DarylRos wrote:

It's not Bernoulli mainly that factors into this, it's Boyle.

I beg to disagree but at the velocities common in dust collection systems
the flow of air is assumed to be incompressible and Boyle doesn't enter
into the calculation. It's not until you have velocities approaching Mach
1 that you start having to consider compressibility.

Pressure dynamics is the same whether it's for gas, liquid or even traffic
patterns.

Maybe so, but Boyle's Law applies to static pressures, not dynamic.

Reduce the size of the pipe, duct or road and you increase the pressure
and reduce the velocity.

You've got it backwards. Reduce the size of the pipe or duct and you
decrease the pressure and increase the velocity.

So the idea that a reduction at one point (be it the smaller pickup at a
saw or a roadblock in the middle of the road), the pressure increases, the
dust, car, water, whatever slows, but then, as the pressure decreases with
the increase in the roadwork, the speed increases.

That may be _your_ idea but gases don't behave that way in ducts.

The traffic analogy was not mine, but worked out by some highway
engineers. They were surprised to learn that traffic flow basically obeys
Boyle's law.

I'd like to see a reference to that.

Which is why you want large main ductwork, this is your freeway. The
smaller gates are your on ramps.

The speed cannot be the same throughout. Just as traffic picks up after a
slowdown. Sometimes when you hit traffic and then it speeds up, you wonder
why. Well, there was a stoppage a while ago, and the system is simply
recovering. It does not stay slow the entire way.

If highways behaved like air ducts then you'd see people going 180 MPH
though construction zones.


--
--John
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(was jclarke at eye bee em dot net)