 So, welcome to the 26th lecture of cryogenic engineering and here we will be talking about a new model now which is cryocoolers, which is I said new model and this will consist of various subtopics and which will be covered over next 6 to 7 lectures. We will talk under cryocoolers, sub cryocooler fundamentals, different types of cryocoolers and their applications. Then we will go in details of sterling type cryocoolers, pulse tube cryocoolers, Gifford MacMohan cryocoolers. When I said in depth means we will not go to the design, we will go in details about sterling cryocooler and I can give you a glimpses of how pulse tube cryocooler, GM cryocooler work and thing like that. Then we will also have a glimpse of various important parameters, very important sub parts of the cryocooler which is a regenerator, heat exchangers and compressor. At the same time lot of work has been done at IIT Bombay and I would like to show the cryocoolers being developed at IIT Bombay so that you can get a direct feel of the hardware which has been developed at IIT Bombay. Also you can see possibly a working cryocooler which can give you an impression about what is the cool down time, what is the temperature it reaches and thing like that. This topic we will try to cover in around 6 to 7 lecture depending on how we go through these topics. Again we have got tutorials and assignments are included at the end of each lecture as what we have been doing till now. So now coming to cryocooler and the first lecture we will talk about what is a cryocooler, alright how does it work, what are different types and thing like that. So what is a cryocooler and importantly why do we need a cryocooler? Cryocooler is basically a device which generates low temperature. So what is the need when you have got a liquid nitrogen, liquid helium all these cryo genes which gives you low temperature then why do we require a cryocooler. We talk about the classification and basics of different cryocoolers. And then lastly I will just cover you know 2 or 3 slides what are different applications of this cryocooler. There are plenty of applications but I cannot cover all of them in detail. So I will give you a field wise usage of this cryocoolers alright. Now let us see what is a cryocooler. So cryocooler is a mechanical device which generates low temperature due to compression and expansion of gas simple thing. It is like any other domestic refrigerator a cryocooler actually nothing but a refrigerator which gives you cryogenic temperature which generates temperature in cryogenic range and actually some people will call it cryogenic refrigerator also. So a cryo refrigerator, cryogenic refrigerator or a cryocooler there are different names they will attribute to this device but essentially what it does it works like a domestic refrigerator. There is a compressor, there is a heat exchanger and there is expansion device. So you can see here in this particular schematic what you have is a compressor then heat exchanger then there is expansion device and lastly we have got evaporator to do the heat exchange with the object to be cooled. So this operates in a closed cycle manner. So you can see the arrows in both the directions. So gas gets compressed, comes to heat exchanger, exchanges heat, gas gets expanded produces cold, gives the cold to the object to be cooled and the gas goes back and the cycle continues. So important thing about cryocooler is it operates in a closed cycle manner which means the mass of the working gas is constant. So once you charge the cryocooler like again in a domestic refrigerator which also works in a closed cycle manner if I charging some amount of gas this gas will be compressed and expanded which generates low temperature. So once you charge the cryocooler with a given amount of gas it will work in a closed cycle manner and generate a cryogenic temperature and whatever object you want to cool. If you want to cool the gas, if you want to liquefy the gas that is also possible using a cryocooler. So cryocooler can also work as a liquefy provided it generates that much amount of cold or cooling effect or refrigeration effect. So as I said it operates in a closed cycle manner which means the mass of the working gas is constant. A cryocooler consists of a compressor, a heat exchanger and an expander as shown in the schematic. So when I say expander actually it houses both expansion device as well as evaporator it combines and this can be called as an expansion unit. The cold generated in the expander so the lot of cold is generated in the expander which houses both expansion device as well as evaporator is exchange between the cold end and object to be cooled. So whatever object is to be cooled this object will be over here. So it is actually evaporated nothing but a kind of heat exchanger which transfers this cold to the object to be cooled. So this is a closed cycle manner in which the cold will be generated and it will exchange this cold through evaporator to the object to be cooled. So cryocoolers are capable of producing temperature as low as 77 Kelvin or below 4.2 Kelvin not only 4.2 Kelvin but below 4.2 Kelvin and they will be used to replace the cryogen. So at many places you cannot go for liquid nitrogen or liquid helium or liquid hydrogen for that matter. You know because of the availability and because this cryogens cannot reach there in time. So what do we do? We use a cryocooler. It generates whatever temperature is of your interest. Any temperature from you know in a cryogenic zone below 120 Kelvin a cryocooler can be designed for that particular application to generate that much of cold and it can therefore replace whatever was earlier done using the liquid helium or a liquid nitrogen or a liquid hydrogen. So that is a very important usage of cryocooler. Like a domestic refrigerator it can generate domestic refrigerator is meant for particular purpose. However cryocoolers can be designed for a particular purpose. It can be customized for a particular purpose to generate a given temperature and also to generate a given cooling effect. It is very important. So why do we require a cryocooler and I will just go through a title of this particular paper. The title shown here is extract from an Asian age on 23rd August 2010. What does it say? It says that earth helium reserves to run out by 2030 earth reserves are obtained mostly in USA through the rocks or through the you know