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  1. #1
    Join Date
    Apr 2010
    Location
    Central Minnesota
    Posts
    18

    Low suction on Trane RTU

    Working on an 7-10 year old Trane Voyager RTU thats about 5 tons i'm guessing, and experiencing low suction pressure. A different service tech where i work came to the conclusion that this unit had plugged orifices and I was sent to install a new orifice kit. When I got there, I did observe the orifices frosted up.

    Here are some numbers before I performed the work...

    ODA: 84 degrees
    Head press: 230-240#
    Suction: 38 - 42#
    Superheat: 50 degrees
    Subcooling: 22 degrees
    RAT: 75 degrees, SAT: 60 degrees (15 TD)
    LL temp: 88 degrees, Suction line temp: 68 degrees
    No temp drop accross the drier and no temp drop up to the liquid header.

    Changed out the orifices with the new orifice kit and weighed in charge...long story short... I'm getting the same numbers as above.

    This compressor had been changed out about year ago because it was shorted to ground...and why was it shorted?? Dont know, didn't get that info.

    I find this to be incredibly odd and not sure what to do next.
    Has anyone else experienced anything like this? Oil logged in evep? This is screaming a restriction in the evap, could it be? Possibly an oversized replacement compressor?

  2. #2
    Join Date
    Apr 2005
    Location
    where the beer flows like wine
    Posts
    2,871
    perhaps there is crap floating around from the previous burn out and it plugged the new orifices right away, I would get rid of the orifices and replace them with a TXV

  3. #3
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    19,558
    Before installing new orifices or a txv kit I would look at pressurizing the suction line with nitrogen and blowing out whatever might have been in the evap through the header connections, so that is a known value, and then the line between the dryer and header. That way, you have eliminated them as possibilities if more diag is needed later, and of course, to help eliminate the need for "more diag later."

    Your numbers suggest a lack of refrigerant in the evap, and I think you are onto something when thinking that the new orifices are blocked. One thing you can due is power the unit down and remove the belt, so you can see where in the evap the frosting occurs, and whether or not it is "even."

    Since you will be opening the system again, I think the TXV conversion kit is the way to go.
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  4. #4
    Join Date
    Jul 2011
    Location
    central Alabama
    Posts
    23
    I agree that txv,s are the way to go. However, I was told by Trane tech that these units run a lower than normal suction. Yours sounds a little lower than even they run, though.

  5. #5
    Join Date
    Jan 2003
    Location
    Kansas City
    Posts
    1,484
    disable the blower and see where the lines start to freeze first. That will be where your pressure drop will be. No temp drop over the liquid line drier does not prove a clear line drier. Make it a habit to install a liquid line tap after the line drier when you open up a system so you can check pressure drop and get an accurate subcool reading.

  6. #6
    Join Date
    Dec 2007
    Location
    houston, texas
    Posts
    3,787
    Did you replace the drier as well?
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  7. #7
    Join Date
    Apr 2010
    Location
    Central Minnesota
    Posts
    18
    Yes, I did replace the drier, and yes, I really wish I would've installed a tap just before the liquid header...will be time consuming to do now. Also, with 22 degrees subcooling measured at the LL header, there has to be a full column of liquid to the orifices...I realize crazy things can happen, but im convinced there's a full column.

    Couple things I did forget to mention. The old orifices looked clean as a whistle, so I'm not convinced this is the problem anymore, and I don't believe retrofitting with a txv will remedy this particular problem.

    Also, I did put an extra 3# of 22 in just to see what would happen which did improve the suction to 55#, but I was flirting with 300 #'s of HP.
    The reason I did this is because I thought I remembered hearing from a Trane rep a few years ago that there were some models of Tranes that had incorrect charges on the nameplate, and that 2# extra needed to be added...has anyone else heard of this?

    I did not get a superheat and subcooling reading after doing this becuase I got pulled off for an urgent service call so I shut the unit down for the weekend. But if my memory is correct, then I'm clinging to the possibility of a dirty condenser.

    Pulling the belt and looking for the frost pattern is a good idea that I will try on Monday. We also have thermal imager that I might bring along too.

  8. #8
    Join Date
    Jan 2008
    Location
    In the work truck
    Posts
    3,071
    Quote Originally Posted by Hook Echo View Post
    Yes, I did replace the drier, and yes, I really wish I would've installed a tap just before the liquid header...will be time consuming to do now.

    Couple things I did forget to mention. The old orifices looked clean as a whistle, so I'm not convinced this is the problem anymore.

    Also, I did put an extra 3# of 22 in just to see what would happen which did improve the suction to 55#, but I was flirting with 300 #'s of HP.
    The reason I did this is because I thought I remembered hearing from a Trane rep a few years ago that there were some models of Tranes that had incorrect charges on the nameplate, and that 2# extra needed to be added...has anyone else heard of this?

    I did not get a superheat and subcooling reading after doing this becuase I got pulled off for an urgent service call. I'm clinging to the possibility of a dirty condenser.

    Pulling the belt and looking for the frost pattern is a good idea that I will try on Monday. We also have thermal imager that I might bring along.

    If you were running 22* of subcooling before you added the extra gas your overcharged..

    Where did you take the high side pressure reading?
    1. Discharge of compressor?
    2.liquid line?
    Gotta have the right tool for the job!

    Where is all the stuff MADE IN THE USA?

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  9. #9
    Join Date
    Sep 2006
    Location
    Ohio
    Posts
    498
    I would also check to see if it is moving enough air. The belt might be loose or some of the supply diffusers could be closed. Check the amp draw of the blower motor.

  10. #10
    Join Date
    Jan 2008
    Location
    In the work truck
    Posts
    3,071
    Quote Originally Posted by hands View Post
    I would also check to see if it is moving enough air. The belt might be loose or some of the supply diffusers could be closed. Check the amp draw of the blower motor.
    That high superheat says restriction..

    I could see your view if the superheat was lower.
    Gotta have the right tool for the job!

    Where is all the stuff MADE IN THE USA?

    "Thats what we do Troy. Incredible, Invisible, Imbelivable things. We are an Unseen, Unknown, Unvincible fraternity of craftsman.."

  11. #11
    Join Date
    Sep 2006
    Location
    Ohio
    Posts
    498
    Quote Originally Posted by Pascone10 View Post
    That high superheat says restriction..

    I could see your view if the superheat was lower.
    You're right, I missed that on his post. High SH and high SC, most likely a restriction.

  12. #12
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,689
    those units do run lower suction 60-65, not in the 40's.

    agreed no airflow issue or SH would be short.


    take bellt off as said and run, see where the restriction is

  13. #13
    Join Date
    Jan 2008
    Location
    Dallas ,Texas
    Posts
    3,725
    Block your condenser off and get you discharge pressure to 300 psi then recheck your readings.
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