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  1. #14
    Join Date
    Nov 2004
    Location
    North Florida/South Georgia
    Posts
    988

    re-read please

    hvacdoctor,

    Please go back & read my post.
    I mentioned that the unit most likely does not have LP bypass capabilities on the start of additional compressor or compressors within the same circuit.

    Scanario: Cir. A starts compressor A1, the bypass timer provides contact closure accross the LP for 2 or 3 minutes, thus allowing the compressor to recover from the startup dip.
    Cir. B starts compressor B1, the same temporary contact closure procedure takes place again just like it did on cir. A, but when compressor B2 or A2 start, there is no temporary contact closure therefore the cir. cannot recover from the dip in suction pressure.

    Trying to correct a problem such as this with low ambient controls alone will often result in setting the operating head pressure range for the cond. fans very high, which will sorta solve the problem, sometimes, but failures will still happen & you will pay dearly on KW consumption.

    Even on the newer models with intelligent control logic, this is a weak link for the reasons mentioned above.

    If the unit is new, you might be able to get Trane to program a chip for you with this bypass feature in it for each time an additional compressor starts as opposed to only when the first compressor in a given circuit starts.
    But I doubt it.

    You can use a manual reset LP, this can be a little tricky to wire in sometimes because often the safety circuit is 24vac & the contacor circuit is 120vac.

    If thats the case, you can add a few relays to the machine with delay on make timers.
    You could use 90-381 type, wire the NC contacts to the pressure control & wire the coil to the contactors circuit with a delay timer in series that doesn't allow the relay to power for 2 or 3 minutes, therefore providing temporary contact closure ANYTIME a compressor starts.

    If you have 4 compressors, you will need 4 relay & 4 timers.

    I hope this is helpful.
    All my leon freaked out!

  2. #15
    Join Date
    Mar 2004
    Location
    midwest
    Posts
    957
    Chiller, you pretty much got it down. This unit has no logic or PLC whatsoever. I have a Honeywell staging control-analoge type controlling it. I will try the time delays with relays.
    An astronaut stuck in space was asked by a reporter, "How do you feel?"
    "How would you feel," the astronaut replied, "if you were stuck here, on top of 20,000 parts each one supplied by the lowest bidder?"

    "do it right or do it twice"

  3. #16
    Join Date
    Aug 2006
    Location
    Mixing oil and fire with a big spoon.
    Posts
    4,336
    a couple of things to think about:

    1. what is the low pressure trip setpoint?
    2. do you use glycol? if so, what type and %? was it designed for glycol or was it converted from water?
    3. have you always had this problem or is it fairly recent?
    4. the model numbers are long! but this might tell me what options you have/don't have...we are all kind of guessing at what you have or at least should have.

    you may have a system problem and not a unit problem...it just shows up first in the unit. if the unit was originally designed for water at 44F and now it is 20% glycol and 42F, that is a big difference to the unit. you may need to replace the low pressure switch to one with a lower setpoint. you may also have a dirty evap, low water flow, etc. which would cause the expansion valves to under feed for a short period of time. chiller temp does not drop fast enough and more compressors come on....bang! low pressure trip. what controls the staging of the units? sounds like it is a little too aggresive. but then again maybe not if you are having system problems and not unit problems.
    "Mother" is the name for God on the lips and hearts of children....The Crow

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