LRod writes:
On Wed, 06 Aug 2008 10:08:40 -0400, blueman wrote:

Anything but PVC for the material.

No one addressed the size, yet, but here are my thoughts on it. For
the short run, it doesn't make much difference. Anything from 1/4" on
up will work. As you get into the longer runs, the smaller sizes wind
up hurting you in the flow department (CFM), even if you are able to
keep a reasonable pressure. And an 8 gallon compressor is unlikely to
get you more than 100 PSI (not because it's 8 gallons, but because
compressors with 8 gallon tanks usually don't have a lot of oomph),
and a longer run at small diameter will be something less than that.
Okay for sprayers--not so hot for bike tires.

The thing about the longer run, is that it winds up being a storage
manifold as well as a delivery manifold, and if you use 3/4" or 1"
pipe, you can effectively increase the capacity of your compressor.

For example (and assuming my math is right--I'm sure someone will
check it), 3/4" pipe has a cross-sectional area of .4415 in^2 and
consequently, a volume of 5.29 in^3 per foot. A 40' run, then, would
have 211.95 in^3, or approximately .9 gallon.

Similarly, a 1" pipe has a cross section nearly double 3/4" (.785
in^2) and a volume of 9.42 in^3 per foot. Your 40' run yields more
than 1½ gallons (1.63) in 1".

Note that that extra storage applies to both the outlet at the end of
the 40' run and the end of the shorter run--it's in the system.


Thanks for the detailed exposition!
So, I guess I'm thinking of biting the bullet and using copper. I'm
trying to decide now between 1/2" and 3/4" (while I appreciate the
extra manifold capicty of 1", I'm not sure it is worth the extra cost
or bulk).

- Would I see much difference between 1/2 and 3/4"?
i.e. Would it be "penny wise, pound foolish to put in 1/2" now just
to save a few tens of dollars on copper piping and fittings?

- Would type 'L' be preferable to type 'M' or is type 'M' good enough
for air at pressures of 90-125PSI?

Now I know from simple geometry that 3/4" has a tad more than twice
the area of 1/2" but I'm not sure of how to do the Ohm's law type math
to see what type of pressure drop I would be seeing with 1/2" vs. 3/4"
for typical uses.

Assume average pressure of 100PSI at the tank with an average steady
state 4 CFM.