Subcritical vs Supercritical steam: Water’s journey from subcritical to supercritical fluid

Super critical boilers are generally used in big power plant > 400 MW. They operate with steam above critical point steam at the pressure above 220 bar or 22 MPa and 374 degc. Water directly converts to supercritical fluid at the operating condition above critical point. A sub critical boiler operates with steam below critical point. Both follow Rankine cycle.

The image is temperature-steam entropy diagram with x axis entropy and y axis temperature. On the left side of the bell curve in red there is single phase water. X in red color is the dryness fraction for steam as steam proceeds to right side in the bell. Inside the bell two phases, water and vapor coexist and on the right side it is the one phase superheated region for steam. The top most point of the bell curve, red sphere, is critical point. Above critical point there is supercritical region

Imagine you have 1 kg water at 1 bar pressure. You are outside the bell curve on left. You start heating the water as water reaches 100 degc it starts converting into vapor. You are now inside bell curve. You have both water + vapor coexisting. Temperature stops rising as heat goes to supply latent of water for water to evaporate. You can see flat blue temperature lines within bell curve. As you move towards right, you have more vapor and less water. X in red color shows the dryness fraction of vapor. At X=1 you have reached the right contour of bell curve. Two phase region ends here. As you move from one to other end in the saturated region in the bell curve entropy increases. The reason is as more water converts to vapor in the two-phase region there is more disorder due to increased randomly moving steam particles and consequently there is rise in entropy. This is an important point to be noted.

When steam is pressured outside the bell curve on right in the superheated region to generate enthalpy, the volume of steam reduces for same mass of water and that limits the available space for steam particles to move around, the entropy reduces. This explains why entropy of superheated decreases with increase in degree of superheat.

Same happens till the critical point. Remember when volume reduces entropy reduces and when volume increases entropy increases.

Second point – Why efficiency of supercritical boilers are more than subcritical boilers? The reason is steam or super critical fluid temperature are obviously much higher than subcritical steam temperature. The Carnot Efficiency, the theoretical maximum efficiency depends on ratio of cold and hot reservoirs. Hot reservoir is the hot fluid feed temperature to the turbine and cold reservoir is condenser temperature.

Efficiency (%) = [ 1-Tcold /Thot] x100% 

Since Thot in supercritical boiler > Subcritical boiler, Supercritical boilers have higher efficiency.