Gentlemen,

Say I'm running an air tool off my compressor via a 20m long airline with
a 10mm internal diameter bore which connects, at the far end, to the tool
via a 10mm ID coupling. Let's say this arrangement is *just* sufficient
to run the tool at its full rated power. Now for whatever reason I change
the coupling to one with say a 7mm ID instead, with everything else
remaining unchanged. Will the tool's performance be negatively affected?
Common sense would seem to suggest so at first sight, but is that
actually the case?



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Cursitor Doom wrote:
Gentlemen,

Say I'm running an air tool off my compressor via a 20m long airline with
a 10mm internal diameter bore which connects, at the far end, to the tool
via a 10mm ID coupling. Let's say this arrangement is *just* sufficient
to run the tool at its full rated power. Now for whatever reason I change
the coupling to one with say a 7mm ID instead, with everything else
remaining unchanged. Will the tool's performance be negatively affected?
Common sense would seem to suggest so at first sight, but is that
actually the case?



Yes. You will have about 1/2 the cross sectional are of the pipe and to
compound the problem a larger proportion of the air will be close to the
walls of the pipe where the greatest friction will be experienced.

On 23/02/2018 20:42, Cursitor Doom wrote:
Gentlemen,

Say I'm running an air tool off my compressor via a 20m long airline with
a 10mm internal diameter bore which connects, at the far end, to the tool
via a 10mm ID coupling. Let's say this arrangement is *just* sufficient
to run the tool at its full rated power. Now for whatever reason I change
the coupling to one with say a 7mm ID instead, with everything else
remaining unchanged. Will the tool's performance be negatively affected?
Common sense would seem to suggest so at first sight, but is that
actually the case?



To a first approximation, you reduce the area by a factor of 2, so the
flow rate by a factor of two, so the power by a factor of 2. I'd expect
you to notice a significant drop in performance.

newshound wrote:

On 23/02/2018 20:42, Cursitor Doom wrote:
Gentlemen,

Say I'm running an air tool off my compressor via a 20m long airline with
a 10mm internal diameter bore which connects, at the far end, to the tool
via a 10mm ID coupling. Let's say this arrangement is *just* sufficient
to run the tool at its full rated power. Now for whatever reason I change
the coupling to one with say a 7mm ID instead, with everything else
remaining unchanged. Will the tool's performance be negatively affected?
Common sense would seem to suggest so at first sight, but is that
actually the case?



To a first approximation, you reduce the area by a factor of 2, so the
flow rate by a factor of two, so the power by a factor of 2. I'd expect
you to notice a significant drop in performance.

My vague recollection is that flow resistance goes up a lot faster than
inversely to the square of the diameter. But I thought the OP was just
changing the diameter of one coupling, and I don't think this will make
much difference compared with the flow resistance of 20m of tubing.



--

Roger Hayter

On Sat, 24 Feb 2018 10:37:51 +0000, Roger Hayter wrote:

My vague recollection is that flow resistance goes up a lot faster than
inversely to the square of the diameter. But I thought the OP was just
changing the diameter of one coupling, and I don't think this will make
much difference compared with the flow resistance of 20m of tubing.

What I was getting at (or trying to) is the question of whether a
restriction in the air line of a mere 30mm in length, say, in a much
longer and wider air-line has the same debilitating effect on performance
as if the whole length of the air-line was the same diameter as the 30mm
long restriction? IOW, is an analogy with "a chain is only as strong as
its weakest link" a valid one?
Let me give another analogy from electronics to better illustrate the
matter. Voltage regulator ICs in TO-220 package have very thin leads in
relation to the current they're expected to be able to pass, but because
these leads are only a few mm in length, currents of say 60A or more can
be successfully accommodated, such accommodation being unrealistic in the
extreme if these same leads were a couple of meters long.
Sorry if this isn't very clear; got the neighbours' kids around here at
the moment screaming and shouting making it hard to focus!





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On Sat, 24 Feb 2018 14:30:52 -0000 (UTC), Cursitor Doom
wrote:

What I was getting at (or trying to) is the question of whether a
restriction in the air line of a mere 30mm in length, say, in a much
longer and wider air-line has the same debilitating effect on performance
as if the whole length of the air-line was the same diameter as the 30mm
long restriction? IOW, is an analogy with "a chain is only as strong as
its weakest link" a valid one?

No...

Let me give another analogy from electronics to better illustrate the
matter. Voltage regulator ICs in TO-220 package have very thin leads in
relation to the current they're expected to be able to pass, but because
these leads are only a few mm in length, currents of say 60A or more can
be successfully accommodated, such accommodation being unrealistic in the
extreme if these same leads were a couple of meters long.

Yup. Length counts...

The analogy between flow and current, and voltage and pressure is valid, though
the "resistors" in an ail line will be nonlinear.


Thomas Prufer

On 24/02/2018 14:30, Cursitor Doom wrote:
On Sat, 24 Feb 2018 10:37:51 +0000, Roger Hayter wrote:

My vague recollection is that flow resistance goes up a lot faster than
inversely to the square of the diameter. But I thought the OP was just
changing the diameter of one coupling, and I don't think this will make
much difference compared with the flow resistance of 20m of tubing.

What I was getting at (or trying to) is the question of whether a
restriction in the air line of a mere 30mm in length, say, in a much
longer and wider air-line has the same debilitating effect on performance
as if the whole length of the air-line was the same diameter as the 30mm
long restriction? IOW, is an analogy with "a chain is only as strong as
its weakest link" a valid one?

No. Flow resistance is a function of both diameter and length (and
viscosity etc) - so a short length reduction to 7mm will not have
anything like the effect that having the whole 20m at 7mm in place of
the larger hose.



--
Cheers,

John.

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On Sun, 25 Feb 2018 01:09:33 +0000, John Rumm wrote:

No. Flow resistance is a function of both diameter and length (and
viscosity etc) - so a short length reduction to 7mm will not have
anything like the effect that having the whole 20m at 7mm in place of
the larger hose.

Thanks John (and Thomas) for this confirmation. It's counterintuitive,
but so are a lot of other things that turn out to be true!
I was suspicious because I have an air tool here that states it should be
connected to a 10mm ID hose via its 1/4" connector.




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On Sun, 25 Feb 2018 01:09:33 +0000, John Rumm wrote:

No. Flow resistance is a function of both diameter and length (and
viscosity etc) - so a short length reduction to 7mm will not have
anything like the effect that having the whole 20m at 7mm in place of
the larger hose.

I know from personal experience when I'm using one of those giant 500ml
'syringes' to pump oil into a gearbox case that using a short flexi-tube
into the 'box requires much less force than a longer tube does. But we
cannot equate fluid mechanics to the laws relating to gases; one category
is compressible, the other isn't!



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On Sun, 25 Feb 2018 01:09:33 +0000, John Rumm
wrote:

No. Flow resistance is a function of both diameter and length (and
viscosity etc) - so a short length reduction to 7mm will not have
anything like the effect that having the whole 20m at 7mm in place of
the larger hose.

Thought of a vivid electrical analogy: think thick copper cabling, short bit of
thin wire (aka a fuse) in series.

Wouldn't work if it were all fuse wire, but does if done as usual.

Thomas Prufer