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Thread: Computer room cooling - Condenser fan failure

  1. #1
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    Computer room cooling - Condenser fan failure

    I have a question concerning operation of a Liebert DX computer room unit during a condenser fan failure.

    Situation:
    Communications room, two upflow Liebert DX 15 ton units.
    The units are configured as a team with lead unit switching every two weeks.

    Two compressors in each unit, two refrigerant circuits, air cooled condensers (also Liebert) with three fans.
    One fan is pressure controlled, variable speed, two are outdoor temperature controlled and constant speed.

    The variable speed motor seized up and the one compressor calling at the time tripped on high head.

    The tech called to the facility noted the fan failure and then disabled one of the two compressors, when asked why he said "you cant run both compressors on only two fans"
    I explained that the coils are length wise on the condenser not stacked therefore all three fans draw air over both circuits.
    He said if he left both compressors enabled it would trip on high head again.

    The outdoor temperatures have cooled down since this event, fan motor still on order.

    My question is: Am i correct when I stated the one circuit does not affect the condensing temperature/head pressure of the other and given that we have cooler weather both compressors should be enabled?

    If I am incorrect what is the reason for this?

    The other concern I have (and got a seemingly strange answer for) is that the setpoints are 68 (return air) and I do not need the room to be that cold. The techs foreman said if we raise the setpoint the head pressure will be higher than if we run the units at 68.

    Again, I can see the pressures being somewhat different running at 68 vs 74 but temps at 74 would reduce the number of compressors online.

    The usual room conditions usually have two compressors in the lead unit running continuously and the standby unit calling with one more compressor periodically.

    Thoughts on this?

    In addition to helping me understand this what are your thoughts on continuing operation while a motor is on order?

    Thanks.

  2. #2
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    Tech correct on disabling one compressor on that circuit. Could lower the ambient fan t’stat or relocate the sensing bulb to the liquid line to help.

    Raising the ra set point will not increase the discharge pressure, should reduce it.

  3. #3
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    Thread Starter
    Thanks for the reply.

    If there is only one compressor per circuit how does disabling one affect the other based on the loss of one condenser fan?
    I want to understand the reason behind that.

    I think the setpoints on the condenser were changed to prevent the remaining two fan from dropping out too soon as it cools outside.
    I hope we can get that motor replace before late fall.

    My experience with the Liebert condensers are the variable speed fan motors live very hard lives. We have had many failures on different units.
    Glad my new datacenter is chilled glycol and in row coolers.

  4. #4
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    You stated the two refrigerant circuits shared fan air flow. Each circuit have 3 fans?

  5. #5
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    Thread Starter
    I think I have that right, two circuits, one compressor each, at the condenser the both circuits have air flow from all three fans.
    "standard" Liebert DX condenser coils.

  6. #6
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    Why not put one fan on a regular pressurestat ( on/off ) other two already on OD thermostat. Why does it have to be a motormaster control type? Are you running the computer room at extremely low outdoor temperatures and need the motormaster type throttling control instead of a regular on/off fan cycling pressurestat?

    Or motormaster on one, regular pressurestat on another and the last motor on OD temperatures. Or one motor comes on when the lead compressor starts.

    As you say throttling a outdoor fan motors takes a whipping.

    Are the compressors the same size in these 15 Ton systems.
    Last edited by Bazooka Joe; 09-12-2022 at 08:31 PM.

  7. #7
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    Thread Starter
    Both compressors are the same size, I do not think they have any capacity controls, just start-stop.

    The standard Liebert configuration for the condensers are p66/Motormaster on one fan, outdoor temp on the other two.
    We could probably configure one for cycling, or even eliminate the variable speed and make that one pressure based.

    Lowest temps we usually get here in VA is 10-20F. some occasional temps in the single digits.
    I need to see how these sequence during the winter.
    In the past our techs have adjusted the temperature controlled fans to compensate for failures and high pressure trips and they run deep into the cold weather.

  8. #8
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    Outside air stat on 1st fan
    Pressure control on 2nd fan
    Speed drive on 3rd fan

    In low ambient I've seen the speed drive ramp too low and cause the motor to overheat!

  9. #9
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    Quote Originally Posted by pecmsg View Post
    Outside air stat on 1st fan
    Pressure control on 2nd fan
    Speed drive on 3rd fan

    In low ambient I've seen the speed drive ramp too low and cause the motor to overheat!
    Vfd fans should not be set up to go below 20-25 htz.
    Normally vfd fan first on to trim pressures.

    Don’t know why anyone would use oa stat on process load or any ac. I’ve always relocated to ll or subcooler circuit tubing or replaced with pressure control. Assumption is indoor load is based on oa ambient.

  10. #10
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    Ive had no issues with 3 fan condensers using a OA Control on 1 fan. 55-60 off!

    With speed drives able to handle multiple fans is a non-issue today!

  11. #11
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    Then there is pollen getting sucked into the coil at certain times of the year. ( At least in my region ) Even on the best designed system. Remember doing PM’s just to keep a clean coil on those computer room systems. Cleaned the coils, then returned a week later to clean again. Thinking not enough air flow across motor does not help, especially if they are open motors.

  12. #12
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    Quote Originally Posted by pecmsg View Post
    Ive had no issues with 3 fan condensers using a OA Control on 1 fan. 55-60 off!

    With speed drives able to handle multiple fans is a non-issue today!
    Had a numbed of Monday morning hot pull downs where ambient stats were an issue. 80-90* chillwater pull downs, fans should control to load.

  13. #13
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    Thread Starter
    Thanks to all for the discussion,

    If this Liebert system has two circuits, one compressor per circuit and common condenser fans how does disabling one circuit affect the other?

    This week the two CRAC units swapped primary/standby roles and now the one with the compressor the tech disabled is primary. The room temp has creeped up a bit due the the temperature differentials when staging up.

    I understand if there are tandem compressors on one circuit, this is not the case on this system.

    Looks like the techs company cant get a new motor for a long time. Ugh!

    Dave

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