Ivor and Olive Lewis on sun 11 jul 04
Dear Friends,
It would be good if we could have an explanation of the behaviour of
the effects of prolonged heating on furnace elements.
Since we may be dealing with several alloys it is reasonable to
suppose that their corrosion and wear processes are not identical.
As I understand things in a general way, repeated heating and cooling
causes grain growth. Provided the initial oxide coating remains intact
it is relatively impervious to gas diffusion. But if it is disturbed
by any cause then the oxidation process is renewed and proceeds along
the grain boundaries. This changes the conduction characteristics of
the element, creating places of increased resistance to current. The
effect is to cause hot spots where elements fail due to premature
melting.
Can anyone add more to this supposition?
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
.
pdp1@EARTHLINK.NET on sun 11 jul 04
Hi Ivor, all...
Why do Electric Kilns not use 'rods' instead of coiled
'wire'?
Would they not be more durable?
Phil
el ve
----- Original Message -----
From: "Ivor and Olive Lewis"
> Dear Friends,
> It would be good if we could have an explanation of the
behaviour of
> the effects of prolonged heating on furnace elements.
> Since we may be dealing with several alloys it is
reasonable to
> suppose that their corrosion and wear processes are not
identical.
> As I understand things in a general way, repeated heating
and cooling
> causes grain growth. Provided the initial oxide coating
remains intact
> it is relatively impervious to gas diffusion. But if it is
disturbed
> by any cause then the oxidation process is renewed and
proceeds along
> the grain boundaries. This changes the conduction
characteristics of
> the element, creating places of increased resistance to
current. The
> effect is to cause hot spots where elements fail due to
premature
> melting.
> Can anyone add more to this supposition?
> Best regards,
> Ivor Lewis.
> Redhill,
> S. Australia.
Jim Murphy on sun 11 jul 04
Ivor,
Yes, I'll add a little something.
For the typical Iron-Chrome-Aluminum alloy elements, it's the Aluminum which
forms a protective Aluminum-oxide layer.
An oxidation atmosphere is required to "grow" or restore the Aluminum-oxide
layer and extend element life.
I read about elements running 10 to 65 C (50 to 150 F) hotter than the kiln
interior. According to my kiln Manual, "The life of elements goes down
logarithmically with kiln temperature." That's kinda like a human running
downhill with a large stone - much FASTER !
I wonder if the Aluminum-oxide layer may help the elements run "cooler", in
part, by reflecting some of the radiation away from the elements - something
like "sunscreen".
A reduction atmosphere may inhibit or destroy Aluminum-oxide layer growth.
Proper kiln ventilation, preferably via remote-mounted powered downdraft
fan, may help maintain an oxidation atmosphere - at least near the elements
themselves.
Best wishes,
Jim Murphy
Ivor and Olive Lewis on mon 12 jul 04
Dear Phil,
Your colloquial use of the interrogative negative in both sentences
makes for confused understanding.
But I am sure the answer to your questions will be available
somewhere.
However, it is the choice of material for the elements and their
service conditions which determines their performance. As a chemical,
Kanthal A conductors running at high temperature will deteriorate in
the same way regardless of whether or not they are thick rods or thin
wires.
Now there was a time when I had charge of four furnaces which were
wound with pure Molybdenum (MP 2620=BA C ) rectangular strip and
immersed in a dry Hydrogen atmosphere.
I suppose the best thing is to treat Kiln elements as expendable items
and account for them correctly as maintenance replacements in the Cost
of Works column on the spread sheet..
Glad to hear you had a good time up north.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From:
To:
Sent: Monday, 12 July 2004 3:01
Subject: Re: Elec kiln elements wearing out too soon? Why?
> Hi Ivor, all...
>
>
>
> Why do Electric Kilns not use 'rods' instead of coiled
> 'wire'?
>
>
> Would they not be more durable?
>
>
> Phil
> el ve
>
>
> ----- Original Message -----
> From: "Ivor and Olive Lewis"
> > Dear Friends,
> > It would be good if we could have an explanation of the
> behaviour of
> > the effects of prolonged heating on furnace elements.
> > Since we may be dealing with several alloys it is
> reasonable to
> > suppose that their corrosion and wear processes are not
> identical.
