Heat Exchanger Network Synthesis - Part 2





Hello participants, so we are back again in our course on heat exchangers fundamentals and design analysis. In that we are working on the topic called heat exchanger network analysis and as I have told that we will do it by pinch technique. Of course I have not introduced what is pinch technique, I have told earlier that we will try to learn this technique of heat exchanger network analysis by pinch technique through a problem. So this problem has already been introduced in the last class let us go to the problem it has got too hot streams the heat capacity flow rate capital c should not be1 from 150 to 60 degrees Celsius another is getting cool from 90° to 60 degree celsius then they are heat capacity rates are given and at the same time the temperature was from 20 to 125 degrees Celsius and another is getting heated from 25 degree celsius to 100 degrees Celsiusyou know what Love plus I have phone this one also I do not like to elaborate much and then this composite cards have been mentionedcomposite cards have been made straight continue at now this was his problem the problem will be solved by a method which is called problem table technical problem table method this is our problem table for the test case which we have taken so you see in this table there are number of number of columns columns are very important and then of course there are a number of tools also inside of the inside of the table we have got you Ciara 231 Coldstream is from 20 degree to 21 25 degree celsius so this is one Colts win and another Coldstream is from 25 degree celsius to 90 degree celsius so that is also where we are ok so we can think of and let me see whether it is 90 degree or 100 degree celsius let us go back to the problem definition 41 Coldstream is 22 125 another is 12:35 21100 sorry 100 so this is what has been shown that one is 22 125 this is 20 and this is 125 and then another one is 25200 okay and respective heat capacity rates as beans have to be configured also what are the heat capacity of these that also has to be considered then there are two hot wheels one hot stream is 150 degree celsius to show your this is 60 degree celsius and another age from 90 degree celsius to 62 streams are also so so 5260 and then from 90 to 60 you can understand that this side we have got the temperature of the hot springs and beside me have got the temperature of the coast now there’s at least four strategic temperature switch is fixed in this particular problem we have got all the temperature we will get some sort of this value without violating the problem statement let’s say 125 is the Coldstream temperature if the transfer is very efficient so hear the Hoxton temperature should be at least 20 degree celsius higher than 125 degree celsius because the 20 degree Celsius is the pinch temperature for this particular celebrity 145 do not know this temperature similarly thehear the Houston temperature is 90 degrees so corresponding corsten temperature would be 70 degree in ideal here it is 60° corresponding Coldstream temperature for tea and beyond these there is no hot spring show only Coldstream is there this is how we will get our our temperature distribution 44xt show you see now we can have different temperature zones and in each temperature on we can think that as if we are having a sip network we are considering a total network of heat exchangers but for a small temperature range we can have his subject was like let’s say here here the hot stream hot FM number one that starts from 150 degree and it ends at 1 45° somewhere in between of course after that also it is very cold so one interval of temperature in which there is only one stream 14 screen 150 degree to 145 degree so that we can m&co shop network similarlynetwork for in which we have got for springs again submit work 5 in which we have got two screens and sub network 6 in which we have got only 150 our team is getting cold from 15145 temperature drop maximum temperature drop it to celsius degree celsius what is the heat capacity at this hot springs capacity of the hot stream is given here for the first half in which is one is 22 degree celsiusgo back to our problem today here there is 18 degree temperature drop and incapacity way is to show here sorry there is a 5 degree temperature drop 150 – 145 to 5 degree and there is a heat capacity weight of 24 – 5 into 22 deficit is equal to – 10 this is not getting going to be next subway sub network subnet 10 from outside I am not supplying any from there is no other subway to work at the top of it showing at 0 and it is given to the lower supply to a 10 kw so this is how we can do similar calculation so difficult here we require .5 + 12.5 into Tisdale what was the output of the previous network this is the input of the this network output of 7811 is the input of sub network to and then the receipt we have to consider that means your input minus the deficit so that will give you -2.5 that will be out -2.5 will be the input of sublet work 3 and again by going by the same kind of logic you can do this problem all the data are you so how we – 107.5 –55 – 55 will be input to laptop network that is our networks and we will have an output of – 67.5 each and every subway to work we have got the van we have gotwe have a whole top 10 seconds of network will have a output of paper into top 10 output of -2.50 some network will Air an input of -2.5 output of 