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Danny Harvey

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Reply with quote  #16 
John,

I used to supply a client with wear strips for some sort of overhead chain conveyor. They had to be clear grain red oak of a specific moisture content and then they applied some sort of oil (walnut I think).

I'm not sure where they came up with the specs but they were happy and so was my sawmill guy.  
electricpete

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Reply with quote  #17 
Here is a photo where Fabreeka was used in the application I mentioned above.  It looks like wood, but it's not.

 
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John from PA

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Reply with quote  #18 
Quote:
Originally Posted by electricpete
Here is a photo where Fabreeka was used in the application I mentioned above.  It looks like wood, but it's not.


Hopefully we will be advised by the op what the product is.  Fabreeka is good down to -65 def F, a long way from the -160 deg C (-256 deg F) mentioned in the 1st post by the OP.
OLi

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Reply with quote  #19 
Liquid LNG is a bit chily I guess? I saw pumps for liquid Ethene and the bearings were composite, way back already.
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Curran919

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Reply with quote  #20 
I had a chat with our local materials engineers to try and assess an alternative to the wood. The wood takes the full load of the pump, so the thermal break obviously can't be a low-density (i.e. foam) polymer, but other hard polymers could replace the wood. There are products like Armatherm FRR that looked good mechanically and thermally. However, as the design temperature is below the embrittlement boundary of most polymers, classic polymers are not suitable here. He started sifting through his brain for ceramics that may be useful, but would be expensive and have low toughness, not suited for industrial application. He couldn't come up with an alternative. That doesn't mean the wood is the best choice, but there is not an obvious alternative. I will hopefully get more info on the wood when I am on site (I imagine in about 3 months...).
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Reply with quote  #21 
Teflon? Wiki say -200
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Curran919

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Reply with quote  #22 
Quote:
Originally Posted by OLi
Teflon? Wiki say -200


Teflon maintains its lubricating properties to -200C, but its mechanical properties quickly drop off at around -100C, apparently.
OLi

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Reply with quote  #23 

Funny in Swedish wiki it says -200 english is very different  https://en.wikipedia.org/wiki/Polytetrafluoroethylene

so maybe we have a colder Teflon...... So put some heating on it? Or maybe wood is good not that I heard of super cold Pine before so it may be something else. 


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electricpete

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Reply with quote  #24 

My gut feel (not a materials engineer, but I can spell materials engineer) is that alternating thin layers of a structural material (steel?) and a suitable insulator might work (still needs work to define suitable insulator).  The insulator remains effective thermally because it is in series with the steel.  The steel allows the insulator to be used in flat thin sheets which moves the stress distribution closer to a pure compressive stress rather than a complex triaxial stress, so brittle fracture is less likely (brittle fracture does not occur under pure compression).  Of course if I’m right about this, then someone somewhere has already made a composite material for ths purpose.

electricpete

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Reply with quote  #25 
on second thought, since when is a thin sheet of brittle material tougher than a thick sheet?!? a lot of things end up cracking if you cut them too thin. There is a disconnect between my intuition and my understanding of the theory. I'm not so sure any more.
OLi

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Reply with quote  #26 
So wood is maybe very good since it has fibers that makes it like the modern composite materials and even if it is reasonable cold it may get brittle but it would likely take some bending before it cracks if the water content is replaced with something else anyway, it maybe keep some of that behaviour even if it get unreasonable cold.
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Reply with quote  #27 
Quote:
Originally Posted by electricpete

My gut feel (not a materials engineer, but I can spell materials engineer) is that alternating thin layers of a structural material (steel?) and a suitable insulator might work (still needs work to define suitable insulator).  The insulator remains effective thermally because it is in series with the steel.  The steel allows the insulator to be used in flat thin sheets which moves the stress distribution closer to a pure compressive stress rather than a complex triaxial stress, so brittle fracture is less likely (brittle fracture does not occur under pure compression).  Of course if I’m right about this, then someone somewhere has already made a composite material for ths purpose.



Just as a thought experiment as I wile away my Friday afternoon waiting for happy hour:

Take a fluid flowing through 10m of 6" pipe, then 10m of 1" pipe. Compare that to a fluid flowing through alternating 1m sections of 6" and 1" pipe. The latter will have a much large resistance curve (analogous to low thermal conductivity). This would be due to recirculation zones, turbulence, etc. However, convert the problem to the analogous 1D heat conduction system, and you do not have the same effect, because thermal turbulence doesn't exist. Therefore alternating thin layers of steel and polymer would be the same thermally as having one block of each. With some caveats:

  • Bad contact between the layers lead to extra contact resistances in the 'collated' configuration and therefore lower total thermal conductivity.
  • If you have the metal layers extend past the polymer layers, you will create circumferential fins that will absorb heat from the 'atmosphere' (assuming no external insulation). This would increase the temperature of both the foundation and the pump baseplate (relative to the uncollated config). If the thermal break is to avoid temperature strain on the foundation, it would help. If your goal is to prevent energy losses (temperature gains) in the fluid, it would hinder. I actually don't even know why they have this thermal break...
John from PA

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Reply with quote  #28 
There are some plastics out there that will hold up to the temperature.  DuPont Vespel is one that is defined as able to withstand cryogenic to absolute zero.  "Cryogenic" range is defined as as from −150 °C (−238 °F) to absolute zero.  See https://www.curbellplastics.com/Research-Solutions/Industry-Solutions/Case-Studies/Vespel-Thermal-Isolator-for-Scientific-Equipment for some information on Vespel.

Some of the "Teca" family also have lower service temperatures that are down around -200.  See https://www.curbellplastics.com/Research-Solutions/Technical-Resources/Technical-Resources/Engineering-Plastics-Manual-Ensinger
electricpete

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Reply with quote  #29 
 
Quote:
Therefore alternating thin layers of steel and polymer would be the same thermally as having one block of each

Yes absolutely. I wasn't claiming any thermal insulating advantage  (to my mind, two quarter inch blocks of insulator separated by a thermal conductor have the same insulating capability as one half inch block of insulator).

I was in my first post claiming an advantage in resistance to brittle fracture.   The reason I thought that is that brittle fracture requires tensile stress, which is reduced if you change the geometry toward a wide thin layer where it is closer to pure compression (analogous to hydrostatic pressure).  In contrast if you have something more like a  tall thin block (not flat thin) loaded in compression at the top and bottom, there are certain planes that are in shear or tension or shear.   Tensile stress is required for brittle fracture. 

In my 2nd post I noted that my intuition suggests something is missing in that line of thought.   

I think what was missing in my first post was that the tensile stress must be compared to fracture toughness to determine if brittle fracture is likely. 
Yes, tensile stress goes down when we move toward thin flat plate, but I think maybe fracture toughness also goes down (even though fracture toughness is something similar to a material property, it also depends on geometriy I think)
So without knowing which one deceases more (tensile stress or fracture toughness), we can't make any predictions. 

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