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RE: Air Compressors and Lurking!

To: "''" <>,
Subject: RE: Air Compressors and Lurking!
From: Randy Rees <>
Date: Tue, 10 Dec 1996 16:10:11 -0800
I have a 4HP sears compressor that if I add an extension cord of less
than 12Ga. it sputters once and shuts off. So I assume that you are
right about the 15-20 amps. But how is it then that they can sell a 6hp
110V compressor?. You've got me thinking, my compressor can be converted
to 220V, so will that help my CFM output any? i.e. a compressor that is
running closer to its rated HP.

>Sent:  Tuesday, December 10, 1996 9:23 AM
>Subject:       Air Compressors and Lurking!
>Personal and private to Dave Deutsch:
>(Call me a whimp will you! Call me a slimmy low life sewer lurker will
>you! Well I am a slimmy low life lurker and proud of it!  I'll only put
>in my two cents when I have to (and when I have some time available). 
>Just remember one thing Dave, I don't get mad I get even.)
>Back to reality.
>I have played around lots with air compressors and have fallen asleep
>many nights to drooling over WW Grainger's catalogues.  The first and
>most important rule is you get what you pay for!
>First of all, HP numbers are the latest marketing persons guess of the
>day.  A real horsepower is 746 watts, meaning at 110 volts it roughly 7
>amps assuming a 100% efficient electric motor.  I haven't seen any 100 %
>efficient motors lately.  In reality you should figure about 9 amps per
>real HP at 110 volts.  Halve that number if you are working with 220
>volts.  So my question is how do you get a 4 HP compressor to run on 110
>volts (4X9=36 amps from your 15-20 amp outlet). (You don't)
>Second of all how many CFM's do you need at what pressure. The real
>thing you need from a compressor is CFM, cubic feet of air per minute at
>a certain pressure.  For the same compressor, the CFM's go up as the
>required pressure goes down.  (The compressor pumps air at about a
>constant volume.  As it pumps more and more volume of air into a fixed
>volume tank, the pressure rises. As the pressure rises the compressor
>has to work harder.  If you don't require the higher pressures, you get
>more air volume at a lower pressure) 
>Determine what you want to do.  Understand that serious sand blasting
>will run into serious $$$.
>If you want to keep up to a sand blaster that has a 1/8" orifice, you
>better plan on 12 CFM at 90 PSI.  You don't blast at 90 (at least I
>don't)  I ran a 7-8 CFM blaster from my real 2 HP compressor steady
>state at about 60 PSI (the specs said the compressor would do 6.7 CFM at
>90 PSI).
>A compressor rated at 125 PSI tends to run between 90 and 125 as the
>compressor cycles on and off.  This means that you can only count on 90
>PSI for your tools.  If you buy expensive efficient tools you will get
>plenty of power.  If you buy cheap, excuse me, inexpensive, tools like I
>do, then your power will be limited by the lower pressure.
>The storage tank is also a trade off.  A small tank means you don't have
>to wait as long for pressure when you first turn it on.  It also means
>you will run out of air faster if your compressor can not keep up to
>what you are doing.  I.E. If you are blasting at 12 CFM and your
>compressor is good for 7 CFM, you can blast 3 times(about) longer with
>your 60 gallon tank than with your 20 gallon tank.  Just remember that
>it will take 3 times longer to get the 60 gallon tank up to pressure
>than your 20 gal.
>A two stage compressor, is more expensive, but provieds up to 175 PSI. 
>That really makes the inexpensive power tools go.  I can't explain why,
>but  the two stage compressors provide more CFM per HP than single stage
>units.  Expect to pay $300-400 for a real 5 HP single stage and
>$800-1300 for a real 5 HP two stage.
>For serious sand blasting I would recommend a 5HP two stage compressor.
>For work other than sand blasting I would recommend a real 2 HP
>compressor with about 6-7 CFM.  Smaller than 2 HP would limit the power
>tools that could be used.
>I now have a 5 HP compressor that draws about 21 amps at 220 volts (more
>than the central A/C on the house) and provides about 16 CFm at 175 PSI.
>Flames to /dev/null (except for David)

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