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This is part III of my video series on building a DIY vaporcompression casacade (i.e. multistage) refrigeration system. In the last two videos, i demonstrated the concepts needed to make a cascade system work and reached temperatures below 110C, but because i used lowpowered reciprocating compressors meant for R600A, I wasn't able to build a system that ran continuously and only managed to evaporate lowtemperature refrigerants in short bursts.

This time, I'm using a pair of rotary compressors extracted from 5,000 BTU window air conditioner units. The first stage will use propane (R290) as a refrigerant, and the second stage will use Ethylene (R1150). The compressors were made for R410A, so they're able to run as high as ~30 bar continuously without a problem. This will allow me to condense Ethylene at the first stage evaporator temperature of around 20C to 25C.

To extract more efficiency (and a lower temperature) out of the system, I use subcoolers on both stages; the return lines to both compressors are wrapped in capillary tubing coming from the condensers.

The lowest temperature I managed to achieve with this system was only 88C, but that's probably because I ran the second stage suction pressure relatively high (around 46 bar) in order to keep the mass flow rate high. Down to about 63C, the efficiency or "coefficient of performance" (COP) is about 10%, so there's definitely some room for improvement.

The best way to improve the ultimate temperature and/or thermal efficiency of this system would be to use a subcooler with a thermal gradient across it that looks basically identical to the regenerative heat exchanger in a joulethomson system. When I experimented with this using my precooled joulethomson cryocooler with 100% ethylene (effectively turning it into a cascade vapor compression system), I achieved nearly twice the thermal efficiency of the device I built for this video.

I originally planned to use this system to liquefy Methane. This is possible at 88C, but would require a very high pressure, and with the marginal cooling power at that temperature, it's probably not practical, so I'll be doing that in my next video using my joulethomson cryocooler.

I'd also recommend using a higherboiling refrigerant for the second stage like R508 (if you can get it), Ethane (R170), Nitrous Oxide (R744A), or even Carbon Dioxide (R744) (although CO2 is only feasible if you keep the second stage evaporator ABOVE the 56C). The boiling points of Ethylene and Propane are far enough apart that the system is reaching pretty far.

Music Used:
Kevin MacLeod George Street Shuffle
Kevin MacLeod Bossa Antigua
Kevin MacLeod Lobby Time
Kevin MacLeod Elevator