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PostPosted: Sep 24, 2017 10:19 am 
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So I stumbled across 'microtube heat exchangers' which apparently promise near PCHE levels of efficiency at a rather low cost, and without the massive loss of material associated with chemical milling.

It essentially seems to be making lots of microscopic tubes (so small flow in this is laminar) and then using swiss fineblanking to make microstrip tube sheets and then diffusion bonding the tubes into them to make modules which can then be connected in series parallel to achieve your objective.

I am thinking about these as a supercritical boiler system for a proposal I have for an Evolved AGR (300 bar DRH, 645C outlet temperature) which can get 50%+ efficiency.
The have the advantage over the PCHE in that they can more readily use a wider range of materials (PCHEs aren't qualified on many things at the moment)

And apparently Microtube heat exchangers can be used with some of the few alloys that can be that can be usd at ~650C at the moment and which aren't full of cobalt (apparently AGR boilers do get a signifcant neutron dose over their lifetime, which excludes Inconel 740H due to neutron activation causing fun issues with maintenance). HR6W and HR35 appear to be the best available, although Alloy 141 looks very promising.

And if they can reallly achieve lower costs per square metre of area than a PCHE - given that a boiler in a conventional AGR costs something like ten percent of the entire plant cost even as it is, this is clearly significant.


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PostPosted: Sep 24, 2017 8:45 pm 
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First thing that springs to mind is the group behind the Skylon spaceplane, Reaction Engines Limited, who specifically want to branch out and sell their microtube heat exchanger tech. They are currently gearing up for assorted engine testing (currently doing construction on the engine test stand), so they will have yet another go for their heat exchanger tech under live conditions, though admittedly their application (cryogenic helium in the tube and hot atmospheric air) is not quite the same...

https://www.reactionengines.co.uk/sabre/technology/heat-exchangers/

Who is currently building microtube heat exchangers in the pressure ranges you are thinking of?

Though one wonders if diffusion bonded stacked foils like those used by Rocketdyne to build rocket injectors (which should be coming off patent...), if using modern laser cutting for the foils, might be a new alternative to conventional chemical milling for PCHE's. Or that crazy femtosecond laser water explosive etching work for fine milling...

http://www.mdpi.com/2072-666X/6/12/1471/pdf


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PostPosted: Sep 26, 2017 11:33 am 
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It appears test work is underway at my steam side pressure for use in recuperators (albeit with a carbon dioxide atmosphere).

If microtube exchangers are the basis of the Reaction Engines technology, that makes me far more confident that the performance required would actually be achievable.


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PostPosted: Sep 26, 2017 5:17 pm 
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One concern that springs to mind is the large number of joints in this type of exchanger. Failure rate better be nil. If you have a million joints, a 10-5 joint leak reliability means average of 10 failures per year. Not good.

In terms of cost, can't imagine that tiny tubes would be cheaper than plates with channels etched in them, given the number of tubes required. I wonder why the PCHE plates aren't pressed/drawn, since the shapes used seem pretty standard, a one time investment in a press and die would look like a good investment. The diffusion bonding process relieves cold working stresses so no need for added process steps.


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PostPosted: Sep 26, 2017 9:54 pm 
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I think the idea here is that whilst you need huge numbers of tubes, they are all functionally identical and can be chopped off the end of a continuous manufacturing line.

Same with the tubesheets, hundreds of thousands of absolutely identical parts.

And turning out millions of relatively simple and identical parts is something we can do very well.

Nickel base alloys are very expensive very quickly when you are throwing away half of the metal (porosity of a PCHE is 50+%).

Heatric apparently make a formed plate heat exchanger which is made of stamped plates diffusion bonded together, but it is supposedly only good to about 200 bar.
So no good for a 45 bar gas pressure and 300 bar steam pressure.


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PostPosted: Sep 27, 2017 10:28 am 
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E Ireland wrote:
I think the idea here is that whilst you need huge numbers of tubes, they are all functionally identical and can be chopped off the end of a continuous manufacturing line.

Same with the tubesheets, hundreds of thousands of absolutely identical parts.

And turning out millions of relatively simple and identical parts is something we can do very well.

Nickel base alloys are very expensive very quickly when you are throwing away half of the metal (porosity of a PCHE is 50+%).

Heatric apparently make a formed plate heat exchanger which is made of stamped plates diffusion bonded together, but it is supposedly only good to about 200 bar.
So no good for a 45 bar gas pressure and 300 bar steam pressure.


