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Welcome to the screencast on humidity charts. Also known as psychrometric chart. Hopefully
we will cover some examples on how to use these charts. How to go reading through them
so that you can utilize them efficiently on problems involving water-air systems. In the
third edition in your book has 2 charts. Figure 8.41 and 8.42, which are on pages 385-386.
These are are both for SI and american units, and I will prominently work in SI units, just
to keep it fairly simple. It is important to note that this chart that you are look
at for SI units. There are 2 reference states. One reference state is being water and 0 degree
Celsius and 1 atm, and the reference state for dry air is 0 degree Celsius, and 1 atm.
Now lets start by introducing a few terms that are worth knowing. The dry bulb temperature,
which is reported on the x-axis. Is the temperature of the air as recorded by a thermometer. In
this chart goes from -10 degrees Celsius to 55 degree Celsius. At absolute humidity is
the amount of water in the air or also known as the moister content. This makes the y-axis,
and we see something like this kind of before. Where we used the saturation pressure of water
and temperature over the total pressure to determine the mole fraction of water in air,
and this is just converted over to a mass fraction. Where we have the mass of water
per mass of dry air. Relative humidity is the relation of the astro moister content
to the saturation content with the given temperature. The left most line of this chart represent
saturation. Now if we look at the other lines that kind of run parallel. You see that would
be 20 percent humidity this would be a line for 40 percent, 50 percent and so forth. So
given a certain temperture and relative humidity you can easily see what the moister content
of that air would be. The dew point can be determined if you cool your system. Moving
horizontally to the left until you have reached saturation. If I chose 25 degree Celsius,
and say 50 percent relative humidity. Then we would fall right here on this chart. Now
the dew point temperature for that system. We would cool our air down until saturation.
This temperature is about 14 degree Celsius. So given air, at 25 degree Celsius, and relative
humidity of 50 percent. The dew point temperature would be 14 degree Celsius. Lets try the following
problem. Find the dew point of the air water system of the dry bulb temperature is 80 degrees
Fahrenheit, and the relative humidity is 50 percent. Since we are given Fahrenheit we
can either convert to Celsius or use the american engineering chart. We go to 80 degrees Fahrenheit,
and 50 percent relative humidity. We are going to say that falls right about here. If we
follow horizontally until saturation. We get to about 58 to 59 degree Fahrenheit. So the
dew point temperature of that system is 58 to 59 degrees. We can also determine enthalpy
of an air water mixture. This is relative to the reference state. If you move along
the diagonal line up to the left you can read the enthalpy is. So for instants, that initial
system we can move upwards to the left, and our enthalpy would be somewhere in between
31 and 32 Btu per pound of dry air. So lets try an example problem where we need the enthalpy
of our air, and I am going to go back to SI units. If a tank contains 10 kg of saturated
air. With the relative humidity of 100 percent. The dry bulb temperature is 20 degree Celsius.
Find the enthalpy of this mixture. Alright, so we can start at 20 degree Celsius saturation.
Moving up to 20 degrees to our saturated line. Then think of this mixture to be about 57.6
kJ/kg of dry air. We know that at this point we follow the thermal moister content, and
we get the moister content to be about 0.015 kg of water per kg of dry air. This tells
us that we have after recalculating a mass fraction of air of about 0.985. We can multiply
that by our 10 kg that we start with to get about 9.85 kg of dry air, multiplying that
by our enthalpy of our air. We can figure out the enthalpy of our mixture is. It comes
out to about 567 kJ. We can also determine the amount of heat to raise the temperature
of the system. Using these enthalpy readings. Lets try the following example. A saturated
mixture contains 100 pounds of dry air. How much heat is required in Btu to raise the
dry bulb temperature from 30 degrees Fahrenheit to 70 degrees Fahrenheit? If we go back to
one of our american engineering units chart. We can read at 30 degrees Fahrenheit, saturation
right about here. We have initial enthalpy of about 10.9 Btu/lb dry air. We need to raise
the dry bulb temperature form 30 to 70. So we need to move horizontally until we get
to 70 degrees. Then we read the enthalpy at that point. If we take that line up so we
can read the enthalpy. We get about 20.8 Btu. lb of dry air. So we know our change in enthalpy.
Is going to be 9.9 Btu/lb of dry air. We are told that we had 100 lb of dry air. So our
Q, or heat needed to do this out be 990 Btu. The chart also provides the humidity air volume.
Provided down here.Which is pretty much specific volume of a mixture in terms of meters cubed
per kg of dry air on our SI unit graph. Now these are the diagonal lines with negative
slope, which then you can read, the volume of the air in water take up. When you use
these values. You need to make sure that you are using the correct units to convert. To
figure out the actual volume of your air. So it maybe helpful to use the moister content.
So you can calculate based on a humid air back and forth from the dry air. There are
2 more features on these charts that I would like to go over. One is the wet bulb temperature,
which is one of the harder attributes to grasp, and is best defined by how it is measured.
Thermometer bulb is rapped in a cloth that is wet with water. So that it is saturated.
There moves past the bulb water will evaporate and the heat associated with this evaporation
will cause a decrease in the thermometer temperature. The final temperature reading upon equiliberating
assume that the cloth stays saturated is the wet bulb temperature. Thus you can see or
realize quickly that in a saturated environment the wet bulb and dry bulb temperature are
the same. Since no evaporation will occur. Now if we go back to our problem. Where we
have say 25 degree Celsius air at 50 percent relative humidity. We can figure out the wet
bulb would just be the following these lines until the saturation point. So our wet bulb
temperature is about 18 degree Celsius. Meaning that, that 50 relative humidity air at 25
degree Celsius would cause enough evaporation to occur. Such that the surface temperature
of that wet bulb temperature. Would actually read the 18 degrees. You can kind of think
of this. Like when you get out of a pool. Your body is wet, and as it starts to evaporate
you get colder. So obviously you are going to feel colder. Depending on the relative
humidity of air. So lets try the following. Two large containers initially contain dry
air at 40 degree Celsius. A small amount of water is added to the container 1. SO that
the relative humidity becomes 30 percent. Lets see where that would be on this chart.
Right about here, that is container 1. Water is also added to container 2 so thus the relative
humidity becomes 80 percent. Lets go right up that line. All the way to 80. It is obviously
out of this chart. Based on these two conditions. Which would have a lower wet bulb temperature.
Even though we cannot see what is up at 80 percent humidity for that case. We do know
that following these horizontal lines. This wet bulb temperature for container 1 would
be much lower then the wet bulb for container 2. So we can safely say that container 1 has
a lower wet bulb temperature. The last thing that I want to mention on this is the enthalpy
deviation. For non-saturated systems you can account for the difference in enthalpy from
the saturated state by adding the correction term written on these plots. You can see the
correction terms by these other lines that kind of run vertically on the graph. So for
instants if we had say 30 percent relative air, at 30 degrees Celsius we would be right
along that correction for the enthalpy deviation. So that when we read the enthalpy. Lets say
about 51 kJ/kg of dry air. We add this correction term -0.4. So that our enthalpy of this system
will actually be about 50.6 kJ/kg of dry air. This is a nice quick way of figuring out non-saturated
systems. What the enthalpy would be for the system. Hopefully that helps you understand
these charts a little bit more, and now you feel confident using them in your future work.