Clar sent me the following:
I’m helping my daughter-in-law set up an exhaust system for her bead making. The system will be 3ft. wideX 2 ft. deep with one 90 and one 45 degree bend and a total run of 6 feet from hood to outside. Velocity should be around 3,000 FPM. We’re looking at Grainger fans 1TDT5-975 CFM and 1TDU1-990 CFM. Unfortunately the charts you reference in your article were not available to me so I cannot calculate losses but in the information I have available I think 7″‘ ductwork is best for this system. What do you recommend? Thanks much.
Okay, first of all, you’ve miscalculated the fan requirements. If the hood measures 3 feet by 2 feet, that’s 6 square feet, and at 125 CFM per square foot, you need a fan capable of a minimum of 750 CFM. The ideal fan would be the 1TDT9 @ 794 CFM free air.
If you use 7″ ducting, your numbers are:
2970 FPM, and a total system pressure of 0.71 inches.
If you use 8″ ducting, your numbers are:
2275 FPM, and a total system pressure of 0.08 inches.
I’d much prefer to see you use the 8″ ducting, as this will also lower your noise levels. High air flow speeds do increase the amount of sound generated, and ideally, I like to see air flows centered on 2500 FPM instead of 3000 FPM.