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Air Compressors: What RPM do most run?

Discussion in 'The Hokey Ass Message Board' started by Flathead Youngin', Aug 10, 2005.

  1. Flathead Youngin'
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    Flathead Youngin' Member

    I saved some threads regarding air compressors and people's tips:

    http://www.jalopyjournal.com/forum/showthread.php?t=51885


    However, I thought there was one that had some suggestions about the RPM a compressor should turn.

    Here's what a guy gave me and the tank I bought....

    *3hp, 115v or 230v, 1750RPM monster motor (almost as big as a 5 gallon bucket)
    *a really nice, used almost none, one lunger compressor (I can upgrade later)
    *what looks like a 50 or 60 gallon tank
    *flywheel/sheave on the compressor is 14" diameter

    What RPM should I run the compressor? Someone suggested an adjustable pulley. Where could I get one of these without paying an arm and a leg (money I could use to upgrade this one later...)


    I'll look on the internet for the ratio formula....

    Attached Files:

  2. squirrel
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    squirrel Member


    the ratio formula is pretty easy, the ratio of the pulley diameters is the opposite of the ratio of the shaft rpm. So if you have a big pulley that's 12", a small one that's 3", 12/3=4 so then the compressor will turn 1/4 speed of the motor.

    swamp (evaporative) coolers use adjustable pulleys, they're pretty common in hardware stores in the southwest.
  3. Flathead Youngin'
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    Flathead Youngin' Member

    Here's a good link when people ask general info about compressors..

    http://www.sawdustmaking.com/AirCompressors/air_compressor.htm

    Here's the formula.....I copied this from a tractor forum.....

    Most industrial pumps recommend 850 RPMs. To calculate shieve size use to following formula
    Sheive(motor pulley) = 850 x 19 / motor RPM
    19 is your pump shieve size
    X= Motor shieve size
    X= 850(19)/ 3600
    X= 16150/ 3600
    X= 4.486 looks like you need a 4 1/2 inch pulley
  4. Bill.S
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    Bill.S Member

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  5. Flathead Youngin'
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    Flathead Youngin' Member

    Thanks, that's good stuff!

    I found on another site where someone claimed that most industrial compressors turn at 1000 or less.......wonder if I'd be safe at 850 with this thing?
  6. blown49
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    blown49 Member Emeritus

    Jeff,

    In your case I think you have to change the answer your equation because your motor speed is 1750 and not 3600. In your first post you said you had a 3 HP motor that was 1750 rpm. I'd probably be more concerned with the condition of the tank, it's pressure rating and condition.
  7. Flathead Youngin'
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    Flathead Youngin' Member


    Yeah, I just cut and pasted that example......I mine is 1750, 14" and needs to turn at 850 (I'm guessing) then I would need a 6.5-7" pulley on the motor...

    I'd still like to hear from someone who knows about the RPM the compressor should turn at.......I really don't like assuming 850 and if it can handle more, I'd like to crank it faster...
  8. squirrel
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    squirrel Member

    could you maybe post another pic or two of the pump? is there a tag on it, or mfgs name? it looks kind of wierd...not like a typical air compressor pump. maybe it's for refrigeration?
  9. krooser
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    krooser Member

    Slow is better....less heat=less moisture in the air...slow is also quieter...
  10. Flathead Youngin'
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    Flathead Youngin' Member


    I'm gettting ready to leave......try to get some pics tonight or tomorrow....

    I just remembered something. My grandpa's old compressor looks exactly like this (but bigger, different color, and a different company: kidding:)) but it is a two cylinder......I wonder if it still has the tag on it and would be close to the same RPM.......

    I didn't see a tag, but I can look again...
  11. 286merc
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    286merc Member

    Most old style pumps are rated to be run over a range of HP motors. The sort of industry standard of 800-900 rpm is for a motor matched to the max hp rating of the pump. If the pump cant do the work of a 3hp motor then you can slow down the motor which will extend pump life as well as cut down heat.

    Pumps are typically rated to use 1.5 to 3hp motors and the next step would be 3-5hp. If your pump is only a 1 cyl then it is probably down in the 1-1.5 HP range and a 3HP motor can run much slower than the standard 800-850. More like 600-650. My old Curtis 2 stage has a tag on it that says 300 rpm minimum, I run at 850rpm with a modern Baldor 5hp 1750 rpm motor.

    Graingers is a good source of shieves and keys.

    A good source of compressor tech info is the Eaton web site. http://www.eatoncompressor.com/page/page/504413.htm

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