 Okay, we are open for discussion. So, I just go in order of how things are here. Center 1-2-0-7, Jawaharlal Institute of Technology, Boravan. Ideabotic saturation process is represented on the psychrometric chart by an inclined line. Yes. Along the constant enthalpy line. But how it is adiabotic saturation? Because during that process there is a change in enthalpy and dBt and omega is also changing. So, how can we say that it is an adiabotic saturation? Please clarify. See, on the psychrometric chart, you said that which line was the same as which line? The WBT and H line. Is that what you claimed? It is shown along a constant enthalpy line. See, the constant WBT line is drawn as the adiabotic saturation line and that line is not the same as the constant enthalpy line. There is a very, very small difference and the difference is arising because you know definitely it is not the same and it is only slightly different because you add a small amount of omega and it does not create so much difference in the enthalpy. But the adiabotic saturation line is not the same as the constant enthalpy line. So, they are different. It is just that when they are plotted they seem to be so close by that you think they are the same, but they are not the same lines. So people normally will plot a small deviation, but they are not the same lines. So, some people may say go ahead and take it as nearly the same, but it is not really the same. What is the practical application of adiabotic saturation process? See, from the practical point of view, what is known is that let us say you want the state of air at any point. So that means I need to know the specific humidity because I need to calculate all my air conditioning processes using my specific humidity. Now if I need to know the specific humidity, I need to know the partial vapor pressure for water vapor and I have two methods to do it. One is using the dew point and one is using the wet bulb temperature. Now it turns out that the wet bulb temperature is the more practical method because that can be easily done and that is why you need to know what the wet bulb temperature is. The adiabotic saturation is very close to the wet bulb process and hence they teach you that. Otherwise from the practical point of view, it is the wet bulb temperature that is always measured and using standard empirical relationship, you get to PV and once you know PV, all other psychrometric calculations can be done easily. So from the practical standpoint of view, the wet bulb temperature is what is needed. So that is the importance from the practical point of view. Thank you. 1148, if you have a question, go ahead. The psychrometric chart is designed for one atmospheric pressure. If the pressure is more than one atmospheric, how to calculate? As I said during my lecture that as long as you stick to the steam tables, you do not really require the psychrometric chart but if you want to draw the psychrometric chart for any other pressure than one atmosphere, you are free to do so because all you need is to measure the barometric pressure at that point. If it is more than one atmosphere, everywhere in the formula where the total pressure comes in, substitute that and get a new chart. So people do this for various pressures and you have psychrometric chart for various pressures. So that is not a big deal. So as I told you for the psychrometric chart, all you require is somewhere you need the P total, you substitute your pressure at that point, you will get all your constant H lines, your constant W lines. They will just be slightly different than what is there at one atmosphere. Thank you. One, two, zero, five. Go ahead with your question. Like carrier equation, you have other equations also. Yes. So this carrier equation that you have given, as you drive under adiabatic condition or the actual condition? No, this carrier equation is just purely an empirical relationship. So it is not assuming anything else. So it is a purely empirical relation. On psychrometric chart, you have the constant enthalpy deviation lines also that you did not discuss. Yes, I mean I have not discussed every possible line out there. So there are small deviation lines which you can try to, you know, because if there are too many lines, there is a clutter and people normally talk about constant enthalpy lines and the deviation lines to get your wet bulb temperature. But I did not discuss all of those because I did not really want to get into psychrometric charts. As long as you can use your steam tables, I think that is probably good enough. Thank you. For evaporative cooling, there is a term enthalpy potential like, you know, temperature potential and all that. So this enthalpy potential term, when you discuss the adiabatic saturation temperature, the evaporation concept, the desert coolers. So that enthalpy potential term would have been discussed by right side. Okay. See, I am not so familiar with this enthalpy potential but what I can tell you definitely is that if you are using a desert cooler, there is going to be a difference in your omegas and the greater the difference in your omegas and the greater the potential, that is the greater the potential for the dryer to pick up more moisture and if you can pick up more moisture and use that much energy to convert that moisture, I think the cooler you can become. Probably that is where all