Short Answer:

The critical point in steam properties is the specific condition of temperature and pressure at which the difference between liquid water and steam disappears. At this point, water and steam have the same density, and no clear boundary exists between them. For water, the critical point occurs at around 374.15°C temperature and 22.12 MPa pressure.

Above the critical point, the substance is called supercritical fluid, and it does not behave like typical steam or water. This concept is very important in power plants, especially in supercritical boilers, where steam is generated above the critical point for higher efficiency.

Detailed Explanation:

Critical point in steam properties

In thermodynamics and phase change analysis, water exists in three main states: solid (ice), liquid (water), and gas (steam). As we heat water at constant pressure, it transforms from liquid to vapor (steam) at its saturation temperature. However, if both temperature and pressure keep increasing, the steam and water get closer in properties until a point is reached where they become indistinguishable.

This special point is known as the critical point. It marks the end of the liquid-vapor phase boundary. Beyond this point, there is no separate liquid or vapor—only a single phase exists known as supercritical fluid, which has properties of both gas and liquid.

Conditions at the Critical Point

For water (steam), the critical point occurs at:

• Critical temperature (Tc) = 374.15°C
• Critical pressure (Pc) = 22.12 MPa (221.2 bar)

At this state:

• Latent heat becomes zero, meaning there’s no phase change.
• The density of water and steam becomes equal.
• Surface tension between liquid and vapor vanishes.

This means that the substance cannot exist as distinct liquid or vapor, and traditional phase change no longer happens.

Importance of the Critical Point

1. End of Phase Boundary

After the critical point, there is no clear separation between liquid and steam. This changes how we analyze thermodynamic properties.

2. Supercritical Fluid Region

Above the critical point, water behaves as a supercritical fluid, which:

• Has high density like liquid
• Can flow like gas
• Is used for efficient heat transfer and energy extraction

3. Applications in Power Plants

Supercritical and ultra-supercritical boilers work at pressures and temperatures beyond the critical point. This allows:

• Higher thermal efficiency
• Reduced fuel consumption
• Lower emissions

4. Improved Heat Exchange

Supercritical steam has excellent heat-carrying capacity and can be used in high-efficiency heat exchangers in power plants and chemical industries.

5. No Latent Heat

At and above the critical point, no latent heat is involved since there’s no phase change. This simplifies some thermodynamic processes.

Graphical Understanding

In a P–T (Pressure-Temperature) diagram, the critical point is at the top of the dome-shaped curve that separates liquid and vapor regions. Below this point, water can be either in:

• Saturated liquid region
• Saturated vapor region
• Mixture (wet steam)

At the critical point, the curve ends, and above it, the fluid is neither liquid nor steam, but supercritical.

Real-World Applications

• Supercritical Power Plants: Steam above the critical point increases cycle efficiency.
• CO₂ Extraction: Supercritical CO₂ is used to extract oils and flavors.
• Nuclear Reactors: Use supercritical water as coolant for high-efficiency.
• Advanced Material Processing: Supercritical water is used for oxidation and waste treatment.

Conclusion

The critical point in steam properties is the highest temperature and pressure at which water can exist as both liquid and vapor. Beyond this point, water turns into a supercritical fluid, which has no distinct liquid or steam phase. This concept is vital in modern power generation, especially in supercritical boilers, where efficiency, fuel savings, and environmental performance are greatly improved by operating above the critical point.