All the books about dust collectors stress having ducts with sufficient
air velocity to keep the wood particles in suspension to avoid
blockages. This means the duct should not be "too large." Sandor
Nagyszalanczy, in "Woodshop Dust Control" quotes 3500 FPM as optimum.
On the other hand, duct with too small a diameter will decrease air
flow thereby reducing collection efficiency because of pressure drops
across the ductwork. He quotes 350 -500 CFM flow rates at each machine
for adequate dust control.
Having just bought a used Penn State DC1B dust collector I was
interested in measuring FPM and CFM values so I could size my duct
appropriately. A Google search for anemometers showed mostly $200 and
up units, which would be a no-go for me. However I found a newly
released unit for about $40.00-$50.00 designed for wind speed
measurements. It's a Lacrosse EA-3010U, on Amazon at
http://tinyurl.com/4s5f2 . The Lacrosse link is
http://www.lacrosse-htawi.com/ea-3010u.html . It's a tiny thing, about
1" x 4". It can be set to measure wind speeds up to 67 mph, and can be
set to read out in meters per second, m/s. Actually it also measures
temperatures and wind chills for you weather fans. (BTW, I have no
relationship with the Lacrosse folks.)
Knowing how much I have spent and will spend on DC equipment I figured
$40. was worth it to get some quantitative measurements, so I bought
one. (Plus, like many WW'ers I love gadgets.)
Here are the formulas for FPM and CFM starting with air velocity in m/s
and duct diameter in inches:
Air velocity in FPM = m/s * 197.
Air flow in CFM = m/s * (duct diameter in inches)^2 * 1.05.
My DC1B showed an air velocity of 6107 FPM at the DC with one 4" port
open and no load, decreasing to 3940 FPM with both 4" ports open.
The air flow was 672 CFM with both ports open, decresing to 521 CFM
with only one port open. Penn State advertises about 875 FPM for the
new DC1B-XL model that superseded my DC1B, so my machine is about in
line with what I would expect.
Now to measure the air flow with duct in place, which will no doubt be
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