this potential comes in. That is, there is some kind of a driving force is how people try to look at it. But finally, if you are very dry, you can put in more moisture and that is going to help you have a better desert cooler. I think that is all that is probably there. I myself am not so familiar with these terms like enthalpy potential, etc. But I can tell you what the rough principle would be for such a case. Thank you. Sir 47. In the calculation of wet bulb temperature that you are told by experimental method that reference is taken that of bulb or with that of air because you told that that equilibrium is reached then only the temperature is calculated, wet bulb temperature is calculated. My question is whether the reference is taken as the bulb or that of air? Yes, see what you are going to measure is directly the temperature of the wet bulb. So technically as you see you are really measuring the temperature of the bulb not of the air. So I think that is where the problem arises. So you are actually measuring the steady state temperature of the bulb. That is the main thing. Thank you. Sir 47. Is there any empirical relation available to calculate the wet bulb temperature based on dry bulb temperature? See the wet bulb temperature is uniquely determined by a dry bulb temperature and your vapor pressure. So using the same correlation with PV and dbt you can calculate your wet bulb temperature. It will only be a trial and error process. You will need to find out or guess a wet bulb temperature, get PV sat at the wet bulb temperature, substitute and see if you get the right PV. If not change your guess it is a trial and error process but you can use the same equation for it. Of course for a particular dbt there will be lot of wet bulb temperatures it will only depend on your vapor pressure. So for a particular dry bulb temperature you cannot have a unique wet bulb temperature. You will have a multitude of wet bulb temperature. Thank you. Sir why the speed of a supersonic aircraft is measured in terms of Mach number only why not kilometer per hour or miles per hour? Actually it can be measured very well in terms of kilometers per hour or miles per hour. There is absolutely no reason why it cannot be measured in kilometers per hour and so on. It is just that at a particular elevation let us say if the aircraft is flying there is a certain temperature at that elevation corresponding to which there is a certain speed of sound that exists at that elevation. And it may be little bit of a convention to mention the speeds in terms of Mach numbers but once you know the elevation where the aircraft is flying and the speed of sound corresponding to that elevation you can always convert the Mach number in terms of the actual velocity namely so many kilometers per hour and so on and vice versa. So there is no hard and fast rule that it has to be one way or the other. Over to you. Thank you. Here in the morning we have assumed that the adiabatic system during the derivation of that Mach number related problem there instead of assuming it as adiabatic if it is some other processes constant pressure or constant volume based where heat is transferred or heat is lost from the system. In that case what might be the possible changes in the final equation? Actually physically it is incorrect to include such effects because physically when you are talking about a sound wave propagation the process is actually isentropic because there is hardly any time for heat transfer. So under that assumption it makes sense to talk about the sound wave propagation. So I think my understanding is that the process actually that occurs is as close to being isentropic as possible and therefore at least my understanding would be that there is no point talking about a heat transfer type process occurring to make it any physically meaningful. Sir you told that enthalpy it is only a function of yes sir enthalpy is only a function of temperature you told sir in your session. Sir is it true for all the processes or is it true only for ideal gases? Yes I think you have already learnt last week that as far as ideal gases are concerned enthalpy is a function only of temperature. So you can safely assume whether a gas is ideal or not based on that you can decide whether to go for this assumption. Now what we can see normally is that at very low pressures even water vapor behaves very close to an ideal gas and hence this ideal gas assumption can be brought in and h can be roughly shown to be only a function of temperature. There is a small effect of pressure but that we neglect and we go ahead. So it is as close to an ideal gas at these low pressures thank you. Sir if a shock wave is moving in certain medium sir okay if that medium suddenly changes to take the infinite density sir. So what happens to the shock whether they collapse or changes? Well if the shock remains as a shock you will still have a density jump across the shock whether the medium into which it is moving is at a lower value of density or higher value of density. So I think the shock will not really necessarily lose its identity is what I can think of. For water vapor if it is reference temperature is 0 degree centigrade in that case what will be the reference temperature to calculate the enthalpy of the air? The reference temperature for dry air is taken as 0 degree centigrade again this is for ease of