different springs hot springs over there and this helium reserve is getting depleted over a period of time. You know the helium gas is very very costly. So if I want to use liquid helium the cost of liquid helium is going up and up. So if I want to have a 4.2 Kelvin I have to really pay a lot of amount. Instead can I use now a cryocooler which generate the same temperature and because why this application is failed very very importantly right now is because of this reason that helium reserves are running out by 2030. So according to professor Robert which is who is from Cornell University helium reserves would be running out by 2030 and therefore the closed cycle cryocoolers will replace liquid helium. What was previously done with liquid helium because the cost of liquid helium is going to go up and up alright it is very difficult to extract helium from air. The cost of extraction of helium gas from air is 10,000 times more and therefore whatever helium reserves are available right now in the rocks for example because this is getting depleted the cost of helium is going to go up and up and that can be completely replaced or its function can be completely replaced by using a closed cycle cryocooler or a cryocooler which generate the same temperature and this is the need of the time. This is a very important function and therefore most of the cryonic applications would now use cryocoolers instead of liquid helium alright. So it is a very important fact a cryocooler can actually it can do away with the usages of liquid nitrogen and liquid helium. So why a cryocooler again taking the same part same topic ahead why a cryocooler importantly I just pointed out if I use a cryocooler I may or may not require a cryogen now. This is very important I will not require liquid helium now which is the most costliest cryogen right now. So it a cryocooler can function to produce a temperature which was otherwise given by a liquid helium. So in this case I will not require any cryogen. Cryocoolers offer reliable and maintenance few operations because of the you know usage over a period of time cryocoolers have become very very reliable and maintenance free now. And therefore they are they are acceptable in most reliable functions required for example in medicine for example in healthcare unit where reliability plays a very important role and cryocoolers have been accepted over there. The cost of cryogen is increasing while the cost of cryocooler is coming down. This is a very important parameter as I said because of the helium reserves are depleting the cost of cryogen is increasing with time okay there are different various losses in transferring of cryogen to the safety requirement again while transferring cryogen. In addition that these gases are becoming rare for example helium, neon, etc while the cost of cryocoolers is coming down because more and more cryocoolers are made and therefore the production costs are coming down and lot of new technologies are coming into picture and therefore the cost of cryocoolers is coming down which is again a good reason to replace cryogens. What was previously done by cryogens now cryocoolers can do the same important thing. There is a scope for new technology which would advance development toward invisible cryogenics. Now this is a very important statement. The cryocooler technology has always been a dynamic technology and every time if you go for any conferences every time you will find that something new is happening something new has come in this field and therefore this technology is always becoming new better and better efficient and efficient and therefore this is a very very important thing that the technology is becoming better and better with time and also for example the physics people or the healthcare unit they do not want to know about cryogenics. They want low temperature no matter how you get it but they do not want to see the cryogenics associated with that thing because for example in hospitals they want low temperature to perform surgery for example or run a MRI machine they are not bothered about how you generate low temperature. So, a new phrase is we would like to go towards invisible cryogenics that means if I am a user from physics department or from let us say from medicine side what I want is I want low temperature and I do not want to worry about how do I get low temperature that means cryogenics for me should be invisible and this is where everybody would like to go. The cryogenics should become invisible and reliable and maintenance free cryocooler can do the same thing basically. This is what we are aiming for. Now coming to cryocooler classification. If I see cryocooler classification the cryocoolers can be classified based on how you get cryogenic refrigeration. So, this is the most important thing. So, I can get cryogenic refrigeration in two ways. One is by open cycle the other one is by closed cycle please understand this slide very importantly now. So, if I want something to be delivered at 4 Kelvin, 10 Kelvin, 25 Kelvin, 30 Kelvin or 80 Kelvin or whatever we can get this cryogenic refrigeration by two ways one is by having open cycle one is by having a closed cycle. And as you open cycle that means the gas or the cryogen which is used is open to atmosphere the gas is delivered to atmosphere closed cycle as I said the cryocoolers where the gas or whatever cryogen we use works in a closed cycle manner it is not left to the atmosphere again. Let us come to the open cycle and under open cycle now we can use stored gas or stored cryogens. So, I can have a stored gas and under stored gas I can have a Joule Thomson cryocooler JT is nothing but Joule Thomson cryocooler. I can expand this stored gas at from high pressure and expand the gas from high pressure to low pressure during which I get a cooling effect and this cooling effect is delivered to whatever to be cooled that means I get cryogenic refrigeration and this gas is let go to the atmosphere. For example, I can use nitrogen here nitrogen at high pressure going through a small constriction or a capillary tube expands it gives the cold and the gas is left to atmosphere all right and this is what we call as open cycle. The other part is instead of stored gas I can go to stored cryogen I can use liquid nitrogen liquid helium for example, I can use this to cool whatever object I want to cool and this gas after delivering the cooling effect it can