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How to approx. Total pressures and Flow (CFM)

  1. Jul 30, 2006 #1
    Here is and Example.

    There is a system with 2-20ft dia. fans, in parallel, pulling air through a bunch of cooling coils with a cross-sectional area of about 800 ft^2.

    One fan is running and the other is isolated by backdraft dampers. In this situation the flow through the unit is 842,770 CFM and a total pressure (static + velocity) of 0.853" wg.

    If the backdraft dampers on the second fan failed and air was allowed to enter through the second 20' Dia. by 10' high fan stack into the unit...how can I approx. the new total pressure inside the unit and the resulting airflow. Also what % of that air is coming through the coils and what % is coming through the open 'hole' in the unit?

    Thanks for any input
  2. jcsd
  3. Jul 31, 2006 #2


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    Very briefly, fans have a characteristic "curve" which you can obtain from the manufacturer. You need to determine the flow resistance which you can do two ways, either by summing the resistances as given per Crane #410 or by using the Darcy Weisbach equation and determining the resistance from your known flow and pressure drop.
    I'd suggest doing it both ways and see if they come out close, then trying to pin down a usable number for resistance.

    Once you have the resistance, you simply put that back into the equation for flow and find the point where that flow will cross your fan curve for a single fan. I realize that's a very brief, so if you don't understand any part of that let me know and I'll clarify.
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