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PostPosted: Feb 19, 2013 4:33 am 
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A paper by Cha et al. on a S-CO2 system to couple to a sodium fast reactor.

http://www.kns.org/jknsfile/v41/JK0411025.pdf

Now this work is for a sodium coolant, but there are some really promising developments in it especially regarding the airfoil PCHE, which achieves 14x lower pressure drop than the traditional zig-zag pattern but surprisingly with a slightly improved heat transfer (+1%). 635 Pascal is almost no pressure drop at all
!

Imagine this technology being used for molten salt primary and secondary heat exchangers, molten salt gas heaters, steam generators...


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PostPosted: Feb 20, 2013 7:46 am 
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Looks like these guys also have a patent on the airfoil PCHE:

http://www.faqs.org/patents/app/20090294113

With some schematics:

http://www.faqs.org/patents/imgfull/20090294113_01


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PostPosted: Apr 07, 2013 10:27 am 
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Sciencedirect has the images available without subscription (but not the text).

http://www.sciencedirect.com/science/ar ... 9308003816


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PostPosted: Apr 07, 2013 4:34 pm 
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The interest in supercritical CO2 power conversion systems is growing. I have attached an article with information about this particular Brayton system. In the article it is mentioned that Pratt & Whitney Rocketdyne is also working on the SCO2 power cycle. However, they do not opt for the printed circuit heat exchangers, because of size limitations. It is also interesting to note that the energy density involved is really impressive in a SCO2 power cycle, similar to the energy density of rocket engine turbo pumps. For more information see:

http://www.sco2powercyclesymposium.org/ ... rocketdyne


Attachments:
SCO2_TMISept_Oct12.pdf [468.53 KiB]
Downloaded 183 times
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PostPosted: Apr 07, 2013 5:30 pm 
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Please forgive me if this is a clumsy question,

Would it be fair to say reduction in pressure drop implies less friction and higher flow rate?

Prior posts I've seen here have mostly presumed tube in shell heat exchanger configurations,

I am guessing this is because high flow rate is important, that high flow rate provides one or more advantages

(relating to delayed neutrons, or noble metal accumulation on hx surfaces, or responsiveness to adjustments in pump rate for load following, or balancing quantities of fissile inventory in-core v out-of-core?)

Very uninformed post here. Grateful nonetheless.


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PostPosted: Apr 07, 2013 8:04 pm 
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How does graphite behave in SC CO2?

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PostPosted: Apr 08, 2013 6:17 am 
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A reduction in pressure drop implies a lower pump power for a given flow rate. Or a higher flow rate for a given pump power.

Friction of the fluid with the HX metal is one factor. Usually the friction of the fluid with itself is a bigger factor. The airfoil design connects many small flow paths, allowing flow to equalize itself in the direction perpendicular to flow, so fluid self friction is reduced a lot.

Hot CO2 reacts with graphite. It makes C + CO2 = 2CO. When the CO2 is cold the reaction rate is tiny.


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PostPosted: Apr 08, 2013 6:39 am 
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Quote:
Prior posts I've seen here have mostly presumed tube in shell heat exchanger configurations,

I am guessing this is because high flow rate is important, that high flow rate provides one or more advantages


Actually tube in shell has vibration issues at higher flow rates. Printed circuit heat exchangers do not have potential for damaging vibrations. Flow rate can be greatly increased with the new airfoil type printed circuit heat exchanger, I'd definately expect this airfoil PCHE to beat shell and tube with ease on all performance criteria.

Higher flow rate is attractive because it makes the exhanger more compact. But it mustn't be too high or there will be erosion from suspended particles (possibly some of the noble metals that are hard enough to abrade) even with airfoils and reduced pumping power.


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PostPosted: Apr 08, 2013 2:51 pm 
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Cyril R wrote:
Hot CO2 reacts with graphite. It makes C + CO2 = 2CO. When the CO2 is cold the reaction rate is tiny.


Hot CO2 does indeed react with carbon. See "Boudouard equilibrium".

Cool CO2 and cool carbon react at rate that is not tiny but zero,
because at low temperature the Boudouard equilibrium ...

2 CO <---> CO2 + C

is on the right. Carbon monoxide is unstable. The reaction that --
although strongly favoured -- at room temperature occurs at a tiny rate
is its disproportionation.

