Exhaust duct riser takeoff - high pressure drop

[jrrocks]

In 10 floor building, a common exhaust duct riser serve each floor pantry. At 10th floor, main riser flow 400 l/s (350 x 250mm) and brach to 10th floor has flow of 40 l/s (150 x 100 mm). When putting this values in ashrae duct fitting (take-off), i am getting more than -50 pascal at brach and main 3 pascal. Does the takeoff pressure drop can go more than 50? For such duct connections?

 

[GT-EGR]

That pressure drop is definitely possible - if you are putting in the correct information into the correct fitting type I wouldn’t argue with the answer since they are the experts beyond any opinions you get here.

Either play with the fitting type in ashrae until you get a pressure you like, or size your fan accordingly.

 

[willard3]

SMACNA also has tables for pressure drop calculations.