PV DIAGRAM OF PURE SUBSTANCE IN THERMODYNAMICS

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.

First of all we must be aware, what is exactly a pure substance? A 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.

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.

Solid phase
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.

Let us consider that we are extracting more heat energy from the system, now pressure will be increased but as system will be in the state of saturated liquid state which is very less compressible and hence change in volume will be less  while temperature will be remain constant.

Similarly, if we consider the curve for another temperature T₂ higher than the earlier temperature T₁ and let us draw the curve for cooling process. We will have similar curve as displayed for temperature T₂ in diagram.

Now if we consider the curve for temperature T₂, 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.

Line which is connecting saturated liquid state will be termed as saturated liquid line and similarly line which is connecting saturated vapour state will be termed as saturated vapour line. These two lines i.e. saturated liquid line and saturated vapour line will meet with each other at the top of the dome and that point will be termed as critical point.

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.

Reference:

Engineering thermodynamics by P. K. Das

Image Courtesy: Google

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