John Jensen on sun 10 jun 01
I recently reconfigured my 15 cu. Ft. catenary arch kiln for fire with
Natural Gas rather than Propane. It worked fine with propane. Now that I
have natural gas, I find that I don't have enough gas to reach past 018. So
I tried dripping waste oil into the burner ports. No luck. I got a lot of
smoke, but no heat to speak of. Anyone had any experience in this sort of
thing willing to talk about it.
John Jensen, Mudbug Pottery, Annapolis, Md.
mudbug@toad.net www.Toadhouse.com
John Baymore on mon 11 jun 01
John,
Hi. These things are sometimes tricky to diagnose from afar with only a
few sentences to go on. I'm going on pretty "thin" information
here........ the more details you can give, the better the answers you'll=
get from everyone on CLAYART.
If you could give things like kiln chamber dimensions, basic circulation
pattern, basic construction materials, number, size, and location of inle=
t
and outlet flues, chimney height and cross section, brand and type of
burner, orifice size, supply line size and the pressure at the burner
manifold, whether the kiln is indoor or outdoor, size of make up air louv=
er
openings (if indoors), any active external vent system used, and so on th=
at
helps us "kiln tech-weenies" to pinpoint the problems. =
That being said..........
I recently reconfigured my 15 cu. Ft. catenary arch kiln for fire with
Natural Gas rather than Propane. It worked fine with propane. Now that =
I
have natural gas, I find that I don't have enough gas to reach past 018. =
So
I tried dripping waste oil into the burner ports. No luck. I got a lot =
of
smoke, but no heat to speak of. Anyone had any experience in this sort of=
thing willing to talk about it.
First of all, if it worked good on propane is SHOULD be able to work well=
on Natural...... if you have things set up right. Might require a few
modifications.
My "gut" GUESS from the limited above facts you mention is that you
probably don't have enough AIR. =
Air Handling Capacity is the starting point around which you design a
combustion system. A combustion system is composed of the burners, the
kiln, the chimney, the external vent system, and it is also affected by t=
he
physical environment surrounding the kiln installation. It matters not,
how much fuel you have if you don't have the air to burn it . Air is
the key. Keep adding fuel and........ without air.......... you get
nowhere except smokey'er and poorer .
I will first of all ASSume that when you switched from propane to natural=
gas you changed the orifices in the burners to compensate for the likely
supply pressure difference, the differing specific gravity of the gas, an=
d
the different BTU's per cubic foot of the two fuels. If you didn't......=
.
this may actually be the cause of many of the difficulties. I also ASSu=
me
that the gas supply line to the kiln burner manifold was re-calculated to=
the proper diameter for supplying natural gas (this will typically be
larger diameter pipe than that for propane). =
(Let me know if these above comments hit the mark.....that old addage abo=
ut
U and ME and ASS )
So........ my semi-educated GUESS here is that when on propane you had a
supply pressure of from 1 to 6 Lbs. Sq. In. G. or so...... and now on
natural you are on 11 Inches Water Column or even possibly as low as 4
inches Water Column. Yes? No? I am also guessing that you are using
atmospheric "venturi" type burners? Yes? No?
Natural gas "squirting" out of what will be a larger orifice at what is
likely a lower supply pressure has less "energy" to impart to the air
molecules it bumps into in the mixing tube and pushes into the kiln. Thi=
s
energy, in the case of atmospheric type burners (not blower driven), is
used to help entrain the primary air for combustion. Less energy equals
less air moved. All things being equal, and making a large
generality....... a low pressure venturi type burner can entrain less
primary air than a high pressure venturi burner. If you are running
atmospheric burners and you lowered your supply pressure....you also
lowered your Air Handling Capacity, even though it is running the same
exact burners as before.
The cheaper (both in cost and in construction) the burners being used, th=
e
more significant the fuel gas change will likely be on Air Handling
Capacity. The little cheapie MR type units are certainly more susceptibl=
e
to this than say a high quality North American venturi mixing tube. Ditt=
o
"home brew" pipe burners.
Running a kiln on low pressure venturi burners is MUCH more dependent on
the flow of secondary air...... which is typically induced by the
kiln/chimney/external vent system design. A marginal kiln design that
could be made to work with a higher pressure system can stop working OK i=
f
the supply gas pressure decreases. A kiln that is DESIGNED to work onl=
y
on one specific fuel at a certain supply pressure can also stop working
correctly if the supply pressure and fuel is changed. The tighter the
design to a specific use....the more finnicky it will be to any fuel supp=
ly
changes. A kiln design that is supposed the be running forced air (blowe=
r)
burners cannot typically easily be changed to low pressure venturis witho=
ut
some mods. And so on.
=46rom the "catenary" comment above...... I have to conclude that it is a=
site-built unit. I don't know of any commercial pre-packaged catenary
kilns . Where did the design come from? Did you "modify" it in any
way? The design may not be adequate to supply as much secondary air as y=
ou
now need with the fuel change. A switch to power burners may be in
order....or there may be some changes you can make to the kiln structure.=
=
More info would help.
Another thing to keep in mind is the effect of depending on secondary air=
for a larger part of combustion needs. This will result in more of the
combustion happening further "into" the ware chamber. This opens up the
possibility of poorer "mixing" in the ware chamber and therefore more
uneven "reduction" effects. The kiln can become more "spotty"....with so=
me
areas experiencing heavy reduction while others are possibly seeing
oxidation....even though by the time the mix hiuts the exit flues....... =
it
is pretty much all fully burned gases. This poor mixing can also
exacerbate hot and cold spots.
