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  1. #1
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    What's this noise?

    Never had this happen before but it's happened 3 times in the past two months. Twice on a freezer once on a display cooler. I plug the unit in, everything starts up like normal. Runs 10-15 minutes. Everything looks good then bam, a sound that sounds like screeching and hissing and pressures go all out of wack. Unplug the unit it stops. Plug it back in and it does the same thing but within 2 minutes. I'm saying the compressors are shot. I'm guessing the valves. I changed one compressor and everything went fine, another one I am waiting for approval, the third didn't want to put the money in to it. I'm really wanting to know what exactly causes that sound. System is totally sealed and the one I changed the comp on had complete charge when I pulled it.
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  2. #2
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    Quote Originally Posted by RichO View Post
    Never had this happen before but it's happened 3 times in the past two months. Twice on a freezer once on a display cooler. I plug the unit in, everything starts up like normal. Runs 10-15 minutes. Everything looks good then bam, a sound that sounds like screeching and hissing and pressures go all out of wack. Unplug the unit it stops. Plug it back in and it does the same thing but within 2 minutes. I'm saying the compressors are shot. I'm guessing the valves. I changed one compressor and everything went fine, another one I am waiting for approval, the third didn't want to put the money in to it. I'm really wanting to know what exactly causes that sound. System is totally sealed and the one I changed the comp on had complete charge when I pulled it.
    Crankcase pressure regulating valve?

    They can make high pitched noises upon startup but usually stop once pressures stabilize

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  3. #3
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    Quote Originally Posted by UmmScott View Post
    Crankcase pressure regulating valve?

    They can make high pitched noises upon startup but usually stop once pressures stabilize

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    Should have been more specific they were reach in units and not equipped with CPR valves.
    Training is a forever thing in a good technician’s life.

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  4. #4
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    Are there any other components? Liquid solenoids? Etc?..
    What brand of reach in?

    Nameplates weighed in?

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  5. #5
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    Some compressors have an internal relief valve. Opens when high side pressure exceeds ---psi. Monitor your high side pressure after startup, and see if it goes too high. It sounds just as you described.
    Experience is what you have an hour after you need it.

  6. Likes UmmScott, c.sherman liked this post
  7. #6
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    Quote Originally Posted by baub View Post
    Some compressors have an internal relief valve. Opens when high side pressure exceeds ---psi. Monitor your high side pressure after startup, and see if it goes too high. It sounds just as you described.
    Didn't know that. Thanks.
    Training is a forever thing in a good technician’s life.

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  8. #7
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    Quote Originally Posted by UmmScott View Post
    Are there any other components? Liquid solenoids? Etc?..
    What brand of reach in?

    Nameplates weighed in?

    Sent from my SM-N920V using Tapatalk
    Nothing extra.
    One was a Fogel single door freezer with aCopeland RS43C1E-IAA-901
    One was a Beverage Air single door freezer with a Copeland something or other
    The third (which I replaced the compressor in) was an Oscartek display case with an Embraco NEK6213GK.

    First and third definitely charged to spec. Second one I'm no sure as someone else worked on it about two months before me and from what I understand got it running then the owner had no use for it so they unplugged it for a couple of months then plugged it back in and called me because it wasn't working.
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  9. #8
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    sounds like internal releif to me also, you should have seen head pressure real high if thats the case

  10. #9
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    I agree, internal pressure relief valve. On initial startup, oil and refrigerant absorbed in the oil may cause the relief valve to pop open almost immediately. Shutting it down will cause the system to continue to whistle a. while until it stops with a clicking sound. This is not a compressor failure.

    The high pressure scenario seems more likely because of the delay. Don't you, as a pro ALWAYS use both high and low pressure gauges?

  11. #10
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    Quote Originally Posted by chrigs View Post
    sounds like internal releif to me also, you should have seen head pressure real high if thats the case
    On on unit (the one I can remember) head pressure shot up right before it happened then while it happened it lowered a bit and suction shot up but until his happened pressures looked good. I didn't keep any notes so I can't tell you what pressures were. I just remember being perplexed because everything looked good for a while.
    Training is a forever thing in a good technician’s life.

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  12. #11
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    Quote Originally Posted by lynn comstock View Post
    I agree, internal pressure relief valve. On initial startup, oil and refrigerant absorbed in the oil may cause the relief valve to pop open almost immediately. Shutting it down will cause the system to continue to whistle a. while until it stops with a clicking sound. This is not a compressor failure.

    The high pressure scenario seems more likely because of the delay. Don't you, as a pro ALWAYS use both high and low pressure gauges?
    Of course I use both gauges. If it's not a compressor failure what is it?I get it not being a mechanical failure but if it's an internal port tripping with no outside causes wouldn't it be he compressor that needs to be replaced?Cleaned condenser and evaporator, fans working fine, unit operating in rooms under 80F, two of the units sealed from factory one ran two years fine the other 3-4 years.
    Training is a forever thing in a good technician’s life.

    I sell knowledge in the form of hours.

  13. #12
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    Dirty coils my friend

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  14. #13
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    Quote Originally Posted by RichO View Post
    I'm really wanting to know what exactly causes that sound.
    Internal Pressure Relief valve.

    Trip pressure depends on the difference between the high and low side pressures and it varies with refrigerant and compressor application. It takes a very high pressure difference. If the trip point pressure difference is regular I would think that it is the valve defective...Pretty rare.

    A slug of refrigerant and/oil can cause a trip on startup or while running if the suction line traps liquid somewhere and periodically releases a slug into the compressor. Careful observation of the pressures at the point of the trip can help differentiate the failure mode. Keep notes when strange things happen. If the trip point pressure difference is NOT regular i would take a hard look at the suction piping for unintended oil traps. A suction line accumulator would solve this problem.
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