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Thread: oil failure

  1. #1
    Join Date
    Mar 2003
    Posts
    373

    Hmm

    I have a unit that is tripping occasionally on oil failure, this is a walk in freezer, with two seperate compressors running (each has two evaporator)and are totally independent of each other, oil pressure is fine, 70 pounds of oil pump pressure, (ie 110 - 40 suction)
    when customer arrives at warehouse in the a.m. sometimes will find it off, hits reset and starts up,
    they are loading meat in the morning after processing it and then taking it out following morning frozen to ship, this is a hot gas defrost system, and I put on a temp. recorder yesterday and found that is tripped at the 12:20 a.m. defrost cycle, hard to tell if it was during beginning or at end of it?
    I have manually put it into defrost and goes through fine, oil level fine,
    any thoughts?????
    hem

  2. #2
    Join Date
    Nov 2001
    Location
    east kansas
    Posts
    8,032
    Unit Short cycling. The constant on and off is allowing the OPS's heater to heat and trip the OPS.

    I had one do this to me when it was short. Cycling on the low pressure switch.

    Maybe, good luck.

  3. #3
    Join Date
    Mar 2005
    Location
    Alberta, Canada
    Posts
    474
    sounds to me like flooded start during low loads. Is this a pump down system? Elect defrost? or is this a vaporizor HG?

    where is the compressors located? warm amb? or cold? Crank case heater? Does the coil pump down before a defrost? is it t-stat controled? solenoid drop? need some more info man

    reasons for oil failure:

    No oil pressure
    Faulty Oil failure control
    Comp failure
    plugged comp pickup screen plugged
    plugged oil supply filter plugged (if external supply)
    plugged oil pot inlet screen (slow fill)(if external supply)
    Liquid refrigerant diluting oil
    Comp crank pressurizing and preventing oil return to sump

    and the one that stumps some people... Compressor power off but control power on. (comp wants to run, but no juice to actually run )

  4. #4
    Join Date
    May 2005
    Location
    ma
    Posts
    1

    Question oil failure

    need more info to help whats the cut in/cut out?
    does it shut off within 10psi?
    does the oil heater work?
    what condition is the oil in has
    anyone added leakfinder to the system?
    as this will thin out with heat
    doea this happen often
    I ran into a problem like this the lag comp
    always failed on oil failure i found
    the controls wired wrong o/f stayed energized
    when the low pres switch opened

  5. #5
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,586
    I have seen intermitant oil failures caused by the compressors internal overload tripping. Does this unit have a current sensing relay for post defrose? Maybe the CPR valve is set too high and over amping post defrost?
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

  6. #6
    Join Date
    Jun 2004
    Posts
    182
    hem057,
    Is this a Copeland compressor? The oil pump on a Copeland won't develope 70lbs differential pressure because of an internal bypass. Post where you are getting your pressures from, because if they are accurate, bryan l is right on the money with a pressurized crankcase. Remember the 40lbs is crankcase pressure NOT suction pressure.

  7. #7
    Join Date
    Mar 2003
    Posts
    373

    just to add to

    I did mention that it is hot gas defrost, so unit runs all the time even in defrost, the only time it would cycle off is if the temperature was satisfied, and they have both set to -15 f., so I don't think they will ever achieve that, I did adjust the No. unit to -8 F, but the owner just called and it had tripped off this afternoon, (usually it trip at night, 12.30 a.m. the other night,)
    and then again it could run for days and never trip, the oil failure control is working, have you ever seen them work intermittent?,
    the fact that it will run for days rules out some of the question being asked by techs, and other question asked are ruled out due to the hot gas defrost, ... above that is,
    no additives to the oil, it is a.b. and with 408, used to be 502 originally, it is a 9RB1 0760 TFE compressor by the way, suction is 15 while I was there, oil discharge at 85, for net oil of 70, during defrost climbs to 45 suction, and the return temp during cooling is 11 F, so that would be around 25 superheat....oil is not thrashing or foaming, and if screen plugged wouldn't we see that in the lack of oil pressure all the time, and if this was wired wrong then the unit would have been acting up for years, I have been doing all there work for last two years plus, so I know it has been running fine?
    thanks for your comments, helps to have a 2nd set of eyes looking at something at times
    hem

  8. #8
    Join Date
    Mar 2003
    Posts
    373

    oops

    I should have read the current replies before posting, I will have to check that total oil pressure now to make sure
    thanks
    hem

  9. #9
    Catch it when it trips.

    Tell us what the oil level sightglass looks like when you come upon the system.

    If there is oil in the glass, and the oil safety is tripped, then you probably have a bad contactor.

  10. #10
    Join Date
    Feb 2004
    Location
    Medford Oregon
    Posts
    807
    Suction pressure should not be used to arrive at
    net Oil Pressure

    Net Oil Pressure is
    Oil pump outlet pressure minus crankcase pressure
    (oil pres can't get to 70)

    compare suction pres to crankcase press, this will not
    give oil press, but will tell you if blow by is occuring
    your difference is probably too high (i.e. too late)

  11. #11
    Join Date
    Mar 2003
    Posts
    373

    again

    the oil level in the sight glass is about 1/4 of the way up the sight glass, have watched it through a complete defrost and the level stays the same, there is a little foam layer on top of the oil 1/8 inch at most, and the contactor is new, replaced it a month ago
    hem

  12. #12
    Join Date
    Mar 2003
    Posts
    373

    tomorrow

    I will get tomorrow the actual reading of the suction at intake to the compressor, the suction on the compressor body, the oil pressure on the oil pump outlet port, and then that should give the numbers you have asked for
    hem

  13. #13
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,586
    Check your amperage post defrost(bet it over amps), add some oil since it looks like your level could be dropping during low load condition!
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

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