again let go to the atmosphere all right. So, that is why we call as open cycle we do not collect it back and compress it back and thing like that. So, here I can use solid cryogen liquid cryogen supercritical cryogen. So, stored cryogen I can use in any form if I go on cooling liquid nitrogen further I will get solid nitrogen here all right which possibly will happen before below 63 Kelvin and this solid nitrogen also could be used to deliver cooling effect. I can use liquid nitrogen I can use compressed supercritical nitrogen gas to deliver this cold all right. So, these are different ways of getting open cycle to get cryogenic refrigeration by using an open cycle. Now, let us come to closed cycle. Under closed cycle you have got two possibilities we have got device which works in a dynamic way we have got a device which work in a static way. When I say dynamic way and static way I am talking about a moving component when I say dynamic that means something is moving either the compressor piston or expander or whatever it is there is one part which is always in motion while static nothing is moving. So, ideally I will prefer to have a static because the moment there are no moving parts there is no there is a maximum reliability there is no wear and tear all right. So, static is the most sought for right, but it has got its own problem. So, dynamic delivers cold very fast and more efficient also, but then you have to pay for it because they will always be some wear and tear. So, we have got a dynamic possibility dynamic closed cycle or a static closed cycle device and under these devices now under dynamic for example, we have got different classification based on the heat exchanger it uses. As I said every refrigerator or every cryocooler will have a heat exchanger and this heat exchanger will be could be of recuperative type or a regenerative type. We will talk about this in detail. So, depending on the kind of heat exchanger they use they can be further classified as a regenerative cryocoolers or a recuperative cryocoolers. Under regenerative now I can have valves between the compressor and expander I can have volvulus unit between the compressor and expander. So, this also makes a different because whenever valves come efficiencies are a problem all right. And under valves now or under volvulus system what you have is different types of cryocoolers and under valves what we have is a gm gifert macmon cryocooler and we have got a gifert macmon type pulse tube cryocooler here. While a volvulus unit is always a sterling cooler or a sterling type pulse tube cooler and we will go through details of these because the cryocooler is a part which is dealing with this quadrant here all right. So, we will go through these more in the coming lectures. Under recuperative what normally you has a joule Thompson or a clouded of a cryocoolers we have dealt with Claude cycle and JT cryocoolers earlier. So, here pointing out gm is nothing but gifert macmon JT is nothing but joule Thompson cryocooler. So, this is what we will be dealing with in the next four or five lectures. While under static we have got various possibilities as I said in static cryocoolers nothing is moving and therefore, we can have a sorption mechanism where gas is actually adsorbed on the sorption material and therefore, sorption cryocoolers we can have radiative cryocooling which is normally used in space we got a magnetic refrigeration we got a cooling due to lasers also. Most of you possibly will may have heard about it about this kinds of cooling devices based on the principles I am writing here. So, you got a no part in this case is moving and therefore, you got a maximum reliability only thing is what you require is very high cost sometimes or the system could be very very less efficient in this case. So, as far as these lectures or this model is concerned we are going to talk about this third quadrant if I call this as third quadrant whatever has been enclosed here this is what we are going to talk about in this model of cryocoolers. Now, depending on the end use application the basic requirement as given below have to be satisfied by the cryocoolers. So, what are these requirement to be satisfied by the cryocoolers? It has to have less weight and small volume depending on end use for example, it can be used in space application where the weight and the volume also are very very important. Fast cool down time and vibration less operation this is also very important characteristic how much time it takes when started how much time it takes to reach down to the lowest temperature and also what is its vibration level noise level these are very important parameters depending on the kind of applications you have in mind. Now, different parameters associated with the maintenance and life is basically MTBM the mean time between maintenance should be as minimum mean time between failure should be as minimum as well as mean time to failure these are typically MTBM, MTBF and MTTF are the terms which are talked about when we are talking about its life and this should be definitely this should be very very high. Continuing further the basic requirements of a cryocooler are if I want to technically see what are cryocooler requirement it should have minimal effect of orientation sometimes the cryocooler or some of the cryocoolers do not work when their orientation is not vertical. If they are inclined for example, pulse to cryocooler which will see it has got its orientation effect acoustic noise electromagnetic interference because some parts are moving all the time and therefore, the devices around this cryocooler can also show some problems because of the interference. So, you have to worry about in what environment this cryocoolers are going to work and accordingly one has to choose or one has to shield this cryocoolers the cryocoolers of course should have high reliability and shelf life cost effective as compared to existing system. Now, sometimes this cryocoolers can be very costly depending on you know what temperature you want to use and different customizations can be made and therefore, the cost effectiveness of this cryocooler has to be justified and this is a normal parameters which one has to consider when one chooses or buys a cryocooler. Now, what are technical parameters when I buy a cryocooler or when I you know decide to design a particular cryocoolers what are the technical parameters