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  1. #1
    Join Date
    Apr 2005
    Location
    Rapid City SD
    Posts
    28

    Hmm

    I have a McQuay with two 10 ton Copeland scrolls on a VAV system that has me stumped? The ambient was 90deg F. Head 225 psi, suction 57psi. At the coil suction line was running 42deg F for a superheat of about 9 or 10. CFM was running about 6000 (a little low for 20 ton) I think that's why the coil was so cold. So this is where it stumps me... the site glass at the TXV was clear except for about every 1.5 minutes it would get really foamy for 15 seconds. So I went out to the outside unit and checked the superheat and subcooling to find the superheat to be less at the end of a 90'(aprox) line set. Suction line was running 34deg F??!(HOW?) The installing contractor also installed a site glass at the condensing unit and the same thing was happening there..site glass was clear but every 1.5 minutes it would foam up for about 15 seconds.
    I thought that the TXV was hunting but the pipe temp at the coil was rock steady at 42deg F and at the unit the pressures were rock steady as well as the 34deg F at the suction line at the unit.
    The subcooling at the condensing unit was 15deg F and was rock steady too.
    How could I be getting intermittent foam at both site glasses without a TXV hunting??? I could see if the valve was opening and using all the liquid and then shutting and stacking it up again. But the valve didn't seem to be hunting. I still don't know how it can get colder on the suction line as it gets out to the condensing unit????? Most of the insulated line was running through a mechanical room that was 78deg F. I was using the same Cooper SH66 therm inside and out for temps.

    Anyone have any thoughts???
    Thanks.

  2. #2
    Join Date
    Dec 2002
    Posts
    603
    Sounds like the suction line exceeds a 3*f temperature drop which indicates a restriction in the suction line such as a kinked line or a line to small and or a restriction in a suction filter/drier.

  3. #3
    Join Date
    Apr 2005
    Location
    Rapid City SD
    Posts
    28
    The suction line I think is 1-5/8 with a 7/8 liquid. But the suction does have 2 DEEP oil traps. Why I don't know, from everything I have learned about piping I don't think it they were needed. The evap sits higher than the condenser. Out of the coil it rises 6'(trap at coil)then goes horizontal for about 40' then falls about 20' at that point it goes outside but not before it has another trap. (that's the dumb one,I can't see any reason for it, its falling!)After it goes outside its horizontal all the way to the unit but it rises up a foot (with 45* elbows) into the unit. (Another oil trap??? I think so!)
    There is no suction line filter dryer.

    Thanks for the info shogun8, that makes sense because of the traps.

  4. #4
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    bigD

    Low suction pressure + low suction line temp = low air flow through indoor coil

  5. #5
    Join Date
    Dec 2004
    Location
    Mid-west
    Posts
    567
    VAV and DX are always a problem due to low air flows, with no capacity control your even in bigger trouble. At low load your txv's are probably hunting causing a little flashing. Run it at full load and see what happens.

  6. #6
    Join Date
    Oct 2004
    Location
    Metro ATL
    Posts
    454
    At 6000cfm what kind of static do you have? If the VAV's are closing and no VFD to control fan speed, the fan could be running on the back of it's curve and thus the low CFM.

  7. #7
    Join Date
    Apr 2005
    Location
    Rapid City SD
    Posts
    28

    Smile

    I work at a hospital doing A/C and refrigeration work that this unit is located at. I have the luxury of hiring a contractor in when I get stumped. So today I had a very sharp tech come and look at it. We found that the superheat was a little low and we closed the TXV down
    1-1/4 turns closed. After doing that the superheat came up to 18degF but the subcooling went to 35degF. (Wow) We proceeded to take gas out as it was overcharged by 11 lbs.
    Subcooling came down to 21degF and the superheat stayed at 18degF. The site glass cleared up when the TXV was closed down and the oil level came up in the compressors when the suction temp warmed up. Hard to believe that just 1-1/4 turns closed would make all that difference.

    Onetime: I aggree that it is always hard with a VAV system as the CFM changes you can't stage enough to keep up with the load.

    jemawalton: The static pressure is controlled by a VFD and was at setpoint of 1.5wc. And with 90degF ambient I don't think that the VAV's were closing down. The discharge temp was about 5degF from setpoint.

    I think that we will still look at taking out the dumb trap at the bottom of the piping. Real bad idea having that in there.

  8. #8
    Join Date
    May 2002
    Location
    Baton Rouge, Louisiana
    Posts
    3,881
    Those superheat and subcooling temps dont seem exactly right still, but you have alot of variables to factor in.

    If that Mcquay unit is in warranty dont let them know your running a 18 degree superheat.
    Quote Originally Posted by MatrixTransform View Post
    very soon it is you that will be pwned

  9. #9
    Join Date
    Apr 2005
    Location
    Rapid City SD
    Posts
    28
    The unit is not in warranty and the superheat was right at the compressor. Doesn't Copeland require at least 20degF of superheat at the compressor??

    Your right the temps are not excactly right but when we went to half capacity it was the best compromise we could do.

  10. #10
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    BigD

    Shutting down txv was not the cure. It only disguised the symptoms. Low air flow through coil is the problem, shutting down txv just allows less refrigerant into coil to raise coil temp.
    The efficiency of the system has now been compromised.

  11. #11
    Join Date
    May 2002
    Location
    Baton Rouge, Louisiana
    Posts
    3,881
    Just to pass on we have a few years old chiller(2 Mcquay)that burnt up a few compressors before we even got the contract, a few after the contract-and we go through a time consuming start up routine and paperwork, that is checked by at least 2 supervisors, than the vp of operations before the start up is okayed and filed away, on the last compressor lost, we were told that they would not warranty anymore compressors because they recommend 8 degrees superheat at compressor and we had it set to run at 12 degrees.

    Just fyi for everyone.
    Quote Originally Posted by MatrixTransform View Post
    very soon it is you that will be pwned

  12. #12
    15 degrees subcooling is overcharged, txv pinched off, low airflo will frost suction line, undercharge will frost evap coil.foam is oil coming out of evap coil. system could be oil logged.

    remove refrigerant till you get about 10 degrees subcooling the recheck.

  13. #13
    Join Date
    Apr 2005
    Location
    Rapid City SD
    Posts
    28
    Originally posted by joey791
    Just to pass on we have a few years old chiller(2 Mcquay)that burnt up a few compressors before we even got the contract, a few after the contract-and we go through a time consuming start up routine and paperwork, that is checked by at least 2 supervisors, than the vp of operations before the start up is okayed and filed away, on the last compressor lost, we were told that they would not warranty anymore compressors because they recommend 8 degrees superheat at compressor and we had it set to run at 12 degrees.

    Just fyi for everyone.
    So why would the guy at Dennis Supply (A/C wholesale house) That went to a school on Copeland put on by Copeland said that Copeland requires 20* superheat at the compressor to assure no flooding back??

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