[Shop-talk] single vs. dual stage compressors

[Randall]

 Karl Vacek wrote :

> be aware that 
> horsepower ratings are often a complete fairy tale, 

Now what could I add to that ? <G>

Things have improved somewhat in that regard, since someone successfully
sued for false advertising a few years ago, but I still wouldn't pay any
attention to "horsepower" ratings for an air compressor.  The cfm is what
matters.

As far as energy efficiency goes; I'm not convinced that single stage units
have that much advantage even at 90 psi.  The example given (15% more air on
5% less apparent power from the 2-stage unit) seems more typical of the spec
sheets I've looked at.  I don't know, but I suspect this is the same problem
as before : it's impossible to build a perfect compressor (infinite
compression ratio), and any unswept volume wastes energy on every cycle.
How much it wastes depends on the pressure ratio; and 2-stage compressors
have a lower ratio (for the same output pressure) than single-stage units
do.

But there is something to be said for not pumping a big tank up to 175 psi
if you don't need it.  I just can't imagine not needing it.  Even a small
die grinder typically needs at least 4 cfm @ 90 psi; which translates to
some 28 scfm in compressor-speak.  And even if I could afford a 28 cfm
compressor, I don't have enough electrical power to turn it!

Randall - in the middle of adding a 30 amp circuit for a measly "7 hp"
compressor, rated under 20 cfm.