Water can exist in the form of solid, liquid and gas as ice, water and steam respectively. If heat energy is added to water, its temperature rises until a value is reached at which the water can no longer exit as a liquid. We call this the "saturation" point and with any further addition of energy, some of the water will boil off as steam. This evaporation requires relatively large amounts of energy, and while it is being added, the water and the steam released are both at the same temperature. Equally, if we can encourage the steam to release the energy that was added to evaporate it, then the steam will condense and water at the same temperature will be formed.
Why use Steam?
Steam is produced by evaporation of water, which is a relatively cheap and plentiful commodity in most parts of the world. Its temperature can be adjusted very accurately by the control of its pressure, using simple valves; it carries relatively large amounts of energy in a small mass, and when it is encouraged to condensate back to water, high rates of energy flow (into the material being heated) are obtained, so that the heat using plant does not have to be unduly large.
Thus steam is most economical,flexible and versatile tool for industry wherever heating is required.
Liquid enthalpy is the "Enthalpy" (heat energy) in the water when it has been raised to its boiling point to produce steam, and is measured in kJ/kg, its symbol is hf. (once known as "Sensible Heat")
Enthalpy of Evaporation (the heat content of steam)
The Enthalpy of evaporation is the heat energy to be added to the water (when it has been raised to its boiling point) in order to change it into steam. There is no change in temperature, the steam produced is at the same temperature as the water from which it is produced, but the heat energy added to the water changes its state from water into steam at the same temperature.
When the steam condenses back into water, it gives up its enthalpy of evaporation, which it had acquired on changing from water to steam. The enthalpy of evaporation is measured in kJ/kg its symbol is hfg. Enthalpy of evaporation is also known as latent heat.
The temperature at which water boils increases as the pressure increases. From this it is evident that as the steam pressure increases, the usable heat energy in the steam (enthalpy of evaporation) which is given up when the steam condenses, actually decreases.
The sum of the two enthalpies is known as the enthalpy of saturated steam. This enthalpy is the total heat energy, which is stored in the steam.