(01-13-2017, 06:22 PM)Arlin Eastman Wrote: What is "Static Pressure"?
Flow resistance. The more you restrict the flow (increase flow resistance), the less air will be moved by the blower.
Here's a fan curve I found online, and is typical of home shop size dust collectors.
Notice that at low resistance (SP) on the Y-axis, say 1" water gauge*, following across to where it intersects the curve, then drop down to the X-axis and read about 1550 CFM. Restrict the flow at the inlet (or outlet, since this curve includes the filter) and you read higher on the Y-axis, look across to where it interects the curve, and drop down to the flow rate on the X-axis, and it will read lower, as you would expect. Worst case is to plug the inlet, and flow will be zero CFM (X-axis), and max SP is about 12.6 inches WG. That's the number that most DC manufacturers publish, but by itself is rather meaningless other than to suggest how the rest of the curve might look (a really high max SP usually means a strong curve).
I can't draw on this notebook, but you can imagine the duct curve to go with the fan curve. Just draw a line across the graph at about the 7" SP mark on the Y-axis. Then mirror the fan curve about that line, so the left end is at zero, and the right end is at about the 13" mark (just for illustrative purposes - actual curve depends on actual ducting), with the curve being concave, like half a bowl. Where the duct curve intersects the fan curve will be the 'operating point' for that particular duct arrangement. Open or close more blast gates, and the curve will shift, being squeezed left for more restrictive, and stretched right for less restrictive. You can see that will less restrictive ducting, the operating point will be lower on the SP axis (Y), and higher on the flow rate axis (X), with the reverse being true with a more restrictive duct curve.
Some DC manufacturers actually publish performance curves, but most don't, and only report max SP. High max SP usually corresponds to a large diameter blower wheel, and is suggestive of higher flow at a give SP than a blower with a lower max SP (and usually has a larger motor), but that's not an ironclad rule, as you can design a blower with extremely high max SP but very low air flow across the board (like a shop vac).
*Water gauge, or WG, is a measure of pressure difference in a U-tube partially filled with water, with one end connected to the duct, and the other either left open (atmospheric pressure), or to another duct location, like inlet and outlet of the blower. The pressure and/or vacuum will pull the water in one direction so the two surfaces are no longer at the same elevation (level), and the difference in height measured in inches is 'inches water gauge'.
You can make your own out of a piece of clear tube.