Why is steam wet

Wet steam area

You have probably already brought water to a boil in a pot with a closed lid and wondered what thermodynamic processes are happening during it. We can already reveal in advance that the wet steam area plays a decisive role and you will also get an explanation of the wet steam and the dew line in this article.

  • What are the wet steam and the wet steam area?
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  • Limits of the wet steam area
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  • Dew line, boiling line and law of the lever arms turned away
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  • Example calculation in the wet steam area
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What are the wet steam and the wet steam area?

The wet steam area consists of gaseous and liquid phases. It therefore represents the visible water vapor on the lid. Wet steam is thus a mixture of vapor and liquid of the same substance, in which both states have reached their saturation temperature. Let's look at the wet steam area in the form of a graph:

As a rule, we use a diagram to illustrate the wet steam area, which has the specific volume V on the x-axis and the pressure p on the y-axis. First, let's look at an isothermal line. That means that we always have the same temperature, for example 20 degrees. In order to be able to do this, we therefore need a formula that represents p as a function of V. The ideal gas law can help us here.

The ideal gas law

We can now transform this by using the specific ideal gas constant and then converting to p:

Limits of the wet steam area

This gives us a hyperbola. Unfortunately, in reality it looks different. If we continue to reduce the specific volume and thus increase the density, we will eventually get liquid. When we come into this area, we call it the wet steam area, as this is where the wet steam predominates. This area looks something like an upside-down parabola. The right arm is the so-called dew line, i.e. the point where the first drop of water occurs. The left arm is the so-called boiling line. This is where the first vapor bubble forms when we reduce the density a little. To understand this, recall the above definition of wet steam again. The intersection is the critical point. Everything above is called “critical fluid” and is characterized by the fact that it is no longer possible to differentiate between liquid and gas.