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  1. #1
    Join Date
    May 2005
    Posts
    8

    Question

    Hi all! One of Our tech replaced the TXVs on a six ton freezer unit with dual EVAP two weeks ago. Instead of using the manufacture recommeded Sporland TXV. He decided to go with Danfoss stainless steel TCAE series. He also replace the oil, and restated the unit. The unit seems to be function normally, except it hardly goes close to Zero. He then tried to adjust the superheat to around 12-13 degree. Now, the unit hit zero temperature but it doesn't go any further (USDA regulation is -10 to -18). We decided to lets it operating at zero through the weekend, but a weird problem has happenned the next morning, the sentronic oil pressure has tripped sometime during the night. Reset the switch and it runs again with out any problems, but it will stop again the next day. We check The oil level and it is correct. Oil pressure is within spec. Sentronic control and sensor are brand new. Whatelse could it be? And why it only happen at night? We are stuck. Any helps will be appreciated.

  2. #2
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,370
    My first question is why were the expansion valves replaced?

  3. #3
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    Originally posted by icemeister
    My first question is why were the expansion valves replaced?
    'Cause he didn't know what was wrong in the first place. It's the #1 reason.

  4. #4
    Join Date
    Feb 2004
    Location
    Medford Oregon
    Posts
    807
    a few possibilities you can check
    1. superheat at compressor too low, at low load conditions superheat drops below 0, (at night) liquid comes back, washes and trips. superheat at compressor should be
    around 25 (20 to 40 range acceptable)

    2. poor quality oil leaves compressor and gets stuck
    in evap after defrost and oil trips. check oil condition
    normally occurs after a refrigrant leak (being suction
    cooled the loss of gas causes overheating which destroys the oil, very few check oil after a leak but should)

    3. flooded start occuring (this one less likely)

    check suction pressure against crankcase pressure
    if it varies, you have wear (if more than 5 lbs diff
    it's too late)

    one thing is certain, you are losing oil pressure
    this will continue until compressor dies unless you
    find cause first

    Good Luck

  5. #5
    Join Date
    Oct 2003
    Location
    tidewater, va
    Posts
    2,140
    X5 Run,


    Did the tech put on pressure limiting TXV'S? I don't know much about Danfoss valves, but if this thing called for pressure limiting TXVs and he jacked the superheat adjustment before he got down to temp, well, you just may be flooding near design. Check and double check wiring to make sure something isn't breaking the wrong side of the circuit, killing the compressor AND allowing power to the heater circuit on oil failure. IS the contactor new? Good luck.

    404

  6. #6
    Join Date
    May 2000
    Location
    Monmouth Junction-NJ-USA
    Posts
    6,005
    [QUOTE]Originally posted by rocket
    [B]a few possibilities you can check
    1. superheat at compressor too low, at low load conditions superheat drops below 0, (at night) liquid comes back, washes and trips. superheat at compressor should be
    around 25 (20 to 40 range acceptable)

    If superheat drops below 0---are you talking about a negative superheat???????? Might be one or two on here that disagree with that theory. LOL
    If you really know how it works, you have an execellent chance of fixin' er up!

    Tomorrow is promised to no one...

  7. #7
    Join Date
    Feb 2004
    Location
    Medford Oregon
    Posts
    807
    didn't mean drop below zero, sorry

  8. #8
    Join Date
    May 2005
    Posts
    8
    Originally posted by icemeister
    My first question is why were the expansion valves replaced?
    First, our freezer temp has never fall below Zero in the last couple years or so. During defrost temperature sometime rised upto 12-14 Degree, depending how much of traffic and new loads in and out the freezer. We was told to bring the temp down to negative temp to be complied with USDA regulation.
    Second, the previous TXV was installed about six years ago, and the adjusting screw has stripped due to numerous of adjustment in the past. So, the idea of replacing TXVs has arrived as the first step of our journey approaching to the ultra low temperature.
    The Danfoss TXVs were recommended by folk at Johnstone, and the information from manufacture website has confirmed that it's the correct replacement. It's fully superheat adjustable with the static superheat at 9 degree. Today, the problem seems to be worser than ever. Box temp constant at 7-8 degree throughout the day, and suction pressure hanging on at 29-30PSI while head pressure is operating at 210PSI @94 degree ambient temp. Which compression ratio seems to be too low for this application. Also, the subcooling is 16 degree and sightglass is clear and full of liquid. I don't have a spec ratio for this compressor. But if my ratio calculation is correct than we are dealing with a leaky valve (valve plate), broken ring or piston. This copeland has three cylinders as I remember. Anyone know what's the compression ratio for a 6 tons copeland compressor for freezer application please post. Thanks guys
    Don

