What is the lowest normal operating suction pressure that is acceptable on R134a chiller running water with no glycol or other low temp additives?
We are operating a 280 ton R134a chiller using a 45 deg F evaporator leaving temperature setpoint. ~630 gpm chilled water flow with an evaporator inlet temperature of 46-55 degrees depending on the load.

We has issues in the past with the mechanical TXVs that were too slow to respond and resulted in liquid flood back during rapid unloading. We are currently installing EXVs and revising the PLC logic to control the EXVs and maintain superheat control best that we can. Bitzer recommends that we run as much superheat as we can on these CSH9571 compact screws, but I am running into issues with running reduced suction pressures in order to maintain 15+ deg of suction superheat.

During the entire load range suction temperature varies between ~45deg at low load and ~48 deg at high load. During lower load operation suction pressures will operate steady state at ~26psig (30 deg sat temp). Evaporator inlet pressures will be no lower than 32 psig at minimum load.

I have always been told to NEVER operate steady state below 28psig (32deg sat temp) to prevent freezing. Due to the water being at least 46 deg at the evaporator inlet and the suction gas being superheated 15 deg (measured suction temp of >45 deg) do I still have to worry about freezing when we operate at 26 psig suction pressure?

The only way I can maintain 15 deg or more suction superheat, but not operate with suction pressure below 28psig is to increase the water temperature which is currently fixed at 45 deg evap outlet due to design requirements and calculations.