We were discussing “

*Triple point phase diagram of water”*as well as we have also seen the concept of*“temperature entropy diagram of water*“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 properties of
a pure substance and therefore we will first look over here the PV diagram of a
pure substance with the help of this post.

*pure substance*is basically defined as one substance which will have homogeneous and invariable chemical composition throughout its mass even if there is any change in phase too. Pure substance will have fixed chemical composition and homogeneous throughout the mass.

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**So,
let us come to the main topic i.e. PV diagram of a pure substance**

As we know that a pure substance will
exist in three different phase i.e.

Liquid phase

Vapour phase or gas phase

In the field of thermal engineering, we
will be interested to discuss the last two phase of pure substance i.e. Liquid
phase of pure substance and vapour or gas phase of the pure substance. However,
solid phase discussion is also quite important but it will be discussed in the subject
of engineering metallurgy. So, we will discuss here the thermodynamic
properties of liquid, mixture of vapour and liquid phase and also vapour phase
of a pure substance.

Now we will consider the arrangement of
cylinder and piston. Let us consider that vapour is available inside the
cylinder and we have provided some constant load over the piston. Let we are
providing heat energy at slow rate to the system i.e. vapour and hence we will
have uniform temperature of vapour inside the cylinder. Temperature of vapour
will increase with respect to time but it will not vary with respect to point.

Let us draw the PV diagram, Pressure
will be shown on vertical axis i.e. Y axis and volume will be shown on
horizontal axis i.e. X axis. We will consider the cooling process in order to
understand the PV diagram for a pure substance here.

We will start our discussion with point
A. At point A, working fluid will be in the phase of vapour. Let us assume that
we are extracting heat energy from vapour with keeping temperature constant or system
is going under cooling process with constant temperature.

As we will extract heat energy from
vapour with keeping temperature constant, system will have various states and
finally system i.e. vapour will reach at a point which will be termed as
saturated vapour state. We have displayed the point B as saturated vapour state
at temperature T1. Process AB will be the isothermal process. Pressure and
volume, both will be changed but temperature of the system will be constant.

Let us consider that we are extracting
more heat energy from system i.e. vapour. We will have a process with constant
pressure and constant temperature. Finally saturated vapour will be converted
completely in to liquid and will reach at a state which will be termed as
saturated liquid state. Therefore at saturated state i.e. at state C, we will
have saturated liquid state.

Similarly, if we consider the curve for
another temperature T

_{2}higher than the earlier temperature T_{1}and let us draw the curve for cooling process. We will have similar curve as displayed for temperature T_{2}in diagram.
Now if we consider the curve for
temperature T

_{2}, we will have similar other points for saturated liquid state and saturated vapour state also.
If we will draw the curve for different
temperature, we will have different points for saturated vapour state and
saturated liquid state. Now we will combine each saturated liquid state points
and after that we will combine each saturated vapour state points. Once we will
combine the saturated liquid state points and saturated vapour state points, as
we have discussed above, we will have one dome type of shape.

Saturated liquid line, saturated vapour
line and critical point have been displayed in PV diagram above. We have also
displayed the curve at critical temperature and also for the temperature beyond
the critical point.

Now we will see the regions of this PV
diagram. Left side region of the dome will be termed as liquid region and we
have displayed this region by L and similarly right side region of the dome will
be termed as super heated vapour region and we have displayed this region by V
in above PV diagram.

We will have mixture of two phase i.e. liquid phase plus vapour phase inside the dome and this region will be termed as liquid vapour mixture region and we have shown this region by L+V in above diagram.

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.

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**Reference:**

Engineering thermodynamics by P. K. Das

Image Courtesy: Google

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Deflection and slope of a simply supported beam with point load,

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