Compressor failure...... cause

[herefishy]

I had a compressor failure (runs, won't pump) on an open display merchandiser. System is Master-Bilt MD-06 with Master-Bilt (matched) C.U., R-22. It is one condensing unit serving one 6' case. An EPR is applied and set to approx. 20degF (according to factory recommendation).

Since I consider compressor failure a sympton, I concentrated on finding the cause. The techs performing the compressor change out, upon start-up found TEV operation erratic, and indicated to me that they interpreted the symptoms to be a restriction. Upon examination of the TEV, the adjustment stem was run all the way in (by others) which evidenced that some previous (others) had been addressing perhaps a similar issue. It was determined that a strainer at the TEV inlet was never installed. We replaced the valve (and installed the supplied strainer) for the sake of it anyway (it was a ODF connection TEV, in case your wondering).

I arrived at the site, and spent awhile "dialing in" the TEV. I made sure the EPR was at 20degF, and upon doing so I realized that the compressor was being unloaded (by the EPR), to the extent that the compressor suction was below 10psi, and even with the low pressure cut-out set at nearly 0psi, occasionally the compressor would cycle on LP. Of course, I will not allow a med. temp R-22 unit to run below 0degF (20psi) on even a pump-down!

I went back to the box, and the thermometer indicated 30degF to 32degF. The temperature control would not cycle. The unit seemed to be constantly "ON". A-HAH !!!

I decided, that the malfunctioning (temperature) control allowed the CU to run continuously regardless of case temperature. The EPR prevented the Evaporator from getting excessively cold or freeze-up, and under extremely low-load conditions, the compressor suffered low mass flow, oil return, etc., etc.,.

Until I could obtain the proper control, I set the low-pressure cut out at 20psi, the cut in at an appropriate cut in range (around 40degF SST), in order to prevent the low return suction pressure scenario at the compressor, and essentially control case temp via the low pressure control.

Today we returned w/control. Checked superheat (at about 9degF) which I felt comfortable with considering I personally set previously, and know that where I left it was at it's most stable setting.

I don't care for the EPR in this application. I asked M-B if they recommended the EPR in this "single case" application, and they said, "definitely". I believe that the EPR isolated the CU from the problems at the case (aside from the fact that the LP cut out was at 0psi), and allowed the compressor to be damaged.

What do you think?