Heat transfer at different temperatures

[raton]

Hi All.
I have a simple question but I am not sure if the answer is that simple.
Is it possible to ensure that a component always incresases its temperature regardless the temperature of the cooling media?

The question comes from the test of an electronic component.
My specification requires that when the cooling media (air) is 60 degrees the component incresases its temperature 15 degress.
The manufacturer has tested the component with air at 20 degrees and the component has increased its temperature 15 degrees, therefore he ensures than at 60 degress of cooling media (air) the component will also increase 15 degress.
Is this correct?

I think it is a good question

 

[corus]

If this is a minimum specification then if the temperature increses by 15 for 20 ambient, then for an ambient of 60, then logically, it must increase by at LEAST 15C, and so is correct.

Tara

 

[raton]

Sorry Tara, you are right but I did not write the question correctly. The at LEAST is the key.

I will rewrite the requirement.
My specification requires that when the cooling media (air) is 60 degrees the component incresases its temperature ONLY 15 degress.

Thanks.

Jaime

 

[desertfox]

Hi raton

As it stands the increase of the component temperature will be greater than 15 degrees if subjected to air above 20 degrees, also the temperature of your component needs to be considered, ie if your component is at different temperatures when the coolng air comes into contact, then its temperature increase will be altered.
Unless I have misunderstood your question.

desertfox

 

[chicopee]

My question: how is the electronic component heated? Is it by the circulating air, or by its internal energy from electrons going abut their business, or both?.

 

[corus]

If you're saying ONLY 15 degrees then you're specifying a maximum temperature increase. I'd say they were incorrect then.

Tara

 

[raton]

The question comes from here.
The electric motor has a cooling system based on cooled air.
When the cooled air is not provided, the electric motor has to be derated and the components should not be heated above 80 degrees.
During the FAT the manufacturer carried out the test without cooled air (at 20C)
With air at 20C the weakest component of the electric motor reached 37C, so it increased by 17C. The manufacturer then assures that if the air is at 60C then the component will only reach 77C.

Is this correct?

Thanks in advance.

 

[ione]

A higher cooling air temperature will lead to a derated heat cooling capacity of the heat exchanger (keeping constant the other parameters). The driving force of the heat transfer process is the temperature difference between fluids involved and so the manufacturer’s statement is too simplistic.

 

[corus]

This would appear valid if the nett heat transfer coefficient to the air remains constant regardless of the temperature of the component. In fact heat loss by radiation and natural convection changes with temperature and so the assumption made is wrong. The difference in values of the heat transfer coefficient probably isn't much at those temperatures, but it'd still be worth checking as 77 is close to the set limit of 80.

Tara

 

[Compositepro]

It is common practice to rate electrical components, particularly motors, by heat rise. For most practical cases the component temperature will rise above ambient temperature by a constant delta T. This is based on the assumptions that the power dissipated is constant and convection coefficients are constant with the small operating range.

 

[IRstuff]

There seems to be a wording usage problem. The term "heated" implies actively increasing the temperature as the intended final effect. "Temperature rise" or similar wording would imply heating as a secondary effect due to operation of the machinery.

Moreover "component incresases its temperature ONLY 15 degress"[sic] is not a reasonable requirement in English. It is almost but not quite the same as without the "ONLY," A better requirement is, "component incresases its temperature by no more than 15 degrees," or, "component incresases its temperature less than or equal to 15 degress," would be the normally accepted wording. A more comprehensive specification would include directly, or by inference, "over ambient temperatures of -XX°C to +80°C," or similar. With clear requirements, it is the required of the supplier to demonstrate by analysis or by testing that the requirement is met. Depending on the contract, the client may have had the right to review and alter the acceptance and FAT procedures to make relevant and meaningful tests of the article in question.

TTFN

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[chicopee]

Raton, an electric motor is not an electronic component. You should be more concerned with semantics.

 

[zekeman]

I think the mfr is right in that if the test at 20 deg ambient produces a delta T of 15 degrees between the component and ambient, it is clear to me the differential would be somewhat less at in increased temperature, since the effective conductance (=1/R) would be greater owing to the increase radiative heat transfer, A simple thermal model,
R1 R2
Telement------------Telectronics---------------Tambient

Telectronics-Tamb)/R2=Q
R1, R2 the thermal resistances.
Q= power delivered to the component

Assuming the power remains constant, then the differential,( Telectronics-Tamb), varies directly as the thermal resistance or inversely as the conductance. And as noted above we expect R2 to decrease with rising temperature.

 

[drawoh]

...

I will rewrite the requirement.
My specification requires that when the cooling media (air) is 60 degrees the component incresases its temperature ONLY 15 degress.

You did not say what kind of degrees you are talking about. They appear to be centigrade.

My crude rule of thumb would be that if [Δ]T were 15C[°] at 20[°]C, then it would be 15C[°] at 60[°]C. This all works as long as all your components do not behave differently at the higher temperatures. This seems to me to be a risky assumption.

In absolute terms, 20[°]C and 60[°] are very very approximately the same (293[°]K versus [333[°]K). Actually, I would expect [Δ]T to be a centigrade degree or two higher at the higher temperature.

JHG