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CFM over a surface.

  1. Jul 8, 2004 #1
    I have a tough question. I'm trying to calculate the cfm over a surface in a room. assuming that the only airflow out of the room is an exhaust vent. I need to know how to calculate that CFM for the surface near the floor of the room, depending on its distance from the exhaust duct and the cfm inside the duct. I have one formula, but it seems incorrect to me:

    O = c (10*x^2 + A) Vx (where Vx is V sub x)

    O is the exhaust volume (at the surface in the room, I believe), cfm
    x is the distance from the center of the hood on the exhaust duct to the surface in question, ft
    A is the hood face area, not including the flange, sq ft
    Vx is the minimum capture velocity, fpm.
    c is a multiple for flanged or unflanged hood, .75 for flanged, 1 for unflanged.

    this formula produces results that are way too large. shouldn't the portion with the x be inversely effecting the results (divided) rather than directly effecting them (multiplied, whatever).

    Any input would be appreciated. thanks.
  2. jcsd
  3. Jul 9, 2004 #2


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    Staff: Mentor

    I'm an HVAC engineer, but I'm really not sure what you are trying to do here. Are you trying to figure out how airflow gets distributed through a room? I don't understand what you men by "surface near he floor of the room". Do you mean hypothetically if you held up a piece of paper edge-on to the flow, what would the flow be? I don't see anything that says how big this hypothetical surface is. I've never seen that equation before. Where did you get it?
    Last edited: Jul 9, 2004
  4. Jul 9, 2004 #3
    Yes. The distance should inversely effect the results with an increase in distance there should be a decrease in CFM at the surface. Are you missing a Log or an antilog in that formula? It should work almost like a sound calculation.
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