Configuring ductwork for least noise
I'm installing a 36x24 hood with variable speed control for a roof-mounted 600CFM blower. I will also be installing a Fantech duct silencer between the hood and the blower. Hood and blower take 10" duct, so that is what I will be using -- overkill for 600CFM, but the 10" duct silencer also has slightly better noise attenuation specs than the 8" version.
The straight line, no-bend distance between ceiling and roof could be about 5 feet, which should allow room for the 36" long silencer plus a short duct run. Obviously, this straight short run gives the best flow. However, adding 10 feet and a couple of 90 bends would decrease flow by less than 4 percent.
Would the noise be less by adding the extra length and two 90's, so that the blower is further away?