Diecast boxes heat dissipation

Absolutely, ludicrously wrong. Raving madman wrong.

And, of course, AlwaysWrong.

John

Rough surface increases turbulence in air flow? Just guessing.

--
Paul Hovnanian  paul@hovnanian.com
----------------------------------------------------------------------
Have gnu, will travel.

It would depend on the temperature. If the heatsink is fairly hot, the tape will increase thermal emissivity (deep IR radiation) a lot. That may make up for the small addition of thermal resistance caused by the tape itself.

From the surface of the heat sink, there's air, and still air has a lot of thermal resistance. A layer of masking tape would be in series with the air, and wouldn't increase net thermal resistance much. So the radiation advantage can win.

There must be a reasonably easy way to test this.

John

Excellent treatment, Don, but remember -- you can't win an argument with an ignorant person.

Cheers, John

get

g

"There must be a reasonably easy way to test this."

I was trying to measure heat flow from a through hole resistor... how much goes down the leads vs into the air. The problem is that once you get a few degrees of temperature difference you start to get convection of the air... And that seems to dominate the heat flow. It's hard to disentangle all the heat flow paths.

George H.

Near room temperature, the effective thermal conductance of a surface of unit emissivity radiating into an isothermal environment is equivalent to about 6 mm of still air--i.e. it's about 4 W/m**2/K. (You compute this by differentiating the Stefan-Boltzmann formula with respect to T.) This isn't very big.

Masking tape is about 5 mils (125 um) thick. Assuming that it acts like plastic, its thermal conductivity is around 0.1 W/m/K, so the layer has a thermal conductance of (0.1/1.25e-4)=800 W/m**2/K.

If it makes the surface emissivity 0.9 instead of 0.2, say, then by covering the surface with masking tape, you gain about 0.7 times the full emissivity result, or 2.8 W/m**2/K worth of radiative thermal conductance. For shiny metal in sufficiently still air, on a flat or convex surface, this might well be a win. (Of course it won't be as good as anodizing or paint.)

Interestingly, for a given pipe diameter, there is a minimum thickness for insulation to be useful--for sufficiently thin insulation, the increase in surface area and emissivity due to wrapping the pipe outweighs the increased thermal resistance.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

Aluminum, especially shiny aluminum, has very, very low emissivity. Putting the tape on it _conducts_ the heat away through the adhesive and the backing, which has much better emissivity, thereby dissipating more heat by radiation. I don't know about the difference in convective cooling or forced-air.

Hope This Helps! Rich

"Never get in an argument with a fool - people might not be able to tell the difference." -- "Unknown"

Cheers! Rich

Is GlimGlam just another nym for the baboon?

Thanks, Rich

I once worked at a plastics extruder. They had a pyrometer, which the Mexican primates called "checkolore temperatura." I made an aluminum swaging die which needed to be hot to work - the pyrometer, of course, showed nothing, so I spit on the die and rubbed some cigarette ashes into my spit, and the checkolore temperatura (In English, "pyrometer") gave a much more accurate reading.

They had a propane torch with a trigger and piezo ignitor; I once asked one of the Mexicans, "Where's la torcha automatica?" and they laughed.

I got them back though - the swaging die I designed was a two-piece thing, with a pneumatic cylinder that opened and closed it (the swaged pipe also had a "chee-chee", so the die had to be separated to get the pipe out.)

We were training the primates, and the boss was saying something like, "Keep your fingers out of here!" so I held up my hand with my middle and ring finger bent over, and said, "Yeah, cuatro cervezas por favor!" and they all laughed.

Yes, I have actually told bilingual jokes. I'm so proud I could poop!

Cheers! Rich

Masking tape radiates thermal IR a lot better than bare aluminum does. This outweighs the thermal resistance of a layer of masking tape.

I already posted calculations of thermal resistance in the case of a typical paint instead of masking tape.

--
 - Don Klipstein (don@misty.com)

A 299 K blackbody radiator surrounded by a 298 K blackbody environment radiates 6 W/m^2 more than it is receiving.

A 323 K blackbody radiator surrounded by a 298 K blackbody environment radiates 170 W/m^2 more than it receives. That works out to 6.8 W/m^2-K, although the effect is nonlinear with temperature.

Most plastics are around or a little over .2 W/m-K. Last night, I found a cite for a typical paint coating having heat conductivity of .15-.17 W/m-K.

So make that 1600 W/m^2-K.

Make that 4.2 W/m^2/K at 1 degree rise above a 25 C ambient, 4.76 W/m^2-K for 25 C rise above a 25 C ambient.

The surface, with surface temperature unchanged at 1 degree above ambient, is getting rid of 4.2 W/m^2 more heat than before, while the metal underneath gets its temperature increased by .00263 degree (presumably by increase of heat input by 4.2 W/m^2 to maintain the radiating surface temperature after improving its emissivity).

If heat input is unchanged, then the surface temperature will drop from

1 degree above ambient by increasing its emissivity. Convection and conduction still have to be figured in for how much a square meter of surface 1 degree above ambient gets rid of via those when the surface is 1 degree warmer than the ambient. However, for unfinned bare metal heatsink without forced air cooling warmed to 1 degree C above ambient, I do expect increasing its emissivity by .7 will reduce its temperature by more than .00263 degree.

--
 - Don Klipstein (don@misty.com)

The t^4 thing changes the tradeoff as the heatsink gets hotter, at least until the masking tape gets gooey and falls off.

Kapton tape is a better deal: about half as thick and harder to fry.

John

If by "the baboon" you mean Nymbecile, yes.

It becomes more emissive, BUT the losses in the interface and migration rate would make the part dissipate the heat source it is sinking SLOWER.

This real easy. Take two one pound blocks of AL in equal shapes.

Tape one up. Heat both in the same oven where they sit at the oven set point after it is arrived at for ten minutes, then remove both and examine with an IR imager.

The taped unit will "look" hotter. Look again in a half hour and I'd bet the the taped unit will retain more residual heat than the bare unit will. Case closed.

I posted it.

Back to my 100 Watt light bulb argument.

Perfect proof. Flash the nichrome for twenty seconds inside the box. Measure the box air temp. Flash the light bulb for twenty seconds inside the box. Measure the box air temp.

Which one will be hotter and why. Same question after one minute and same question each minute after that.

How long do you think it will take before the 100W bulb heats the box air to the same level as the nichrome?

The real question is Why is it not an equal rise time curve, despite being at an equal wattage?

Never mouth off like one either. People will know for sure that you are the fool you are mouthing off like, you mouthy fucktard..

The GriseTard has had a bur under his saddle since the bar fight pic he posted, which I commented on.