Hello,

Does anyone read whats before them before they press the tit to print? In the same issue, page 599 the picture captions are all wrong. The errors that have crept into the magazine of late begger belef. Stupid mistakes. Come on sharpen up in there please.

Regards,

Bob

Not so, dry steam is represented by the whole of the righthand curve between saturated and superheated steam on the temperature/enthalpy diagram. Dry steam can exist at low pressures, as well as high. Dry steam simply means there is no water vapour present.

Subcritical means that bubbles can form in the liquid on the way to forming steam. Supercritical implies that the steam forms directly from water without bubble formation. As discussed the critical point for steam is at 374°C and 3208psig.

Andrew

Well it's in the right thread now, that will confuse people.

Neil

Don't worry, entropy confuses everybody. All you need to remember is that it stays constant in a reversible process, in a real one it increases. It never decreases.

I'll wait for a thermodynamicist to let loose before I confuse the situation by attempting to describe your graph. A more useful picture in terms of crtiical temperature is

**LINK**

Sorry it's in French. As long as you are above 0.006 atmospheres, moving to the right will change from solid (ice) to liquid (water) to gas (steam). Crossing a solid line needs heat input (latent heat), or if crossing from right to left gives heat out. Above the critical pressure, no line, no latent heat. Below 0.006 atm, ice goes straight to gas, imagine dry ice which is carbon dioxide. This happens in reverse when you get snow and hoar frost formation, water vapour at a low partial pressure freezes directly without going through the liquid phase.

Edited By duncan webster on 09/04/2017 20:21:01

Edited By duncan webster on 09/04/2017 20:25:42

Steam at around 1000psi only applies to water cooled reactors such as the pressurised water reactor at Sizewell B (around 69bar and 285 deg C) The CO2 gas cooled AGR reactors in the UK have final steam conditions not too far away from contemporaneous fossil fuelled plant, the limitation not being one of 'meltdown' but very long term degradation of the graphite moderator, so the gas temperatures were maintained at close to those used with the previous generation of UK designed Magnox gas cooled reactors. The Magnox reactors did have lower final steam temperatures and pressures although nowhere near as 'wet' as Sizewell B.

Most coal and previously oil fired generation in the UK built from the 1960's onwards uses a standard of around 166bar and 568 deg C, the AGR's operate at 540 deg C with the same pressure. The UK did however build one large scale supercritical coal fired boiler at Drakelow C that operated at around 240bar and 590 deg C, some half a century before the metallurgy had really caught up

Most of the new build lignite fired plant in Germany constructed from 2007 to around 2015 has been supercritical yet still not without some degree of boiler metal issues.

No-one has waded in with info on SoD's graph, so here goes. Of more use is an Entropy-Enthalpy curve, known as a Mollier diagram.

I've only ever used it for designing things like nozzles for turbines, and that was a very long time ago. You could use it for designing reciprocatinfg engines, but there are other ways. Pick a point to suit your steam conditions, say 10 bar, 300C. You can read off the Enthalpy as 3052 kJ/kg (I cheated and used steam tables). Assuming isentropic expansion, draw a vertical line through this point down to the casing pressure, say 0.05 bar, and you will find that the Enthaply has dropped to about 2120 kJ/kg, and the dryness fraction is ~0.85. The loss of Enthaply is 932 kJ/kg, this can be used with appropriate adjustment for nozzle efficiency to work out the steam speed. No I can't remember how to do the efficiency bit, I'd have to look it up. I remember tomes by Kearton and Stodola being useful, but as I said it's a very long time ago. The charts we used were much bigger (covered a desk top), and so could be read to a much higher accuracy. I daresay it's all done by computer nowadays. The red line gives the interface between wet steam and superheated. Note that lines for pressures above critical (220 bar) do not cross the red line