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Thread: centrifugal blower with forward curving blades

  1. #1 centrifugal blower with forward curving blades 
    Forum Radioactive Isotope sculptor's Avatar
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    I have a centrifugal blower with forward curving blades. I intend to blow hot air from the top of the greenhouse down about 28 feet into the lowest part of the house.
    Ok, the blower came with an output restricter which is removeable. the route down includes 2 90 degree bends and a short horizontal run.
    the duct sise is 20% larger than the 11x 13 inch outlet of the blower.
    now the question:
    Will the duct resistance create enough back pressure so i can I safely remove the outlet damper/restricter for more air flow without the motor overloading?


    Last edited by sculptor; March 8th, 2014 at 09:55 PM.
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  3. #2  
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    Increasing the air flow will increase the draw in your motor. Check the data plate of your motor and take an amp reading, then, in most cases you can reposition the discharge restrictor to optimize your output without overloading your motor.


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  4. #3  
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    Quote Originally Posted by keeseguy View Post
    Increasing the air flow will increase the draw in your motor. Check the data plate of your motor and take an amp reading, then, in most cases you can reposition the discharge restrictor to optimize your output without overloading your motor.
    Thanx keeseguy

    I had thought to hang an amp meter on the ac line to the motor and check the meter as the airflow stabilized, then watch it for awhile longer, ignoring the readings as the blower was ramping up to max air flow.

    Does this seem a good and safe approach?
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    Sounds like what I would do. Is this a new blower package, or one you fitted to your needs? The reason I ask is because there are several things you could check to ensure efficent operation. Is it belt or direct drive?
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  6. #5  
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    belt driven
    2 speed motor
    it was a used blower which one of my helpers gave me
    I cleaned it up
    changed the mounting location for the motor so it would fit the allotted space
    (which meant changing the belt size---so, so far, i've got $15 invested)

    Thought to set it up then take readings and check airflow on low and high speed settings with and without the baffle
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  7. #6  
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    Quote Originally Posted by sculptor View Post
    I have a centrifugal blower with forward curving blades. I intend to blow hot air from the top of the greenhouse down about 28 feet into the lowest part of the house.
    Ok, the blower came with an output restricter which is removeable. the route down includes 2 90 degree bends and a short horizontal run.
    the duct sise is 20% larger than the 11x 13 inch outlet of the blower.
    now the question:
    Will the duct resistance create enough back pressure so i can I safely remove the outlet damper/restricter for more air flow without the motor overloading?
    What happens if there is no restrictor?
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  8. #7  
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    When you ajust that belt make sure you have a good amount of deflection when tightened. Typical blower bearings are bronze, slide type bearings, if you tighten your belt too much it pulls the shaft against the bronze and it will wear the bearings fast. You possilbly also have an adjustable sheave on the motor, if so, make sure it is closed enough that the belt doesn't run too deeply into it, if so it will pull in the exiting belt causing vibration and lead to the same type of adjustment issues as and over tightened belt.
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  9. #8  
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    Quote Originally Posted by Robittybob1 View Post
    Quote Originally Posted by sculptor View Post
    I have a centrifugal blower with forward curving blades. I intend to blow hot air from the top of the greenhouse down about 28 feet into the lowest part of the house.
    Ok, the blower came with an output restricter which is removeable. the route down includes 2 90 degree bends and a short horizontal run.
    the duct sise is 20% larger than the 11x 13 inch outlet of the blower.
    now the question:
    Will the duct resistance create enough back pressure so i can I safely remove the outlet damper/restricter for more air flow without the motor overloading?
    What happens if there is no restrictor?
    without it being hooked into the duct work, it didn't seem to speed up enough to tax the motor, and the amp readings seemed steady
    There was almost no change for the motor with or without the baffle/restricter, except that air speed with the baffle on was faster(but i suspect at a lower volume)
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  10. #9  
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    The baffle is likely to prevent air noise in the application it was originally in. If your amp draw with no duct work is good, you will be golden once the duct flow restriction slows the volume. That is when, if you have the variable sheave you can adjust that with your amp reading to increase your blower speed and your volume.
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    Quote Originally Posted by sculptor View Post
    Quote Originally Posted by Robittybob1 View Post
    Quote Originally Posted by sculptor View Post
    I have a centrifugal blower with forward curving blades. I intend to blow hot air from the top of the greenhouse down about 28 feet into the lowest part of the house.
    Ok, the blower came with an output restricter which is removeable. the route down includes 2 90 degree bends and a short horizontal run.
    the duct sise is 20% larger than the 11x 13 inch outlet of the blower.
    now the question:
    Will the duct resistance create enough back pressure so i can I safely remove the outlet damper/restricter for more air flow without the motor overloading?
    What happens if there is no restrictor?
    without it being hooked into the duct work, it didn't seem to speed up enough to tax the motor, and the amp readings seemed steady
    There was almost no change for the motor with or without the baffle/restricter, except that air speed with the baffle on was faster(but i suspect at a lower volume)
    My only experiment with blowing air through a tube (as it sounds like you are doing) was that the flow through the tube "doubled" when the fan was not inside the pipe but back from the entrance a little.
    I have not seen anyone use this feature since, in most cases the fan is in the tube or the opening taking air from one area to another, so look at that too if it is air volume you need.
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  12. #11  
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    oops
    It seems that my amp meter was lying to me, so I took it apart, cleaned the contacts and put in a new battery.
    Ok now?
    the motor is a 6.5 and 7 amp motor low and high speeds
    with the restrictor on --4 amp start up + or- .4 amp-then 1.1-1.2 as she ran
    with restrictor off------4 start up +or- .3 amp-----then 1.2-1.3 amp as she ran
    Sitting on the bench--wide open, no ducts, I let her run for an hour---same 1.2 to 1.3 with an occasional jump to 1.4 amps

