I'd like to explain in this video the pressure- enthalpy diagram for single component system
and then specifically use the diagram for water to talk about some details.
So what I've shown here is pressure versus enthalpy.
Notice that this is log of the pressure, so that we can really see all the details of
this diagram, and enthalpy in kJ per kg.
So the red point, this is the critical point and this is an important part of the diagram.
And so what we want to do, let's first look at some isotherms to get an idea of how enthalpy
depends on pressure at constant temperature.
So if we look at 100 degrees C and we look over here at the line which is liquid, so
this is liquid, we notice the enthalpy is essentially independent, to a reasonable approximation.
At very high pressures we see some slight increase, but liquids are incompressible,
and therefore, not surprisingly their properties don't change much with pressure.
Now we go from liquid, saturated liquid, so this line along here is the saturated liquid
line.
And we go to saturated vapor, of course a big change in enthalpy due to the heat of
vaporization.
And now for the gas at 100 degrees C the enthalpy is pretty much independent of temperature
and this is behaving very much like an ideal gas because we are at relatively low pressure,
100 kPa and lower.
We go to 200 C it's very similar behavior, however at 300 C things are different.
So the liquid phase shows behavior that's essentially enthalpy doesn't depend on pressure,
significantly.
We have the phase change at 300 degrees C. But notice now that as we lower the pressure
at constant temperature, at 300 C, so going along this line, as we lower the pressure,
the enthalpy decreases.
Or the other way of course, at constant temperature as we raise the pressure, we actually decrease
the enthalpy.
So down here at the low pressures again this is behaving like an ideal gas.
Enthalpy is independent of pressure.
But at the higher pressures, and these are of course, pretty high pressures, two MPa
is 20 bar pressure, and we go from 2 MPa up to about, let's see, 8 MPa there's a significant
change in enthalpy.
If we look now at at isotherm that's in the supercritical region, namely above the critical
point, so let's look at the 400 degrees C isotherm.
Low pressure, ideal gas, but notice now, there's no phase change, but enthalpy certainly at
400 degrees C shows a rather large change in enthalpy with a change in pressure as we
go for example from 10 MPa up to, maybe 35 MPa.
A very large early change in the enthalpy for this supercritical fluid.
So enthalpy certainly depends on pressure in addition to depending on temperature.
So let's look at a couple other aspects of this diagram.
These green lines, for example this line, labeled 30% corresponds to 30% quality.
In other words this corresponds to 30% of the water is present as vapor, 70% as liquid
at each of the different temperatures and corresponding pressures.
Another thing we want to look at is this saturated line.
Notice as I increase the pressure the enthalpy increases for a liquid at saturated conditions
because the temperature is also increasing.
To stay on the saturated line, pressure and temperature both increase, to stay on the
saturated line at a higher temperature I have a higher saturating pressure, and therefore
the enthalpy increases.
In contrast, if I look at the saturated vapor line, this line over here, saturated vapor,
notice that the enthalpy is almost independent of presssure.
Even thought the temperature is changing.
So we are changing the temperature, we go from 100 to 200 C, and of course if we went
from 100 to 200 C at constant pressure there would clearly be a change in enthalpy, that
would be significant.
But going from 100 to 200 C while increasing the pressure, we see almost no change in enthalpy.
So there is a very slight dependence when we have both pressure and temperature changing
saturated vapor.
The final thing I'd like to use this diagram to show is, and one of the reasons that this
is a useful diagram, is if we look at, say a throttle process.
So if we start with a saturated liquid and we throttle it, namely adiabatic process,
flow process, enthalpy doesn't change so delta H equals 0 for a throttle process, adiabatic.
And that means if we started with a saturated liquid and we lowered the pressure we would
end up with a vapor- liquid mixture, and we would drop the temperature significantly by
throttling and lowering the pressure.
And of course this is what we take advantage of in a refrigeration system by dropping the
pressure through a throttle we get a large temperature decrease and we can get to low
temperatures where we can take advantage of this for refrigeration.