Hi Rallas
That is correct ,you never let the suction pressure go below the freezing point of water ie (R134A 32DegF =28 PSIG ) as in the chiller barrel you can get areas of poor water movement which can end up with localised freezing around the tubes and then crushing the tubes, causing failure. Ideally you should have a Low pressure switch set at 28 PSI to stop the machine in that situation.
Your problem seems due to the fact your trying to run the chiller with much too high superheat. This has 2 effects 1) you run with a low suction gauge 2) you lose system capacity.
It is good your fitting Electronic expansion valves as with them you can run with lower superheats and with increased capacity
You can easily run with superheats down to 8 or 10F or possibly lower providing you use a complete dedicated EEV superheat controller. A custom programmed PLC may end up giving a poorer and slower control than the original TEVs