iandol on mon 27 nov 00
>feriz delkic recommends using a rolling pin and with as much pressure =
that you have, flatten the fibre, install, then spray<
Am I missing something here?
My belief was that entrapped air gives the insulation, not the ceramic. =
Air is responsible for the volume and has a lower thermal conductivity =
than fibre. So, if the fibre is compressed, threads will have a greater =
area of contact and conduct heat from the kiln faster.
Also, I would expect the compression to fracture the fibres, reducing =
their integrity and weakening them.
But then,I have only experience of building a few fibre kilns so I must =
be wrong.=20
Ivor Lewis
John Hesselberth on tue 28 nov 00
Cameron Harman wrote:
>Ivor
>
>Higher density fiber has better thermal resistance than lower
>density fiber, hence a more compact fiber is a better insulator.
>Fiber blanket is usually 8 or 10 pounds per cubic foot density,
>formed fiber, such as boards can be much higher, such as 18 or 24
>pounds per cubic foot.
Hi Cameron,
Are you saying that a 1/2 inch thick blanket that is 20 lbs per ft3 will
transmit less heat than a 1 inch thick blanket that is 10 lbs per ft3
(same weight of fiber in both)?? Or are you saying that a 1 inch blanket
of 20 lbs per ft3 will transmit less heat than a 1 inch blanket of 10
lb/ft3 material??
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is, perhaps, still necessary to say that the very best glazes cannot
conceal badly shaped pots..." David Green, Pottery Glazes
Cameron Harman on tue 28 nov 00
Ivor
Higher density fiber has better thermal resistance than lower
density fiber, hence a more compact fiber is a better insulator.
Fiber blanket is usually 8 or 10 pounds per cubic foot density,
formed fiber, such as boards can be much higher, such as 18 or 24
pounds per cubic foot.
The process of heat transfer inside a refractory is VERY
complicated involving all three modes of heat transfer
(conduction, convection and radiation). It is difficult to explain
in simple terms the mechanism that occurs, but the measured
results show that the higher density fiber is a better thermal
insulator.
Radiation passing through air at higher temperatures transfers
more heat than the fiber at the same, elevated temperatures. The
compression reduces the chances for this transfer. It is, however,
desireable to a certain extent. As I say, the mechanism is very
complicated. People have been trying to model this internal
process for more than fifty years and have only achieved limited
success so far. Models have been made for the refractory as a
whole with much greater success.
You cannot hurt the fibers by compressing them. There are two
reasons for this, firstly, the ceramic fibers are extremely
strong, a phenominum of very thin fibrous materials. Secondly,
they are relatively short and flexible and are not too prone to
breakage anyway.
Cameron
--
**********************************************************
Cameron G. Harman, Jr. 800-556-0766 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
get your free ezine: http://www.kilnman.com/ezine/ezine.html
THE place to go for solutions to your drying and firing problems
**********************************************************
Lee Marshall on tue 28 nov 00
In a message dated 11/28/00 6:01:42 AM Central Standard Time,
iandol@TELL.NET.AU writes:
<< eriz delkic recommends using a rolling pin and with as much pressure that
you have, flatten the fibre, install, then spray< >>
ok that's fine for building a kiln, but what do you do with a geil kiln? i'd
really like to protect the fibre but paul keeps telling me not to get near
the kiln with itc, that it will flake off. i've got about $10,000 invested in
this kiln and don't want to mess it up.
lee marshall
in tennessee where we know him best and he lost
lmarsh1220@aol
Gavin Stairs on tue 28 nov 00
At 01:17 AM 11/27/00, you wrote:
> >feriz delkic recommends using a rolling pin and with as much pressure
> that you have, flatten the fibre, install, then spray<
>
>Am I missing something here?
>
>My belief was that entrapped air gives the insulation, not the ceramic.
>Air is responsible for the volume and has a lower thermal conductivity
>than fibre. So, if the fibre is compressed, threads will have a greater
>area of contact and conduct heat from the kiln faster.
>
>Also, I would expect the compression to fracture the fibres, reducing
>their integrity and weakening them.
>
>But then,I have only experience of building a few fibre kilns so I must be
>wrong.
Hi Ivor,
There are a few factors at work here. What you say is generally true, but
is heavily modified by other things.
First, fibre tends to shrink when heated to its operating
temperature. This means that fibre batts tend to leave gaps after a while
unless installed under compression. These gaps are damaging to the kiln
wall containment, so all fibre manufacturers recommend installing under
some kind of compression. This effect would also crack or delaminate any
surface coating like ITC. So, the pre-compression Feriz is suggesting is
partly to off-set this.
Second, if the overcoat is applied only to the superficial layer of fibre,
it is only weakly bonded to the whole batt. Feriz' treatment will mean
that the ITC coat has a chance to penetrate somewhat. As the fibre
rebounds during drying, the ITC and fibre will form a graded junction over
the first mm or so of the fibre batt, which is much stronger than just a
surface layer.
This is not very degrading to the insulation value of the fibre, because
fibre is actually a relatively poor hot surface insulator. By that I mean
that the air spaces are much less effective at temperatures at which
radiation is the dominant heat transfer mechanism. At these temperatures,
the air gaps are efficiently bridged by the radiation, and the insulation
value of the fibre plummets. This is also true of IFB, which also relies
on air spaces for insulation. Hard brick maintains its rather poor
insulation value even at high temperatures because it is a solid
material. Hot face fibre is still somewhat better than hard brick, but not
as much as you might like.