that govern the choice of a cryocooler. So, if I want to design a cryocooler or if I want to buy a cryocooler what are various parameter I will look for all right. So, important thing is cooling effect and the cooling effect is how many milliwatts of cooling effect at a particular temperature or how many watts of cooling effect at a particular temperature. For example, I require 50 milliwatts at 80 Kelvin do I want it if I want to use this thing this is my design specifications. So, one has to really know and mind you that for this we are paying if I say instead of 50 milliwatts I want 500 milliwatts the cost of the cryocooler will be very very high. So, therefore, one should be clear for what exact end use application a cryocooler has to be designed or cryocooler has to be bought. There is no point in buying a very high capacity cryocooler if our requirements are very very low because one will pay for this cooling effect because size of compressor size of expander size of entire cryocooler will change as soon as one demands higher cooling effect at any temperature. The next parameter which is very important is what is the compressor power requirement it could be in watts it could be in kilowatts. So, to obtain this cooling effect compressor power also is to be you know decided in fact this upon this is nothing but the COP of a cryocooler is not it. So, compressor power requirement is a very critical parameter because the power requirement sometimes could be available or may not be available depending on in what environment you are working. Do you have a three phase requirement for example, do you have a single phase requirement this is also a very important parameter when one decide to buy a cryocooler. Now, many times the compressor needs to be cooled there is the after cooler do we require a cooling water do we require chilled cooling water chilled water requirement. So, when I buy a bigger compressor I may go for a cooling water requirement and therefore I will require a chiller and that is also an additional cost. So, I should know do I require is it a air cooled compressor or a water cooled compressor there are different parameters associated with this and one should be aware of all these technical parameters. Then one has to worry about service requirement of a compressor because this compressor could be oil filled compressor the moment they are oil filled the oil needs to be changed over a period of time or even the absorbers needs to be cleaned or replaced over a period of time and this talks about service requirement. Maybe after ten thousand hours of working or fifteen thousand hours of working this is required to be done and therefore one has to know what are the service requirement because when the compressor is being serviced your cryocooler will be down and if your application for example is in health services the down time is very costly so one should be aware of such parameters and of course the vibration level and the cost which I have talked about all this will constitute kind of technical parameters or it will govern the choice of a cryocooler to be used for a particular application in mind. Now, let us come to the classification of cryocoolers and as I said earlier the cryocoolers are classified based on the kind of heat exchanger it uses. So, every cryocooler will have some kind of heat exchanger and every heat exchanger the heat exchanger could be of two types a regenerative type and a recuperative type and I will talk about this in the next slides. So, if you have got a regenerative type of heat exchanger then we can have different cryocoolers like we can have sterling cryocooler, given macmon cryocoolers and pulse tube cryocoolers they all come under category of they all use regenerative heat exchanger to cool the gas where heat exchange happens while a recuperative heat exchanger under this category what falls is Jules Thomson cryocooler and Claude Cycle and Brayton cryocoolers also I will talk about this. So, this is a very important cryocooler case which uses regenerative cryocoolers regenerative heat exchanger based cryocoolers and sometime they are called as regenerative type cryocoolers also while this is a recuperative type of cryocoolers. So, based on the heat exchanger the cryocoolers use we have got a regenerative heat exchanger recuperative heat exchanger and different types of cryocoolers come under that. So, a heat exchanger is a device in which the warm fluid gets cooled due to heat exchange with cold fluid as you know this. In most of the cases the process of heat exchange occurs at a constant pressure it can either be a regenerative heat exchanger or a recuperative heat exchanger depending on the kind of heat exchanger between the fluids. So, you know the basics of heat exchanger where warm and cold fluid exchange heat. Now, in a recuperative heat exchanger the flow direction of two fluid is constant and is simultaneous. So, you can see that this is a recuperative type of heat exchanger where the hot fluid goes and the cold fluid comes down and this direction of hot fluid is always constant it will not change over a period of time and both these fluid the hot fluid and the cold fluid are moving simultaneously both of them are present at the same time. The two fluids are separated by a solid boundary across which the warm and cold fluids exchange heat. Now, this is also very important that the cold and hot fluid are actually segregated they are away from each other and the heat exchange happens between some cooperative for example. So, we can have a tube into in which one tube is housing a hot fluid the other tube is housing a cold fluid and through the thickness of this tube or the surface area of this tube the heat exchanger is happening. So, actually they are basically separate they are housed in a separate tubes sometimes or separate channels or separate sections. The direction of the fluid flow may either be counter flow cross flow or parallel flow as explained in earlier lectures. If you have seen my my liquefaction lectures we have talked about the recuperative heat exchanger there and these directions can be parallel to each other cross flow counter flow kind of recuperative heat exchangers alright. So, these are the recuperative heat exchanger which are normally used in Joule-Thompson cryocooler. Now, let us see regenerative heat exchanger and in the regenerative heat exchanger a matrix is used as an