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Thread: Centrifugal Fan Arrangement Question

  1. #1
    Associate Engineer
    Join Date
    Oct 2016

    Centrifugal Fan Arrangement Question

    Hi Everyone,

    Not sure if this has already been discussed (can't find it if it has been), but I have a ventilation system problem and could use some advice.

    What I have is three separate branches feeding into one main trunk. Each separate branch has it's own "booster" fan, not all the same size. All three feed into one larger fan. I'm having issues keeping the system negative, as if the sum of the three flow rates into the main blower are exceeding the main blower's capacity. From the OEM drawing, supply fans are 1960, 500, and 4350 cfm. They feed into a 6500 cfm main blower. I realize that there are pressure losses and air density issues to contend with, but is it as simple as the sum of the three branch fans air flow must be less than that of the single main fan to keep the system negative?

    This is on an emissions control system, so positive pressure anywhere in the system cannot be allowed. If it's not as simple as summing the three branch flow rates, what information would be required to determine why the system is not under negative pressure?

    System layout is attached. The "wet" system is the problem area that will not stay under negative pressure. Point "A" in lower left is just an open ended snorkel that should be sucking in fumes, but instead it is blowing out air. The condenser has been throroughly cleaned and verified free air flow through it. Thanks!

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    Last edited by cawsonmatt; 09-06-2019 at 08:58 AM.

  2. #2
    Associate Engineer
    Join Date
    Sep 2019
    Very good documentation, I am going to take a look

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