From: kpmartin@thinkage.on.ca (Kevin P. Martin)
Subject: Re: Air Compressor Questions
Organization: /etc/organization
Distribution: na
Date: Fri, 16 Oct 1992 15:33:07 GMT

In article <8070@vtserf.cc.vt.edu> greenfie@vtserf.cc.vt.edu (Steve Greenfield) writes:
>I just bought a Sears/Craftsman air compressor that was 3.5 HP and
>had a 25 gallon tank that was rated at 125 PSI.  I think it came
>with a 15ft hose which was 1/4" and rated at 200psi, I believe.  I
>have never used a compressor before and I have a few questions and
>concerns.

The critical figures for a compressor are the PSI (the tank pressure
in Pounds per Square Inch that the compressor switch aims for) and
the volume in SCFM (the number of Standard Cubic Feet per Minute
that the compressor can produce at that pressure). It is possible
to calculate from these how many horsepower the motor must produce.
I suspect that your 3.5 HP figure refers to Sears HP, not actual
HP (you can only get about 2HP from a normal 120V outlet).

You normally run the air through an adjustable regulator to get
some fixed pressure lower than the tank pressure. The pressure
you want depends on the tool being used.

In addition, each tool is rated in SCFM as to how much air it needs
for continuous duty. But many tools are not used continuously (like
an air drill). In order for the compressor to be able to keep up, you
want the compressor's SCFM to be no less than the average consumption
for the tool.

> ... I am confused about sizes of hoses, fittings and tools.
>The literature that came with the compressor does not go into
>details ... other than the SCFM ratings.  ... All the pneumatic
>tool ads say 1/4" NPT (?), 3/8" hose size along with the PSI and
>CFM ratings.
>The compressor literature did say that you should not use an extension
>cord but to use more air hose instead.

The main point is that a tool's listed working pressure is to be set at
the tool, not at the regulator. The air flow in the lines causes a
pressure drop at the tool. If you have a higher (peak, not average)
SCFM tool, longer air lines, narrower air lines, or more connectors,
you get less pressure. This is analogous to voltage for an electric
tool: if you have higher current consumption, a longer extension cord,
a lighter-gauge cord, or many connectors, you will have a reduced
voltage at the tool. In both cases, you get impaired performance
from the tool.

However, with the air tool, you can turn up the regulator pressure
to compensate for the losses (you can't turn up your line voltage),
and the energy lost does not cause overheating of the lines, tool,
or connectors (unlike for electric tools). This is why they suggest
longer air lines rather than a longer power cord.

As for connector sizes, NPT refers to National Pipe Thread, the same
sizes used for threaded water pipe. The actual pipe outside diameter
is about 1/4" bigger than the nominal size (if you want to see a
sample, almost all shower heads fit on 1/2" NPT).
The sizes for flexible air lines refers (more or less accurately)
to the inside diameter of the tubing. The sizes for "soft" copper
tubing and flexible steel lines (like brake lines) refer to the
outside diameter.
You can get hose ends that have nipples to fit in the hose, and
result in a male NPT end. You can also find finished hoses ending
in both male or female NPT. The quick disconnects are available to
fit both male and female NPT. You should likely put male QD's on
all your tools and one end of each air line, and female QD's on the
other end of each line and on the conpressor outlet.
One note, though: There are two incompatible QD systems that you
are likely to encounter (and others that you won't encounter because
they are too rare or too large for you). At least one company refers
to them as "A" and "M" type, but they are ofter known as "long" and
"short" respectively. The "long" and "short" refer to the length of
the plain tube at the end of the male QD. The "short" one is about
1/4" long and the retainer ridge has a symmetric shape. The "long" one
is about 3/8" long and the ridge has an asymmetric sawtooth shape.
--> Make sure you always buy the right type, and make sure when you
	start out that this type is (and will remain) readily available!
	The wrong NPT size can be corrected with adapters, but the wrong
	QD just plain won't fit.
The selection of "A" or "M" connectors, 1/4" NPT threaded connections
and 1/4", 5/16", or 3/8" hose go together well (in the sense that you
can get direct combinations of these sizes without needing extra adapters).
You should probably go with 3/8" hose; it is heavier, bulkier, and a bit
more expensive, but you're less likely to be in the situation of needing
more regulator pressure than the compressor produces...

>Stephen L. Greenfield                     Bitnet:   FEATS@VTVM1
              -Kevin P. Martin, Thinkage Ltd., Kitchener, Ontario, Canada
Newsgroups: rec.woodworking