Dave Murphy on tue 25 apr 00
------------------
To All:
I have just built a new gas kiln (propane) with the following stats:
60 cu ft 9=22 IFB 40 stacking
roman arch
2 atmospheric injection mixers (looks like the ones on page 197 in the =
=22Kiln
Book=22 by Olsen
16 foot 9=22 square brick chimney
I drilled out the orfices to approx size 10 wire - propane coming in at =
11=22 of
water pressure
I seem to be using alot of gas and also have trouble getting a good =
reduction
flame out the peeps on the one side
without smoke temp is good but clilmbs very fast in the early stages of the
firing. I have an impression that the orfice is too large and too much gas =
is
flowing. Any advice would be appreciated. I am using the criteria in the =
Olsen
book for BTUs needed for a kiln this size but I think I am over powered. I =
also
would like to know if anyone out there thinks uping the pressure would be
advised. Up here in Ontario we are allowed to have up to 2lbs. of pressure =
to a
device attached to a house.
Still learning TIA
Barbara Murphy
Craig Martell on wed 26 apr 00
Barbara sez:
>I have just built a new gas kiln (propane) with the following stats:
>I drilled out the orfices to approx size 10 wire - propane coming in at
>11" of
>water pressure
>I seem to be using alot of gas and also have trouble getting a good reduction
>flame out the peeps on the one side
>without smoke temp is good but clilmbs very fast in the early stages of the
>firing. I have an impression that the orfice is too large and too much gas is
>flowing.
Hi:
My calculations indicate that you are a bit underpowered. Your burners are
putting out about 507,000 btus peak with a #10 orifice at 11"wc. If you
accept a minimum peak output of 10,000btus per cubic foot as the correct
amount, you'd need 300,000 per burner. So, you have a shortfall of about
93,000 btus. Your kiln should still fire to cone 10 Ok but it might take a
bit longer. This theory assumes that your burners are mixing enough air to
make the rated btus for your orifice size. If you upped the pressure to
2psi, you would have to size the orifices down to a #28 which would give
you a bit more than 300,000 per burner. You need to know if your burners
will deliver that sort of output, and that they can be used with high
pressure. No matter how big one makes an orifice or how much one ups the
pressure applied at the orifice, a burner has a peak output that it will
not surpass. If you go beyond this point, the burner cannot entrain enough
air to supply good combustion and secondary air is not going to do enough
either.
I'll let the kiln nerds take it from here. Oh, I mean "nerd" in a non
derogatory way so put your magnums back in the holster.
later, Craig Martell in Oregon
ferenc jakab on wed 26 apr 00
----- Original Message -----
From: "Dave Murphy"
To:
Sent: Wednesday, 26 April 2000 5:11 am
Subject: New gas kiln trouble
----------------------------Original message----------------------------
------------------
To All:
I have just built a new gas kiln (propane) with the following stats:
60 cu ft 9" IFB 40 stacking
roman arch
2 atmospheric injection mixers (looks like the ones on page 197 in the "Kiln
Book" by Olsen
16 foot 9" square brick chimney
Barbara,
I built to Olsen's specs too (Loosely to his specs). Your speculations about
the orifices may be correct but I found that the problem with my kiln (a
catenary arch ) was the chimney. It is simply too big with too much draw. I
think Olsen's plans were adapted from wood kilns, firstly, and secondly were
formulated in an age when energy was cheap. If your chimney has a baffle,
and it should, then do an experimental fire up with the baffle closed. With
both burners set to a low flame (as in early stages of firing) open the
baffle until it just starts to draw. You will find that this setting should
do for most of a firing. Keep an eye on the flame through out the firing and
when you notice a little spontaneous reduction, i.e. your flame is yellow
instead of blue then open the baffle just a little more. If you have a
pyrometer then you will notice a temperature stall or drop if the baffle is
opened too much or closed too much. In my chimney I open the baffle only
about an inch to start and rarely more than 2"-3". Every kiln is different
of course.
Another thing that occurs to me when I re-read your post is that your
burners may not be getting enough primary or secondary air. primary is drawn
through the burner it self, and secondary is drawn past the burner at the
port into the kiln. Is the kiln port large enough? Is the air mixture into
the burner adjustable? I use venturi burners with an adjustable air baffle
for this reason.
