Jeff Spitzer wrote:

Russell Kent wrote:

I'm not certain I agree with this assertion.  Ceramics have the useful

property

of having a low thermal conductivity, even in thin layers (witness the

relatively

thin tiles on the Space Shuttle).  If one were to coat the inside of the

exhaust

manifold only, then the ceramic layer would slow the conduction of heat

to the

underlying metal.  Since mild steel has a substantially higher thermal
conductivity than ceramics, it should wick the heat away and try to dump

it in

the air around the outside.  Assuming you have a good flow of air around

the

outside, this should allow the exhaust manifold to maintain a lower

operating

temperature by causing the exaust gases to retain more of their heat.

Coating *both* the inside and outside of the exhaust header will

definitely raise

the operating temperature of the exhaust manifold, since the outer layer

will

slow the manifold's ability to dump heat into the cowl air (which might

be useful

for cowl temperature problems).  The inner layer would still slow the

conduction

of heat into the manifold though.

Coating only the exterior of the manifold will make the manifold's

operating

temperature be at least as high as coating both inside and out; it would

probably

be higher.

Can any thermal engineers / product enginners out there add some facts

to this?

Russell Kent

The fact that some ceramics have an extremely low conductivity (space
shuttle) does not mean that *all* ceramics share the same values.
Regardless of the thermal conductivity, the resistance to heat flow will
always be a function of the thickness.  My point was that without
significant thickness (let's say .03++) then the ceramics won't
substantially lower the pipe temp.  Cermamics that are applied this thick
tend to crack at high temp differentials and with big differences between
metal and ceramic CTE.  Witness the shuttle *tiles*, not shuttle ceramic
dip.  Ask the manufacturer why and how the coating is applied.  I expect it
is inside and out, very thin, and done to prevent corrosion.

Jeff Spitzer

 
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