> Al Gietzen wrote:
 >
 > I was considering getting my manifold coated on the inside in an effort

to

reduce wall temps and
 > heating in the cowl.  Before committing to the cost I want to know how
much it will really help;
 > seemed suspicious to me that the very thin coatings (.005-.010") would
provide anywhere near the
 > temp reductions claimed by Jet-Hot.
 >
 > I estimated the heat flux from a 1600F wall by conduction, natural
convection, and radiation to
 > total about about 5 watts/sq-cm.  Most of this is by radiation.  If you
have air blowing over the
 > surface it might go up by a few W/sq cm.
 >
 > I called Jet-Hot to get the thermal conductivity of the coating.  They
refuse to provide any
 > engineering data of any kind.  They did tell me that the coating on the
inside would only be about
 > 0.001"; and about .005" on the outside.
 >
 > It is hard to imagine that the thermal conductivity could be less than a
ceramic, so using that, a
 > .001" coating gives a temp drop of a whopping 6 C, or about 10 F drop in
wall temp.  Adding
 > another .005-.010" on the outside may drop the temp a total of about

100F.

 >
 > So unless you're doing it to make your manifold and pipes look nice (or

I'

ve made an error in my
 > calcs - entirely possible) these coatings are a waste of money;

especially

a .001" coat on the
 > inside.  No wonder they don't give out any engineering info.
 >
 > Al Gietzen

 Except for one thing Al and I am not saying this because I necessarily

think

its true.
 I am just saying it is a possibility.  Steel and SS tends to oxidize at

high

temp.
 If you can keep the oxygen away from the surface they might not corrode so
badly
 and perhaps last longer. Mild steel flakes off in large chunks when used

for

 rotary engine exhaust.

 Paul Lamar

-------------------------------------
There is a way that such a thin coating can work, although I have no way of
knowing whether they are doing it that way.

If the coating has a LOW EMISSIVITY the radiated heat will be much reduced.
This is the only effect that could cause a significant temperature drop by a
thin coating.

emissivity = absorptivity = (1- reflectivity) so a low emissivity coating is
a highly reflective coating in the appropriate wave length (infra red for
exhaust manifold). A shiny SS surface has low emissivity, however it quickly
oxidizes to a high emissivity surface.  if the ceramic coating prevents that,
it COULD be effective.

The emissivity as a function of wavelength needs to be measured in order to
determine whether it is effective or not.

Bill Schertz
KIS Cruiser #4045

 
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