<VV> Harbor Freight/Central Pneumatic

[Dave Keillor]

Ditto to what John said.  I have an older (30 years?) 1 hp compressor that's
rated more CFM than today's 5-6 hp compressors that are rated on
locked-rotor torque, not actual output.

One thing that John didn't mention is two-stage compressors.  They have a
low pressure stage and a high-pressure stage (you can recognized them by
their two different size pistons).  They're much more efficient on a hp to
cfm basis -- and more expensive, too.

Dave

-----Original Message-----
From: jryall@juno.com [mailto:jryall@juno.com]
Sent: Tuesday, June 15, 2004 2:19 PM
To: virtualvairs@corvair.org
Subject: Re: <VV> Harbor Freight/Central Pneumatic


At the risk of starting a long thread about compressors, here goes: 
There's a lawsuit going on now about the way the compressor manufactures
rate their motors.  Most 'home' compressors are rated at 'peak'
horsepower.  The Harbor freight unit is rated this way.  I don't know
about the Lowes unit, I couldn't find it on their website.  Peak
horsepower is basically 'stall horsepower'.  The motor isn't turning, but
is developing a lot of torque just before the fuse blows.  Obviously, it
can't pump any air when it isn't turning.  The amount of air you need
depends on the rating of your tools, and how continuously you use them. 
A small tank 1 hp compressor will run an air chisel (for about 15
seconds) and won't be satisfactory for that use. Ditto for an impact
wrench.  It probably won't be able to remove the lug nuts that you
tightened by hand last year.  I have a Coleman compressor from Home
Despot that says 5 HP, but the amps rating on the motor calculates out to
2.2 hp.  7.1 CFM @ 40, 5.7 CFM @ 90.  It is adequate for small tools and
air chisels, but is nearly useless with a 1/2 inch drive impact wrench. 
My electric impact wrench is much better.  Trouble is, the air compressor
was bought to match the impact wrench rating.  somebody is lying here. 
Either the compressor doesn't put out the volume it is rated at, or the
impact wrench uses more air than it was rated for.

The real way to compare the output of compressors is to look at the CFM
ratings.  SCFM stands for standard CFM, where the output is corrected to
some industry standard.  It is probably a more reliable figure than
(nonstandard) CFM.

The Central Pneumatic compressor is a direct coupled compressor.  In the
picture is looks to be a single cylinder unit.  I don't like these and
prefer a belt drive with at least two cylinders.  I believe the belt
drive compressors last much longer in use.  Of course, home use is easier
on tools than industrial use, and you might not use it up for a long
time, so it's your choice.

Horsepower, if I remember my numbers right, is (volts * amps)/748.  Check
the label rating to see what the actual horsepower is, so you'll know how
badly you are being cheated, but go by the CFM ratings.  Occasionally, a
unit that goes up to 150 psi is really handy with that impact gun, but be
sure to use a regulator to reduce the pressure, under normal use.

Other things come into question.  If you have to pay freight from Harbor
Freight, then factor that into the cost of the unit.  It may only come by
truck to a dock.

My preferences:  I like a belt driven compressor.  They are less noisy
and tend to last longer.  'Oil-less' compressors are cheap disposables,
and have to be rebuilt frequently.  Better to use an oil separator/filter
unit if you are painting.  It's difficult to find a real workhorse
compressor in 110 volts.  Yes, you can make do, but they are really
horsepower (and CFM) limited.  My 5hp rated 2.2 actual hp compressor has
to have a 35 amp circuit simply because of that 5 hp startup torque.

Rules of thumb for air compressors:  
1) Always buy a larger unit then you think you need.
2) A large tank helps by delaying the moment the compressor comes on, but
also takes longer to fill.
3) Don't forget that next year you will buy a sandblast cabinet and need
a lot of air.
4) If the wife says you can have an air compressor, don't tell her that
there are smaller cheaper ones than the one you want.

John Ryall

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