Use of Spin-ins, scoops and air extractors 1

[krb]

I am looking for input on the use of spin-ins, scoops and air extractors in branch take-offs in low pressure ductwork. We don't use them generally, but I see them called for on alot of projects designed by other engineers. My feeling is that they restrict airflow and cause greater friction loss than the use of 45deg branch take-offs and air tites. Any comments?

 

[Blackstick]

For tapping a rectangular low pressure duct with a round branch duct a conical spin in tap is the way to go. Try to avoid a round 90º tap without the conical fitting, you need the evase to ease the air as it turns the corner, the hard 90º edges create a lot of turbulence in the branch which translates to pressure drop. For rectangular low pressure duct taps the 45º entry tap is the most cost effective and most efficient for airflow. SMACNA did some tests and determined the old fashion method of using extractors at the branch take-offs actually have higher pressure drops at the branch and also causes an excessive amount of disturbance to the airflow downstream in the main. This also causes problems if there is another branch take-off close by. Take a look at SMACNA’s HVAC SYSTEMS DUCT DESIGN, chapter 5, DUCT DESIGN FUNDAMENTALS for further information and photos of a duct setup using a clear plastic side, duct tap extractors and smoke to trace the airflow. My copy is the 1990, third edition so a more recent copy of the manual may be slightly different.