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Old 11-13-16, 11:00 AM   #1
marx290
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Default Building a 100 Year Refrigerator

Haven't been on the forum in some time; I have been busy. Began a project in June to construct a modular, long lasting refrigerator. It has gone through many iterations, and there are many more to come. The refrigerant is propane. All heat exchangers are intended to be passive. The evaporator is currently in a freezing brine tank to store cooling power; that is likely to be part of the final design. The compressor is about 1/10 hp, 120VAC, but it will eventually be changed to a DC model. As I work on improved performance and develop better manufacturing techniques, the machine will become more robust and easier to work on. I'm pursuing an aesthetic quality as well; it will be installed in the top of a hand built wooden refrigerator cabinet some day.

The system runs 1 to 1.5 hours at a time, and 3 to 5 times per day.

I have experimented with several different ideas including two-phase thermosiphons and paraffin phase change materials in the condenser side. All had marginal success, but were scrapped due to either limited benefit or unnecessary complication.

I thought this was a good place to share some pics as I just built a new condenser yesterday, and I'm going to take a week or two off to get some reading done. I may start a side project which is an intermittent absorption ammonia refrigerator.

Thanks for taking a look.

This one one of my first jaunts into capillary tubing, Charts to reference guidelines for lengths don't normally include r290, so I was kind of guessing at first. Unfortunately, I guessed far too large of a bore, and wasn't understanding why cutting the length down wasn't getting me anywhere. Not to mention the high compressor amp draws and ridiculous discharge temps. It was a troubled time. I'm in better shape now. I believe that r404a is a reasonable comparison for sizing cap tubing for r290.

This setup was originally built to cool a brine. The brine was used to test a two phase thermosiphon. I was successful in chilling a bucket of water remotely using this thermosiphon containing propane as a working fluid. Think of a thermosiphon and a heat pipe wrapped into one. Later, I cut a hole in a small minifridge to begin a modular cooling unit project.



I got a little fancier with this design. Back in June of this year, I decided I would begin to actually construct a machine I have been thinking about for a long time. It is intended to eventually be a modular cooling unit which can be installed in a a variety of refrigerator cabinets or in a custom made wooden cabinet. The design principle of dropping it into the top has obvious parallels to the GE Monitor Top of the 1920s and 30s, as well as a few other manufacturers at the time who were making their machines more easily serviceable; it was a different time. I also enjoy the obvious benefit of discharging the heat at the top, where it belongs. The evaporator underneath is immersed in a strong solution of propylene glycol and water- intended to act as thermal mass. Later versions used a much weaker solution what begins to freeze at about 26 F. This has the effect of holding the cabinet temperature down for several hours.




Limited surface area of the brine tank was resulting in poor temperature recovery when opening the door or adding heat load. Rather than modify the tank (Like I would do later on), I chose to experiment with a larger plate heat exchanger. I had two aluminum plates and no way to easily solder copper to them, so I just made several serpentines with the tubing and sandwiched it between the plates using some bolts. This evaporator was quite effective at pulling the cabinet temperature down rapidly, but lacked the holdover of a some thermal mass. I have considered adding some phase change material to a plate evaporator like this, but to date I have not done so.



I left the evaporator alone for a while to focus on the condenser. I have been a long admirer of any kind of static heat exchanger. I am trying to build as many of the components myself, and an aluminum finned air cooled coil just doesn't fit the bill. I was going to have some fun with this. I believe technology should be beautiful craftwork, I tried my hand at some fancy copper work. The hyperboloid shape was chosen because I simply like the shape. Figuring out how to make it was another matter. I drove from Portland, OR to Portland, ME to deliver a car, and along the way I worked this weird thing out. What I liked about it was that the hyperboloid curve was not cut, it just is a natural result of these straight lengths of copper arranged in this way. Pain in the *** to build though, and future versions will be assembled differently.





This is a new evaporator and brine tank. There is simply a coil of 1/4" copper tubing in the bottom of this 2" half steam table pan. To increase surface area I soldered stainless steel fins to the bottom and sides. the tank is filled with a weak solution of propylene glycol and water which begins to freeze at about 26 F. This will hold the temperature in the cabinet for many hours.


Because the machine now runs for perhaps an hour every 4 to 8 hours (depending on ambient temperature), then all those hours of heat load need to be discharged in about an hour. I like this type of cycling- more or less, as the evaporator is better fed when it runs for a longer period of time rather than short cycles. I decided to try a phase change material on the condenser side as well. I figured if I could store cooling power in the brine, I could store discharge heat which could be dissipated over several hours at a lower temperature. Didn't exactly work out that way.

I chose petroleum jelly as a phase change material. The hydrocarbon chains of white petroleum are generally shorter than candle wax or canning wax, but longer than mineral oil. This makes for a melting point around 90F. 6 pounds of the stuff in a glass jar with 50 feet of 3/16" copper tubing made for my condenser. During the run, the hot gas condensing would melt the jelly into a clear liquid. But how to cool the wax before the next cycle? I chose to use a two phase thermosiphon- again with propane. Just for an added measure, I used an air cooled coil at the top. Several wraps of 1/4" copper tubing in the jelly was connected to the coil up top. Liquid propane vaporizes in the hot jelly and condenses in the air cooled coil up top. The system ran at pretty high head pressures and subcooling was a serious problem. On a positive note, the thermosiphon worked beautifully!


White petroleum is a poor thermal conductor- liquid or solid. I tried a more direct heat exchange between two cone shaped coils sandwiched together with some sheet metal cones. The lower one is the VC condenser. The upper one is the evaporator of another two phase thermosiphon. The air cooled coil of the last thermposiphon seemed to work so well, I decided to use the hyperboloid condenser this time.

Subcooling was still a major issue, but again the thermosiphon worked well.


Continued on a later post.

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Last edited by marx290; 11-20-16 at 02:29 PM.. Reason: captions
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