auraCodes/indian-english-hindi-tts-60min huggingface dataset:
So, uh, you may recall that, um, when we discussed, uh, moist air or rather, uh, defined the specific enthalpy of moist air, we actually defined, um, uh, defined.
Enthalpy of moist air, it would be in units of say kilojoule per kg air.
Okay? 
And we can then approximate  HA using the calorifically perfect assumption. 
And HV may be approximated as HG of T. 
However,  if you look at psychrometric applications, then
CP times T2 plus omega times T2 and CP times T1 plus omega times T1. So you notice combinations like this. So let me just show this with a different color.
Omega times H2 and you notice this combination CP times T1.
So you notice this combination. So,  what you are seeing is actually the specific enthalpy  defined on a per dry air basis or per kg of dry air basis. Here also you see the same thing.
This term, uh, these two terms go together and then
You can see that these terms go together.
Okay? This of course becomes Cp times T. This is hg of T.
Okay? And this is nothing but the specific enthalpy of moist air in units of say kilojoule per kg air.
in the usual manner. But in the applications or examples that we have seen so far, we notice that it does not  when you when you look at the  the energy equation that we have applied to different examples.
We notice that the terms do not appear in this combination but appear in a combination where the specific enthalpy is defined on a per unit mass of dry air. So basically instead of dividing by M, we divide by mass of dry air.
So if you do that, then we end up with this combination HA plus omega times HV, which is in units of kilojoule per kg dry air.
Okay? There is nothing wrong with this. Okay? So this is perfectly all right, okay? But this is specific enthalpy on a per unit mass of moist air. But in applications, when we apply energy balance to practical problems and
Okay? This is far more useful in psychrometric applications than this, okay? But there is nothing wrong with this also.
And H star is what we will use in our examples from now onwards, okay? But we will clearly indicate. So this is actually H of moist air on specific.
specific enthalpy of moist air on a per kg on a per kg moist air basis or per kg air. This is H star which is specific enthalpy of moist air on a per kg dry air basis.
Now, the new terms that we have introduced in psychrometry.
Uh, or like this. So this is a list of new terms that we have introduced. First one is the, uh, the partial pressure of water vapor.
PV psychrometric operations, unit operations remains constant. Generally, we assume the mixture pressure to be constant. Furthermore, in all the examples that we are working out, we have assumed the mixture pressure to be 1 atmosphere. But mixture pressure is usually a constant, which means that PV depends only on omega.
Temperature T and omega. By temperature, what we mean here is the dry bulb temperature.
May be written as HA plus omega HV and HA itself may be written as CP times T plus omega HG of T.
So we may write for the water vapor, uh, partial pressure of water vapor times volume of mixture equal to MV or V times T. So V itself may be written in terms of the, uh,
Omega times RV times T over PV, and if I replace PV in favor of omega using this relationship, I get finally something like this.
This one here. So if you look at this one again, I can gather CP times T2.
And the dew point is nothing but T sat of PV, and PV depends on omega, so this depends on omega only.
But the, uh, the important point that emerges from, uh, looking at the list of expressions here is that all the quantities depend only on two independent properties, T and omega.
Which is the basis for developing the psychrometric chart. So basically the psychrometric chart has two axes, vertical axis is omega.
And the horizontal axis is T, which is the dry bulb temperature, of course.
Okay, some of them interestingly enough depend only on omega. Let me denote this using a slightly different color.
And this also depends only on omega. All the others seem to depend on T and omega.
So basically the psychrometric chart, as you can see, has uh
and dry bulb temperature on the x-axis and lines of constant values of each one of these variable is given in the is depicted in the chart. So basically the chart
Plots, lines of constant PV, lines of constant phi, lines of constant H star, lines of constant VA, wet bulb temperature.
Now, if you look at this and this, because these depend only on omega, uh, that means if the axis is vertical, this can be shown very nicely in the axis itself. So let me just erase this.
