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Air Compressors and Lurking!

Subject: Air Compressors and Lurking!
Date: Tue, 10 Dec 96 12:23:30 -0500
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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