Fan Sizing and V.A.V.'s
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  1. #1
    Join Date
    Feb 2010
    Location
    New York City
    Posts
    70

    Fan Sizing and V.A.V.'s

    A supply air fan , controlled by a Variable Frequency Drive, supplying multiple V.A.V.'s
    If all the V.A.V.'s are 100% open Is the Fan generally Sized so that it will
    operate at Less than 100% Speed,And still maintain the desired staic setpoint

  2. #2
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190
    The reason you have a VAV system is YOU ARE NOT GOING TO HAVE ALL THE BOXES CALLING AT THE SAME TIME
    It is called diversity, that means the East boxes call in the morning with the west not needing the max until PM then the sun is no longer on the east. 80% of the max peak loads is the rule of thumb for the max demand with the partial load needs less than that
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  3. #3
    Join Date
    Oct 2001
    Posts
    6,932
    the only time the VAV boxes are at 100% are during morning warm up and or night setback for full fan and to make a preset setpoint...basically a constant volume system.typical VAV systems run 1.5" of static at 55F and when the space is satisfied the VFD is running a 30Hz signal to the fan.as the VAVs move around from minimums to maximums the VFD will react to that duct static build up maint.that 1.5"
    "when in doubt...jump it out" http://www.youtube.com/watch?v=U1qEZHhJubY

  4. #4
    Join Date
    Apr 2012
    Location
    Paper Street Soap Company
    Posts
    2,298
    They ramp down to maintain static and its how the old pnuematic systems with variable inlet guide vanes and pnuematic reciever controlers worked.

    Lowered amp draw too as the load in the building decreased.

    I still have a couple of small office buildings that are from the ground up pnuematic. Once they're calibrated correctly and as long as the air drier and air compressor dump works they're practically bullet proof.

    Only time I get zone issues us when some engineer monkees with the T stats.

  5. #5
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,343
    Achicagoperator

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