Any CFM experts have a guesstimate

[Bob Slater]

I just finished installing an in line duct fan to clear a small room in my shop of bad air. It is 8 inches in diameter and supposedly flows 675 CFM . I have been using it without the run of duct that takes the intake to just over my work table and it seems to move a good amount of air. I ran the ducting tonight. About a foot and a half away from the wall I reduced the duct from 8 to 6 inches because I had some 6 inch spiral pipe. The total run is about ten feet to the outlet. It seems to move less air now. Any guess as to how much I degraded the potential by using 6 inch duct as opposed to 8 inch? I will spend the $$ to upgrade if you think it will be a noticeable difference. thanks, Bob.

[Rob Russell]

Well, the drop in square footage from an 8" pipe to a 6" pipe is about 40% of the surface area. That sounds like a big change to me.

[Dustin Lane]

With a little bit of me guessing I would say you could use volume of a cylinder ratios to get a good idea. So with an 8 inch you'd have 16 pi in^3 compared to 9 pi in^3 with the 6 inch (using pi r^2 * h, assuming h to be 1). With that you'd have cut the CFM potential nearly in half.

This is all based on my knowledge of blood flow thru veins and arteries. Seems close enough to me.

[Bob Slater]

Thanks guys. Those performance cuts are too much when I think about why I installed the fan in the first place. I am just going by gut feeling and remembering how hard it is to breath through a long snorkel. I will redo the ductwork for max performance, as I want this fan to clear the air as fast as possible. Going 8 inch the whole way. I would also think these motors don't have an over abundance of extra torque.

[Frank Hasty]

As an individual who had some experience with similar problems in a previous incarnation I can tell you that an 8" pipe has almost twice the carrying capacity of a 6" pipe for the same flow and pressure loss.
So your decision to increase to the larger diameter is a sound one.

An even better solution would be to use 10' dia pipe instead of the 8". The reason is fans push air against the friction caused by air velocity in the pipe, and I have a hunch that your fan is a typical "propeller" fan. These fans usually do not do very well when pushing against much friction, their capacity drops off quickly. So even with the 8" pipe you will be moving a good bit less than 675 cfm.

Good luck!

Frank Hasty

[Jim Finn]

"propeller" fan. These fans usually do not do very well when pushing against much friction, their capacity drops off quickly. So even with the 8" pipe you will be moving a good bit less than 675 cfm.
Frank Hasty[/QUOTE]............................I agree 100%. Longer the duct the worse it gets. Squirrel cage fans are the best type to use.

[David Epperson]

As had been noted the reduction in duct diameter will increase the friction...on the output side of the fan. But if I read correctly this added restriction was on the inlet side. Which means that he fan is now in a partial vacuum and simply cannot get enough air to push at full capacity. An alternate solution here might be to just install another 4" or 6" inlet port into the 8" line. The performance at the 6" work point may degrade a bit, but the overall air moving performance should jump back to normal.

[Bob Slater]

Good points. The fan in question is kinda like a squirrel fan, as the vanes are located around the perimeter of the airflow.
http://www.tjernlund.com/ductboost.htm
It is the M-8 model.

[Bob Slater]

Just upped the whole thing to 8 inches with some new spiral pipe and a one piece 90 degree elbow. It sucks now. Much better. Looks quite impressive too.