I decided I might as well spend a bit more time this evening and do
something to back up my opinions. I said not all air compressor
horsepower are equal. Campbell-Hausfeld offer a popular low price range
of compressors, probably on a par with Sears (maybe they make them for
Sears, for all I know). I will compare two 5 hp units, a 4 hp unit, and a
2 hp unit, all from CH.
Campbell-Hausfeld 2 hp Single Stage: 7.2 cfm @ 40 psi max 125 psi
(115/230 v, 15 amps @ 115 volts) 5.6 cfm @ 90 psi
Campbell-Hausfeld 4 hp Single Stage: 8.1 cfm @ 40 psi max 125 psi
(115 v only, 15 amps @ 115 volts) 6.4 cfm @ 90 psi
C-H 5 hp Single stage (vertical): 11.4 cfm @ 40 psi max 125 psi
15 amps @ 230 volts 9.0 cfm @ 90 psi
C-H 5 hp Two stage (vertical): 14.7 cfm @ 90 psi max 175 psi
? amps @ 230 volts 14.3 cfm @175 psi
The first observation is that the CH "2 hp" compressor has nearly the same
air output as the CH "4 hp" compressor. In fact, if these specs are
correct, both use the same amount of current, so the horsepower cannot
really be different at all.
At 90 psi, the relevant pressure for sandblasting sheet metal, the two
stage 5 hp unit has over 160% of the output of the single stage 5 hp
compressor, even though both have the same horsepower, because the two
stage design is more efficient. So figure that horsepower is at least half
advertising hype--focus on the output, that's what matters. Also,
efficiency is expensive--the two stage unit is more than twice the cost of
the single stage.
Upgrading to a 5 hp single stage is not really all that expensive--Harbor
Freight offer the 5 hp single stage compressor for $450 with free shipping
in the 48 states, whereas the 4 hp is $365. 20% more money buys almost 50%
more output.
But enough about supply, what about demand? Eastman gives the
following requirements for their sandblasters.
90 pound capacity pressure sandblaster
w/ 3/32 inch nozzle: 7 cfm @ 80 psi
w/ 1/8 inch nozzle: 15 cfm @ 80 psi
w/ 5/32 inch nozzle: 25 cfm @ 80 psi
As a physiologist, I have to say that it is nice to see that the flow
through a tube really is proportional to the fourth power of the
radius, as I teach the students (ok, it's not perfect, the flow is not
laminar, but it sure isn't a linear function).
In practical terms, a 3/32 nozzle will do good work relatively slowly, but
will clog more often than a 1/8. I found I preferred to use a 1/8 nozzle
to avoid having to clear clogs all the time, even though using the 1/8
nozzle meant I waited for my 2 hp compressor a *lot.* I guarantee you you
will not want to sift the blasting medium to prevent clogs; therein lies
madness. By the way, a caution: do not use real sand! Therein lies
silicosis.
The 5 hp single stage compressor should just about keep up with the 1/8
nozzle, if one takes into account that there are pauses in sandblasting as
one moves himself, the parts, or whatever. With most single stage units
under 5 hp, you are almost certainly going to have to stop and wait after
a short spell of blasting for the compressor to refill the tank. You have
to consider whether this is a problem. How much do you really have to do?
Does ease of moving and dollar savings of a smaller unit compensate for
their limited capacity? Most people will probably decide the smaller
units will do. Some of them (the type A personalities like me) will
regret the decision; others won't.
If an air tool is one that is used in short bursts, almost any compressor
with a tank will supply it. Other air tools tend to be used for long
stretches, and some of them require a lot of air. Some examples from the
Eastman and Harbor Freight catalogs:
Eastwood Dual action sander: 6.5 cfm @ 90 psi
Central Pneumatic DA sander: 4 cfm @ 90 psi
Central Pneumatic Random Orbital sander: 12-15 cfm @ 60-100 psi
Eastwood Angle sander: 8 cfm @ 90 psi
Eastwood Cut off tool: 4 cfm @ 90 psi
Binks 2001 Standard Spray Gun (Eastwood): 11.5 cfm @ 30 psi
I bet my cutoff tool uses more air than this, because I spend a lot of
time waiting for air. It cuts steel like butter, yes, for about 45
seconds, then it cuts steel like, well, like steel.
I have a 2001 spray gun, and should say that it may use either more or
less air than this, depending on the nozzle chosen. 11.4 cfm @ 30 psi
should be in the ballpark, though, and would require the C-H 5 hp single
stage compressor. A unit under 5 hp should be fine for applying
primer, where stopping and starting marks can be sanded out. When it came
time to apply the final finish, you could make do with a small unit, but
you would have to mask the car into sections that you could finish before
the compressor fell behind the gun.
These considerations are behind my frequent urgings to SOLers to analyze
their needs, and then add a margin for later needs, before they go out to
buy a compressor. Any compressor is a huge improvement over none. If you
use it hard, however, your appetite for air will grow and grow. I started
with a 1/2 hp Sears I bought second-hand in 1955. I used it 30 years, for
everything. I wanted a bigger one for 29.9 of the 30 years. Toward the
end, the poor thing was so worn it struggled to build enough pressure to
turn itself off. When my 16 yr old daughter tried to park the car in the
garage for the first time, and crushed the old faithful compressor, I
smiled and bought the biggest one I could afford--well, the biggest one my
mean old wife would *let* me buy (2 hp, high efficiency). And I realized
almost immediately that it was not big enough. I want more, more,
more..... And I want an HVLP sprayer.... It never ends, does it?
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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