Fouling in Heat Exchangers - Part 2
Hello everyone, if you recall we were discussing A very important practical aspect of heat exchangers. That is fouling in heat exchangers and we have seen that how or we have discussed how fouling can deteriorate the heat transfer performance it has got a fake both on pressure drop and rate of heat transfer and how we have seen some snapshots photographs taken from different sources that how fouling deposits sit there on the passages of heat exchanger and how it changes the geometry. So once we have the basic idea regarding fouling let us proceed and let us see some details of it so next slide if we gosuperior not only the cross-sectional area but the fastest condition that will all change if we consider that only there is change in the passage area that is only the hydraulic diameter is affected then from the basic physics from the basic science a transfer coefficient that is that is inversely proportional with hydraulic diameter and tell tabby that is pressure drop that is inversely proportional who is the cube root file with the third power of hydraulic diameter now I have given the entire formula how the pressure drop is to be calculated and if we consider that do there is a change in the passage area by the mass flow rate of the fluid that we have kept constant then we can find that delta p that is proportional to The tourist industry proportional to b ^ 5 the ratio of pressure drop if we take the registry operator drops of a clean heat exchanger or other a fouled heat exchanger and the clean heat exchanger that means delta pa / data PCs is the founding condition and see is the clean condition then these friction factors are coming into picture and then diameter mean it is inversely proportional to the fifth power of the diameter now if we consider the following does not affect the friction if I receive verification that it’s is equal to f f and and also consider that the reduction in tube inside diameter dutifully is only take to 20% then the resulting pressure drop increase will be approximately 70% to 205 showing at 10% for a 20% reduction in the in the diameter will give 100 per-cent increase in pressure drop assuming that friction factor has not changed and also we have we have assumed that we are maintaining the same mass flow rate we are assuming that willonly one that cannot be only twice so at regular interval we have to have some sort of heat exchanger cleaning cleaning up this passage is called cleaning how we will do this thing some something we will get there at 18 a.m. in detail technology how cleaning offices can be done what we can find that other equipment heat exchanger also need some sort of maintenance regular maintenance departments out of the seat exchanger and this is very crucial because if we are if we if we offer a blind eye to this aspect there could be large deterioration of the heat exchanger performance and that will have it say on the overall plant performance the entrancekeep constant which amounts to that we have increased the velocity if we increase the velocity vector that is one part and second part is we have seen we have seen the photographs that the position of violence that makes the surface event and generally the similarity Gibson increaseis that been some sort of people that housing has taken place and heat transfer coefficient has increased not at all because the following which will deposit on the surface they are highly insulating generally in most of the cases for many of the cases not most of the cases in many of the cases we neglect the resistance or to transfer from one flew into another part of the resistance which comes due to the solid will definitely because the solid material has got high conductivity and it is very thin the solid metallic wall etc are very thin in many cases but when there is the position of power and the violence are insulating material there conductivities Andrew and the thickness can increase to English to some to some final text so we cannot regulate the deterioration of in overall heat transfer due to the appearance of a new resistance convective heat transfer coefficient probably is increasing but there is appearance of a new resistance due to the definition of the violent and that we cannot make it to that side effect of housing in all the cases almost all the cases that it will deteriorate the performance as a heat exchange device impact is the increase in convective heat transfer is alone rather than increasing thermal resistance due to the presence of insulating layer is considered it is very difficult to get the resistance due to falling from a tree what will be the name of the following layer composition and that’s out what could be the thermal conductivity all these things it is not easy almost impossible to determine if that is so the design and dependent handbooks codes and standards and sometimes and many times on experience that how one can do things are there how one can rate the performance of fidget exchanger in the presence of and how much extra area or extrawhat should be the cleaning city up to how much time we can allow the Poland to deposit on the surface so that my deterioration of the heat transfer performance is within the level of my expectation and then I have to definitely definitely clean the heat exchanger so that it work for us in a better way so there are many aspects of fouling fasting at the design stage I had to take some sort of a precaution because I'm so that ruling does not take place in a ramp into it precaution so that mine design is slightly oversized to some extent oversized true that even with filing also I am capable of having the plant performance what I have paid for and then I have to also decide regarding the cleaning strategy or cleaning cycle training schedule so all these things should come from some sort of simple calculation where do the phenomenon is very complex so some sort of simple calculation because many simplifying assumptions has to be taken and we have to depend largely on handbooks codes and standards how to take care of Audi or the extra resistance due to family that has been discussed earlier when we had the design of heat exchangers simple heat exchangers so that part I will not discuss other other aspects of housing that we discuss it has to be kept in mind that all the his standard Suffolk heat exchanger all the data book of heat exchanger will have information regarding regarding the housing register and we have to select the appropriate housing register from those and obviously the engineer’s experience regarding this goes a long way selection of the proper housing sector has how many distance we go to the nextthe decrease in overall heat transfer coefficient because a new resistance element is coming into picture probably two new resistance elements will coming to pick check because to fluids are flowing through its up throwing separated by a solid surface and on both the sides of this solid wall there will be depositing of founding father without using liquid is more significant than its supposed to be considered liquids liquids and gas liquid there will be hiring more compared to the guide it is Indian but they’re also process heat exchanger and clean condition as you is equal to 1501 circle with an example that is our who is one by you that is 6 into 10 to the power – 1 – 413 – 4 metre square tell me what we have got the overall resistance for this particular application how many distance let’s say hot side and cold side combined together is 3 into 10 to the power – 4 1 metre square kelvyn park what is considered so here it is 6 and here it is 3 so you see 50% extra heat transfer area is chargeable to Foley okay becausewhat are the transfer coefficient of a gas to gas heat exchanger is much lower compared to a liquid heat exchanger so far is of this order and then howling the distance is 3 into 10 to the power – 4 which is equal to that what