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  1. #1
    Join Date
    Dec 2006
    Posts
    35

    Too low superheat for 5 ton Liebert a/c

    5 ton Liebert used in Data room-uses 22-air cooled rooftop-variable speed condenser fan-according to records comp and Externally equalized valve changed only 1.5 yrs ago-due to remodeling contracters moved lines to rooftop over a few feet-then workers noticed comp very cold and sweating-tech I know bled air from lines on roof and that helped but not long afterwards the compressor died-I install new comp but I have 73psi on low side with 48F on suction line which means my superheat is too low-unit cools with 55f to 60f discharge air but I worry about superheat and how to make sure this compressor will last--any thoughts--any way to check expansion vaive and/or valve sensing bulb-any way I can make sure variable speed for condensing fan is working properly--Im concerned about liquid fllodback and I know Copeland wants at least 20 superheat thanks!!

  2. #2
    Join Date
    Jan 2008
    Location
    Hells Kitchen, Phoenix Arizona
    Posts
    318
    you have 5 degrees SH and copeland wants 20? what is the valve set for? Sounds like you have a bad/wrong txv...alot of txv's are set to 12 - 15 I don't know about Liebert...I would try cleaning the bulb and the suction line and strapping it down tight and wrapping it...Then recheck...5 degrees doesn't sound right...

    PS. It might be overcharged, whats the subcooling???
    "Overkill is an often underrated achievement", Will Hayden -- Red Jacket Firearms

  3. #3
    Join Date
    Aug 2006
    Location
    Vancouver , Canada
    Posts
    297

    Hmm Superheat woes

    9yriceman:

    You are way short on info and sounds like there are too many assumptions here - you say a tech ' bled air from lines on roof ' ?? what the heck?? Is there air in the system?? Was the drier changed? What are the pressures? Proper TXV? Subcooling? etc., etc., etc. - - -

    If there is air in the system, the charge will have to be pulled, get a new drier in the thing, change out the TXV if you are at all suspect it is not the proper TXV ( should probably be a Sporlan SVE4CP100 or equivalent ) If somebody put a 5 ton R-22 TXV it is likely too big ( remember they are rated at 100 psi delta P ) - make sure it is the right size - you don't want to lose this compressor as well. If you are not sure how to determine there is air in the system or how to get it out of the recovery cylinder - ask.

    I have said it before - when installing the TXV bulb, clean both surfaces, install it in the PROPER location, put a bead of heat sink compound between the bulb and pipe, and wrap the works with Armaflex tape or equivalent to keep the air off of it.

    Good Luck -
    Superheat and subcooling tell it all !

  4. #4
    what is your subcooling. how do you know your not overcharged.
    subcooling tells you how much refrigerant is in a system.
    10 degrees subcooling is what you need, if its more, then remove gas.
    73 lb suction seems high for a computor room. The temp should be in the low 70s. Sounds overcharged.

  5. #5
    Join Date
    Apr 2007
    Location
    GA
    Posts
    13
    Quote Originally Posted by airworx View Post
    what is your subcooling. how do you know your not overcharged.
    subcooling tells you how much refrigerant is in a system.
    10 degrees subcooling is what you need, if its more, then remove gas.
    73 lb suction seems high for a computor room. The temp should be in the low 70s. Sounds overcharged.
    This is not true. This computer room like many can be putting out too much load for the Liebert to handle causing the suction pressure to be high. There are too many variables here. I've never seen ANY computer rooms sized properly.

    If you shoot the return air grill on the Liebert with your Raytek and shoot the discharge air grill If you are putting out a 15 degree split or better I would leave it alone!

  6. #6
    Join Date
    Sep 2007
    Location
    Balto, MD.
    Posts
    200
    Very few Lieberts run a 10 degree subcooling, 12-15 maybe. This can be true with a lot of equipment manufactures also. On some Trane systems the subcooling is as high as 14 to 22 degrees. As stated in the IOM booklet. Atleast this is stated for the Trane RAUC 20 to 60, and Voyagers 27 1/2 to 50 ton units. Sometimes we are only guessing if we don't have the recommened specs from that manufacture.

  7. #7
    Join Date
    Jan 2008
    Location
    Hells Kitchen, Phoenix Arizona
    Posts
    318
    Charging charts are a good thing...Almost every unit with an expansion valve is charged with subcooling...Duct split doesn't mean much...And since the txv is trying to maintain a fixed superheat, subcooling is the most reliable way to tell if its charged properly...
    "Overkill is an often underrated achievement", Will Hayden -- Red Jacket Firearms

  8. #8
    Join Date
    Jan 2007
    Location
    Charm City--the city that bleeds
    Posts
    2,790
    Did the folks who moved the lines possibly kink them? It may not be an issue... just asking. But they should not have bled anything from the lines.

    Is this a single fan Liebert condenser with a P66? Or is it a Lee-Temp condenser(with the receiver on the side)?
    You'd have to add up the rated charges and calculate the lineset I would venture to guess. Don't sweat the superheat if it's close.
    This isn't an old challenger with a two-stage evap coil, I hope. They eat compressors for breakfast.
    Last edited by The Doctor; 01-27-2008 at 07:30 AM.
    It's great to be alive and pumping oxygen!

  9. #9
    Join Date
    Dec 2006
    Posts
    35

    THANKS so far for help! More info....

    tech told me he bled air from line by shutting unit off for a while, then going to the roof and used his guages to bleed air from valves at condenser-I did change filter drier when I installed new compressor and pulled a long, double vac on it.//my high pressure is 220psi///I do have at least 15 degrees difference betwen return and discharge air but the low superheat worries me that this compressor wont last,either--it is an old unit, over 20 years old

  10. #10
    Join Date
    Sep 2007
    Location
    Balto, MD.
    Posts
    200
    Adjust the superheat then!

    At room temperture of 68 degrees the lowest you want to see is 10 degrees. I like to see near 15-16 myself. Make your adjustments in small increments. The compressor belly portion should allways be warm even if the end bell is cold.

    As far as subcooling on both the Challenger300 and Deluxe system/3 they call for subcooling of 5-10 degrees with a full sight glass. But you older system may run higher.

  11. #11
    Quote Originally Posted by 9yriceman View Post
    according to records comp and Externally equalized valve changed only 1.5 yrs ago-due to remodeling contracters moved lines to rooftop over a few feet!!

    make sure it is the good valve, and replace it.
    adjustable valve ?
    Don't interrupt me while i'm talking to myself

  12. #12
    Join Date
    Jan 2007
    Location
    Charm City--the city that bleeds
    Posts
    2,790
    Quote Originally Posted by 9yriceman View Post
    tech told me he bled air from line by shutting unit off for a while, then going to the roof and used his guages to bleed air from valves at condenser-I did change filter drier when I installed new compressor and pulled a long, double vac on it.//my high pressure is 220psi///I do have at least 15 degrees difference betwen return and discharge air but the low superheat worries me that this compressor wont last,either--it is an old unit, over 20 years old
    Did you answer : Is this an old challenger with a two-stage evap coil?
    They eat compressors for breakfast

    Is this a single fan Liebert condenser with a P66?
    Or is it a Lee-Temp condenser(with the receiver on the side)?

    You'd have to add up the rated charges and calculate the lineset I would venture to guess. Don't look for superheat if it's a two-stage evap coil until you defeat that overengineering.
    It's great to be alive and pumping oxygen!

  13. #13
    Join Date
    May 2001
    Location
    Western Wa
    Posts
    1,819
    Good point on the two stage coil.

    I'd also be for having look at air flow.
    God Bless our Veterans

    God Bless the USA

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