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Darksider's Realm

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    selecting an air compressor


    Posts : 13
    Join date : 2009-04-30

    selecting an air compressor

    Post  autocrosser on Sun May 24, 2009 2:28 pm

    First decide what you will use the air for. Pick a compressor that will handle the heaviest use without running continuously (see duty cycle below). Buy the largest compressor you can get the first time. I have never heard of anyone complain that they bought too large a compressor, but compressors are the #1 tool to be upgraded in a short time of ownership because of disappointing performance of too small a unit.
    VERY heavy volume user: sand blaster.
    Heavy air users: paint spray gun, inexpensive versions of air tools like impact wrenches and sanders. (Cheap tools use oversized air motors to compensate for low tollerence construction and inefficient materials/design, wasting a lot of the air that goes thru them.)
    Medium users: High quality air tools.
    Low users: tire inflating, blow gun, nail guns, short burst tools.

    Very heavy use will require a minimum of a 7 HP 2 stage compressor and frequent waits for rebuilding pressure. This size compressor will allow near continuous use of Heavy use tools.
    Heavy use with pauses or continuous use of Medium use tools will require 7 HP single stage or 5 HP 2 stage.
    5 HP single stage will run medium use tools with occasional breaks for pressure rebuild and/or cooling.
    110V compressors with wheels or carry handles that plug into a standard household outlet will only handle low use tasks, no matter what their hype or specs say.

    Electric motor HP is also often overstated. Don't go by the HP rating, but find the spec for the outlet or breaker size called for in the hookup instructions. 220V 20A = ~ 3 HP, 30A = 5 HP, and 50A = ~ 7 HP for practical comparison use.

    Air output is also over hyped especially on the cheaper units. Most compressors are rated at X cu ft/min at Y pressure free air. This means how much air it puts out directly at the compressor manifold outlet without any restrictions. regulators, driers, oil seperators, piping, flex hose, tool type and design will all quickly and drastically reduce that output measurement at the point of use. It's easy to reduce the flow to useless levels. Cheap compressors measure output by calculating the swept volume x rpm. They boost the output by overdriving the compressor speed and soon burn out.
    Given the same motor HP the following rules of thumb are reasonable assuptions:
    a 2 stage unit will put out more air than a single stage with less effort because of efficiency. 5 HP 2 stage is approx = 7 HP single stage.
    Multicylinder compressors are more efficient than single cylinders.
    Don't even consider an oilless compressor. The compressor should have a crankcase full of oil like a gas engine.
    Cast iron cases and cylinder liners with ringed pistons will be more efficient and last longer than aluminum compressors with lapped pistons.

    Duty cycle: compressor specs should also include the rated duty cycle. This is given as a %, and represents the amount of time the compressor should be running vs not running. a 20% duty cycle means the compressor should not be run continuously for more than 12 minutes without being allowed to stay off and cool for 48 minutes out of each hour. Only HD overbuilt industrial grade compressors are rated for continuous operation. You would not want to use a compressor that has a just adequate capacity and a 20% duty cycle for painting your whole truck, and have to let the compressor cool for 48 minutes after every 12 minutes of spraying, the paint would dry in the gun/on the truck while you waited! Exceeding the duty cycle will destroy the compressor in short order and probably result in an excessive amount of oil and water to be pumped into the air line from the overheated compressor, and hot air to be sent thru the gun drying the paint too quickly causing a lot of overspray and orange peel.
    Are you a member of one or more of the warehouse clubs like Sam's, Costco, or BJs? some of them sell stationary compressors in the tool dept. Check all branches within reach, each frequently carry different inventory based on the local demographics. For example we have 3 Sam's stores within 20 miles around us. One stocks large compressors, the other 2 don't???
    Check out buying online from an out of state dealer as well. IR for example drop ships for free from their closest warehouse no matter who you order from, and the sales tax savings from purchasing out of state can be significant. I think mine had a charge of <50.00 extra for lift gate/pallet truck delivery inside my garage door.
    Kaptn' Obvious

    Posts : 24
    Join date : 2010-05-19
    Location : Austin, TX

    Re: selecting an air compressor

    Post  Kaptn' Obvious on Fri Jun 11, 2010 3:09 am

    I'm about to start compressor shopping. I know what I'm looking for and I know what my usage will be, However I don't know what to buy. To sum it up I'm looking for a low use paint ready rig. It needs to be able to run 2 die grinders at 100% load. I also need clean air. Nothing pisses me off more than trying to clean something off and have my air hose shoot oily water all over it...

    I have done zero shopping. And I don't plan to. I'm looking for a quality system I can count on.

    You guys should see Tim Allen compressor system they put in at work. It's a 100 hp BEAST from Ingersoll Rand. It pumps out almost 500 cfm @ 145 P.S.I. It's duty cycle seems to hover around 30%... Can you say overkill?

    Any help picking a unit suited for me would be appreciated...

    Posts : 13
    Join date : 2009-04-30

    Re: selecting an air compressor

    Post  autocrosser on Fri Jun 11, 2010 2:15 pm

    For your use I would recommend a 5HP or (better)7HP two stage compressor such as IR 30 series with at least a 60 gal tank.
    Water in the lines can be caused by several factors:
    1. Overworked compressor. The longer and more often a compressor runs the hotter it gets, in turn heating the compressed air more. Hot air holds more water than cool air, the higher the temp differential the more water will condense out as the air cools. solution: Buy a bigger compressor, put it where it can draw in cooler drier air.
    2. Water buildup in the tank. Solution: Open the tank drain at least once a week or as often as necessary to keep the tank dry. If you are needing to drain it often, see #1 and/or add an automatic drain valve to the drain line.
    3. inadequate drier/seperator(s). solution: first see and fix #1 and/or #2, no drier is going to be able to fix those problems. Install a large capacity dessicant drier between the tank and the main distribution piping, check and replace dessicant frequently. Unless you live in a cool/very dry climate a mechanical (spin) type drier is not adequate.
    Install a second drier at each outlet drop where dry air is needed.
    Use a refrigerated drier for spray gun lines.
    4. Improper sized/installed distribution piping. Solution: Use 3/4" HD copper piping for main distribution trunk install near ceiling and strongly (3/8"/ft) pitched towards the end. Keep as short and straight as possible. End the trunk in a drain only drop leg with a ball valve. Open the drop leg valve after each use to drain. service drops should be plumbed with 1/2" copper pipe with the branch T installed pointed up off the trunk and the service drop elbowed up over and down. T the secondary drier and quick connect off this drop at least 12" above the end of the drop and end the drop with a ball valve for draining.
    5. Blowgun spits water/vapor onto surfaces. Solution: Using too high air pressure at blow gun. When compressed air rapidly expands it cools a lot. That cold air condenses atmospheric moisture onto the surface, like huffing onto a cool window or mirror. Install a small regulator onto your blow gun between the quick connect and the gun as a permament addition. Turn that regulator down to 30# or less. You don't need 90-150# coming out of the blowgun and it's dangerous, can actually cut off or blow a hole thru a finger! affraid

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    Re: selecting an air compressor

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