Visualising supercrtical phase changes
Visualising supercrtical phase changes
(OP)
With sub-critical boilers I find it easy to visualise the change from water in the downcomers to a mixture of saturated steam and water leaving the furnace tubes.
My problem is in visualising what happens with supercritical boilers when liquid changes to steam. In this regard the pressure/temperature phase diagram is not very helpful.
In its simplest form the pressure/temperature phase diagram is clearly a single line (saturation line) starting at the triple point and ending at the critical point with liquid on one sde and vapour on the other.
If water above the critical pressure is heated to say 700K what happens at the critical temperature, does the liquid instaneously become steam.
Any help would be appreciated.
athomas236
My problem is in visualising what happens with supercritical boilers when liquid changes to steam. In this regard the pressure/temperature phase diagram is not very helpful.
In its simplest form the pressure/temperature phase diagram is clearly a single line (saturation line) starting at the triple point and ending at the critical point with liquid on one sde and vapour on the other.
If water above the critical pressure is heated to say 700K what happens at the critical temperature, does the liquid instaneously become steam.
Any help would be appreciated.
athomas236





RE: Visualising supercrtical phase changes
If you start over with cold liquid but at pressure above the critical pressure and add heat, you move smoothly to higher temperatures and enthalpies without ever seeing a phase transition (no vertical "cliff" to climb). Basically, the high pressure fluid just expands in volume smoothly as you add heat. Above the critical point (pure substance), the stuff is just "fluid," with no phase changes at all.
This also can be used to explain to someone how you would go from low pressure cold liquid to low pressure high temperature vapor without ever seeing a phase transition. You raise the pressure to above the critical pressure with little temperature change (pump), add heat and let the fluid expand isobarically until the temperature is well beyond the critical temperature, then reduce the pressure with little temperature drop until the fluid is hot and at low pressure, ending up with what you would conclude is "vapor." You've walked around the critical point. On that cut sheet of paper (P-T diagram), the path that avoids the "cliff" is obvious.
With a multicomponent fluid, the same thing can be done, except that the plotted lines of constant vapor fraction are separate within the two-phase region and meet at the mixture critical point. In the region of vaporization with distinct phases, there is no vertical "cliff" but a steep uphill, corresponding to large increase in enthalpy and only a small increase in temperature. The critical point usually is not at the top of the envelope, but off to one side. The lowest pressure that just touches the top of the envelope is called the cricondenbar, and the temperature that just touches the right edge of the envelope is called the cricondentherm. This plot can be used to visualize retrograde condensation and other things. Take a look at the API Technical Data Book - Petroleum Refining, Chapter 4 ("Critical Properties").
RE: Visualising supercrtical phase changes
athomas236
RE: Visualising supercrtical phase changes
Phase (P,T) diagrams show a final point on the liquid-vapor boundary line corresponding to the critical point, which for water is 374oC and 218 atm.
To athomas236, the opposite process would probably be easier to imagine. Above 374oC liquid and vapor cannot coexist in equilibrium, and any isothermic vapor compression wouldn't result in condensation. The density would gradually increase with no transition to a liquid.
Just to enlarge the picture (I hope not muddle it) let's add that phase (P,T) diagrams don't show critical points (CP's) for the solid-liquid boundary lines, no matter how high the pressure. IMHO such CP's haven't yet been found.
RE: Visualising supercrtical phase changes
Subcritical is your wife's reaction when you return home "late from work" smelling of beer. The phase change happens over time as she has gradually figures out that you have been out with the boys.
Supercritical phase change is your wife's reaction when you return home late smelling of perfume.
RE: Visualising supercrtical phase changes
That's a good one. I hope you never get caught in that supercritical state.
Cheers.
RE: Visualising supercrtical phase changes
For water flowing thru a supercritical steam generator (ie a boiler), some issues associated with this phenomena are:
a) for ensembles of 2 to 1000+ tubes absorbing heat and operating in parrallel between to equal pressure headers ( so called parallel channels) we have both static (Ledineg) and dynamic flow instabilities to contend with - prediction of flow distribution between parallel tubes requires sophisticated analyses
b) for heated components undergoing a high heat flux, there is a severe deterioration in the physical properties across the fluid boundary layer between the metal surface ( fluid temp at wall temp with low k, visc, rho) while the bulf of the fluid in the middle of the channel is cooler and has a low velocity shear force due to hi rho. This can lead to psuedo nucleate boiling , espescially when combined with an unusually low particular cahnel flow rate due to the effects of item (a) above.
c) There is also a change in fluid solubility across the boundary layer near the wall of a heated tube, such tha there will be a net transport of impurites from the bulk of teh fluid ( at midchannel) to the heated wall, leading to a rapid buildup of crud on the heated tube's wall. this crud acts as an insulator and causes the tube metal to run very hot and fail in a short service life.
Of course these peculiar changes in physical properties can be used to advantage, and there are many supercritical extraction process that are used to process pharmacueticals etc.