The Cincinnati Fan Engineering Data guide, readily available from their website resources, state that performance issues can arise if elbows are to close to the inlet and outlet of the blower housing.
Duct turns should be in the direction of flow. Ours are at 90 degrees.
There should be 4 fan diameters of ducting between the discharge and any elbow. We have less than one.
I believe the importance of these considerations is demonstrated Bill Pentz inclusion of the neutral vane and air ramp and Bill's advice to have the inlet duct to be as long and straight as possible - 5ft.
For actual performance, these numbers may not mean all that much, but for noise they may.
I am certainly not an air expert, but it seems logical that having as much straight ducting running into and out of the blower housing would improve efficiency by reducing turbulence. Turbulence in an air stream would generate noise.
A standard box fan is extremely noisy/cfm compared to the squirrel cage fan in your furnace or even in your car because of the excessive turbulence.
Don
Duct turns should be in the direction of flow. Ours are at 90 degrees.
There should be 4 fan diameters of ducting between the discharge and any elbow. We have less than one.
I believe the importance of these considerations is demonstrated Bill Pentz inclusion of the neutral vane and air ramp and Bill's advice to have the inlet duct to be as long and straight as possible - 5ft.
For actual performance, these numbers may not mean all that much, but for noise they may.
I am certainly not an air expert, but it seems logical that having as much straight ducting running into and out of the blower housing would improve efficiency by reducing turbulence. Turbulence in an air stream would generate noise.
A standard box fan is extremely noisy/cfm compared to the squirrel cage fan in your furnace or even in your car because of the excessive turbulence.
Don