Liebert model ct543a-as serial 311590-006 This is a three module chiller used in process at hospital. I happened across this on a PM, so this condition could have been occurring for a few months. #2 module was running but obviously not pumping so I put my gauges on and it was in a vacuum. The diagnosis was fairly simple a broken cap tube on TXV sensor bulb. The compressors are semi hermetic about 15 ton Carlyles. I replaced powerhead and contactor and compressor came on line ran beautifully, good amps and pressures. A penn low pressure switch is used on this machine but apparently only on a pump down situation because after the call ceased the control functioned properly. Megged compressor no damage, amazing. No jumpers on machine. Time delay parrallel to low pressure. About a year ago this happen on another Liebert chiller I was servicing. Is the only low pressure protection on this model the freeze stat, which would not be affected by the sudden loss of pressure? Don't have schematic in front of me now.
Most likely, b/c of the pumpdown feature, there would be no lp switch for low refrigerant but if the unit was in a vacuum and the contactor was not welded, then the pressure switch is a paperweight or wired wrong. If there is a low refrigerant psi sensor, it would be on the highside of the system open on loss. set around 30psi or so. Depending on the control logic, the lp may be the operating control while the solenoids are controled via the processor or aquastat.
Liebert LP problem
I am not familiar with the Liebert chillers, but have run into the same situation on a couple Deluxe 3s ovet the years.
Liebert has a time delay built into the board ( on some equipment ) that runs the compressor for 3 minutes regardless of the suction pressure to ensure there is liquid getting to the txv - then the thing won't short cycle itself to death. It is my understanding this is for remote condenser equipment in low ambient situations; however, at the site I am at now, we have 25 Deluxe 3s that are water cooled - same feature and not really necessary.
Try pulling a wire off the LP control on the other system and then initiate a call for cooling - it is a possibility.
Superheat and subcooling tell it all !
i have had a few issues with the pumpdown controllers on leibert systems
seems like sometimes the controller designers outsmart themselves with these particular pumdown controls
i know that on a few leibert systems, i have had to remove a tiny capacitor off of the processor board
in order for the board to function and react properly to pumpdown and low pressure situations
this was per leibert tech support
you need to run the system through all the sequence of operation and make sure the controller is operating properly
and if it isn't get on the horn with leibert tech support
the ones i had problems with ran fine for a few years and then all of the sudden start having control problems!