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Super heat not stabile

I was on a start up recently where we came across unstabile superheat.
The system is a vav system. four stages of cooling. Glasses clear, suction pressure only flucuating a pound or two. Freq drive condenser fans. We saw the superheat flucuate from 4 deg to 20 deg in 30 secs and back the other way.

We checked everything, bulbs in the proper place tight no oxidation on pipe. etc. any adjustment tried made no change. Spoke to sporlan valves are ove15 4 valves on system. They had no answer other then to play with the valves.
I thought that the problem could be caused by too high of an air velocity across the coil. We were told system was air balanced. Boxes were open and main freq drive was in auto control. So that about rules that thought out.
Does anybody have any ideas??


  • Techman
    Techman Member Posts: 2,144

    HI ED . Check for a "full glass" right at the TXV, afull glass at the condensing unit doesn't mean a full glass at the metering device .
  • Empire_2
    Empire_2 Member Posts: 2,343

    What is your head pressure when all this is happening? Are you sure it's properly charged? Sounds like the valve is "Hunting".

    Mike T.
  • Ted_13
    Ted_13 Member Posts: 40
    valve or orifice could be oversized

    if the valve or orifice is oversized it will hunt. Try moving the bulb further down the line.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    The Mystery of the Rising and Falling Superheat

    One question: Is the evaporator located below the condensing unit?

    The reason I ask is that the situation you describe (being, of course, blind to everything else going on in the system) is indicative of an improperly piped system. By this I mean that, if the evaporator is located below the condensing unit and the suction line is not properly trapped, you will be acucmulating oil in the last tube(s) of the evaporator as well as the suction line before the line turns upwards toward the compressor and condenser.

    As the amount of oil in the line changes, so will the temperature that is being sensed by the thermal bulb. If the temperature at the outlet of the evaporator coil changes, your superheat readings will obviously be out of whack, in addition to the position of the needle in the TXV.

    Keep us posted.
  • Ed Dicristofano
    Ed Dicristofano Member Posts: 2
    Unstable superheat

    I checked with sporlan regarding the valves, sizing is correct. All 4 valves do the same thing.

    Evap is located inside and is slightly higher then cond unit. The suction pressure is not hunting, 1-2 pd swing. head pressure is on freq drive fans. head was fairly stabile at 200-210.

    Unit is piped properly. trapped at suction outlet and inverted at top in the suction header.

    One bank of the compressors has a sight glass on the oil equalizing line and oil maintained a steady level all day.

    It appears like I said air velocity is too great on the evap. Its not on the coil face long enough to allow valves to stablize.

    I spoke with the control contractors tech yesterday. Boxes were open and he had the freq on auto. so if any boxes did close the freq would back down. Air side has been balanced, but i have not seen a report. and as we all know its easy to right down numbers that work. lol.
    Any other thoughts???????
  • Empire_2
    Empire_2 Member Posts: 2,343
    Let's do this if you can

    Using your diff pressure meter, tap 2 holes into the duct before the evap and 1 on the down side of the discharge. Let's see if the VAV dampers are the problem. The system may be doing what is expected, but at least we can verify if the air flow/static is changing, due to different vav calls for cooling.

    I just have this feeling that the to much air flow scenario isn't the problem.

    Mike T.
  • don_163
    don_163 Member Posts: 67
    I'm with mike

    Superheat swing..aka hunting.It sound like the valve needs to be adjusted open.Reduce the adjustement spring pressure in this will give you a higher volume.

    This should help the evaporator feed be less erratic.

    Happy hunting..Pun intended.
  • Techman
    Techman Member Posts: 2,144

    Hi Ed.How goes the SUPERHEAT battle? Another possibility,Do the TXV's require the use of a "distributor"?Or maybe debris,but ehough for 4 valves? Let us know what you find out!
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    In Agreement with Mike T and Don

    I do not think that the evaporator airflow is having this effect on the systm but would love to hear how your search is going.

    What are the capacities of the evaporators and the condensing units?

    Thee weeks Don.... You getting ready??? Give me a call.
  • Jim Bergmann_2
    Jim Bergmann_2 Member Posts: 79
    TXV Problem

    I have seen similar problems like this before, they are sometimes hard to diagnose. Attached is a training sheet I use for the 523 refrigeration analyzer, the solutions may help. Also I would verify the suction line is warmer at the compressor inlet than the evaporator outlet. In other words the suction line should continue to warm (add additional superheat) as it goes toward the compressor. You may have a flooded coil due to low load or other reasons and the reason your superheat appeared to change so fast was liquid was flashing in the suction line. A quick check is to grab hold of the suction line. If the line continues to feel warmer as you hold it, there is only vapor in the line. If the line grows colder as you hold it, liquid is flashing and absorbing heat from your hand in the process. The system would be flooding. A digital instrument will allow you to data log the system over time and see if these problems are load dependant.

    Jim Bergmann
    HVAC/R technical Specialist
This discussion has been closed.