Exhaust Fan: Sizing pulleys/motor
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  1. #1
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    Default Exhaust Fan: Sizing pulleys/motor

    I have this exhaust fan:

    Made by a company called: “Coolair” in Jacksonville, FL. Doesn’t match anything they currently produce.

    I got it with a 1/2-HP/1725 rpm motor.

    As that motor was mounted on plywood, I’m hazarding that it is not the original motor for the fan.

    The blade design seems like this fan should move a lot of air, it always worked, but I think it can work better.



    Someone gave me this motor some years back, I’m tired of tripping over it and think this may be a good application for it:


    It’s a cheap Chinese/Taiwanese thing, but ought to work for this application.



    Now! On to my question(s)

    What is the best way to determine the operating rpm of the fan blades?

    Obviously I could just size the pulley commiserate to the rpm change in the motors, but as I’m also quadrupling the HP I’m wondering if I can turn the fan blades faster, before I have cavitation issues.

    Hoping one of the smart fellers here can put me on a better path.




    Be safe




    Jeremy

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    Just my $0.02. looks similar to an engine cooling fan which regularly reach over 2500 rpm even on larger engines. My guess would be that your motor will be groaning before you over speed them blades if it's as big as it looks.

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    I'll toss in my two cents as well. The driven sheave is integral to the fan assembly so you can't really change that. I'll guess that the original motor was a 1725 unit which you want to replace with a 3450 unit. To stay within any 'guessed at' fan RPM's, you have no options to re-sheave to allow for a respectable fan speed...you need to find a 1725 RPM motor to get you into the ballpark.

    Stuart

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    Any reason not to call Coolair? Hard to imagine they wouldn't have more information on this than any of us.

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    I put in an email to Coolair… but I also know well enough not to expect too much from cold-calling a company and asking about an obsolete product. Been there, done that.

    Yes. The driven sheave is integral… no changing that. As I’m reworking the whole motor mounting plate no matter what, I can add a jackshaft if necessary. Or find another motor.

    The problem I’m having is still knowing what rpm the blade needs to be turning to even start.

    A buddy of mine that used to build airboats and ultralights gave me a couple suggestions and said he’ll look into it further and get back to me. Problem is, he’s used to variable pitch technology, a luxury I don’t have.



    Be safe



    Jeremy

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    There many online calculators where you input speeds and pulley diameters to get what you want. Speed x diameter of driver pulley equals speed x diameter of driven pulley.

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    I get all that… it still all hinges on knowing what speed the fan blade itself needs to be turning.

    Jeremy

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    tip speed needs to stay subsonic.

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    Found this:
    https://eurovent.eu/sites/default/fi...figure%206.pdf

    Couple basic rules from that link.

    CFM proportional to fan rpm. Makes sense
    Power consumed to drive fan varies by the RPM^3.

    So doubling the motor rpm will require 8X more power.
    You have 2 hp when you used to have .5 or 4X more.
    So you are going to have to gear it down. But how much?
    x^3 = 4. X=? 1.58 faster than the 1725 motor or 0.79 x the 3450 speed.

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    Whatever motor you decide to use on the fan, put an adjustable sheave pulley on the motor.

    Then, find the motor data plate and find out what it's full load amperage (FLA) is.

    After assembly and when you first start the motor... clamp an AC ammeter around one of the power wires supplying the fan motor... Start the motor, observe the amp draw....adjust the pulley diameter larger or smaller, whichever one adjusts the current draw to right to the FLA.

    Load on the motor is the big thing... the fan will only spin as fast as the motor can efficiently supply the power to do so. If it's FLA is 4 amps, adjust the pulley so that the motor is drawing just under 4 amps.

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    Quote Originally Posted by JoeE. View Post
    put an amp clamp around the line feeding power to the fan motor and start that motor... observe the amp draw, and adjust the pulley diameter bigger or smaller....whatever it takes to get the operating amp draw of the motor to within that specification.
    Exactly; it's the amps that matter. hook it up and measure the current, then you'll have a pretty good idea.

    It you throttle the air intake, creating a partial vacuum, the current (load) will be reduced. or if there's a filter fitted for instance.

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    See if you can find or borrow a three phase motor and vfd. Then you can run a test at different speeds to see what works best.

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    Isn't the limiting factor for RPM the shear strength of the rivets that hold the blades to the hub? Not saying to run it that fast, been around cooling fans when the blades let go - very violent and not fun.

    You need to measure to load on the drive motor to decide if it's appropriately sized for the fan.

    Higher the RPM, the more HP required. In other words, the more air you're moving, the more HP you need.

    Edit: to elaborate a little

    You can turn that fan with a hundredth of a horsepower, but at significantly reduced RPM. If you try to spin too big a fan or at too high an RPM, you'll run out of HP and overload the motor.

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    Before you overload that motor you will overload your ears. Big and slow and you will still be able to hear yourself think.

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    Quote Originally Posted by jermfab View Post

    A buddy of mine that used to build airboats and ultralights gave me a couple suggestions and said he’ll look into it further and get back to me. Problem is, he’s used to variable pitch technology, a luxury I don’t have.



    Be safe



    Jeremy
    Way, Way too much power density in your friends stuff. You planning on taking this up to "go once around the pattern" with it ?

    Yours looks like a simple ventilating fan or a swamp cooler fan,
    and you failed even the simplest job of giving us a "DIAMETER"....Duh.

    Why not search at Grainger's for similar size & construction ?
    EDIT: Here, 15 seconds on the duck:
    Belt Drive Exhaust Fans With Drive Package - Grainger Industrial Supply

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    Quote Originally Posted by gbent View Post
    Before you overload that motor you will overload your ears. Big and slow and you will still be able to hear yourself think.
    Right. I have a Dayton 42" fan in my welding area. I keep ear plugs in when it's running.

    Sent from my SAMSUNG-SM-G891A using Tapatalk

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    I'm thinking a 3" and 10" pulley would get a little over 1000 rpm.
    3 and 12" about 850 or so.

    I would try slow and safe at first.

    Hang a camera on the building so you might know where it went.

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    This is an attic exhaust fan from the 1950 or so. It is designed to operate quietly and efficiently. I will check the motor specifications on the one that I have later today or on Monday and post.

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    Here is an example with 28", 2 1/4 hp, and RPM of 1600.

    Error | DNS Resolution | Northern Tool + Equipment

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