I recently bought a (used) Oneida 1.5 HP internal DC (yay!) and ran
into this dilemma. Oneida's website recommends 5" ducting for this DC.
This has something to do with the FPM becoming too low for 6" ducting
(I'm not too sure).
My question is, is it okay to go with 6" ducting for a smallish shop
(the ducting will be confined to about 11' X 18' area, with ducts no
longer than 15 feet or so).
BTW, do the performance curves published on Oneida's website include
the pressure drop due to the Cyclone and the filter? (I hope so)
I believe the 5" is specified to maintain the required "transport
velocity" in the duct. As you go from 5 to 6" the area increases by about
40% therefore the velocity in the duct drops by the corresponding amount
allowing dust to settle out in the duct. Over time the ID of the duct will
be reduced but not in a uniform way and you will probably notice a drop in
air flow because of increased resistance. Cheers, JG
Anil Kalagatla wrote:
Thanks for your response. Does this depend on the length of the
ductwork? Does this mean that the increased CFM due to 6"" is not
worth the potential settling of the dust in the ducts? Also, does it
make sense to "clean out" the ducts once in a while (not sure how one
would go about it).
The transport velocity only depends on duct length to the extent that
static pressure drop increases. You want to maintain a minimum of 3500
fpm in mainline ducts and 4000 fpm in branch lines or chips will settle
out of the airstream.
You can compute what the velocity in a duct will be using the formula:
Vduct = CFM/A
Where Vduct = velocity of air in duct
CFM = Cubic feet per minute
A = Area of duct
Area of duct can be computed as:
A = Pi * r^2
Where Pi = 3.1415927...
r = radius of ductwork (i.e. for 6 inch pipe, r = 3 inches, for
5" r = 2.5 inches)
The answer to your question is that the sizing of the ductwork is a
balancing act between volume and velocity. In addition, you want to
take into account static pressure drop (one of the reasons 4" ductwork
is at a disadvantage). The link
<http://cnets.net/~eclectic/woodworking/cyclone/ contains a very
helpful spreadsheet for computing these numbers. Used in conjuntion
with a volume vs. static pressure drop curve for your dust collector,
you can size your ductwork to provide the capacity you need.
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