Monday, 15 August 2022

Portable air conditioning

Portable air conditioning

It sounds like just the thing. A small, battery-powered box that makes a room cold in just a few minutes.

 Except, it doesn't.

A real air conditioner is a heat pump. It pumps heat from one place to another. It works by compressing a fluid (which causes the fluid to get hot), it move the hot fluid to another place, then it uses convection to cool the hot fluid; so the heat is dissipated outside the room. Then the fluid is decompressed and that makes it get cold, and that cools the room. You can see the effect for yourself using a bicycle tyre pump. Refrigerators and freezers work the same way.

These portable units, don't have anywhere outside to dump the heat. Anyone with any sort of physics knowledge would know right away that a "portable air conditioner" cannot work, just as a perpetual motion machine can't work. Of course, most people have very little knowledge of physics.

So what do these units do?

If you blow air over water, the water gradually evaporates, and this cools the area by using the latent heat of vaporisation. Your sweat works the same way; the sweat evaporating from your skin, cools your skin. Not by much, but it does work.

So you could get the same effect by letting a fan blow over a basin of water ... and that's pretty much what these units do. Or they use ice, which means that they're piggybacking on your freezer, and your freezer will have to work harder to make all that ice.

If it comes with a hose that you poke though a window, then it could actually be a portable air conditioner, in which case it will cost around £500. If it's an evaporative cooler, then it'll be more like £30 (£15 to £20 if you get something from eBay), and if you pay much more than this, you're overpaying. The Beldray costs £20. The Cooledge costs £65, far too much, but most of these evaporative cooling units are grossly overpriced - and over-claiming.

There's a downside to evaporative cooling that no-one mentions. The air it blows out is a little bit cooler, but it is also a bit more humid. And that means that your own personal cooling system (your sweat) won't work as well as it does with dry air.

 Yes - evaporative cooling works. But don't expect it to do very much





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