Don Jung on sun 29 dec 02
Paul,
Wow, lots of questions... I'll just share some of my experiences that
address some of the questions you are asking... I asked myself the same,
when I did a similar thing.
Even temp:
For such a small kiln (16CF), a 'lazier' flame made a big difference for me
with my 13 CF kiln. Tried firing slower, repositioning the burners,
preheating, more gas, more air, less gas, less air, stacking differently...
Atmosphere control:
No problem, damper works great for secondary air control and burners have
primary air control. Follow Nils venturi flue design and port / flue
sizing... full control of red/ox.
Soak or ramp control:
Not as easy as an electronic controller on an electric kiln. I found I
could control temp by adjusting gas and damper and hold it... but the cones
will continue to fall since you still have heatwork over time. Never wanted
or tried to fire down, can't see why not if you wanted to.
Weather:
I can't control the weather, but I have fired in sun, wind, rain, cold, hot
and even some snow. My kiln can be shielded from rain with just the chimney
sticking out but I hate getting wet so I usually look for good weather to
fire. I haven't found it to impact the draw of the chimney much at all.
The damper moves from 1/2 to 3/4 in to get the draw I need.
Quiet burners:
No problem... can't even tell they're doing anything until there's a yellow
glow coming from the ports.
Nat gas vs propane:
I like higher pressure propane. The equipment costs less and there's more
flexibility/control. For a 16 CF kiln... no need for more than 5 PSI
propane and a couple of burners. Calculate the BTUs you need (+ 50%) and
drill the size of orifice to match the PSI needed for the BTUs / burner.
Olsen and Nils books have the charts to figure out what BTUs your burners
will put out. Burner design is the same, just the orifice is different.
Propane uses a smaller orifice because of the higher pressure.
Designing a burner:
A venturi works wonders and it doesn't have to be tooo exact. Sounds like
you understand the principle and with readily available cast iron piping,
you could make one very easily. I get plenty of velocity and have to close
off half the primary air to create a lazier and more reduced flame. I think
it would work well with low pressure gas as well, but haven't tried it. In
theory, if you use large enough pipes for the burner, the primary air would
be enough to combust all the gas, but it's better to use both primary and
secondary. Burner tips or flame retention nozzles are an excellent addition
to the design. They help keep the flame from blowing out and create
turbulence to mix the air and gas. Fairly easy to make and even simpler to
buy a couple. As for forced air... for high pressure propane, no need,
especially for a small kiln. Keep it simple, no electrical motors, natural
draft chimney.
I don't understand the two burner system with one above the other..? I
think you want to use one as a pilot in some sense. Build some wind shields
around the port and burner and use standard BASO valve and pilot. The flame
retention tip works wonders and I've never had a problem... knock on wood.
You could use Nils miniature flattop or Olsens fastfire kiln designs with
two burners. Goes to cone 10 in 4 hrs or drag it out to 9 hrs.
Preheated / forced air:
Don't need it. Thought about running air buy the chimney to exchange some
of the heat before it goes to the burner ports... but makes it
complicated... keep it simple.
The forced air can be added later if you really can't control the secondary
air with the damper but I'm amazed by the difference in just a minor
adjustment of the damper to create the atmosphere needed. Focus on making a
good flue/chimney and all that other stuff is minor.
Oh, and flame speed doesn't mean even temp. A slower lazier flame, I've
found is better and forced air would make that more difficult to achieve.
Secondary air:
Yes, it depends on the draw of the chimney and damper position, but in a
similar way, the burner also adds gases to the kiln, so if it's turned up or
down, the secondary air will go down or up if the damper is not changed.
You sound like a candidate for an Oxyprobe if you want precise control. $
savings with forced air... if you're doing reduction, you're not burning
efficiently anyways... Oxyprobe will help. The preheated air might, but how
much... I don't know.
Calculations:
I figured out the pressure, orifice and BTUs at one time, built it and filed
it away. It's all in Olsen's and Lou's books. If you determine the gas you
want to use, the time you want to fire, the temp you want to get to and the
size of the kiln in CF, and someone like Marc Ward can tell you or sell you
what you need.
cheers,
Don
Paul Taylor on mon 30 dec 02
Dear Don and all
>Wow, lots of questions...