intermediate heat exchange medium between the warm and the cold fluid. So, you can see here in addition to the warm fluid and the cold fluid what you have is a matrix in between. So, what happens here the flow is periodic that means I will have a hot fluid going through first for some time and this hot fluid will give the heat to the matrix and this matrix will take the heat and store the heat. After some time when the hot fluid has gone the cold fluid will come which could be the same fluid at lower temperature or a different fluid also. This cold fluid will come and this cold fluid will pass through the regenerator and it will take heat from this matrix and it will get warmed up during this period. So, we got a hot flow followed by a cold blue and the hot and the cold fluids are actually not moving at the same time. What does it mean? It means that they are oscillating flows. The flow is in this direction for some time and flow then in this direction it will stop, it will come to 0 and then the cold fluid will start after some time. So, it is very important to understand that the heat exchange is happening through a matrix and therefore, we say that the flow is periodic in nature. So, in this case the flow is periodic in nature alternating between the warm and the cold fluids across the matrix. It is important to note that it is an example of indirect heat transfer that means there is no direct heat exchange between the warm fluid and the cold fluid. The warm fluid gives the heat to the matrix and then the matrix passes this heat to the cold fluid. So, there is a indirect heat transfer between the hot to the cold fluid through the matrix. This is the way the regenerative heat exchangers work. Now, in recuperative case if I want to see now recuperative cryocoolers you got a compressor, a heat exchangers and Joule-Thompson expansion. So, this is Joule-Thompson cryocooler which uses a recuperative heat exchanger. The first recuperative cryocooler shown here is a Joule-Thompson cryocooler which we have seen earlier in a Linde-Hampson cryocooler also the cycle also. The gas is compressed in the reciprocating compressor and is expanded isenthalpically based on its inversion temperature. If you remember that Joule-Thompson cooler will produce low temperature only when the gas temperature is below its inversion temperature and you get a cooling at this point. So, simple Joule-Thompson cryocooler will have a compressor or you get a high pressure side and a low pressure side with walls over here you can see the high pressure gas will come it will get expanded by this expansion device produce the cold and the low pressure gas will go back through a recuperative heat exchanger it will pre cool this incoming high pressure gas and it will go back and cycle will continue this is the way a Joule-Thompson cryocooler work the hot and the cold food exchange heat in a recuperative heat exchanger in this case. Now, you can at the second cycle which is Brighton cycle instead of having this wall we can have an expander which is a moving expander and it has got an inlet wall and exhaust wall and depending on this expansion this will produce work output over here this will not produce work output here, but this will would work output and therefore what we call is a Brighton cycle and the third cycle will be a combination of this. So, this cycle is a Brighton cryocooler cycle similar to Joule-Thompson cryocooler except that the expansion device produces work and the third is a combination of these two. So, we have got a Brighton work output also at the same time we got a JT also and we have got a three recuperative heat exchanger and this is what we have explained earlier Claude cycle if you recollect all right. So, we have got three devices three different cryocoolers which work on recuperative cryocoolers. So, the next type of cryocoolers now we have is a regenerative cryocooler as I said the regenerative cryocooler we use regenerative heat exchangers and there are various types. So, here we will deal with first what we call as Sterling type cryocoolers or Sterling cryocoolers here. So, here you can see that the schematic is shown we have got a compressor here and the compressed gas will come through the regenerator this is the regenerative heat exchanger. At the same time what we have got is something called as displacer which is also moving the compressed gas will come over here the heat of compression will be removed in this after cooler here the gas will come to the top of displacer over here and during the motion of this displacer this gas will be pushed and this will come through regenerator and the gas will come to the expansion space over here alright. And as soon as the pistons go back the gas will get expanded at this point when the displacer comes down this gas will go back and the cooling effect will be delivered in the evaporator at this time. We will go through the details of working of this Sterling coolers, but just to tell you that there are two moving components here when the compressor piston and expander or a displacer at this point this displacer is just displacing the gas when it comes to the expansion chamber over here or expander over here this expansion volume it will just force the gas to go out piston compresses the gas while displacer just displaces the gas it is not compressing alright. So it will just displaces this gas from this it will come through regenerator come through the expansion volume or expansion chamber the gas will get expanded depending on the motion of the piston at this point and then the displacer will come down and it will push this cold gas out delivering the cold effect at this point here the evaporator is housed over here and this is the way the Sterling cooler works. So it consists of a compressor, regenerative heat exchanger and a displacer and the compressor piston and the displacer maintains a fixed phase angle between their motion. So this red line shows the relationship between the compressor piston and the displacer this is a very important design aspect if they are in phase they will not produce any cooling effect at this point and therefore a very important or very fixed phase angle a correctly maintained fixed phase angle between the motion of the piston and the displacer will be maintained over here and that is a part of your design parameters which will produce cooling at this point. This phase difference is vital