So what is shown here, let me erase this also, just okay. So what is shown here, this is
PV, but instead of showing it in kilopascal, this is shown in millimeters of mercury, and the reference value 760 millimeters of mercury for barometric pressure is given there. So this can be easily converted into kilo units of kilopascal.
So PV depends only on omega, since omega is the vertical axis, PV axis is also vertical.
So this is the dew point, so this is dew point 0, dew point 10 degrees Celsius, 20 degrees Celsius and so on.
Okay? So that is shown parallel to the omega axis. So these two are straightforward and  so we had denoted them using this green color. So let me just show them again. So this is
In green, this is in green. They are dependent only on omega, so we know we can show them in the vertical axis.
Quantity whose dependence on temperature we don't really know is this Hg.
If you actually go to the property tables, let's say steam tables, and for the range of temperatures that we are going to likely encounter in psychrometric applications. We already mentioned that. So for the range of temperature,
kg, it varies only by about 100 kilojoule per kg, which is less than a 5% variation. So what happens is across the entire range of temperatures that we are seeing, Hg varies by less than 5%, which means that essentially Hg is a constant, more or less.
Which means that all H star equal to constant lines will depend linearly on T and omega, which means the H star equal to constant line.
is a line with a negative slope, meaning in the second quadrant.
Of course, when we draw this line, you know, we are drawing accurately. But this argument that we are making is just to get an idea of what these lines will look like.
Uh, this constant, uh, H star equal to constant, what does it look like on a, uh,
with the negative slope, which is why we have shown the chart in the second quadrant.
So H star equal to constant line looks like this. So it's uh these lines are uh it has a negative slope.
Or we can argue that HG is more or less a constant, so H star equal to constant is going to appear more or less like a straight line in with a negative slope in the psychrometric chart.
Now VA on the face of it appears to depend both on T and omega, but if you look at the second term here.
Omega is typically a number of the order of about, you know, 0.001 to maybe 0.01, 0.02, not very big, okay, when compared to 0.622, which means that essentially this term
Uh, depends on omega and T. However, for the range of values of omega and T under consideration, the slope varies very little and hence lines of constant VA are nearly straight lines but steeper than lines of constant H star.
So the next expression that we are going to look at is the expression for wet bulb.
Again, uh, this quantity, uh, omega minus omega, uh, wet bulb is likely to be a very, very small number, okay?
is going to be a very small number. So the second term probably is negligibly small, which means that lines of constant wet bulb temperature will coincide with the lines of constant H star. So as you can see here,
And you can see lines of constant wet bulb temperature here. So for example, 20 degrees Celsius, 15 degrees Celsius, 10 degrees Celsius. So you can see that they are almost parallel to
Lines of constant H star and that is what we are seeing in the chart also. So basically these are the quantities  contours of which are depicted in the psychrometric chart.
So let us summarize what we have said so far. So we have argued that lines of constant H are straight lines, almost straight lines with a negative slope in the second quadrant in T omega coordinate space.
And we also argued that 0.622 plus omega depends very weakly on omega because omega itself is very small in psychrometric application. So PV depends almost linearly on omega alone.
The quantity omega minus omega wet bulb is usually very small, which means that lines of constant wet bulb temperature almost coincide with lines of constant mixture specific enthalpy in the psychrometric chart.
So in this  lecture, we are going to  discuss  psychrometry.  Okay, so what do we mean by the word psychrometry? Basically,  psychrometry is the study of the changes in the state
and content of moist or humid air. And the primary application where psychrometry is applied is in heating, ventilation, and air conditioning, that is HVAC.
Okay. 
Now, at the same time, if the  humidity level is high, that is also not very comfortable. 
Although temperature may be okay, but  high humidity level is also not good because you feel sticky. 
Again, there is a sense of discomfort. 