107.5 and then the next server network that is certainly 285 four-wheel had an input of minus 107.5 and output of 27.5 we want to make any input and negative ok so we want to do it in such a way that no input is negative negative means what is the meaning of input is negative input is coming from a submit work at the top to the sub network at the bottom and if we see this ad networks they are different temperature ranges that means input is the energy which is coming from a sap network of higher temperature to a sub network of lower temperature heat flow from higher temperature to a lower temperature and we can never make it negative as best we can make it 0% and never make it negative that is the violation of second law of thermodynamics network is based on second law of thermodynamics we have to keep that in mind that it is based on second law of thermodynamics we can never allow heat to flow from a lower temperature to a higher temperature or always we should see that it is flowing from higher temperature to lower temperature or at least there is no heat flow from higher temperature to lower temperature tonight works like that ok to this thing what we had to do the maximum negative input we can see 107.5 so what we do that here at the beginning we had class 107.5 so if I had it 107.5 then output to EV 107.5 + 10 that means 117.5 and the same way we will proceed somewhere we will get output is equal to zero that means no energy is flowing from a higher support network to El WhatsApp network as output is new for medicine supply to work 3 then the input to submit work for that will also become zero and this is the pinch point of the heat exchanger network this is very important point this is called the pinch point of the heat exchanger network and once we get the pinch point then it follows automatically and all the input quantities are positive if all the input quantities are positive we will get all the output quantities are also positive and or at least two inputs are positive or 0 outputs are also positive then what we have given at the info as 107.5 this is nothing but from outside we have pumps certain amount of energy so what is this this is nothing but your hockey bility so hot you to eat in we have to supply 107.5 whatever you need it maybe he’ll work so that is the amount of circulating we have to supply and day in this heat exchanger network that means 6 sub network what we will find that there is a output of 40so what is that true this is supplied to something this has to be supplied to something because I have to maintain all the temperature sensor so what it could be done that this could be supplied to some quarters so basically 107.5 is the heating requirements or the heat exchanger network and 40 or particular what is the cooling requirement of the heat exchanger network and in between there is a pinch point ok to pinch point we are getting to temperature is 1 is 70 and 90 that means that the pinpoint my temperature will be hot hot streamside temperature will be 19th and Court Street outside temperature to be 70 exactly there is 20 degree celsius of temperature difference this is the pinch pinch point hear the heat exchanger network orbit exchanger is most efficient because he transfer is taking place at the minimum temperature difference is we will have some sort of a temperature difference which is more than 20 degree celsius today’s if you have understood they’ll probably you have understood the half of the problem let's play this is the play one side is energy and another side is your there could be so basically my hot streak is ending over here so this is your hot utility code strings are being heated so this is your hot utility Coldstream sir anything over here so this is your old utility this is our energy then it becomes cold utility it becomes hot utility and this side it is temperature should this becomes your pinch temperature difference and in the present problem this is your 20 degree celsius so this is your cold utility here the hot stream is to be cooled what stream is getting cold partially with the help of Coldstream but intercooling may not be possible so this is where the hot stream is getting cold this is where Coldstream is being heated and this is the pinch point where we are tapping your minimum amount of temperature difference between the hot Steam and the cold streamonce again so if you have up understood the problem table method so let us go to let us go to the next slide what we have got here that Sunday to work one Saturday to work to submit to a3 that is above the pinpoint the red line is the pinpoint okay so this is the redline this is your pinch point some network forcibly 285 and 7816 is below the people and hear from some network one energy is coming to submit workbook from Sumner to work to it is coming to submit work 3 bus from Sydney to 8327 8140 amount of energy start network 425 some amount of energy is coming sub network 5278 work 6 some amount of energy is coming and we can see how much energy is coming at the talk we had given 107.50 watch that is the hot is it at the bottom we are expecting 40 clover that is the cold utility and we have got zero heat exchange at this point this week all ate the hot end of the problem this week all as cold end of the problem and if we see the hot scream and Coldstream so they can be sewn like this so this is the hot and problem this is recording problem so SIM network it flow diagram for