calculation and we would have preferred that the reference temperature for water vapor is also at 0 degree centigrade. However you realize that there is a problem that you know at that temperature okay you do not have a liquid vapor interface and it is only at 0.01 degrees that you start to have a liquid vapor interface that is the triple line. So it does not matter we just say okay 0.01 degrees and 0 degrees there is not too much difference. So saturated liquid at 0.01 degrees is a reference 0 for water and temperature is equal to 0 Celsius is a reference for dry air. So this is the convention by which most psychometric charts are made thank you. 1105 what are the physical significance of wet bulk temperature and dew point temperature and question number two so you have shown a formula for the to determine the vapor pressure so there are so many formulas to determine vapor pressure so which one is correct and which one is valid region we have to take it for consideration to solve the problem. Okay so I mean I do not know what you would mean by a physical significance see what you really need for calculating most air conditioning processes is that you need to know what are the comfort condition and hence you need to know what is the relative humidity. So as far as we are concerned the dew point temperature is there purely to calculate the omega so that we can get the relative humidity and how much water vapor is being carried by the air so that is why we need the dew point temperature if someone wants to make dew out and you know remove water out of the air for a separate process you know where you know in deserts people want to get water out then you would require a practical practically the dew point temperature for that purpose otherwise in our case we are using it purely to get omega and similarly the wet bulk temperature is also to get omega both these quantities are there only so that we can get omega and using that we will get our relative humidity and this is used for all our calculation. Now regarding which equation to use for the wet bulk temperature these are all empirical relations and it is not so easy to tell you know which one would be the best fit but we would prefer to go with the carrier equation it is a reasonably well known equation and that is why we are preferring it I do not think that we have really some kind of solid proof that this is far better than the other one or something it is just that we are going by a really well known equation which is the carrier equation thank you. Sir normally shock wave application I heard that in aerospace department only so where we can find in mechanical departments of this shock wave application and for what mach number in mechanical department we have our yeah you are right typically the compressible flow and applications of shock wave really fall under basically a high speed flow category situation which mechanical engineers typically do not deal with you are absolutely right about that normally mechanical engineers do deal with incompressible flow which is usually a low speed flow where you normally do not see these kinds of phenomena only when you are talking about situations like for example propulsion type application where you have a nozzle or maybe even some sort of a wind tunnel application where you are using a supersonic wind tunnel that is where you will need these kinds of ideas but on the whole I will agree with you that mechanical engineering need not always get into these kinds of situations. Hypersonic flow means what is the mach number? Hypersonic flow typically as a rule of thumb people will say that when the mach number goes beyond 5 if it is mach number of 5 or more we consider the situation to be hypersonic and this is purely based on the rule of thumb there is no reason to say that if it is 4.85 it is not going to be hypersonic so roughly speaking above 5 is when people will take the situation to be hypersonic thank you. Sir by designing any conditioning system we will speak about something by a bypass factor of coils right sir so I want to know what does bypass factor means. Yeah I mean there is a question in fact the last question in the exercises with the bypass factor so what really happens is of course I mean the moisture is flowing over coils which have the refrigerant flowing in it and the coil temperature is really low and you expect that the moisture will condense on to the coils and hence the specific humidity will decrease. Now not every part of the moist air is in contact with the coils because the coils are not you know of infinite area and what we just assume is that after passing over the coils the air will again mix and there is one common temperature and specific humidity. Now the bypass factor is just purely you know a fudge factor which we will say is how much amount of air has actually been in contact with the coil so if all the air has bypassed the coils then we say if the bypass factor is 1 which means the coils have had no effect on the moist air and it just leaves with the same temperature and pressure same temperature and specific humidity. If the bypass factor is 90 percent or 0.9 we say ok 10 percent has really come in contact with the coil and its temperature has gone down to that of the coil and hence when we mix you know it will be a mixture of the net air plus whatever has come in contact with the coil and