Just to put in my two cents' worth regarding plumbing for air lines. I have
1/2" black iron pipe in my garage and it works great. Main reasons for going
with iron are (1) it's cheap, (2) you absolutely need metal piping to help
condense out the water, and (3) every repair shop, dentist, and dry cleaner
(folks who use lots of high pressure air) I talked to said iron is the only way
to go. Copper may be OK, but it's really expensive and I'd worry about burst
strength (copper is really soft stuff). PVC is a definite no-no; with 125-150
psi air it'd burst in a minute (and besides it's illegal to use it in air lines
here in Virginia).
Oh, and you got the straight scoop about water drainage. Slope the lines away
from the compressor, use tees off of vertical pipes to connect your equipment,
and ball valves at the end of each vertical run to drain the inevitable water.
Another trick I learned to reduce the water going into the traps is to run your
horizontal feeder line relatively high up, and connect verticals by teeing off
the TOP of the feeder, go up a short (6") stub, over and down to your
verticals. Doing this you pick off the relatively dry air from the top of the
pipe and reduce water.
And by the way, I'm using a 60gal Sears vertical 6hp oilless compressor. Works
great for bead blasters, tools, paint, etc. Especially good for paint because
there's no oil to worry about. I run my air tools off a separate line that has
an in-line oiler attached.
Hope all this helps...
'67 Mustang (dearly departed)
'61 T-Bird (still in boxes)