So, hot face fibre has some deficiencies which can be mitigated by the
suggested treatment, to some degree. The coating provides a hard surface,
which controls fibre debris and in-flux of kiln atmosphere into the
fibre. Since the kiln atmosphere can't penetrate easily, and the coating
is highly refractory, it provides some protection against alkali flux
vapors, so long as it remains intact. It may also beneficially alter the
albedo of the surface, which may increase the insulation value of the wall,
depending on the fibre.
In return, there is a slight increase in heat conduction over the
consolidated surface layer of a few mm. Not much of a trade off.
Gavin
Gavin Stairs
Stairs Small Systems
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
phone: (416)530-0419 stairs@stairs.on.ca
Cameron Harman on wed 29 nov 00
John asked about the heat transmission of fiber based on thickness
and density:
First of all you must compare fibers made the same way, that is, a
board will be different thermal conduuctivity than a blanket.
However, given blanket to blanket comparison the comaparisons are
made on the same thickness. For construction considerations
different thickness may be used as a result of the different
thermal conductivities. Since thermal conductivity is a direct
measure of heat transmission, it is the most useful way of
comparing fiber materials of the same ilk, that is blanket to
blanket, e.g.
For Thermal Ceramics Kaowool blanket here are some data taken at
2000 degrees F. At 3 pounds per cubic foot density the thermal
conductivity is about 4.5, at 6 lbs it is 2.7, at 10 lbs it is
2.0, at 18 lbs it is 1.3 and at 24 lbs it is 1.0. As you can see
there is a steady decline in the thermal conductivity as the
density increases.
A fiber board of the same density will be higher conductivity.
Pyro bloc at 10 lbs is 2.8 (up from 2.0 in blanket) but Kaowool
board is nearly the same as blanket.
Please email me off line if you wish further data or information.
Cameron
--
**********************************************************
Cameron G. Harman, Jr. 800-556-0766 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
get your free ezine: http://www.kilnman.com/ezine/ezine.html
THE place to go for solutions to your drying and firing problems
**********************************************************
Gavin Stairs on sun 3 dec 00
I just noticed that this reply went direct to Cameron rather than to the
list. Hope that no-one minds a return to an old thread. Gavin
At 07:19 AM 11/29/00, you wrote:
>...For Thermal Ceramics Kaowool blanket here are some data taken at
>2000 degrees F. At 3 pounds per cubic foot density the thermal
>conductivity is about 4.5, at 6 lbs it is 2.7, at 10 lbs it is
>2.0, at 18 lbs it is 1.3 and at 24 lbs it is 1.0. As you can see
>there is a steady decline in the thermal conductivity as the
>density increases.
....
If Cameron doesn't mind me butting in with a bit of theory...
Insulating wool is a network of fibres, The fibres contact each other only
here and there: the body of the wool is mostly air. Compressing the wool
does not increase the contact much until the felt becomes very dense, so
the conduction paths are not greatly altered. Or at least altered, but not
very fast. Cameron's numbers above are for a constant thickness of similar
felt. If we convert the numbers to a constant weight of fibre, we can see
what the compression is doing to the effectiveness of a given weight of fibre:
lb/cu.ft BTU.in/hr.ft2.F BTU.lb/ft2.hr.ft2.F
3 4.5 1.12
6 2.7 1.35
10 2.0 1.67
18 1.3 1.95
24 1.0 2.00
The first column is the wool density. The second is Cameron's heat
conductivities. The third is the first column times the second, divided by
12in/ft.
What the third column tells us is that, as Ivor and John surmised, the
effectiveness of a unit mass of fibre decreases as it is compressed, but
not very fast. In fact it goes about proportional to the logarithm of the
density.
Since fibre cost is about proportional to mass, this would seem to indicate
that the economical use of fibre is to have thick, light batts as opposed
to thinner, denser batts. However, if we need to fit our fibre into a
defined space, or if the inside is small, such that the insulation volume
increases more rapidly than as the thickness, then there may be good reason
to compress the fibre.
Inside an insulator, it is the fact that there is an air gap which
dominates the radiative heat transfer, not how big it is. So shrinking the
air gaps does not appreciably change the insulation value. The change in
the third column is due mostly to increasing contacts between fibres, and
resulting shortening of the conduction path.
As a practical matter in designing a small kiln, all fibre has to be
compressed somewhat for the reasons given in my previous post. In a
commercially manufactured kiln, the fibre will have been installed in such
a way as to provide this compression, probably by accordion folding the
fibre into a panel somewhat smaller than the relaxed size of the fibre
batts. If you want to coat such a fibre wall with a refractory, I would
advise first contacting the manufacturers (kiln and coating) for
advice. This is especially true if the unit is under warranty, and you
want to preserve the warranty protection. If they prove unhelpful, then
you should at least dampen the fibre before spray applying the
refractory. It is probably not helpful to roll or otherwise compress the
surface, unless the wall has seen appreciable service or is in need of
repair. It may be that the fibre wall has some sizing in it, which should
probably be burned out before application of any top-coat, so one or a
couple of high heat cycles should be run before attempting the coating in a
new kiln. Coating of an old kiln wall is more risky, in that the fibre
strength is probably degraded, and there is more need to help it out by
increasing penetration of the coating refractory. In this case, roller
application, or heavy water soaking or some other treatment may be in
order. In any case, what is needed is to effect penetration of a few
millimeters, not to soak the whole wall.
Hope this is of some use to somebody.
Gavin
| |
|
