Two stage system troubleshooting

[nike123]

It's not related -
If your compressors and cap-tubes are right then your problem is as Gary says - oil type and return provisions. This very topic has been discussed before either here on this discussion board or on one of the old Usenet groups - I forget where - but changing the oil type solved the problem.

I do want to discuss the principle of subcool occurrence with Mad Fridge - but I will start another thread for that - good luck

OK, I appreciate your reasoning.

[nike123]

Nike, add a little bit R134a into the system to rinse the cap tubes. The R134a remains liquid and flushes it.
Why R410a for the LP?

I am there simply as possible source of valuable information. To me, it is much more important to gain some general knowledge on two stage cascade.
I don't have any influence on design and commission.
My primary goal is to gain some knowledge on cascades from concrete example from life.

[nike123]

Nike, add a little bit R134a into the system to rinse the cap tubes. The R134a remains liquid and flushes it.
Why R410a for the LP?

I don't know reasoning! I presume that is because of availability.

[nike123]

Even the best of oil separators will allow some tiny percentage to pass through. If that oil remains in the evaporator, then over a period of time sufficient oil will accumulate to cause logging problems. For this reason some small amount of oil carrying fluid must be added to ensure oil return, even with a good separator.

I have advised as you suggested!
We will see in next few days if advice is accepted.

[Gary]

With a good oil separator, only a small amount of oil will be flowing through the evaporator, thus only a small amount of carrying fluid is needed.

With no separator, large amounts of oil will be flowing through the evaporator, thus large amounts of carrying fluid will be needed... enough to substantially alter the pressure/temperature relationship of the refrigerant, resulting in higher evaporating temperature and higher box temperature.

Low stage oil return is a serious problem in cascade systems, which must be addressed.

[Gary]

It looks like the sub cooling is only there to reduce the pressure drop across the high stage evap (no direct increase in performance).

I would expect some reduction in flashing at the interstage, increasing the effective heat transfer surface area... but the same effect could be accomplished by slightly increasing interstage size. I'm thinking the reduced flashing is not worth the added complexity.

[mad fridgie]

I would expect some reduction in flashing at the interstage, increasing the effective heat transfer surface area... but the same effect could be accomplished by slightly increasing interstage size. I'm thinking the reduced flashing is not worth the added complexity.

Correct it comes down to how they have selected the heat exchanger

[Peter_1]

In the past, we made several cascades for Meissner coils (R501/R13, R404a/R23 and R1150/R1270) and we still service +/- 20 cascade and autocascades these days. Our experience is that you must have a very, very dry system. The smallest quantity of water in the system will, together with the oil clog your system.
As Gary said, an oil separator isn't always working 100%
What's in the Visio file? The same information as in the pdf?
Is this used as a CPU cooler.

[nike123]

What's in the Visio file? The same information as in the pdf?
Is this used as a CPU cooler.

No! It is used as freezer for medical samples.
File for download is PDF of diagram I made in Visio. That is actual diagram of system with some informations on it.
I will send you password for download at private message.

Other PDF is model from Coolpack.