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RE: Air Compressors

Subject: RE: Air Compressors
From: (John T. Blair)
Date: Wed, 11 Dec 1996 18:26:04 -0500
At 09:58 AM 12/11/96 -0500, W. R. Gibbons wrote:
>On Wed, 11 Dec 1996, BOB NOGUEIRA wrote:
>> -- [ From: Bob Nogueira * EMC.Ver #2.5.1 ] --
>> If you hook up two 2-3 Hp compressors each with an out put of ,say,  9
>> CFM both feeding a common storage tank, will the result be in the
>> ballbark of 18 CFM ?
>> Bob Nogueira
> I've been thinking about this, since I have one good 2 HP already.
> I don't see why you could not simply create (out of iron or copper
> pipe) a manifold that has an inlet from each compressor and an
> outlet (with a regulator) for your sandblaster or whatever.  You 
> could install a shutoff valve on one compressor so you used just one
> for small jobs, and crank both up when sandblasting. 
>Can anyone think why this would not work?  

The manifold idea might work.  However, IMHO you can't discount the
accumulator.  The larger the storage tank the more of a buffer you
have to smooth out the air flow, similar to a capacitor in an 
electrical circuit.  While it will take longer to charge the tank
the bigger it is, you will have more are to draw from.

The other day a neighbor wanted some help removing the crankshaft
pulley on a GEO Storm to do a timing belt.  I asked if he had a
compressor, he said yes.  So I get my "good" impact wrench, impact
extension and socket and go over to snatch it bolt off.

His compressor was supposeidly rated at 120 PSI (don't know what 
the cfm was).  There was no accumulator, the compressor ran all the
time.  Hooked up the air hose and pulled the trigger on the impact
wrench.  The gun spum right up and down.  There was not enough air supply to
sustain the gun for a few seconds.

Since my 5hp 30 gal. compressor is built in under my work bench and
I've got too much stuff in the garage to roll the compressor out
even if I could get it out from under the workbench, I went to
another neigbor's and borrowed his 2 hp 2 to 4 gal tank.  (This
one was movable.)  It had a big enough accumulator to sustain the
gun to break the bolt loose.

And yes, as you drain the accumulator, it will take a while to recover.
This is where the size of the compressor (displacement)
and HP come into play.

A 500HP motor turning a 50cc compressor will not get you much

By the same token a 1hp motor can't really turn a 350 cubic inch
displacement compressor either.

Compressor design appears to be a compromise of the size motor,
displacement of the compressor jugs and the ratios of the pullies
on both.

BTW, another neighbor made a compressor from an old car A/C
compressor and an electric motor.  Can't remember if he had a 
pressure cut on/off switche on it or not.  But for small/short
jobs it did the trick.  I wouldn't try sandblasting with it, but it probably
could handle spray painting a car.


John T. Blair  WA4OHZ          email:
Va. Beach, Va                  Phone:  (757) 495-8229

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