Hi all -
I'm trying to troubleshoot this machine in a new (circa 2009) university campus office/lab building. The chiller seems to be oversized - it generally runs at between 15 and 30% of load. A previous thread seemed to have a similar problem, but no resolution:
The behavior we are seeing is that the chiller goes into a short cycling mode at low load - only at night. The machine will oscillate between off and on for hours on end - a particularly bad bout saw 24 cycles in 10 hours. This can't be good for the motor. I suspect the cycling repeats due to the differential to start and stop settings, which are +/- 5*. The chiller setpoint is 50*, and inlet temps vary between 55 and 60. However, it's not clear what exactly pushes the machine into the short cycling state in the first place - shouldn't the chiller be able to operate all the way down to 10% load or so?
The thread mentioned above does propose the solution of lowering the chilled water setpoint by 2-3*, but I'd rather avoid that solution and figure out what in the controls might cause this in the first place.
I should mention that there is a pump in series with the chiller inlet (constant speed, not on a VFD), and the building management system aims to keep the differential pressure across this pump at 32 PSI by using a PID loop to actuate a bypass valve that sits in parallel to the chiller, which shunts chilled water back to the inlet and reduces the delta-T of the chiller. Does this make sense?
I'm an electrical guy by training (see above, "shunt", "series", "parallel", etc.), so I've probably left out some important details. Please let me know how I can help you help me.