Boiling of Water - continued.

After all of the water is converted into steam, the temperature of the steam is still 212 deg F. Now if we add more heat to the steam, we will see an increase in its temperature. Why does this happen? It happens because at this stage, no change of phase is taking place; therefore addition of heat will result in increase in temperature. Therefore, heat being added at this stage is sensible heat (because a thermometer will be able to detect it).

As heat is added to the saturated vapor its temperature increases. Let’s say the temperature increases to 220 deg F. Steam at this temperature is referred to as the “superheated steam”. Since its temperature is 8 degrees greater than the saturation temperature, we will say, “the steam has 8 degrees of superheat”.

Superheated steam is defined as steam that has temperature that exceeds the saturation temperature, for that particular pressure. In practical applications where we require dry steam, we superheat the steam. This allows the steam to lose some of its heat and still remain in vapor form. For example, if steam at 220 deg F, loses some of its steam and its temperature drops to 215 deg F, it remains dry (because its new temperature is still above the saturation temperature).