to have a efficient operation. Now the next cryocooler what we call is a Gifford-Mackman cryocooler and what you can see here there is only one line high pressure low pressure was travelling from this while here we got a high pressure line and a low pressure line separate line because the compressor kind of compressor used here will have two walls inbuilt walls over here which are actually direction dependent walls. So high pressure gas comes over here and we have in addition one more wall in between the compressor and the expander. If we call this as expander and this is compressor we have got some wall here which will allow the high pressure gas to come out for some time and then high pressure gas will come over here and the displacer is housed here and suddenly after some time the high pressure gas will stop because this wall will stop the high pressure gas to come out and it the low pressure wall will open and the gas here will get expanded producing low temperature and again the displacer will move down and the gas will be displaced back and it will go to the low pressure and the cycle will continue. So high pressure gas will come for some time and the low pressure gas will go after some time. So whatever is the compressor used here we have got a relationship between the rotary wall or a wall which is used here some wall kind of a mechanism and a displacer motion this is very important. So this schematic Gifford-McMahon cryocooler the required phase difference again as which was done over here between the compressor piston and a displacer. Now here it will not be compressor piston and a displacer but between the wall mechanism which is used over here and the displacer. The displacer will move according to the wall mechanism when does high pressure gas come in when does low pressure gas go out. So required phase difference is maintained by relative motion between the wall mechanism and the displacer motion in case of a Gifford-McMahon cryocooler. The sterling cryocooler works at high frequency because you can see here the frequency of the piston will be same as that of the mains basically maybe 50 hertz maybe 120 hertz whatever and a displacer has to move with the same frequency as that of the piston. Therefore this is called as a high frequency machine the sterling cryocooler therefore work at high frequency while the Gifford-McMahon cryocooler they do not work at high frequency it will move at a frequency of this wall at what frequency the wall opens high pressure wall and the low pressure wall mechanism works. So this is normally works between 1 to 5 hertz so they are actually called as low frequency machines they got different advantages and disadvantages also. So sterling cryocooler work at high frequency while GM cryocooler works at low frequency. In addition to this sterling and GM what we have is also pulse tube cryocooler. So what you can see in pulse tube cryocooler is there is no displacer in the expander here there is no mechanical displacer over here and therefore we do not require a drive for this also in earlier two cases because the displacer is moving we will require a drive mechanism for this displacer motion also. So pulse tube cryocooler works without having any mechanical displacer instead gas works as a displacer there is only a tube filled with gas alright and all of them have got a regenerator over here. There is no need for a mechanical drive in this case because there is no mechanical displacer and therefore one complete moving unit is you know got rid of we have just got rid of one moving part basically which means this is a more reliable system there are no wear and tear there is no need of a mechanical drive for this. So naturally the vibration in this case are much less as compared to what they are for sterling and GM coolers right because there is a displacer and there is no displacer in this case. As a result the magnitude of vibration in pulse tube cryocooler is less as compared to sterling and Gifford Mach 1 cryocoolers simple thing we will go through the details of all this cryocooler in the coming lectures but I just wanted to highlight the differences or different regenerative cryocoolers. Now this pulse tube cryocoolers can be with wall and without wall which means they can be of sterling type pulse tube cooler which is without wall and also they could be of GM type pulse tube cooler which means there is a wall elements between the compressor and expander. This is called as sterling type pulse tube cooler this is called as GM type pulse tube cooler. As I said this is a high frequency machine this could be a low frequency machine. So the pulse tube cooler can be of sterling type or a GM type as shown in this figure. The phase difference is achieved using a inertons to move what you can see is all these things when you got a gas moving over here one has to have some relationship between the gas and the piston motion in order that the cooling if it gets produced and therefore there are different kind of phase shift mechanisms which are could be inertons tube or if is double inlet walls you may have heard of these things and we will study this phase shift mechanism in detail in the further lectures but what you have to see here because there is no mechanical driven displacer you got a gas which is moving up and in order that a particular phase difference gets maintained between this gas motion in relation to the piston motion we have got a phase shift mechanism which is actually introduced by the inertons tube or if is or a double inlet wall and also you can see a reservoir on the top. These are nothing but all the devices which actually introduces this phase difference or nullifies this phase difference we can see that in the later part. If I see all the four cryocoolers you can see a sterling cryocooler a GM cryocooler the difference between you can identify the difference because of this wall mechanism in between or having HP and LP line over here sterling type pulse tip cooler and a GM type pulse tip cooler. The pulse tip cooler can be highlighted with the fact that there is no displacer shown over here alright and GM can identified by the fact that there is a wall over here in this case wall over here these are very important things and all these four cryocoolers are very important to understand how do they work. Now with this background we can see now what is the comparison of efficiencies of a sterling