Wednesday, October 14, 2009

How Emissivity affects Heat Transfer and Surface Temperature

"I know the higher the emissivity a surface has the greater its ability to absorb heat (IR), however, is a painted surface able to reject or shed heat better? An example would be does a painted engine run cooler than a unpainted metal engine or hotter or the same temp?"

If an object has a higher temperature than its environment, then increasing its emissivity will certainly lower its temperature. As an example, lets look at the motor in the illustration. It has a cover made of stainless steel and is very shiny. Let's assume its emissivity is 0.1.  We have measured its steady state temperature at 150 F. The rate of heat generation of the motor has to be exactly balanced by the rate of heat loss from the motor. So, in this case for simplification, we will only consider convection and radiation. The numbers are shown in the illustration.

Now, if we painted the motor with flat white paint, the heat transfer will change. We still have to get rid of the 66 Btu/hr being generated by the motor, but if we assume a new emissivity of 0.9, then the huge increase in the radiation loss will reduce the surface temperature to 118 F as shown.

1. It is instructing you chose flat white for your comparison, and also puzzling. It is not so reflective as shiny stainless steel, but is still white, which our everyday experience tells us reflects more heat than a darker color. Could you add a third example above by painting it black? Where does your surface temp stabilize, and why? THANKS !

2. We are interesting in emitting more heat, not reflecting it here as we want the motor to be cooler. The emissivity of stainless steel means that the motor has a poor efficiency for getting rid of its heat by radiation.

Painting it white (or black) raises the emissivity considerably enabling better heat transfer by radiation. (White paint emissivity is about the same as black paint emissivity).

The white paint will actually keep this motor cooler as it emits infrared well, and also reflects visible light well (so it won't be absorbed and turned into heat). The black paint absorbs visible light well, and thus, will not keep the motor as cool. Indoors this would not be a big issue, but if the motor were in sunlight outdoors, it would be.

3. Great article! What about powder coating? Would this effect the results any different than paint since it is essentially a layer of melted powder/enamel bonded to the metal vs. paint?

1. I would expect a powder coating to have a somewhat lower emissivity than the white paint. If that is true then it would not be as effective. It all depends on the emissivity of the powder coating (and to some degree its thermal conductance).

2. From what I found the powder coat is essentially .750-.9 emissivity. The goal is to lower the manifold temperature by allowing it to radiate more heat. From what I understand that is what emissivity is. Can you correct me if I have miss interpreted this? What is at play here is the manifold is physically heated through conduction as it is bolted to and touching the engine.

What I don't want is for the powder coat to have an externally cooler temperature but "trap" the heat in the metal of the manifold and not allow it to escape or radiate.

3. Raising the emissivity of the manifold will allow it to cool more efficiently and lower its temperature.

4. good blog !! it's having wonderful information about your experiment of Heat Transfer.

5. based on the above experience we can conclude that emissivity and temperature are inversely proportional. Am in right

1. That is not correct. Emissivity at any wavelength is a value that is dependent on material, and surface roughness. If the material properties change due to temperature or anything else (chemical action, etc.) then the emissivity of the changed material may be different.

2. Thanks a lot for the explanation Mr. Gary.

6. If heat loss is the same, how is it the temperature is lower on the white painted motor? How do we determine where stabilization is when we are removing the same amount of heat in both cases? According to your explanation, heat removal by conduction does not have the same effect as heat removal by emission, is that a fair conclusion?

7. The temperature of the motor casing decreases because the amount of heat lost by radiation has been increased by a factor of over 5 times. Since the temperature difference between the surface and the air has decreased, the heat lost by convection has decreased about 47%.

Although fairly negligible in this case, the lower temperatures also mean heat loss by conduction is reduced as well.

It all boils down to the conservation of energy. Energy into a system must equal the energy out plus the energy stored.

8. Ok, so can we assume the emissivity relation to temperature of the material is stable with temperature and change or increase rapidly with a phase change occuring? For example, if the melting point of a designated material is 150degC, can I assume that it's emissivity is 'a' from 0-150degC and switches to 'b' past 150degC? Or is there a linear relation between emissivity and temperature? Thanks for your help!

1. Sorry if my question is not making much sense. The example should help you understand it I guess...

2. That really depends on the material. However, for many materials, the emissivity should be relatively stable in a specific waveband as long as the heating is not causing surface changes (such as oxidation). When there is a phase change, a change in emissivity would be expected.

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