Ellen Baker on sat 22 feb 97
Timothy -
My husband, a professional mechanical engineer who specializes in
furnaces & thermodynamics, reviewed your Feb. 19 request for advice about
slow firing at your high (8300' !!) altitude. He suggested that I relay an
analysis of the problem(s) and possible solutions, in general terms so that
you can sift through advice with a better sense of what's going on.
Problem 1 -- Rarefied Atmosphere (low oxygen & low pressure). The air is
so "thin" at 8300' that your kiln cannot "harvest" the heat in your fuel
efficiently using natural draft burners that rely on supply air at ambient
pressure. There are some things you can do to improve on your heating rate
perhaps (see Concept 2, below) but an unassisted flame will only achieve
limited efficiency at your altitude.
A minor contributing complication of Problem 1 is that at high altitudes,
because air pressure is low, your kiln's "chimney effect" is less effective
than it would be at sea level. One person suggested that draft would
improve if you raised your chimney, and that's true -- but the problem with
increasing the "through-put" of your kiln is that you will lose heat "up
the pipe" faster. If you leave the draft of your wood stove open all the
time, it won't get very hot, right? Therefore, beware: raising the
chimney without heating the kiln better will only aggravate your problem.)
Problem 2: Fuel. Under natural draft conditions, you would be better off
burning propane as it is a more "concentrated" fuel than natural gas -- it
actually delivers more energy at a given pressure. Using natural gas, you
will be forced to correct your combustion problem more radically than
you'll need to if you switch back to propane. You should reconsider your
choice of fuel before proceeding with any changes.
Concept 1: "Turbocharging." You've probably seen the compressors spinning
at the fronts of turbofan and jet engines. Compressors do for airplane
engines flying at high altitudes what forced-air blower systems do for
kilns: they "concentrate" the flow/supply of oxygen meeting the fuel
stream. Further, they tend to increase overall pressure inside the kiln.
At your altitude, you will probably need to force air to get all the heat
you should from your fuel. (In industry, sometimes a fan that can
withstand high temperatures is installed on the "exit" end of the furnace,
to help balance the "throughput" of the hot gasses inside the kiln -- it's
very effective but expensive to install and justify unless you have
tremendous production.) The suggestion of switching to power burners is a
good one.
Concept 2: Smaller Orifice Size. Burners work sort of the way that
candles work. When a candle wick is the right size and length it produces
a bright light with a minimum of smoke. You couldn't use a 1/4" rope for a
birthday candle's wick. As you surely know, hot wax vapors actually burn
in a jacket (a sort of an "aura") around the wick -- where oxygen meets the
gas. When you blow too hard on the candle wick, it blows out. Similarly,
the stream of gas that flows from a burner orifice forms a sort of "bloom"
that must be sized properly for the amount of air flowing up through the
air inlet in the outside end of the burner -- and of course the efficiency
of the flame will be affected by air pressure, density, etc. That's where
the tables come in. Thank goodness it's possible to predict through
calculations what should work best. Those folks who have suggested that
you need to go to a smaller orifice size are surely correct, so I've been
told.
Concept 3: More Burners. If you do not "turbocharge" your kiln with a
blower type burner system, you may want to add more burners to get more
heat. Of course, this would still be inefficient from a fuel-consumption
perspective, but it is an option.
CONCLUSION: You can improve the situation. Do what you can, practically
-- at a minimum, go to smaller orifice size, much smaller if you go back to
propane. You may want to consider adding burners. If you can afford it,
go to power blowers and even, perhaps, power draft (out the chimney) as
required.
Hope it all works out! Ellen Baker, Glacier, WA orion@telcomplus.com
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