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what does this paragraph mean in cengel thermodynamics (it is on page 119): 

Above the critical state, there is no line that separates the compressed liquid region and the superheated vapor region. However, it is customary to refer to the substance as superheated vapor at temperatures above the critical temperature and as compressed liquid at temperatures below the critical temperature.

where is  this  region " it is customary to refer to the substance as superheated vapor at temperatures above the critical temperature "? i attached a pic. i don't know whether i understan it or not

Annotation 2020-03-15 180656.png

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  • 3 months later...

Fluids above the critical temperature are called "super critical." We sometimes call a liquid above the critical pressure "super critical." For example, in most large coal-fired steam power plants, the feedwater entering the boiler is about 4400 psia (30 MPa). The critical point is where the saturated liquid and vapor are indistinguishable; that is, a distinction is physically meaningless. There are no bubbles formed when boiling a liquid above the critical pressure. This is why you must have special equipment to clean (i.e., "polish") the feedwater in a supercritical coal-fired plant, as the boiling process doesn't naturally separate out impurities and they build up over time. The pressure at the bottom of the sea is above critical. There are "vents" in sea floor where the water is spewing out at near 700ºF (371ºC), yet there are no bubbles. You can find videos of these sea floor vents online. Refrigerants have a much lower critical temperature and pressure. You can also find online old black-and-white videos of a supercritical refrigerant flowing in a clear pipe undergoing a transition from "liquid" to "vapor" but you can't see any difference or tell where it happens, which is completely different from what this process looks like at subcritical pressures. Interesting things happen around the critical point, which is why we study this region and some designs focus on this. For example, there is considerable current interest in supercritical CO2 systems.

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