One of my favorite cartoons is of an aireal view of a garden maze. The sort
you get in stately homes in England. In the maze is a circle of people all
following each other and one person is saying to the man behind him 'the
bloke' at the front knows the way.
Once upon a time our knowledge of glazes was a bit like that. We all
followed on with using the same glaze recipes - now it is different because
people asked lots of questions and realized that there were different
criteria for different glazes needing specialized firings
I am wondering if a lot more questions could be asked about kilns.
>I'll just share some of my experiences that
> address some of the questions you are asking... I asked myself the same,
> when I did a similar thing.
I set my criteria for a kiln that has temperature accuracy and atmosphere
control on cooling and heating as opposed to economy and ease of firing. I
am glad to see someone has done something similar and. It's experience that
I am after not conjecture so you post is most welcome.
> Even temp:
> For such a small kiln (16CF), a 'lazier' flame made a big difference for me
> with my 13 CF kiln. Tried firing slower, repositioning the burners,
> preheating, more gas, more air, less gas, less air, stacking differently...
This is the most useful information . I have my suspicions that this is
true because it is the technology I use in my production kiln and if staked
properly. It fires relatively evenly although my glazes are designed for a
maximum firing range and the glazes I am hoping to test further are not so
tolerant . I am hoping to do better next kiln If I can.
>
Weather:
> I can't control the weather, but I have fired in sun, wind, rain, cold, hot
> and even some snow. My kiln can be shielded from rain with just the chimney
> sticking out but I hate getting wet so I usually look for good weather to
> fire. I haven't found it to impact the draw of the chimney much at all.
> The damper moves from 1/2 to 3/4 in to get the draw I need.
>
umfortunatly here we get all the weather you have listed in the same
afternoon.
>> Atmosphere control:
> No problem, damper works great for secondary air control and burners have
> primary air control. Follow Nils venturi flue design and port / flue
> sizing... full control of red/ox.
I think what I want is evenness of atmosphere as well as temperature.
> I don't understand the two burner system with one above the other..? I
> think you want to use one as a pilot in some sense. Build some wind shields
> around the port and burner and use standard BASO valve and pilot. The flame
> retention tip works wonders and I've never had a problem... knock on wood.
> You could use Nils miniature flattop or Olsens fastfire kiln designs with
> two burners. Goes to cone 10 in 4 hrs or drag it out to 9 hrs.
The two added venturi burners were to get the kiln past 950c if the
electricity failed and for cooling control. Venturies are less violent. I
imagine it's easier to hold a low kiln temperature with them at say 600c.
I think The cornell of my question is : although I understand that the
forced air burner is more efficient, and it's easier to measure the inputs
than the venturies, and they are less susceptible to the vagaries of the
wind (???). I am wondering if the extra flame speed - giving better heat
transfer- promote an even temperature or does the speed of the flame
transfer heat unevenly to the load by forming hot currents through out the
kiln; Or does the increased flame speed cause so much a swirl that the heat
is distributed as evenly or more evenly than a bank of soft flamed venturi
burners. I was hoping to get information from people with experience of
both.
I think the kiln one uses sets a tone for the ware one makes . The glazes
I am using seem to need obsessive accuracy to get the effects I want. For
years I had hoped it was a matter of chemistry because I was under the
illusion that the chinese kilns that fired these glazes were unevenly fired
and haphazardly controlled - now I know they were not,
I will contact Mark Ward but I expect to pay for his valuable time. I am
doing as much research so my questions can be as succinct as possible.
again thanks
No longer following the person in front
--
Regards from Paul Taylor
ps. my server is a disappointment. If a perconal reply is bounced back
please contact me at tamlin1952@go.com
http://www.anu.ie/westportpottery
Phone International 00 353 98 21239
From the Republic of Ireland 098 21239
Paul Taylor
Westport pottery
Liscarney
Westport
County Mayo
Ireland
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