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  1. #1
    Join Date
    Mar 2004
    Location
    midwest
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    958

    Unhappy

    have a large walk in storage Freezer designed for -15F. We currently are at -8F. System has (2) 30HP Hussman SLD 0301 L4M air cooled condensing units with clean coils with matching remote electric defrost evaporators. One of the system's had R404A gas added every few months or so due the flashing sightglass. We recently leaked checked the condensing unit found no leaks, then added dye to system checked entire condensing unit had piping down to the evaporator and still no leaks. Then found 1/3 ice covered evaporator, found defrost needed repair fix it and cleared coil with hot water. Changed the defrost setting from every 6 hours @30 minutes to 45 minutes 4x per day. Is that too long for freezer defrost????

    Tech thinks that iced up coil could have given us low charge indications on the sight glass. He pulled out 20Lbs or so of gas but the system sight glass is still clear.

    Also performed pump down test on copeland 6DT3-300E COMPRESSOR. COULD ONLY GET SYSTEM INTO 15 PSI VACUME WHILE THE OTHER UNIT PUMPED DOWN RATHER QUICKLY DOWN TO 25PSI.

    BEFORE WE OPEN UP COMPRESSOR WE WANTED TO LET THE SYSTEM RUN A FEW DAYS WITH A ICE FREE CLEAR EVAPORATOR COIL BEFORE OPENING UP THE COMPRESSOR.

    ANY HINTS WITH CHARGING 404A SYSTEMS? ANY ADDITIONAL HELP OR IDEAS WOULD HELP.

    [Edited by hvacdoctor on 09-29-2004 at 11:49 AM]

  2. #2
    Join Date
    Jun 2004
    Posts
    182
    What's your suction pressure? Is it low enough to achieve -25°evap (-15° box)? The pump down test really doesn't tell you alot about the compressor. The biggest thing here is after you shut the compressor off did it hold the low side pressure or did it raise?

    If you want to see if the compressor is operating correctly look at the amp draw. Measure and record suction and discharge pressures, voltage and amperage. Compare this information to the proper compressor curve sheet. If you can, post the information here and we will check it for you.

  3. #3
    Join Date
    May 2000
    Location
    Monmouth Junction-NJ-USA
    Posts
    6,012
    Need to know superheat at evap. sub-cooling, td over coil etc. Can't remember whos service bulletin but i have on somewhere about running POE systems in a vacuum. Maybe Dave can help out here. I only looked it over briefly but said it was a definite no no.
    If you really know how it works, you have an execellent chance of fixin' er up!

    Tomorrow is promised to no one...

  4. #4
    It takes what it takes to defrost the coils. You are stuck with the limitations of this sysytem. Make sure all sources of moisture from infiltration etc have been reduced as much as possible. Is the box in good shape, or is it part of the problem?? Typically when a system has been designed properly (how often does that happen?), the equipment has been sized to allow removal of the heat load with the time alloted for refrigeration vs defrost. In other words, instead of doing say 25 tons in a 24 hr period, the capacity of the equipment is "upped" so that it can do 25 tons in an 18 hr period. This would allow for 6 hrs of defrost. Based on the changes you made to the time clock, the total amount of time spent in defrost has not changed. It may be that 3 hrs total defrost time is not enough.

    Iced up coils can definetly cause a sight glass to flash. Check the reciever's liquid level as well as the sight glass to determine correct charge after the coils are clear. Assuming the reciever is not insulated, the liquid level can be determined by heating the reciever's "end bell" with your torch and then carefully touching it to see where the metal turns from hot to not hot. The transition between hot and not hot is the liquid level, but be careful not to burn yourself when doing this.. The receiver should be 1/3 to 1/2 full. If after filling to this level you still have a flashing sight glass-we need to talk again. Use compressor capacity curves to determine if compressor is pumpimg properly. While the pump down test you described is useful, it is not a definitive test.

    404a at -25 or even as low as -50 is still in a positive pressure.

  5. #5
    A pump down test is practically a waste of time.
    Like others have stated in many other threads, the compressor curve charts are the only way to know what capacity a compressor is operating at.

    As far as the charge goes, a system will require the most amount of refrigerant once it is down to temp and about ready to shut off.
    And this only counts when the whole entire set up is designed to work together for all the factors involved.

    Structure, load conditions, coils/condensing units... they ALL need to be working together in perfect harmony.
    Otherwise you got a mis-match. And this will cause prolonged run, at the very least!
    At the worst, it will never hold the product to your desired specs. And THAT my friend, is where the nightmare begins!


    I agree with West-Dude. He is right.

    Once you've got the coils consistently clean and the system is fully charged... you can expect 100% refrigeration to occur.
    An old rule of thumb was to desgign equipment to run 60% of the time. Then shut off.