so on. If there is zero bypass it means everything was in contact with the coil and it has reached the coil temperature. So the bypass is just a first factor telling us how much probably air was actually in touch with that coil so that is all about it. Thank you. Sir what is the Mach number for incompressible flows? So the question is about Mach number for an incompressible flow. So this answer depending on who you are asking you will get this answer but roughly speaking for incompressible flow the Mach number tends to 0 it is a very very small value. So you know technically speaking you can say that if the Mach number is less than 0.3 we end up treating the flow to be incompressible but if it is truly incompressible flow the Mach number is actually very very small tending to 0. Thank you. 1128 B19 Akpur, go ahead. Sir in psychrometry we have humidity ratio that degree of saturation and relative humidity and one more term I have come across that is mass of water vapor per meter cube of dry air. So out of these four terms related with moisture contained we generally do emphasize on relative humidity like in air conditioning calculations we take relative humidity in weather data also we have the data related with relative humidity. So why it is so? Why other terms are not generally taken? Yeah this is purely a matter of how comfortable we are. So for example the relative humidity gives you an idea of how moist the air is. So if it is 100% relative humidity you know that you know it is really humid as far as it can take and you can no longer push moisture in it. Whereas if I give you specific humidity there is clearly no clue on what net moisture can you know get in. And since the relative humidity is expressed in some kind of a percentage it is a pretty good idea of how dry the weather is and we seem to be comfortable with it. So that is why for comfort conditions just to tell how moist the air is relative humidity is a very very convenient quantity. However let me tell you that if you want to do calculations it is a specific humidity which is important because that is why on the basis of which we calculate all our enthalpy. So for calculation purposes the specific humidity turns out to be a better or more convenient quantity whereas to tell people about how comfortable things are relative humidity turns out to be a better quantity. Thank you. My second question at atmospheric pressure how can you say water vapor will behave as a perfect gas? Yes so at this point at least definitely the water vapor is not at atmospheric pressure it is very very at a very very less pressure the partial pressure of water vapor is very very low and most of the dry air is at atmospheric pressure and let me tell you that at atmospheric pressure if you are at reasonable super heat even steam still will behave as a reasonably good ideal gas. So you can actually get to the steam table and you know calculate the properties if you are reasonably super heated above 100 degrees even at atmospheric pressure you will see that the ideal gas behavior still remains. So I mean finally water oxygen and nitrogen they are reasonably super heated even you can draw a dome for them and figure out where it will behave like an ideal gas and where it will not behave like an ideal gas. And at atmospheric pressure there is a reasonable good ideal gas kind of behavior irrespective of anything. Thank you. 116. What is the practical usefulness or the significance of entropy we have been making these calculations for many of these systems engines and turbines and such but what is real what is really the usefulness of this number. Yes so from an engineering point of view we take entropy purely to see whether process that you have defined is practical or not. So for example you can create any process that you want but whether it will really happen or not is what we are interested in it and you can be sure that if it is an adiabatic process then unless the entropy remains the same or increases and in all practical purposes it should increase then we can say this is the process which is possible and let me tell you you can invent any kind of process and you can believe that it may happen but I am telling you most possible would happen just because you think that they may happen and the use of entropy is only to see whether that process will happen or not. Now in cases of turbines and compressors it is also used to figure out how much we are deviating from a very efficient engine or an efficient turbine. So there is something called an isentropic efficiency and if you are very close to the isentropic efficiency that means you assume an adiabatic turbine and if you can maintain the entropy same then you can get the maximum work output. So the further away from isentropicity you are the less work output you get and hence you can always think that yes I have a machine which is not that great and how is it that I can get closer to the isentropic situation. So again this is a practical use you know that you can work on something which is not totally out of the world and still try to go towards isentropicity. So that is the whole idea just for our engineering take we need to know how close to practical or how close to implementation we are. Thank you. See how it can be helpful and useful to evaluate the efficiency or the performance of any system. For instance again if we dig further