Ok so here goes with another long post. I got an okay to share some details about a process chiller we built for condensing CO2 at -24F; I guess we all realized that we have no interest in selling chillers outright and hopefully someone rips off the design so we can just buy one lol.
Capacity is approx 80k btu at -24F, but it obviously can run colder. We’ve ran it down to -60F and then it started freezing the glycol.
The low stage:
We used a Copeland ZO88KCE to pump r744 (CO2). I can’t tell you our cost but I think retail is around $3k. Unfortunately, despite being a freaking awesome refrigerant, they’re not real common so the Americans at least won’t be able to find them used/new old stock.
The evaporator heat exchanger is a duda brazed plate, I think it’s a 60 plate 4x24" but I don’t remember exactly. They’re so cheap, just oversize it. We use the exact same HX for the interstage heat exchanger too but again, only because it’s oversized.
Expansion valve is a sporlan SER-C; sporlan will tell you that you only need an SER-B because of the pressure drop allowed but that is a crock of shit, ate a whole rev number for us lol. Depending on who’s building it you can run it off their Kelvin II superheat controller or you can run it off your own PLC. A little note here: the stock temp transmitter only goes to -60F and below that the controller will close the valve, blowing all your refrigerant and oil out the relief valve if you don’t address your logic accordingly; you’ve been warned. For lower temp operation, a mechanical TXV might be better but it will probably hunt like a mofo because there is not a product designed for these temperatures at this size.
We did not need a fade out tank because of how much evaporator piping we had, but there’s little downside to having one. I HIGHLY recommend wiring an unloader valve to run between the discharge (before the condenser) and suction of the evaporator to keep it from spiking pressure on startup because you will obviously want to keep the condenser-side volume as low as possible (this will mitigate the standstill pressure issue). You will also want a HP and LP cutout obviously, but you also need a pressure switch to ensure the low stage is sufficiently cold before it starts; this is much more effective than using a temp transmitter to do the same.
Now, this low stage hasn’t been “ratified” by Copeland but with our parameters it was well within their operating envelope. However, as you try to run this colder, oil management may become an issue which I have yet to address. Builder beware.
Pretty much all condenser piping is .035 wall 5/8 OD 304l welded, all evaporator piping is 1-1/8 OD of the same. The mueller HP copper can also do the pressure but it was waaaayyy more expensive than the stainless. Relief valve is set for 750 psi, service valves are where they always are.
High Stage:
We ran a ZPT166KCE tandem r410a compressor for the high stage. This allowed us additional capacity that we actually used as a dryer for the system air compressor. This is a much more common compressor with plenty of cross options.
Evaporator has been discussed already. For the condenser we used two 4-5 ton rated goodman slab coils. TXV sized out as a ERZE-6. Same deal with all the piping as the low stage, stainless is cheaper than the HP copper so, yeah, we did that.
Unloader circuit is helpful for the high stage as well if you don’t want to run a liquid accumulator.
We charged for the interstage to be operating around 32F so that you could visually see which stage was “winning”. If you bumped that up a bit, you could eliminate the entire high stage by using the facility chilled water (if it’s an option) which is usually around 44F. The next thing to do would be an R23 or N2 “even lower” stage but I haven’t had any time to select components for that.
If anyone has specific questions let me know. I can draw up a PID if y’all like but it’s probably not necessary for anyone that actually plans to build one. Total parts cost will probably come out in the $6-7k range, but could be quite a bit lower for a smart shopper/scrounger. It’s enough to condense 4+ kg/min of CO2 at -24F so it’s a pretty big unit.