Ductwork Design Tips - For those who want to do it themselves (Remember: We'll do it for you.)

Once the basics of duct design are understood, the process can be carried through to ANY size duct system. Air losses are so small with the K&B clamp and components, that no additional allowances need to be made. The following example provides the basics of design. It is based on 4500 FPM.

In this system, we are using openings of 4", 6", and 8". In a majority of systems, the dust portal of the machine is measured and used as a "given".

The size of duct "A" is of course 4" because the CFM in the system is constant until the next branch at which point the 6" line feeds into the system. At this point the collective CFM is 1277. Using the velocity/volume chart we look for 1277 CFM under 4500"/min. However we find that it falls between duct size 7" and 8". By rounding to the nearest number, we find that 7" is the closest number. "B" is therefore best sized at 7".

Branches, also called "T on Taper", are sized by the large end and called out by A-B-C.

The branch "BR 1" is therefore a 7" branch, tapering to a 4", with a 6" lateral. This is listed as a 7-4-6.

Continuing on the 8” line feed into the 7” line (B) and the collective CFM is now 2848. Referring to the chart, the best selection for ‘C’ is an 11” duct and the branch (Br2) is called out by 11-7-8.

Using the same process, any size system can now be sized by just adding the collective CFM and using the chart. Even if you have multiple branches as for example a 18” and a 22” branch running coming together. The total CFM would be app. 19838. This then would require a combined duct size of 28”. See chart.