Changing the fuel gas supply seems like such a simple thing a first
glance....... but it is sort of more of a major variable than many people=
realize. If you changed the height of a chimney on a downdraft from 25
feet down to 10 feet you would CERTAINLY expect the preformance of the ki=
ln
to likely change. But gas is gas....right? Nope. Propane at 2500
BTU / Cubic Foot, STP.......vs....... Natural at 1000 BTU / Cubic Foot,
STP. That alone tells some of the story. They are both gases...... but
quite different in physical and chemical properties. And quite typically=
supplied at different pressures.
If you understand all this ..... fuel gas changeovers can be quite
accomplishable. But you have to take all the differences into account an=
d
have the system ready to accept the impacts and demands of the changeover=
.
Burning oil is no different than gases in the regard for air needs. If y=
ou
don't have enough air...... all you get is smoke. =
One DIFFERENCE with oil is that you have MUCH more difficulties in mixing=
the fuel with the air, even if the air is present in sufficient quantitie=
s.
Home oil burners get around this by utilizing high pressure oil sprayed
(atomized) in a fine mist out of tiny orifices along with a high speed
blast of air from a blower. This breaks the oil into really tiny droplet=
s
and mixes it very well with the air. Even so....... gas furnaces burn
cleaner because of the better air/fuel mixing in the combustion chamber
.
A drip oil system has no such atomization deal going on. So you have to
set up a way to get the oil into tiny droplets and mix it with the air. =
The typical system for drip oil involves dripping the oil onto a slanted
(into the kiln) steel plate or series on step-like plates that is/are ver=
y
hot. Getting the plates hot is either done with a wood fire first.....or=
a
small gas burner. Once the kiln is hot....they are self-sustaining. The=
oil "sizzles" and splatters and breaks up into droplets as it runs across=
the plate(s). Sometimes a SMALL amount of water is added to the drip
stream to facilitate this sizzling action.
Once the oil is sizzling.....you need a GOOD airflow across the plate(s) =
to
pick it up, move it into the kiln, and to combust it. This airflow is
totally dependent on the design of the rest of the system. Generally
speaking, this requires downdraft or cross draft design attached to a
significant chimney. Updraft is possible........ but a little harder to
design well. If your kiln has marginal entrainment of air....... that ca=
n
cause sudden problems like this, and also result in oil drip not working
much or at all.
WATCH OUT with drip oil systems! If all of the oil you are dripping onto=
the plates is not getting burned........ which requires careful managemen=
t
of the drip valves........ the unburned liquid oil can seep down through
the cracks in the floor refractories and end up pooling on the floor unde=
r
or near the kiln. These hot oil pools have been known to catch
fire......and that can ruin your whole day in a hurry.
Another probelm with drip oil systems is the buildup of clinkers (carbon
deposits) that can plug inlet flues. These can grow when you are not
looking . If you are getting clinkers...... you don't have enough ai=
r
or your drip plates are not atomizing the oil well enough, or you are
dripping oil too fast, or the bag wall is too close.
If your kiln is a TOTAL of 15 Cubic Feet...... that is pretty small to
utilize a drip oil system. Oil has a long flame....it takes time for tho=
se
little droplets top burn away. Oil kilns typically have larger fireboxes=
than gas kilns to allow for this. With a really small kiln..... you end =
up
with more firebox than kiln . Also...... there is property of
combustion reactions called "flame quenching"...... which is when a burni=
ng
flame hits a RELATIVELY cold refractory surface. The surface absorbs th=
e
heat from the flame and it stops the burning in it's tracks. This is a
PRIME cause of clinker buildup in oil kilns.....and one reason that oil
kiln fireboxes are typically "deep" in front of the inlet flues.
If you HAVE to use oil...... you probably should switch to at LEAST a low=
tech forced air system to help with both above issues.
But rather than that....... let's see if we can't solve your gas problem.=
=
Much simpler way to fire . Hopefully one of us here on CLAYART will g=
et
you going again.
BEst,
...................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JohnBaymore.com
JBaymore@compuserve.com
John.Baymore@GSD-CO.com
"Earth, Water, and Fire Noborigama Woodfiring Workshop August 17-26,
2001" (Full- now on waiting list.)
Steve Mills on tue 12 jun 01
Going on my experience with both sorts of kiln;
apart from the serious increase in gas pipe/jet/burner size needed to
compensate for the much lower pressure (my 55 cu.ft. gross truck Kiln
was fed by a 4 inch pipe with 2 natural draught burners with 1/4 inch
jets), a propane Kiln is in effect a forced draught kiln because of the
higher gas pressure, whereas a natural gas one relies on the draw from
the chimney to get the fire into its belly. In short; a propane Kiln
pushes, and a natural gas Kiln sucks!!
you will need to increase the chimney length as well until it pulls
(there is the maths to do this, but I can't do maths!)
Steve
Numerically challenged in
Bath
UK
In message , John Jensen writes
>I recently reconfigured my 15 cu. Ft. catenary arch kiln for fire with
>Natural Gas rather than Propane. It worked fine with propane. Now that I
>have natural gas, I find that I don't have enough gas to reach past 018. So
>I tried dripping waste oil into the burner ports. No luck. I got a lot of
>smoke, but no heat to speak of. Anyone had any experience in this sort of
>thing willing to talk about it.
>John Jensen, Mudbug Pottery, Annapolis, Md.
>mudbug@toad.net www.Toadhouse.com
--
Steve Mills
Bath
UK
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