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  1. #1
    Join Date
    Feb 2006
    Location
    Phoenix,AZ
    Posts
    2,893

    High pressure trip w/ low charge

    How is this possible on a 30RAP106. The IOM says it could!

    This is what it says:

    T126 Alert Circuit A High
    Head Pressure
    Compressor operation outside
    of operating envelope. Circuit shut down
    Automatic, only after
    first 3 daily occurrences.
    Manual reset thereafter.
    Reading from OAT
    sensor must drop 5 F
    (2.8 C) before restart


    And these are the possible causes:

    Faulty transducer, low/
    restricted condenser
    airflow, low refrigerant charge,
    faulty EXV

  2. #2
    Join Date
    Mar 2010
    Location
    Morgan Hill Ca.
    Posts
    1,219
    Quote Originally Posted by KB Cool View Post
    How is this possible on a 30RAP106. The IOM says it could!

    This is what it says:

    T126 Alert Circuit A High
    Head Pressure
    Compressor operation outside
    of operating envelope. Circuit shut down
    Automatic, only after
    first 3 daily occurrences.
    Manual reset thereafter.
    Reading from OAT
    sensor must drop 5 F
    (2.8 C) before restart


    And these are the possible causes:

    Faulty transducer, low/
    restricted condenser
    airflow, low refrigerant charge,
    faulty EXV
    Think about it for a minute.....

    Makes perfect sense to me. What would happen if you had just enough refrigerant to keep the system from dropping out the low pressure switch. No condensing, no cooling, no cooling the pressure just goes higher and higher to to heat build up.

    GT
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  3. #3
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    Feb 2006
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    Phoenix,AZ
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    Quote Originally Posted by GT Jets View Post
    Think about it for a minute.....

    Makes perfect sense to me. What would happen if you had just enough refrigerant to keep the system from dropping out the low pressure switch. No condensing, no cooling, no cooling the pressure just goes higher and higher to to heat build up.

    GT

    Have you seen it?

  4. #4
    Join Date
    Mar 2007
    Posts
    3,108
    I have seen it on an RTAA; not the scenario GT described, but a high pressure trip as a result of low-charge and a hot chilled water loop. The condenser fans on the RTAA are staged based on system delta T converted to delta P. Low charge = low differential pressure = no condenser fan action = high pressure trip
    "There is no greater inequality than the equal treatment of unequals."

    -Thomas Jefferson

  5. #5
    Join Date
    Feb 2006
    Location
    Phoenix,AZ
    Posts
    2,893
    On this particular machine! Would you expect to see a temp. drop across the Liquid line dryer, if undercharged? I got a 12 degree drop across it and sight glass is flashing. Could the temp. drop be because the dryer is so close to the EXV. And again the T126 lockout boggled me. I'm thinking dryers plugged. Now i'm not sure?

  6. #6
    Join Date
    Mar 2007
    Posts
    3,108
    I have never seen this chiller, but I would say that any liquid line drier that is dropping 12 degrees is a problem. How close are we talking? Close enough to the EXV where the condensation from the valve body is affecting your temperature readings? I would think that 8-12" would be enough distance to get a reliable reading.

    I would expect to see a temperature drop across a plugged liquid line drier only get worse as the charge is decreased. When you have a low charge, the liquid is closer to the saturation point and will easily flash into a vapor with resistance to flow.
    "There is no greater inequality than the equal treatment of unequals."

    -Thomas Jefferson

  7. #7
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    Feb 2006
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    Phoenix,AZ
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    Quote Originally Posted by Tech Rob View Post
    I have never seen this chiller, but I would say that any liquid line drier that is dropping 12 degrees is a problem. How close are we talking? Close enough to the EXV where the condensation from the valve body is affecting your temperature readings? I would think that 8-12" would be enough distance to get a reliable reading.

    I would expect to see a temperature drop across a plugged liquid line drier only get worse as the charge is decreased. When you have a low charge, the liquid is closer to the saturation point and will easily flash into a vapor with resistance to flow.
    The EXV is probally a good 2 feet away!!

  8. #8
    Join Date
    Mar 2010
    Location
    Morgan Hill Ca.
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    1,219
    Quote Originally Posted by KB Cool View Post
    Have you seen it?

    Yes I have, it was a Carrier Aquasnap and it had a low charge and kept tripping the high pressure cut out.

    Virtually ANY liquid line drier with the refrigerant at saturation at a high rate of flow (read velocity) due to a high evaporator load and low refrigerant charge will read a higher than expected TD, not sure about 12*F but way more than 2* I think the highest I have seen was 7*F and the drier was fine. Once the system was topped off after a leak repair was made it dropped right back down to less than 1*F

    Liquid line driers are designed to filter liquid, not gas, so if the refrigerant is flashing off in the drier, you will see a large temp. drop.

    GT
    If a day goes by and you have learned nothing, I hope you got a lot of sleep.

  9. #9
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    Feb 2006
    Location
    Phoenix,AZ
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    2,893
    Quote Originally Posted by GT Jets View Post
    Yes I have, it was a Carrier Aquasnap and it had a low charge and kept tripping the high pressure cut out.

    Virtually ANY liquid line drier with the refrigerant at saturation at a high rate of flow (read velocity) due to a high evaporator load and low refrigerant charge will read a higher than expected TD, not sure about 12*F but way more than 2* I think the highest I have seen was 7*F and the drier was fine. Once the system was topped off after a leak repair was made it dropped right back down to less than 1*F

    Liquid line driers are designed to filter liquid, not gas, so if the refrigerant is flashing off in the drier, you will see a large temp. drop.

    GT
    That makes sense!! Thanks for the info!

  10. #10
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    Feb 2006
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    Phoenix,AZ
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    Well guys, pulled the charge and cut out of the dryer and i could barely blow through it!!! I'm still baffled by the head trip.

  11. #11
    Join Date
    Aug 2002
    Location
    Greenville SC
    Posts
    434
    If you have Micro Channel Coils, any restriction will cause a high pressure trip. Cut your drier open and see if a magnet will pick up the fiber filter material. If it does, get ready to change out your compressor.

  12. #12
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    Feb 2006
    Location
    Phoenix,AZ
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    Quote Originally Posted by chilbrig View Post
    If you have Micro Channel Coils, any restriction will cause a high pressure trip. Cut your drier open and see if a magnet will pick up the fiber filter material. If it does, get ready to change out your compressor.
    Yeah, it is a Micro channel. But, when i first got to the job. I think my head psi was no where near the tripping point!! But, anyways this what my boss had me do. What a PITA!!! I know the charge amount on the data plate is no longer correct!!
    Attached Images Attached Images

  13. #13
    Join Date
    Jan 2005
    Posts
    604
    Well, I had this alarm. I called for explaination.

    The official reason that all those symptons will flag a High Head alarm, "..we're computer programers...picked the five conditions and chose what we thought would be the most common....."......"essentially the compressor is running outside it's design envelope".
    Hope that helps. Hahaha

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