To me it's the ability of having intricate flow channels that would not be economical with tubes that is a big advantage. The best example is the work on airfoils PCHE. Pressure drop can be cut an order of magnitude without loss of heat transfer rating. This is easily worth it just for reduced expenses for pumps and pumping power, even if you waste >50% of the material. It also makes possible natural circulation trains that extend the benefit even further.

The airfoils can be placed very tight together so you should get good bonding between plates. Pressure rating should be pretty good.

The other thing is tubesheets. These are trouble. Many tiny tubes in a tight pitch leads to poor ligament efficiency, requiring very thick tubesheets for a high design pressure. This can be an issue for thermal stresses. You'd probably need a headered arrangement.


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PostPosted: Sep 27, 2017 3:30 pm 
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Apparently a team at Sandia believes it might be possible to cast an airfoil PCHE type structure in a single piece as the lack of discrete channels allows the casting cores to interconnect. Apparently called 'Cast Metal Heat Exchanger' / CMHEs and projected to see an 80% reduction in costs for similar performance/compactness.

Now that would be something


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PostPosted: Sep 27, 2017 3:44 pm 
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E Ireland wrote:
Apparently a team at Sandia believes it might be possible to cast an airfoil PCHE type structure in a single piece as the lack of discrete channels allows the casting cores to interconnect. Apparently called 'Cast Metal Heat Exchanger' / CMHEs and projected to see an 80% reduction in costs for similar performance/compactness.

Now that would be something


Yes, this would be remarkable. It surprises me a little though - imagine it's difficult to prove the airfoils meet tolerances without being able to inspect each plate first. But definately you could cast individual plates, or hot press them, cold drawing, then diffusion bond, sure something will work.


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PostPosted: Sep 27, 2017 3:53 pm 
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I imagine QC on a monobloc casting would be primarily empirical. If it performs on a test rig and passes pressure proofing it would be accepted, even if some airfoils are out of tolerance


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PostPosted: Sep 27, 2017 4:46 pm 
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E Ireland wrote:
I imagine QC on a monobloc casting would be primarily empirical. If it performs on a test rig and passes pressure proofing it would be accepted, even if some airfoils are out of tolerance


Good point. You may be right about this - pressure and flow test could be the manufacturing QC. Current standards such as asme section XI require periodic in-service inspection of welds and other joints, but TMK not the base material itself. So a one-piece forging or casting would not be required to be inspected in-service.

Still, I'd be amazed if intricate fins spaced very tightly can be cast in one block.


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PostPosted: Sep 27, 2017 7:53 pm 
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Cyril R wrote:
E Ireland wrote:
I imagine QC on a monobloc casting would be primarily empirical. If it performs on a test rig and passes pressure proofing it would be accepted, even if some airfoils are out of tolerance


Good point. You may be right about this - pressure and flow test could be the manufacturing QC. Current standards such as asme section XI require periodic in-service inspection of welds and other joints, but TMK not the base material itself. So a one-piece forging or casting would not be required to be inspected in-service.

Still, I'd be amazed if intricate fins spaced very tightly can be cast in one block.


Considering the advances in 3D printing sand casting molds, it would then becomes an issue of the sand grain size affecting surface roughness of the airfoils. There might be other tricks, like that DMG-Mori hybrid style 3D CNC/printer, that prints a little, then machines a little in spots that are still accessible before being covered by new material to improve the finish of the mold surfaces.


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PostPosted: Sep 29, 2017 10:41 am 
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One area in a hyper efficient modern plant that might be able to use these microtube exchangers [to return to the original topic!] is in a condenser assembly.

Currently I propose using direct contact condensers with an advanced heat exchanger coupling the feedwater-spray side to the seawater cooling side.
THis allows a much lower delta-T than in a conventional condenser and with proper design placement will largely eliminate seawater and air ingress into the feedwater system [place the heat exchangers closer to the shore than the rest of the plant, at a lower elevation, alongside the modular nature of the plant allowing leaing segments of the Heat Exchange system to easily be taken out of service rapidly], but to show substantial economic advantage requires a very high efficiency heat exchanger.

PCHEs are the obvious choice for this duty due to their enormous effectiveness, but their pressure and temperature capability is not really used ina situation where the heat exchanger temperatures are below 20 celsius, and the pressures on both sides are at or about one bar(abs).

It occurs to me that if you put the seawater outside the tubes and the feedwater inside the tubes (reversing the traditional arrangement) you could still provide much easier access for cleaning than in a conventional condenser [you just need to steam the outside of the tubes to remove biofouling etc, whereas the area constrained inside of the tubes remain clear] and certainly better than practice for a PCHE - which is still considered suitable for filtered seawater service but only marginally.


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