into it entropy can be again associated or related with volume as well as state of order or disorder. So for instance if I have a system or I have a certain amount of matter or gas and if I increase or decrease the volume in a sense increase or decrease the state of order or disorder I don't really see any value in it in the sense that one will be more efficient or will have more sense of orientation so as to derive or achieve more efficiency from the system so that's my primary contention on it. So let me give you an example see just you know just following this thing that there is order and disorder will not help you so much but I tend to give a regular example that let's assume that you have a stone and there are 10 people pushing it and all of you are pushing in different direction the stone is not going to move too far or maybe it will never move but if all of you stand on one direction pushing it you will all have put the same amount of effort but would have moved the stone far away so that is the kind of thing that you get if you try to evaluate entropy. So I have a turbine if I ensure that the entropy remains the same you yourself can see from the steam table that I will get the maximum work output if I start increasing the entropy your work output will decrease and you know that you are not doing something so correct and you can try to get towards the maximum or the isentropic case and you will ensure that you can get as great work output as possible and that is it's just an engineer's way of looking at it this is what we can do so a regular physicist will probably never find a use for entropy but as an engineer we realize that there is this ideal situation and if we get as close to it as possible we will get maximum work output from a heat engine and we should try to go there so that is the most practical use of that you can think of thank you. Hello sir so one question you want to ask regarding the psychrometry chart you have taught us the psychrometry chart in the previous session one very important thought about the psychrometry chart is that it is very useful under the conditions of normal atmospheric pressure we are having any two values and we can calculate each and every value of psychrometric properties if the value of atmospheric pressure is changed under those situation can you explain me how this chart will be useful for us to calculate the different properties of vapors like that. I think I have already repeated this point two three times that this is the only reason that we are emphasizing steam tables that is because we don't want you to use the psychrometric chart in a thermodynamic course the psychrometric chart can be used only at regular one atmospheric pressure so for example if you go to a hill station and the pressure is only 90 kilo Pascal you will have to make a different psychrometric chart which will be used only there so whereas if you have the steam tables you will have no such problem so right now the emphasis is please use the steam tables don't look at the psychrometric chart because that can be used only at one atmospheric pressure if the pressure is you know even 10% off then sorry you can't use the chart at all so thank you my question is that the adiabatic process in psychrometry is correlated with the constant WVT and enthalpy constant how it is correlated so I think again I have probably answered this question earlier the adiabatic wet bulb temperature is an adiabatic process it's not necessarily a constant wet bulb temperature okay there is only a very small difference when you plot it on the psychrometric chart and hence people tend to think that these two lines are the same but these two lines are not really the same they are actually different things they are no you can just see that the difference arises because if you change the specific humidity in a certain range it's not going to change the enthalpy too much so that is why you think that it's nearly a constant wet bulb temperature line whereas in reality this is not correct thank you 1088 go ahead sir this is regarding the velocity of sound and psychrometry so it is given that the velocity of sound is given by the Newton's formula Laplace correction it is the under root gamma bulk modulus of medium upon the density of the air so it is practically found that the velocity of sound is more or travel faster in a moist air than a drier so the question is that the velocity is inversely proportional to under root of density of air so it is a faster in a moist air and it is less in a drier so how the density is less than drier moisture density is less than drier that is the way I understand it is if you are dealing with moist air you will have some amount of liquid also present along with the air and consequently the mixture that you are dealing with will have properties very different from that of the air and in fact if you see if you are dealing with a pure liquid sort of a situation the speed of sound is much much higher than what is it in air I mentioned that in air it is roughly about 350 meters per second but if you look at liquid it goes to something like 1500 meters per second so moist air is something in between these two extremes is what I will say and therefore the speed of sound would actually be higher than 350 meters per second but less than 1500 meters per second this is what I would like to mention thank you