Types of Steam — As we known from the previous article, water has three main phases: solid, liquid, and gas; which we can easily determine through various conditions of temperature and pressure in the phase diagram of water. However, water vapor themselves are actually still subdivided into three: saturated steam, superheated steam, and supercritical.
Saturated steam is a condition in which water vapor has an equilibrium pressure and temperature with liquid water. In other words, saturated steam is a wet steam, which still mixed with liquid water-phase molecules.
This saturation vapor begins to form just as the water reaches its boiling point, until all the energy from latent heat is absorbed by water. While all latent heat has been absorbed by water, and the amount of vapor phase has reached almost 100% of its, that is the end of the vapor phase of saturation. The process of reaching almost 100% of the vapor phase occurs at stable pressure and temperature. Furthermore, if thermal energy continues to being absorbed by saturated steam, there will be an increase in temperature and encourage steam to turn the phase into superheated steam.
In the phase diagram of water, saturated steam phase is shown only as a line of saturation curves which is the boundary between liquid water and superheated steam.
Superheated steam is a phase of water that has passed through the saturation phase by absorbing more heat energy, so that the entire water fluid has a pure gas phase. Superheated steam contain high thermal energy, and is very popular used as an energy source of power plant steam turbine.
The dry and pure phase characteristics of this gas become the absolute requirement of steam before it can enter the steam turbine. This is because superheated steam is able to avoid the damage caused by the erosion of wet moisture as in saturated steam.
Supercritical steam is a water phase condition above the critical point of water, which is 22.1 MPa and 374°C. There are two characteristics of supercritical steam: water has no latent heat value, as well as the same volume-specific value between water and supercritical steam. It can be said that this supercritical steam phase has an indistinguishable phase between liquid nor gas.
Supercritical steam is also used at modern steam turbine power plants which is pursuing high efficiency. The main challenge of using this supercritical steam is on the design of boiler tube materials. The boiler tube should be very resistant to thermal stress causing by the generation process of supercritical steam.