Liquid Line temp question
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  1. #1
    Join Date
    Feb 2007
    Location
    Napa
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    111

    Liquid Line temp question

    Originally posted in the wrong forum....


    I have a Carrier 12 ton split system. Original issue was bubbles in the sight glass, SC and SH also indicated low charge.

    Charged system to clear sight glass. Ambient temp at troubleshooting time was 65-75 and minimal load in the zone.

    Before initial charge of system, head pressure was at approx 160 psi and suction was 75-78. Motor Master type c/fan control (Hoffman) was running c/fan at full speed even at lower ambient when initially started troubleshooting based on the LL temp (as per the manual, it goes to high speed at LL temp over 80 degrees).

    After charge, Head pressure increased slightly (but still had c/fan running at full based on LL temp), but suction pressures stayed about the same. This is odd, since everything indicated low charge except the suction pressure at initial beginning of troubleshooting

    What is also odd is that as my LL temp increases, my head pressure decreases and as the LL temp decreases, head pressure increases.

    There are 2 non adjustable TXV's for the stacked evap coils and they seem to be metering properly and do have temp/pressure drops through both TXV's (indicated by temp drop across TXV). I will do diagnostics on those today to confirm though.

    I doubt that there are any non condensables, as my head pressure is running more on the low side, even when ambient was closer to 75. I did have about 29 degrees air temp diff across condenser coil, but only about a 13 degree drop across the DX with both coils active.

    We do have re-heat coils which is more of what we use to maintain temps. Cooling is more to help maintain humidity levels since we have no other source for de-humidification, so there is an artificial load when temp is low enough in suite to activate RH coils.

    I did pump down the compressor using the service valves, and was able to get it into a slight vacuum, but pressure did slowly begin to climb every time (pumped down 5 times), however, I have had service valves leak by in the past around here, so I am not ready to condemn the valves just yet, unless it corrosponds to the questionable LL temp/pressure issues.

    I cleaned the snot out of the condenser coils, which are 2 coils sandwiched together. I have not yet pulled the top of the C/U to seperate the coils and do a better cleaning. This unit runs one of our clean rooms, and can't shut it down completely to do that for another month.

    I will update with more specific information relating to pressures, temps, SH & SC, but was more curious about the LL temp/pressure issues to start with as I can't really do much more until I figure out what is causing that as I can't get the c/fan control to control in low ambient with out switching to a pressure switch...

    Updated readings:

    Carrier M#38AKS012-601 S#2597G00125
    amb temp 66
    C/U temps 65/93
    Suction Pressure-75
    Head w/fan on 175-210 (fluctuates high to low approx every 60 seconds)
    Suct 73-74 psig line temp 48-60 degrees (as head pressure increases, suct line temp decreases)
    LL 95 degrees @ 175 psig, 78 degrees @ 205 psig
    Discharge line temp 155-160 degrees (odd how this temp only flucuates 5 degrees, but LL temp fluctuates 30 degrees)
    As head pressure drops, sight glass begins to flash at unit, but no flashing in either sightglass at air handler near filter dryers.

    Suction pressures only fluctuate 1-3 pounds, discharge (actually Liquid Line) fluctuates approx 30 psig every 60 seconds, line temps fluctuate at about the same rate.

    Moving on to TXV...will update with any info...

  2. #2
    Join Date
    Feb 2007
    Location
    Napa
    Posts
    111
    Also, 1 mechanical unloader which appears to be working properly, but was wondering what the effect on the sysyem that would have if it were failing or leaking by. Amp draw indicates that head is loaded. There is a slight fluaction in the compressor amperage as pressures move around, but less than 1 amp, so it doesn't appear the the unloader is acuating which should be indicated by a larger amp deviation.

  3. #3
    Join Date
    Mar 2011
    Location
    North Carolina Piedmont Area
    Posts
    447
    What type of compressors do you have? Sounds like an 06D semi hermetic. What is the strategy for loading and unloading the compressor? Do you know what the setting is for the hydraulic unloader. Usually around 60 PSI to unload. It sounds like compressor is running unloaded and not staging correctly. This may explain why you have a low head and high suction.

    I would try to load the compressor and see what it does. Definitely find out what the settings is of the unloader.

    Also what is the temperatures at suction inlet of compressor, motor section, oil sump, each head located at the top of head and underside of head, and finally discharge temperature.

    What is the superheat at inlet of the compressor.

    ____________________________________
    Troubleshooting is not part of the repair....understand the symptoms and you will find a solution.

  4. #4
    Join Date
    Feb 2007
    Location
    Napa
    Posts
    111
    Compressor is an 06D, and unloader is set 60ish/7.5ish. It is unloading if I adjust, based on amp draw, and is currently running fully loaded. Just not sure if it were leaking by enough to cause it to act like this, but not enough for it to show up on the amp draw. My experience usually has the unloaders failing completely, so I don't know if that is even a possibility.

    Amp draw is low based on name plate (19.? FLA) running about 14 amps, but is consistant for the last 2 years amp readings from P/M's at the same time of year.

    TXV's both look good, but still need to get tools for measuring line temp from the roof. By hand it felt like one outlet was cooler than the other so I want to verify line temps.

    I cannot access the indoor coil as I can not shut the unit down or open the access doors because that would affect room pressure in the clean room, so I can not visually verify that the coil is loaded up, but it is cold and sweating coming out of the coil. The sensing bulbs are also inside the unit, so I can not access those at this time either. Since the TXV's are non adjustable, there isn't much else I can do to verify operation of the TXV's.

  5. #5
    Join Date
    Jul 2008
    Location
    Southern California
    Posts
    138
    Your amp draw will be under FLA because the head pressure is low. Your expansion valves are hunting because you don't have the required 100 psig MOP. The suction is less that 100 psig from the discharge. If the unloader is operating I would look for a broken or leaking discharge valve on one of the cylinders, this will cause the suction to be high and the head to be low.
    Effort is the 1st step towards failure.

  6. #6
    Join Date
    Feb 2007
    Location
    Napa
    Posts
    111
    Carrier too agrees with failing valves, so will be replacing the compressor. I am sure that some will say I should just replace the valves, but this is a criticial piece of equipment, so since I will not have the down time when we do shut this down for repair to spend a lot of time looking at the rest of the compressor, I am just replacing the whole compressor to ensure reliability. Failure is not an option...ha ha

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