Okay, so for
deals with that, okay? So essentially it looks at the state of moist air, which means air with water vapor in it, and also the content of moist air. So if the air is too dry, then we may want to add more water vapor to the air to make it more comfortable. If the air is too hot,
Humid, we may want to remove some water vapor again to make the environment more comfortable, okay? So comfort depends on both temperature as well as humidity, and that is what we are going to study in this module, okay?
Okay, how temperature, state of the air and content or in other words temperature and humidity of air can be controlled in these sorts of applications. So if you go to big buildings, the temperature and humidity are automatically controlled by the central air conditioning system.
Automobiles also nowadays have very sophisticated systems where you can set the temperature and everything is regulated, so you have a very good sense of comfort, okay? So we will see how, uh, how this effect is achieved.
In other words, how do we control the temperature and the moisture content in the air? Number one, how are these quantities measured so that appropriate control actions can be initiated? So we will look at both these aspects.
in this module. 
In addition to HVAC,  psychrometry is also of importance in drying and also in cooling towers used in thermal power plants. 
We will look at these applications, but by and large it's used in HVAC.
Okay? So the next question that arises is what is moist air? Okay, what do we mean by moist air or humid air? Because
Okay? So humid air does not mean there is a lot of water vapor in the air or moist air does not mean there is a lot of water vapor in the air. Moist air or humid air refers to the fact that we have air, just dry air plus some amount of water vapor.
Okay? The amount of water vapor, again, uh, is what we are actually going to quantify and try to control in practical applications.
So let us take, uh, say air at a temperature of 25 degrees Celsius and one atmosphere pressure, okay? So it contains a certain amount of water vapor at the same temperature, but depending on the amount of water, it is at its own partial pressure PV.
Okay? So let us look at the state of the water vapor and  PV TV diagram like this. So you can see that the water vapor is at a temperature of 25 degrees Celsius and at some partial pressure.
Okay, note that it lies on the superheated side, or in other words, the water vapor is superheated because the partial pressure is so low.
Because the amount of water vapor that is in the air is very, very small.
pressure of the water vapor, and as we increase the amount of water, the pressure increases. So the isobar moves in this direction, and as we decrease the amount of water vapor in the air, the isobar or corresponding to the partial pressure moves in this direction.
So the water vapor in the room is superheated on account of the fact that its partial pressure is quite small.
steam that comes into mind and you know, temperature is very high. But
Uh, that need not be the case. So here we have air at room temperature. The water vapor is also at room temperature, 25 degrees Celsius, but because its partial pressure is so low that it exists in the superheated state. Okay, that it's important to keep that in mind.
And the range of temperature that we are looking at is usually between 0 degrees Celsius and about 60 degrees Celsius.
for these conditions,  so 60 degrees Celsius, the isotherm corresponding to 60 degrees Celsius is here, and the isotherm corresponding to 0 degrees Celsius is here. So this is the range of
Uh, this illustration was actually discussed in the previous course when we talked about properties of pure substances, particularly steam.
and R134A. So roughly, this shared area denotes the region in which the vapor obeys the ideal gas equation of state to within 10%.
And we will assume that, uh, in this course, and that is a very safe engineering assumption to make. Okay, there is no need to use the tables. The departure from the tables is very small, so we can essentially use that and we will use that.
So these are the two important points that the water vapor in the room is superheated on account of its partial pressure being very small.
The water vapor is superheated and we will assume it to be an ideal gas which obeys PV equal to RT.
So, moist air itself may be treated as a mixture of two ideal gases. Once we say that we are going to treat the water vapor as an ideal gas, we may take the moist air to be a mixture of two ideal gases, dry air.
which is usually denoted with the subscript A, and water vapor denoted with the subscript V.
Okay, so the air in this room is taken to be a mixture of two ideal gases, dry air itself and water vapor. Note that dry air itself is a mixture of two ideal gases, O2 and N2, but we need not worry.
And this enormously simplifies the thermodynamic analysis of psychrometric applications.
mixture molecular weight. All these concepts have been discussed in great detail in the previous course, okay? So no new concepts are being introduced here. The only special thing is that moist air is a mixture of just two ideal gases.
