[freerpg]

Hey guys just wanted to find out a few things. I see there are re-usable instant heat packs. you click a disk shake it, it gets hot.... then boils it and it starts all over again
so my question is if I try something similar with an ice pack would it work?
ice packs are UREA and water, could I boil the water out? or would a vacuum be better to extract it?
I am assuming that due to the fact of adding these 2 compounds together it gets cold, then trying to separate them will make it hot.... unless they bond in some sort of reaction so that there is no water or urea left at all.... anyway any help on this would be greatly appreciated.
the end goal is to make reusable ice packs for beer coolers, possibly even reusable cold compresses.
I have tried the old citric acid and baking soda mixed with water, but that wasn't possible to replicate as it reacted giving new compounds.

[Borek]

In general recovering the urea (or whatever the dissolving compound is) is doable, but I don't see how to make it practical.

[freerpg]

would a vacuum pump work? or would boiling it be better?

[billnotgatez]

Any way you do your process, it would probably cost you more to revive an instant ice pack.
I might try vacuum first.

[vmelkon]

You would have to cut the bag open in a beaker. Wash the plastic bag and remove it and throw away. Dry the urea by heating it gently. You have to make a new water bag that is sealed. I do this using a metal sheet and I gently heat the sheet from under while the bag's edge is on top. The plastic melts and seals but it isn't "perfect" like what you get out of the factory.
You also need another bag. Put the water bag in and put the urea crystals in and seal again by melting the plastic.

****I'm not saying that I recycle my ice packs. I have done plastic bags for other purposes such as to seal a mercury container inside a plastic bag. Then I pressure tested the bag to make sure there are no leaks.

[Enthalpy]

In general recovering the urea (or whatever the dissolving compound is) is doable, but I don't see how to make it practical.

Neither do I. Drying is feasible, but a ready ice pack must then be reconditioned with several sealed compartments joinable at will, at this looks too difficult for the end user.

One more hurdle: mixing with water is quick for a fine powder, but the drying method may leave a solid block or a coarse powder. A parry would be a liquid, not a powder, to mix with water. Glycerine makes cold upon mixing but viscosity renders it difficult to mix; a few methyls more without alcohols should improve that, the same way as propanediol is thinner than glycol.

Example not checked:

----- EDIT OOPS -----

The original query is not about reusable cold packs but other uses like air conditioning.

Then, yes, do it. Perhaps drying at the end user if easy enough; I feel heat more convenient than vacuum. At some places you could even use sunheat, that would be a sales argument and it looks cheap. If not drying at the end user, you might distribute dry (powder or rather) liquid to your customers in the morning and collect it in the evening or rotate the stock at the next morning, just like ice blocks were sold before air conditioners existed.

A liquid is easier to handle by a machine: pump it to the roof, mix it with water... Cool the dry liquid by blown air first, then have a heat exchanger to cool the dry liquid by the used mixture sent to the hot dryer.

The cycle is less power-efficient than usual ones because evaporating water takes much energy while mixing absorbs just a bit, but sunheat is free.

[Intanjir]

For an ice pack the easiest thing for a consumer would be to regenerate by boiling water out and then refilling somehow with tap water. I imagine two bags, end to end, with a one way flap valve between them. Squeeze on the water bag to have it enter the urea bag. Alternatively just have some kind of resealable bag with urea in it and let the user add water to it from whatever source they like. A one-way valve is still a good idea to help avoid leaking urea water during use but this will require that the consumer has some means of applying pressure when refilling. You will of course need some method to allow the steam to escape when regenerating.

Besides using a liquid one might overcome the surface area problem by having the urea chamber be quite porous, ie a sponge.

[Enthalpy]

A beer cooler has to remove much heat. The equivalent of an ice pack would be big. Fortunately, better methods exist (already).

It involves a liquid (for instance water) in one chamber and a humidity absorber in an other. When both chambers communicate, absorbing vapour means the liquid evaporates continuously and gets cold. Evaporation absorbs more heat than mixing.

This is described somewhere on Wikipedia and is available commercially.

It must be possible to regenerate the humidity absorber and refill the liquid chamber. Did I read that this is already done too?

[Intanjir]

The humidity absorber would release heat as it captured fluid.
So this is a kind of heat pump, with hot and cold regions.
Regeneratable by applying a temperature difference.

If there was a means for liquid to directly flow between chambers then this could be operated continuously and would constitute a heat pipe.
https://en.wikipedia.org/wiki/Heat_pipe

You wouldn't want liquid to flow directly though since this would be a significant convection and, completely contrary to a heat pipe, we want to make a temperature difference not equalize one.

This can also be operated continuously using a temperature difference. In which case we have an Absorption Refrigerator or rather an Adsorption Refrigerator.
https://en.wikipedia.org/wiki/Absorption_refrigerator