Feri.
Dave Murphy on thu 27 apr 00
Hey:
It just occurred to me that this particular burner probably does have a
maximum BTU rating. I think it would probably be impossible to find out.
They are injection mixers and in the Olsen book they talk about an absolute
rating of about 100,000 BTU's each and I did use them on a 30 cu ft kiln. I
could have fired that kiln in about 4 hours (not that I wanted to) by
opening up the mixing pin and allowing more fuel in. I would imagine that
there would be a limit to that too. Do any of you know how much fuel per
firing you are using? I am using about 200 litres/ 12 hour firing (which is
about average) It is more expensive now that propane is at 49.9 cents per
litre.
Barbara
----- Original Message -----
From: Craig Martell
To:
Sent: Wednesday, April 26, 2000 12:53 PM
Subject: Re: New gas kiln trouble
> ----------------------------Original message----------------------------
> Barbara sez:
>
> >I have just built a new gas kiln (propane) with the following stats:
> >I drilled out the orfices to approx size 10 wire - propane coming in at
> >11" of
> >water pressure
> >I seem to be using alot of gas and also have trouble getting a good
reduction
> >flame out the peeps on the one side
> >without smoke temp is good but clilmbs very fast in the early stages of
the
> >firing. I have an impression that the orfice is too large and too much
gas is
> >flowing.
>
> Hi:
>
> My calculations indicate that you are a bit underpowered. Your burners
are
> putting out about 507,000 btus peak with a #10 orifice at 11"wc. If you
> accept a minimum peak output of 10,000btus per cubic foot as the correct
> amount, you'd need 300,000 per burner. So, you have a shortfall of about
> 93,000 btus. Your kiln should still fire to cone 10 Ok but it might take
a
> bit longer. This theory assumes that your burners are mixing enough air
to
> make the rated btus for your orifice size. If you upped the pressure to
> 2psi, you would have to size the orifices down to a #28 which would give
> you a bit more than 300,000 per burner. You need to know if your burners
> will deliver that sort of output, and that they can be used with high
> pressure. No matter how big one makes an orifice or how much one ups the
> pressure applied at the orifice, a burner has a peak output that it will
> not surpass. If you go beyond this point, the burner cannot entrain
enough
> air to supply good combustion and secondary air is not going to do enough
> either.
>
> I'll let the kiln nerds take it from here. Oh, I mean "nerd" in a non
> derogatory way so put your magnums back in the holster.
>
> later, Craig Martell in Oregon
>
Julie Holmes on wed 3 may 00
Dave - I just got back from a kiln-building workshop. The teacher, Tracy
Dotson, might be able to help you with your problem. He has about 30 years
of experience building kilns and has lots of info to share about firings,
etc. I think his email address is thepottery@aol.com. If that doesn't
work - you can call him in NC at 828.765.8222. He says he really likes
getting calls from people in the midst of firing a kiln - but I"m sure he
would be glad to talk with you, too!
Good luck -
Julie
----- Original Message -----
From: "Dave Murphy"
To:
Sent: Tuesday, April 25, 2000 3:11 PM
Subject: New gas kiln trouble
----------------------------Original message----------------------------
------------------
To All:
I have just built a new gas kiln (propane) with the following stats:
60 cu ft 9" IFB 40 stacking
roman arch
2 atmospheric injection mixers (looks like the ones on page 197 in the "Kiln
Book" by Olsen
16 foot 9" square brick chimney
I drilled out the orfices to approx size 10 wire - propane coming in at
11" of
water pressure
I seem to be using alot of gas and also have trouble getting a good
reduction
flame out the peeps on the one side
without smoke temp is good but clilmbs very fast in the early stages of the
firing. I have an impression that the orfice is too large and too much gas
is
flowing. Any advice would be appreciated. I am using the criteria in the
Olsen
book for BTUs needed for a kiln this size but I think I am over powered. I
also
would like to know if anyone out there thinks uping the pressure would be
advised. Up here in Ontario we are allowed to have up to 2lbs. of pressure
to a
device attached to a house.
Still learning TIA
Barbara Murphy
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