Okay? So this is an application of mixture of ideal gases.
Its own equation of state, dry air PA equal to number of moles of dry air times RT over V. Notice that we are using the Dalton's model. So all the components occupy the same volume and are at the same temperature, only their pressures are different.
And since we have only two components, we may write the total pressure of mixture is nothing but the sum of the partial pressures of its components. So PA plus PV. And the total number of moles is also the sum of the number of moles of the individual species.
And so that is we can use that for both PV and PA.
So,  if you are not familiar with this, I  suggest that you  look at the video  in the first course which deals with mixture of ideal gases and then  revise these concepts before you proceed.
So the first new term that we are going to introduce, I'm sorry, introduce in psychrometry is the
Okay? The humidity ratio denoted omega is defined as the ratio of the mass of the water vapor to mass of dry air in a given sample.
sort of don't realize that you are talking about MV or masses of different substances in the numerator and denominator, okay? So it's always a good practice to denote this as MV over MA, kg vapor per kg dry air.
Now, we can write this since, um, by definition, molecular weight is, uh, mass divided by number of moles, we may write MV as NV times MV, and similarly for the dry air.
And if I divide the numerator and denominator by n, in this case, for instance, if I, if I do this.
So I divide the numerator and denominator by n, then nv over n may be written as
As we learned in the previous course, sum of all mole fractions should sum up to 1. In this case, there are only two components, so YA plus YV equal to 1, and I can replace YA in the denominator with 1 minus YV.
Now, if I substitute the value for molecular weight of water vapor 18 and for air 28.97, we get this to be
Omega equal to 0.622 times PV over P minus PV kg vapor per kg dry air. So this is an expression that we will use quite extensively.
Another term that is used  in the context of  synchrometry and HVAC application is relative humidity denoted by the Greek letter phi.
from practical experience,  you may  probably have  noticed the following. Let's say that we have a certain amount of water vapor in the air in this room. And we keep adding more and more water vapor, and beyond a certain point,
I will explain that in a minute. But the air appears to be quote unquote saturated. You cannot add any more water vapor to the air in the room. So if you try to add any more water vapor, it simply condenses back to liquid water. It doesn't stay in the vapor form or superheated form.
Okay? 
So, that means that there is a maximum to the amount of water vapor that this air can hold at this temperature. 
We have not changed the temperature of the  of the air in the in the room. 
So the temperature remains constant.
We try to add more and more water, which means that the partial pressure of the water vapor increases and the partial pressure of air decreases, but the mixture pressure remains the same, okay? So, so we keep adding more and more water vapor.
And it appears that there is a maximum amount of water vapor that we can add.
In the given sample, to the maximum mole fraction of water vapor that can be present at the same temperature.
can only hold so much of water. So that is the maximum amount of water vapor that the air can hold at this temperature. So the ratio of YV over YV saturated at that pressure and temperature is called relative humidity.
And as I said, the mixture pressure remains the same. So when the mixture appears to be saturated, the mole fraction of water vapor is a different value. It is likely to be higher because we have added more water vapor. So YV sat.
Times P is the partial pressure of water vapor at that state, okay? So the ratio comes out to be PV over P sat of T.
Okay, remember, once the water vapor, amount of water vapor reaches its maximum value,
Precisely meant by that is that the partial pressure of water vapor. The amount of water vapor is such that the partial pressure of water vapor is equal to the saturation pressure.
Okay, so that is called relative humidity. So relative humidity 100% implies that the air is
Any value less than that, we can say the air is not saturated, there is room for adding more water. So if it is too humid, so we say the relative humidity is high, then you feel sticky.
So 80%, 90%, 100%, you feel sticky. If the relative humidity is 20 or 30% and the temperature is also in a reasonable range, then we feel comfortable, okay? So relative humidity measures not only the amount of water vapor present, but also
The amount that can be present in relation to the temperature. So it brings in both the effect, the mole fraction of amount of water vapor and the temperature. Whereas the humidity ratio talks only about the amount of water vapor, not the temperature of the mixture.
