WHAT IS STEAM?

A molecule is the smallest amount of any element or compound substance still possessing all the chemical properties of that substance that can exist. Molecules themselves are made up of even smaller particles called atoms, which define the basic elements such as hydrogen and oxygen.

It is convenient to consider the molecules as bounded together by electrical charges. The degree of excitation of the molecules determines the physical state of the substance.

In ice, molecules are locked together in orderly lattice-type structures and can only vibrate. Continuous addition of heat causes the vibration to increase to such an extent that some molecules will eventually break away from their neighbors and the solid starts to melt to a liquid state. The heat that breaks the lattice bounds to produce the phase changes while not increasing the temperature of the ice is referred to as enthalpy of melting. This phase changes phenomenon is reversible. 

When freezing occurs the same amount of heat is being released back to surroundings. The density increases upon melting in the case of H2O, which is why ice floats on water.

As the temperature increases and the water approaches its boiling conditions, some molecules attain enough kinetic energy to reach velocities that allow them to momentarily escape from the liquid into the space above the surface, before falling back into the liquid. Further heating causes greater excitation and the number of molecules with enough energy to leave the liquid increases. As the water is heated to its boiling point, bubbles of steam form within it and rise to break through the surface.

The density of steam is much less than water. When the number of molecules leaving the liquid surface is more than re-entering the water freely evaporates, At this point, it has reached a boiling point or its saturated temperature, as it's saturated with heat energy.

At constant pressure, the addition of more heat does not cause the temperature to rise any further but causes the water to form saturated steam. The temperature of boiling water and steam is the same, but the heat energy per unit mass is much greater in the steam.

Increasing the pressure effectively increases both the enthalpy of water and the saturation temperature.

The relationship between saturated temperature and pressure is known as the steam saturated curves.

Water and steam can coexist at any pressure on this curve both being at the saturated temperature. Steam at a condition above the saturated curve is known as superheated steam

Temperature above the saturated temperature is called the degree of superheat of the steam.

Water at a condition below the curve is called sub-saturated water.

If the steam is able to flow from the boiler at the same rate that it is produced, the addition of further heat simply increases the rate of production. If the steam is restrained from leaving the boiler and heat input is maintained the energy raises the pressure, in turn allowing the saturation temperature to rise.

The amount of heat required to changes the state of water at its boiling temperature into steam is called enthalpy of evaporation

It involves no changes in the temperature of the steam/water mixture and all the energy is used to change the state from liquid to vapor.

The process of evaporation is also reversible. The same amount of heat that produced the steam is released back to its surrounding during condensation when steam meets any surfaces at a lower temperature.

Enthalpy of steam = This is the total energy in saturated steam, and it is simply the sum of the enthalpy of water and enthalpy of evaporation.

Steam with a temperature equal to the boiling point at that pressure is known as dry saturated steam. If the water content in steam is 5% by mass, then the steam is said to be 95% dry and has a dryness fraction of 0.95.