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Monday, 3 October 2016

QUALITY OR DRYNESS FRACTION OF STEAM

We were discussing “Triple point phase diagram of waterand TV diagram of pure substancein 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) = MV/ (MV+ ML)
Where,
MV - Mass of vapour
ML - 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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