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  1. #1
    Join Date
    Dec 2004
    Location
    Colorado
    Posts
    3

    gravity fed walk in cooler

    First post but I’ve followed this site for a long time. One of my customers has a gravity fed 8x8x8 walk in cooler using MP-39 with remote condenser. The liquid line solenoid is controlled by an air sensing thermostat mounted on the wall about 6 feet up. The remote condenser is shut off by a low pressure control – I’m in Colorado so it gets pretty cold.
    The problem has been, as long as they can remember, is that the evaporator coil would freeze up fairly often and keeping an even product temperature has been very difficult to obtain. I installed a defrost control and at present I think it defrosts 3 times a day for 45 minuets. After installing the timer the coil no longer freezes up but a constant temperature is still a problem. Apparently very slight changes in temperature result in too warm to freezing of the product.
    I’m looking for suggestions on how to correct this problem. I’ve thought about using a constant cut or a Johnson Controls A419ABC thermostat with the sensor placed in the evaporator coil [best location for bulb placement] or using another low pressure control . When the 419 first came out if I placed the sensor in the evaporator coil it seemed to have a short life span and I quit placing them in the coil. Any suggestions would be appreciated.
    Thanks Mike
    MK

  2. #2
    Join Date
    Oct 2007
    Location
    The rainy state
    Posts
    172
    what kind of superheats are you running. second, defrost is this pump down? welcome to the site.

  3. #3
    Join Date
    Jun 2005
    Location
    Seatte Wa
    Posts
    48
    Static coils without a constant suction pressure can be tricky if your running this off a single compressor I would really get rid of that fan cycle control and install a headmaster that bypasses at 100 psi.also install a balanced port TXV with some sort of liguid line stabilizer (heat exchanger).

    Check that solenoid valve it should not be used to cycle temp,I would cycle temp off line voltage from the stat and only pump down on defrost I would go for something like 120 min off cycle maybe at night when there is very little load on the box,or you could just use valve as a migration stop and also just kill Comp with Timer but you may have to watch for fllodback after defrost.

    A regular stat on the wall should work just fine I like to put the A19 with remote bulb so I can have all the controls right at the condensing unit.

  4. #4
    Join Date
    Dec 2004
    Location
    Colorado
    Posts
    3
    Chilly 123
    Super heat is approximately 10* and when it goes into defrost liquid line solenoid is deactivated and compressor is shut off when suction pressure drops to just above 0 psi

    Devildawg
    It is a single compressor with fan cycle and no headmaster. If I understand you correctly your saying only use the liquid line solenoid to pump down during the single defrost in the middle of the night and use the thermostat to switch the compressor contactor. The TXV is the same one used when R-12 was in the system and not a balanced port valve. Would tying the liquid line to the suction line work as the heat exchanger that you mentioned? What kind of a dog is a devildawg?
    Thanks Mike
    MK

  5. #5
    Join Date
    Oct 2007
    Location
    The rainy state
    Posts
    172
    well mk i don't think tying the liquid line to the suction will help you if your getting 10 deg sh at the coil. 10 deg sh is pretty good. indication your feeding the coil well. need some more info, head pressure, ambient temp. I agree with devildawg, lose the fan cycle control, and go with the head master. you maybe dipping your head pressures too low at night with the fan cycling ctrl. causing your evap to starve at night, and the box has to run all day to play catch up!

  6. #6
    Join Date
    Jun 2007
    Location
    Williamsport, PA
    Posts
    165
    Install a solinoid on the suction line. This will stop the overshoot caused by pumping out the entire evaporator. Leave the liquid line solinoid installed. That will keep the coils from filling with liquid during the off cycle, protecting the compressor from slugging at start-up.
    I have had good luck using suction stops on meat cases and deli cases with gravity coils and single condensing units.
    Gravity coils do not work well when box is subjected to high load periods. If this box is not being used for hanging meat or some other product that would be sensitive to the fans blowing then replace the coil with a standard evaporator.

  7. #7
    Join Date
    Dec 2000
    Location
    Flat Rock, NC
    Posts
    463
    These units are normally equipped w/ suction accumulators to prevent floodback on start-up and are controlled w/ pressure switch only. Sometimes they use a liquid line solenoid that closes when compressor is off to prevent migration. Pressure is the only good way to control temps. No defrost control is needed when set up properly. Shoot for box temps of 38-42 deg at best.

  8. #8
    Join Date
    Sep 2002
    Posts
    2,731
    If that walk in had r12 and gravity coils its got to be old as hell what is the insulation shape and do you have air leaks

  9. #9
    Join Date
    Mar 2007
    Location
    Madison, Wisconsin
    Posts
    2,260
    I had a similar box, but it was a freezer, also had problems with uneven temps. I put on a digital defrost timer with 4-30 min defrosts. It had an electric defrost. Then I took the sensing bulb and placed it on the wall about 2' from the door and 3/4 of the way up the wall, set the differential to 6*. Ran fine for three years until I tore it down. This was also an old R-12 converted to MP 39, but I think we added a R-134A TXV.
    I r the king of the world!...or at least I get to stand on the roof and look down on the rest of yall

  10. #10
    Join Date
    Aug 2007
    Location
    Chicagoland Area
    Posts
    43
    When using an air defrost for walkin coolers make sure the fans continue to run while in the defrost mode, and your drains are clean. I have run into that same situation, the evap would freeze up forming ice on the inside of the coils not totally defrosting the coil.

  11. #11

    gravity coil pressue?

    at what pressure does a R 12 unit work at with a 1/2 hp water cooled unit?

  12. #12
    Join Date
    Aug 2005
    Location
    Rutland,Vermont
    Posts
    30
    The biggest problem I think you may be having is a gravity coil is used for high humidity, and low load change application. These were used alot in pizza shops for dough, vegetables, and some flower cases. With high load you would have no off cycle defrost and freeze up is inevitable.What was this case set up for(with R-12 10-20 years ago originally?

  13. #13
    Join Date
    May 2003
    Location
    Utah
    Posts
    722
    Quote Originally Posted by tazzman1945 View Post
    When using an air defrost for walkin coolers make sure the fans continue to run while in the defrost mode, and your drains are clean. I have run into that same situation, the evap would freeze up forming ice on the inside of the coils not totally defrosting the coil.
    This is a gravity coil......there are no fans.

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