Direct Contact Heat Exchangers – Part 2
So friends we were discussing cooling towers which is a direct contact heat exchanger and as I have told you psychometry is very important and in a cooling tower we can we can theoretically get a cold water temperature as low as oil temperature. So basically it is like this if we see that, cwt is greater than equal to or greater than equal to or rather let us put it in slightly different manner it is like this we can put it in a slightly different manner let’s a dry bulb temperature DVT is greater than equal to cold water temperature in a cooling tower and cold water temperature is greater than equal to quit working so that means the temperature in between your rival temperature and and this difference we are calling as approach so let us go back to our slide and what do you want to do that little bit of air water vapour mixture thermodynamics these are all the things I have given this only for a recapitulation sure we have defined the humidity ratio and give me one can get in terms of partial pressure of paper and pressure of the year this relationship ending time of this we are doing coming in and hard work is going outshow in certain quantity of dryer the amount of water vapour than changing most of the properties are almost all the properties of water that is defined based on the most of the dryer so this is also an important thing you have to keep it in mind and dryer quantity to the cooling towers that remains constant then conservation of mass of water water water is coming in alone with my amount of water is coming in then what are you going out and along with along generally small amount of water is what arebecause hot water is getting cold and you see hot water I have considered constant mass flow rate which is not exactly true but for all practical purposes it is to 4 + water vapour that is there it has got some temperature so CC into all these things that will get the enthalpy of air and similarly for outgoing and also we can get it so we can make some sort of balance what is happening okthe amount of water vapour will get into a are showing that is going it is bringing some amount of latent heat that hsg is coming believing this will matter much in our veterans make sure we are what we are doing for the bulk of the order for the total of the heater we are trying to do some sort of energy balance for a small you can see that we have taken the height of the tower is small portion of the cooling tower by the area is a what is this in this area is nothing bad any physical area it is the interface area which the air and water show for all practical purposes it is very difficult if not impossiblewe we hit hard with this much amount because there will be changing the water flow rate higher flow rate of water is entering here and higher temperature for hot water so it is hard temperature similarly here this is Samantha LP and this is the change of enthalpy and this is the change of mass flow rate because some amount of operations and basically enthalpy change of water that is equal to the enthalpy change of air water vapour mixture and by integration you can get this kind of thing what we can do hyper we transfer coefficient of water and you do not know we cannot make it but only thing that is and to Delta that is for the height for delta why the amount because we can also applyand then what we do similarly transfer from the bank to air interface for a are also we get this one now police defusing the moist air mass transfer process if you like this show will be there is some sort of a mass transfer coefficient into a into a area that gives mass transfer coefficient butdefinition has also already been given and whose number is equal to one fourth Gear water system the heat transfer coefficient for that comes that now becomes easy because we can have this kind of relationship as number is equal toNLP difference that has been broken and the difference in specific humidity that has also been brought in with this finally we can get in North edsa I’m not a is the air flow rate bhi is the difference in enthalpy and that is equal to this mass transfer coefficient in to divide into a shape into ha88bn Thursday of this week he is the enthalpy of air okay what does it mean energy transfer due to basically which is due to mass transfer and what will be the energy transfer due to mass transfer is the enthalpy of water and he is the enthalpy of air should basically from a chef that means from water some amount of moisture will go to hear so this is the enthalpy difference and this is the mass transfer coefficient and this way increase the overall length LP of here so that’s why we are getting THC we can go to thewhat is coming in and then if we do this integration we can get the the tower height but for getting the tower height we have to move this quantity equation give the required height of the tower the inside of the equation is called markell integral I am and the expression on the right-hand side is called the packing fraction IP this is how it has to beboobies with Samsung everything will be get done we can write the equation this is called Michael schooling required I would like you to follow any kind ofthis quantity is very difficult to design outside determined from the fundamental physics nowadays people are even tell exactly see it can be determined but it is not easy and not whatever we determine that is not reply so we couldn’t our manufacturers have gone bad no cooling tower that will depend on benefits what are the things it will depend on cooling tower performance let's say cooling tower performance it will depend on wbt it will depend on approach it will depend on the type of field near using it will depend on if I mainlyit is not only having a human kid is also having some sort of a dimension late to get to dimensional volume how much is the weight and height so all these things haven't play produce some sort of the car ok used latest plastic little bit and then later see how it is done through and it is very difficult tobecause we do not have idea regarding data through a&t sorry bro is the air density metre and we do not have the idea of what is done element that is determined by the cooling towel manufacturers we're doing it today and then we can have height of the air back in company nidhi plan you have to decide if you cannot make itdesign is done to have some idea I mean it is important to have some idea and that’s why I have eaten this explanation explanation we can go to the next slide show hearing c the previousABC enthalpy of saturated water vapour mixture at the park water temperature and h e c h a s t what age is the enthalpy of air which is unsaturated and he is the one that is down volume per unit the cross-sectional dataexercise can be defined but the light inside we cannot be defined and we have to be to depend on the supplier of the heat exchanger so basically there is there for a given wbt and even approach given kind of design what we can do the move the left hand side and the right hand side will be a function of NYC which is nothing but entered wymt ok so this is nothing but it let me write it true that you can understand this is nothing but a Luigi basically what you can do suppose you have got a cooling tower that means feel volume extra are given you our own design condition is a given temperature and and approach so what you get this is your Michael straight line is Marshall’s integral what you gave that is your left-hand side of the equation which I was doing and how the cooling tower volume all mass transfer coefficient plus volumeexperimentally and again let’s say for a wet bulb temperature and approaching manufacturing manufacturing function sure he will get the black let me to I mean if you worried that she will get the performance of the cooling this is how you know how this thing is done and where is almost we are towards the end of the think we wanted to design for indirect or direct contact it is suggested by the analysis which I didn’t say that you want to remind how much water will evaporate so what we can do we can I see you weekend week we know the ambient air condition and one can assume that the water definition of saturated condition at the temperature of becoming hot water already clean lines are coming this is the cooling tower to make it let’s say this is our cooling tower okhot water is coming and this is the cold water temperature cold water is going out so this is ambient air and this is ear out this is the old psychrometric chart show and me and there is somewhere at this condition andno problem what will happen to see we have some letters through the air and water but hot water is coming from the hot water is coming in this election and a ambient air is going in this direction in contact with water anddear Megan kulman take a 19 from the water but it wouldn’t it make it why because the amount of moisture is coming late and Katie’s coming but there is no sensible heating so he’ll make it good but ultimately here we come out almost close to the hot water temperature and in the saturated condition change h a I s h a n s o n y a is greater than 8 in and what temperature would water temperatureplay these things cannot be explained to every great detail in a full stock exchange but I expected to give I attempted to give you some beans or how it works I have not given much attention for the very methodical derivation of the word equation which is the air intake scoop you can pick it up but you want to understand one thing that how the energy changes taking place in this kind of direct contact extension which is cooling tower sensible and latent heat transfer is taking place and later transferred to a place that transfer is not that important we are having the maximum amount that is why you need cooling tower there is only20% or 80% of the circulating water but we can call the water to regret texting so cooling tower is not only a heat exchanger it is also a water conservation device and before plan we only sacrifice 3% of the separating water and a great amount of cooling tower is the device where the heat due to second law or according to the second law it has to be dumped in the atmosphere and that is how heat is being dumped and ultimately it is being dumped in the cooling tower. With these few words I end my lecture and thank you for your kind attention.