Getting heat out of a box
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From: markp on 6 Aug 2010 16:28 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:cqio56pbb9ofsj818ecc1mk205j0fnodep(a)4ax.com... > Something like this? > > ftp://jjlarkin.lmi.net/L650_HS_proto.JPG > > ftp://jjlarkin.lmi.net/L650_HS_pretty.JPG > > John > Similar to that John, but with much larger heatsinks on both side of the box along the full length of the box (unless I can get away with a large heatsink on the top, but then there's going to be very little natural convection so probably inefficient as it'll be mostly by radiation). As a matter of interest, do you know how much power those hestsinks dissipate, or roughly the temperature rise internally? BTW the environment will just be still air externally, I can't rely on any air movement other than natural convection. Mark.
From: dagmargoodboat on 6 Aug 2010 17:08 On Aug 6, 3:28 pm, "markp" <map.nos...(a)f2s.com> wrote: > "John Larkin" <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in message > > news:cqio56pbb9ofsj818ecc1mk205j0fnodep(a)4ax.com... > > > Something like this? > > >ftp://jjlarkin.lmi.net/L650_HS_proto.JPG > > >ftp://jjlarkin.lmi.net/L650_HS_pretty.JPG > > > John > > Similar to that John, but with much larger heatsinks on both side of the box > along the full length of the box (unless I can get away with a large > heatsink on the top, but then there's going to be very little natural > convection so probably inefficient as it'll be mostly by radiation). > > As a matter of interest, do you know how much power those hestsinks > dissipate, or roughly the temperature rise internally? > > BTW the environment will just be still air externally, I can't rely on any > air movement other than natural convection. > > Mark. Why not make less heat? You said your 100W AC-DC converter is dissipating 20W. That's 80% efficient. 90% under full-load (when dissipation is highest) isn't unreasonable. You're using 10W--could you use less? Etc. -- Cheers, James Arthur
From: Ian Iveson on 6 Aug 2010 18:29 Jim Thompson wrote: >>Fins inside would certainly help. I'm rusty at my thermo, >>but the way >>you'd calculate this would be to figure out the thermal >>resistance for >>each metal to air interface (component to air, air to box >>inside, box >>outside to air) and add them up -- you'll find that the >>thermal >>resistance of the box's aluminum itself is insignificant, >>unless you're >>really aggressive about fins. > Configure a "endless" tunnel (with internal fins), fan > driven. Fins > on the outside. And pray a lot ;-) Or thermally bond the culprit components to the top, attach the potential victims to the bottom, stop air from circulating inside, and blow on the outside if necessary. That'd also relieve the inside from the heat generated by the fan. Depends on how effectively the hot bits can be heatsunk to the top extrusion, and how much the circuit is open to layout changes, but generally it would be better to isolate the heat generators from the bits that don't like heat, no? Just to be contrary. Different strategies, horribly complicated situation for analysis. Experienced thermal engineers are probably making good money. Ian
From: Grant on 6 Aug 2010 20:04 On Fri, 06 Aug 2010 10:50:54 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >Something like this? > >ftp://jjlarkin.lmi.net/L650_HS_proto.JPG > >ftp://jjlarkin.lmi.net/L650_HS_pretty.JPG > >John On a smaller scale, this photo is of the inside of box bits from a ~150W power converter. The mains side power switch heat spreader to the left, the other heat spreader is for the secondary TO247 dual schottky rectifier. Insulation is that non-woven 0.4mm thick paper like stuff. The aluminium plates are 2.5mm thick. One on left measures 86 x 56mm. A screw each side of the power device applies pressure through an elastic pad (blue, dunno what it's made of) and PCB to the TO220 or TO247 case onto the heat spreader. http://grrr.id.au/image/heat-spreader-and-insulator.jpg The box was lightly finned extruded channel with sheet metal ends. http://grrr.id.au/image/heat-spreader-and-insulator-outside.jpg No fan, sealed box but not airtight. Made in Taiwan. Grant.
