In article , Jack Stein wrote:
Doug Miller wrote:


That said, compression fittings are a poor choice where high flow
rates may be required.

I never heard this before, and not sure I understand why? Not saying
it's incorrect, just that I don't get it? They don't appear to restrict
flow at all?

Actually, they do. Take apart a compression joint in copper tube and take a
look.

I happen to have a hunk of 3/8 copper tubing with a compression fitting
in my junk pile, which you made be go down and look at... couldn't see
any notable issues that would preclude it's usage in a high flow
situation, other than it was 3/8" in pipe. I guess there might be some
negligible constriction which is difficult to see and might be important
in super small pipe,

One man's negligible is another man's noticeable, I suppose...

but in normal home air systems in this discussion,
a very non-issue.

Compression fittings are used in high pressure conditions, which I guess
would infer high flow rates?

Pressure and flow rate are completely different. Pressure is the force moving
a fluid (air, in this case) from one place to another; flow rate is the amount
of fluid moving per unit time.

I tend to think that the amount of air that would flow through a 3/8"
pipe under 1000 lbs of pressure is more than would flow through the same
pipe with 2 lbs of pressure? Is that wrong? I know volume and pressure
are two different things, but I'd think one effects the other?

No, you're not wrong, and indeed the pressure does affect the flow rate -- but
flow rate is also determined by the size of the pipe. You're going to get a
higher flow rate through a one-foot diameter pipe at 2psi than you will
through a 3/8"-diameter pipe at 1000psi.

Most easily illustrated by example:
Low pressure, low flow rate -- the water in your rain gutters.
High pressure, low flow rate -- a mountain stream.

See, right there I don't get it? In my mind, to get high pressure you
need to restrict the flow.

That's not correct.

I don't see a mountain stream or a river as
high pressure?

Steeper gradient = higher force (due to gravity).


I admit I know next to nothing about fluid dynamics, but
thats my point... For example, if I have a high pressure hose, and put
a hole in the hose, the water will shoot out the hole into the air, and
the more pressure, the higher it will go. The mountain stream is not
shooting up in the air, just flowing... low pressure, high volume. My
mind says the only way to get high pressure in a mountain stream is to
force in through a pipe of some sort.

I should have made it more clear that I meant that as an analogy to try to
make the point. I did not mean that the mountain stream is literally under
higher pressure.

I do realize that the deeper the
stream, the higher the pressure at the bottom of the stream from the
weight of the water.

Now that I think about it, for Doug, the disconnects at the end of your
air hose are compression fittings. Not particularly the type we are
discussing but thought I'd throw that in.

Obviously a very different type from what we're discussing.

Well, if you think about it, the main difference is a spring holds the
compression fitting together instead of threads. Pipe dope is not
needed anymore there than on a standard brass compression fitting.

Except that you won't be unscrewing one of those. Threaded fittings *are* from
time to time unscrewed, and doping them on assembly makes disassembly easier.