This page calculates the head loss coefficient and head loss of a fluid in across angled lovers inside a pipe.
The head loss hL can be calculated as:
hL = kloss x (um2 / (2 x g) )
Where kloss is the head loss coefficient, um is the flow velocity in the pipe, and g is the gravitational acceleration. kloss is dependant on the Reynolds Number Re. If Re is smaller than or equal to 50, then (1 - 50 / Re) is taken to be zero:
kloss = + (1 - 50 / Re) kqu
Where kqu is a function of the angle and the number of louvers based on a table of values (Ref), and Re is the Reynolds Number and is calculated as:
Re = um x Dh / v
Where v is the kinematic viscosity of air and Dh is the hydraulic diameter of the tube. A tube of square cross section has a hydraulic diameter equal to the width of the tube.
Idel'chik, I E. and Fried, Erwin., Flow Resistance, a Design Guide for Engineers, Taylor & Francis, Washington D.C. 1989. p 306.