Quote Originally Posted by Mike Burke View Post

I have 4" spiral pipe for my whole system. Should I be expecting to change duct size out of the SDD for my main run ?

Mike,

I don't know how long your runs are or the rest of your setup but the longer the run with 4" the more of a detriment there is to your performance. Attached is a matrix showing the pipe to duct size relationship. If you can operate in the 4000-4500 FPM columns you will best optimize the ducts you have. You will notice that going from 4 to 5 inch ducts you will increase the area by almost 50%. Ina 4" duct you will probably have a velocity higher than 5000FPM (assuming the runs are fairly short). Going too large is also not good as the velocity to keep things airborne requires more CFM. So a 6" duct @4o00FPM needs a blower with 784 CFM. But you have to consider the rest of your system's resistance. So with the SDD, piping, filter, etc you probably have 6-9" of resistance. For that you will need a 3-5HP blower. So for a 1.5-2HP blower you are best to use 5" duct as far as possible. But you should start with Bill Pentz' static pressure calculator (excel spreadsheet) and let it tell you how much resistance your system has.

When connecting your elbow you will best be served by using at least a 6-7 diameter duct and remove the 5" reducer on the blower.
Duct data chart.JPG