cryocooler and a GM cryocooler. So you can see a sterling cryocooler has got AC power supply which is out from our mains and what you get here also is oscillating pressures. So essentially there is no conversion from AC to AC but there is a conversion from AC to OC. So what goes in is the oscillating motions over here which is 20 to 140 hertz and at the same frequency the pressure will come out oscillating at the same pressure which is fed to the expander. So there is no conversion from AC to DC or DC to AC whatever oscillating motion is here of the piston the same oscillating motion of the pressure will go to the expander and this AC to AC this conversion alright could be about 85 percent alright. This AC to AC basically is a power transmission from AC and what you get ultimately is a PV pressure volume diagram. So this conversion could be 85 percent over here and therefore sterling coolers are supposed to be very efficient over here. On the other hand if you go to GM cryocooler your supply is main supply again and therefore this is oscillating but here got walls in between and therefore what you get from here is AC to DC conversion we get HP line high pressure line and low pressure line defined over here. So actually we can call this is a conversion from AC to DC because of the walls and assuming that the wall efficiencies are 50 percent normally we got a power loss of 50 percent over here and again what is fed to the cryocooler is AC form because what we have want is a pressure fluctuations at this point. So again this DC is converted to AC that means oscillating pressures which are fed to the expander due to the existence of this wall mechanism which is a specific characteristic of a GM cryocooler. So if I give 100 watts at this point efficiency of this wall assumed to be 50 percent what I get is only 50 watts at this point and again 50 percent efficiency at this point. So what I get ultimately is only 25 alright and therefore overall efficiency of a GM cooler is much less as compared to what it is a sterling cryocooler and why it is less because of the presence of the walls. This presence of the walls actually brings about inefficiency in a system and what is delivered as 100 watts input and what is delivered at the outlet is only 25 watts. So GM cooler is a very inefficient in cycle so one has to feed lot of wattages over here in order to get cooling effect at this point alright. So because of the presence of walls it is a very important to understand the deficiency of a GM cooler is much less as compared to what it is for sterling coolers. So let us go through each cryocooler and just see advantages and disadvantages of various cryocooler. So a GT cryocooler is used for cooling infrared sensors on missiles, surveillance cameras. It can be used for cooling semiconductor electronics and also recently it is being used for cryo surgery because the high pressure gas just goes in it gets expanded produces cooling and it can just go out to the atmosphere or it can be recompressed also. So there are various usages GT cryocoolers are normally used also it can be used for to liquefy natural gas. The recent developments in GT cryocoolers are instead of using a single gas we can use mixed refrigerant and if I use mixed refrigerant my pressure limits are coming down alright. So this is a very recent phenomena very recent developments happening in this field and also instead of a moving compressor one can use sorption compressor also. So gas can be adsorbed the gas can be dissolved. So sorption compressor is a latest phenomena people are looking at as a result of which nothing will be moving in a GT cryocooler. So these are the different highlights of GT cryocooler the advantages of are nothing moving on the cold side. So no cold moving part steady flow operation and therefore no vibration you got a high pressure line and low pressure line nothing oscillating no pressures are oscillating there is no oscillating flow in this case and therefore no vibration the cold it can be miniaturized. So lot of research is going on to miniaturize this end can we have a micro heat exchanger for example and this is lot of developments are happening on this and this is a very good advantage of a GT cryocooler. These advantages are one has to depend on the inversion temperature. So real gas behavior we cannot use the ideal gas real gas behavior has to worry we have to worry about its inversion temperature. It requires high pressure of the order of 200 bar if I use a single gas and therefore the pressure requirements are very high in this case and because of the orifice or a capillary tubes sometimes it can get clogged it can get blocked with the impurities and therefore this is susceptible to clogging and this is a very important thing that many a time this has to be cleaned sometimes the blockage can happen if the water vapour comes with the gas it can get ice formed over here and it can block the flow of the gas this is a very important thing and this is one of the disadvantages of a GT cryocooler. Of course the efficiency is also low in this case now if I see sterling cryocoolers here they are used very predominantly to cool the infrared sensors in satellites in space application. So all the satellite experiments normally will be using sterling type cryocoolers or sterling coolers it can also be used to cool the high temperature superconductors and also we have seen earlier that the sterling cryocoolers several sterling cryocoolers working together can be used for nitrogen air hydrogen liquefaction we have seen this in air liquefaction earlier. So sterling cryocoolers have been used for quite some time and their usages have been very very significant. The recent developments are linear compressors which is dry compressor being used which uses a flexure bearing in this case we got a gas bearing also nowadays to you know soothe in the flow of the gas between the compressor and the displacer compressor piston. So gas bearing effect also can be realized in sterling cryocoolers. We can do easily multi staging to reach a lower and lower temperature and that is where hydrogen can be liquefied if I can go for a two stage unit to generate 20 Kelvin this can be used for various end usage application. So advantages of sterling cryocoolers is basically high efficiency small size and weight is a reliable operation practically zero maintenance if possibly designed correctly. The disadvantages