    amazingly, she took another .5 amp for startup, and another .2 amp while running when in slow speed both with and without restrictor/baffle
    (I ain't completely confused yet, but I am working on it)(I had assumed that slow speed would = less amp draw)

    The assembly will sit in an area with approx 5 sq ft opening the blower outlet is about 11 x 13 inches--143 sq. in.
    the duct is initially trapezoidal(to fit available space), then 8 x 18 or 144 sq. in.

    OK
    back on the scaffold to install it and test airflow at the bottom

    When you don't know what you're doing before you do it, life is still a daily adventure.

    Thanks for the inputs and wish me luck
    rod
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  13. #12  
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    If your motor is rated for 6.5 or 7 amps and your are only pulling 1.5. I wouldn't trust my amp meter, it should pull more then that with the belt off.
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  14. #13  
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    Quote Originally Posted by keeseguy View Post
    If your motor is rated for 6.5 or 7 amps and your are only pulling 1.5. I wouldn't trust my amp meter, it should pull more then that with the belt off.
    Or it could simply be that the motor is not working anywhere near its max rated load.

    Electric motors are not really my field, but depending how this one is wired, it might be a type that draws more current the larger its mechanical load is - in the case of Sculptor's motor, up to a max of 7 amps. So if its mechanical load was low, then so might be its current draw.

    Motor aside, Sculptor, how is your heat recycling idea going? I live in a three storey house, and have also been thinking about recycling warm air from the top storey to the lowest floor, to save on heating bills.

    OB
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  15. #14  
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    Quote Originally Posted by One beer View Post
    Quote Originally Posted by keeseguy View Post
    If your motor is rated for 6.5 or 7 amps and your are only pulling 1.5. I wouldn't trust my amp meter, it should pull more then that with the belt off.
    Or it could simply be that the motor is not working anywhere near its max rated load.

    Electric motors are not really my field, but depending how this one is wired, it might be a type that draws more current the larger its mechanical load is - in the case of Sculptor's motor, up to a max of 7 amps. So if its mechanical load was low, then so might be its current draw.

    Motor aside, Sculptor, how is your heat recycling idea going? I live in a three storey house, and have also been thinking about recycling warm air from the top storey to the lowest floor, to save on heating bills.

    OB
    I have had the blower attached to the duct blowing approx 120 degree F (49-50 degrees C)air down to the lowest part of the house for several days.
    It then migrates up through the house keeping all several degrees above ambient.
    That "lower part of the house" was built as a bunker by the man who started building this house---he made a video wherein he stated: "and, when you are down here, you are surrounded by a minimum of 10 inches of concrete on all 6 sides. He was thinking bunker, and I thought "thermal mass"(heat sink). The blower set up warms that thermal mass up over 72 degrees F(@22C). Then, it warms the house all night long.

    The solar space was designed as a bastard space --- not the best as a greenhouse, and not the best as a solar collector.
    I am looking forward to giving it a real test next winter. My plan was that it would (at a minimum)halve the heating load for 1/2 of the winter, while providing me with oxygen and the rather pleasant smells of growing plants and the refreshing ambiance of a solarium. One problem we have here in Iowa, is that it is cloudy for much of our winter, during which time, if the set-up can keep it's self and it's plants above freezing, I'll be satisfied.

    The first couple days of operation, I tried to stay near the blower, and retested the amp draw several times per day.
    All seemed well, and I'm free to do other things again.
    This is the third "solar" structure I have built since studying the basic designs at the Design Department at Southern Illinois University under Buckminster Fuller in the early 70s.

    Good luck with your plan to conserve the energy used in your 3 story building.
    Please share your deliberations, plans, and process.
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  16. #15  
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    Thanks Sculptor, I will, but don't hold your breath because I still work full time and don't have as much free time as I would like to do stuff like this.

    I too am interested in such things, and I first saw the idea for recirculating warm air in an aircraft hangar. Four tubes about 6m long and about 20cm in diameter made of polythene with small fans on the top were hung in the corners of the hangar and blew the warm air from the ceiling down to floor level.
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