[Brox]

Hello!

I've been tasked with creating an on demand cold pack that can stay cold (between 32-55 F) for at least 60 minutes. There are volume constraints where the pack must be ~ 7x12 cm at the most.

I've been experimenting with various amounts of ammonium nitrates e and created a small 2 oz "core" of PCM rated to phase change at 42 F. The core didn't freeze (did not get cold enough) but froze with 162 F PCM and stayed that way for an hour. Only problem is that 62 F isn't cold enough.

I can't seem to find any info on using salt water or alcohol for the reaction to lower the temp. I'm open to urea as well. Really, any safe substance that can fit the size, time, and temp requirements.

Any suggestions? TIA!

[Borek]

cold pack that can stay cold (between 32-55 F) for at least 60 minutes. There are volume constraints where the pack must be ~ 7x12 cm at the most.

Are you sure it is doable?

Without stating what is the operating temperature it is not even possible to evaluate how much heat the cold pack should be able to absorb, that would give some estimates on the thermodynamics of the idea. Size limits the mass, amount of heat over the mass gives specific heat capacity, specific heat capacity can't take any value but is rather limited to a range of values.

[Brox]

Ok, good point. The operating temp will be at least 99-101 F. One full side of the pack would be compressed on a person exercising so core temps would rise to ~101 F and ambient temps will be weather based but typically between 80-100 F.

Right now I have a standard 70 g cold pack made from super absorbent polymer and water. That's works very well when frozen. However I need an instant option.

Let's rephrase: currently, I'm getting 20 mins of effect from a standard ammonium nitrate and water pack. Total is ~120 ml in volume. The temp drops to ~40 F and slowly rises over time losing a couple degrees per min.

If all things equal, is there a different solution that gets colder than the standard ammonium nitrate and water pack?

[Brox]

Well, I figured it out myself using barium hydroxide and ammonium nitrate. Temps drop well below freezing with a lot less mass than a standard instant cold pack.

Just so I know, was my initial question not answerable or not in the spirit of this forum? Lots read it and no answers

[Borek]

More likely that nobody didn't know the answer good enough, so they didn't post.