I have a 60 ton air cooled water chiller with 4) 15 ton scroll compressors. There are two circuits with two compressors in parallel on each circuit. One chilled water evaporator tube bundle with two circuits in it. The problem is in fluctuating suction pressures in circuit 1, with the drop low enough to trip the low pressure lockout switch. It runs for a day or so but the suction pressure goes from 58 to 48 in 60 second cycles. Circuit 2 runs pretty steady between 60 and 63 psig.
The dryer has been replaced and the sight glass shows a dry system. Thought the TXV could be the cause and it has been replaced only to have the same issue. Water side is common to the other circuit which does not have this problem. Superheat running 17-18 degrees, subcooling running between 8 and 13 degrees. refrigerant was weighed in to factory nameplate but it looks like it could use more refrigerant based on the subcooling. Before the TXV was replaced tried adding more refrigerant than nameplate and it did raise the suction some but it still had the fluctuating situation. I am leary of overcharging by 15 or 20 pounds on a system that is supposed to have a 65 pound charge. Any thoughts?