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The critical point of water occurs at 647.096 K (373.946 °C; 705.103 °F) and 22.064 megapascals (3,200.1 psi; 217.75 atm; 220.64 bar). [ 3 ] In the vicinity of the critical point, the physical properties of the liquid and the vapor change dramatically, with both phases becoming even more similar.
Triple points mark conditions at which three different phases can coexist. For example, the water phase diagram has a triple point corresponding to the single temperature and pressure at which solid, liquid, and gaseous water can coexist in a stable equilibrium (273.16 K and a partial vapor pressure of 611.657 Pa).
At the critical point, (304.1 K and 7.38 MPa (73.8 bar)), there is no difference in density, and the 2 phases become one fluid phase. Thus, above the critical temperature a gas cannot be liquefied by pressure. At slightly above the critical temperature (310 K), in the vicinity of the critical pressure, the line is almost vertical.
Steam is also useful in melting hardened grease and oil residues, so it is useful in cleaning kitchen floors and equipment and internal combustion engines and parts. Among the advantages of using steam versus a hot water spray are the facts that steam can operate at higher temperatures and it uses substantially less water per minute. [13]
The SCWR operates at supercritical pressure. The reactor outlet coolant is supercritical water.Light water is used as a neutron moderator and coolant. Above the critical point, steam and liquid become the same density and are indistinguishable, eliminating the need for pressurizers and steam generators (), or jet/recirculation pumps, steam separators and dryers ().
Far from the critical point the gravity causes liquid drops and gas bubbles to rapidly settle towards the interface and surface tension causes drops and bubbles to rapidly merge to larger ones, which settle even faster. But as critical point is approached, the density difference between liquid and vapour diminishes and so does the surface tension.
Data in the table above is given for water–steam equilibria at various temperatures over the entire temperature range at which liquid water can exist. Pressure of the equilibrium is given in the second column in kPa. The third column is the heat content of each gram of the liquid phase relative to water at 0 °C.
All data used in this section were obtained from the NIST Chemistry WebBook. [8] It is useful to note that for N 2 the normal boiling point of the liquid is 77.4 K and the critical point is at 126.2 K and 34.0 bar. Overview of the temperature and pressure dependence of the compressibility factor for N 2