Glowing aluminized steel manifolds 2

[Wicsteve]

We supply a lot of exhaust tube manifolds for the small engine industry. In the last few years, many of these manifold tubes have been converted from steel to aluminized steel. Engine exhaust gas temperatures in these tube are hot (about 1100 F or even higher). Some of our customers have noticed that the aluminized steel tubes tend to glow bright red where the old standard steel tubes did not. Then after the aluminized steel ages (surface turns dark grey), the glowing problem goes away (or at least isn't as apparant). Is there some characteric of aluminized steel that could explain this? The affect of the darken coating might explain some of this but I'm confused by the reaction of a virgin steel tube vs. a virgin aluminized steel tube.

 

[unclesyd]

Did you change the gauge of the tube material?

Is the Aluminum applied pre or post fabrication?

 

[btrueblood]

If the aluminum coating is fairly fresh when the manifold is installed, it will have a low thermal emissivity (0.3 to 0.5), which will tend to push the tube wall temperature higher. As the surface oxidizes over time, the emissivity will climb towards 1.0, the tube wall will radiate heat more rapidly, tending to cool the tube to a lower temperature.

 

[EdStainless]

It's no fun when some give a detailed correct answer right off the bat.
The only other part is that the appearance may be a factor, you can simply see it more clearly in the brighter material.

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Plymouth Tube

 

[Wicsteve]

I can accept that oxidation and difusion change the aluminum coating and that after the surface turns grey and rough that radiation is improved (no surprise here).

I understand that radiation is a factor, but what surprises me is why a new 'plain steel' manifold (unpainted and 'bright steel' in color) seems to react quite a bit differently than an new aluminimum coated steel.

Wall thickness of both pipes are approximately the same but not exact.

 

[btrueblood]

Steel has inherently higher emissivity than aluminum, comparing "clean", unoxidized material.

 

[MSUKeith]

We have found that when the effects of aluminized coating alloying with the base steel (turning gray or even black) are evaluated with both a contact thermocouple and an IR thermocouple or camera that convection and conduction are the main heat transfer mechanisms, radiation plays a much smaller role. It does effect how the eyes view 'glow', particularly at temnperatures where the sample is just beginning to glow. Obviously an improper emissivity setting for the surface condition of the sample can give wildly incorrect IR termocouple results.

When in doubt turn out the lights - an aged part will probably have a similar glow to new. Plus its a valid excuse to get in a good nap!

 

[btrueblood]

MSUK,

I have had the experience I quoted, which differs somewhat from yours, but I was using actual thermocouples to measure the temperature of the tube wall. If the emissivity of the surface varies over time, how can you trust an optical measurement?

 

[Wicsteve]

MSUKeith's description seems to be consistant with what has been observed by our customers.

I'm not certain that I believe there is all that much difference in emissivity of type II alumininized steel and new, clean CRS steel. I haven't found any published value for aluminized steel emissivity and steel's published data seems to vary quite a bit depending upon surface finish.

I did run a quick and dirty calibration curve for our Raytek non-contact thermometer, changing it's emissivity settings, then plotting temperature deviation from actual temperature. While aged aluminized steel differs substancially in it's characteristic curve, the two curves for aluminized steel and straight, (washed)CRS steel lay almost on top of one another (or at least until we get to very low emissivity settings). Yes, I know this isn't a very good way to verify emissivity and I can't seperate instrument error from the effects of the real emissivity. However, if I believe my data I would have set the value for our thermometer's emissivity setting at 0.6 for both materials, then set E at .95 for the aged aluminized steel. [In reality, I always paint a spot on the part surface with a flat black, high temperature paint when attempting to take IR temperature readings of exhaust systems.]

 

[MSUKeith]

We use ASTM E1933 spec to set emissivity on IR equipment. We also have a book of samples for bare stainless and aluminized steels showing the various heat tints and/or alloying rates with the associated emissivity. This gets us in the ball park just based on comparing visual appearance to the book. Its also a good tool to judge time at temperature for a part.

Many times we have muffler chambers with extreme differences in heat tint on the same muffler depending on internal tube routing. The emissivity of the IR device needs to be adjusted to read accurately between the two chambers as well as adjusted for aging.

 

[EdStainless]

Steve's comment is right on. Never trust emissivity settings.
When you care black a spot and set for 0.95. I have used carbon black in MEK for painting these spots.
Or if you really want the right number use a fine gauge contact thermocouple.

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Plymouth Tube

 

[Wicsteve]

I want to try to summarize for my own understanding and perhaps add a question or two to the discussion.

1) Probably the two most important heat transfer mechanisms involved is conduction and convection. Heat radiation is probably less important for a couple of reasons -- a) The tube diameters are relatively small (1 to 1-1/18 inch diameter and usually less than a foot long) so surface area isn't that great; and b) the manifold tubes are surrounded by other hot surfaces (engine, heat shields, and muffler) so the amount of radiated heat transfer between relatively equal hot surfaces is minimized.

2) Since aluminized steel is essentially almost all CRS, then manifolds made of either aluminized steel or uncoated CRS, should have nearly identically heat transfer properties (excluding any minor radiation effect). That probably means that both aluminized steel manifolds and uncoated crs manifolds are likely to reach approximately the same maximum temperature for any given engine running condition.

3) A new aluminized steel manifold won't immediately darken upon heating. A new uncoated steel darkens immediately at temperatures above approx. 400F (first blue, then grey before starting to glow).

Can I assume, that the darkening surfaces of heated crs causes a shift in it's emissivty value (upward) and in turn affects the amount of visible light being absorbed by a glowing surface? So once glowing, a material with a higher emissivity value, appears darker than a material with a lower emissivity value even if both materials reach roughly the same temperature.

 

[redpicker]

Wicsteve,

You are correct. Emissivity is the compliment of reflectivity, so a material with a higher emissivity will have a lower reflectivity, and will appear darker when there is an appreciable amount of reflected light. That is, in a dark room, at temperatures high enough to glow, the material with the higher emissivity will appear brighter (since most of the light coming from the surface is being emitted). In a bright room, the material with the higher emissivity can appear darker (if most of the light coming from the surface is reflected). This will depend on how much radiation is being emitted and how much is being reflected.

rp

 

[786392]

Is this not the 'Basic' Concept of "Black Body Radiation"
I wonder?

Best Regards
Qalander(Chem)

 

[IRstuff]

You should do the math. I get that a cylindrical blackbody 1 1/8" diam x 5 ft. at 700°F radiates 2500 W, while it only convects 236 W.

There are still a multitude of other possibilities, including, surface micro-quality, which affects radiation and convection, as well as potentially different interior surfaces, which can affect heat transfer between the gas and the pipe.

Without careful measurements and analysis, this discussion can endless.

TTFN

FAQ731-376