Different CTEs

[drodrig]

Hi,

We have a PE1000 plate with an aluminium plate on the top (900x900mm2 - 35x35inches2). The aluminium plate is cooled by a brass plate-pipe down to -40ºC (-40F). This brass system is in a pocket of the PE1000 plate; there is insulation filling the pocket below the brass.

The aluminium plate is screwed against the PE1000 all around the perimeter. An O-ring seals the perimeter from Isopropanol which is on the top of the aluminium plate.

We chose PE1000 because of its working temperature and the good resistence against the isopropanol, but we overlooked the thermal expansion, which happens to be almost 10 times bigger than the aluminium.

I'm worried this will end up failing. From a quick simulation I get 40MPa when the tensile strength at break is 30MPa.

I would like to ask for some thoughts to solve this. I was thinking about puting an intermediate material (which will keep the PE1000 not so cold) or using Teflon to allow displacements.

thanks
regards,

 

[SwinnyGG]

I would already be concerned about cracking that UHMW plate between the screws and the o-ring

Why would you not make the surrounding tray the same material as the top plate (aluminum)

 

[drodrig]

Why would it crack in the location?

The UHMW material was chosen because of the low thermal conductivity, this is exposed to room temperature and we would get ice/water easily

thanks
regards,

 

[IRstuff]

You might want to talk to your supplier and get an actual curve vs. temperature. Many materials have much lower CTE at cold temperatures. Most of the CTE values I've seen for this material applies for the range of 23C to 100C, so -20C might have some chance of having a much lower CTE.

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[drodrig]

Yes, I finally found the value and it is a bit lower, but still 6 times higher than aluminium

I'll start trying to let it expand without the connecting screws to the alu plate and see that happens

thanks
regards,

 

[gruntguru]

Can you move the screws outside the o ring perimeter or move the o ring inside the screws? Then clamp the aluminium using a picture frame so the aluminium can slide past during expansion. Alternatively, over-sized holes thru the Al with nuts (again requires relocation of the o ring).

je suis charlie

 

[drodrig]

Gruntguru,
I can't change the position of o-ring / screws because the Isopropanol would go thought the screws holes.

Allowing displacement is a good idea, maybe with some teflon sheets.

What do you mean with picture frame?
thanks

 

[mighoser]

You can use a glass or ceramic reinforced UHMW with low CTE. Tivar 88 or Tivar Ceram-P

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