what to use for thermal insulator??
what to use for thermal insulator??
(OP)
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.
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.





RE: what to use for thermal insulator??
RE: what to use for thermal insulator??
RE: what to use for thermal insulator??
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.
Bob Sander
robert.sander@solvay.com
RE: what to use for thermal insulator??
Good luck! Sometimes being an engineer is helpful in design!!!
RE: what to use for thermal insulator??
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
RE: what to use for thermal insulator??
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.
RE: what to use for thermal insulator??
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 ~~~
RE: what to use for thermal insulator??
RE: what to use for thermal insulator??