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I often carry around a couple of reusable hand warmers when hiking or just around the outdoors when it's cold. They don't take up much space, they're easy to reuse (just boil) and they provide a fair bit of heat for a while.

However, try as I might I can't find any equivalent reusable packs that work the other way - i.e. they set of a reaction that draws in heat and makes them cold for a while, and then (I'd expect) you could put them in the freezer for a while to reverse the process.

I wouldn't really use them for hands, but they could come in useful for cooling you down generally - placing at the back of your neck for instance.

Do such things exist? Technically I don't see why it's not possible but I haven't seen any around at all.

EDIT: After a bit of looking around I stumbled across these - perhaps they do exist after all! Has anyone used them? It looks like they're geared more towards medicine than general use but if the effect's the same that's fine by me.

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@MatBanik Well I didn't specifically mean for hands - back of the neck for instance cools me off quickly, and something that portably reaches sub zero temperatures does it a lot more quickly and reliably than perspiration! –  berry120 Feb 4 '12 at 16:27
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You may find this interesting reading, though I doubt it can be practically applied: Icy Ball –  Mr.Wizard May 12 '12 at 7:09
    
We have little packs like this amzn.com/B000P3GXLM in the freezer for when the kids get bonked on the head. May also work for you. –  Brad Patton Dec 19 '12 at 18:39
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4 Answers

up vote 5 down vote accepted

Instant ice packs, like this one, which are usually used for treating sprains and strains sound a bit like what you are looking for. They use a chemical reaction to cool, but, they are single use only.

Such instant ice packs work by mixing ammonium nitrate with water. The hydration of ammonium nitrate is endothermic (absorbs heat). Perhaps unintuitively, you would need to apply more heat to dehydrate the ammonium nitrate and keep it separate from the moisture you are removing to be able to reuse. I guess this would be difficult to achieve in a simple reusable pack.

Personally, I like using my buff dipped a stream or river and worn round my neck to cool down.

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Thanks for the answer - does the pack I linked to in the edit look reputable to you? –  berry120 Feb 5 '12 at 18:19
    
@berry120 I can't find any detailed information on them. If I had to guess they are the medical equivalent of those cool packs you keep in your freezer then use in your picnic hamper to keep your sarnies cool. Manufacturer's product information sheet. –  Graham Feb 5 '12 at 20:42
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During the 1996 Summer Olympics, there were vendors all over Atlanta selling these:

Cool bandannas.

I think they're filled with polymer beads similar to the ones found inside diapers that hold moisture once you get them wet. I wouldn't say it kept me completely cool, but-- being "completely" cool in 96-degree heat is a fantasy, unless you're in air conditioning. I would recommend these-- or something like them-- as all you need to do is dunk them in a cold creek, and you're good to go.

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Reusable heaters rely on heat of crystallization of sodium acetate, that is, heat you obtain by crystallization of a solution. Coolers rely on heat of solvation, which is the heat that gets absorbed (in this case) when ammonium nitrate is dissolved in water. You cannot make an easily reusable hand cooler, but technically the system can be restored to its original form if you evaporate the water and recover the ammonium nitrate.

My suggestion is to find a small amount of ammonium nitrate and put it into a metal can with a reusable cap on top. Add water in the same amount (to make it sort of porridge-like) if you want cold. Be careful, because it can get really cold. Once done, put the can on a warm surface and let water go away slowly. Ambient temperature evaporation is safer (and you may get very nice crystals). Be careful. Ammonium nitrate can become dangerous if heated too much. It is commonly used as a fertilizer, but has a huge record of explosive behavior. For this reason, don't buy too much or you may end up on the FBI list.

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When water evaporates, it cools the surrounding surface. This is primarily why we sweat - our body is using this process to cool itself down.

So, a buff (or mitts) dipped in water can act as pretty effective coolers as they dry out. To 'recharge' simply dunk them in water again...

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