System Or Fan effect from Smacna

[Tremblay]

HI.

Not sure if I am at the right forum for this question. System or fan effect when a ventilation duct is attached at the outlet of a ventilation fan depends of SWSI DWDI with at least 4 different configurations. Pressure drop will vary with % of Length (Lmin). Smacna show 4 positions: Up, down, right or left. Can anyone explain why the left and right positions (B,D) having a different pressure drop values? I can't see why a left or right turn would make a difference of pressure drop.

Thanks in advance for answers.

[WAYNE3298]

The difference is the turbulence of the air. I know that doesn't tell you much but if you look up "System Effect" in the AMCA manual you will better understand. They have some examples of different things that create system effect and show you how to determine the system effect factors.
Unfortunately all things that create system effect have not been analyzed but fan discharge ducting has.
I hope that helps.

[Tremblay]

Wayne. Tks for the answer. I had a deeper look at the AMCA publication 201-90 and I may have a partial answer. The inlet side (on SWSI configuration) will create more energy loss fan effect on the b configuration(same side as inlet). D is opposite side of inlet. Interesting correlation between flow pattern at the outlet of a fan depending of inlet configuration. Thanks.

[WAYNE3298]

You are welcome Tremblay. It's great you have the AMCA publication because system effect comes in many ways and can kill the system performance. Air rotation verses duct elbow direction especially at the fan and blast area are covered pretty good and are important.

[bradluke0]

The worst case ( which Wayne discusses in his post above ) is what we call "breaking the back of the fan ". Kind of hard to explain but here goes....Say you are looking at the side of a fan and it is rotating clock wise and the air is being discharged to the right . You would not want to have an elbow close to the discharge which turns up .

[WAYNE3298]

That's what I was trying to say Brad but couldn't come up with the words.

[bradluke0]

I knew you had it but couldn't quite spit it out....hehe.