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PostPosted: Apr 09, 2013 11:38 pm 
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We did some fairly extensive study on PCHE's for high-temperature heat transfer applications. They are extremely difficult to make work. The issue is the very large thermal stresses and plastic deformation that occurs during thermal transients. If one of the loop pumps trips, and the other does not, so flow continues on one side, the thermal transient is severe (as the PCHE equilibrates to the inlet temperature of the flowing fluid). So shell-and-tube heat exchangers remain the practical choice.


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PostPosted: Apr 10, 2013 2:15 am 
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Per Peterson wrote:
We did some fairly extensive study on PCHE's for high-temperature heat transfer applications. They are extremely difficult to make work. The issue is the very large thermal stresses and plastic deformation that occurs during thermal transients. If one of the loop pumps trips, and the other does not, so flow continues on one side, the thermal transient is severe (as the PCHE equilibrates to the inlet temperature of the flowing fluid). So shell-and-tube heat exchangers remain the practical choice.


Thanks for pointing out a problem area Dr. Peterson.

Pump trip logic is an important part of the plant protection system of any molten salt cooled or fuelled reactor. We'll want this, to prevent excessive cooldown rates but also to prevent freezing. Preferably there would be a double redundant process and safety related trip.

IIUC, a big transient problem for PCHEs is the short flow path resulting in large axial temperature gradients. With the new airfoil PCHE developed by the South Koreans, a much longer flow path can be accomodated. Such a PCHE could be just as long as a tube in shell heat exchanger, yet have a much lower pressure drop.


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PostPosted: Jun 07, 2013 1:11 am 
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This might be of interest regarding heat exchangers.

REL, the people trying to develop the SABRE advanced air breathing rocket engine for their Skylon SSTO rocket, have been doing interesting heat exchanger research. A PDF I was directed to illustrated some of their work on stamped foil based "plates" for an advanced PCHE. Please see page 11-13 in the following PDF.

http://www.reactionengines.co.uk/tech_docs/Heat%20exchanger%20design%20in%20combined%20cycle%20engines%20IAC-08-C4.5.1.pdf

If the practicality of stamped foil based PCHE production improves, would this necessarily reduce the cost, as it uses less material and has less waste material in production? Though microchannels are probably not such a good idea for a salt->SCO2 heat exchanger...


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PostPosted: Jun 07, 2013 4:27 am 
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Asteroza wrote:
This might be of interest regarding heat exchangers.

REL, the people trying to develop the SABRE advanced air breathing rocket engine for their Skylon SSTO rocket, have been doing interesting heat exchanger research. A PDF I was directed to illustrated some of their work on stamped foil based "plates" for an advanced PCHE. Please see page 11-13 in the following PDF.

http://www.reactionengines.co.uk/tech_docs/Heat%20exchanger%20design%20in%20combined%20cycle%20engines%20IAC-08-C4.5.1.pdf

If the practicality of stamped foil based PCHE production improves, would this necessarily reduce the cost, as it uses less material and has less waste material in production? Though microchannels are probably not such a good idea for a salt->SCO2 heat exchanger...


Thanks for the link, yes that's a typical cross flow PCHE arrangement.

PCHEs are much cheaper than tube in shell. They cost more per kg of material, but are so much more compact, the net effect is a large reduction in cost. INL has done cost estimation and found that the PCHE is more than 50% cheaper than tube in shell.

There are also many savings in space, and for a nuclear reactor, also shielding and containment savings, and for a molten salt reactor, also large fissile inventory savings.

Microchannels are a good idea for salt-gas exchangers. The small channel diameter makes it much stronger and the maximum break flow much smaller. Also, the weakest bonds are the diffusion bonds between adjacent flow channels, which contain the same fluid. not the strong bonds/welds at the sides or the plate itself.

One downside with microchannels is the small tolerances that must be kept. So, expensive corrosion resistant alloys must be used. But because the PCHE uses material so efficiently, it is not a big deal to go for these more expensive alloys.


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PostPosted: Dec 28, 2013 9:42 pm 
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In case anyone missed it, nextbigfuture had a report on recent s-CO2 turbine work by Sandia and others. There seems to be an increasing chance that these will be available by the time anyone has a working MSR to drive them.


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PostPosted: Dec 29, 2013 3:49 pm 
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Luke wrote:
In case anyone missed it, nextbigfuture had a report on recent s-CO2 turbine work by Sandia and others. There seems to be an increasing chance that these will be available by the time anyone has a working MSR to drive them.


Thanks luke, great news. It'd be interesting to see how these pilots are going to deal with the extreme mechanical stresses produced by such compact turbomachinery and high mass flow rates of the S-CO2 cycle.


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