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Side opening on a shrouded industrial fan

Mike Lish | Posted in General Questions on

I need to redirect exhaust flow 90 degrees. Rather than fabricating ducting offsets from a plenum box fitted to the exhaust opening, I am considering doing away with the plenum by blocking off the fans exhaust opening, and creating a side opening in line with the fan blade tips. This will allow straight line ducting. Will this reduce the fans efficiency.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mike,
    I'm not sure that I am visualizing the situation properly. A photo or sketch would help clarify the situation.

    But here are the important principles: If the fan is well designed, it is designed to operate as manufactured, without modification. Trying to get the airflow to move at 90 degrees to the way the fan designer intended will almost certainly lead to turbulence and increased static pressure (i.e., reduce efficiency).

    What you probably need is either a different fan, or a long-sweep 90 degree duct fitting.

  2. Mike Lish | | #2

    Here is my further explanation to help you visualise. I am installing an intake fan for a self build paint spray booth for cars in a domestic type garage. The fan is located at ceiling height above the entrance, next to a side wall, and I will be installing a duct to direct its air flow 90 degrees to its exhaust opening to the full width of the entrance. Because the fan is bolted to an inside front wall, I am considering blocking off its exhaust opening and cutting a hole into the shroud directly opposite its fan blade tips. this will allow a straight run on the ducting, supported by the wall, right up to the side of the fan. I take your point about reduced efficiency owing to turbulence however, my general understanding of fans is that the highest percentage of blade efficiency is at the blade tips, so if a new opening was created at the blade tips then will any reduction of efficiency be marginal and acceptable.

    Thanks for your response.

  3. GBA Editor
    Martin Holladay | | #3

    Mike,
    Sorry, but my answer remains the same.
    (a) I'm not sure I'm visualizing this properly.
    (b) A photo or sketch would help.
    (c) Modifying a fan is likely to reduce its efficiency.
    (d) It would be best to purchase a fan that can be used without modifications.

  4. Charlie Sullivan | | #4

    I agree with Martin--we need a sketch or a photo or something to have any idea, but the answer is likely to be that it reduces efficiency regardless of what the sketch looks like.

  5. Drew Baden | | #5

    Regardless if the picture is clear, it's clear that what you want to do is block off the exhaust port and create a new one on the side of the shroud. You now know this will reduce efficiency and you need to get the fumes exhausted from your garage. Either way, please wear breathing protection. I know someone who developed COPD and is now fatally ill because he was doing the same as you--trying to remove paint fumes with a degraded fan-- but was NOT wearing breathing protection. This happened over a period of years.
    On a non-paint related adventure, I moved an exhaust 90° from a bathroom ceiling fan and it seemed to do the job just fine. It's a leaky house, though. If it were a super tight house, and my health depended on it, I wouldn't have relocated the exhaust.

  6. Mike Lish | | #6

    OK gents, Having anticipated your answer I have ordered a plenum with a 90 degree outlet. its inlet will fit snugly into the fan shroud opening. I will have to angle the extension ducting from the plenum outlet to its supporting wall. Owing to a weight issue, I will construct the ducting using 3mm hardboard (fibreboard) which will have a grid mesh opening along its length to support a length of blanket type filtration. The aim is to eliminate jet streams by broadening the air stream across the full width of the garage. I have also installed an extractor fan at the opposite end of the garage. The combination of industrial intake and exhaust fans ought to provide sufficient air flow. I made up a basic barometer to gauge a slight positive pressure when balancing the two fans with their respective speed controllers.

    Thanks for your input.

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