Thermodynamics Temp/Enthelpy in HTS

[Homer Simpson]

Possibly the wrong forum, but talking specifically about Heat transport system (PWR, candu)

On T-H diagram, At 3.2 MPa, the maximum enthalpy exists for saturated steam (2802.3 KJ/kg). This explains why the curve bends back in higher and lower than this.

Question: this implies that above 3.2 MPa, if you INCREASE pressure, you would cause saturated steam to go into superheat, and below 3.2 MPa the opposite.

Is this true?

Our plant system operates above 3.2 MPa in the Heat Tranport system, with only slight amount of boiling at channel outlet, about 4% steam quality 310deg C. Obviously high pressure is required to keep the water as liquid.

So wouldn't the above make a really unstable sort of situation? For instance, let's say flow is reduced in a channel and that channel goes into dry out. Increasing HTS pressure would cause it to go further into superheat?

To me, It doesn't add up. Am I missing something?

Thanks,

 

[theCandyman]

Are you using a Mollier diagram? To me it looks like increasing pressure on the saturation line always goes toward superheated conditions. Am I misunderstanding what you're asking?

 

[Homer Simpson]

http://www.spiraxsarco.com/images/resources/steam-engineering-tutorials/2/2/fig_2_2_3.gif

Above is example the T-H diagram I am looking at. You can see on the right side of the curve, the highest H value is at 3.2 MPa (horizontal lines are constant Pressure). The saturation line on the Mollier diagram shows this, I think, as well - as the saturation line has a peak at about 30 bar:

http://www.engineeringtoolbox.com/docs/documents/308/mollier-diagram-water_2.png

so any pressure increase above this pressure, at saturation, would make the superheated. I think the whole idea of it just threw me for a loop at first, since as an operator its just engrained that a drop in pressure will cause voiding, increase will collapse voids, which after thinking about it for a while still obviously holds true in all situations since there is no operating scenario in which the bulk of the coolant can gain enough heat to reach the saturated steam area anyways.

I guess I was thinking about each vapour bubble as an individual 'system'. If we were running with some boiling, then I got to thinking "this does't make sense, an increase in pressure will cause the vapour to go more superheated" and my flawed thinking connected this to "more boiling in the channel", which is of course wrong... more pressure may indeed push those individual vapour bubbles into superheat, but the increase in pressure will also collapse their volume. And of course, the bulk of the coolant is way below sat steam conditions, and will mix in with the vapour bubbles.

Thats what I've convinced myself anyways.

 

[theCandyman]

Hmmm, I might be wrong, and I don't have much confidence dealing with this since all of my thermodynamics class dealt with T-S diagrams, but here's my guess.

After thinking about it some, if you increase the pressure, you decrease the specific volume, and this lowers the enthalpy; so if you apply this to your diagram, if you're at the critical point, you'll move down and to the left . Remember that the pressure lines outside of the two-phase region aren't straight like the one in it, they are almost parallel to the wet steam on the lower enthalpy side and perpendicular to the dry steam line on the higher enthalpy side. So in the case you mentioned in your first post, you'll end up in the sub-saturated water region.

If I'm wrong, hopefully someone can give you the right answer or the correct reason.

Edit: After reading the rest of your second post, I see we were on to the same idea. Increase pressure = decrease volume.

 

[Homer Simpson]

Thanks Candyman, I'm used to looking at these on T-S as well, from college days, it makes it more useful for sketching out cycles and thinking about effeciancies, too bad 'entropy' is a little mysterious at times.

Seems we are on the same wavelength, so that's reassuring. I should have specified the actual pressure at the plant, it's about 10 MPa at the channel outlet, so in this case we are well below the 22.1 MPa critical point, though I've heard of thermal plants that do operate boilers in the 'super critical' range