Phase Change Heat Exchangers – Part 1





Welcome back today we are going to start a new topic and we will be dealing with this topic for quite a few lectures. This is a very important topic as far as heat exchangers are concerned and basically we are going to look into some difficulty ,again there are many variations but we have classified them into our we have we have assembled them for the sake of understanding in one group and which I am calling us which I am calling as heat exchangers involving first show in some heat exchanger we will see that there is a change your face so those kind of heat exchangers we're calling it is it is dangerous go to the next slide show face changing fluids will change its base at least onein a very special kids one can have multiple multi-screen exchange and then the requirement of its qualification as a face changer texted that is that at least one of the food with its base I change your face could be made from solid to liquid liquid liquid but many will see that person is taking place either by condensation or iPhone condensation is also present in case of being called an experience that are particularly in a country like India and a lot of water because the hot and humid here comes in the context of the cooling coil it gets dehumidifier and the inside and the water vapour present in the air condensers this dehumidifier coil on all the cooling and dehumidifier coil is also taking part in what's the minimum amount of time we had this time probably we will try to13 in the exchange will be changing its fares from the inlet of the village idiot in the exit of the kittens always if we call this mode of heat transfer or this condition is condition of the following will always be his condition from the beginning of thewe will consider in our discussion so either we are dealing with the condenser or we are dealing with boiler that we are dealing with the condenser the condenser temperature changes as it is also orwhat is him to see let’s see the past and meant the dog can’t hear the ftfc that is if it’s changed and this is the clinical typically this is a parallel to the direction of the condensing free so then it becomes counter-current flow but will not change the performance of the heater will not change if we keep the heat exchanger design constant then the inlet and outlet temperature of the fluid that will not change it is independent of orientation this is one thing we have to keep it in mind that instant heat transfer when there is purely face change for a particular tree their independent operation same thing for a boiler has been getting any response from the company fromand if we go for effectiveness and you only here with president will have this relationship very easy to remember so this is the relationship between a DUI and effectiveness this is effectiveness and this is a new number of transfer evening and this is different for a simple system and I found two parallel and counterflow it does not matter which way you are this is a cross-flow so that is that was a cross-flow condenser and this is the condensing fluid inside this will be the temperature in ocean and this will be the temperature distribution condensing through it and as it is in the face in condition there will not be any change in the temperature from inlet to outlet and there will not be any variation of temperature also along the inlet play me or along the exit this is the coolant which is gaining heat so when at the inlet it will have uniform temperature low temperature at the outlet there will not be any variation of temperature variation of temperature at the outlet is is characteristic of class plus heat exchanger by here we will not get here we will have this kind of temperature distribution and so obviously you see that there could be other reasons for selecting the arrangement whether we will go for parallel flow flow or cross-flow for assitant heat exchanger but normally all of them are equivalent now let us take up a problem a condenser is required for an old learning paper having a latent heat of 250 kilo joule per kg flowing at 5 kg per second the paper is pure and may be assumed to condensate at its saturation temperature of 80 degree celsius the condenser in school by buy-side pulling water specific heat has been in with an inlet temperature of 20 degree celsius and outlet temperature of 35 degree celsius the overall heat transfer coefficient that has been calculate the cooling water flow rate the value of Delta t and the surface area what is the eighth LU4 this case water flow rate we have got to fill that in North CP school and we can find find out and this is equal to this is the total heat gain by the Texas oil gained through heat transfer by the cooling water stream to this is the heat which is rejected by Steam also hit rejected by Steam that will be equal to Eastern floral and deleted it because it is interface condition right hand side everything is known and from the left-hand side it has been told that the water temperature of the water is 20 degree celsius and outlet temperature is 35° Celsius so from there we get the mass flow rate of water that is 19.84 KG per second floor of water is known at this point next what we can do we can calculate the log mean temperature should log mean temperature we have calculated 12 it will be at a constant temperature that is you’re at and other to eat there will be the temperature of 20 and outlet temperature of 35 show other three also we can get the temperature and from there we can calculate the log mean temperature reaches 52.1 degree celsius in this casethe heat exchange is always equivalent to that of a pure counter-current flow thus we have got this kind of relationship this is very important and you can load this so this is one important point and this is also important to know for a po really phase change heat exchanger already I have explain what is Budapest in this is very important to me okay so please try to correlate the problem which we have done at the end of our discussion on film cubit exchanger that was much more it because this inside heat transfer coefficient pressure drop etc we wanted to take care of but that was also across floor heat exchanger they are also we have calculated lmtd so please look into that problem go back to your to my order lecture and please look into that problem but whatever I have underlying these points are very important and this is how we will get the Delta t now let us go to the next page surface area we can calculate this is our problem very simple problem but it gives us