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  1. #1
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    Suction Running in vacuum - Mixed Refrigerant - Revco -40 freezer

    Hi,

    I'm currently working on a Revco -40oC freezer (Single compressor model) (Model: ULT1740) that runs with a mixed refrigerant R134A and R404A at about (R134A)30%:70%(R404A) ratio.

    The compressor is running fine, no leaks, made sure of this.

    However the freezer gets stuck at -32oC and fails to cool further. Set temperature is -40oC I have evacuated the entire system, put vacuum for about 4hrs and charged the system again, but similar issue. After the system reaches -15oC it takes about 2-3 minutes to cool every degree and once it reaches around -25oC it takes 10 minutes to cool every degree and this time taken rises exponentially until it gets stuck reaching -34oC. All of this is observed under NO LOAD conditions-empty freezer.

    Things observed,

    • Suction side pressure is under vacuum. Even with correct amount of refrigerant charge it is running under vacuum

    • Frosting observed on suction line

    • Cooling happens very quickly from ambient temperatures to upto -10oC

    • I have not changed the filter drier yet.

    • It is a capillary tube system



    Please advise what the issue might be and also corrective measures to be taken. I'm quite confused over this and wish to resolve the issues.

  2. #2
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    May 2014
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    I'd call Revco. Quite often there are tricks to charging these. -40 is the ragged edge for 404A. I'm guessing the 134a is only there to move the oil in the ultra cold evap. And 30% sounds like too much, or said another way, there's not enough 404A in the system. Back when I was doing ultra low temp work, we were using R-12.

    The idea is at those ultra cold temps, the R-12 (or in this case 134a) is a liquid in the evap. And as a liquid, it thins the oil out so the oil can keep moving through the evap. And back in those days, it was only 10%. Sometimes less; you'd break the vacuum to 0 psig with the low pressure refrigerant, then weigh in the rest with the high pressure refrigerant.
    I do a triple evac with nitro to remove non condensables.

  3. #3
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    Unless it is an auto cascade system. In which case, there would be more than one metering device. Or there will a "box" that is sealed in foam with tubing going in and out.

    But probably not auto cascade -40. Like I said, that's the ragged edge for 404A.
    I do a triple evac with nitro to remove non condensables.

  4. #4
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    Thread Starter
    Quote Originally Posted by BBeerme View Post
    And 30% sounds like too much
    Exactly 213grams of R134A and 496grams of R404A.

    I'm really unsure how to proceed next... Any ideas?

  5. #5
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    Does it have a nameplate static charge? You should be running about zero on your suction imo.
    Honeywell you can buy better but you cant pay more

    I told my wife when i die to sell my fishing stuff for what its worth not what i told her i paid for it

  6. #6
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    Yes.

    As I was doing the therapeutic cleaning of the kitchen from last nights dinner and food prep for the day, lots of memories came flooding back. If it is a restricted cap tube, then you most likely also have a weak compressor, because there was no cooling coming back.

    That said, just a weak compressor will do exactly what you are seeing. So I would replace both.

    All that said, on these ultra low temps, you do not want to see a suction temp over 60*F. That's one thing you check for if doing a PM on these units.

    Had a So-Low cascade system one time where the suction was 65*F. Called the manufacturer. They said, yup, we run those units right at the theoretical limit of running safely. They were Bristol compressors and held up pretty well, actually surprisingly well.

    Late for work, gotta go.


    Quote Originally Posted by bibeku View Post
    Exactly 213grams of R134A and 496grams of R404A.

    I'm really unsure how to proceed next... Any ideas?
    I do a triple evac with nitro to remove non condensables.

  7. #7
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    Thread Starter
    Yes... It is mentioned on the name plate... Over a period of 8hrs of observation I noticed the suction pressure going from vacuum to 0 and back to vacuum... And it kept oscillating like this for every once in 30mins or so...

  8. #8
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    Quote Originally Posted by BBeerme View Post
    All that said, on these ultra low temps, you do not want to see a suction temp over 60*F. That's one thing you check for if doing a PM on these units.
    It was way less than 60*F for sure as the suction line was clearly frosting...

  9. #9
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    I would of changed the dryer just eliminate that part of the equation I’m with bbeerme weak compressor or partial restriction or both
    Honeywell you can buy better but you cant pay more

    I told my wife when i die to sell my fishing stuff for what its worth not what i told her i paid for it

  10. #10
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    Thread Starter
    Thanks for the help...

    I will be replacing the filter dryer first, in that case...

    Also, it is a 12 year old freezer and the location is around 500kms from my office, so I need to go prepared...

  11. #11
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    One thing that is intriguing me is that, if the suction is showing no pressure (and vacuum), how come the suction/return line is frosting? Could you please let me know on this...

  12. #12
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    Quote Originally Posted by crazzycajun View Post
    I would of changed the dryer just eliminate that part of the equation Im with bbeerme weak compressor or partial restriction or both
    After you change the drier I'd have a vacuum gauge on it when you evacuate, go as low as you can. I shoot for 250 microns or better with touchy stuff like that, you really can't go to far. If you can't get it there change the vacuum pump oil.

    Strange that it uses 134a...I hate that stuff...wouldn't surprise me if cap tube was restricted which makes deep evacuation and a clean system all that more important. Usually its a little propane they put in. Curious to see what you come up with.

  13. #13
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    I must say that when I pulled vacuum for 4hrs, I got a good vacuum of 29.89inHg (Approx. about 750 microns).

    Say the capillary tube is restricted, would the suction/return line still be frosting?? Because, the suction line in my freezer was frosting well.

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