I have been asked to take on the service and maintenance of a large Dunham Bush chiller (circa 400kw) that is fairly new - 4/5 years - and operates on R407c.

Had a look at the beast today and noticed one or 2 things that didn't seem right.

The discharge pressure under load was 229psig (about 45C saturation) but the discharge pipe was measured at 84C - some superheat or what! and the customer says that the unit keeps tripping but they don't know why?

The second stage control circuit stat was set at 87C and the sensor is connected to the discharge line about 3" from the pot. The pot is a Hartford vertical screw with part winding start and unloader valves on a sort of sliding solenoid arrangement.

The adjacent machine (same size) is currently out of service because the sliding unloader has released into the screws and seized the pot!!!

Oil is not apparent in the sight level - where has it gone in a sealed machine with no sign of leaks.

I had the machine running and cycling for over 3 hours today and the pressures, temperatures and current draw were all OK. Any one know why (before I do the PH chart) the machine has such a high superheat?? and why it is running so close to the limit of the control stat??