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In this video we are going to look at the definition of vapor quality and compare mass
fraction vs. volume fraction of vapor. The example we will use will be steam because
we have data that will make it easy to make that comparison. So quality is the mass percentage
that is vapor. A quality of 1 means it is saturated vapor. A quality of 0 means it is
saturated liquid. We have to be at saturation conditions when we are talking about quality
because we are talking about both vapor and liquid being in equilibrium. Let's consider
steam exiting a turbine at 2 bar and it has a quality of 0.9. What that means is there
is 0.9 kg of vapor for every 0.1 kg of liquid. Since it is at saturated conditions that means
from the steam tables we are at 120 degrees C to 2 significant figures that is the saturation
temperature. On a mass basis 90% of the steam is vapor. The question is what is the percentage
on a volume basis. What we are going to do is look up in the steam tables the specific
volume of the liquid and the vapor at 2 bar and 120 degrees C. These are specific volumes
for liquid, capital "L", vapor capital "V" and these are just from steam tables. We can
say if we had a kg that had a quality of 0.9 then how much liquid and how much vapor volume
do we have. The volume of the liquid that we would have, this is now just a total volume
of the liquid, it is a tenth of a kg of liquid with a specific volume of this many cubic
meters per kg. We have 1.06*10^-4 m^3 of liquid and likewise for vapor we can do the same
calculation but we are using now the value for the vapor volume. For steam with a quality
of 0.9 here is the volume of the liquid, here is the volume of the vapor and here is the
total volume. If I wanted the volume fraction that is liquid that is going to be the volume
of the liquid over the total volume. I have calculated the volume fraction and I have
also calculated the volume percent. So 0.1 percent the volume is liquid. This is the
volume fraction that is liquid which is now the volume percent that is liquid. Which means
that the volume percent that is vapor is 99.99 percent essentially. This is quite a bit of
difference when we compare mass fraction vs. volume fraction. Let's just do the calculation
from one other condition, the same type of calculation. A quality now of 0.1 so 10% is
vapor and 90% is liquid on a mass basis. Let's look at the volume percent. It is the exact
same type of calculation. Here is the exact same calculation for quality of 0.1. We calculated
the volume of the liquid and volume of the vapor to add and get the total volume and
then I have calculated a volume percent of liquid. Volume of liquid divided by the total
volume times 100. Even when it is 90% liquid the volume percent liquid is only 1% and still
98.9% vapor. by volume. The finally thing then to mention about quality is we calculate
the volume from quality by using the volume of the vapor, the total volume and the volume
of the liquid. The fraction that is vapor, 1-q, is the fraction that is liquid. We can
do the same thing for enthalpy where we are using values from the steam tables for specific
enthalpies, cubic meters per kg. The same thing for entropy and any of the other state
variables.