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  1. #1
    Join Date
    Aug 2009
    Location
    eugene oregon
    Posts
    10

    compressor superheat

    i was wondering what the superheat at my compressor should be. it is a 404a low temp freezer line up of 25 cases. i am thinking that i need 20 to 25 degrees, but i have been told differant. i had 12 degrees when i arrived. the reason i was called out was that a pipe going to compressor has alot of ice on it. i know that it is no big deal. see it all the time, but i do have a small frost line on the end bell of the compressor. so that is why i checked compressor superheat to see if there was a problem. check cases and all cases have good air flow and holding temp. with my compressor superheat at 12 degrees i thought that was a little low. found 3 cases with real frosty suction line in case. checked superheat in cases and #1 had 2 degrees #2 had 2 degrees #3 had 3 degrees. adjusted case to run at 6 to 7 degrees of superheat. now at my compressor i have about 52 degrees superheat and i am going back today to check the other cases in the line up. does anyone have any thoughts on that or did i do the right sets?

  2. #2
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,567
    Most manufacturers say 20 degrees is the minimum at the pump.

    Cool compressors are happy compressors.

    I think 52 is a bit high for a well insulated suction line.

  3. #3
    Join Date
    Aug 2009
    Location
    eugene oregon
    Posts
    10
    i still have to check the other cases and i think they might have superheats alittle high, but dont know until i get there. you think i took the right steps on this one? what should i have done differant? with high superheat at the compressor i know that is can in time get to warm and could case damage but all can happen?

  4. #4
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,567
    I would re-check everything and here is why.

    Your SH jumped too much for the small adjustments that you claim to have made.

    By adjusting a couple of cases a couple of degrees, you got a 40 degree increase at the machine?

    Start with charge. Do you have a solid column of liquid? If not, put your wrenches away until you do.

    You checked airflow, but did you verify that there was no ice in the cases? Really?

    What do the driers look like? Suction filter? TXV screen?

  5. #5
    Join Date
    Sep 2002
    Posts
    12,288

    Let's herd these ducks into a line

    Let's start with this:

    # 1 - Do Not Assume
    # 2 - Do Not Chase Your Tail on this thing

    Phase One -
    Clean condenser coils with minimal rise across them.
    Stable head pressure
    8-12 liquid subcooling at the inlet to the TXV's

    Stage Two -
    Clean and securely re-mount each TXV's sensing bulb.
    Clean the line where the sensing bulb attaches.
    Clean evaporator coils of all dirt
    Freshly defrosted evaporator coils.

    Stage Three -
    Set up a monitor on the compressor SSH
    Set the SSH at each evaporator;s suction connection to 15-16.
    Adjust each coil/TXV one at a time.
    I like to start with the most distance coil - but it doesn't really make any difference.

    As you are setting each coil's SSH - watch the compressor SSH monitor for anything radical happening. If you see it - stop and check the last two things you worked on to verify that what you did is still accurate.

    Stage Four -
    Check the compressor's final SSH. I like to see it VERY stable at 18-20. If you can't make it stable - then add about 5-6 to those numbers. But try to make it stable. <g>

    PHM
    ---------


    Quote Originally Posted by cws38 View Post
    i was wondering what the superheat at my compressor should be. it is a 404a low temp freezer line up of 25 cases. i am thinking that i need 20 to 25 degrees, but i have been told differant. i had 12 degrees when i arrived. the reason i was called out was that a pipe going to compressor has alot of ice on it. i know that it is no big deal. see it all the time, but i do have a small frost line on the end bell of the compressor. so that is why i checked compressor superheat to see if there was a problem. check cases and all cases have good air flow and holding temp. with my compressor superheat at 12 degrees i thought that was a little low. found 3 cases with real frosty suction line in case. checked superheat in cases and #1 had 2 degrees #2 had 2 degrees #3 had 3 degrees. adjusted case to run at 6 to 7 degrees of superheat. now at my compressor i have about 52 degrees superheat and i am going back today to check the other cases in the line up. does anyone have any thoughts on that or did i do the right sets?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  6. #6
    Join Date
    Oct 2008
    Location
    Canada
    Posts
    83
    15-16* SH at each evap? why so high?

  7. #7
    Join Date
    Mar 2007
    Location
    tx
    Posts
    1,088
    If like the RSES Avatar said, you do have a full column of liquid, it is not uncommon at all to see ice build up on a suction line and compressor end bell of a freezer. After all, that line should be anywhere from -30* to 0* F. I see it a lot for sure if insulation is broken down covering the suction line. 2*-5* is very normal super heat settings on a freezer at the evap. And I never like to see above 20* of superheat at the compressor in any application. Was there a reason for the call other than someone saw ice on that suction line?
    Bad information is worse than no information at all.

    There are three kinds of people in this world. Those who can count and those who can't!

  8. #8
    Join Date
    Sep 2002
    Posts
    12,288

    My thought was -

    To make the compressor happy and long-lived we want about a stable 18-20*degrees SSH at the compressor. Unless you have a desuperheater or a piss poor suction line install - you're not going to gain more than a few degrees in the suction line run.

    So you can't have the evaps at 10 and the compressor at 20.

    Can you? <g>

    Wet saturated suction gas will wear the rings out of a compressor in no time due to oil washing. The bearings won't be far behind due to oil dilution. Refrigerant is a solvent for oil. As the lube oil is under refrigerant pressure it Always contains some dissolved refrigerant. But that is designed for in the bearing loading numbers. You can't add more or the oil becomes too thin to lube properly. A dry suction gas the way to avoid that problem.

    I would much rather sacrifice a little evap performance, and of course; compensate with added compressor run time, than I would tear up a compressor with low SSH.

    Slugging is obvious - wet suction is insidious.

    PHM
    --------


    Quote Originally Posted by Jettmac View Post
    15-16* SH at each evap? why so high?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

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