250F Heating Application: Need two flexible, different high temp fabrics

[bcavender]

I have a heating project that needs a flexible application of continuous heat as a two dimensional secondary operation takes place. To make the tool easy to construct, it would be helpful if the material could be sewn with a high temp Kevlar thread. The physical forces of tension in the fabric will be small (5-10 lbs max) as it will essentially be a wrap. The heat source is simply internal resistance wire between the two fabrics.

First Material:
On the side facing the work, I need a highly conductive fabric material that can easily handle 250 degF (max) and is thermally conductive enough that it will diffuse the heat from the wires that will be appx 8mm apart so there isn't more than a few degrees difference between being right over a wire to 4mm between. Low heat capacity, high thermal conductivity with very low/no electrical conductivity as the wire will not have separate insulation to promote heat flow. Thickness can vary as necessary.

Second Material:
The backing fabric simply needs to act as a thermal and electrical insulation to minimize energy loss and provide support for the heater wires ... and be sew-able. Reflective properties would be a bonus.

My research turns up a number of fabrics that can fill the backing need for electrical/thermal insulation, but finding a material that can conduct/spread the heat is far more difficult.

I would greatly appreciate any comments/suggestions as to what materials might be worth considering for this application.

All comments welcome!
B

 

[SWComposites]

Sounds like you need a metallic fabric.

 

[MintJulep]

For example:

https://nano3dsystems.com/product/copper-conductive-fabric/

https://www.tomoegawa.co.jp/english/product/new_product/copper.html

 

[bcavender]

Yes, the tricky part with a metal fabric is the electrical conductivity with the resistance wires. Adding an electrical insulator to either the fabric or wire slows heat xfer. (But this is worth pursuing as I know nothing of what is available on the market. I'm thinking this problem has already been solved and I just haven't run across the product.)

The best idea I had found so far is a silicon rubber filled with zinc oxide that raises the SiRubber sheet thermcon to ~8 W/mK. (ZnO thermal conductivity=50W/mK) The hesitancy there is that zinc ox is electrically conductive so if there was a breakdown in the SiRubber a short could dump a lot of energy in one spot with suboptimal results. I doubt silicone rubber could stand up to sewing as well. So that's really a dead end.

Possibly there is a fabric with a deeply embedded non-conductor to allow thermal conductivity, but retain its fabric resistivity ... Hmmm ...

Thanks!
B

 

[MintJulep]

Would something here work for you?

https://www.watlow.com/products/heaters/flexible-heaters/silicone-rubber-heaters

Why design a flexible heater yourself if you can buy one from an expert.

 

[bcavender]

Mint,
Watlow has some great custom work there. I will give them a call Monday to see where they stand on sizes/pricing.

Part of the issue is my customer wants to keep as much as possible proprietary… that said, possibly integrating some off the shelf or semi custom products could satisfy both needs.

Thanks for the recco!
B

 

[Compositepro]

Your requirements are unrealistic and probaby unecessary. Where there is heat flux, there is always a proportionate temperature gradient. In any real electric heater the wire will be hundreds of degrees hotter than than what is being heated. If not, there is almost no power being supplied. In almost all cases where an object is heated that requires very precise temperature control or is heat sensitive such that it must not get above a certain temperature, a cirulating fluid is used for heating. This means a water or oil bath or jacket, or an air circulating oven.
Metals are often heated using direct flames without overheating. Paraffin is heated using a double boiler.
If you give details of what you are trying to accomplish you can get much better advice. But your ideas are probably too confidential reveal details and in the end nothing will be developed due to spending all your effort learning about the realities of heat transfer.

 

[LittleInch]

Electrical wires 8mm apart in a flexible blanket with no electrical insulation???

Not a good idea IMHO. Why are you worried about heat transfer from the wire?

If you want even hear spread just use a metal chain mail or thin mesh to even out the heat.

Think electric blanket here...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

Almost seems like putting the object in a vacuum bag and doing a oil-bath sous-vide might be worthwhile.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[TugboatEng]

Silicone rubber coated aramid sounds like what you need. Silicone is conducive of heat, insulative against electrical while aramid insulates against heat and electricity while also having very high tensile strength. You can add metallic facing if you need refractory or low emissivity properties.

 

[bcavender]

I wish I could be more specific, but confidentiality is up against my beans/cornbread stop.

One of the basics I can say is very fast cycle times are necessary. Ovens/dips block/contaminate the secondary operations as well as potential for heated product damage during transfer in/out. The customer also targets the capital cost per heating station to essentially be driven into the dirt to pop their investment IRR and be able to survive potential unlevel foreign competition.

Minimizing the gradient, because of cycle time requirements, also becomes a bit more involved when a 10% PID temp overshoot will create a product reject. Getting a lot of energy in there at their pace calls for extra low proportional and pretty high derivative parms to get to critical damping … I will say that our first feasibility shot nailed the low capital hurdle, but the heater/mass couldn’t hit the cycle time by a factor of 2.5 … thus chucking mass and getting the heat coupled just as close and even as possible is the challenge.

8mm spacing is tight, but it’s the next feasibility test (driven by W/in^2 and trying to achieve some standardization in total resistance to simplify power drive equip configs & min capital constraint) is to simply sew Kevlar separators in the fabric. Simple, low tech, low skill assy. Clearly there’s always a risk that the design will choke, but if successful, their capital/trade secret goals will be in hand.

As for Aramids, it’s worth digging further because of its strength/heat tolerance. (Though at conductivities <1 W/mK, like Nomex, I was hoping to find something a lot better out of the chute), but it sure looks like the options are limited. Admittedly this is new stuff to my experience/pay grade, but I’m willing to learn. I need to get down on a transfer calc given the thinnest the facing fabric could be … maybe there’s a partially loose/open aramid weave that would pass IR or take up a thermal conductor between strands … hmmm.

Appreciate all the good comments folks. It helps the process a lot