-40F mod for small freezer?

Hi there,
I am hoping to find out whether it is possible to modify a normal household freezer in such a way that it will be able to cool a couple gallons of water, spread out in trays, from room temperature to -40 within a few hours. Is this feasible? I was considering changing the coolant to propane, as it seems happier to go colder, then maybe changing the capillary tube? Any input would be greatly appreciated. I don't have a freezer picked out yet, since I don't know what I need to accomplish this.

Not going to happen. Domestic insulation is way too thin. Compressor/ evaporator and condenser are not sized for 40 below 0. The best you could achieve with the thermostat bridged out is -26 C (-15 F)

okay, what if I took the cooling unit from it and put it into a different, heavily insulated casing? Could it be done then?

Nope. 6" insulation would be a starting point though. Domestic freezers run small fractional HP compressors on R134a or R600. ( In OZ.) Neither compressor or gas type is suitable for -40 C or F.
Conventional commercial kits aren't designed to run -40 either.
Why do you need -40?

Well, I have been building a lyophilization chamber, and apparently very low temperatures assist in avoiding meltback and establishing the crystalline structure of the frozen solution (or something like that). So what I need to end up with is a decent-sized chamber which I can get down to -40 C or F (same thing). I was really hoping to do this without a very expensive cascade refrigeration unit. It will be thickly insulated. Is there any combination of tubes, restrictors, and refrigerants that could reach those temperatures powered by a household compressor?
Thanks

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Originally Posted by Thisisausername

Well, I have been building a lyophilization chamber, and apparently very low temperatures assist in avoiding meltback and establishing the crystalline structure of the frozen solution (or something like that). So what I need to end up with is a decent-sized chamber which I can get down to -40 C or F (same thing). I was really hoping to do this without a very expensive cascade refrigeration unit. It will be thickly insulated. Is there any combination of tubes, restrictors, and refrigerants that could reach those temperatures powered by a household compressor?
Thanks

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I picked up a cheap R404a condensing unit on clearance from E-bay. Coupled with a very, very well insulated eski (cooler, chilly bin, whatever it is called in your part of the world) and a Propane (R290) charge I can get -40C.

I use a long coil of 1/4" copper as the evaporator submerged in a couple of litres of Ethanol.

You'd be very hard pushed to turn a domestic fridge/freezer to get it anywhere near that low because the refrigerants available are just not suited to temperatures that low.

Have you thought about a couple of kilo's of dry ice to experiment with? That'll theoretically get you into the -70C's. If you can build a well insulated cabinet that holds dry ice for a sufficiently long period, you'll be able to build a refrigeration unit to go in there. Don't seal it or as the dry ice sublimates it'll pressurize and explode - neat party trick but it can ruin your day if you're not expecting it.

To test cabinet insulation, the cheap and easy way is to freeze a defined qty of water (I do it in 1kg lots), put it in the cabinet with a remote thermometer and put it outside in the warm. As soon as the temperature starts to rise from 0C you know the ice is gone. You can calculate the heat infiltration by working out how how long it took the ice to melt knowing the latent heat of fusion for water. It' s a bit agricultural, but as long as you pre-chill the inside of the cabinet with ice before you to your test block it can be surprisingly accurate.

To experiment without spending lots of £ or $, the easiest way would be, as Brad says above, Dry Ice or liquid Nitrogen (via a suitable regulator) depending on what is cheapest/most available in your neck of the woods.

As an example, a size V bottle of liquid Nitrogen will keep a 1.5m4, well insulated, freezer at -80C for about 24hrs.

Just remember, if you do go for one of these options MAKE SURE YOU ARE IN A WELL VENTILATED AREA as both Dry Ice and Liquid Nitrogen absorbs Oxygen and are known to suffocate unaware operators.

:cool:

Thanks for the input. You may be interested to know, I actually already did make a sort of prototype which used a dry ice and ethanol bath to work! It did not function, due to manufacturing problems (the materials I was using did not stand up to the cold). I would like to end up with a working unit, rather than an experiment, that I can use to store my own foods in quantity. Unfortunately, the process would use a great deal of dry ice each run, which gets too expensive very quickly, so I have not put the effort into rebuilding the dry ice bath unit. Anyway, I'd love to not deal with supercooled methanol again. Nasty stuff, that! I do so wish Liquid nitrogen equipment was available, but it is not.

@Brad, So what constitutes very very well insulated? what kind of insulation did you use? I have a small freezer unit being donated to me soon. I was sort of intending to remove the guts, charge them with propane, and see how cold I could get them.

The insulation test is very interesting, thanks!

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Originally Posted by Thisisausername

@Brad, So what constitutes very very well insulated? what kind of insulation did you use?

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I had some (quite a lot actually) leftover high density polyurethane board insulation from making an ice box on a boat, so I put a small cheap eski inside a large expensive eski and filled the area between them with high density polyurethane.

I know a lot of the solar power / efficiency people get domestic fridge/freezer units and glue much thicker poly insulation on the outside. Of course this only works in those units that have a remote condenser rather than built into the skin, and you'll want to disable the seal heater if it has one (one of my fridges pumps the discharge gas around a pipe just under the sealing surface to keep it defrosted).

If you are really keen, go and talk to people who do custom fridges for caravans or boats. A mate of mine made an ice box years ago out of aluminum and had a pro blow it full of urethane insulation. The insulation was about 8" thick all around, but it held dry ice for 21 days while they went 4wd'ing.

There is *loads* of information out there on refrigerator design and lots of is freely available. There are also some university studies on insulation and efficiency. Unfortunately I don't save the links when I find this stuff, I just download it and read it.

Experimenting is fun. Dry ice is a cheap (ish) way to test out theories. My biggest concern about hacking up a domestic fridge/freezer is by the time you get the evaporator below about -20C your refrigerant mass flow will be so low I'd be very worried about cooking the compressor. You might want to put a fan blowing across the compressor and condenser for your tests as you are going to need all the help you can get.

Also, if you are going to charge it with propane you'll likely be using a compressor rated for either R600a or R134a. Both have significantly lower pressures than propane, so unless you keep your condensing temperature down (thus the fan) you run a real risk of damaging something that is not rated for the pressure. If it's an R600a compressor, I'd be concerned about the required motor HP for the displacement burning the compressor out on propane.

Don't let me discourage you because doing stuff like this is how I got going too. I just want to point out some of the not so obvious pitfalls to hopefully stop you blowing a hole in a pipe (or worse).

Get a cheap set of gauges and a vacuum pump from E-bay and go for it.