# Dynamic response of once through steam generator

Hi, Can anyone point me to a source of information on, or know anything about, likely transfer functions between feedwater flow and hot gas
temperature, and exit temperature for a once-through waste heat steam generator. It's just a coil with hot gas flowing across it, no steam drum. The front end of the coil heats the feedwater, and the back end does some superheating. It's at (very) subcritical pressure.
Anything, even if approximate, would assist. Specifically, which of the two disturbiances might cause a quicker response? Is the response likely to be more dead time or low pass? TIA
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On Thu, 19 Feb 2009 19:49:06 +0900, Bruce Varley wrote:

I dunno this stuff (what does "subcritical pressure" mean?), but:
It's basically a flash boiler with a fixed heat input -- might "flash boiler" make a good search term for the web or technical library?
My intuition told me what would happen, but then I started thinking a bit deeper about it and realized that my intuition was making too many assumptions, and was about to make an ass out of u and me.
I'd like to see this modeled thermodynamically, by someone who actually understands thermodynamics (IOW, not me).
If (as my intuition originally assumed) the output is infinitely compliant (i.e. the output pressure is fixed), then I would assume that the immediate response of extra water inflow would be extra steam outflow, and would push colder steam ahead of it. Conversely, if the output flow is held constant then I would assume that the immediate response of extra water inflow would be higher pressure, and the pressure would induce a temperature rise. But I dunno.
It does feel, to my oh-so-dependable intuition, that the response to a squirt of inflow would be more immediate (and more complicated, once the tubing itself gave up or gained heat) than the response to a change in the exterior heat.
--
http://www.wescottdesign.com

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On Thu, 19 Feb 2009 10:30:40 -0600, Tim Wescott wrote:

Google on supercritical boilers. This is the other side of the water critical point.

Thanks for the comments Tim (and to other posters). When we discussed this later we came to the conclusion that the physical configuration is a bit of a red herring. You immediately think 'transport delay', but I now think that even though it's a 'long pipe' it may still behave more like an ordinary boiling mass, with energy storage in the contents and the surrounding metal. As soon as boiling commences, the plug flow transit time becomes fairly insignificant.
I think you're right about the relative response times, feed rate would give a quicker response.
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Bruce Varley wrote:

In G. Shinskey "Process Control Systems", 1st edition, pages 246 ff you find a short discussion of the special behavior of that boiler type as well as of the influence of relevant variables, He also offers a control scheme. Maybe it is a good starting point.
Dieter
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The system seems a lot like a fired furnace used in many process industries. The flue gas side usually has a very fast response. The process side np\normally has a slower response with some transport delay. You might look at fired furnaces or boiler economizers.
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