John Baymore on mon 25 oct 99
------------------
(snip)
Re. the orifices, my husband asked Summit kilns to set the orifices ot 7=22
natural gas pressure. Summit says 11=22 is normal but that 7=22 would =
actually
give us more fuel and not be a problem at all.
(clip)
I think you somehow got this backwards. Seven inches Water Column of gas
pressure is LOWER than 11 inches Water Column. So if the orifice size
remains the same, at 7 it will pass LESS gas than at 11. There is no
problem adjusting the size of the orifice to get the same amount of gas
through it at either pressure... at the lower pressure it will be a
slightly larger hole. If you had it drilled for a certain BTU figure at 7
and you actually have 11....then you have a little higher BTU output coming
through the orifices than you thought you did.
Primary air is the air that is mixed with the gas before combustion begins.
In the type of burners you probably have (inexpensive one piece casts
similar in look to the MR750) it is entrained by the mechanical energy
supplied by the gas flowing out of the orifice. At 7 inches WC ....there
is not much kinetic energy there... so the percentage of primary air
possible is pretty low....particularly in a cheap casting of the mixing
tube/burner unit. Assuming you have the burners I think you have, the
control of the flow of this primary air is done via a mechanical shutter
that you can close and open at the end of the burner away from the
retention nozzle (where the flame is). Remember when it is full open....it
is still only entraining a small percentage of primary air.
This type of burner has what is called a =22mechanical linkage=22 of primary
air to gas. This means that as the gas is turned up, the amount of primary
air entrained increases also without you having to do anything else. If
the burner was set at 40 percent primary with the burner running at 2
inches WC, then if you turn it up to 4 inches WC the primary air ratio
stays the same..... 40 percent. The cavaet to this is that the less
precise the cast of the venturi in the mixing tube, the less precise and
linear this linkage is. Cheap burners don't do this quite as well as more
expensive ones. That is one factor (among lots of others) that you are
paying for in more expensive burners.
Secondary air is the air that gets mixed with the already burning flame.
It is entrained mostly by the function of the draft of the kiln. It flows
into the mixture around the burner nozzle at the point it enters the kiln.
On a small updraft.... this draft is minimal..... and again makes the kiln
subject to being affected greatly by things like stacking variations.
Flames dependent on lots of secondary air are poorly mixed..... meaning the
consistency of the atmosphere from one part of the flame to another is
greatly variable. This variability can make great variation in reduction
effect results in the kiln chamber.
At 7 inches your burners will be ever so slightly less able to entrain
primary air than at 11 inches...... so the kiln will be even more dependant
on secondary air......and with the type of burners I suspect are on that
conversion...... not really very precise casts....... you are probably
relying on well over 50 percent of your air coming in as secondary. This
will make the kiln very sensitive to stacking variations, damper settings,
and weather.
I think it would be useful for you to pick up a book or two that covers
fuel kiln firing and invest in some reading time. If it is still available
=22Gas Kiln Firing=22 by W. Ritchie is one...... Nil's =22Art of Firing=22 =
is
another......... =22Kilns=22 by Daniel Rhodes is yet a third.... and =22The =
Kiln
Book=22 by Olsen is another. Those should give you a lot of background.
Focus on the sections on combustion theory and kiln operation for now.
A gas kiln is very different from an electric kiln. Most electric kilns
that studio potters use closely resemble what might be refered to as a
=22consumer appliance=22...... in other words they have been made very =
simple
in operation and there are few controls to deal with. With the advent of
the computerized controller....they are sort of like programming your VCR
=3Cg=3E. A gas kiln is more of a hands-on unit.... and requires much more
understanding of the =22how's and why's=22 of things in order to get it =
working
properly. With computers, commercial gas kilns could be made simpler to
control ....more like a =22consumer appliance=22....but the cost of doing =
that
would raise the price considerably.
If an electric kiln is analogous to say ........running a VCR......... then
maybe a good analogy for a gas kiln is a car. Once you understand all the
controls and how they relate to the operation of the car, driving is not so
hard. But there is a lot more that you have to know to drive a car than to
run the VCR. When you were first confronted with a car..... you probably
thought it very complicated.... now it is a piece of cake. YOu can and
will learn if you put in some effort. Don't expect to get the fuel firing
bit down in the time that it took to learn to run the electric kiln. The
knowlege of certain aspects of firing that you learned from firing an
electric kiln is transferable...... but you have to add onto that base.
I think it is important to note that there is no such thing as =22THE one
size fits all=22 firing schedule...........even for electric firing. Each
firing is dictated by such things as the particular ware being fired, the
desired results, and the specific kiln and it's associated design
parameters and control systems. This becomes even more evident in
fuel-fired kilns. On an electric kiln, you typically have only two
controls....... BTU input via heat settings (high, medium, low .........or
the ratio of time ON to time OFF with digital controllers) and air flow via
something like an Enviro-vent being either on or off. On a gas kiln you
have a more complicated inter-related series of controls.... gas input,
primary air input, secondary air input, and damper (which simultaneously
(sp?) controls secondary air, atmospheric conditions, heat distribution,
and convective heat losses).
