Getting heat out of a box
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From: John Larkin on 7 Aug 2010 11:58 On Thu, 5 Aug 2010 15:54:40 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> wrote: >On Aug 5, 7:56�am, "markp" <map.nos...(a)f2s.com> wrote: > >> ... I've got a design that dissipates about 30W and need to >> encase it in a sealed box (not hermetically, but to all intents and purposes >> there can be no airflow through the box). >> >> Current thoughts are an aluminium extruded box with large heatsinks > >If the box has >30 square inches of external free-air surface, a >circulating >fan inside the box would do it. Or, does it have to be small? Sounds scary. A square inch of surface in free air can have theta of over 100 k/w. And that's just the outside of the box... ther would be a similar theta on the inside.
From: legg on 7 Aug 2010 23:09 On Fri, 06 Aug 2010 10:05:17 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 5 Aug 2010 15:54:40 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> >wrote: > >>On Aug 5, 7:56�am, "markp" <map.nos...(a)f2s.com> wrote: >> >>> ... I've got a design that dissipates about 30W and need to >>> encase it in a sealed box (not hermetically, but to all intents and purposes >>> there can be no airflow through the box). >>> >>> Current thoughts are an aluminium extruded box with large heatsinks >> >>If the box has >30 square inches of external free-air surface, a >>circulating >>fan inside the box would do it. Or, does it have to be small? > >Scary. A square inch of surface in still air can have theta over 100 >K/W. > >John ~160, actually. RL
From: legg on 7 Aug 2010 23:29 On Sat, 07 Aug 2010 08:58:30 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 5 Aug 2010 15:54:40 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> >wrote: > >>On Aug 5, 7:56�am, "markp" <map.nos...(a)f2s.com> wrote: >> >>> ... I've got a design that dissipates about 30W and need to >>> encase it in a sealed box (not hermetically, but to all intents and purposes >>> there can be no airflow through the box). >>> >>> Current thoughts are an aluminium extruded box with large heatsinks >> >>If the box has >30 square inches of external free-air surface, a >>circulating >>fan inside the box would do it. Or, does it have to be small? > >Sounds scary. A square inch of surface in free air can have theta of >over 100 k/w. And that's just the outside of the box... ther would be >a similar theta on the inside. > Actually, in a situation where the major dissipators are more closely coupled to the box wall, the box wall temperature can become a fairly accurate indicator of internal air temperatures. Dissipation from other isolated internal sources into this air has to be restricted simply due to the increased internal anbient. The result is that the sealed box wall becomes the dominant regulator, however backwards this may sound. RL
From: John Larkin on 7 Aug 2010 23:06 On Sat, 07 Aug 2010 22:09:31 -0500, legg <legg(a)nospam.magma.ca> wrote: >On Fri, 06 Aug 2010 10:05:17 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Thu, 5 Aug 2010 15:54:40 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> >>wrote: >> >>>On Aug 5, 7:56�am, "markp" <map.nos...(a)f2s.com> wrote: >>> >>>> ... I've got a design that dissipates about 30W and need to >>>> encase it in a sealed box (not hermetically, but to all intents and purposes >>>> there can be no airflow through the box). >>>> >>>> Current thoughts are an aluminium extruded box with large heatsinks >>> >>>If the box has >30 square inches of external free-air surface, a >>>circulating >>>fan inside the box would do it. Or, does it have to be small? >> >>Scary. A square inch of surface in still air can have theta over 100 >>K/W. >> >>John > >~160, actually. > >RL Well, I didn't want to be alarmist. It's complicated, depending on size and orientation and such. 100 is plenty enough to toast the box in question. John
From: John Larkin on 7 Aug 2010 23:16
On Sat, 07 Aug 2010 22:29:04 -0500, legg <legg(a)nospam.magma.ca> wrote: >On Sat, 07 Aug 2010 08:58:30 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Thu, 5 Aug 2010 15:54:40 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> >>wrote: >> >>>On Aug 5, 7:56�am, "markp" <map.nos...(a)f2s.com> wrote: >>> >>>> ... I've got a design that dissipates about 30W and need to >>>> encase it in a sealed box (not hermetically, but to all intents and purposes >>>> there can be no airflow through the box). >>>> >>>> Current thoughts are an aluminium extruded box with large heatsinks >>> >>>If the box has >30 square inches of external free-air surface, a >>>circulating >>>fan inside the box would do it. Or, does it have to be small? >> >>Sounds scary. A square inch of surface in free air can have theta of >>over 100 k/w. And that's just the outside of the box... ther would be >>a similar theta on the inside. >> >Actually, in a situation where the major dissipators are more closely >coupled to the box wall, the box wall temperature can become a fairly >accurate indicator of internal air temperatures. Dissipation from >other isolated internal sources into this air has to be restricted >simply due to the increased internal anbient. The result is that the >sealed box wall becomes the dominant regulator, however backwards this >may sound. > >RL If the box has, say, 5 k/w to the world (30 sq inches, 150 K/w per) and you dump 30 watts from inside, the wall temperature averages 150K above ambient. That's bad enough, without the parts inside having another bunch of theta between themselves and the hot wall. That's why it's best to bolt hot stuff directly to the inside of the walls, and not use convenction, even with a circulating fan, inside. So I guess we agree. This one uses oversize transistors and a heat spreader plate to avoid needing a heat sink. The chassis is a bit too thin to dump the heat into directly. ftp://jjlarkin.lmi.net/Chimera.JPG John |