oil pressure low

Hey guys (and gals?),

I am new to posting here. I have spent several sleepless nights reading some of the threads though. I appreciate the time you take to help others.

I have a small walk-in freezer box, approx. 16' x 18', with a small Copeland compressor. Recently, we have had trouble with the unit tripping on low oil pressure. When we put our gauges on it, we have plenty of pressure, about 35psi over crankcase. So we replaced the oil pressure switch. But we still get occassional nuisance trips.

I believe the unit trips when coming out of defrost. One technician suggested that it was a problem of the oil being thinned by refrigerant flooding back to the compressor. Can anyone explain the dynamics of this problem? What is the proper fix for such a problem?

Thanks for your help!

Regards,

Steve

Hi Skuech.
Speak to your technician again. He is likely onto something.
If you are using a pump down system for defrosting, an incorrect sequence of events can lead to refrigerant carryover.
Particularly if your room stat shuts the comp down on stat satisfied and not the liquid line solenoid valve.
Are you able to log when you get your nuisance trips?
Have you checked your crankcase heater another area which may contribute/
A bit more info on what is going on would help.
Grizzly

Hyup, check your liquid line solonoid valve to make sure it is not passing , make sure your evaporator is free of frost or any other blockage, then put it on a manual defrost. If the solonoid is passing it will not pump down and when the defrost period has ended liquid will enter the compressor and go into the crankcase. It will mix with the oil andmake it thin, the oli pump will not be able to pump the liquified oil and it will trip on low oil pressure. Let me know if this has helped.

Should have a suction accumulator and crankcase pressure regulator in line to avoid the chance of oil thinning or dilution. CPR will limit load on compressor after defrost.

Thanks guys, it gives some things to check out. I will let you know what we find.

I have seen this happen before. If the evaporater is dirty the suction can fall rapidly after the derost cycle is over. This can cause the oil to foam and trip out on Oil Pressure.
Worth a look.

Hey guys,

Just wanted to reply back to let you know what we found.

While my technician was observing the machine coming out of defrost one day, he noticed that the contactor was not pulling in all the way. We replaced the contactor and have not had the oil switch trip since (2 weeks).

Thanks again for your comments and suggestions.

Regards,

Steve

Thanks for the update Steve, glad to hear that it has been resolved.

hi skuech

you mean defrost contactor does not comes off after defrosting finished . am I right.
in this case compressor works more and your room temp. does not comes down well ,
but it is not reason you oil pressure comes off .
you problem is return liquid to compressor. you have to adjust your TEX valve .

If the contactor is not making a solid connection, the voltage to the compressor is low and the compressor trips on internal overload. The oil pressure switch times out and trips. Then the compressor internal overload cools and resets, but the oil pressure switch does not reset, so everyone thinks it is an oil pressure problem. I have seen this many times caused by faulty contactors, undersized wiring, bad connections, etc.

Quote:

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Originally Posted by Gary

If the contactor is not making a solid connection, the voltage to the compressor is low and the compressor trips on internal overload. The oil pressure switch times out and trips. Then the compressor internal overload cools and resets, but the oil pressure switch does not reset, so everyone thinks it is an oil pressure problem. I have seen this many times caused by faulty contactors, undersized wiring, bad connections, etc.

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Thanks for that Gary.
Your explanation makes a lot of sense!
Grizzly

Thanks Gary,

You stated the case much more eloquently than I could.

Thanks again to all for your comments.

Steve

hi gary
I agree with you if compressor has internal overload , and some small compressor has got this system and mainly hermetic compressors have this system and most of them has sensors (PT) for over heating inside part wiring .
but how about in normal and big size ?

Quote:

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Originally Posted by mbc

hi gary
I agree with you if compressor has internal overload , and some small compressor has got this system and mainly hermetic compressors have this system and most of them has sensors (PT) for over heating inside part wiring .
but how about in normal and big size ?

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Bigger comps usually have a number of thermistors in the windings and a box to monitor them such as a Kirwan INT69, some even measure the correct phase sequence on three phase scrolls. Seen these on comps from 2hp upto 40hp, which is the biggest i work on.