John Baymore on sun 22 mar 98
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Dan,
Difficult to completely diagnose kiln stuff without seeing the actual
installation. There are just so many variables in this stuff.
However........... that being said.........
(snip)
They fired up nicely when test fired but I recall the same
burner racing like a jet engine at the school I taught at, but they had
a 2=22 gas pipe feeding the burner. The same burners on my set-up burn
rather casually and I haven't gotten to the point of doing a test firing
for temperature yet. My pipe is the standard residential 3/4=22 and I
think the pressure is why the burners don't =22roar=22.
(clip)
Number of possible factors here.
It IS possible that the pressure of the natural gas line at the school was
higher than the one at your home. Industrial locations frequently have
mains pressure that is 11 inches water column. Sometimes even higher. The
institution may have had gas fired boilers and for that reason the main
pressure was of the higher variety. Higher orifice pressure relative to
atmospheric means more potential energy available to entrain primary air.
The school may also have had only 4 inches of pressure from the main, but
for the linear run of the supply pipes there from the main to the kiln, the
pressure drop at the burner manifold may have been negligible because of
proper pipe diameters.
Pipe imparts friction.... the longer the pipe and the more ells and the
like the larger the diameter to maintain pressure and flow. At you
residence, the mains pressure may still be at 4 inches WC, but the pressure
drop may have reduced that significantly at the burners. Lower pressure,
lower primary air.
Third possible factor is that the burners may have been installed on a kiln
with significant draft, and your kiln does not have that same =22pull=22. =
An
atmospheric (aspirating or venturi) burner uses the kinetic energy of the
gas exiting the orfice to entrain the primary air for combustion. At the
lower pressures typically used with natural gas, this energy from the
expanding gas flow is pretty small. These burners can only entrain a VERY
small portion of the air needed for combustion. The rest comes from air
induced to flow by the action of the draft of the kiln. Stronger draft
means more air. Some air from the draft goes through the burner as primary
(smaller amount)... and some comes in as secondary. More air means more
complete combustion. Hence the =22roar=22.
(clip)
1. If the gas orifices were made slighly larger and forced air provided to
the burners, would they produce more heat?=7F
(snip)
Key here is the addition of forced air. With it, the answer is PROBABLY,
and actually barring some wierd factors like bad mixing and flow backing up
in the mixing unit and spilling out into the room, it is almost certain.
Without the =22forced air=22........ NO. Emphatically. In designing
combustion systems the standard of measure is called air handling capacity.
You can only burn fuel if you have air to burn it with (technically,
oxygen =3Cg=3E). No air, no heat. Takess about 10 cubic feet of air per =
1000
BTU's of heat valuer in a fuel. Conveniently..... natural gas has about
1000 BTU's per cubic foot. So 1 cu ft natural needs about 10 cubic feet of
air (at STP).
Drilling out the orifices would let more gas flow through the hole in a
unit of time. This will increase the available POTENTIAL heat per unit
time. This would likely provide only a tiny increase in entrained air.....
assuming the configuration of the design of the throat of the burner would
=22accept=22 the increased flow. In certain cases drilling out an orifice =
will
actually decrease the amount of air entrained.
Venturi burner design looks for the disbursing jet of gas to hit the burner
structure tangental to the curve at the narrowest point in the throat for
maximum efficiency of entrained air. For that reason venturi burners have
a specified range of orifices that are within acceptable limits of
performance. Go outside this range in either direction, and the percent of
entrained primary air drops off significantly.
So you will have to add air into the gas stream (and mix it adequately) to
burn all the fuel. If these burners were originally designed as aspirating
types, they might not be optimum if cobbled up into a forced air
configuration. You actually could get aerated gas mixture backing out of
the assembly and into the room in certain cases. Be careful.
Trying to cram too much air through the existing system might be hard.
Might be better and easier to switch burners, or come up with a home-made
design that is specifically set up for forced air. Not all that hard.
(clip)
2. If the manifold feeding the two burners were of a larger diameter,
say 2=22...then reduced back down to 3/4=22 would this add more available
gas volume to the burners?
(snip)
It might, slightly, depending on the exact configuration of the
manifolding. But the differences might be pretty minimal. Most of the
=22drop=22 is probably coming from the supply run between the mains and the
burner manifold.
That (3/4 inch supply run) would be the first place to change, probably.
(clip)
3. Are there devices that increase the weak residential gas
pressure...like there are for weak water pressure?
(snip)
Yes.... but they are expensive and =22techy=22. Not usually the province of
the studio potter.
If your street main will supprot the VOLUME of draw necessary at peak, then
there is no reason that you can't get a combustion system to work on 4
inches WC. Best to go forced air at this low pressure....... gives you
more control options.
Call the gas company and ask them what size main is coming to your house,
at what pressure, and what peak draw it will support into your property.
Ask the kiln manufacturer about peak volume draw on natural gas for the
unit you have. If you built it, use 10,000 BTU / cu ft / hr. for an
insulating brick kiln, 17,000 BTU cu ft / hr. for a hard brick kiln, and
7,000 BTU / cu ft / hr for a fiber kiln as a good estimate of gas
consumption. Measure the total cubic footage, not just the stacking space.
Divide the BTU's by 1000 to get approximate cu ft / hr for natural gas.
Compare the suppy capacity in CFH to the CFH kiln requirement. If the
supply is greater.... you can fire the kiln. If not, you need a bigger
diameter main in the street, more main gas pressure, or a smaller kiln.
You'll need a blower to supply about 10X the CFH of gas to realize the heat
potential in the gas, and if you want oxidation options at peak flow, you
need even more air. That is the amount of air actually exiting the system
into the gas stream, not just the blower rating. The air flow is subject
to the same friction losses as gas going through pipes and ells and so on.
If the run from the blower (s) to the mixing unit is short, the losses are
minimal.
(snip)
4. If the nozzle dia. of the burners were reduced would a more aggresive
flame be produced, and would that mean higher temperature capabilities?
(clip)
No. See above for a lot of it.
If the blower can deliver adequate volume against the static pressure
generated by the smaller nozzle crosssection, you will still get the flame
in the kiln but at a higher velocity and in a tighter pattern. Which might
or might not be desireable depending on the exact kiln design.
If the blower can't =22push=22 through the restriction, you'll get less gas/
air mixture into the kiln.... and less heat. Possibly some backup out of
the system into the room if there are any =22holes=22 present in the =
combustion
system prior to the retention nozzle.
Temperature climb capabilities are basically a measure of heat generated
compared to heat losses. The bigger the differential the faster the
possible firing rate. Flame temperature of a fully areated gas mixture is
the prime determinant of maximum temperature in a simple kiln. Also at
some point the maximum firing temperature is determined a bit by the total
volume of combustion byproducts that can =22fit=22 into the available =
chamber
space in a unit of time, and the heat transfer capabilities of the heat
exchange medium........ in our case the stack of ware.
(clip)
Running larger dia. pipe from the service point to the studio is not an
option I wish to choose.
(snip)
This unfortunately is the culprit that I find in most of these cases. It
is the most probable =22offender=22 in this case, not being able to see and
hear and touch and sense the actual installation.
Hope this information is of help. I am sure you'll get lots more from
others too. Good luck with the installation.
Best,
........................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
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