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  1. #1
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    anybody seen weird pressures before

    one of the other techs in our office came across this on a dx chiller, scroll compressors, remote condensor 3 stories above, Sporlan OROA head pressure controller, r22.

    the discharge line is at a lower pressure than the liquid line pressure. i am not talking about 1-2 psig lower either...like 20-25 psig lower. i do not know all of the details for sure, but his is a pretty good tech and would have waited for pressures to settle out before questioning it. he checked it after it was running (the unit runs 24/7/365) for a while. he checked on 2 different gage ports on the discharge line and 2 different ports on the liquid line. it was warm enough outside to use 100% liquid line, but i do not know for sure if the OROA valve was opening or not, but this tech would have mentioned that if it was...hell, he wouldn't have even asked me any questions if it were! he is good.

    has anybody seen something like this before? i have only once for sure. it was on a dx recip chiller with a water cooled condensor. total line set was maybe 5 feet.

    now before anybody tells me to buy a new set of gages because they are cheap or something (who will be the first to NOT read this entire post before commenting? ) i use (and he used) the same gage for checking both. i have a small ball valve in my manifold set (kinda like the hoses with the ball valve in the end) that allows me to shut off the high side hose and open the liquid line hose and read that pressure. it is very quick and it uses the same gage.

    any thoughts?

    good luck (to me this time )
    my 1st time jumping out of a plane...http://youtu.be/Kv38G0MHsGo

  2. #2
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    Feb 2003
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    I bet the oroa valve was backed off a bit, he probally measured the discharge down stream from the oroa valve. liquid line pressure probally has some of the discharge gas going into it is why it was higher. if he measure the discharge right off the compressor it will not be lower than the liquid line.
    but I am not there to see where all the access is at. confused now?

  3. #3
    Join Date
    Jun 2001
    Location
    Greeneville, Tennessee
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    HeadMaster

    The OROA-5-180 is flooded head pressure control type valve. When head pressure falls below the valve's setpoint (180 psig in this case), the valve backs up liquid into the condenser which effectively eliminates some of its condensing surface, and thereby maintaining head pressure. The OROA will also bypass hot gas to the receiver when the pressure difference between hot gas pressure and receiver pressure exceeds about 15 psi. In this way, receiver pressure will be maintained when the valve is backing up liquid into the condenser.

    If your condenser was sized at a 15F TD or lower, and its pressure is being controlled at 180 psig, then one could expect some hot gas bypass to the receiver at a 75F ambient. If hot gas bypass stops at say an 80F ambient, you're probably ok. If not, you might be a bit short of refrigerant.

    If the condensing pressure is significantly above 180 psig and you are observing hot gas bypass, then something else of interest is going on here. Two possible problems: (1) something is restricting liquid flow thru the valve; or (2) you may have a high pressure drop across the condenser coil causing the OROA to bypass all the time. If you are simply fighting with a condenser coil with a high pressure drop, you can solve the bypass problem by adding an ORD-4 differential valve in the hot gas line connecting the discharge line to the OROA.

    When the OROA is bypass hot gas to the receiver, both hot gas and liquid from the condenser are being mixed in the receiver which maintains receiver pressure. But the hot gas bypass cannot occur unless there is a pressure difference between hot gas and receiver pressure.

    Note that if the OROA dome has lost its charge, the valve would act little more than a tee fitting. You would see low head pressures on a cold day, and no hot gas bypass.

    Liebert has used these valves for years.

  4. #4
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    all pressures were taken before the OROA. the liquid line pressure (before the OROA) is higher (about 225 psig) than the discharge line pressure (about 200-205 psig) which is taken at the compressor. values taken at the receiver would be subject to the OROA valve at some point in its stroke (whether working properly or not) and i do not know what those values are as they were not part of the subject question.

    has anyone seen this before? (jayguy, you already asked that! ) i am wondering if the pressure tap is at angle that would cause a higher/lower pressure than expected (kinda like a pitot tube in an air stream that is tilted weird or something and measuring velocity pressure) or is there some kind of eddy swirl creating a lower pressure in the discharge tube? just some possible thoughts as to explain this phenomenon.

    thanks!
    my 1st time jumping out of a plane...http://youtu.be/Kv38G0MHsGo

  5. #5
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    Quote Originally Posted by jayguy View Post
    the discharge line is at a lower pressure than the liquid line pressure. i am not talking about 1-2 psig lower either...like 20-25 psig lower.
    This can't happen, at least for an extended period of time.

  6. #6
    Join Date
    Oct 2003
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    tidewater, va
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    Jayguy,
    I have seen this before on an old Carrier split heat pump. I remember thinking "Hey, how can this be?" The liquid pressure was about 12 to 15 psig higher if I remember right and after talking to the boss, all it could be chalked up to is WEIGHT of liquid refrigerant. If your scenario is how I read it, remote condenser ABOVE the unit where you are getting readings, and a full liquid column, it has to be the weight of the liquid. someone may shot this theory down with a law, but the fact remains if you took the reading, and the other guy took the reading...it is happening.


    r404a

  7. #7
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    Jan 2004
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    Lancaster PA
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    R22 liquid lines lose .5 PSIG for every foot of rise.
    R22 liquid lines gain .5 PSIG for every foot of drop.

    So 3 stories is 30 foot plus. So yea, you could gain 15 plus PSIG.
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  8. #8
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    Quote Originally Posted by beenthere View Post
    R22 liquid lines lose .5 PSIG for every foot of rise.
    R22 liquid lines gain .5 PSIG for every foot of drop.

    So 3 stories is 30 foot plus. So yea, you could gain 15 plus PSIG.
    While reading the posts,I was thinking "could it just be the weight of the refrigerant? " so I vote for beenthere on this one. If anyone cares.haha

  9. #9
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    Mar 2005
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    ..
    Last edited by stanbyyourword; 10-10-2008 at 08:06 AM.
    Keep it simple to keep it cool!

  10. #10
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    yep , its the column of liquid i`d bet install a tap at roof and compare stan
    Keep it simple to keep it cool!

  11. #11
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    Quote Originally Posted by r404a View Post
    If your scenario is how I read it, remote condenser ABOVE the unit where you are getting readings, and a full liquid column, it has to be the weight of the liquid.
    Ok, I stand corrected. If you're condensing unit is way way up there, it can happen.

  12. #12
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    Quote Originally Posted by r404a View Post
    ...The liquid pressure was about 12 to 15 psig higher if I remember right and after talking to the boss, all it could be chalked up to is WEIGHT of liquid refrigerant...

    r404a
    this is what i was thinking too on this unit, but it does not explain the old dx water cooled unit...no matter...as long as something tries to make sense. i will mention this to the other tech and see if he can get a reading "up top".

    thank you very much to all who have posted.

    it was good luck to me!
    my 1st time jumping out of a plane...http://youtu.be/Kv38G0MHsGo

  13. #13
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    I love this site. I learn something new everytime I'm here.

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