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  1. #1
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    CHILLED Water Pump, VFD or No VFD ?

    Question; we have 3 Dunham Bush WCFX chillers. Only 2 run at a time. 1 is a back up. All run off 1 chilled water pump without a VFD. The coils in the air handlers of course have flow control valves. Would it be best to have that single pump run off a VFD or does it really make a difference. What would be best and why. Thank you all for your resent help on condenser water screen size.

  2. #2
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    I would think you would have to compare system gpm vs pump gpm to even see if it was feasible. If it is how are you going to control the drive. You might have this moved or maybe start a separate thread in the tab section once you have numbers.
    Honeywell you can buy better but you cant pay more

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  3. #3
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    Feb 2016
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    Is this a constant volume or constant pressure system?
    No man can be both ignorant and free.
    Thomas Jefferson

  4. #4
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    I am Not a big fan of varying water flow rates through chillers as it tends toward refrigeration control issues.

    Are the flow control valves at the coils two-connection or three-connection?

    If three; are the control valves bypass or diverting?

    What is the rest of the model number?
    How many compressors per unit?

    PHM
    --------


    Quote Originally Posted by Chilly56 View Post
    Question; we have 3 Dunham Bush WCFX chillers. Only 2 run at a time. 1 is a back up. All run off 1 chilled water pump without a VFD. The coils in the air handlers of course have flow control valves. Would it be best to have that single pump run off a VFD or does it really make a difference. What would be best and why. Thank you all for your resent help on condenser water screen size.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  5. #5
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    Apr 2019
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    Thread Starter
    Yes, constant volume. Pump is running at 35psi. We will be calculating the GPM from the circuit setter locations on each coil. Control valves divert the flow if not needed at a said coil. 270 ton unit to medium screw compressors per chiller model# states; WCFX46T. Water cooled, R134A Dunham Bush chillers.

  6. #6
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    I would install a VFD to get the soft start but you need to run it at a constant speed.
    No man can be both ignorant and free.
    Thomas Jefferson

  7. #7
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    Thread Starter
    Yeah understand. We have a soft start on the pump now. I was just wondering if it was better to control temperature using a VFD on the pump.

  8. #8
    Join Date
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    also, with a VFD on the primary loop, engineers love to run the chiller barrel flow rates at the low end of the required flow range. As Mikey said, you end up with refrigerant control issues, which leads to service calls.

    Plus you’d be spending thousands of dollars to implement the controls scheme and components.....there goes any perceived savings.

  9. #9
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    Thread Starter
    Good point 71chops. Thanks Mikey! Just going to stay with what we got...constant even flow.

  10. #10
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    Add to that............If the chiller isn't controlling the pump flow the 2 controls tend to fight each other.

  11. #11
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    If you had a primary secondary system, you could keep the primary a constant volume loop and change the secondary to a variable volume loop, but you would have to remove all the 3 way valves, install 2 way, install a bridge, and an end of loop bypass.

  12. #12
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    Feb 2016
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    If you want a constant volume system to cool the entire building it has to run at a constant GPM. The biggest chilled water problems I have seen is adding on to a constant volume system by adding a chiller and pump then trying to run one pump and chiller at low loads.
    No man can be both ignorant and free.
    Thomas Jefferson

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