T-v Diagrams in Thermodynamics – Explanation and Reading

What is the T-v Diagrams in Thermodynamics?

Thermodynamics is a very important area in engineering. There are different kinds of topics in thermodynamics. One of these topics is the property diagrams of pure substances. Also, it is very important to read these property diagrams to make comments about thermodynamical systems. The first property diagram is the T-v diagram in thermodynamics.

Image Sorce: Wikimedia.

What is a T-V Diagram?

T-v diagrams are the diagrams that show the properties of substances such as water with the changing temperature and the changing specific volume. For each point of temperature and pressure, there are specific features of these substances.

It is very important to read the T-v diagrams properly for an engineer. So here, we explain the T-v diagrams with the example of water.

How to Read T-v Diagrams?

T-v diagram of water.
Image Source: Yunus A. Çengel, Michael A. Boles – Thermodynamics, An Engineering Approach-McGraw-Hill Science, 8th Ed., Pg: 120

To understand the T-v diagrams, let’s talk about the water. You can see the T-v diagram of water above. You can see the clear explanations on this diagram.

As you see above, one axis of the diagram is T and another axis is v. Also there is a solid green line on this diagram. This solid line is the line that includes the critical point of pure water.

What is a Critical Point?

The critical point is the point where saturated vapor and saturated liquid phases are at the same point. At only these T and v properties, this substance can remain both as saturated vapor and saturated liquid phases.

For example, the critical point properties of water is Pcr = 22.05 MPa, Tcr = 373.95°C and Vcr = 0.003106 m3 /kg.

Saturated Liquid Line and Saturated Vapor Line

As you see in the T-v diagram of water above, the right side is the saturated vapor line. And the left side is a saturated liquid line.

So, the water at the left side of the saturated liquid line is compressed liquid in which there is no vapor. If the water is at the saturated liquid line, this water is a saturated liquid that is about to start a phase change.

If the water is between the saturated liquid line and saturated vapor line, this is the mixed liquid-vapor region where phase change occurs.

And also, at the saturated vapor line, water is ready to completely convert into a vapor state. On the right side, water is in the superheated vapor state.

So we can understand that, between these two line sections, the substance is in the phase change process.

Changing Pressures

You can see the dashed lines on the diagrams which are showing the phase change of water at different pressure levels. So, with the changing pressures, the saturation points of substances change. If we increase the pressures, a phase change occurs in higher temperatures.

What Happens above the Critical Point at T-v Diagrams?

Above the critical points, we can not talk about an exact phase change. Because we can find this fluid as the gas state. With the increasing temperatures, the density of gases decreases. But in general, we are calling these fluids superheated vapor is the temperature is above the critical temperature. Also if it is below, we call them compressed liquid.


So, it is very useful to use T-v diagrams in thermodynamics to read the different properties of liquids. The general explanation of these diagrams can be made like the above.

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