Anybody familiar with engine powered air compressors?

Discussion in 'Shop Talk' started by HermitJohn, Jan 20, 2007.

  1. HermitJohn

    HermitJohn Well-Known Member

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    The homemade air compressor I bought for $12 at an auction not long after moving here gave up ghost couple years ago. Electric motor seized and the compressor was an old truck air brake compressor and wasnt ever very strong. I've gotten by with tiny "emergency" compressor that came with some junk car I bought long ago, but need bigger more durable one.

    I've been thinking of replacing motor and compressor on the homemade one with a 5hp engine and an old York air conditioning compressor with oil resevoir (have two or three good Yorks setting around that will never be used again). I thought engine instead of motor cause I have couple spare 5hp engines but no spare big electric motor.

    Thing is how is pressure cut out handled on an engine driven compressor? On an electric, a pressure switch just cuts current to the motor. On an engine driven one, you wouldnt want to kill the engine ignition, as you would have to restart it again manually. I suppose an electric clutch like already on air conditioner compressor, but then there would have to be a battery and a way to recharge the battery unless I wanted to use car battery every time I wanted to use air compressor. So how are small commercial engine driven compressors cycled? Some sort of pressure release valve that keeps tank from being pressurized beyond a set pressure?
     
  2. boonieman

    boonieman Well-Known Member

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    Most commercial air compressors that are gas/diesel powered are completely different from a household one. The engine on those doesn't shut off to control the pressure. Most of those types of compressors have loading valves built into the compressor. When the pressure reaches a certain setpoint, the loading/unloading valves open (very similar to valves like a car engine has). When the unloading valve opens, the air inside the cylinder doesn't get compressed, it just gets vented to the atmosphere. When pressure is needed, the valves close, allowing the compressor to do it's thing.

    I would definitley use a pressure relief device. I would also think you could use a pressure switch to turn off the engine. It would be easiest if the engine you have has an off/on switch. You could wire this switch thru a pressure switch using the normally closed set of contacts. Of course, like you said you would have to restart the engine each time, but it seems do-able to me.
     

  3. moopups

    moopups In Remembrance

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    Ayep, they have a blow off valve. Set it where ever your comfortable with, usually about 110 pounds. Chrysler air conditioner pump are also popular if you can find one, it has to be the oil bath type.
     
  4. HermitJohn

    HermitJohn Well-Known Member

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    Ok, I didnt mean "COMMERCIAL" , but "factory made" small engine driven air compressors. For example one cheap china tool place here locally sells a small 5hp engine driven compressor for under $400. I highly doubt they used a specialized compressor with a mechanical unloader valve or at least nothing very exotic at that price. I was thinking of just buying one of those, but the cheapness gene in me resisted spending that much money and the engine was one of those China clones of a Honda so no idea how well it would hold up.

    As to just killing engine ignition with pressure switch. Fine for inflating tires once in a while but pain in rear for anything else such as a die grinder or small cutoff tool, not that I would be using it for much more than inflating truck tires at most.
     
  5. agmantoo

    agmantoo agmantoo Supporter

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  6. PyroDon

    PyroDon Well-Known Member

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    hermit heres a couple picks of how mines set up
    [​IMG]
    notice thers an unloader on the top of this T then a line runs up to a spring loaded throttle kick down in the next picture . below the engine controling unloader and just above the tank is the over pressure pop off .
    the unloder unloads the compressor as well as kicking the throttle back to idel when pressure drops (mines adjustable and st at 135 psi) the unloader closes providing pressure to build from the compressor as well as kicking up the throttle to run RPM. with a 5.5 hp honda Im getting 14.5 CMF at 110psi which has been plenty to run even an air hungry impact wrench off an 8 gallon tank
    [​IMG]
     
  7. Gideon

    Gideon Well-Known Member

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    A farmer nearby has a small pump like your old one but uses a water tank(50 gal)for air reserve. He just pumps it up and shuts off the supply line so he has a constant supply of compressed air without waiting for the tiny pump. What ever setup you use, put in a relief valve to vent pressure in case of bypass failure. wc
     
  8. HermitJohn

    HermitJohn Well-Known Member

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    Ok, makes sense. Looked around and Graingers wants $40 plus shipping. Another place wants $80. Ebay has no name one $25. Throttle control is much easier to find and around $15.

    Not sure what a junked out small engine driven compressor would go at auction. Not that common of an item.
     
  9. Jim S.

    Jim S. Well-Known Member

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    If you've got an engine and you've got your valve, and you are going to mount it all on your old tank, all you need is a 2-stage compressor unit brand new from Harbor Freight. I see them on sale in the local store here from time to time for $99.

    Here's the unit at the regular price online:

    http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93786

    :hobbyhors :hobbyhors :hobbyhors :hobbyhors :hobbyhors

    You'd have, what, $175 tops out of pocket in it?

    On the other hand, a 3 hp motor from H-F online is $99...
    http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=7570

    :shrug: :shrug: :shrug: :shrug: :shrug:

    Or if you choose to stay electric, you can look in the Yellow Pages and call around to motor repair places. Sometimes they have stuff that doesn't get picked up or whatever, at bargain prices.