> > As I understand things in a general way, repeated heating
> and cooling
> > causes grain growth. Provided the initial oxide coating
> remains intact
> > it is relatively impervious to gas diffusion. But if it is
> disturbed
> > by any cause then the oxidation process is renewed and
> proceeds along
> > the grain boundaries. This changes the conduction
> characteristics of
> > the element, creating places of increased resistance to
> current. The
> > effect is to cause hot spots where elements fail due to
> premature
> > melting.
> > Can anyone add more to this supposition?
> > Best regards,
> > Ivor Lewis.
> > Redhill,
> > S. Australia.
>
>
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Ivor and Olive Lewis on mon 12 jul 04
Dear Jim Murphy,
You are correct about it being the Aluminium in the metal alloy of the
elements which provides the protective coating, preventing more oxygen
from coming into contact with the Iron and Chromium atoms.
But remember, volatile fluxing agents are able to destroy the
continuity of the Aluminium oxide layer, allowing oxygen to penetrate
deeper into the metal. As I said, this happens along the weaker
pathways between the grains of metal.
Heat energy, no matter where it is generated, transferred or stored,
must obey the rules of the game. One of these is that it flows from
regions of high temperature to regions of low temperature. How well it
does this by radiation in relationship to the surface of a conducting
element is a function of surface emissivity, which in turn depends
upon the wavelength of the energy being radiated and absolute
temperature (K) of the radiating body, which in this instance is the
layer of Aluminium Oxide.
But I don't think elements can be made "hotter" by radiant energy that
is reflected back to them from cooler parts of the kiln. That would be
contrary to the rules of nature.
Over long periods of time Atmosphere may have a considerable influence
on the wear and tear of elements. Although we regards plain air from
the atmosphere as being "Neutral" or "Oxidising" it is possible that
the 0.03% Carbon Dioxide can have an effect. I wrote recently that
above about 450=BA C. Carbon Dioxide can do strange and seemingly
unusual things, one of which is to decompose into Carbon Monoxide and
Oxygen (2CO2 >=3D> 2CO+O2). You and other readers may wish to speculate
on the effect of small quantities of the additional gas on the
behaviour of an aluminium oxide film. I will keep an open mind.
Such interesting problems we try to solve ! !
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From: "Jim Murphy"
To:
Sent: Monday, 12 July 2004 5:30
Subject: Re: Elec kiln elements wearing out too soon? Why?
> Ivor,
>
> Yes, I'll add a little something.
>
> For the typical Iron-Chrome-Aluminum alloy elements, it's the
Aluminum which
> forms a protective Aluminum-oxide layer.
>
> An oxidation atmosphere is required to "grow" or restore the
Aluminum-oxide
> layer and extend element life.
>
> I read about elements running 10 to 65 C (50 to 150 F) hotter than
the kiln
> interior. According to my kiln Manual, "The life of elements goes
down
> logarithmically with kiln temperature." That's kinda like a human
running
> downhill with a large stone - much FASTER !
>
> I wonder if the Aluminum-oxide layer may help the elements run
"cooler", in
> part, by reflecting some of the radiation away from the elements -
something
> like "sunscreen".
>
> A reduction atmosphere may inhibit or destroy Aluminum-oxide layer
growth.
>
> Proper kiln ventilation, preferably via remote-mounted powered
downdraft
> fan, may help maintain an oxidation atmosphere - at least near the
elements
> themselves.
>
> Best wishes,
>
> Jim Murphy
>
>
______________________________________________________________________
________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your
subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Joseph Herbert on tue 13 jul 04
I believe that Amaco (the art materials company, not the Indiana remnant of
Standard Oil) sold front loading electric kilns with bar (not wound)
elements to the public school market. I have seen a cabinet the size of a
refrigerator that had a 3 cubic foot kiln at the top, about eye level.
Seems like the elements were bars in the walls and door (?!?). I expect in
their target market the relative lack of (frequent) maintenance was
attractive.
Of course, this is from memory and I seem to be having that "memory of
convenience" in which I may be remembering things as I wish them to have
been or as is convenient for the current argument.
I always wanted to have a marriage of convenience but I could never find
anything to marry convenience to. Convenience always seemed so good on its
own. A marriage of convenience and responsibility - that doesn't seem
likely or much fun. A marriage of convenience and moral bankruptcy - seems
redundant. A marriage of convenience and expediency - that might have
worked but neither of them had the time for it.
Stay well,
Joseph Herbert
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