Steam Terminology
Steam is a gaseous of boiled water. To produce steam, heat energy is needed which will change the phase / state of water into steam. In industrial applications, steam is generated from a boiler. The water will be contact with heat energy and become steam by reaching its boiling point at the pressure. For example, water heated at a pressure of 125 kPa will evaporate at a temperature of 105.97oC. When water is heated at a higher pressure, its boiling point will also increase. Vice versa, at a lower pressure water will boil at a lower temperature. See the table below.
The process of changing the phase from water to steam:
1. Sensible heat, the heat required to raise the temperature until the boiling point is reached. So it is called “the specific enthalpy of saturated liquid”. If the heat is continued, the temperature will stop rising and the water will evaporate. The enthalpy value at this point is indicated on the steam table with the symbol “hf”.
2. Latent heat: the heat required to change the water phase to steam. The value of thermal energy in this process is called “the increment of enthalpy for evaporation”. This enthalpy value is indicated by the symbol “hfg” on the steam table.
At this point it means that heat energy is supplied through two phases, the value is called “the specific enthalpy of the saturated vapor” and is shown on the steam table with the symbol “hg”.
Then hf + hfg = hg in units of kJ/kg.
Steam in such a state of saturation can still contain even more energy by continuously giving it heat. Applying heat will increase the temperature above the saturation temperature at that pressure. And the vapor phase will turn dry (like a mirage). This is called superheated steam. The enthalpy of this superheated steam increases with the increase in temperature.
Types of Steam are determined based on the condition of the phase, the pressure-temperature relationship and the process of occurrence. There are 3 types of steam:
1. Wet steam, this kind of steam is still mixed with water in it, where the vapor fraction is still below 100%. And the temperature is still below the saturation temperature (boiling point). Usually this type of steam is the initial steam when the evaporation process occurs.
2. Saturated steam/dry (saturated steam), steam that is no longer mixed with water in it, which means that the vapor fraction is 100%. The vapor temperature is the saturation temperature at the vapor pressure. The saturation vapor phase occurs after the evaporation process.
3. Superheated steam, which is steam whose vapor fraction is already 100% (dry). However, the temperature is above the saturation temperature at its vapor pressure. The advanced hot vapor phase occurs after the saturation vapor phase, i.e. the temperature continues to rise with constant pressure. What distinguishes between saturated steam and superheated steam is their energy content, where superheated steam has a higher energy content (enthalpy) than saturated steam.
In the power generation process using steam, the steam used should be superheated steam. This is because in the steam piping system there must be heat-loss, thus preventing condensation of steam before it reaches the turbine and after it leaves the turbine (for plants that use a reheating cycle).
