## Material vs Ambient Temperature

## Material vs Ambient Temperature

(OP)

All,

I have been having a discussion with a coworker about temperature rise over ambient. We have two opinions but have not been able to find any test data or proof for either case. If we have a bar of any material and we apply a current through it(lets pretend 5000A), we know the bar will heat up some amount due to the current flowing through it (lets pretend it rises 150*F)and it was already a set temperature (ambient 75*F) before we applied the current. We then find the temperature of the bar after current has been running for a few hours to be 150*F + Ambient (75*F) = 225*F

He claims that no matter what the ambient temperature is in the room the bar will ALWAYS rise the 150*F due to the current being forced through it. Example we crank ambient up to 80*F then the bar should read 230*F

I am claiming that is not true and that the higher the temperature the less rise we should expect over the ambient. Now, it may be a very small difference but I do not believe the temperatures simply sum. If the room could theoretically be at 500*F I do not believe the bar would be 650*F.

Can someone help me prove one way or the other? There has to be some temperature limitations here and if we find that they DO NOT SUM, what is the formula for finding this?

This reminds me of velocities, everyone says velocity is just V1 + V2 but that only works at very low speeds, where the actual equation shows to much greater detail what the velocities actually are. That is what I am hoping to find here. Any help is greatly appreciated!

Thanks,

Jason

I have been having a discussion with a coworker about temperature rise over ambient. We have two opinions but have not been able to find any test data or proof for either case. If we have a bar of any material and we apply a current through it(lets pretend 5000A), we know the bar will heat up some amount due to the current flowing through it (lets pretend it rises 150*F)and it was already a set temperature (ambient 75*F) before we applied the current. We then find the temperature of the bar after current has been running for a few hours to be 150*F + Ambient (75*F) = 225*F

He claims that no matter what the ambient temperature is in the room the bar will ALWAYS rise the 150*F due to the current being forced through it. Example we crank ambient up to 80*F then the bar should read 230*F

I am claiming that is not true and that the higher the temperature the less rise we should expect over the ambient. Now, it may be a very small difference but I do not believe the temperatures simply sum. If the room could theoretically be at 500*F I do not believe the bar would be 650*F.

Can someone help me prove one way or the other? There has to be some temperature limitations here and if we find that they DO NOT SUM, what is the formula for finding this?

This reminds me of velocities, everyone says velocity is just V1 + V2 but that only works at very low speeds, where the actual equation shows to much greater detail what the velocities actually are. That is what I am hoping to find here. Any help is greatly appreciated!

Thanks,

Jason

## RE: Material vs Ambient Temperature

But there are two reasons why they actually will not.

Two heat transfer mechanisms determine the temperature rise above ambient of a heated object: natural convection and radiation.

Natural convection flow is driven by the temperature difference between the surface and the surrounding air. This driving force remains constant as the ambient increases until you get significant changes in the material properties of the air (big density changes or ionization). So if there were only natural convection, the two temperatures would always just add together.

Radiation depends on the difference of the absolute temperatures to the fourth power. As the ambient goes up, its absolute temperature gets larger and so does the absolute temperature of the solid surface. Radiation from the surface to the surroundings increases as the ambient goes up, even if the difference between the ambient and the solid does not change (eventually it has to if the radiation cools the surface).

I don't know if this helps justify your gut feeling, but the radiation component of heat transfer agrees with your gut. The temperature difference between the solid and the air will do down as the ambient increases.

## RE: Material vs Ambient Temperature

## RE: Material vs Ambient Temperature

However, you do have a point, also. As temperature rises the amount of black body radiation from the object and the environment increases in proportion to the fourth power of the absolute temperature. This will reduce the delta T between the object and the environment. This is why vacuum furnaces operating at high temperature have fairly good temperature uniformity but lower temp. vacuum ovens do not. But for practical purposes this effect is often ignored.

## RE: Material vs Ambient Temperature

Radiant heat transfer does not become dominant until you reach higher temps.

The difference in convective heat transfer between 225F (380K) and 300F (420K) is insignificant.

We resistance heat and stretch straighten metal parts at 1300-1400F, at that temp each part shape is a separate control problem because radiation is a big issue

= = = = = = = = = = = = = = = = = = = =

P.E. Metallurgy, Plymouth Tube

## RE: Material vs Ambient Temperature

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

## RE: Material vs Ambient Temperature

Ted

## RE: Material vs Ambient Temperature

It's likely that anything you've seen in the past is due to an increased resistivity reducing the power dissipation.

Nevertheless, there are also some changes to the heat transfer coefficient; the components of the heat transfer coefficient are temperature dependent.

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

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## RE: Material vs Ambient Temperature

http://www.fusionpoint.be

http://be.linkedin.com/in/fusionpoint

## RE: Material vs Ambient Temperature

If ambient = 75, and (now) you pass current through the bar, it heats up then stabilizes at 225 deg. (Heat in (current) = heat out (radiation losses at (225 + 273) to 75 deg ambient (75 + 273) PLUS + convection losses between 225 deg surface and a 75 deg ambient).

Then, you are asking what happens if ambient becomes 200 deg, right? Is the new bar temperature = 350 deg?

## RE: Material vs Ambient Temperature

## RE: Material vs Ambient Temperature

If you maintain constant voltage instead of constant current, then heat generation will decrease with increasing ambient temperature because current will decrease with increasing bar resistivity.

Ted

## RE: Material vs Ambient Temperature

TTFN (ta ta for now)

I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg

FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm