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Comment by ladberg | Hacker News Reader

The author doesn't really explain it well because his explanation breaks some physics laws, so it doesn't "suck" your body heat away.

Instead, it lowers the amount of heat radiating onto you from your environment by replacing hot walls with cold walls, which will lower the temperature of your skin.

It's efficient specifically because it doesn't cool by convection (like every other cooler). This allows the designers to insulate it so they only have to offset temperature gained by incoming heat radiation when keeping it cool. That also means it won't cool the air, just the objects near it.

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Thermal radiative coupling is bidirectional. Sucking away is appropriate and important as you do actually have to displace the energy somewhere. It works the same way when sucking air.

A simpler non-radiating equivalent would be a reflective foil, or a polished metal - you could enclose yourself within it, get no radiating heat from environment, yet it wouldn't help cool you, because your radiation has no place to go.

I still feel like "sucking" is misleading because it doesn't change how much heat you radiate, only how much heat is radiated on to you. A better analogy would be like a curtains for heat.

I think that the analogy is used because people underestimate how much heat is lost to radiation. Human body radiates energy away at about 800W. It's not noticeable, because normally it is balanced by a similar flux of thermal radiation coming in from the environment. If the apparent radiative temperature of the environment is reduced it can indeed feel like it is sucking heat out.

You're off by about an order of magnitude there. The human body uses ~10MJ per day which is ~100W. This not only includes radiation losses but also convection and evaporation.

I am not. As I said this outgoing radiation is mostly balanced by incoming radiation. But it does illustrate the theoretical maximal capacity for radiative cooling, when stuck naked in interstellar space.

That's a bit misleading. It's like saying i can blow at a pressure of 16 psi in the void, but when I blow into a balloon whith the atmospheric pressure of the environnement, I can only reach 2 psi. The 16 psi is purely theorical.

That seems like a false analogy. In the case of thermal radiation you actually are emitting photons with a total power of 800W independent of your environment.

My bad, you're right.

> normally it is balanced by a similar flux of thermal radiation coming in from the environment

The parent comment wasn't talking about net power dissipation. 800W radiation would be the case if the environment was at 0K. Usually, the environment radiates about 800W back at you.

Wow, I'd never realised the total radiative transfer was that high! I guess this explains why those silver foil emergency blankets work so well...

> It works the same way when sucking air.

This is a good analogy because (being pedantic) you can't suck air either. You can only make a region of lower pressure, into which the higher-pressure air pushes itself.

May I then ask what can you suck, by your definition?

Good question! 'Sucking' here is a bit like 'centrifugal force' - it's a real thing, but only in the right frame of reference.

In the case of centrifugal force, it's a real thing in a rotating reference frame, but doesn't exist in an inertial reference frame (because in that frame the acceleration is towards the center, ie. centripetal).

In the case of suction, it's a real thing when your zero-pressure reference point is above 'absolute zero' pressure, such as when you're measuring gauge pressure. In this case you can have an area of 'negative pressure' which is 'sucking' the surrounding fluid into it.

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