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Thread: Trane SFHCC55

  1. #1
    Join Date
    Jul 2004
    Posts
    68
    Have a 55t rtu with 2 circuits. Each has 2-14t scrolls with 1 txv on each circuit. I noticed on the 2nd circuit when only one compressor starts my txv hunts and the sight glass flashes then goes full. Once the 2nd compressor starts all is normal. Is this due to a 28t txv trying to feed the evap with a 14t comp.? I also noticed my superheat gets to the point where liquid may be returning. I have no IOM's for this unit. As far as circuit 1 I don't know if it is doing the same thing. Does anyone know if this unit is designed to allow only 1 compressor to run on each circuit? From what I can tell both comp's on each circuit run together and are staged thru a time delay.

  2. #2
    Join Date
    May 2000
    Location
    Stafford, Va. USA
    Posts
    213
    Hi Tony,
    That unit will stage the compressors on independently of of each other, compressor or circuit. If one compressor on a circuit trips a safety, then both compressors for that circuit will be locked off until reset.
    The sightglass will not indicate that the circuit is fully charged or low on refrigerant. The subcooling must be checked with both compressors of that circuit operating. The subcooling should be in the neighborhood of 15 degrees.
    I hope this helps.
    Allen

  3. #3
    Join Date
    Jul 2004
    Posts
    68
    Hey Allen, thanks for the info. I thought about it last night and figured it might be overcharged. Arrived to the job and took readings. Found 40 deg subcooling. adjusted charge to 18-20 deg subcooling. Like you said you can't go by the sight glass. Now with one compressor I have solid superheat and no hunting. Sight glass does clear at higher head psi. Seems the old saying of clearing the glass with txv doesn't apply here.

  4. #4
    You can never go by a sight glass only subcooling.

  5. #5
    Join Date
    Jul 2004
    Posts
    68
    Hey Rog, I noticed trane in your name. I have a question on the charge data on the name plate. It says TOTAL CHARGE EACH CIRCUIT- CIRCUIT 1 53lb CIRCUIT 2 (is blank) This unit has 2 circuits. My local Trane rep is also confused. We figured each circuit holds half of the 53. Do you have any info?

  6. #6
    I'll check eagle on that.

  7. #7
    Join Date
    May 2000
    Location
    Stafford, Va. USA
    Posts
    213
    Hi Tony,
    The nameplate indicates 53 lbs on circuit 1, circuit 2 will be another 53 lbs. Both circuits are the same.
    Allen

  8. #8
    Join Date
    Jul 2004
    Posts
    68
    Hi Allen, When I recovered the circuit to replace the compressors, approx 27lb was recovered. We then weighed in the 53lbs (no leaks found when we questioned the 27lbs). After start up we encountered the above metioned subcooling and hunting problems. Adjusted the charge close to the 27lb and all seems ok. I checked with our local rep and they thought each circuit would be half of the 53. They are checking into it deeper to let us know.
    Some Trane mystery?????

  9. #9
    Join Date
    Jan 2004
    Posts
    1,176
    There is no way the charge is less than 53# per circuit.Even at 2# per ton that is 46# and it is usually more than 2# per ton.The expansion valves on these do hunt.I have done new S/U's where they hunt and fixed old ones where they hunt and have changed valves and moved bulbs and still hunt.You can feel it on the suction line when it opens and closes and you just have to keep messing with it and get it dialed in as best you can.You are better overfeeding than underfeeding so try for 15 degrees at the compressor.

  10. #10
    Join Date
    Jul 2004
    Posts
    68
    "2lb per ton" never heard that one. txv's should never hunt if all is correct with the system. and if you overfeed your probably going to flood.

  11. #11
    Join Date
    Jan 2004
    Posts
    1,176
    Just take a typical DX system and check the charge to ton ratio,it is usually always higher than 2# and sometimes over 3.

  12. #12
    Join Date
    Sep 2002
    Posts
    1,389
    It will go real close to 2-2.5#/ton if the unit has subcooling sections in the condensers. The bulbs need to be taken apart and inspected for cleanliness. They have to be clean and well tightened as well as re-insulated. Especially running half capacity on a twice as large valve.
    Do what you will, but my humble advice is to take the time and check them. You will walk away clean and not have flooding worries later.

  13. #13
    Join Date
    Jan 2004
    Posts
    1,176
    I just yesterday got both expansion valves on a new 50 ton unit to stop hunting by taking a gallon of oil out of each circuit.Superheat tracked fine and oil level is still ok, seems it may have been overcharged.My compressors went from sweating and slugging liquid to being smooth, no more noise changes or fluctuating suction pressures.

    [Edited by acjourneyman on 07-28-2004 at 10:46 AM]

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