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  1. #1
    Join Date
    Oct 2002
    Posts
    81
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    Variable Primary Flow - Minimum Flow

    Curious how you guys typically set up minimum flow bypass for chilled water. If the low flow safety (DP switch, flow switch) is set to open at the chiller's spec'd minimum flow, what setpoint would you use to control the minimum flow bypass?

    Example - flow switch opens at 400 gpm. What bypass setpoint would you typically try to maintain to avoid nuisance low flow trips?

  2. #2
    Join Date
    Dec 2014
    Location
    New Mexico
    Posts
    456
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    The chiller manufacture has that numbers. They have tested what their chiller will tolerate and not freeze up at. You adjust the flow switch to that.


    Sent from my iPhone using Tapatalk

  3. Likes yorkie820, WillyTech liked this post
  4. #3
    Join Date
    Oct 2015
    Posts
    52
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    You might just have to experiment until you find the sweet spot. You don't want to set your bypass GPM setpoint so low that you risk tripping out your chillers (low flow is typically a manual reset at the chiller, in my experience), and you also don't want to set it so high that you inefficiently rob your zones of water.

    If it were me, I'd start at 500 gpm and run a couple cycles of your full range of operation, all in auto control and see what happens.

  5. #4
    Join Date
    Jan 2003
    Location
    USA
    Posts
    2,489
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    Ditto. Its a T&E session. Set it 50-100gpm over the absolute min and see what happens.
    Propagating the formula. http://www.noagendashow.com/

  6. Likes roadgear16 liked this post
  7. #5
    Join Date
    May 2012
    Posts
    32
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    Given a clean sheet of paper I would install differential pressure transmitter(s) across the chiller evaporator section(s) and modulate the bypass valve to maintain something just slightly above the manufacturer's minimum. DP is: less expensive, easier to maintain, more accurate, more robust.

  8. Likes WAYNE3298 liked this post
  9. #6
    Join Date
    Feb 2006
    Posts
    283
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    I would add 100 gpm like stated and see how well the system handles it. I'm also a fan of evap DP over flow meters. I've never found a flow meter that wasn't a piece of garbage.

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