Re: GAS-L: Re: Fw: RETORT FOR CARBONISING COCONUT SHELLS - REG.

[AJH <andrew.heggie@dtn.ntl.com>]

On Mon, 16 Sep 2002 11:37:52 -0300, Kevin Chisholm
<kchisholm@ca.inter.net> wrote:

>A temperature of 300 C = 570F would seem to be OK.
>Ordinary mild steel, such as would be had in 45 gallon
>drums, doesn't scale significantly until well above
>700F.

You do need to consider that a temperature differential must exist
across the retort wall for heat transfer to take place. The poorer the
heat exchanger (and a 200 litre drum is poor) the bigger the
temperature drop, in fact it is one reason that larger retorts using
external heating are not practical, let alone the fact that char in
the retort is not a good heat transfer medium either.
>
>As a starting point, it would thus seem that there is
>no need for retort materials to be anything more
>expensive than mild steel. 

Our ring kilns are mild steel but they do suffer over time.

I meant to comment on one of Crispin's posts about case hardening
using char. When this is done the carbon diffuses into the iron to
form a eutectic mixture, this happens well below the melting point of
iron. There is an urban legend at one of our university's, I do
actually believe it is true because of my closeness to the source.
Anyway De Beers had loaned a largish diamond for research into
dislocations in the carbon crystal (the department was contracted to
research growing diamond "lens" for use in optical equipment in
extreme environments, the results were regularly seen on tv about ten
years ago). The crystal needed to be "excited" at about 750C. The
researcher knew he was safe from "graphitising" till about 1200C so he
raised its temperature ultrasonically on a steel transducer. At some
point around 600C he was horrified to see the diamond diffuse into the
steel. He had to show the small patch of cast iron in the middle of
the steel transducer to explain to his sponsors where the diamond had
gone. I am sure I have some of the technical facts wrong but you
should be able to follow the salient points.
>
>Does anybody have strong feelings to the contrary?

Not me, my understanding is that for low temperature work mild steel
is fine, it could become brittle through carbon diffusion at higher
temperatures. There does not seem to be any case for stainless on the
inside of the retort as its corrosion resistance is dependant on an
oxide layer forming. With the type of retort based on Jury's posting
the outside temperature in the flaring zone could reach 900C. Now
titanium that's another matter, you could run some high temperature
experiments with a retort made for that :-).

I have also had good results when using vitreous enameled stove pipes.

AJH

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