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I have a chimney with a single 100 cfm exhaust fan. If I would add a second similar fan in the same chimney,in line with the first fan, what would happen? Would I move 200 cfm? Would I still move 100 cfm? Or would my cfm just increase slightly, and if so...how much?

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With fluid-/aerodynamics, exact figures are often hard to state.

E.g.: Your fan is rated for 100 cfm in absolutely ideal conditions, i.e. airflow without any obstructions. The fan in your chimney will not achieve 100 cfm because of ducts that the air needs to flow through. They act as an obstruction. How much does it achieve? No idea, it depends on a lot of factors.

If you add another fan, your airflow will most likely increase. Not by 100 cfm (same reason as above) but most likely with a substantial amount. My best estimate would be that the airflow will increase roughly by a factor of $\sqrt{2} = 1.41 $. This is a rough estimate, but without knowing much more about the situation, you're not likely to get a much more accurate estimate.

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  • $\begingroup$ My setup is a 24" tube installed vertically exhausting from a room through the roof and outside. I currently have one fan installed in the tube. Barring any external conditions, back draft, etc... Would adding a second fan in the same tube blowing the same direction essentially double my airflow? $\endgroup$
    – Geo
    Jan 25, 2023 at 16:50
  • $\begingroup$ I just used 100 and 200 cfm's as an example. In reality it will be much more than that. $\endgroup$
    – Geo
    Jan 25, 2023 at 16:53
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    $\begingroup$ @Geo all of this info belongs in the question ... it does not belong in comments $\endgroup$
    – jsotola
    Jan 25, 2023 at 17:15
  • $\begingroup$ @Geo I can almost guarantee that it will not double the airflow, a substantial increase is likely, but we cannot say how much exactly. $\endgroup$
    – Chris_abc
    Jan 26, 2023 at 8:06
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Here's the fan curve for an axial fan. If your original operating point was at 1, 250 kcfm and 8" pressure, then you'd hope that 2 fans would take you to point 2, 4" and 350 kcfm. But, your system pressure would rise due to the increased cfm, so in practice you'd end up near point 3, say 310 kcfm, as the back pressure is roughly going to vary with cfm^2.

axial fan

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