we have configured in case of liquid is equal to that but the distance is now is now onlyso you see that absolute value of howling the distance that is not very I mean that is important but that is not the only thing to be considered how much data duration of heat transfer will take place that depends not only on the absolute value of having resistance this example shows that very clear ok what could be very detrimental falling in a clean condition or otherwise clean condition could be very detrimental the condition that may not be that man serious serious problem examples and examples what’s impractical feel they will get familiar with this kind of examples in this coursetravelling is very important so many cases we will not be able to go into much detail some basic idea regarding found in different kind of founding there a strategy to mitigate them doors will be the concern for the pregnant course let us go to the next slide again we are assessing following is less than the liquid side folding this is what we have shown by calculation also I will clean clothes system clean very clean closest like refrigeration system and the refrigerator refrigerator is circulating through some sort of a closed system and there are filters etc liquid soap powder small compact heat exchanger compared toArgos compact heat exchanger design 14 condition they cannot be designed for I mean they cannot be used for very dirty condition but asserts the tendency of houses will be late in the glove compartment and see where they are operating should be also not encouraging competition because if there is any plot is obviously one can understand the consequencessuspended particle diameter should be should be much less and the minimum passage area should be at least three times more than this suspended but what does it mean it means of course ID system designed level we have to take care we have to also take care in selecting a proper filter in a circulating system there should be filtered that what should be what should be the efficiency of the filter that we have to decide based on this kind of consideration but what happens many cases we think of only the particle which can be there in the circulating liquid but this particle sometimes argument we have conceded only the waist size of a particle but being too during circulation and which time they can be agglomeration there can be what’s the position with different reason that I’m taking care of all these things taking care I’m in taking even after taking all the precautions there could be some sort of a falling in case in case of competitive the cleaning anyhow there should be a winning strategy for the heat exchanger let’s go to the next play some sequence events ino nucleation this is very important shuffling means a new species is coming that was not there in the original liquid or original fluid or that was thoroughly mixed harmoniously mixed in the history that is getting separated out and that is getting deposited on the surface it transfer surface that is how to pouring again let me tell you that generation of new species or separation of the species from the circulating fluids and getting it deposited already transferred surface that is so for that thing many cases what is needed the first event of fouling is heterogeneous nucleation heterogeneous nucleation because it will not take place every where it has it has to take place for the falling to be more rather falling to be effective effectively there could be falling if these new creation take place on the solid surface all the solid surface when the new creation is taking place this is called a heterogeneous nucleation so heterogeneous nucleation is in many cases that is the inception of the filing that does not take place immediately you start operating the brand new heat exchanger it takes time so that is the latest after that there is transport transport is the transfer of the key components like oxygen from the bulk fluid to the heat transfer surface ok so this is transport so there should be some sort of a transport different mechanisms like diffusion there will be diffusion because of a concentration gradient if the electric field electric field it may be due to potential difference of different components and an electrolyte is present so electrophoresis diffuse porous is sometimes the material diffuse and then content so very solid surface for this motion is diffuse porous is sedimentation it need not be explained what is sedimentation the Olympics supposed all particles are there in there is some sort of infection and the particle gets deposited on the solid surface turbulence down sweet beautiful and some particles in the damn floor direction etc are responsible then first there could be some sort of education then there could be some sort of transport then these things should get attached very surface condition and blue conditions are responsible for this attaching does not give the I mean it is not the final stage of how so what is getting attached that will get removed also give Lucy a four-star Boulevard solution again it is getting salt getting resolved in the dissolved in the fruits cream and then erosion so what is what is depositing that may permanently not stay on the surface because their tooth removal and this this process is a dynamic process there is d position there is removal etc and then ultimately they deposited layers should get aged they should consolidate on the solid surface on the heat transfer service so mainstream that is most of the cases are being we can be found in deposit so then some sort of a permanently we get an amount of farmland deposition on the surface and we call it for me we calling family so this is why we get during falling influence of operating variable only quit cycling shall we have used some symbols the arrowhead is upward direction that means city increases arrowhead Inn in the downward direction that means decreased arrowhead in the direction that meansI like depending on other condition and when there is no arrowhead but not much information is available we do not know what will happen temperature or on the precipitation it may increase it may decrease temperature that decreases high temperature at the Arctic region obviously particulate that may increase or decrease concrete proofs of increase decrease and both way that has been out of that I am not going on explaining each and everything so these are available to users are information and from this information let us take another example pressure pressure in case of chemical reposition so it will increase but biologically it has shown increasing trend in some cases it has shown decreasing train in some other places similarly last mass concentration impurities pa this is just for information please go to it is informative but nothing much to be explained here from your common sense you can call it that why a particular factor is increasing a particular type of howling so that you can read now influence of operating on that site found similar situation has been shown here with you is you this thing new parameters for Westside bowling particularly if so what kind of owl is it going to be passing there are all devicesone has to be very much concerned that what is the effect of fungi because we deal with commercial product there are different compositions the gases that a high temperature and even there are on Bond and wonderful particle effect which can get deposited so different type of housing can take place it is not a clean grocery so we have to be careful and what happens in case of fuel I mean what is the effect of poor Larry issue that has been discussed so I think we have got some idea regarding calling to our discussion has not ate we will continue for at least one more lecture with fouling so far so far, I thank you for your attention and we look forward to move some more thing regarding fouling in the coming lecture. Thank you