are dry or no lubrications many times vibration due to mechanical displacer now this is the moving part and therefore you can have some vibration at this end because of which it cannot be used in some places where vibrations are not acceptable. The gm cooler as we know we have just seen the gm cooler which has got a wall mechanism over here it is widely used in cryo pumps it is a very important usage of gm cryocoolers it is being widely used in cryo pumps it is also used on MRI machines and NMR equipments the very important thing about gm coolers on every MRI machine you felt gm cooler being used for shield cooling well NMR also it uses for cold probe purposes. So gm cooler is being used in various applications for cooling of superconducting magnets result developments one can go for four Kelvin coolers use of rare earth regenerator materials in the regenerator for second stage and the advantages are it can reach four Kelvin in two stages it has got proven reliability for quite some times it is a low frequency machine as you know and therefore the vibrations are at low frequency here the disadvantages are however it has got a vibrations which is not acceptable sometimes at various applications less efficiency as we pointed out every gm cooler is basically of less efficiency because of the presence of walls and of course it is a noisy one can know that the displacer is moving up and down all the time because it uses sealing also at this point which is also at disadvantages in a gm cryocoolers yeah here is a disadvantage it is requirement sealing at low temperature and as you know sealing at low temperature is not a very easy task and the sealing is rubbing basically at this point which is also very very disadvantages first tip cooler has a biggest advantages of having no displacer and therefore no mechanical drive and therefore no vibrations or less vibration in this case it can also be used for cooling infrared sensors it can be used in space application it is just being used now you know the usage has just started importantly however it can be used to recondense liquid helium and liquid nitrogen which is very important because the vibrations in this case will be very very minimum and therefore it can whatever helium get boiled off it can get condensed whatever nitrogen is boiled off it can get condensed and that much of cooling effect can be offered by pulse tube cryocooler the recent development pulse tube cryocoolers can easily reach below 4 Kelvin it can be miniaturized so we can have a high frequency sterling type machines which can be used for cooling producing cold the advantages are no cold moving parts as you know here nothing is moving no sealing requirement because there is no displacer here there is no sealing requirement at low temperature which is very much required in GM coolers less vibration at the cold end because there is no mechanical displacer at this point the disadvantages are there is no much reliability data available because pulse tip coolers have been recently used after 2000 in space application for example so we do not have a time data over a period of time to show its reliability as compared to what we have with sterling coolers sterling cooler has a orientation if it is a one of the most disadvantages thing about the sterling type pulse to cooler or pulse to cooler is this being a gas phenomena there is no displacer it has got its inclination effect if it is used with gravity against gravity it does show some listening of cooling effect with orientation and this is the parameter this is the design aspect which is being considered which is under research at the moment having seen all this different cryocoolers let us just see the overall applications of various cryocoolers so cryocoolers are used in falling areas are just shown in space military night vision camera the infrared cameras infrared detector cooling in space and satellite which is a very important aspect of cryo cooling in space the gas industry usage cryocooler for gas cooling liquefaction storage and cryo pumps the very important usage of cryocoolers similarly in medicine cryogenic catheter cryo surgery wherever at low temperature you want to do surgery cryocoolers can be used they are also used in superconductivity so we can have a magnetic levitation train it will use cryocoolers superconductor transformer it will use cryocoolers there are all aspects of research being carried out they will all use cryocoolers to give cold all the time they will use cryogenic also and they may use cryocoolers also so trains superconducting transformer motors and generator will use cryocoolers for its applications in the energy sector also at smes where superconducting magnet energy storage devices you can have energy also and CERN large hydrogen collider usage cryocoolers are being used in forms of cryo pumps or also as cryocoolers in this sector of course cooling of MRI NMR and superconducting magnets cryocoolers are predominantly used right now in fact the cryocoolers being used in MRI is one of the most important boost for the cryocooler industry because this is the most commercial application right now which has really you know initiated or attracted cryocoolers to be produced in large number and because of which the cryocooler costs have come down lastly giving the summary a cryocooler is a mechanical device which generates low temperature due to compression and expansion of gas we need cryocoolers because it eliminates cryogenic requirements and offer reliable operation cost of cryogen is increasing where the cost of cryocoolers is decreasing heat exchangers can either be regenerative or recuperative type depending upon the kind of heat exchange in the recuperative heat exchanger direct heat transfer whereas in the regenerative heat exchanger it is indirect heat transfer the example of recuperative cryocoolers are jetty, brighton and cloudy cryocoolers example of regenerative cryocoolers are sterling gefford macmon pulse tube cryocoolers which we just saw the cryocoolers are used in various application in space gas medicine superconductivity applications finally we have got some self assessment exercise for you at the end and please go through kindly assess yourself for the lecture and also we are given answers for this so please go through these questions assess yourself honestly and see if your answers matches with the answers given at the end thank you very much