    Once your freezer drops back down near it's desired temperature, you will begin to see how much time the compressor(s) will be running.
    Depending upon the size of your freezer box, this may take days before your there.
    And once it is close, it may require some adjustments to get it the rest of the way.
    Clear coils and properly charged condensing unit along with properly feeding TX valves will get you there.

    Measuring your superheat and subcooling will be the best thing to do when your at that design temp.
    But prior to that ... your system is running an uphill battle and those components will be maxed out unitl they settle in near design temp.


  6. #6
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    Originally posted by rayr
    Need to know superheat at evap. sub-cooling, td over coil etc. Can't remember whos service bulletin but i have on somewhere about running POE systems in a vacuum. Maybe Dave can help out here. I only looked it over briefly but said it was a definite no no.
    Dave will be back, (Yes, I'm going BACK to ork, again..... ) but, in the meantime, read this thread:

    http://hvac-talk.com/vbb/showthread....537#post602537


  7. #7
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    By the way, is the product being loaded into this thing at temp, or are they attempting to pull the product temp to a final temp with it?

  8. #8
    Join Date
    Aug 2001
    Location
    Eastern Washington
    Posts
    2,990
    Originally posted by condenseddave


    Dave will be back, (Yes, I'm going BACK to ork, again..... ) but, in the meantime, read this thread:


    [/B]

    AH-Haaaa! Your true identity shows itself Daavvve! your Mork, from Ork,....... say, could you introduce me to Mindy??? I allways wanted to meet her.

    Sorry bout your little egg buddy on that other thread. Musta been one of your missing probes.

    [Edited by TB on 10-03-2004 at 02:26 AM]
    TB
    Everyone knows something I don't.

    2 Chronicles 7:14
    14 if my people, who are called by my name, will humble themselves and pray and seek my face and turn from their wicked ways, then will I hear from heaven and will forgive their sin and will heal their land.

  9. #9
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    Originally posted by TB
    Originally posted by condenseddave


    Dave will be back, (Yes, I'm going BACK to ork, again..... ) but, in the meantime, read this thread:


    AH-Haaaa! Your true identity shows itself Daavvve! your Mork, from Ork,....... say, could you introduce me to Mindy??? I allways wanted to meet her.

    [/B]
    If I knew Mindy, I wouldn't be going back to work at that time of night.

    That third person thing was in jest regarding Ray's reference that I 'd be along.

  10. #10
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    Kinda like the suspenders, though.

  11. #11
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    And I really need to fix my "W" key. It's getting annoying, now.

  12. #12
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215

    Scattered thought #4

    The only thing I remember reading about not running anything in a vacuum is the Copeland scroll AE's. It'll immediately cause shorting across the compressor terminals, due to there being no resistance in a vacuum.

    You should never run ANY compressor in a vacuum, really, for the same reason, unless that compressor is designed for it.

    I know of some old dipping cases (R12) that need to run in a vacuum to maintain their case temps, but none with 404a. I mean, a 5 pound suction on 404a is a 40 below coil...

    POE is a touchy oil, the lower the suction, the touchier it gets. The key is to maintain a good superheat, to prevent the discharge temps from getting out of hand.

    All that said, it's not real relevant to the original question, I guess.

    There is no set standard that I know of for a compressor to be able to pull against a closed SSV. Also, there are other factors that can effect the outcome of a vacuum deacy test.

    BUT, if you have 2 identical compressors, one goesto -25, and the other struggles to -15, your first order of business is to take the operating suction and discharge pressures, write them down, and record the operating voltage and amperage of each phase, on each compressor, write them down, and average the readings on each. Then get your Copeland wholesaler to give you photocopies of the curve sheets, or go to the Copeland Energy calculator and see if the compressor is inefficient. I suspect that it is. If it is, pull the heads, look for broken valves, (ICED coils usually FLOOD) loose, or broken reed pins, bad gaskets, bad plates, broken pistons, excess oil on top of the piston, etc. Fix or replace the pump, and be on your merry way. Apparently, the system wasn't designed to run 100% on just one unit, but was well designed to hold well below zero with one bad one.


    Here is an example of the report you get when using the Energy Calculator progrma from Copeland:


    Note that I used the compressor number you gave. This does NOT mean that you should see the same amperage as the example above. You have to do your own curve based on what conditions you are seeing.



  13. #13
    Join Date
    Mar 2004
    Location
    midwest
    Posts
    958

    Lightbulb

    Thanks for the replies. ? about pump down test. Why don't you recommend the pump down test to check compressor eff? The reason is we installed a new compressor last year on the other unit it pumped down rather quickly -25psi while the unit in question took 2x as long to get down to -15psi??? Just curious about this.

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