From: games@max.u.washington.edu
Newsgroups: rec.woodworking,rec.autos.tech
Subject: Re: Air Compressor Wisdom (long)
Date: 25 Aug 92 17:24:13 GMT

In article <BtJFHK.1I4.2@cs.cmu.edu>, br@cs.cmu.edu (Bill Ross) writes:
> 
>     I got a wonderful, huge old compressor at a garage sale.  It has
>     about a 35 gallon tank with a 7 Amp motor driving a 2 cylinder
>     compressor.  It looks like it is circa 1940-1950 and must weigh
>     over 150 lbs.  I seems to run pretty well and it will produce
>     120 pounds of air and hold it.  I plan on cleaning it up, replacing
>     wiring and gaskets and painting it.  Since I've never had a compressor
>     before, I've got some questions:
> 
> 	What accessories do I need?  Are an air filter and oiler
> 	necessary?
> 
> 	If I'm going to be running high-volume tools like an impact
> 	wrench, is a 1/4" hose sufficient?  Should I look for 3/8"
> 	hose and fittings?
> 
> 	This compressor has an oil sump.  Can I use motor oil in
> 	this or do I need to find special compressor oil?
> 
> 	What types of tools work well with air compressors?  Which 
>         should I stay away from?  For example, if you were needing
> 	a drill, would you buy electric or air powered?
> 
>    Thanks for the help
>    Bill
>    br@cs.cmu.edu

Well, I used to work in a large shop that didn't do normal things, and after a
while, I sort of became the compressor guru.  This didn't mean that I was an
expert on the subject, just better than everyone else in the shop.

Answering your questions in no particular order.

The first and most important measurements of an air compresser with respect to 
tools is CFM/M @ PSI.  That is, how many cubic feet of air can it put out in a
minute at a given PSI.  Most air tools are rated at 90PSI, and will run a
little higher (like up to 110 for some), and other like to run a little lower.
(I can't even remember what it was now, but we had something that LIKED to run
down at 80)  Paint sprayers tend to run at 25-40 PSI  (O.K., mebe not all, but
OUR big one did.)  Now, at that pressure, they all require a certain minimum
air flow to keep them going.  A little drill used sporadically won't tax a
compressor, because it can recharge the tank while you are doing something else.
Most little air tools require from 2 to 6 CFM.  However I am the proud owner
of a 9 inch grinder that eats 13CFM at 90PSI, but I run it at more like
95, so it actually eats a bit more.  This is more than most small compressors
can keep up with.  It will empty a small tank in no time flat, and then
the compressor can't even generate enough air to keep it going.
(At our shop we had a compressor that could keep up with 3 of these things)
If your compressor can't keep up with one of these, and you want the tool 
anyway, think of the tank as a buffer of air.  Just go to a junk yard, and buy
a big nasty tank (50 to 80 gallons) (test it there before paying for it), and
add it to your system.  Then your compressor can build up the buffer enough to
get a good shot out of the bigger tools.

This leads to the answer to the last question...  Avoid air tools that take
more air than your compressor can provide.  However at our shop, we had a 
rule of thumb for all tools (given a huge compressor), If you need to buy a
tool, and you can buy it to run with air, DO SO.  The reason for this is that
air, as it escapes from the tool expands, and in doing so absorbs heat.  This
in turn means that the tool will run cooler than its electric counterpart,
hence it will last longer.  Also, if you stall the tool, there will be no
motor coils to overheat, and short out, so there will be no damage.  (I have
burned up an electric drill this way in my younger days).  Also, there are 
commercial type tools that are far more common in air power than in electric.
20,000 RPM Die Grinders are an example.  It seems to me that nailers used to 
be this way, but are not so much any more.  powered pop rivet guns seem more
common in air than electric.

As far as hoses go, I think that we used 1/2 hose for everything.

Care and feeding of an air compressor.  Basically an air compressor works much
like the engine of a car only with a modified cam.  The intake valve opens on
the intake stroke, and then it closes, and the output valve opens on the 
compression stroke. (forcing the air into the tank).  As such, there is no such
thing as AIR COMPRESSOR oil.  However as with all mechanical things, the oil
you use should be matched to the application.  Call an air compressor shop to
see what they reccommend, but I seem to recall that ours ran slower than
a car engine (500 rpm on the compressor head), but it was indoors in a 
constant temperature, so we used something like a medium weight (not multi-
weight due to not having temparature extremes) non detergent motor oil.

For the tools, we used a very light weight tool oil, because they run very
fast. (20,000 rpms for some tools.)  Use it sparingly, a little goes a long
ways.

As far as other accessories go, get a regulater end near where the tool is
so you can adjust the pressure that is allowed through.  Make sure you have a
good water trap system, water(read rust) in an air tool is death.  Make sure
that you have a drain on the bottom of the holding tank, moisture will condense
out of the air in there, and tap the tank to see how full it is, drain it
when you can tell there is water in it. One thing I did was to blow air
out of a spray nozzle every morning for 30 seconds or so before running any
tools, in order to discharge any condensed water in the piping system.

We found that certain times of year were much worse than others for water 
in the system problems.

I hope that this is helpfiull.  None of this information is based on
pure science, this is all observed behaviouour from an existing system.
(and based on my memory from when I worked there)  I am not sure who is
in a better position, you or I, as I have a lot of air tools, that I
bought while working there, and I have not yet bought my own compressor for
home.  Or, you, who has a compressor, but no tools.

	Well, good luck with the new toy.  Compressors can be very usefull
	in all kinds of suprising applications.

			John.
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