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  1. #14
    Originally posted by hvacdoctor
    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.
    You can use "pump down" as only PART of the diagnosis!! Yes this will tell you how well the suction valves are working, but no 2 compressors will be identical.
    Use the "time" it takes to pump down. Then shut it down and watch how fast it "leaks back" and how fast it takes for it to get to "operating" pressures. Use this information with with the compressor curve (amp draw) superheat and subcooling. with all this information you should be able to determine if the compressor is efficient or not.

    My 2 cents...

  2. #15
    Join Date
    Aug 2001
    Location
    Eastern Washington
    Posts
    2,989
    granted, this "curve sheet " thingy is new for me, but one way I proved before a leaky set of valves, instead of leaky solenoid was the temp rise of the suction line, as the hot gas leaked through the compressor valves, warmong considerably the suction line.
    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.

  3. #16
    Join Date
    Aug 2001
    Location
    Eastern Washington
    Posts
    2,989
    Originally posted by westcoast refer man


    Iced up coils can definetly cause a sight glass to flash.
    can you explain this?? seems to me that if the coil is frosted over the txv will be shut tight to keep SH up in a coil not absorbing heat due to low air flow. If its a fixed orfice, pressures will drop creating lower flow, ...unless...the lower temp in the system lowers in turn the refrigerant volume enough thatit mimics a low charge condition???

    not arguing, just not understanding the physics

    Originally posted by westcoast refer man
    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.

    just slap your hands on the hot reciever and the red will indicate refrigerant level like an upside down thermometer




    Another thought: you got headmasters on the systems that could be leaking or open. Txv sticking open?
    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.

  4. #17
    Join Date
    Mar 2004
    Location
    NJ
    Posts
    1,214
    Originally posted by westcoast refer man
    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.

    well I just learned something I was gonna suggest remove all the charge pull a vacuum and recharge it. If you have been adding freon for awhile because of a low sight glass the system could still be overcharged even after what has been taken out by your "tech" which may be why it takes longer to pump down. If the system worked fine before on a 1/2 hour defrost every 6 hrs why not try to locate the real problem instead of create more by playing with defrost cycles. Someone mentioned door seals/humidity level, another cause of icing of the coil could be gas backfeeding thru a solenoid valve if that valve is bad, bad txv, inefficient compressor(which is the hardest to diagnose for me) without a sure sign of compressor inefficiency(high amp draw,bad pressure readings, I try everything else first. Unfortunately by adding refrigerant as your cure it makes the diagnostic starting point more difficult as it just added to the problem and who knows may have damaged the compressor compounding one problem with another.

    [Edited by thehumid1 on 10-08-2004 at 04:16 PM]
    thehumid1-------I live in NJ, a state where it's free to come in but you have to pay to leave!

  5. #18
    [QUOTE]Originally posted by TB
    [B]
    Originally posted by westcoast refer man


    Iced up coils can definetly cause a sight glass to flash.
    can you explain this?? seems to me that if the coil is frosted over the txv will be shut tight to keep SH up in a coil not absorbing heat due to low air flow. If its a fixed orfice, pressures will drop creating lower flow, ...unless...the lower temp in the system lowers in turn the refrigerant volume enough thatit mimics a low charge condition???

    not arguing, just not understanding the physics

    TB, no I can't explain the physics on this one, but in my many years of experience I have found this to be true. Perhaps I should have made that statement differently, which is the problem with a message board; people can pretty much take you only in the literary sense. Let me rephrase my comment- I've seen many an evaporator iced up with the sight glass flashing. In those situations, I always get the evap clear and clean before I consider the charge. Plently of times, but not every time, I've found that the system was fully charged after having defrosted. In other words, don't assume you're short on gas if your evap is iced.

  6. #19
    Join Date
    Aug 2001
    Location
    Eastern Washington
    Posts
    2,989
    OK then, I'll float my boat along side you there, I agree with that, and have seen that myself too.

    Also seen a chiller compressor puke cause they added more refrigerant while it was allready flooding the compressor, just because the sight glass was flashing.


    I guess the jist of these posts is, just because the sight glass bubbles a bit don't guarantee a low charge, just guarantees a need to look into why its bubbling.
    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.

  7. #20
    Join Date
    Aug 2004
    Location
    MICHIGAN
    Posts
    12
    You make mention that box is utilized as a storage freezer.Assumming product is frozen when loaded into box,and system was designed,and installed properly,ie; proper refrigeration line sizing,piping practices, dehydration,excessive infiltration etc.,and product does not interfere with air patterns in box.Would be helpful to know suction and discharge pressures, liquid line temp.,suction temp.at txv bulb,at inlet to compressor,and what type of head pressure control.If system has been losing refrigerant, R404a is a blend,and will fractionate.If you do not know how much freon has been lost,adding freon may be futile.You would be better off replacing entire charge.As far as charging correct amounts into system,your equipment manufactures data will tell you how much refrigerant evaps. take,and cond. units take with 80%of reciever filled.Estimate piping lengths,and refer to charts on how much freon they hold.(Bohn offers this info. in thier installation manual).This will get you started,fine tuning now can be done when box pulls down to design temp..You say that your defrosts are set for 30,or45 min..Keep in mind that these are fail safe times,your defrost/termination are the controls you want determing adequate defrosting,not failsafe time.You may have to replace non-adjustable thermo-disc with adjustable type,depending on your frost loads.

  8. #21
    Originally posted by degenetron
    If system has been losing refrigerant, R404a is a blend,and will fractionate.If you do not know how much freon has been lost,adding freon may be futile.
    http://hvac-talk.com/vbb/showthread.php?threadid=61424
    I asked this very question just a week ago. And according to the guru's here in our community, 404 is a solid refrigerant.
    It does not fractionate.
    A leak will then leave a system low of charge but not to where it has to be totally recovered and replaced with virgin refrigerant.

    This may be news to a lot of people and it may rattle some cages, but so what.

    Just one more of the "wives tales" has just bit the dust!

    THAT is why we all come here. To share information and to learn.


    And if I ever got on a job with a guy who took a sizeable compressor and front seated the suction service valve to test for capacity ... I'd have to say he never visited this site ... or if he did ... he didnt read what was printed here.



    In my long years in this industry I have only replaced one compressor which I deemed bad and later discovered to have been fine. Only one!
    And the way I tested it and subsequently condemned it was by front seating the suction service valve and watching it pull down into a vacuum.

    That was a long time ago.
    I dont like violence, but acting ignorant is just about as bad as acting violent.
    Cause folks should know better ways to behave themselves.
    But I'll tell ya, had someone slapped me up side the head and rebuked me for making that test on that compressor ... it would have saved that customer a lot of money.
    But as it was ... the customer paid for my mistake. And it was a mistake I shall remember for ever and never repeat.

    If you see a guy front seat that valve for such a test ... maybe he needs a slap up side the head.

    [Edited by R12rules on 10-09-2004 at 10:12 PM]

  9. #22
    R404 is a blend.
    125, 44% / 143a, 52% / 134a, 4% made by Dupont with a name of SUVA HP62/FX70

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