an idea how to calculate the basic calculation of a selfie now I have coined some sort of a word or terminology for the practical heat exchanger welfare office team takes place is not like this show in heat exchanger we will have change your face but before that and after that there could be single phase heating or cooling condenser superheated vapour at 3 so we have to dance but the food is not exactly at the saturated condition when it is entering it is it is the other fluid is in entering at superticket state if it is entering anticipated stay over here then it has to be brought to the situation condition and this is single-phase cooling for the free then there will be first in the transfer and then again there could be circling and this is again single phase and this is how the coolant temperature coolant is a single phase do it so cool and temperature will continuously increase I have four linear but this car this car can be non-linear this can be nonlinear and this can only hear from them but these cards refrigerationbut obviously it will end up with a superheated stay independence so you see that the car will be something like this and this either it can go to this rain or it can come out out of like this while and boiler so here’s subcooled water goes to the boiler and then there will be liberation liquidcoming from the evaporator and all the three parts will be pregnant what kind of pictures because I have done at the beginning of this class making a misconception that because one side temperature is not changing one thing that ok once I feel temperature is not changing things are a little bit busy but things are not easy because her other things are more complex because we have got this kind of abrupt change in the temperature if there is a change in the temperature car then what happens basically we are having feet so we can think that this is one heat exchanger this is a purely faced in heat exchanger and this is another it is so let me write heat exchanger 1 heat exchanger 200 having heat exchanger three so I have to do this then there is another thing this side it is temperature and decide it is late showwhat is the minimum terminal temperature difference between allowed that is how I have to this is one kind of a decision we design engineer this place at one end of the heat exchanger that means this terminal temperature difference is taking place at one end of the heat exchanger in case of this testing heat exchanger where there are subcooling and superheat in the minimum temperature at this place called peach this restriction to the design and this is happening inside so this is very important and we had to maintain a feed temperature probably I will elaborate this as we proceed on and that’s another complexity is added to your face contest we will elaborate this thing with it or change it exchange we have seen one unique feature that the minimum temperature difference between the two weeks that can take place somewhere in between the danger particularly when securing or superheating or both are involved in assisting heat exchanger so once again I like to draw your attention to this particular figure so I had taken in a typical case of a boiler where theit goes to the security of condition then vaporization take place and then we are having superheated vapour superheated liquid husband but if it is in the spirited conditions which it is superheated vapour so probably in a boiler we will find this kind of phenomena or in a condenser also we can find out the security and then the first Indian and then superheated region in the reverse order that means in case of a condenser it will start from superheated vapour it will go to the face in June and then it will be in the subcooler condition depending on the process required now the point which I like to make is that in that initially let’s say we are considering a condenser so initially the fluid is at the first its density is low and then it is in the two-phase region in the quantity of the producers and the quantity of liquid increases and then it comes out totally in the sub-region she can imagine that from the beginning of the from the start of the heat exchanger to its 8 there is a large change in density of the fluid of red mixture volume flow rate is changinglet me elaborate this let us go to the next slide in the next slide I have shown the schematic diagram and cutaway view of a orphan automotive condenser you know incarcerate mobile there will be four condition towards your condition there will be a refrigeration system and in there there will be a condenser compression refrigeration system so there will be a compressor up after the company said the compressed paper that means which is at superheated state and high-pressure that we enter the condenser show in a post at 2 and then it will become liquid and in many cases it will come out of the condenser out as support quit so you see from superheated vapour to support liquid there is lot of change in density lot of changing Florida how do we take care of it so we’ll take care of it very interesting we take care of it in this particular heat exchanger by providing different pass so you see there are different passes this is the this is the first pass and then this is the second part and then it’s past 3 and then this is past wellLastPass where the superheated vapour is flowing there are 14 keeps all the cubes are of same dimension SO14 tubes means large cross-sectional area as the density is lower for vapour we have to provide large cross-sectional area then in the second pass the paper is now being cold and it will go to the two-phase region in the second pass there are 70 drastic reduction in the number of views and in the third pass there are six tubes and forth pass the food will be in the subcooler liquid condition then there will be only for passport there we go to show you see this is one opportunity to see a cutaway view of the heat exchanger this is a cross-flow kind of heat exchanger which is very common in refrigeration application refrigeration condenser so you say refrigerant flows and the tubes are also very evening you can see that use these are platitudes and in one tube there are seven passengers so basically what we are seeing across floor to visit exchanger using microbes because these messages are all small dimension this is also a competitive danger because the area density area of heat transfer area density per unit volume is very large and so this is cross-flow refrigerant is flowing in this direction or in this direction depending on the which pass