From: Nunya on 6 Aug 2010 22:23
On Aug 6, 10:57 am, Tim Wescott <t...(a)seemywebsite.com> wrote: > On 08/06/2010 10:44 AM, Nunya wrote: > > > > > On Aug 6, 10:13 am, Tim Wescott<t...(a)seemywebsite.com> wrote: > >> On 08/05/2010 05:17 PM, markp wrote: > > >>> "whit3rd"<whit...(a)gmail.com> wrote in message > >>>news:db6a66cd-1a00-45f7-a7c3-8a6b80e2d110(a)d17g2000yqb.googlegroups.com.... > >>> 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? > > >>> Do you mean and area of the sidewalls of the box externally> 30 square > >>> inches (that's equavalent to 5.48" x 5.48" for ease of visualisation), with > >>> an internal fan, will get rid of 30W? What's the max temp rise above ambient > >>> internally do you think, and would I need to put heatsink fins inside the > >>> box as well as outside? > > >> Fins inside would certainly help. > > > Total bullshit. He said no moving air inside. > > Please quote a post. The one I can find said that he'd 'really prefer' > no moving _parts_. Which is understandable -- fans wear out. It'd be > interesting to do an MTTF calculation for the assembly with > electrolytics (and elevated temperatures) but no fans, and with > electrolytics and fans (and hence lower temperatures). You have never heard of hermetically sealed, mil EL caps? There are other ways of capacitive storage as well. > > All radiation must be external. > > > Remember the SR-71B? It dropped 50°F when they went from > > bare skin to matte black IR radiative paint... on the OUTSIDE > > of the airframe. Even though the skin was the source of the heat. > > Yes, that's basic thermo. Ahhh... basic... internal radiation coupled to non-moving or slowly moving internal air that is already hot is going to "work"? Hahahaha... You cite! > > Conduction cooling relies on operating AT elevated temps, > > Passive heat flow itself requires a temperature differential. So _any_ > cooling "relies" on operating at elevated temperatures. You missed the point. Try *more* elevated than convection coupled methods. > > and designing to reliably do so within a window of > > EXTERNALLY radiated energy acceptability. So, it needs to > > be mounted onto a conduction plate externally, or operate > > at a temp where it can be free standing and still run. > > Again, you assume radiative or conductive cooling externally, when > (unless I missed a post) the OP has not said anything at all about the > environment he's putting his electronics into. To remove heat, there MUST be external coupling. Either to the air or to an attachment (conduction) plate or both (ideal). > Should he take a Colman > cooler, aluminize the inside, put his electronics into that, and shut > the lid, then he won't get good cooling no matter what he does with the > _inside_ of the box. You obviously do not know much about conduction cooling. One takes all of the heat generating internal sources and couples them to a LARGE thermal MASS INSIDE the case, and couples that to that same integrated thermal mass that is the outside of the case. This should result in a fairly homogenous temperature throughout with a few slight variations near the actual source elements, which increases as the designer falls below the minimum mass spec'd for conduction cooling efficiently. A well done design may or may not benefit from an external matte black finish, and may or may not benefit from the inclusion of some certain sized "fin array" incorporated into the case exterior. Metal conducts better than air, so an internal fin array and trying to use air internally to perform cooling as you stated earlier, was a bad method. Using METAL MASS to CONDUCT the internally generated heat into the mass, and then conductively pass it to the outside of the mass, and then to the conduction plate that you mount it onto. THAT MAXIMIZES the amountof internally generated heat that you can bring to the outside to dispense with it in whatever way you can. So conduction cooling IS the best INTERNAL management method, and the external design is all that one would need to fancy with based on space considerations, etc. Then, one presumes that the outside temp is lower than the heat generating elements of your product If it isn't the discussion is moot for conduction or air operated internals. He DID specifiy a sealed enclosure in both cases, so AIR inside would have to STAY inside. Something I fear you forgot to notice as well. > > The hardest part is usually trying to comply with the upper > > end of any claimed thermal operating envelope. > > True. Unless you don't pay any attention to the lower end of the > envelope, in which case you'll be getting calls from your first Alaskan > customer come winter. That depends on how you bottle it up, and what parts might change operation under cold. Failure under cold is easily managed. Don't let it happen. I did suggest conformal coating, but full encapsulation would work as well, just be far less servicable. There are a few encapsulants that are 'approved' for 'space use' in the US. For HV circuitry, there is only one. CONAP. http://www.cytec.com/conap/index.php That stuff will stop a sharp point, thick blade knife with a forceful blow, and I think it would even stop bullets. The knife tip only penetrates about a quarter inch. Anyway, it does NOT gas,and it does NOT detach from a surface very easily once it has made adhesion and cure. |