Rebuilding Two Stage Cascade Phase Change Cooler

Hello,
Recently I was very lucky to be given a two stage cascade unit by the overclocker Unseen. He mentioned he was very active here in the day so I decided here might be a good place to start asking questions, or more accurately looking for advice.

The unit in it's current forms has R404A in both stages, Unseen mentioned he was unable to get something like R1150 for the second stage, a problem I do not have. I have some welding I want to do on the aluminium chassis it's mounted to, and adding some heavy duty coasters and then some rework to do on the stainless steel cover (namely routing the eval line out the top not side of the unit). Most of my questions pertain to charging a cascade unit, do I treat the unit just like a normal phase change unit when charging, just one stage at a time? Whats the superheat settings I should be aiming for? Talking a bit with Bartek, I'll probably replace the evap head with one of his.

I am rebuilding this unit up for a very high load capability not lowest temperature possible, so he recommended a shortened capillary tube, currently it's pretty long. I also noted the lack of an oil return line and oil separator, is that something that is needed or is it's omission normal? I plan to use R404A and R1150 in the unit, first and second stage respectfully, but are there better options?

I've never really actually torn a phase change unit down with the intention of rebuilding it, so I'm a bit green when it comes to this. Any advice would be welcomed beyond "Don't try this it's very hard" I included a photo of the unit from the top.

Oh, another bit of pertinent information, the high heat load could be an i9 7900X, I do need a permanent GPU bench to quickly bench GPUs so around a 5.4-5.5GHz i9 would be nice, so I'd hope the unit can maintain a clock that, I'm confident it can.

Welcome to XS,

The first stage should be dialed in for a load on the second stage but it sounds like you need a shot of R1150 in the second stage along with something to help the oil return.

I would use a little butane, maybe 5 to 10 grams or change the oil to low temp PE oil, charge it like a normal low temp condensing unit, 20% R404/R507 and 80% R1150 (guessing) this ratio will work but you can wing it from here. Watch out for cheaper "low temp" pe oil that only goes to -65c

While running set it to +7 PSI

Lets see the block, it might be mine. I see you have Danfoss compressors, excellent choice. Insulate the brazed plate heat exchanger and everywhere the suction line is exposed. Add a air heat exchanger to strip off more heat from the discharge line of your second stage, you are sending hot compressed gas directly into the heat exchanger now.

Currently both stages are filled with R404A, not R1150 in the second stage, that is the gas I will use, but yes. This unit used to be a freeze drier for from a company called Millrock Technology, all the parts in this unit currently are Danfoss, with the Alfa Laval heat exchanger. They used to be insulated with neoprene but I removed it to do the rebuild work, which I haven't started yet. The unit is still operational but gets pretty icy, I used that to see what parts got cold, and the heat exchange was barely cold, so even now there is an imbalance somewhere. I need to repaint them too, they where a bit rust on top, which is a shame but they'll look like new when I'm done. Here is a small collection of photos from the unit. There are a few pics of the evap head. There is currently no oil return line, which is something I am very sure is a major issue, while it might be hard to add, I'd rather have that to ensure there is no oil in the evap head and to ensure the sump is always full.
https://drive.google.com/open?id=1ZI...HY0D5l4Dmk5Zc9

That appears to be my block with different mounting, the line set also looks familiar.

Hopefully my advice works for you.

The line seems to be corrugated stainless steel, while not as long lasting, I think I'll use a similar solution because I can braze onto and this is for a single system so not moving around. Would you like the evap head back? I plan to use this on modern hardware so a more modern design would be needed (especially for an i9 7900X) so this head would be not so much needed, however the mount is unusual.

It seems a really solid foam material to press it down. I can't say I had good luck on modern (or even LGA 775) with it, I noted large amounts of play even when tightened. The contact area was also very badly pitted after a few years of not so nice storage so it needs to be replaced I think. The adding another air heat exchanger before the phase change exchanger is some solid advice, so that is something that I'll do, although I might have to make that myself because room is for of at a premium, I might move the internals around when I redo the copper tubing.