[Shop-talk] compressor/sandblast guru question

[Randall]

> Maybe a guru can enlighten us.

I am certainly no guru; but my opinion is that as long as the compressor can
keep up with the load, you don't need a large tank or a larger compressor.
It won't hurt the compressor to run continuously, in fact it is probably
worse (in terms of overall wear, power consumption etc) to keep starting and
stopping it.

The large tank is primarily an advantage in two ways: it lets you work
longer with tools that take more air than the compressor can produce, and it
allows for longer on/off cycles.  In other words, it is actually an
advantage to have the compressor off for longer, followed by being on for
longer.  (Among other things, every time the compressor stops, a certain
amount of compressed air is vented to the atmosphere, so it doesn't have to
start under full load next time.)

Compressor "hp" ratings are indeed like lawnmower "hp", except maybe worse.
CFM ratings seem to be a little more consistent to me, but you have to be
very careful about what kind of CFM they are talking about.  Air tools (like
blast cabinets) seem to typically be rated in ACFM at their inlet; while
compressors are generally rated in ACFM or SCFM (by which they really mean
ACFM) at their inlet.  The problem is that the ACFM at the outlet of a
compressor (where the air tool is attached) is many times lower than the
ACFM at the inlet!

> Ibve got a Quincy 310 compressor sitting on a 80 gallon tank... using a
> 5 hp motor.

This probably falls into the fake hp department.  According to Quincy, their
310 compressor is not suitable for more than 3 hp.

> Itbs turning about 800 rpm

According to Quincy's data sheet, that should be about 8 ACFM @ 175 psi;
about 11 ACFM free air.  For the sake of argument, let's assume 10 ACFM @ 90
psi.  ACFM at the outlet is roughly the ACFM at the inlet times the ratio of
absolute pressures.  90 psi is roughly 6 atmospheres "gauge" or 7
atmospheres absolute, so the pressure and CFM ratio is roughly 1:7.  So the
actual output is roughly 10/7 or 1.4 cfm.

My small, bench top blast cabinet draws about 4 ACFM @ 90 psi.

So no, I don't think you have enough compressor for continuous operation of
the blast cabinet, unless your nozzle is a lot smaller than mine is (which
is certainly possible).

A cubic foot is about 7.5 US liquid gallons, so your 80 gallon tank holds
about 10.6 cubic feet.  If it is at 175 psi and you regulate the output down
to 90 psi, that will expand to roughly 19cf @ 90 psi.  If the gun is drawing
4 cfm and the compressor supplying 1.4 cfm, then you'll get about 19/2.6 or
7 minutes of blast cabinet operation before you'll have to stop and wait for
the compressor to catch up.

Depending on what you want to do, 7 minutes may be enough.  I rarely spend
that long on a single part; and the compressor could catch up while you are
preparing the next part for blasting (removing all the loose scale, dirt,
grease, etc).

PS Feel free to "beat up" on my numbers.  I've used only rough
approximations to a lot of things, including using 15 psi for atmospheric
pressure (it's closer to 14.7) and ignoring the effects of temperature
changes and humidity.  But I think the result is good enough to answer your
question.

-- Randall