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Re: Air Compressors and Lurking!

Subject: Re: Air Compressors and Lurking!
From: "W. R. Gibbons" <>
Date: Tue, 10 Dec 1996 19:48:03 -0500 (EST)
On Tue, 10 Dec 1996 wrote:

> 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

This was very informative.  I saw an ad last week for a 6 hp Devilbis at 
an incredible price, $380.  I feel deprived with my older Sears 2 HP, so 
I wrote down the specs on my Sears and then went to look at the cheapo 
"6HP" unit.  Here's the skinny on the Devilbis and a couple of more 
expensive Colemans the store also had for sale:

Old Sears 2HP single stage:  12 amps@220v  7.5 SCFM @ 90 psi
Devilbis 6HP single stage:   15 amps@220v  10.5 SCFM @ 90 psi
Coleman 6.5 HP single stage: 15 amps@220v  10.9 SCFM @ 90 psi
Coleman 6.5 HP double stage: 22 amps@220v  19  SCFM @ 100 psi

Conclusions:  HP ratings no longer mean diddly squat.  I'd be wasting my
money to buy either of the new single stage units for a 50% increase in
air.  When I start blasting with my pressure blaster, I spend 3/4 of my
time waiting for air.  Spending only half of my time does not seem worth
the money.  The two stage machine was more than twice the price of the
others, but I would have to buy that to really provide the air for serious
sandblasting.  For now, I figure I will stick with the old Sears; it does
pretty well. 

A big tank is not all that much benefit for sandblasting, though it might 
be for spray painting.  You might be able to get all the way around the 
car without stopping if you have a big enough tank.  Sandblasting will 
use it all up quickly, then as Sheldon points out, you have to wait 
longer for it to recover.  If you need the maximum output of the 
compressor for a considerable time, the size of the tank doesn't matter.

   Ray Gibbons  Dept. of Molecular Physiology & Biophysics
                Univ. of Vermont College of Medicine, Burlington, VT
        (802) 656-8910

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