John Rodgers on mon 12 jul 04
Air density is a big governing factor in "fuel" firings. The combustion
of fuels and the release of the the heat energy is, if course,
dependent on the amount of oxygen present. Altitude reduces air density
and thus affects the amount of oxygen available per cubic foot of air.
Air temperature affects the density in that the hotter the air is the
more spread out the O2 molecules are, and thus the air is less dense and
there is less oxygen per cubic foot to burn. Moisture or humidity -
water molecules - displace the oxygen and thus there is less oxygen per
cubic foot for combustion.
I think one is right on target with keeping track of the ambient air
temperature, barametric pressre, humidity, etc. Only with sufficient
information, can one make the corerect judgements for adjusting a fuel
firing. Otherwise, one is just guessing. The more data you have, the
better control you can excercise.
In my earlier years, I was a pilot and did a lot of flying. Aircraft
performance had to be calculated to be sure the machine was capable of
taking off and landing safely within the confines of whatever runway was
available. Passenger and cargo loads had to be adjusted, fuel loads
adjusted, aircraft gross takeoff weight for given conditions had to be
calculated. It was/is possible to for an airplane to perform beautifully
at Laguardia airport on a cool day - almost sea level - and that same
airplane loaded the same way not be able to get off the ground in Denver
( a mile high) even on the longest runway. Why?? becasue of what is
known in aviation circles as "Density Altitude". This is calculated and
then related to aircraft performance. The numbers are based on
temperature and elevation, and aircraft performance is based on those
numbers. On a cold snowy day in Denver, density altitude is low. On a
hot summer day at 2 pm in the afternoon, density altitude in Denver will
be well above the actual 5280 ft elevation. It may be for example near
9000 feet. Thus the airplane would be performing as if it were at 9000
feet. The engines are rated for their horsepower at sea level on a
standard day ... atmospheric pressure at 29.92 in. Hg and temperature at
69 degrees F. The density altitude under those conditions is Sea Level -
or Zero Feet. That 1000 Horsepower performance is based on the amount of
Oxygen that goes into the engine for combustion under those conditions.
AT sea level on a standard day, a 1000 Hp engine produces 1000 HP. If it
takes 1000 horsepower to lift the airplane from the ground on a 5000
foot long runway. How would it do at a density altitude of 5000? How
about 9000? O a hot day in Denver in the summertime, with a Density
Altitude of 9000 feet, you airplane engine will be producing maybe 50%
of the 1000 horse power it can produce at sea level. The air density is
so low that there is not enough oxygen for the full horsepower to be
developed. Add to that the reduced lift on the wings that occurs in thin
atmosphere, and you have a situation where it's possible for the
airplane to not even be capable of geting off the gound at all
Apply these concepts to Fuel-fired kiln performance, and you begin to
see the need to measure the atmospheric conditions. Elevation, Ambient
Air Temperature, Atmospheric Pressure, Humidity - all factors in
controlling a firing in a fuel fired kiln and getting best performance.
Related to this, is a concept brought forth by Roger Graham in Ceramics
Monthly a while back.
Jet aircraft use instruments to measure the temperature of the air going
ito the engine - Turbine Inlet Temperature or TIT, and the the
temperature of the outgoing exhaust gases - Exhaust Gas Temperature or
EGT, as well as fuel/air ratios (lean vs rich). Together they are used
to calculate an Exhaust Pressure Ratio or EPR that is an expression of
how much power is being produced. So instrumentation for measuring what
goes in and what comes out is important.
Roger has done some work using a standard automotive Oxygen sensor
found on the moden sutomobile that works in conjunction with the
catalytic converter to reduce harmful exhaust emissions. Roger has found
that the O2 sensor is capable of satisfactorily ( and significantly
inexpensively - like $40 for a new O2 sensor vs $600-800 for a Kiln
Oxyprobe) monitoring the oxygen content of the exhaust stream on a fuel
kiln. The data gathered from such a probe, combined with other
atmospheric data provides a good basis for solid control to improve
performance of fuel fired kilns.
In an automobile, the O2 sensor signal is used by an onboard computer to
control the amount of fuel and air coming into the engine under
different load conditions. On a fuel kiln, Roger - or you, or me, as the
case may be, comes to be the computer and controller , since we do all
the interpreting of the data, and the controlling, ie, turning up the
fuel, moving the damper, etc. But the idea is the same. Data inputs from
sensors - makes for a better performing kiln, better firings. And the
use of the inexpensive O2 automotive sensor to measure the O2 in the
exhaust stream to determine Oxidation, Neutral, or Reduction firing is,
in my humble opinion, a real stroke of genious.
Regards,
John Rodgers
Chelsea, AL
Ivor and Olive Lewis on tue 13 jul 04
Dear John Rodgers,
Two points.
First, your essay on the behaviour of aircraft in relation to altitude
might make some sense to us and be of relevance if you gave examples
of pressures and densities and related these to a specific kiln
situation.
Second, though the Automobile Oxygen Sensor may be substituted for an
Oxygen Probe devised specifically for use in a ceramics situation,
there is still a hidden assumption when these instruments are used
that the absence of oxygen indicates the presence of gases of a
reducing nature. Or in other words, we assume when oxygen is absent
Reduction is assured.
For those who do have problems of relocation to different altitudes
the following fact my be useful. Given a Standard Atmosphere is
1013.25 Millibars at Standard Mean Sea Level when the temperature of
297 Kelvin it would appear that pressure varies with increased
elevation by - 3.5 Millibars per 30 metres.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
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