers alright and maybe I got some fins here you can see the kind of fins that we have there are aluminum fins and I will show you that so you can see the layers of fins put together across this layer these are fins here so maybe you can just pass it behind you can just show around different types of fins they are all imported fins and aluminum fins basically and they are put together by layers and then they are brazed together now we have only one place where this can be done and right now where the brazing is possible aluminum brazing is a very special class of manufacturing so you can see that aluminum braze fin heat exchanger are the most compact fins so you can see one fluid is going this way the other fluid is going that way so when the gas goes through this one fluid goes through this like that it can exchange it with the one fluid above this or below this also one fluid is going through this so they can have heat exchange between this so one fluid going this way other fluid is also going that way so you can see the fluids are going different layers and layer by layer we can actually have fluid A, fluid B and fluid C also going through this can you see this small heat exchanger you can see different layers here so one layer here other layer is open from that side and comes out here third layer so alternately you have got different layers when you see this here the other layer is on this side so one fluid is going here other fluid is here cross flow heat exchanger basically you know the cross flow heat exchanger right they are making a 90 degree angle so you can see a typical plate fin heat exchanger these are all fins now if you see latest cryogenic journal we have given lot of analysis on this I just would like to have one more slide and stop here Bhabha atomic research center are under sanctions to make such heat exchangers because such liquid helium containers are used for heavy water tritium separation under that pretext we were under sanctions now we have developed this heat exchanger and we have developed the entire liquefier in BRC and I will show the engineer there just finished PhD he actually defends in the next two weeks so you can see different I am just circulating the fins here you have got all the fins which is going up and down now here is the heat exchanger which could be as small as a small duster or which can be long and little bit so you can see different streams coming here stream 1 stream 2 stream 3 fluid 1 fluid 2 fluid 3 you can have layer by layer the heat exchanger working down alright and you can see the typical heat exchanger can you see here so all these are heat exchangers this is the heat exchanger this is the heat exchanger the 6 heat exchanger if I had told you that in order to liquid helium temperature I have to have a train of heat exchangers and these are all heat exchangers here which are made layer by layer okay and then everything is surrounded by multi layer insulation which you possibly can see on the satellite also similarly you got insulation across because you want to have radiative heat minimized so these are all multi plate fin heat exchanger multi layer plate fin heat exchangers I will stop here I got 5 more minutes you can ask me questions in the next class in the next lecture tomorrow I will talk about refrigerators we are talking about liquefiers today we will talk about liquefiers and a case study any questions any simple whatever you feel stupid question also is most welcome oh brazing is a jointing phenomena the way you got a soldering right so soldering is a low temperature brazing at high temperature on 4 and A and the third is welding you understand welding you know welding so stainless steel to stainless steel will be welded copper to stainless steel will be brazed there is no melting of the material you can actually come into picture like flux third metal and then they will fuse together so brazing is a jointing phenomena normally done at 400 degree centigrade or 500 degree centigrade for aluminum it is done at 506 degree centigrade because aluminum will get see when you anyway will not go into the manufacturing of that but normally aluminum and copper are brazed because they get oxidized immediately any more questions about heat exchangers could you understand the role of heat exchanger in the liquefier could you understand the high effectiveness requirements in a liquefier I talked about what is the lowest heat exchanger effectiveness that we can tolerate 85% yeah very good and what is the best that we can have 100% can we have 100% what was the yield if you remember I talked when 100% it was for lindenhampton cycle and what was the lowest that we got at 85% very good so you remember lot of things here except a few who were sleeping I have noted down who were sleeping also anyway so I hope this was interesting for you this was a different thing for you tomorrow we will talk about not the liquefier but the refrigerators and the heat exchanger also plays a very crucial role thank you