What is super-critical and sub-critical pressure in boilers ?

The Critical pressure in boiling a liquid is that pressure above which there is no clear change of state between the liquid and the vapour phases.  Simply put, water turns into vapour without boiling.  Above a pressure of 22.1 MPa, water reaches this state. 

In supercritical boilers, water is boiled at a very high pressure.  At that high pressure, there is no clear distinction between the water and vapour phases .  The fluid can  no longer be called liquid or vapour.  It becomes what is known as a super-critical fluid.

Supercritical Boilers are generally used in Turbine systems.  When the supercritical fluid drives the turbine, it loses pressure.  As the pressure drops below the critical point, the supercritical fluid becomes a mixture of water and steam which then passes through the condenser.

Supercritical Boilers consume less fuel as compared to Traditional Drum Boilers.