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  1. #1
    Join Date
    Dec 2002
    Location
    ON., Canada
    Posts
    194
    A used copeland condensing unit,M#C7D8-0750TFC001, was installed for a new walk in freezer. The unit use to be 502 and it's been changed to one shot. The mineral oil was removed and AB oil installed. The txv is new and the head says 502, but I did notice that the bulb is installed after the equalizing line. While I was charging this system up I noticed that the liquid pressure out of the receiver would be 310psi(118F) and the suction pressure before the CPR valve would be 36psi(5F), box temp@32F, compressor superheat 25F, evap superheat 22F, 6F subcooling after receiver, and sightglass full. After running for about 10-15mins the pressures would drop to 270psi(108F) and 25psi(-6F) with bubbles in the sight glass. I would also have 7F evap superheat with 9F compressor superheat before the accumulator and CPR valve. This would happen every 15mins, the changing in pressures. It seems like I have a partial restriction once in a while, I think. The beginning passes are all freezing evenly, but the last passes are still warm. Could the evap be flooded with oil? How can I check? Or should I be relocating the tx bulb to before the external equalizing line. I still need to return to this job Monday, any input is appreciated.

  2. #2
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,374
    Your first mistake is to use "One Shot". I would have opted for R402B (HP80) or R408A as a good interim replacement refrigerant. The change to AB oil is a good thing. A better long term solution would be to go with R404A and POE oil, but it would require a couple more oil changes.

    Since you have a CPR valve, I assume the TXV has a Z charge and not a ZP pressure-limiting charge in the thermostatic element. These two can fight each other on pulldown and get you all confused.

    When you measured the 7 Deg SH at the evap was the box near the desired temperature, like 0 Deg F? Did you set the CPR by using the unit RLA rating at pulldown? It's possible that these fluctuations may be from the CPR throttling. If it's pressure range is too close to where you're operating, you could get such varying conditions (I would think).

    I doubt it's oil logged if your getting fairly good coverage at the coil.

  3. #3
    WHO did this project?
    WHEN was this done?
    WHY was hotshot chosen? Was that what somebody had leftover from a party somewhere... and they didnt wanna go buy some better blend?

    Lose the hot shot.
    I assume the unit is in great condition. In other words, wherever it last operated, it was doig it's job perfectly.

    You say a few passes of the coil are NOT cooling? Okay ... it sounds like the TX is not opening enough to feed the whole coil.
    That could be several things.
    Undercharge.
    Undersized.
    Partly restricted.
    If the coil utilizes an orofice inside the distributor, maybe that is where there is trouble ALSO. Maybe an undersized orofice is in there presently.


    If this box is a keeper, then the best thing is 404 for the blend.
    How's the piping job? Is is short and clean? If yes, then do what is necessary to change over to four oh four and POE. In the long run, you'll be glad you did.

    If you "inherited" this project AFTER the install ... what Icemeister said to do is whatya outa recomend to the customer.
    And make sure you get some money UP FRONT! Seriously!!!
    Who knows what lurks beneath the surface of mechanical refrigeration .....? Only time will tell....... Even Popa Dice dont know what's in there .... and he happens to know quite a lot about mismatched-change-overs.... just ask Jack!


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