DIY Cascade Refrigeration System (Part III)
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Published 2024-04-11
This is part III of my video series on building a DIY vapor-compression casacade (i.e. multi-stage) 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 low-powered reciprocating compressors meant for R600A, I wasn't able to build a system that ran continuously and only managed to evaporate low-temperature 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 4-6 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 sub-cooler with a thermal gradient across it that looks basically identical to the regenerative heat exchanger in a joule-thomson system. When I experimented with this using my precooled joule-thomson 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 joule-thomson cryocooler.
I'd also recommend using a higher-boiling 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
All Comments (21)
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If my voice sounded funny in this one it's cause season allergies are terrible rn. For those asking, I was originally gonna put out part III of the joule thomson series but there were some delays. It's workable right now for liquid methane, and should be usable for liquid nitrogen pretty soon.
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this guy makes nothing but refrigerator videos, and I have no clue why, but I love watching every single one.
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Honey wake up! Hyperspace Pirate uploaded! Its time to watch for 13 minutes video about refrigiretors! Yay!
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Love the dry humor in these videos!
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Glad to see those R410s going to stay with their forever family.
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The concept of people doing high tech things with low tech gear is deeply satisfying.
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Definitely my favorite YouTube series. Not even close. Honestly this is what I thought engineering would be when I started, turns out most of us have boring jobs at defense companies. Living the dream man, keep it up!!
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Now that i started to have thermodynamics classes in uni this is even more interesting.
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It's always a good day when Hyperspace Pirate uploads a video!
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You've made a lot of videos on refrigeration and I haven't watched them all so I don't know if these comments are relevant, so these are offered FWIW.... It is necessary to remove moisture from the sealed system. A filter-dryier helps but only has limited capacity. You should put the entire system on a vacuum pump and pump it down to less than 500 microns--and hold it on vacuum for a half hour or so. This will remove most of the moisture and non-condensibles (mostly nitrogen and CO2). The capacity of a filter-drier assumes this was done. For maximum efficiency, you should do a "triple evac." This means pulling a vacuum down to 500 microns or less (and making sure it holds) and then breaking the vacuum with a small amount of the intended refrigerant. Doing this three times makes sure that when you charge the system, the only thing that remains in there is refrigerant. The oil in the compressor is chosen for its compatibility with the refrigerant (which is why compressors are labeled for "R-134s, 410a, R-22, etc.). The compressor doesn't care what the refrigerant is but the label is there to indicate of what type of oil is in the compressor (PAG, POE, mineral oil, etc.). The refrigerant and type of oil have to be compatible. I'm not sure that's the case here (but I haven't looked it up). Soldering is not used much any more in HVAC-R because of leaks, vibrational cracks, call backs and so forth, but is fine (if the pressures are not too high and you don't mind finding and fixing occasional leaks). Sta Brite 8 is a silver-based solder. It's expensive, but it works. I use silphos brazing material but it really requires brazing temps (oxy acetylene). For copper/copper connections, the phosphorus serves as the flux. Some installers use MAPP gas but IMO the joints have to be heated too long when brazing with anything other than oxy/acetylene.. With oxy/acetylene you can get in and get out without ruining components nearby. For that reason, it's a good idea to protect other components by spraying them with cooling gel (or using wet rags)--and being fast--to prevent heat from damaging valve seats and other stuff downstream. Damage to valves and other components from overheating may not be obvious at first.
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I have a real 2 stage cascade system on my shelf here from a -80c fridge. The second stage has 2 large resevoirs on the low side that add volume to the system when its off to reduce the standing pressure when its at room temperature. Just a tbought for ya. Then it can have more liquid available when its running, possible moving highter BTUH, and at the same time not exploding when its on the shelf warm. Also, if your in Canada (gta) I'm in the hvac industry and can get you compressors and what-not used.. slightly abused.. for free.. lol Edit: I should watch the whole video before commenting 😂
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Hey, I see you took my advice on pull down resistors! I'm deeply honored.
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I congratulate you on moving forward. In the space of time since you started, many YouTubers have started their channel, made some cool videos, hit their first "peak", then burnt out and quit. I think YouTube should reward guys like you, who KEEP producing consistent content. Just my opinion, but hey... Good job!
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What do you mean I'm not HVAC certified, don't worry about the leaks, if the refrigerant wants to stay, it can come back on its own
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THE FRIDGE DUDE IS BACK!! exciting stuff
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Nice set up. Remind me on some cold traps we sold some 30 years ago. All time running at -104C and stayed alive for 15-20 years. Much simpler set up using a single small condenser . Used an insulated evaporator flattened coil outside the drum. Not a storage tank for the low stage Ethen, just the system volume was enough. The oil separator was just a vertical placed small strainer with a microscopically bored cap tube for oil return for an all time oil return. They brazed the discharge line into the side of the strainer and placed the capillary tube in the bottom, worked well. Look forward to your next step 😊
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you are my favorite channel on youtube
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Really awesome stuff, cool to see what can be done with parts ripped out of window units and some forethought. You clearly have done your homework but for brazing I would highly recommend having a flow of nitrogen gas through any of the fittings and areas you're sweating. The black copper scale you see on the outside of the tubing is also present inside, and with the solvent nature of refrigerants and oils, can be dissolved or broken up making its way into the compressor and/or metering devices. This could cause issues with your capillary tubes, and as others have mentioned pulling a vacuum and ensuring you hold below 500 microns for a decent amount of time could avoid some other problems with moisture and refrigerant acidification. All in all, really good work!
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The problem with using a heat exchanger on the suction line to subcool the liquid side is that it raises suction superheat. The higher the suction superheat, the lower the efficiency and the lower the system capacity. Ideally you'd want a thermostatic expansion valve on each stage to keep superheat of each as low as possible.
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I used to build very large cascade refrigeration test chambers. Every bit from the structural steel frame to the copper brazing of the loops. I didn't do any of the electronics outside of mounting them and didn't ever run or setup the cooling systems outside of testing when it was done. I really enjoy this series because it helps me to better understand what I used to build.
