I'm working on a project (school level) and in it I'll be dealing with the pressure generated by water when superheated.

I could understand the concept of superheated water but superheated steam is hard to understand. Are both inter-related, i mean that when water is superheated then it changes into superheated steam?

Or is it something different? And since in my project I require high-pressure-energy to be generated from water, so which one will be better, superheated steam or superheated water?


1 Answer 1


Boilers and similar steam generators usually produce wet steam, which is a mixture of saturated steam and liquid water droplets. Saturated steam is steam that is in equilibrium with liquid water. The temperature of the steam is equal to the temperature of the liquid water; it corresponds to the boiling point at the given pressure. At normal pressure ($p=101\,325\ \mathrm{Pa}=1\ \mathrm{atm}$), the temperature is $T=99.974\ \mathrm{^\circ C}$. The corresponding specific enthalpy of the steam is $h = 2676\ \mathrm{kJ/kg}$; the specific enthalpy of the liquid water is $h = 419\ \mathrm{kJ/kg}$.

If heat is supplied to the wet steam, liquid water vaporizes. At constant pressure, the temperature remains constant until all liquid water has been vaporized, i.e. until the steam is dry.

If further heat is supplied to the dry steam, the temperature and the specific enthalpy of the steam increase. Such steam at a temperature higher than its boiling point is called superheated steam. For example, superheated steam at normal pressure ($p=101\,325\ \mathrm{Pa}=1\ \mathrm{atm}$) and a temperature of $T=150\ \mathrm{^\circ C}$ has a specific enthalpy of $h = 2777\ \mathrm{kJ/kg}$.

Superheated water is liquid water under increased pressure at temperatures above the boiling point at normal pressure. For example, water at a temperature of $T=150\ \mathrm{^\circ C}$ and a pressure of $p=506\,625\ \mathrm{Pa}=5\ \mathrm{atm}$ is liquid. The specific enthalpy of the liquid water is $h = 632\ \mathrm{kJ/kg}$.

The energy content of superheated steam (e.g. $h = 2777\ \mathrm{kJ/kg}$ at $T=150\ \mathrm{^\circ C}$ and $p=101\,325\ \mathrm{Pa}=1\ \mathrm{atm}$) is much higher than the energy content of superheated water (e.g. $h = 632\ \mathrm{kJ/kg}$ at $T=150\ \mathrm{^\circ C}$ and $p=506\,625\ \mathrm{Pa}=5\ \mathrm{atm}$) because of the latent heat, i.e. the enthalpy of vaporization $\Delta H_\text{vap}$.


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