This would only be true if there were significant liquid hold-up. A while back I mentioned about checking how much internal liquid volume it takes before it is seen exiting the separator. Did you ever perform such a test?
Otherwise having a large phase separator gas volume should not be an issue, and if anything, aids in separating the liquid from the gas.
I find it strange that the suction accumulator would be a problem. On rotaries it never seems to be an issue.
Looking at your diagram, I see you had a 32.5 psig suction versus a 289.4 psig discharge pressure. This ratio doesn't look bad to me, especially considering the load. So although lengthening the cap tubes will bring the suction pressure down, theoretically allowing the refrigerants to evaporate colder, it probably will also mean less heat load capacity.
Once again looking at your diagram, your temperature split on the primary mixed refrigerant side of the evaporator is starting to look pretty big at 27 degrees C. A more typical temperature gradient under full load should be 10-15 degrees C. So this would suggest three possibilities...
- Insufficient liquid feeding the evaporator
- Excessive system pressure drop (hard to say without downstream pressure readings)
- You need a bigger compressor
If the evaporator HX had poor heat transfer as you suggest, then I would expect to see the feed and return on the primary side to be very close in temperature (very little gradient). the fact that the temperature gradient is large, suggests that very good heat transfer is taking place (assuming that you are not simply running out of refrigerant). Personally I think you still need more low boiling refrigerant flowing in the system. And judging by the compressor discharge temperature of 108 C, you might need a tad bit more butane (you might find that mixing a bit of n-Butane with the iso-Butane would also help in this area). Large systems usually require either a high static balance pressure (225+ psig) or sufficiently sized expansion volume, in order to provide enough low boiling refrigerant when condensed.
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