  9. #9
    Join Date
    Feb 2004
    Location
    Medford Oregon
    Posts
    807
    first it appears that this box was designed for 0, now the health code wants colder. the chance of existing equip working is 0 (no way). you will need new equipt to meet
    the health code requirements. resize the load and get quote
    on new coil and unit

    as far as compression ratio goes it will vary thru out the
    operating range of compressor

    get curve sheet on this compressor, add 14.7 to high and add 14.7 to low then divide (answer = hi/lo) do this for your conditions - that will be your current ratio

    better to look at amp draw on the curve sheet, will tell you more


  10. #10
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,370

    I agree with rocket......

    You can go to Copeland's website and download their Compressor Performance Calculator, select your compressor model and plug in the refrigerant, suction and discharge pressures and it will tell you what that pump should be doing, and quite accurately. (You should also be able to do that at your Copeland distributor)

    It eliminates a lot of guessing and speculation about the compressor so you can move on to the real issues.

  11. #11
    Join Date
    May 2005
    Posts
    8
    Originally posted by rocket
    first it appears that this box was designed for 0, now the health code wants colder. the chance of existing equip working is 0 (no way). you will need new equipt to meet
    the health code requirements. resize the load and get quote
    on new coil and unit

    as far as compression ratio goes it will vary thru out the
    operating range of compressor

    get curve sheet on this compressor, add 14.7 to high and add 14.7 to low then divide (answer = hi/lo) do this for your conditions - that will be your current ratio

    better to look at amp draw on the curve sheet, will tell you more

    Rocket! I'm totally agreed with you on this issue. As we've already explained to the incharge supervisor during face-to-face that the unit may not capable pull and keep box temp constant at the new low temp requirement, or the original design was not intented to be operated at this ultra low temp. However, our supervisor would like to give it a shot, and that's how we got ourself in to this situation. An inquiry for compressor info has been faxed to manufacture technical support. We expect to receive it tomorrow or so. In the mean time, we're keeping an eye on it while chewing the pencil. Clock is ticking.....

  12. #12
    Join Date
    Oct 2003
    Location
    tidewater, va
    Posts
    2,140
    x5,

    As I was reading the posts and replies I thought of a box I had a few years back. Customer SWORE up and down to a junior tech that the box held -10....for some reason he chanmged the compressor. Guess what? It didn't work. I remember pulling a curve and seeing that this SOB was going full throttle and kept the box closed all night...so on and so forth. I contacted the factory and they said that the unit good come down to 0 or close to it and was DESIGNED TO BE A HOLDING BOX. That it, it was never meant to PULL Down, just to hold product at that temp, frozen products that arrive that way and WAS NOT DESIGNED FOR BARELY ANY DOOR TRAFFIC. Customers lie. For some reason, they think that lying will make a tech do a better or faster job. All they get is more trouble.

    What gas is in this thing...I don't recall seeing that. What happened with the oil failure situation? Good luck.

    404

  13. #13
    Join Date
    May 2005
    Posts
    8
    Originally posted by r404a
    x5,

    What gas is in this thing...I don't recall seeing that. What happened with the oil failure situation? Good luck.

    404
    R404a,
    The unit is running with r-22 in it as of original design. Our last attemp for the day is increased operating superheat by 6 degree on top of 9 degree static superheat. I also swapped out the low pressure control for more accurate cut-off. Don't know if the oil switch still strip until tomorrow morning. I'm expecting the compressor info will be faxed to us by tomorrow. All related document are missing. Nameplate is scrapped off and whoever rebuilt it nametag is in place. Thanks

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