Originally Posted by
David L Morse
The 1.8" measurement is the pressure drop of the test duct with no restrictions. A test duct has an entrance and so it has entrance loss. A round duct will have an entrance loss of about 1.2 (bell mouth) to 1.9 (plain end) times the velocity pressure in the duct. 1500 CFM in 8" is 4300 fpm, giving a velocity pressure of 1.1" wg. Depending on the shape of the entrance you would expect a static pressure loss of 1.3 to 2.1" wg at that flow rate.
Thanks for that David. I never paid attention to how significant the entry loss is on a plain end duct. When I measured my unit and it was about 2" with an unrestricted test duct (which matched Bill's assumption of ~2" for his cyclone) I figured I was right the right ballpark. But you are correct I need to measure between the blower and cyclone and not the room pressure as a baseline.
My mistake was going back to what someone told me about fan curves and that they will never change, it is just the SP you add to the fan and where you land on the curve. But this reminded me that is false. Through manufactures fan/system curves and my own testing I found the fan curve can change significantly depending on the changes to the inlet/outlet of the fan/blower (like adding a cyclone). The curve to consider must be created where your ducting begins.
I guess this is why manufactures put 7 & 8" inlets on 1.5 & 2 HP cyclones, to get better fan curves. Not because someone will put a 7" main on a 1.5HP cyclone.
Thanks again,
Carl