We were discussing “

*Triple point phase diagram of water”*and*“**TV diagram of pure substance”*in our previous posts. Now it’s time to go ahead to discuss another topic in the category of thermal engineering.
Today we will see here the concept of
quality or dryness fraction of steam with the help of this post. Dryness
fraction or quality of liquid vapour mixture is quite important to understand
before going for determining the thermodynamic properties of working fluid in
thermal engineering applications.

Let us recall the concept of

*“temperature entropy diagram of water*”. We have drawn T-S diagram in our recent post and we have understood the basic fundamentals of this diagram over there. Therefore we will only draw here the T-S diagram once in order to understand the concept of dryness fraction.
It is not necessary to draw the T-S
diagram only for better understanding of dryness fraction of steam. You can
also understand the basic of dryness fraction with the help of “

*PV diagram of pure substance”*.*T-S diagram of a pure substance*

As we can see in above figure, we have
shown here two constant pressure lines i.e. for 1 MPa and for 10 MPa.

Critical point is displayed here at the
top of the dome and as we have already discussed that critical point will be
defined as a point where saturated liquid state and saturated vapour state are
identical. Saturated liquid line and saturated vapour line meet with each other
at this point. We must note it here that above the critical point, we will have
gaseous phase of the working fluid.

Saturated liquid and saturated vapour
lines are also displayed in above figure. Region left side of the saturated
liquid line will be termed as compressed liquid region or liquid subcooled
region and similarly region right side of the saturated vapour line will be
termed as vapour region and these two regions will be single phase region.

Region falls under the dome will be
termed as saturated liquid-vapour mixture region or simply wet region.

We have also mentioned the various
regions such as liquid subcooled region, superheated vapour region and 2 phase
region or saturated liquid-vapour mixture region in above figure.

###
**Let
us come to the main topic i.e. dryness fraction or quality of the liquid vapour
mixture**

As we know that if working fluid will be
in the liquid subcooled region or in the superheated vapour region, we will
require two independent thermodynamic properties in order to secure the third
thermodynamic properties or any other thermodynamic properties because working
fluid will be in the single phase in above two regions.

Let us consider for the region of 2
phase i.e. wet region or liquid-vapour mixture region. One more information will
be required along with two independent thermodynamic properties of the pure
substance for determining the other properties of the pure substance. This
additional information will be termed as quality or dryness fraction in the
field of thermal engineering.

Let us consider that we have 1 kg sample
of two phase mixture, if x kg is the mass of vapour then mass of liquid will be
(1-x). x will be termed here the quality or dryness fraction of liquid-vapour
mixture and it will calculated as mentioned here.

*Dryness fraction (x) = Mass of vapour / Mass of the sample*

*Dryness fraction (x) = M*

_{V}/ (M_{V}+ M_{L})
Where,

M

_{V }- Mass of vapour
M

_{L }- Mass of liquid
Value of dryness fraction (x) will vary
between zero and one. If value of dryness fraction (x) is 0, it indicates that working
fluid is at saturated liquid state and if value of dryness fraction (x) is 1,
it indicates that working fluid is at saturated vapour state.

###
**Let
us consider the following figure to understand the significance of dryness
fraction **

We have displayed the saturated liquid
states by m and saturated vapour states by n in following figure. As we have
seen above that value of dryness fraction (x) is 0, it indicates that working
fluid is at saturated liquid state. Therefore, value of dryness fraction x will
be zero for point m.

Similarly, point n which indicates the
saturated vapour state will have value of dryness fraction equivalent to one
i.e. x= 1 for point n.

Let us consider the point a, b and c in
above figure. Point a, b and c indicates the value of dryness fraction 0.25,
0.50 and 0.75 for different pressure value. For example point a will indicate
that liquid vapour mixture quality will be 0.25 for different pressure lines.

Do you have any suggestions? Please
write in comment box.

We will see another topic in our next
post in the category of thermal engineering.

###
**Reference:**

Engineering thermodynamics by P. K. Nag

Engineering thermodynamics by P. K. Das

Image Courtesy: Google

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