Going to look at a couple of examples using a psychrometric chart as a way to increase
your understanding of the way a psychrometric chart works, and what kind of information
it provides. The first one is "how much heat's required to raise the dry bulb temperature
of 100 kg of saturated air from 10 degrees C to 35 degrees C?" So let's do this one first.
We're going to take advantage of this interactive simulation, which is available online, and
I've given the link here for the simulation, and now let's go to that simulation and do
this first problem. So notice in this psychrometric chart I am only displaying some of the information
you'd typically see. I've removed grid lines and specific volume to make it a little easier
to read the values. So our starting point we're at 10 degrees C, and we're on a saturated
line, and we see the enthalpy per kg is 29. Now what we want to do is move horizontally,
I want to keep the moisture content the same. Just because we're heating it we're not adding
any water, but we're going to be lowering the relative humidity as we raise the temperature
because we can hold more moisture at the higher temperature, and so we're going to keep this
moisture content at .007 kg per kg dry air, so I am going to move the point. OK, so that's
pretty close, now you can see the relative humidity is only about 21% when I've raised
the temperature. The enthalpy now I read from this blue line is 53. Again, that's kJ per
kg. So now let's go back and look at the calculation. So we started out we were at 29 kJ per kg
for the enthalpy, as we raise the temperature we added energy, so we're now at 53 kJ per
kg, so 53-29, 24 kJ/kg multiplied by 100 kg, so 2400 kJ is the amount of heat that must
be added, so this is Q for this problem. Next let's compare air and see which has the lower
wet bulb temperature, so let's go back to the simulation. By clicking here I can get
a little more control over moving these sliders, so I've selected relative humidity, 30%, a
temperature of 40 degrees. This is the wet bulb temperature, that's 25 degrees, essentially
along a constant enthalpy line. Let's see if I can display the enthalpy lines. Now I
want to move to 30 degrees, and 60% relative humidity, so I move the slider and I can get
more control, 60% relative humidity, and I want to lower the temperature to 30 degrees, and
again I can get control this way. So now 30 degrees and 60% relative humidity, wet bulb
temperature again, I can read it off here, it's 24 degrees, so they almost have the same
wet bulb temperature, very different conditions, but I've lowered the temperature and raised
the humidity, so the wet bulb temperature is 24 degrees, let's write those down. So
the wet bulb temperature of this condition, 25 degrees C, here almost the same, 24 degrees
C, so both temperature and humidity of course affect the wet bulb temperature. When we get
a lot more information from the psychrometric chart, but this gives you a couple of examples,
again go to the simulation on your own, try various conditions to better understand the
behavior of air that contains moisture.