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PostPosted: Aug 10, 2017 5:47 pm 
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Joined: Jun 19, 2013 11:49 am
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So, traditional steam generators are simple kettles with u-shaped tubes in them, this functions effectively as a co-current heat exchanger. (Babcock and Wilcox used OTSGs, but that appears to have died when they were bought by Westinghouse, and they also have issues with very small water inventories)
As such, the steam outlet temperature is normally approximately equal to the reactor inlet temperature. This represents a significant loss of exergy as the outlet temperature in a CANDU is 310C, but the steam temperature is only 260C - or in the case of an APWR it is 325C and 283C.

There is currently research ongoing in building compact steam generators using a PCHE - but there seem to be problems with flow stability, especially during various transients.
They don't appear to like phase changes.

So how about placing a counter-flow heat exchanger at the top of the steam generator, where the primary fluid could be contacted with the single phase flow that is the steam flow from the stop of the steam generators.
Heat Transfer Coefficient for a water/high pressure gas heater with a PCHE is 1-4kW/square metre-kelvin.
And 100 bar is low enough pressure to still get 1300 square metres per cubic metre.

Which translates to 3.25MW/cubic metre-kelvin.

If the pressure drop of a PCHE is too high you could use a Heatric hybrid heat exchanger, with the primary fluid in the etched channels and the fin formed channels containing the primary steam.

In the APWR (a four loop PWR) will produce 625kg/s of steam. With a thermal power of roughly ~1100MWt.
A superheater that increases the steam output temperature of ~100MWt.
Considering that the steam side inlet temperature of the heat exchanger is 283C but the reactor inlet temperature is ~288C, even with a one celsius outlet temperature difference the AMTD is 3C.
Which means that ~100MWt would require a superheater of only 10 cubic metres, assuming a PCHE and not a hybrid heat exchanger.
Considering the steam generator has a diameter of at least five metres at the top, we are looking at a vertical height addition of less than one metre even with the headers.

And having 324 celsius steam at 67 bar will get significantly better efficiency than at 283 celsius and the 41C of superheat will probably allow the high pressure turbines inlet seperators to be dispensed with.
And the principle is generalisable to any reactor that uses a traditional steam generator.


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