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  1. #1
    Join Date
    Jul 2005
    Location
    pembroke,NY
    Posts
    33
    I had a call walk in wasn't cooling. when I arrived at site, found evap coil. frozen. Defrosted,checked T-stat in cooler box, that was set lower than 30*. Box temp was 48*,and I could not get it below 46* before pizzaria employee went into it. So I set low pressure controll to have unit shut off at 15 psig and turn on at 30 psig, on a r-12 system. I believe unit was never shutting off the way T-stat was set, so I set it like this. It has a defrost clock that was working. My boss said this wasn't a good practice, but didnt go int detail why. I'm really green, only been in field fo 3 1/2 weeks. I would like opinions of exp. techs on how I might have done things better.

  2. #2
    Join Date
    Aug 2002
    Location
    Southold.calm
    Posts
    6,333
    For r-12
    CI 36
    Co 16

  3. #3
    Join Date
    Nov 2003
    Posts
    501
    It's got a stat and a low pressure cut out switch, both being used for temp control? I've never seen this before. I would use one or the other unless the low pressure switch is used as a method of protecting the compressor in case of a leak.

    If you use the low pressure switch for temp control, I'd set it this way:

    Turn the cut in to where the compressor comes on at that moment. Turn the differential WAY up so that it runs and runs. Put your temperature meter probe in the return air stream. When the air reaches a point about 4-5 degrees below your desired cut in point, raise the differential to shut off the condensing unit. When the temperature on your meter shows your desired cut-in temperature, turn the cut in pressure slowly down until the switch closes. That just set your cut in pressure.

    Next, turn your differential high again, to allow the condensing unit to run. When the air reaches your desired cut out point, turn the differential down slowly until the CU shuts off.

    Cycle the unit several times without touching the pressure switch to ensure that the temperatures at which it cuts in and out are where you need. Adjust SLIGHTLY as necessary.

    Hope this helps!

    John

  4. #4
    Join Date
    Jul 2005
    Location
    pembroke,NY
    Posts
    33
    Thanks dorrmann.

  5. #5
    Join Date
    Jul 2005
    Location
    pembroke,NY
    Posts
    33
    What about the fact that I couldn't get the box down to desired temp. It was already 8:00 at night and box would not come down with people going in and out. I wanted to set by temp. but wasnt able unless I stayed all night. Thats why I set by pressure. Could I have done diffrently in this situation.

  6. #6
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,764
    pressure switch on that setup is for loss of charge. control temp by thermostat. double check time clock, mark dial and make sure time is advancing. set defrost longer if needed to control frost build-up

  7. #7
    You can have a low charge, clogged drain pan, dirty evaporator coil, loss of one or more evap fan motors ... there are many reasons why a coil would freeze over.
    Besides the t-stat being set too low ....


    If the box wasnt pulling down while you were on site ... it could be a capacity issue. Like weak compressor. But I doubt it.

    You dont usually have bad valves when you freeze your coil over.
    Although low charge would also give you low capacity. Once every last bit of ice is off the finned surfaces of the evaporator coil .... THEN you can begin inspecting the compressor for what is wrong and why it froze over.

    #1 is check for low charge. Take some temp measurements and pressures then come back here and tell us what you found.

  8. #8
    Join Date
    Jul 2003
    Location
    ALABAMA
    Posts
    230
    is this a cooler or freezer? No defrost should be necessaary on a r12 walkin cooler.(air defrost).
    It sounds like you are using this as a cooler with the setting on the control,however the low pressure switch on walk in that has a t'stat should be used as a safety.

    It sounds to me like either the box is low of freon or another problem (fan,coil dirty,stat to cold) since the cooler would not pull down. What are your pressures?

  9. #9
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    If you have a functional thermostat, "bottom out" the pressure control, (Set it for about 7 CI and 2 CO) and use the Tstat for temp control, and the defrost clock for defrost.

    You'll drive yourself insane trying touse two different controls for the same purpose on the same machine.

  10. #10
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    Originally posted by cphc
    is this a cooler or freezer? No defrost should be necessaary on a r12 walkin cooler.(air defrost).
    At a 18-25 degree F coil, you most certainly DO need a time-off defrost, during which the fans remain running.

  11. #11
    Join Date
    Jul 2003
    Location
    ALABAMA
    Posts
    230
    Not really. With a 20 degree coil your box should run around 35 degrees if you cycle the unit off at this temp.
    Put a 8 to 10 degree diff. and the air in the box is now 40-45 at start up again. This temp should defrost any small amount of ice. Ice really should never be on the coil of a med to high temp box anyway. That why I ask if this was a freezer.

  12. #12
    Join Date
    Sep 2002
    Posts
    511
    The T-stat in the walk-in cooler is to control the liquid line solenoid that feeds liquid refrigerant to the TXV metering device.
    The pressure switch is to prevent the evaporator coil from going to low of a pressure and freezing the coil up.
    It will shut it off at a pre-set,in this case 16 psig and come back on at 36 psig and allow for air defrost of the evaporator coil while the refrigerant is boiling off and cooling the walk-in cooler.

    The reason why the cooler wasn't pulling down while you where there is two fold.1)there where people going in and out of the box and 2)as a box gets colder the pull down of the temperature gets slower and you may have to wait over night for it to pull down to the desired temperature.
    especially after the box traffic stops and the door remains closed for a longer period of time.

  13. #13
    Join Date
    May 2004
    Posts
    1,886

    cphc

    If your letting your coolers get up to 45* then you will be getting some call backs. Health Dept will have a cow if it above 41* if it contains any perishable items.

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