No subject

Fri Aug 22 10:58:37 CDT 2008

considered; conduction through the walls (with subsequent convection to =
the
air) and radiation.  This is the one place in our installations where =
the
radiant heat transfer becomes the dominant mode.  The convection away =
from
the pipes is not much affected by the coatings.

=20

The coatings are only about 0.007" to maybe 0.010" thick. They are =
ceramics,
which have low conduction coefficients, but the temperature drop is =
directly
proportional to the thickness, and less than 10 mils of ceramic will do =
very
little - I don't remember the numbers any more, but consider maybe 15 - =
20
degrees.  Negligible.

=20

The radiant heat transfer goes as the 4th power of temperature, and is
directly proportional to the 'emissivity' of the surface.  The =
emissivity of
a somewhat oxidized SS may be 0.2 - 0.25.  If you increase the =
emissivity,
more heat will be transferred away, and the pipe will be cooler.  A flat
back coating has an emissivity of maybe 0.9 - 0.95; so the =
recommendation of
that coating is to make the pipe cooler - at the expense of putting more
heat to the surroundings in the cowling.  A white, or bright silver =
coating
has low emissivity, maybe 0.07 to 0.12.  The pipe will be hotter, but =
more
heat will stay in the exhaust.  OK, the cooler pipe will transfer less =
heat
by conduction/convection to the air, but at temps of 1200F or more the
radiant heat transfer will dominate.

=20

The same factors apply to the inside of the pipe.  Emissivity is also a
measure of the heat absorbed by the surface, and it is important here
because the inner wall is looking at very hot gas.  A low emissivity =
coating
on the inside will definitely reduce the wall temperature and the heat
transferred to the area around the pipe inside the cowling. So a white =
or
bright silver coating is good.   Problem is, it won't last long - after
awhile the inside of the pipe becomes coated with that grey exhaust =
particle
stuff; and the emissivity maybe end up at about 0.5 or so, about the =
same as
if the pipe wasn't coated.

=20

My judgment was the money was better spent elsewhere.

=20

Disclaimer: All the numbers in the above statements are approximate and =
from
memory.  The conclusions are based on my engineering judgment. Your =
results
may vary.

=20

Al

=20

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<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>Here's my 2-cents worth on exhaust coatings.&nbsp; =
I
considered this for my installation some years ago.&nbsp; I was looking =
for
ways to reduce the heat into the cowling from the exhaust system.&nbsp; =
I got
info from Jet-Hot and others.&nbsp; Lots of fantastic claims are made, =
and I
know there are those who swear by it &#8211; but, I did the math and =
decided
not to bother.</span></font></p>

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>For the most part, coatings on the outside are
cosmetic.&nbsp; It does make for a very nice looking exhaust.&nbsp; =
</span></font></p>

From a heat transfer point of viewTwo modes of heat
transfer need to be considered; conduction through the walls (with =
subsequent
convection to the air) and radiation.&nbsp; This is the one place in our
installations where the radiant heat transfer becomes the dominant mode. =
&nbsp;The
convection away from the pipes is not much affected by the =
coatings.

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

The coatings are only about 0.007&#8221; to maybe =
0.010&#8221;
thick. They are ceramics, which have low conduction coefficients, but =
the
temperature drop is directly proportional to the thickness, and less =
than 10
mils of ceramic will do very little &#8211; I don&#8217;t remember the =
numbers
any more, but consider maybe 15 - 20 degrees. =
&nbsp;Negligible.

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

The radiant heat transfer goes as the =
4th power
of temperature, and is directly proportional to the =
&#8216;emissivity&#8217; of
the surface.&nbsp; The emissivity of a somewhat oxidized SS may be 0.2 =
&#8211; 0.25.&nbsp;
If you increase the emissivity, more heat will be transferred away, and =
the
pipe will be cooler.&nbsp; A flat back coating has an emissivity of =
maybe 0.9 &#8211;
0.95; so the recommendation of that coating is to make the pipe cooler =
&#8211;
at the expense of putting more heat to the surroundings in the =
cowling.&nbsp; A
white, or bright silver coating has low emissivity, maybe 0.07 to 0.12. =
&nbsp;The
pipe will be hotter, but more heat will stay in the exhaust.&nbsp; OK, =
the
cooler pipe will transfer less heat by conduction/convection to the air, =
but at
temps of 1200F or more the radiant heat transfer will =
dominate.

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

The same factors apply to the inside of the =
pipe.&nbsp; Emissivity
is also a measure of the heat absorbed by the surface, and it is =
important here
because the inner wall is looking at very hot gas. &nbsp;A low =
emissivity
coating on the inside will definitely reduce the wall temperature and =
the heat
transferred to the area around the pipe inside the cowling. So a white =
or
bright silver coating is good.&nbsp; &nbsp;Problem is, it won&#8217;t =
last long
&#8211; after awhile the inside of the pipe becomes coated with that =
grey
exhaust particle stuff; and the emissivity maybe end up at about 0.5 or =
so, about
the same as if the pipe wasn&#8217;t coated.

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>My judgment was the money was better spent =
elsewhere.</span></font></p>

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

Disclaimer: All the numbers in the above statements =
are approximate
and from memory.&nbsp; The conclusions are based on my engineering =
judgment.
Your results may vary.

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>Al</span></font></p>

<p class=3DMsoPlainText><font size=3D2 face=3DVerdana><span =
style=3D'font-size:11.0pt;
font-family:Verdana'>&nbsp;</span></font></p>

<p class=3DMsoPlainText><font size=3D2 face=3DNimrod><span =
style=3D'font-size:11.0pt'>ity/maillist.html</span></font></p>

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