Does unsaturated steam necessarily carry water?
Thread Content
I would like to ask the experienced colleagues: does unsaturated steam necessarily contain water? Here’s the situation: our factory uses steam generators of the heat transfer oil type to produce steam, with the heat transfer oil being used for secondary heating. The temperature of the heat transfer oil ranges from 190 to 210°C. Both the temperature and pressure of the steam are monitored (the steam pressure after the pressure reducing valve is 1 kilogram, with a temperature of around 110°C). By referring to the saturated vapor pressure table, it can be seen that the steam is not saturated (saturated steam at 1 kilogram of pressure should have a temperature of 120°C). Our question now is whether this unsaturated steam necessarily contains water mixed in it
1 kilogram of saturated steam should be at 120°C; since the current temperature is clearly below 120°C, it indicates that it is not saturated steam. The same situation occurs with us – the steam contains water. This is due, on the one hand, to problems with the steam generator, and on the other hand, to an undersized gas-liquid separator as well as a lack of automatic and timely water removal mechanisms. There was a significant improvement after enlarging the gas-liquid separator and replacing the water strainer.
If the temperature of 1 kilogram of steam is not 120°C, does that mean there must be water in this steam?
The more it is transported, the more it cools down – as the temperature drops, visible water will naturally condense within the pipes.
Is there a theoretical basis for this? Unsaturated steam must contain water, right?
When a liquid evaporates in a confined space, the liquid molecules move through the surface of the liquid into the space above, turning into vapor molecules. Due to the chaotic thermal motion of steam molecules, they collide with each other as well as with the container walls and the liquid surface. When colliding with the liquid surface, some molecules are attracted by the liquid molecules and return to the liquid, becoming part of it again. At the beginning of evaporation, more molecules enter the space than those that return to the liquid. As evaporation continues, the density of vapor molecules in that space increases, which in turn leads to an increase in the number of molecules that return to the liquid. When the number of molecules entering the space per unit time is equal to the number of molecules returning to the liquid, evaporation and condensation are in dynamic equilibrium. At this point, although evaporation and condensation continue to occur, the density of vapor molecules in the space no longer increases; this state is known as a saturated state. A liquid in a saturated state is called a saturated liquid, and its corresponding vapor is saturated vapor; initially, it is wet saturated vapor, and it becomes dry saturated vapor only after all the liquid water in the vapor has evaporated. During the process in which steam moves from unsaturated to wet saturated to dry saturated, its temperature does not increase; however, when heating continues after reaching dry saturation, the temperature rises, and the steam becomes superheated.
The watch should be fine; it’s a new one that was just replaced
The pressure is higher than the saturated vapor pressure corresponding to this temperature; if there are no non-condensable gases, it seems that everything should be water. Just like the pressure is at atmospheric level, and the temperature is below 100 degrees.