Getting heat out of a box
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From: whit3rd on 8 Aug 2010 03:44 On Aug 7, 8:58 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 5 Aug 2010 15:54:40 -0700 (PDT), whit3rd <whit...(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. Not true, of course. The inside has forced airflow, not 'free air', and the only way you'd get 100K/W would be on a horizontal surface with no convection. No box is entirely horizontal surfaces, and no plausible scenario raises the case temperature by 100K without initiating convection. A wall-wart in a plastic case with circa 10 square inches of area has no difficulty in shedding 4W. Metal should dump heat faster.
From: dagmargoodboat on 8 Aug 2010 09:29 On Aug 8, 3:44 am, whit3rd <whit...(a)gmail.com> wrote: > On Aug 7, 8:58 am, John Larkin > > > > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > On Thu, 5 Aug 2010 15:54:40 -0700 (PDT), whit3rd <whit...(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. > > Not true, of course. The inside has forced airflow, not 'free air', > and > the only way you'd get 100K/W would be on a horizontal surface with > no convection. No box is entirely horizontal surfaces, and no > plausible > scenario raises the case temperature by 100K without initiating > convection. > > A wall-wart in a plastic case with circa 10 square inches of area > has no difficulty in shedding 4W. Metal should dump heat faster. Well, back to the question at hand, internally the OP should mount the hot parts to the box walls, and externally he needs a heatsink that gets him to ~1C/W. Even better than getting the heat out of the box is... not putting so much in in the first place. -- Cheers, James Arthur
From: markp on 8 Aug 2010 10:35 ><dagmargoodboat(a)yahoo.com> wrote in message >news:422e8204-6a5e-45fa-9cc3->df28034e5222(a)h28g2000yqd.googlegroups.com... >On Aug 8, 3:44 am, whit3rd <whit...(a)gmail.com> wrote: >> On Aug 7, 8:58 am, John Larkin >> >> >> >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> > On Thu, 5 Aug 2010 15:54:40 -0700 (PDT), whit3rd <whit...(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. >> >> Not true, of course. The inside has forced airflow, not 'free air', >> and >> the only way you'd get 100K/W would be on a horizontal surface with >> no convection. No box is entirely horizontal surfaces, and no >> plausible >> scenario raises the case temperature by 100K without initiating >> convection. >> >> A wall-wart in a plastic case with circa 10 square inches of area >> has no difficulty in shedding 4W. Metal should dump heat faster. > > Well, back to the question at hand, internally the OP should mount the > hot parts to the box walls, and externally he needs a heatsink that > gets him to ~1C/W. > > Even better than getting the heat out of the box is... not putting so > much in in the first place. > > -- > Cheers, > James Arthur I think that is what I'm intending to do. I'll mount the higher powered stuff directly to the side walls, and probably use an aluminium backed PCB thermally coupled to the side walls too so that lower powered devices have a thermal path to the heatsinks. The AC-DC converter will be a baseplate cooled type mounted probably directly on one of the walls (or find some way of thermally connecting it to both walls with a thick bracket maybe and mount it horizonaally). Mark.
From: John Larkin on 8 Aug 2010 12:48 On Sun, 8 Aug 2010 00:44:12 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> wrote: >On Aug 7, 8:58�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Thu, 5 Aug 2010 15:54:40 -0700 (PDT), whit3rd <whit...(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. > >Not true, of course. The inside has forced airflow, not 'free air', >and >the only way you'd get 100K/W would be on a horizontal surface with >no convection. No box is entirely horizontal surfaces, and no >plausible >scenario raises the case temperature by 100K without initiating >convection. > >A wall-wart in a plastic case with circa 10 square inches of area >has no difficulty in shedding 4W. Metal should dump heat faster. Why would metal dump heat faster? Convection would be the same, radiation generally better for plastic. Warts I have handy; areas exclude the side that faces the wall. 7.5vdc 350 ma transformer type, about 9 sq inches exposed. 60% efficient? dissipates 2 watts maybe? Panasonic phone wart, 12vdc, 100 mA, claims 5 watts input, about 17 sq inches exposed, dissipates maybe 3 watts? 12 volts, 1 amp DC, "class 2", transformer type, 26 sq inches, could dissipate 10 watts operating? Powers speakers and runs pretty warm at zero volume. Very rough trend is 3 sq inches per watt dissipated, not far from your 2.5 ratio. So one watt per square inch sounds dicey. My ROT is 150K/W per square inch of exposed area, but that can vary enormously. Any others data points? John
From: legg on 8 Aug 2010 14:01
On Sat, 07 Aug 2010 20:16:49 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >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. Yes. 30 in^2 is a small package to dissipate 30W. If this were a conventional isolated converter, you'd not find much free air left inside this size of a package anyways. RL |