what to use for thermal insulator??

Hello,
I am assisting in the design of some heating heads.
We are using a 2000W rod type heating element, and
want to apply heat through an aluminum plate
(1/4" thk., touching the heating core and about 1/2 to 1" in height, x 8" lg.).
(Temp. range about 400 to 700 deg. F.)
I need to isolate the plate, to insulate the rest of the
head ass'y. from the heating element. (So the rest of the
heating head doesn't act as a heat-sink, drawing heat away from the plate...). The project Eng. wants to use a Ceramic/Mica composite, for one because it is 'machinable'. (The product spec says the thermal condutivity of this ceramic is about 2.4 BTU @ 75deg. , but not being an engineer experienced with thermal condutivity, the numbers don't tell me much...)
The debate is whether an air pocket would do a better job of insulating.

What is your opinion / advice ?

We need a good insulating material, which would be reasonably easy to machine (preferably without diamond tooling), that is easy to obtain and not overly expensive.

Is simply using an air pocket a better choice than any
ceramic composite? How much of an air space do you think would be necessary to minimize heat transfer?

Thank you very much - in advance - for your kind reply.

 

[IAMIS]

My engineering level does not make me an authority on this but I can offer an opinion. I believe an air pocket is only a good insulator if the air can be kept still and there is no radiant heat. Also, two factors of heat conduction are time and distance. If you see charts on the thermal conductivity of a material like a mica composite, you will see a certain thickness of the material separating two different temperature zones over a range of time. The length of time that your heating element is on and off will influence the insulator you choose. Air, being a fluid, I think, tends to form currents. If the flow can be prevented, as in the case of aerogels, it is then a good insulator. (continued) [sig][/sig]

 

[IAMIS]

That is the golden puzzle, "How do we keep the air from flowing (cheaply)?" This is why the vacuum bottle in a thermos works. I would say tne mica may be a decent choice. It is readily machineable with carbide cutting tools. One limiting factor is that it is recommended that it be sandwiched between two layers of another material to prevent delamination. I am currently working at a company that makes reflow solder ovens, and we use mica composites in a few locations. A company that sent me some good information is Cogebi, Inc at

http://www.cogebi.com.

(is that selling?)Hope that helps. [sig][/sig]

 

[mgoblue]

I do not understand what you are describing. I cannot picture in my mind what your head, plate, and element look like together.
However, the conductivity values are the basis for selecting one insulator over another. You have the one value from the project engineer for the plate. You can obtain another value for still air from reference books. Then you can compare. Since I don't understand your mechanical design, it is difficult to understand how you would enclose your insulating air layer. Assuming you need some metal plate to enclose the air, you must also take into account the thermal conductivity of that material in evaluating the air gap.

I know that is not much help but there is just not enough information.
[sig]<p>Bob Sander<br><a href=mailto:robert.sander@solvay.com>robert.sander@solvay.com</a><br>[/sig]

 

[dprice]

At the temperatures you propose I think the radiant effects will control. YOU can estimate radiant transfer by using an equation. I do think the project engineer is on the correct path. The ceramic should provide a radiative shield and thermal insulation. Being easy to machine is a good feature. YOu can estimate the total heat loss by converting your units you have the heat flux, however I think you have your units wrong. The units shoudl be BTU/area*thickness* delta T. YOu can convert BTU to kw*hr and estimate if the mica hass sufficient insulation. You can go to a basic physics book to understand the units calculations etc.

Good luck! Sometimes being an engineer is helpful in design!!!

 

[socrates]

Sean: The question that really needs to be answered here before you can correctly choose your insulator is how much heat transfer can your system ie. heating head tolerate. Thermal conductivity is measured in kW/m*degC or Btu/hr*ft*degF. I looked up Cilica Brick and found that at 392 degF it had a conductivity of 0.68 B/hr*ft*F with a
delta K/delta T of about 389E-6 up to 1,112degF. I made the following assumptions;

Ceramic Mica has approximately same deltaK/deltT
insulation is same sq Ft as al plate......8sq in = 0.056sq ft.
that you want the cool side at about 75deg F. approx Room temp
that the insulation is about 1 in thick

My results were as follows;

q = k*A*delta T/delta X
thermal conductivity of your ceramic; B/hr*ft*F
K @ 400 deg F = 2.53 resulting q= 553 B/hr
K @ 700 deg F = 2.64 resulting q= 1,109 B/hr

then I tried Cilica Brick and this is what I got;
thermal conductivity of CB; B/hr*ft*F
K @ 400 deg F = .683 resulting q= 148 B/hr
K @ 700 deg F = 0.80 resulting q=336 B/hr
You asked about the efficacy of using air. I would definitely advise against that despite the favorable insulation properties of air; as you mentioned your boss wanted a material that was machinable and air does not fall into that catagory. Also you would have to deal with leakage and expansion. If you haven't already done so there are three things that need doing;
1. verify that the temperature of the Al plate at the insulating surface is indeed 400 to 700 deg F.
2. figure how much heat the heating heads can absorb.
3. find the insulator that fits the bill both thermally and handling wise.
I hope that I have been of some assistance to you and good luck.
Sincerely,
Dave Sigel, ME dsigel@pacbell.net

 

[ratbuns]

Sean

As a project engineer designing commerical food service equipment, we face this situation alot. As was mentioned above your time and radiant heat will play a part. Air is an excellant insulator, but it DOES need to be moved! The radiant heat will heat the air, thus no insulating value. If you can move the air across your plate with a small muffin fan, your okay. But chances are, you can't. So I'd recommend the ceramic/mica composite.

Good Luck,
Mike Rayburn
Wells Mfg. Co.

 

Thanks all for your replies.

We have indeed decided against using Air as an insulator.

Wish I could comment on the new design (using the ceramic),
but it hasn't yet been tested.


Thanks!


~~~ PLEASE consider this topic closed ~~~

 

I don't know if you will view this as I was just this minute reading your submission. For your information there are other liquid ceramic insulators availlable that may be of use in your application. One is SuperTherm and the other is SP2001F. They are manufactured by a company called Superior however you will need to find the distributors as this is the only way to purchase their products. The products are manufactured in the US. I only know of one located in Wester Canada. wolfclan@hushmail.com

 

I advise to spray the part that you want to insulate with Zirconium Oxide. It can handle temperatures as high as 1000C. But it is only a passive insulator. Meaning cannot be at high temperature for a long time. I am currently in the same position as you. I need to find the right insulator at 600F.