To learn to fire most effectively in the shortest time possible, you need
some instrumentation to augment your human senses, and some calibration on
the controls. Good guages on the gas pressure are a start. Calibration
markings on the primary air shutters is a good idea. A reasonably accurate
pyrometer is crucial. A calibrated damper is important. With these you
can work toward making reasonable cause and effect observations, and hope
to get some reproducable results.
And nothing beats an oxy-probe for speeding up the learning curve on fuel
firing..... but the cost factor is certainly there.....which makes it a
=22hard sell=22 to potters, many of whom tend to be always looking for the
cheap solution (whihc is often the most expensive in the long run =3Cg=3E).
I'd also suggest, as I did in my last post, that you could use a local
fuel-firing =22mentor=22.
In learning to fire you're going to lose some pieces. As you embark on new
things, the learning is the important part.... not the product. One thing
I often tell my students.... remember ..... it is only dirt. If you are
thinking about what you do, with each piece you make you learn more....so
even if the end product is not =22saleable=22 or =22perfect=22 or =
=22successful=22 or
whatever you want to call it ........ you have GROWN from the process of
making it. It is not a total loss. Even in seeming failures there is
learning available if you are open to it. Try to relax, enjoy the new
challenges, and learn.
A while from now you'll wonder why you were so concerned at the time =
=3Cs=3E.
BEst,
......................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40compuserve.com
John.Baymore=40GSD-CO.COM
B. Acton on mon 20 dec 99
I have a 10 cubic foot electric kiln that needs lots of electrical
repairs. Instead of doing that I was considering modifying it somehow
to create another use for it such as a raku kiln and so on. Has anybody
out there converted their electric kiln to something else and how did it
work out? I'd be glad for any input.
Bob Acton
Dale A. Neese on tue 21 dec 99
I just pulled out all the elements, cut a hole in the side near the base for
a burner. Made a light weight lid out of expanded metal and fiber blanket.
Cut a hole for the flue. The kiln was an old electric with a blank ring of
bricks. I can adjust the size of the kiln to fit the pieces I fire by
removing a ring or two. Worked very well.
Dale Tex
Joanne Van Bezooyen on wed 22 dec 99
Well.....Bob, let me tell you......I recently converted an old electric kiln
to natural gas and was assisted by many over Clayart with learning to fire
it. It is probably all in the archives under "cold conversion kiln", so I
won't re=iterate....Good luck.
Joanne in Tucson
B. Acton wrote:
> ----------------------------Original message----------------------------
>
> I have a 10 cubic foot electric kiln that needs lots of electrical
> repairs. Instead of doing that I was considering modifying it somehow
> to create another use for it such as a raku kiln and so on. Has anybody
> out there converted their electric kiln to something else and how did it
> work out? I'd be glad for any input.
>
> Bob Acton
Theodore Banton on wed 22 dec 99
I converted my olymoc electric to gas rather simply. I went and got some
hard firebrick- because I wanted to increase the usable space. I bought 2
mr750 venturis and cut a hole in the top. i use propane and can fire cone 6
twice off an 80 lb tank. Marc Ward is the guy to talk to.
MudPuppy on fri 2 jul 04
I just did this...
Here's where I got my burners, and they have both natural gas and propane
available.
Mine is propane.
I'm pretty happy with teh actual burners, though I'm a novice at firing in a
reduction enviroment and am still configuring everything...
http://www.flclay.com/Summit%20Raku%20Kiln%20Con.html
I bought the GV-18 BTW
Cathi Newlin, Mercer, Mo
mudpuppy@box49.com
http://www.box49.com
----- Original Message -----
From: "Naomi Rieder"
To:
Sent: Friday, July 02, 2004 7:45 PM
Subject: electric kiln conversion
> Can anybody direct me to information on how to convert an electric kiln to
a
> propane fired downdraft kiln? So far I've only found electric to gas, but
> not downdraft.
> MTIA!
>
> Naomi Rieder
>
>
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Naomi Rieder on fri 2 jul 04
Can anybody direct me to information on how to convert an electric kiln to a
propane fired downdraft kiln? So far I've only found electric to gas, but
not downdraft.
MTIA!
Naomi Rieder
Naomi Rieder on sat 3 jul 04
Cathi,
Thanks for your info about the Summit burners. I'll have to phone them about it because, from what I understand, and I may be totally wrong, the placement of the burners underneath the kiln, without a provision for a chimney (as Steve Branfman shows in his book} is not a downdraft. I was hoping to find out what the difference is in the set-up between an updraft and a downdraft.
Naomi Rieder
Gary Navarre on sun 4 jul 04
> Can anybody direct me to information on how to convert an electric kiln to
> a
> propane fired downdraft kiln? So far I've only found electric to gas, but
> not downdraft.
> MTIA!
>
> Naomi Rieder
>
Naomi,
For what it's worth I converted an old electric to a woodburner. Don't
know what kind of draft it's called but it eventually worked, although
not too practical except for tests and to prove to myself I could do
it. I sold most of the pots.
The main difference between gas and propane is the size of the
orifice, propane needs to be smaller, unless my disflexia is kicking
in and it's the other way around.
The flame came in under one edge, hit a loose bag wall to push it up,
down through the load, and out the bottom edge hole on the other side.
I had to add a couple of layers of brick outside to insulate for ^10.
The fire box was my old standby, a hob with a pignose.
Gary Navarre
Navarre Pottery
Norway, Michigan
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