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  1. #1
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    Trane 3D Scroll Tandem 50 Ton Split

    First post - I'm a fairly green refrigeration apprentice. Had a question on the Trane RAUC C50 EB2 - 50 ton split unit with four 3D scroll compressors, 2 in tandem with eachother (15T with 10T comps in tandem x2). I am just doing a maintenance on the unit and noticed that the sight glass is flashing on the two liquid lines. Now this is occuring when just 1 compressor of the 2 in tandem are running. I was curious as i've never worked on tandem compressors before, and where they share the same liquid line is throwing me off. If just 1 of these compressors runs, would that cause the sight glass to flash? Or is it a sign of being undercharged? I would assume the liquid line regardless would always need the full coloumn of liquid to feed the TX valve and ensure theres some subcooling going on. But then it makes sense that it would flash as the liquid line doesn't change size regardless of one of both compressors running and if just one is running it isn't pumping nearly half the refrigerant feeding that one coil, making it flash in the oversized line. Obviously quite confused. Any help of clarification on how this actually works would be greatly appreciated.

  2. #2
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    Sep 2005
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    Quote Originally Posted by Apprenticanic View Post
    First post - I'm a fairly green refrigeration apprentice. Had a question on the Trane RAUC C50 EB2 - 50 ton split unit with four 3D scroll compressors, 2 in tandem with eachother (15T with 10T comps in tandem x2). I am just doing a maintenance on the unit and noticed that the sight glass is flashing on the two liquid lines. Now this is occuring when just 1 compressor of the 2 in tandem are running. I was curious as i've never worked on tandem compressors before, and where they share the same liquid line is throwing me off. If just 1 of these compressors runs, would that cause the sight glass to flash? Or is it a sign of being undercharged? I would assume the liquid line regardless would always need the full coloumn of liquid to feed the TX valve and ensure theres some subcooling going on. But then it makes sense that it would flash as the liquid line doesn't change size regardless of one of both compressors running and if just one is running it isn't pumping nearly half the refrigerant feeding that one coil, making it flash in the oversized line. Obviously quite confused. Any help of clarification on how this actually works would be greatly appreciated.
    Always go by your subcooling to check charge (superheat will vary based on current load). A sight glass will flash sometimes at half capacity, it will also flash at first when a system has been off for a period of time and you have a high load. Your txv will be wide open. Years ago I made the mistake of charging a system just going by the sight glass. The system had a high load and I was charging it until the sight glass cleared up, after it satisfied and went to come back on the compressor immediately tripped on high head due to too much liquid stacked up in the condenser, I had overcharged it by about 10 lb.s (lesson learned). So get your system at full capacity and close to load setpoint, check your superheat and subcooling, and that will tell you if it's undercharged or not.

  3. #3
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    Quote Originally Posted by _knight_ View Post
    Always go by your subcooling to check charge (superheat will vary based on current load). A sight glass will flash sometimes at half capacity, it will also flash at first when a system has been off for a period of time and you have a high load. Your txv will be wide open. Years ago I made the mistake of charging a system just going by the sight glass. The system had a high load and I was charging it until the sight glass cleared up, after it satisfied and went to come back on the compressor immediately tripped on high head due to too much liquid stacked up in the condenser, I had overcharged it by about 10 lb.s (lesson learned). So get your system at full capacity and close to load setpoint, check your superheat and subcooling, and that will tell you if it's undercharged or not.
    Ok thanks for the advice man, appreciate it.

  4. #4
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    The charge should have been weighed into that system, it's very difficult to charge a system with multiple stages in the same circuit using only the traditional charging methods. First, the system must be operating at full capacity and fully loaded and I would think that would be rare for a 50-ton split system. I wouldn't worry about a flashing sight glass unless the system performance was suffering, ie it's 75 degrees in the space and you can only get 65 degree supply air with all stages online.
    Saving the world...one service call at a time.

  5. #5
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    what type of AHU do you have? Is it Trane? VAV or CAV? Indoor or outdoor? Intellipak or modular (if Trane)?
    Saving the world...one service call at a time.

  6. #6
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    let's get him all confused. when a unit is running, you should have a clear sight glass. period. sure, you will have some flashing and then some clearing until the system gets stabilized. no matter how many compressors are running, you should have a clear sight glass...physics are physics and expansion valves REQUIRE AND ARE SIZED based on liquid refrigerant. please keep in mind that a flashing sight glass does not mean under charged...it means liquid and vapor are present. if the sight glass is after the filter-drier, the filter-drier could be partially plugged up. if you have a txv-based evaporator, your suction superheat is going to be very close to the same superheat no matter how many compressors are on.
    When the rich wage war, it's the poor who die - Linken Park

  7. #7
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    worked on tons of those units. good possibility that condenser is halfway clogged with dirt, and/or cottonwood. Did you feel the lines? If they burn your hands then you need to wash the condensers. Very common for that unit.

  8. #8
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    Quote Originally Posted by mallron View Post
    what type of AHU do you have? Is it Trane? VAV or CAV? Indoor or outdoor? Intellipak or modular (if Trane)?
    It's running on an Air Wise AHU in the mechanical room and it's on the Metasys VAV system. It serves a 4 story TD bank in southern Ontario. It definitely was not at full load but that's what made me nervous of when it does get hotter here and the other 2 stages need to run. It isn't having any issues as of yet to satisfy all the zones, cycling the 2nd stage compressor to maintain a 13.5* - 17.5* celcious DAT.

  9. #9
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    That's what i would have thought, is that you would always need the full sight glass to ensure theres full liquid serving the TXV's. I will have to check pres/temp drop across the LLFD to see if that could be causing the flashing.

  10. #10
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    Quote Originally Posted by superfittertech View Post
    worked on tons of those units. good possibility that condenser is halfway clogged with dirt, and/or cottonwood. Did you feel the lines? If they burn your hands then you need to wash the condensers. Very common for that unit.
    The lines are normal hand luke warm to the touch, except for when the solenoid opens and the unit starts, for the first say 20 seconds of the unit running it is hot like discharge line but i'm assuming thats because it hadnt had time to condense before sending that hot vapour down the line. The coil isn't caked dirty by any means but could use a washing as it hasn't it a couple years.

  11. #11
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    Sorry, that was in response to Jayguy's post.

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