WardBurner@aol.com on fri 21 feb 97
Timothy,
Where to start?.....For one paragraph, you asked enough to have the answer
take a whole chapter.
First, the rule of thumb is to decrease orifice size by one orifice number at
5,000 ft and another orifice size every 2000 ft there after. Less oxygen
means less fuel that can be burned. But, your burners were set up for propane
to start off with which means the orifices were already too small, so you did
right by increasing them. But, increased to what? The TR-80 usually comes
with a #18 orifice for LP use. On 11"wc pressure the burner should put out
about 250,000 BTU, enough for one of these to fire your kiln quickly. To
complicate things the TR-80 has a adjustable needle orifice. Now the second
problem, though minor. The TR-80 is a compound injector better suited for
high pressure gas. You now would be better off with a TR-8 single stage
injector which comes with a 13/64" orifice for NG at 7"wc and an output of
90,000 BTU's. You would need a slightly smaller orifice, say a #9 in a TR-10
because of your altitude.
Here I'll take the time to apologize; there is so much info to give you folks
on this subject that I'm going to assume somethings are understood...I
realize they are not, but to keep this as short as possible this may be a
little heavy on the technical side and leave many things out.
Propane to Natural gas conversion has, sometimes, severe consequences. You
take a fuel that has 2500 BTU's per cubic foot delivered at, say for
instance, 10 PSI and switch to Natural gas. You now are running a gaseous
fuel that has 40% the BTU value, delivered at 1/40 th the pressure. That's
right....2% of the pressure of 10 PSI. These are big changes that you and
your drill bit can't take care of. I'm not picking on Timothy here....I see
this lack of knowledge in "famous professors", students and even in folks who
conduct kiln building workshops. I ain't smarter, I just took the time to
learn this stuff 'cause I didn't know. There is a concerted effort in the
clay community to avoid issues related to math. Off the soapbox and back to
work....
The TR-80 is designed to use 30-50% entrained primary air which means the
rest of the air needed for combustion has to come from the secondary source
of the area around the burner ports. Over 5000ft, you should make your burner
ports and flue 50% larger than they would be at sea level. A 5" x 5" port
minimum for this size burner head. A 50 sq. in. flue or bigger and it
wouldn't hurt to add some height to your chimney.
A 26 cubic foot fiber kiln (you didn't say how thick, but I'll assume 6" of
fiber) should need between 7,000 and 10,000 BTU's per cubic feet to fire to
cone 10. The lower figure will give you a 10-12 hour firing, the higher a 6-8
hour firing. So you need a total of 182,000 to 260,000 BTU/Hr. These burners
are probably too large for this kiln. I would need to know what size you
drilled out your orifices to. Burners that are too large with improper air
supplies can fail to get a kiln to temperatures. As far as Venturi vs Forced
Air, here's the blurb outa my catalog;
<<large demand natural gas kilns. These burners do not rely on the action of
the gas and chimney to pull primary air into the burner. Whether using
natural gas or propane, power burners are not affected by weather conditions.
Also, power burners do not need chimney height to create draft. The chimney
needs only to be high enough to remove flue gases from the kiln building or
shed. Because all primary air is supplied and mixed by the blower, power
burners are more efficient. Their high volume and long flame length help to
produce more even firings. This volume and flame length also make it possible
to run kilns on two burners, replacing 4 to 6 Venturi burners. On the down
side, power burners need electricity, making power failures problematic. With
proper safety equipment and installation, these problems can be averted.
Power burners are designed to operate on 7" WC pressure for natural gas and
11" WC pressure for LP (propane).
Venturi: These burners are best when used with LP, but work well on natural
gas in small kilns or with an adequate number of burners. One of the main
advantages of Venturi systems is operation without electricity. Since there
are no blowers, Venturi burners run quieter under low pressure than power
burners. Although generally not as efficient as power burners, they may be
turned down lower for preheating. (Power burners can be supplied with a small
Venturi pilot mounted to the head for this purpose). Venturi burners can
produce different BTU outputs with changes in pressure (see pg. 18). Higher
pressures reduce efficiency, but provide more flexibility than the set high
end output of power burners.>>>
We also have an orifice chart that is the product of a whole lota number
crunching by yours truly. It covers pressures from 4"wc to 25 PSI, 139
different orifice sizes from #80 to 1/2" and an altitude adjustment from sea
level to 11,000 ft. Please specify LP or natural gas.
Timothy, as with most tech. issues, I'll need to speak with you on the phone.
We have only scratched the surface here. Anyone else out there who has
technical problems should phone for concise answers specific to their
problem, but I'll still be happy to give general answers, like this, to the
list.
Marc Ward
Ward Burner Systems
PO Box 333
Dandridge, TN 37725
USA
423.397.2914 voice
423.397.1253 fax
wardburner@aol.com
Timothy wrote:
<<< I
live at 8300' of elevation and am having a bit of trouble getting a fiber
downdraft kiln (26 cu. ft total space, 20 cu. ft stacking space) to cone 10
in a reasonable period of time. I converted the kiln to natural gas. It had
previously been fired with propane by its previous owner. I am using the
same Eclipse, venturi type burners (2, TR80s) with the orifices drilled out
a bit. The burners may be a bit undersized(?), taking into consideration the
altitude. Can anyone provide guidelines on the # of BTUs actually needed to
fire a fiber kiln this size to cone 10 and the merits of a venturi burner
system vs a burner with forced air etc.>>>>
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