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I read the recent exchanges on wrapping exhaust manifolds, and can offer the
following (worth 2 cents):
1) At the temperatures the pipes run at (red hot) the major portion of heat
loss from the surface is through radiation to the cooler surroundings, not
through air cooling. (Radiant heat loss goes as the fourth power of the
surface absolute temperature, meaning not much loss at room temperature, but
huge loss at high temperatures, witness light bulb filaments). I would expect
that the exhaust pipe temperature is probably a hundred or more degrees F
cooler than the average exhaust gas temperature within. The gas heats the
pipe, and the pipe radiates to the outside world. So the pipe must be cooler
than the gas.
2) Wrapping the pipe will substantially reduce the radiant heat loss from the
pipe surface, and so the pipe wall will be hotter than unwrapped (no science
needed here), and will come closer to the average gas temperature within the
pipe.
3) Here is the tricky part: When measuring high gas temperatures with
thermocouples, it is very easy to get significant installation effects that
affect the readings, sometimes substantially. It is VERY DIFFICULT to get a
true, accurate temperature reading that is within better than 20F-100F at
these temperatures. Turns out we don't care. We look at comparisons and
trends.
Imagine you are an ant (roasted) on the end of the thermocouple where the
junction is reading temperature, and look at the surrounding world. The
temperature is the result of a dynamic heat flow balance: heat in from the gas
flow (which is pulsing up and down every engine rev with difficult to predict
effects on heating rates around the thermocouple) while the thermocouple loses
heat out by conduction down its length to the cooler outside world and the
thermocouple is radiating to relatively cooler exhaust pipe walls (assuming
they are unwrapped). To the ant, the outside world looks cooler, and heat is
lost from the junction. When heat gain and loss are equal, you get a steady
temperature reading, but lower than the gas temperature. Both losses,
conduction and radiation, lead to junction temperature (and thus readings)
lower than if the pipe and junction were perfectly insulated. In other words,
you are always reading a temperature below the exhaust gas temperature. The
conduction error depends on the length of the thermocouple (penetration into
exhaust pipe) so shorter penetration would read lower temperature than a
longer penetration.
4) So now you wrap the exhaust pipe. The pipe wall temperature rises, so the
radiation loss from the thermocouple to the wall lessens, so the thermocouple
reads a higher temperature even though the gas temperature is the same. (Also
the gas temperature will be very slightly higher due to less wall cooling,
although at a short distance from the cylinder head, the cooling effect is
negligible.) Thus the thermocouple temperature rises and you see it in the
cockpit. Gas temperature has not changed at all.
5) Most aviation installations are very similar: same penetration depth into
exhaust pipe, same distance downstream from head, and the materials of
construction are all very similar. So it it's no surprise that they all read
about the same at the engine same operating conditions. The errors are all
about the same. As a result, "rules of thumb" evolve about what constitutes
"good" or "bad" temperatures based on field experience. What really counts is
the trend in temperature or the comparison of one operating condition to the
other. You are not getting real absolute accuracy with these installations.
6) Wrapping the exhaust pipe will not affect the mixture ratio (lean or rich),
and will have little effect on gas temperature, especially near the heads.
There is a lot of energy in the exhaust gas, and little surface area in the
exhaust manifold to get it out. Go downstream 5 feet and maybe you will
measure a modest temperature drop. Not in 2 inches from the heads.
7) Here's the key consideration in my mind: exhaust tube life. Most exhaust
systems are made of 321 stainless steel. If you look at the oxidation rates
and strength versus temperature for all high temperature alloys in the range
of 1200F-1500F, you find that their strength is dropping rapidly with
temperature, and oxidation rates are going up. The slope of the curves are
quite steep in this temperature regime. Thus a 50-100F increase in pipe
temperature could have a significant decrease in lifetime (failure due to
thinning or cracking). Going to Inconel tubes gets you a couple of hundred
degrees of reserve and extends strength and lifetime significantly, but cost
is high.
Summary: my guess is that wrapping exhaust pipes is probably OK, if you are
willing to remove the wrapping periodically (every 200-300 hours, maybe more
often) for a close inspection for cracks and scaled areas where oxidation is
eating your wall thickness. I would like to hear from anyone that has 1000
hours on wrapped exhaust pipes. I doubt they will last that long, but I could
be wrong. If they do last, then go ahead: reduce the heat load on everything
else under the engine. You would not want to be near that glowing mass of
tubing, and neither does the local hardware, especially if the hardware is
made with epoxy.
Forewarned is forearmed.
Fred Moreno
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