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  1. #1
    Join Date
    Dec 2011
    Location
    Fort Worth, TX
    Posts
    80

    High superheat with NU22B

    I changed out one 6 ton circuit on a 2 circuit 12 ton RTU and converted a 5 ton split system both to NU22B. The building owner wants something less expensive than R22 to use when we find units with issues. There are about 65-70 units. Both of the units I changed over have high superheat. I followed instructions and started at 80% of original charge and on the RTU I ended up with about 2 lbs over. I never got below about 35-40 superheat. It seems to cool fairly well as these units are about 15-30 years old depending on which end of the facility I am working on.

    If I remember correctly, the RTU had an orifice and the split system had a txv. Both had the same thing with high superheat. Any ideas? Is this normal? At the time of conversion the indoor temp was about 90-95 and the outdoor was about 100-105.

  2. #2
    Join Date
    Nov 2004
    Location
    up in the hizzy
    Posts
    1,295
    Nu22b has less capacity than r-22, with such big load the units will struggle, try adding more juice and see if SH comes down, also try opening the txv a couple of turns

  3. #3
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    5,674
    Are you using the r22 pt chart or nu22b dew point and bubble point

  4. #4
    Join Date
    Dec 2011
    Location
    Fort Worth, TX
    Posts
    80
    I am using the NU22b chart

  5. #5
    Join Date
    Dec 2011
    Location
    Fort Worth, TX
    Posts
    80
    It seems like the evaperator coil is starved. ???

  6. #6
    Join Date
    Jul 2005
    Posts
    5,156
    Quote Originally Posted by Jbart View Post
    It seems like the evaperator coil is starved. ???
    Yep, but that's a given because that's just another way of saying the SH is too high.

  7. #7
    Join Date
    Nov 2004
    Location
    up in the hizzy
    Posts
    1,295
    when the makers of the "flavor of the month" tell us to "charge 80% of the original charge", all they doing is attempting to mimic the original refrigerant pressures and temperatures disregarding capacity, they figure most of the units out there are over sized so a 20% drop in net refrigeration effect wont be noticed.

    If you add more juice SH may drop but the rest will go out of whack.

  8. #8
    Join Date
    Dec 2011
    Location
    Fort Worth, TX
    Posts
    80
    Thanks for your comments but I'm looking for answers. 20% reduced capacity has nothing to do with my high superheat. Has anyone used this stuff?

  9. #9
    Join Date
    Sep 2002
    Posts
    11,568
    The air entering the evaporator is 90-95 and you want to know why the SSH might be high? <g>

    Let the units run to stabilize and check them again 'at conditions'. Give it twenty four hours of continuous operation - looking for 72-75 inside temps - and then take all the readings again.

    PHM
    ------




    Quote Originally Posted by Jbart View Post
    I changed out one 6 ton circuit on a 2 circuit 12 ton RTU and converted a 5 ton split system both to NU22B. The building owner wants something less expensive than R22 to use when we find units with issues. There are about 65-70 units. Both of the units I changed over have high superheat. I followed instructions and started at 80% of original charge and on the RTU I ended up with about 2 lbs over. I never got below about 35-40 superheat. It seems to cool fairly well as these units are about 15-30 years old depending on which end of the facility I am working on.

    If I remember correctly, the RTU had an orifice and the split system had a txv. Both had the same thing with high superheat. Any ideas? Is this normal? At the time of conversion the indoor temp was about 90-95 and the outdoor was about 100-105.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  10. #10
    Join Date
    Dec 2011
    Location
    Fort Worth, TX
    Posts
    80
    But the R22 circuit has no problem with high superheat, only the NU22B

  11. #11
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    17,029
    That's because the new chemistry is a compromise. The high superheat is caused by the compromises being made. You were not expecting identical behavior and cooling ability.

    Were you?
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  12. #12
    Join Date
    Dec 2000
    Location
    Flat Rock, NC
    Posts
    463
    I believe that 407c would result in better flow rates and lower superheat in this application.

  13. #13
    Join Date
    Aug 2006
    Location
    Mixing oil and fire with a big spoon.
    Posts
    4,015
    Quote Originally Posted by codgy View Post
    I believe that 407c would result in better flow rates and lower superheat in this application.
    I believe that R22 would work best in this situation. It seems that everyone wants to switch old, crappy and under-performing units to a new 'miracle' refrigerant and then they get all surprised by the problems. The problem isn't the refrigerant...it's the unit. Fix the unit and move on. All the time and money that you are putting in this unit with refrigerant that wasn't designed for it seems like a waste. R22 isn't that much more expensive especially on such a small charge unit. If you don't understand things like 'temperature glide' and txv/orifice metering designs, then you aren't doing anyone a favor by re-engineering a unit.

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