convection cooling 1

[lau183]

I have a aluminum box that is 19in x 7 in x 11in. inside the box approx 110 watts is being dissipated. the aluminum thinkness is .225in and i was wondering how much heat can the box dissipate by natural convection alone at 25 c, 50C, and 70C.
At ambient i measure 50 C on the hottest portion of the box after about 2 hours and it seems to reach a state of equilibrium. i took a fan at about 100 cfm and sucked the air off the top of the box and the temp dropped to about 35 c in about 30 min box weighs about 50 lbs and has an aluminum motherboard and 44 pwb card mounted vertially inside (not a solid aluminum box). im just looking for a rough number any help is appreciated thanks in advance

ps please dont spam me because i cant seem to figure it out myself

 

[IRstuff]

is 50 C the hottest spot on the box or in the box?

TTFN

 

[lau183]

50C was hottest spot on the box on the back 10x7 side fairly uniform there. the front 10x7 side was more like 40c. inside the box there are most definitly hot spots that are approaching 70c near the back of the box but the skin of the box never went greater than 50C in an ambient of 25C. i was trying to think of it like a 110w lightbulb encased in a block of aluminum.

 

[IRstuff]

I think that you've pretty much reached the limit for the design and configuration of the electronics that you have. Unless you have extended temperature parts, 70C is the upper limit for commercial integrated circuits.

Additionally, 65C case temperature would be a burn hazard, so you're pretty close to maximum allowable case temperature. Assuming that your ambient was 25C, your temperature coefficient to the hottest external spot is 0.23 deg/W. To the first order, the delta temperature will be the same for different ambient temperatures, so with a 50C ambient, the case temperature will reach 75C, ouch!, and the internal max temperature will reach 95C, which is the limit for extended temperature IC's.

TTFN