Subject: Re: [Q] which resistance wire for electric furnace ?

From: C++ Freak <[email-masked]>
Date: 6 Jun 1996 13:14:26 GMT

I got Kanthal wire from a friend of about 1 mm thickness with
3.7 Ohms/meter.
When using 15 meters at 230 Volts power it will drag 4 Amps and thus
yield 900 W (60W/m). When using 7.5 meters it's 8 Amps and 1800 W (240 W/m).
Using the blackbody radiation formula, the theoretical wire temperature 
is 760 K *above the ambient temp* in the first case (thus 790 C)
and 1077 K in (thus about 1100 C) in the second case.
Are these values useful in practice ?
And until how much temp can Kanthal withstand (in air) ?

Klaas

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From: [email-masked] (Jim Naylor)
Date: Wed, 05 Jun 1996 12:11:04 -0600

In article <[email-masked]>, [email-masked]
(John M. VerBurg) wrote:

>...there are a few interested, I could post them.
> 
Please do!  Thanks.

-- 
Jim Naylor                             [email-masked]
Wichita KS

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From: [email-masked] (Peter Brooks)
Date: 7 Jun 1996 16:56:58 GMT

John M. VerBurg ([email-masked]) wrote:

:    Now that I think about it, What about Titanium wire. Aldrich also sells
: it relatively cheap. I think the m.p. is about 1800C. I've never heard of
: anyone using it though. Does any know why this might be?   

Probably because Ti will oxidize very quickly.  We build integrated circuits
at work here, and some of them use a Ti film for (mumble, mumble) reasons.
When we run the wafers through the aluminum alloy/anneal process, we have to
make sure that the circa 500C tube is not contaminated by O2.  When it is,
I've seen the thin Ti film oxidize away completely.

Pete Brooks

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From: [email-masked] (Peter Brooks)
Date: 10 Jun 1996 17:41:44 GMT

C++ Freak ([email-masked]) wrote:
: I got Kanthal wire from a friend of about 1 mm thickness with
: 3.7 Ohms/meter.
: When using 15 meters at 230 Volts power it will drag 4 Amps and thus
: yield 900 W (60W/m). When using 7.5 meters it's 8 Amps and 1800 W (240 W/m).

I'm incredibly rusty on the blackbody formulae, but you do need to find out
what the temperature coefficient of resistance is for Kanthal.  It's incredibly
unlikely that your wire will still be at 3.7 ohms/meter at 1100C.  Note that
many ovens have to deal with this--one term is inrush current, which will
be a lot higher than the current at design temp.  Not too far removed from
what you have to deal with in motors, except for the fact that the physics
are completely different.  :-)

Pete Brooks