Beginning the Modular Refrigerator

Beginning a few weeks ago, I’ve embarked on a project to construct a modular, long lasting cooling system for  domestic purposes.  I’ve outlined my reasoning for this elsewhere on this site.  100 Year Fridge  The majority of my documentation is on YouTube at this time, so I thought it prudent to begin sharing some of the details of the design in a less “rambling crazy person” format.

The first cooling unit was not an all-in-one package, and much more crude, so I won’t be including much of it here.  The unit I built was essentially a smaller brine chiller.  That is, it was designed to chill an insulated container full of anti-freeze and water.  Initial tests were a partial success, as I was able to lower the temperature far below freezing, but not in anything resembling efficiency or longevity.  The system was comprised of a 1/4 HP compressor salvaged from an ice machine, a small air cooled condenser, a short length of capillary tube, and a short length of 1/4” copper tubing immersed in the brine tank.

Construction was for two purposes.  Initially, I wanted a brine chiller so I could test simple two-phase thermosiphon.  I won’t go into it now, but the thing worked just fine, and demonstrated the principle could be used to refrigerate remote liquids (and possibly air) from a distance.  The original refrigerative apparatus was consolidated to a single platform with the condenser unit mounted to the surface of a board, and the evaporator suspended from underneath.

After this proof of concept, I returned to the long term goal of making a successful cooling unit for a refrigerator.  A mini-fridge, originally thermoelectric, was re-purposed by cutting a round hole in the top, and dropping the cooling unit in.  The evaporator was far too small, and the compressor too big to cool air directly, the machine is designed to chill a brine tank which in turn cools the contents of the refrigerator.  Overcharge of the propane refrigerant was a problem, but was necessary due to the improper size capillary tube.  I acquired some new 0.031 inch inside bore tubing and found 48 inches to work well.  Nonetheless, the evaporator was still too small, and the compressor too large, resulting in excessively low evaporator pressures.

The above picture is more current and includes a different compressor.

The above evaporator was the original one, pieced together from old parts.  The large object towards the top was intended to be a sort of accumulator to increase the volume and relieve some issues of flood back.  It was determined that the evaporator was simply too small for both the application and the large compressor.

While I still had the 1/4 HP compressor, I replaced the evaporator with a much longer one.  The original was about 88 inches, while the new one was 210 inches long.  To make it stiffer, I used some thin copper wire and laced it around the tubing in three spots.  It worked quite well.

To further support the coil, I suspended it from the bottom of the condenser board with wire and two screws into the wood.  The bucket of coolant is raised to the roof of the cabinet, and a wire shelf is placed underneath.

My original idea was to make a freezer.  Running this setup down to below freezing temperatures is pretty easy, and I was able to freeze a few pounds of water, but not very fast.  The problem is that the refrigeration system had no problem pumping heat from the coolant until it was below 0 Fahrenheit.  Going much lower didn’t seem to be worth it until the air temperature dropped further, so I would shut it down and wait for the cabinet temperature to drop.  In retrospect, I could have driven the coolant temperature further, but there are other issues to contend with.  With a few containers of tap water in the cabinet, the air temperature didn’t drop far below 26 F.  Over the course of several hours, the bucket temperature was maintained between 0 and 10 degrees, and this stored cooling capacity was allowed to chill the cabinet.  Without the heat load of the freezing water, the air temperature would easily lower, but with the water the cooling unit barely stayed ahead of the cooling task.  No doubt the limited insulation of the refrigerator cabinet was a large contributor to the problem, but still larger was the fact that the compressor only runs for half an hour every two hours, rather than running continuously until the desired cabinet temperature was reached.  I felt better suited to this task would be a forced air heat exchanger or a larger surface area static evaporator, similar to a chest freezer.

Another problem with a brine tank freezer is defrost.  With the thermal mass, a shut down freezer would take a rather long time to defrost, short of adding an external hot gas defrost or electric defrost to the outside of the tank, which is not an option.

It has been decided to pursue refrigeration only, right now.

I took the time to replace the 1/4 HP with a smaller 1/8 HP compressor from another small mini-fridge.  This one had an iso-butane charge (r600a).  Charge was gone, but the compressor was still good, albeit a little loud.  Displacement is about half of the old one, and half the wattage.  Run time is longer, with a higher suction pressure.  Much better for the application.  Specs here.

This setup works fine as a refrigerator.  It does not take long at all to bring the coolant and cabinet air temperature down to the 30’s.  At first, I was keeping the coolant between 15 and 25, but the air temperature was dipping into the low 30’s, and below freezing.  Today, I have found that by running the compressor for 10 minutes at 100 watts, I can keep the coolant temperature between 20 and 30- with the air temperature between 35 and 38 degrees.  The holding period is about 3 to 3.5 hours.  I’m fairly pleased with that on a 70 degree day.

The next step is to move on to a larger cabinet.  I have one twice the current size.  I am going to replace the beer bucket with a stainless steam table pan, and design a support mechanism so that it hangs from the condenser assembly.  Additionally, the unit will be offset to one side rather than right in the middle.  I will us some plexiglass, or similar insulative but rigid material, to form a baffle around the brine tank in order to increase convective air currents.

I intend to eventually find an even smaller compressor, or switch back to the BLDC compressor.  In the mail, are a pair of temperature controllers, and an 8 channel thermocouple temperature logger with USB connection and graphing software.  I am looking forward to to finally getting some collected data for a device.  Furthermore, the sealed thermocouples in the refrigerant lines should produce some compelling information regarding the thermodynamic performance of this and other refrigeration machines.  I think it may be possible to acquire some pressure transducers and collect head/ suction pressures as well.

The next project in the foreseeable future is the elimination of the air cooled condenser for a handmade static condenser, constructed of copper and maybe brass.

More details on the progress to date, and further as videos are added, can be found in the “Modular Cooling Unit” playlist on my YouTube channel.

Thanks for reading!

-M.C. Pletcher