this this particular problem this is this is the problem number and submit were combined into hot and cold here so hot and cold region weekend so you see pinch point there what are the two point then divides the problem into two problems two problems are now independent because no way they are connected 1 is 0-1-1 heat exchange one part of the heat exchanger network is not transferring it to the other part of the heat exchanger get what they are not connected to pinpoint divides it into two different problems with a very important this is a very profound Kinder observation that by the pinch point we are having two different heat exchanger network one is at the top which we call hot end of the problem and another is at the bottom which we call the cold end of the problem and now we can solve them independently we can have independent solution without wondering what is happening in the cold then we can have the solution of heartland and without wondering what is happening at the at the Houghton we can have the solution of the cold let’s say we are having two sections that is possible we will discuss it and called it we are having two solution solution latest call eab and cold day we are having two solution Lucas coly c&d so what I can do I can join any hot and solution to any golden solution show my total solution could be at all my total solution could be busy or my total solution could be AC okay this is how to exam tothis has to be kept in mind this is a very important aspect this is a very important aspect of speech analysis alright so let us go to the next slide rule of pinch analysis point divides the AC and heat exchanger network into two independent problems to be solved separate just now whatever I have told and explain so let’s say this is my pin point and I have got a and then I can have let’s say d and e two different solution of the Golden you can combine the solution to this is one very important aspect of very important aspect of pink analysis loan hip-flores allowed at cross DP this is a very very important thing that once we identify the beach point by mistake by wrong design by our some sort of fancy design we should not allow any heat to be transferred from one side of the pitch to the other side only to be used in the hobbit so you have got one hot solution we have got one cool solution or other network for $10.00 network and cool names of the network should know what utility local duty is to be used in the hot and similarly know what ability is to be used in the cold so three rules we have got first rule is that no heat flow is allowed across the pinch point and they’ll know old utility is to be used in the hot end and no hot utility is to be used in the coldest and violation of let me explain a little bit for that what we will do let us go 2 I think I think I weigh soccer I will go to this solution problem table method here what we have got we have got the pinch point so here we have got TP point now what I try to do so in the pinch point you see that I I tried to send something across the pitch point how I can do that let us do one thing at the top at the hot and what I have done I have said or I have added 107.5 hot so instead of that later and there’s 10 till 14 to something if I do this you will go on adding this and you will not get here over here you will get some positive that makes this is not letting second law because from what are some network only and it is coming to the WhatsApp network this is not valid and second law of thermodynamics flow across the pitch point out point 120 degree celsius based on that 20 degree celsius we have got a pinch point at constant temperature of 90 degree and cold temperature of 70 degree to hear you will find how much wood floor 110 – 107.5 so that will be .5 2.5.5 kw of heat flow you will get across the bridge point and ultimately what you will get your cold utility that will increase to 42.56 basically then you are getting a double penalty how you are getting a double penalty instead of 107.5 kilowatt of hot utility you are using 10 10 kw of water at more hot if he were using 2.5 kilowatt more hot hot when using and instead of 40two penalties one minute I’m supporting pretty and once in the dumps of Cody this is called a double penalty and we should not have heat transfer across the pitch point to this is very important as quickly go back to our 2 ourdesigner beach air show start at this point this is also I do not know whether I have told it or not but it is very important I do not recall whether I have sold it or not but this is very important that it has to be kept in mind design of each age should start at the beachboy few few information few messages said they are not only information they are very important rules we have to keep it in mind being point divides the heat exchanger network into two independent problem these problems can be solved and supersolve independently and while solving this problem we should start from the pinpoint and go towards either towards the hotel or quartz according to this is one message second message is no heat flows to be aired across the pinch point that message is no hold-ups should be used at the hottentot message is no hope in creating is to be used at the cold so these are the things to keep in mind. Well we are at a very interesting point of heat exchanger network analysis, thank you for your patience and we will proceed with the same problem in the coming lecture.