Well you're right Boardy, you're starting to pick these things up
The size of those compressors is quite good, and for the cost of Rotaries in the UK it would be nice to use them, as that's the biggest expense really.
For the heat exchangers, if they are real ones, ie if they're manufactured ones with what looks like corrugated pipe inside, then they'll do the job ok, but are normally about 1/2ton max. Not bad, but I've always found that they're better for precooling. They do get used though. If you find that they don't do a great job then you can just make your own heat exchangers with pipe in pipe coiled, and use those on the suction return and run the Low stage's gas through it from just after the desuperheater, and just befroe the heat exchangers. It gave me about 10c colder under load when I used a slhx, and that ain't bad.
The captube length looks decent, though about 5-6' is what I set up my cascade with on the base stage with .031". Some have had success with longer, but if you need 2 stages run off that thing, more capacity is good. Even if it means that the temps on the base aren't as good.
If you don't want to have to worry about what oils are being used, a little 290/propane in the stages will ensure good oil carrying.
The oil seps will be fine if they're copper wool filled, but if not some can slip through.
If you did use the compressors, expect a little loss with the frequency being lower here compared with the US, but about 3/4HP and about 1/3 to 3/8HP on the smaller one would be about what you'd expect, which is great.
Really other than making sure that all of his brazing will hold pressure, and finding out whether the evaps are decent, the only really bad thing is the suction lines. Those flexi's are normally only rated to 150psi or so max. Static pressure at room temp on the Low stage gasses can beup to 200psi. That means that they could leak, or even burst, when the thing is charged up and that's not good. So I strongly recommend getting braided suction flexi's.
Anyways, if you just vacuum the High stage, and check that it holds, then pressure test and it holds that, then you can just charge it up, and see if that works at least. Just make sure your low stages are sealed up and in vac or with some nitrogen in there so that when the Low stage pipework is really cold you don't draw moisture in you'll see how it goes, and it'll give you a little experience on the base, and how to get it to hold. After that, see if you can get the Low stage that's complete tested and charged, even if it's just with 290 or your 402a, and load that too, then you can see how the base stage handles the load, and charge it so it will.
Your biggest challenge is the High stage. Getting it to hold that load, and then reworking the charge to handle both loads. If you tend to get floodback when the Low stage is only running on circuit(ie when cpu but not gpu evaps) then you may want to either add a larger suction accumulator or change the captube for TEV so that you don't get liquid floodback, but still have enough capacity to run the stages under load.
lol I could type all day on this, but really, just treat it as one stage at a time, start by making sure the High stage will work, then the cpu low stage, and tune the high to just that low, then add in the gpu low stage, and get the high to support that next, then you're only working on one at a time, and you'll pick up most of what you need to know as you go.
Just don't jump into a heavy gas like ethylene, or any of the flammable low gasses right away til you're sure you've got it sussed, as I've seen others do that and it scares me as 1150, 170 are both highly flammable and risky to start with. 508b is very good, but highly expensive. I don't know if BOC has R23 or not, but it's a good cheap starter if they do. Otherwise, CO2 is dead cheap and will give you some good experience and it won't kill you if you have issues and end up with a bit of wasted gas.
Cheers
Gray
