Can loop flow rates be too high?

Discussion in 'Maintenance and Troubleshooting' started by newgeohp, Jun 28, 2010.

  1. newgeohp

    newgeohp New Member

    More Q?

    After three weeks my installer got back with me. The recommendation from WF was to set the unit to dehumidify mode (reduce fan speed) and see if delta T air side goes up. I was a little surprised but figured why not; let’s see if the laws physics apply to my new unit. So I changed it over (900 to 700CFM) and wow Delta T did go up with lower fan speed, just like they were supposed to, but the unit output is still 70% of SC rated capacity in either normal or dehumidify mode for both stage 1 and 2 @ 46% humidity.
    A friend recommended that I measured the temperatures of the U bends on the coils with an IR temperature meter. The center 25 U tubes ranged from 15-20°F and had lots of condensation on them, but the bottom 8 U tubes had a temperature of 50-55°F and no condensation on them. The Top 8? (I couldn’t see them well) also had little condensation on them and their temperatures were higher than the center coils. Is this normal? See attached picture.
    Are the coils broken up into groups? It looked like there were about 40 U bends, if they are broken up into 5 groups of 8 and one is clogged maybe that would explain 20% of the lost 30% of capacity? I’m an electrical guy and don’t know anything about the refrigerant side of things, so laugh all you want.
    Thanks Rich.
  2. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    If you are truly confident of...

    your heat transfer calculations on both air and water sides of the unit and both are low, then an undercharge or blocked evaporator distribution tube are both possibilities, though I'm skeptical of both.

    You are correct that "coils are broken up into groups" more specifically, liquid refrigerant is divided into several (4-8 or more) individual paths and fed into widely spaced parts of the evaporator.

    Your IR measurements of various parts of the evaporator are suspect for the purpose of gauging system performance for several reasons, chief among which are:

    1) Opening the air handler cabinet so as to be able to take pictures nearly completely defeats airflow and thus heat transfer across the evaporator - blower pulls 80+% of air through the removed cabinet panel rather than through the evaporator...absent heat load it cools and exhibits icing

    2) IR thermometers are notoriously unreliable as to actual vs perceived contact spot and measured surface emissivity

    3) The distribution of liquid refrigerant will cause quite different temperature readings throughout the surfaces of the evaporator coil, all of which the transiting airflow will tend to average together by turbulent mixing downstream of the coil.

    Upon what do you base your figures of 70% of SC capacity? Are you certain the deficit isn't being made up, partly or wholly, in the form of latent capacity?
  3. newgeohp

    newgeohp New Member

    Thanks for the input Curt,
    I am pretty confident in my calculations, and I to don’t trust IR measurements either but I do trust the delta between the measurements on the same type of surface. I also was concerned that the coil did not ice up evenly. The installer came out yesterday to take measurements and it turns out the coil or source line for that section is blocked.
    I will update when I find out more.
  4. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Coil should not ice at all. Evenly or otherwise.
  5. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Agreed - I figured it might stem from airflow bypassing the evap with a panel removed.
  6. Pump can be speed controlled

    For under $100 you should be able to get a solid state speed controller to tweak pump speed just where you want it. I have bought DDC controlled single phase motor controllers for $40. Fixed (manual adjusted0 ones about $25-30.
  7. teetech

    teetech Member Forum Leader

    I also was concerned that the coil did not ice up evenly. The installer came out yesterday to take measurements and it turns out the coil or source line for that section is blocked.I will update when I find out more.

    I assume he referring to checking the "frost pattern" of the distributor tubes and coil absent air flow.
  8. newgeohp

    newgeohp New Member

    It's fixed

    Well the guys just left after installing my new coil and the data looks good so far.
    My delta air temps are up from 15°F to greater than 20°F and I’m within 10% of the spec sheet now. (Good enough for me) The delta water temps are now matching the spec sheet also.

    I’m glade the unit is working properly now I just hate it needed major surgery this early.

    Thanks for your input throughout this process.

  9. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    No matter how hard we try, there is allways the "Friday widget" when it comes to manufacturing. Gladd you got resolution and are satisfied with performance.

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