Fwd: OT: Items in a metal box are cooler?

Oh yeah, in fact you can put a metal box inside a metal box and get even more cooling. This can be continued ad infinitum. But you can't get down to absolute zero, that would violate laws of thermodynamics.

No, of course not.

Jeroen Belleman

Human senses are amazing, and one can pretty well estimate the thermal conductivity of a surface by touching it. Metal feels cooler than paper, even when both are settled at ambient.

Fingers are amazing at sensing textures, too.

Of course a metal box doesn't make itself cooler than a plastic one. That would violate COE and the box would get all drippy.

There are millions of people that you can do battle with.

But if you had a big enough chunk of metal it would solve all our energy worries. ;)

Cheers

Phil Hobbs

What "FACT" is this? Under what set of conditions do you believe it will be true?

In the general case, if a metal object is sitting in a room where the room temperature is stable, the metal object will come into thermal equilibrium with the rest of the room. They'll be at the same temperature. Heat energy will enter, and leave the metal object at the same rate. (If you want them to have different temperatures, you have to add work to the system to supply heat, or to "pump" it away somehow.)

If you touch the object then, it can quite easily _feel_ cooler than whatever it's sitting on, or cooler than the room air, even though they're at the same temperature. This has to do with the fact that your body's temperature is higher than the metal, and the metal will conduct energy away from you body faster than the air (or the table). As this happens, the metal will warm up to a higher temperature than the ambient, and if you've put something inside it, that object will also heat up.

Now, there are definitely conditions under which a metal object can _temporarily_ be cooler or hotter than the ambient air. For example, if you place a dark-colored metal object outdoors in the afternoon, and wait until the sun goes down and the sky becomes dark, the metal will cool down faster than the air does. This happens because it's an efficient radiator of infra-red, and radiates its internal heat away into the (dark) sky quite efficiently. The metal can drop well below the air temperature, and if it cools enough, water from the air will condense on its surface as dew.

The opposite thing happens in the morning, when the sun comes up - the metal will absorb sunlight efficiently and will heat up faster than the air does.

So, you can make a refrigerator of sorts - a well-insulated box with a set of heat-radiator fins on it. During the day, keep the box well-covered and insulated. When the sun goes down, remove the insulating cover, and let heat radiate away into the dark sky. Won't work as well if the sky is cloudy at night, though.

This trick doesn't require metal, of course. Solar pool-heating systems can be used to cool a pool by running water through the (plastic) panels at night - this is sometimes done in hot climates to provide a cooler pool during the day.

Polished metals, especially copper and brass, are excellent reflectors at thermal wavelengths. Plastics are nearly black in the thermal IR. So if there is a radiant heat source around, the metal will feel cooler than some other things.

Unless you paint it.

No, mainly it's cooler than your fingertips and a good conductor of heat.

If you want to fail attempt an assault on the laws of thermodynamics.

Capital letters (or shouting) will not turn a falsehood into a truth.

If you count a normal refrigerator as "similar": it's a metal box that's cooler on the inside, then "yes", something similar is reasonable.

This wrong idea is not new.

Putting a lie in capital letters does not make it true. Metals feel colder to the touch because they have a higher specific heat capacity.

They also burn you more effectively when hot like an oven shelf.

You mean those closed minded people that understand thermodynamics?

I have a metal box in my kitchen and the temperature inside is definitely cooler than ambient. I use it for storing things other than medicine.

Amazon has a nice dual thermocouple thing...

which would let you verify the observation.

Which is impossible, of course. It would be a perpetual motion machine.

A big metal box could add some thermal mass to smooth a heat spike, but not for long.

If the metal is the radiant heat source, do I want to paint that metal black, or leave it bare, for cooling? I'm thinking of a motorcycle engine's air-cooled cylinder and head.

I've read inconclusive blather about black radiating more heat, but the paint also being an insulator.

If you put a metal box inside a metal box, it would get even colder! Use that to store ice cream.

Black anodize would be good to increase radiation from aluminum.

Things that are black in the thermal IR spectrum are good radiation emitters and good absorbers, high emissivity. Shiny metals, opposite.

Paint is almost black in the far IR, but as you note is also a thermal insulator. It will make convective air cooling worse.

Anodize is very thin and both radiates and conducts well.

"Black" here means black at thermal wavelengths, not necessarily visually black. Water is black at these wavelengths.

Thin black plastic garbage bags are nearly transparent in the thermal IR. That can be handy.

does radiation really make a significant difference at normal heatsink temperatures?

The radiation power goes as the 4th power of the temp, so it starts to matter as a heat sink gets pretty hot.

Try this:

I think that's right.

Engines mostly cool by conduction, not radiation. So the color is not very significant.