In order to define the negative pressure at the fan , we can use the following tables:
Equivalent straight length of pipe elbows (bends) Chart for determining the air velocities
The assumed standard diameters, elbow radiuses, and equivalent straight pipe lengths of 90 o elbows (bends) are given in the table below.
Based on the values of the 90 o elbows, it is possible to estimate the equivalent straight pipe length of other angles using the following factors.
Finally, from the Moody chart, see below, we can get the Darcy-Weisbach friction factor f D as...