3-stage cascade

[before]

First of all; I'd like to thank you so much for your long and friendly post.

As a service technician who regularly works on cascade systems usually -80C two stage Revco’s, Herasus and the like and a few -150C Revco multi-cascades. Have you heard of them? Very interesting, the crux is that they only have 1 compressor, 3 heat exchangers and a mixture of about 6 different refrigerants inside and achieve -150C. How it’s done is a little more complicated.

Auto-cascade is very interesting indeed.

I've found this pic about . Which refrigerants are used?

Anyway back to your 3 stage cascade and some advice on compressor from what I can tell you are using rotary compressors, these types of compressors are design for use on mostly air-cons, were they operate at medium to high back pressures with a larger volume of refrigerant being pumped through the compressor. In your application the compressors are operating at a very low back pressure sometimes even in a vacuum with a small volume of refrigerant being pump through the compressor, they were never designed with this type of operation in mind. Not to say that they won’t do it, they will. It’s just that they won’t last a long time doing it. My advice to you is to use low back pressure (LBP) reciprocating compressor. They are designed to run on LBP and have smaller volume of refrigerant pump through. They are used in most 2 Stage Cascades, and are much happier running at your operating conditions.

I've already heard about LBP compressors but those are pretty difficult to find for me. Actually, I can easily get rotaries; but LBP are rare (at least for me).

Of course, using rotaries can be a real matter while trying to get temperatures that correspond to very low succion pressures. For instance, with my other cascade, a 2-stager unit, the second stage filled with R1150 can do about -115C (~0.5bar) loaded. The result is interesting during a certain amount of time, but the compressor temperature is more than reasonable...

[before]

Here are some pics of the revised unit.

Click the thumbnail to view full-size image

[before]

BTW I've got a weird problem with 3rd stage. Here's how it's running.

- Goes down quickly to lets say -45C (takes very few seconds)
- Stops going down
- Goes up to lets say -35C
- Goes down again not so fast to about -90C
- Stops (or goes down very very slowly) during compressor suc side is freezing (takes a while)
- Goes down to -130C (slowly but surely)
- Goes up to about -120C
- Goes down to -131C
- Goes up to -122C
etc.

What's wrong in your opinion? Could it be the HX between 2nd and 3rd stages which is not good enough? Evaporating temperature and flow of 2nd stage are great... something like -98C in / -98C out unloaded achieved very quickly. Also, adding more R14 into low stage doesn't make any improvement; it gets worst...

[before]

I've replaced the coax HX by a brazed plate one (12pl).

The cascade works way better now. I've to find the right adjustment.

I couldn't resist to take a very quick pic.

Click the thumbnail to view full-size image