So relative humidity is a very important term that is used in psychrometric applications and also if you listen to weather forecast, let's say temperature predicted tomorrow is let's say 26 degrees Celsius, relative humidity 50%.
In this chart, let me just clear some of these lines here.
So let's say we have water vapor at 25 degrees Celsius and certain partial pressure.
Okay, let's say that the partial pressure is less than the maximum value. And now, if I add water,
More water vapor to this air while keeping the temperature constant, the state of the water vapor.
moves like this. Because as I add water, there's a partial pressure increases and the state
Uh, the partial pressure keeps moving like this. So we keep moving along the isotherm into different isobars, okay? Because we are keeping the temperature constant, we are moving along the isotherm.
Okay, notice that this state lies on the saturated vapor line. So this is the saturated.
So this state lies on the saturated vapor line. We cannot add any more water beyond this point. If you do, the water will condense right back into a liquid.
The partial pressure corresponding to the given state, which is PV, divided by this pressure.
25 degrees Celsius, okay? Notice that this is the saturation pressure corresponding to 25 degrees Celsius. That is called the relative humidity corresponding to this state.
Okay? The dew point temperature is defined simply as the saturation temperature corresponding to the partial pressure of the water vapor.
So we have, uh, we have moist air at let's say 25 degrees Celsius and there's a certain amount of water vapor. So its partial pressure is PV.
So the saturation temperature corresponding to PV is called the dew point temperature.
Which means that the partial pressure of the water vapor remains constant. So we are as opposed to constant temperature process that we looked at in connection with relative humidity. Now we are looking at a constant.
Isobar like this until I reach this state point. So once I reach this state point, again it's on the saturated vapor line, and if I lower it any further, water will begin to condense. So this temperature, the corresponding temperature here,
This temperature is called the dew point temperature, and that is nothing but the saturation temperature corresponding to PV.
This is why, um, you see dew or dew drops on grass and leaves in the early in the morning.
Okay, so the air contained a certain amount of water vapor in the evening.
There is no more evaporation, so that is the maximum amount of vapor that you are going to see, right?
So as the temperature begins to drop during the night, the amount of water vapor remains the same.
Okay? The temperature begins to drop, and then if it drops close to the dew point temperature during the night, you will start to see dew drops on leaves and other surfaces.
Late in the evening is sufficiently high and the temperature drops sufficiently low during the night, then you will definitely see dew early in the morning.
Uh, as moist air is heated at constant pressure, it becomes drier, okay? So you can see this here.
So as we heat moist air at constant pressure, that means we are moving along in this direction. Okay, let me just change the color here.
So if I heat moist air at constant pressure, then I'm moving along this isobar like this. So as I keep moving up, uh, you can see that it becomes drier.
Okay? Because it's distance, so let us say heat it up to this point.
The saturation state corresponding to this lies over here. This corresponds to phi equal to 100. So this is phi equal to 100.
For this state. So as I move up here, you can see that here this state is closer to the saturated vapor line. Here it is far away from the saturated vapor line. So as I heat air, moist air at a constant pressure, the air becomes drier.
As a cool moist air at constant pressure, uh, the state gets closer and closer to the saturated vapor line, so the air becomes more and more humid.
Not only the amount of water vapor, but the temperature also. So relative humidity actually brings out the fact that perception of humidity.
Okay? So what practical HVAC systems try to do is to heat and cool.
And humidify and dehumidify air to achieve an ideal combination of temperature and humidity for maximum comfort of the occupants.
Humidify by adding water, dehumidify by cooling the air so that the water vapor condenses out.
Okay, humidification is done by adding steam to the air. Dehumidification is done by cooling the air so that the water vapor condenses out. So this is what HVAC systems do. But in order to do this, you need to know what the temperature is and also what the amount of water vapor in the air is.