Trying to calculate temperature of unit

[ENGINEERRMECH]

Hello Forum-
Thank you for taking the time to review my question.
I'm trying to determine what the maximum inside temperature of a new product will be and to see if the product will need a fan.
The heat sources are various electrical components which account for 3W and a an intermittent stepper motor of 6W.
The dimensions of unit are 11.5inX5.5inX3in and made of ABS/PC blend. The unit as of right now is sealed, but not air tight by any means, i.e. no vent holes on the top of the unit.
The operating temperature of the unit is 10C to 40C.
I was trying to calculate the temperature of the unit say after it was running for an hour at 3W using Q=m*c*(T2-T1) and solving for T2, but I get a crazy final temperature of ~3500C? I don't understand how to take into account the losses of convenction and conduction, or if I even need to. I then wanted to calculate the temperature change as a result of turning on the 6W stepper motor for 30 seconds.
Any help would be appreciated, I really want to understand this and would really like to see the equations that would be used.

Thank you and Kind Regards,

ENGINEERRMECH

 

[IRstuff]

You should be using something like 3W = htc*area*deltaT, where htc might be as low as 2.5W/m^2-°C, area is the exposed surface area of the box.

TTFN

FAQ731-376

 

[ENGINEERRMECH]

Hello IRstuff-
Sorry, I haven't done a heat transfer problem for quite some time. Do I need to subract the T2 in your equation 3W=htc*area*deltaT from Q=m*c*deltaT because of the heat transfer to the outside the box? I was first hoping to calculate/graph the inside temperature of my box, but didn't know how to account for convective, radiant, and conductive heat loss.

Thank you,
ENGINEERRMECH

 

[IRstuff]

No, the equation is for the convection from the box.

TTFN

FAQ731-376

 

[ENGINEERRMECH]

Hello IRstuff-
I calculated T2 using the convective heat transfer equation in the attached file. My T1 started at the lowest operating temperature of 10C and I calculated htc for my material by dividing k by material thickness per wikipedia. I don't know if I trust the 80.853C result for T2. Is T2 the temperatue outside the unit at the unit's surface? How is the time that the heat source is on taken into account? I know these are basic questions, but thank you for helping.

Thanks and kind regards,
Engineerrmech

 

[IRstuff]

Something more along the lines of:

http://files.engineering.com/getfil...48e9-4860-b958-595fde0c6612&file=box_heat.mcd

TTFN

FAQ731-376

 

[Twoballcane]

What temp are you trying to calc? Is it the air inside the box or the top of the electronic component like a flat pack? In most thermal studies for electronic packaging, it is the top of the electronic device to outside ambient air. So your thermal circuit should be from the top of the device, thru the air, thru the box, and then to ambient air. Plus you have to take into account conduction from the device thru the board to a heat sink (if you have one). In any case, the main reason for (in most of my analysis) is to make sure your device junction temperature is not exceeded.

You also looks like you have two different scenarios going, steady state and transient. In this case you find your max temp at steady state and then add on the temp rise due to transient. I recommend investing in Dave S. Steinber's Cooling Techniques for Electronic Equipment. This will walk you thru the analysis

Tobalcane
"If you avoid failure, you also avoid success."

 

[zekeman]

Your first equation looks reasonable in spite of assuming convection. I get your coefficients for the combined radiation and convection for 2 walls.

So the temperature you should get is the inside temperature of the enclosure is the steady state value.
For all other ambient temperatures, just add the drop
to the value.

How did you get a drop of 70 deg and why don't you include the stepper motor power giving it a duty cycle? I get closer to 10 deg C as the drop, giving 20C as the inside temperature.

Oh, I just noticed the problem.. You used that suggested value which is WRONG for this application of natural convection/radiation

Redo the problem to include the stepper and you should be just fine

Finally, why would you need the time value of temperature when the steady state is more conservative. It would probably not be helpful and unnecessarily complicated.