# Where to reduce the duct diameter



## kayak (Jan 2, 2016)

Running a 6 inch main trunk running along the ceiling. Wye on the ceiling to feed two 4 inch pickups at the table saw, one for the overhead blade guard, the second for the under cabinet pickup. Should the duct work for the under cabinet pickup be reduced from 6 to 4 right at the table saw to keep the large diameter as far as possible, or at the ceiling in order to up the air velocity on the vertical drop?


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## ORBlackFZ1 (Dec 25, 2013)

The simple answer is: Since you will be using both the blade guard pickup port and the cabinet port at the same time, both should have 4" pipe run to them after the "Y". 

The reason is because of the cross sectional area of the two 4" pipes (2 x 12.566 + 25.632 square inches) is approximately the same as the 6" pipe (28.27 square inches).

For the more complex answer, you will have to find a Mechanical Engineer that specializes in Fluid Mechanics. There is additional friction because of the surface area of the inside of the 4" flexible pipe vs the 6" metal pipe. There is also turbulence due to the "Y", etc.

Eric


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