What is superheated steam? The answer is very simple: superheated steam is the real gaseous phase of water. If saturated steam is a water vapor that still mixed with liquid water, or even supercritical steam that is indistinguishable with supercritical water, the superheated steam is dry H2O gas that has no moisture content at all.
To understand how water can turn its phase into superheated steam, we just need to understand the phase diagram of water above. Superheated steam becomes one of the water phases besides solid, liquid, saturated steam, and supercritical. Also according to the diagram, superheated steam can be derived from liquid water, solid water (ice), and also supercritical depending on environmental conditions.
Water Liquid Phase change to Superheated Steam
Liquid water can turn the phase into superheated steam after crossing the saturation curve line. To get past the saturation curve line, there are several ways that can be done. First with fixed pressure, the environmental temperature is increased. Second at a fixed temperature, the environmental pressure is lowered. Finally, room pressure is lowered along with rising room temperature. But all these processes must meet one condition, the line of change of environmental conditions must cut the saturation curve.
Solid Water Phase (Ice) change to Superheated Steam
Ice, can directly change the phase to dry superheated steam without having to pass through the liquid phase, and vice versa superheated steam can directly change the phase to ice without having to pass through the liquid phase. This change of phase must be at a lower pressure than the triple point, ie 0.61 kPa, or 0.006 atm, almost vacuum pressure. This principle explains how hail can occur. When the cloud above the Earth’s atmosphere which has vacuum pressure, it cools rapidly so that the water vapor changes instantly into ice.
Supercritical Steam Phase Change becomes Superheater Steam
Supercritical steam can also turn the phase into superheated steam by lowering the pressure. Supercritical steam has pressure and working temperature above the critical point, which is more than 22.1 MPa and 374 ° C. Thus, suppose a certain amount of supercritical steam is at 25 MPa and a temperature of 600°C decreases the pressure to 18 MPa, the steam will turn into superheated steam.