Material vs Ambient Temperature

[AlwaysConfused1]

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

 

[tkordyban]

In general, the two temperatures do sum.

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.

 

[JStephen]

If you simplify the problem enough, the temperature rise is the same. If you look at it in more detail, the thermal properties of the surrounding air and of the bar itself, the electrical properties of the bar, all vary to some extent with temperature. At higher temperatures, you also have an increasingly larger effect from radiation heat transfer. Net result: At higher temperatures, it's not immediately obvious if the temperature differential would be more or less than at ambient, but it wouldn't stay exactly the same, either.

 

[Compositepro]

Your question is confusing to read and that is probably because you were somewhat confused when writing it. In general the temperature rise will be 150F above ambient.

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.

 

[EdStainless]

If you are talking about ambient temps in the range of 70-100F and a temp rise of 100-150F, then they will add.
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

 

[dgallup]

Within some range of ambient temperatures they will simply sum. At some point you are going to have a lot of other physical phenomenon taking place which will greatly change the results. The bar may start to rapidly oxidize, the bar may start to sag and stretch, the bar may even melt or burst into flame. All these things will happen at different temperatures depending on the bar material and atmosphere. You are not going to find a simple formula to describe the temperature change for all materials, atmospheres and ambient temperature.

----------------------------------------

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.

 

[hydtools]

The heat generated is directly related to the product of I^2 x R which does not change with ambient temperature unless resistance R does. Heat transfer depends on temperature difference. In your example the bar needs to be 150 degrees above ambient to reach thermal equilibrium, heat generated equals heat transferred. Therefore, the bar temperature always rises 150 degrees regardless of ambient temperature for the same I^2 x R.

Ted

 

[IRstuff]

Copper has a fairly large temperature coefficient for resistivity:

http://www.endmemo.com/physics/resistt.php

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)
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[kingnero]

The electrical resistance will alse change with temp, so if you were to force a constant current through this bar, the temp rise (that your coworker assumed constant) will also change when you have different starting temperatures.

http://www.fusionpoint.be

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

 

[racookpe1978]

I am no clear of your question either.

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?

 

[AlwaysConfused1]

Thanks for the responses everyone. Looks like we are both correct to a degree. At lower temperatures we can sum and be accurate but at very high temperatures there are so many variables that come in to play that it would not be a simple addition.

 

[hydtools]

Once heat transfer reaches equilibrium, the final bar temperature is as you say simply the sum of ambient plus bar temperature increase. Bar temperature increase may be higher starting from higher ambient temperature if bar resistivity increases and current is kept constant resulting in more generated heat.
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

 

[IRstuff]

This

https://www.electronics-cooling.com...on-heat-transfer-coefficient-on-a-flat-plate/

shows a fairly substantial increase in heat transfer coefficient over temperature. Since resistance increases with temperature, which means, for a constant voltage, power decreases. Thus, the deltaT will be lower with constant voltage, as well as with constant poewr.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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