Getting heat out of a box
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From: Bill Sloman on 6 Aug 2010 08:11 On Aug 6, 12:56 am, "markp" <map.nos...(a)f2s.com> wrote: > Hi All, > > I'm looking for a company that could help solve some thermal management > issues. Basically 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 on the > outside, and a method of thermally bonding or piping any higher powered > components directly to to the box inside. I also need an AC-DC supply which > I think needs to be a baseplate cooled type and bond that directly to the > side of the box that has the heatsink. I'm unsure of size of box needed or > even whether this is a viable solution. > > I know it's a bit vague, but anyone know a company who could help > (preferably one with a vested interest in solving it, like an aluminium > extrusion box manufacturer)? Sounds like a job for heat-pipe. The heat generated in the box boils water (at sub-atmospheric pressure) which condenses on the inside of the heat-sink extrusion that forms the outside of the box, and wicks back to the heat source where it boils again. The heat transfer rates can be remarkably high, as long as water vapour is the only gas in the heat-pipe. We got ours from Isoterix of Woolmer, Northumberland, back in England in 1992. There are other people who make and sell heat-pipe assemblies. -- Bill Sloman, Nijmegen The
From: Tim Wescott on 6 Aug 2010 12:44 On 08/05/2010 07:56 AM, markp wrote: > Hi All, > > I'm looking for a company that could help solve some thermal management > issues. Basically 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 on the > outside, and a method of thermally bonding or piping any higher powered > components directly to to the box inside. I also need an AC-DC supply which > I think needs to be a baseplate cooled type and bond that directly to the > side of the box that has the heatsink. I'm unsure of size of box needed or > even whether this is a viable solution. > > I know it's a bit vague, but anyone know a company who could help > (preferably one with a vested interest in solving it, like an aluminium > extrusion box manufacturer)? What's your airflow outside the box? There's not much use getting the heat to the skin of the box if it doesn't have a place to go after that. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
From: John Larkin on 6 Aug 2010 13:05 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
From: Tim Wescott on 6 Aug 2010 13:13 On 08/05/2010 05:17 PM, markp wrote: > "whit3rd"<whit3rd(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. 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. There are nice tables of area vs. airflow vs. thermal resistance which are helpful, but figuring out the airflow is impractically difficult. I used to work in a group that had a very similar problem to yours, compounded by the fact that most of the parts inside the "box" were on a moving platform and thus _had_ to dissipate heat to the internal air. The mechanical group always did lots of fancy calculations _second_, but _first_ they populated the space with resistors, light bulbs, fans and thermocouples to get a baseline for their numbers. > The box is going to need to be big for the components, probably 12" x 4" x > 3" external dimensions or so. That's a good 120 square inches available if > you ignore the base and the two end plates. Those other free-air surfaces > can also have quite large surface area heatsinks on them too. > > Having said that, I would really like it to be rugged and not have any > moving parts in it at all. What do you think the chances I could keep all > internal temps below, say, 20 degrees above ambient if I were to bolt the > high powered stuff directly to the box walls inside? The 30W is spread out a > bit component wise, but 20W of that comes from the PSU. 20 degrees above ambient sounds pretty ambitious in itself. It depends significantly on airflow on the outside of the box -- convection makes a huge difference to cooling, you can't do this calculation without taking the outside of the box into account. Using convection inside the box is certainly convenient, but it multiplies the number of metal-to-air junctions by three -- instead of having the box outside to air junction, you have the component to air, the air to box inside, then the box outside to air. How much this will increase your overall thermal resistance depends heavily on how well stirred the air is inside the box and the airflow outside the box. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
From: Jim Thompson on 6 Aug 2010 13:28
On Fri, 06 Aug 2010 10:13:58 -0700, Tim Wescott <tim(a)seemywebsite.com> wrote: >On 08/05/2010 05:17 PM, markp wrote: >> "whit3rd"<whit3rd(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. 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. > [snip] Configure a "endless" tunnel (with internal fins), fan driven. Fins on the outside. And pray a lot ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Spice is like a sports car... Performance only as good as the person behind the wheel. |