In remembering my old pipefitting days, I recalled a time when I was on
a chemical plant job and told this old joke to my new welder and his
very green helper. I finished the line about turning the building over
and the helper asked: "Anyone get hurt?"
That was funnier than the joke.
And believe me, I really have run 6" screw pipe for fire suppression
systems in the laboratories. The main run is 6", it really requires a
72" pipe wrench (which weigh about 75 pounds each) and it is all
suspended in the ceiling to supply the fire sprinklers. The drain lines
are the coolest. They are 2" pyrex (chemical resistance).
The 1/2" main buys you a lot. Many people assume that if one part of
the pipe is small, then the rest of it might as well be small too
because only so much air can get through the small part anyway. Their
thinking is erroneous.
The air can move through a very short section of small pipe at great
speed. But every inch that it moves along at great speed slows it
down a little from friction against the walls. If the whole length is
small, the friction losses for all those inches add up and the air
will move much slower.
However, if the pipe is big enough, the air inside is just loafing
along, not moving fast enough for the friction along the sides to be
significant. Then, the friction losses for the long section of big
pipe become insignificant, so you only have a few inches where it's
moving really fast. You can live with the friction losses for a few
By the way, if you remember (A=PI R Square), the 1/2" pipe is really
FOUR times larger in volume than the same length of 1/4" pipe.
Therefore, the air is moving at least four times as fast in the 1/4"
pipe as it is in the 1/2" pipe. In fact you get an additional
advantage from the bigger pipe because the air at the center of the
pipe can go faster than the air around the edges. The value of this
phenomenon increases with pipe size. Maybe more than you really
wanted to know, but some of this is counter-intuitive.
"Even an old blind hog finds an acorn every now and then."
forgot to mention that the compressor only has a 1/4" nip, so does
that mean having a 1/2" main run buys me anything, given I will have
to have a reducer at the compressor itself, or does it matter?
Yeah, that smaller nipple restricts the flow. Larger pope/hose on the other
side will only give you a split second more volume.
The 1/2" on the other side will not add but it will certainly help to not be
as restrictive as 1/4" pipe would be.
Correct - the effect depending on how much of the larger diameter pipe
is downstream of the restriction. The more pipe volume downstream of
the restriction the larger the effective tankage.
A quarter inch orifice in the line will restrict the flow more than if
the orifice wasn't there, but nowhere near as much as if the entire
line were the diameter of the orifice. The larger the pipe, the less
pressure drop per foot for a given flow rate - or - the greater the
flow rate for a given pressure drop. And, ignoring start/stop
transients, steady state performance is independent of where in the
line the restriction/orifice is located.
Looking back at all the posts so far, and the fact thay you say your
output is , "1/4" nip", (I assume that means a 1/4" pipe fitting), I
suggest that before you install all that pipe, you make darned sure that
the compressor will handle all the tools you intend to run from it in
the future, right from the tank. I say this because that 1/4" outlet
makes me think that the compressor setup may not be very powerful; ie:
it may only deliver 1 or 2 cfm at rated pressure.
"DO NOT USE PVC PIPE MADE FOR WATER LINES!!! "
Responded someone. I beg to differ. John and I had and John still has
a shop fitted with schedule 40 PVC lines for his large compressor. The
large I.D. of the PVC pipe serves as additional volume and the runs
and drops provide access all over the shop.
Each outlet is fitted along a vertical drop that terminates in a weep
valve below (for draining the lines.
We used automotive ROS, Grinders, Binks Spray Guns, die grinders for
years and years without a problem. I can't recall the exact sprecs on
the compressor, but it was a two-stage with an 80 gallon tank or damn
close to it - Tall as I.
Air pressure is measured at the compressor, but should be the same if
you moved the gauge to the end of the line.
Holding capacity is initially limited by the attached tank, but
increases the moment you attach an air hose so the tank size never
equals the working volume, now does it?
You let a little air out and nothing happens, a bit more and at some
point the overall pressure in the lines and tank frops below the
refresh setting and your pump begins to run to pump in sufficient air
to raise the pressure to the high cut-off point.
Restrictions due to a smaller diameter hose (or PVC piping) could,
indeed impact the performance. But you are going to use 3/4" PVC or
larger and avoid such a complication.
My bet is that your little compressor is the only "choke point" in
your planned approach and that will be remedied down the road when you
can afford to connect a bigger, more powerful unit to the PVC piping
installed to over yourself today.
Provide for drainage and filtration and you'll do just fine.
Just because you are getting away with it does not make it right. I have
seen PVC pipe explode for no apparent reason with 100 PSI air pressure. I
guy I know has scars on his face from a different PVC pipe rupture. The
stuff is dangerous. When it ruptures it goes off like a hand grenade spewing
PVC shrapnel all over the room.
Manufacturers and OSHA have warnings against it's use. I used PVC in the
past, but never again after seeing the carnage when it fails.
I know guys that go to the bar and drink until they can barely walk, then
drive home. They have been doing it for years with no problems, so I guess
it is ok.
What he says...
It is _NOT_ suitable for air, at least if there are any exposed runs--if
all were underground or otherwise contained entirely, it would at least
not present the imminent threat altho still unapproved...
As well as just the simple bursting scenario, it is subject to similar
disintegration failure on impact, etc., ...
On Sun, 3 Feb 2008 11:46:12 -0800 (PST), Hoosierpopi
Do NOT use PVC. It is EXTREMELY DANGEROUS if it is shocked while under
pressure . It fragments violently, and packs a FATAL blow if you are
in the (rather extensive) range of fire.
It also deteriorates in contact with oil, and you cannot guarantee
your air is oil free. This makes it more brittle.
Posted via a free Usenet account from http://www.teranews.com
On Sun, 3 Feb 2008 11:46:12 -0800 (PST), Hoosierpopi
OK, as long as you don't have to satisfy any OSHA regs and aren't
worried about flying plastic shards in the event that the pipe does
"(T)he Plastic Pipe Institute, in its Recommendation B dated January
19, 1972, recommends against the use of thermoplastic pipe to
transport compressed air or other compressed gases in exposed plant
"It is our position that PVC pipe shall not be used as a means of
transporting compressed air. This position follows the manufacturer's
own statements that PVC is unsuitable for compressed air systems. "
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