Compressor failure...... cause

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?

Hi, Herefishy:)
just one comment about the L.P. cutout I would never have thought of setting the cutout at to cut out at anything other than 0 to 5 psig, that is a new one on me.
As for EPR's it is not recommened by any maufacturer to fit a single EPR on a single compressor/evaporator system. Why is it necessary to fit one in this case? Would it not be simiplier to switch off on stat or if product dehydration is a problem back the stat up with hot gas bypass.
On the pumpdown thing again, I would often sett L.P. switches to control at pressures on common/multiple evap/compressor systems, but generally to ensure that the refrigerant was mostly pumped out I would set the L.P at near 0 psig. In the past I have controlled freezers on L.P. cutout (the old Barker cases come to mind) but that was because the coils were under sized and they wouldn,t cycle properly on tstat control.
Regards. Andy.

Hi Andy,:)

Medium temp. R-22, pumping down to 0-5 psig puts you at around -40degF SST ! Imagine that SH during the pump-down period, WOWIE! Take into consideration the minimum rated suction temperature of the (medium temp) compressor. I was informed by a knowledgeable dealer/design guy that the 0-5psig cut out is not a good idea, and detrimental to typical med. Temp R-22 stuff.

I'll bet your 0-5psig Medium temp R-22 pump-downs take a while, huh? Have you paid attention to the sound of the compressor when it's pumping down to 0? I don't like that sound, myself !

As for why M-B would spec and ship the EPR on these single case units? I think that maybe it is to protect their thin evaporator coils from freeze-ups, maybe? I have had some MB evaporator coil failures, too many I thought. I have one technician who condemned two leakers within two weeks. I was accusing him of not actually finding the leak, and pointing to the coil. I called MB to ask if this was a problem. They said, "has the coil ever 'frozen up'?",

Me, "well, yes come to think of it."

They, "Then it may very well be leaking."

LOL!!!

It would sure make me happy to pull all the EPR's out of these single case systems! I thought it was wrong to begin with.

Hi, Herefishy:)
you do have a point about the pumpdown pressure on R22 especially. In my defense most systems I work on are semi's and well fit for the load during pumpdown. Also R22 is not fitted to anything of size on this side of the pond now as it is banned in all but aircon splits. R404a pumps down much nicer with a cooler discharge temp.
The smaller systems I deal with from time to time are generally not pumpdown anyway with hot gas def used in intergrals quite a bit instead no pumpdown required.
But I take your point about the pumpdown settings and I will be setting my switches higher on my med temp applications.
Regards. Andy.

Oh,..... back to the EPR thingy.....

Quote:

---

...it is not recommened by any maufacturer to fit a single EPR on a single compressor/evaporator system.

---

... But M-B DID recommend it, and to my knowledge, spec'd and supplied it. All these particular systems at each location of this particular national chain (of which there must be 20 stores which I service) are fitted as this system I describe.

Hi, Herefishy:)
I was refering to the valve manufactuer not the cabinet manufactuer who are generally a law onto themselves.
Maybe the Professor can shed some light on the subject?
Regards. Andy

P.S
I still think the EPR is a poor way to protect the intergrity of a evaporator!!

on a past call that I contacted factory tech support on (don't remember what brand) for a self contained unit I asked why they had a suction control on it and I was told they had to have it to meet a UL (underwriters lab) standard. I believe it had to do with amp draw. Tech suport told me to go ahead and remove it. Sory I can't remember what brand or application.

Alright Magnumman
I think you may have crossed lines here as I think you are referring to a crankcase pressure regulator wheras they are talking about an EPR

Your right I had just gotten home from repairing a unit with a Cpr valve and had them stuck in my head.