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Thread: low pressure ?

  1. #1
    Join Date
    Dec 2013
    Posts
    6

    low pressure ?

    Consider a chilled water piping system where chilled water is being supplied to a fan coil unit.

    If the coil is new (no fouling) , the pressure difference between the inlet and outlet of the coil can be used to calculate the flow rate through coil.

    I have often talked to air conditioning equipment guys who say 25 psi pressure is too low, you need at least 35 psi of pressure.

    Shouldn't the only thing that matter be the difference between inlet and outlet pressure, and not the absolute value of supply pressure at the coil inlet?

  2. #2
    Join Date
    Aug 2006
    Location
    Mixing oil and fire with a big spoon.
    Posts
    4,015
    Generically speaking, the only thing that matters to a coil is differential (psiD). With that being said, higher pressures do have some benefits (like air removal) but most of those benefits show up in heating water.

  3. #3
    Join Date
    Aug 2009
    Location
    Prattville, Alabama
    Posts
    1,757
    Jay's right, but I want to add that you should also keep in mind what the pressure is at the lowest pressure point in the system. Typically that will be at either the highest physical location or at the pump suction. It should be at a positive pressure, preferably at about 10 PSIG to give some room to drop. If it is less than 0 PSIG a leak at that point will suck in air, creating other problems, and make finding that leak much more difficult.

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