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  1. #1
    Join Date
    Apr 2008
    Posts
    169

    Liebert 30 ton DS’ LPCO question

    I wanted to get some input on something I’ve noticed during service and checks on the ‘DS Copeland scroll compressor units. Part of our service routine is to document operation of the high and low cutouts.
    I first noticed on an earlier occasion of pulling the liq. sol coil allowed the comp to pump down approaching zero (gauge).
    Copeland does not like these to run anywhere near that low (below about 20 or so psi if I recall.
    So calling tech support was advised the system would (should) protect from loss of charge for example, that perhaps it was delay’ between my gauge reading and the system’s response, and it would be safe to run a test briefly enough to try it again.
    Did a test on a second different unit, it did the same thing even letting it linger a bit longer before pull the plug’.
    These have a Low Pressure-to-analog transmitter by the way.

    I am skeptical about the gauge lag’ is this case. On a normal stop from cooling /pump down I can always reliably see them pulling down typically to around 4-10#. Liebert’s control target cutout point is actually quite a bit higher than that and if I’m not mistaken part of that difference may be at least partly system response lag.

  2. #2
    Join Date
    Apr 2011
    Location
    Coastal Maine
    Posts
    888
    It is system response lag in my opinion. On the DS units, it is a Pressure-to-digital-to-analog. The signal is routed through the triac array to start the shutdown sequence. Emerson owns Liebert and Copeland, so I am sure that it is all figured in. I would say that as long as it is shutting down when you start pumping down, then the switch is working, just not as fast as we would see it in a traditional LPCO.

    Good Luck.

  3. #3
    Join Date
    Apr 2008
    Posts
    169
    I came up based on info gathered when I first got exposed to these systems they were not to be pumped down in ways we could get away with in recips. Or that indicated they could be rather quickly damaged (how quickly..?) running at low suction.
    From Copeland for example;

    "Do not operate compressor without enough system charge to maintain at least 0.5 bar (7psi) suction pressure. Do not operate with a restricted suction.
    A minimum suction pressure of 1.75 bar (25psi) must be maintained during charging. Allowing pressure to drop below 0.5 bar for more than a few seconds may overheat scrolls and cause early drive bearing damage.

    Something I found at another forum (I do not know if this valid BTW--

    "Residential scrolls have a unique feature (Copeland only) which only allows the compressor to draw a compression ratio of greater than 11:1 when the floating seal drops, you'll notice an equalization of pressures as it stops moving refrigerant. So, when your compression ratio hits this 11:1 you won't be able to go down lower.
    It is designed to prevent a "micron level" vacuum. At micron levels, you have removed all insulation around the motor (refrigerant acts as the insulator) and the motor becomes an emitter, the shell becomes a collector and the terminals can arc (inside or out). This is also possible in reciprocating compressors but it hasn't been as big of an issue because it takes a long time (if ever) you reach those levels by closing a valve or blocking the suction line.
    In a scroll, they are close to 100% efficient so it can pull down in a snap!"

    So, at this point it seems there's a couple different ways to pick here;
    I could ignore it..
    Or call bluff' and see how long it actually takes the system to stop?

    IDK. 'Lag- but why the difference? It occurs to me maybe they wrote some additional delay for other than a normal control stop?


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