it is and then it is air-cooled or rather the transfer is to Air Show air passage is provided by these things so these are the different type of fence priority the defence they are attacked to be flattened you to onto the fence side through the inside air passes through this is a very unique heat exchanger it has got so many speciality let me point out all of them one by one but first thing it is a first in heat exchanger second thing in this face changer text injured both 3D and in all the three regions are there superheated vapour DG and then to face really and then they support region third it is across floor heat exchanger it is a compact powerfit exchanger flatten tubes are used and in these tubes there at 7 passage passages in each to and the outside is made by things and then there are four passes and the number of tube purpose is not uniform to take care of the changes also one thing I like to mention that we have started our discussion on face then heat exchanger most of the cases I will deal with condenser that means liquid is condensing the weavers is the evaporation or boiling of liquid do we call evolution but the exactly reversible will be boiling of liquid and boiling up liquid is needed in many places but where we are boiling the liquid with the direct application of heat like firing coal or nuclear power or or some sort of oil or in some cases electric heating in small size unit we call it a boiler so where particularly where there is firing we call it but we can have boiling in other places also like in a refrigeration system but that we call as discussed condenser are not very different from them so I think one can pick up but not discussed like to briefly discuss boiler before I go to the next line this shows the schematic diagram of a boiler boiler boiler pressure of the working fluid is below the critical point and then I saw the super critical boiler pressure of the working fluid thatI have give me a name to this life boiler radiators to heat exchanger basically heat extended their slightly different because in this subcool condition particularly if we had any from the critical point there is a large difference between the density of liquid and vapour in a particular situation or particular situation the same problem that I have to handle a fixture in the boiler and then I have to findtext yes then these three are three different type of this is your economy and economy will find the type of study is not much different from that which is not this section but this looked kind of a section here of course we are having a completely different kind of arrangement which I will explain later why this is the main part of the boy life we do not have much hope to discuss this in this particular course because violets are considered as fire detection yet and in our course fired heat exchanger we have excluded only a glimpse of it I will discuss it and then after that we will have been saturated vapour which will be taken to the super 8 in condition at the corresponding pressure in this room supercritical boiler it is sometimes called once because the food passes continuously maybe through different sections of the tube and it is liquid circulation or liquid and double circulation from the other kind ofmany kind of her operations play one is far less word innovation will be there and that isn’t the heat exchanger part of it where the from the hot gas or from the hot shots of commercially it will be transferred to the coal or oil or some suitable fuel will be fired it will be generated increased body and the wall of the furnace we can have the Tuesday the vertical tubes and surgical team liquid will pass and when it is passing through this vertical cube if we go to theit will get it but it will not get completely evaporated partly it will get better than this come to the circular section which is called Kingdom choir tour and liquid will get1 and 1 is equal 1/8 oz and circulation is continuously today’s ride and to be down here there could be a perm it could be my assistant buy pump or the circulation could be assisted by voice so this is completely a different kind of heat exchanger though it is heat exchanger but it is a completely different kind of heat exchanger what we find invisible boy let’s go back to the next line so here’s what we can see who this is b pharmacy all these walls are to be kept cool so what we do then we provide the water tube through this through that’s why they called water wall so it absorbs the heat some amount of steam is general did you go to the gym and then to becomesome of them are many of them are chasing destiny this is to make this thing simple because it will be easy to understand but actual boiler there will be a lot of complexities and five play outside quite complex so I will show another slide to give you some sort of an idea so what kind of that we can see here here we can see superheat superheated show the super heater will be a single phase heat exchanger this boiler evaporated section that will be a twofer sitting here we can have a water heater which is a single phase exchange but what we can help fill water heater so these are in two facilities go back to sort of this is how it looks you can see the vertical lines these are all tubes and huntersville is surrounded by these tubes so that the heat exhaustion is there by this by the water flowing through this tube and tube wall is also cape ok and here we can see through this novel the fuel is injected either in the form of pulverized fall or in the form of oil and conversion takes place if you closely look into this figure you will find otheror maybe some of biliary combustion is there an ideal partner to this could be something planned and the exhaust gas off the guest buying which is at high temperature it is coming so what are you want to show that in boiler there are three operation disa pool water is to be brought to the security condition the saturated liquid is to be operated and then the saturated vapour is to be super heated and do in small kind of heat exchanger we can do it in Simi texted back in a boiler there at 3 laboratory section this is the economizer by at the liquid temperature is increased and it comes to the saturated condition then this is the boiler wired to circulation we generate steam evaporator coil vaporizer and after this thing innovative we have got the superheated the saturated steam temperature is increased this is how we are having introduction to phase change heat exchangers, we will solve some problems in our next class. Thank you for your attention.