Subject: Re: Gas forge temps

From: [email-masked]
Date: Tue, 6 Aug 1996 11:30:53 -0400

First a thought on relative emissivities of flame & refractories:
  Not specific data, but emissivities & absorptivities of various materials
are a function of frequency.  For "black boxes", absorptivity & emissivity
are both equal to one across all temperature (frequency) ranges.  This is not
true for real world materials.  The reason that things (especially iron
things) heat up in the sun is that their absorptivity at sunlight frequencies
is greater than their emissity at lower frequencies (ie. ambient outdoor
temperatures).  If the frequency being emitted by a flame at combustion
temperature is above the best absorption frequency for the material being
heated, then radiant heating efficiency will be poor.  You can't heat a
mirror.  I think what you're after is the emission spectrum of burning gases
(and products of combustion), the emissivity of hot refractories (probably
close to black box radiation at that temperature) vs. the absorptivity of the
metals being heated.  Emissivity/absorptivity data for metals is relatively
easy to come by (might try CRC handbook of chemistry & physics).  I'm not
sure about emission spectra of hot gases, but also shoul not be too tough.

Second, back to the forge.  I agree with your combustion analysis, but still
need to see some radical improvement in forge design to come from this
discussion.  Also, you seem to imply that a naturally aspirated burner,
properly designed should burn as hot as a blown burner.  This is generally
not the case in practice.  Is this because of poor design, necesary
tradeoffs, or something we haven't discussed concerning the behavior of the
flame inside the forge?

You also discuss losses due to the need to heat all the extra freeloading
noncumbustibles.  Does anyone know of experiments with forges that use waste
heat to preheat fuel, air, or both?  Any predictions on performance boost?  

Pete Engle

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From: "Steven O. Smith" <[email-masked]>
Date: Tue, 6 Aug 96 10:49:04 MDT

> Also, you seem to imply that a naturally aspirated burner, properly
> designed should burn as hot as a blown burner.  This is generally
> tradeoffs, or something we haven't discussed concerning the behavior
> of the flame inside the forge?

Yeah, I second that point. 

> heat to preheat fuel, air, or both?  Any predictions on performance boost?  

The Sandia forge (plans from ABANA) preheats the incoming air (I think
this is "recuperative"). I tried this with the forges I'm building. It
only made a 125 degree F difference in the final, fully warmed up
forge temperature, at a great increase in the complexity of the
plumbing. There is also a reliability issue in that the air pipes
going through the chimney eventually scale away.  I was preheating
both air and fuel after mixing. I also had a problem with flashback in
the air/fuel tubes once they were fully warmed up. I decided against it.

--
Steven O. Smith
[email-masked]