Off-grid food freezing – heat exchanger suggestions?

[InterBlog]

Another cross-post here, because there are many people on this forum with engineering skills.

To make an 8-year-long story short, I carry frozen food into the backcountry using the method shown below (original thread here). I freeze vacuum-packed food pouches into monolithic ice blocks formed in a mold that we built to conform to our Yeti cooler which rides on our Class B’s rear hitch carrier.

This is one of the best off-grid mods I have ever developed, as it revolutionized our food logistics. We eat fabulously well in the wilderness, miles from any food source.

I am now aspiring to improve this method even further. I think I can extend the life of the ice blocks if I can find a way to lower their temperature while off-grid, without needing machinery or electricity.

For this idea to work, I cannot simply embed (for instance) a segment of plain copper pipe into the ice blocks. The object must function like a heat exchanger, so it needs to have fins, or plates, or some structure to that effect.

Also, whatever its configuration, the channel(s) into which the nitrogen is poured cannot be too narrow, because of the surface-area-to-volume issue and the boil-off that will occur (I worked with LN2 in a laboratory setting for years, so I know how it behaves). I have to be able to get the LN2 in there such that it sits long enough to suck heat out of the ice blocks before dissipating.

Trouble is, I cannot find heat exchanger material similar to that shown. The only sources I've identified are Asian mass market wholesalers such as Alibaba and the like, and as a consumer, I can't procure from sellers of that type.

But maybe I am not thinking widely enough. Ideally, the object identified to serve this purpose will be aluminum, but it doesn't necessarily need to look exactly like the examples shown below.

Does anyone have any ideas on what I might procure for this purpose? Thanks.

[rvsprinterguy]

Fin tube is a heat-exchanger pipe, which might be what you're looking for. Since you live in refinery-abundant territory, I'll bet there's a stocking distributor or two in your immediate area. I'm from Tulsa, and I know such is the case for that town.

[hbn7hj]

Obviously you have ways of procuring LN2 in small quantities that we don’t have!

The only other option is a compressor freezer and solar panels.

Certainly an interesting concept.

[MsNomer]

I'm thinking, too, it wouldn’t take that much power to keep a solid frozen block frozen.

[rvsprinterguy]

Interesting topic. Apparently, sometimes you can buy LN2 at tire shops (I know N2 is what Costco fills tires with, then they put on those cool green valve caps). Industrial gas retailers like Airgas and Praxair are said to be potential sources, but bring your own dewar.

[booster]

It has been a long time since school, but IIRC liquid nitrogen behaves somewhat like water in that the major amount of heat absorbed is at the phase change from liquid to gas. If that is truly the case, you would want/need the liquid to vaporize in the heat exchanger so it would absorb the most heat. Perhaps a drip feed system a super insulated box on top of the heater exchanger set vertially????


Be aware that thermal shock on the materials may cause them to fail. Extruded aluminum often is full of stresses in the first place so that could be an issue.

[InterBlog]

Originally Posted by hbn7hj

Obviously you have ways of procuring LN2 in small quantities that we don’t have!

The only other option is a compressor freezer and solar panels.

Certainly an interesting concept.

I believe Airgas will sell LN2 to anyone who has a properly-rated container. I think it's about a buck a liter. I haven't looked into it yet because, without a heat exchanger, I can't use it anyway.

Yes, solar and compressor is my Plan B, and I have a separate thread describing how I'd like to configure a tiny off-grid trailer. The problem is that, with global shortages of everything, I may not be able to obtain a trailer before the end of the summer. Even if we do, we may not be able to obtain all the parts for a supplemental solar system.

And even if it gets built, we will probably carry a small cheap residential-grade freezer, and I'm not sure I want to run the compressor while underway. It might be better to Yeti the ice blocks and then transfer them at the destination where the freezer compressor can run without getting pounded around on the roads.

[InterBlog]

Originally Posted by booster

It has been a long time since school, but IIRC liquid nitrogen behaves somewhat like water in that the major amount of heat absorbed is at the phase change from liquid to gas. If that is truly the case, you would want/need the liquid to vaporize in the heat exchanger so it would absorb the most heat. Perhaps a drip feed system a super insulated box on top of the heater exchanger set vertially????


Be aware that thermal shock on the materials may cause them to fail. Extruded aluminum often is full of stresses in the first place so that could be an issue.

Good points. My engineer husband brought up the phase change issue, particularly in the context of the ice itself.

In trying to apply the K.I.S.S. principle, he also asked, "Well, what if you just leave a gap around each ice block and pour the LN2 into the gap? That would give you a lot of surface area."

I said, "Say goodbye to the Yeti, because it will shatter." That's the kind of thing one learns after handling LN2 occupationally.

What I am proposing here is crude, quick, and dirty. It may or may not contribute significant benefits toward the goal.

Incidentally, we tried using dry ice one year. Total failure because heat exchange potential turned out to be so poor - it sublimated without much impact. They sell it in grocery stores all day long for freezing purposes, but it doesn't work in my application.

[InterBlog]

Originally Posted by rvsprinterguy

Fin tube is a heat-exchanger pipe, which might be what you're looking for. Since you live in refinery-abundant territory, I'll bet there's a stocking distributor or two in your immediate area. I'm from Tulsa, and I know such is the case for that town.

^^ Thank you for this excellent suggestion. I added the term "fin tube" to my Google search, and discovered a manufacturer about 50 miles from me. I may be able to buy a couple of short larger-diameter samples from them.