Damn. I'm sure I calculated a much lower Re once, but I can't find my
notes to see where I made the mistake (I assume it was me, but I'll
check yours). Probably got screwed up on the whole lbm/lbf thing...

G

Bill Leonhardt wrote:

"G. Lewin" wrote in message
...
SNIP

Now, as for the important answer of which is more important for moving
chips: turbulence effects vs. friction effects? I can't say. But if you
work through the calculations, you find that the "recommended" flow
speed usually works out to the transition region between laminar and
turbulent flow. Coincidence? I suspect (and this is pure conjecture)
that some amount of turbulence is necessary to keep dust from sticking
to the sides of the duct. Obviously, though, the bulk motion of the air
is what moves the dust from A to B.




This didn't seem quite right to me so I took a look at the numbers. IIRC,
recommended duct velocities are 3000 to 4000 fpm.

Reynolds number = Re = (density)(velocity)(diameter)/(viscosity)

At 70 deg F: density = 0.075 lbm/cu ft viscosity = 0.044 lbm/ hr ft

A lower limit could be 3000 ft/min in a 4 inch duct.

Re = (0.075 lbm/ cu ft) (3000 ft/min) (60 min/hr) 4 in) / (0.044 lbm/hr ft)
(12 in/ ft)

Re = 102,273

Since transition from laminar to turbulent flow (in internal duct flow) is
in the range 2,000 to 10,000, this is clearly turbulent. Higher values for
the flow rate and/or duct diameter will yield higher Re numbers.

I would expect you would want to stay away from laminar flow, and certainly
stay away from transition for good performance.

Bill Leonhardt