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Dear Hasan,
I know that you know how to calculate your project based on each method. Then I would not going through calculations and want just directly answer your question:
Frankly, it is an engineering trade off :) when:
1- If there is considerable difference among lengths of ducts in different branches, and the initial costs are important for the owner of project, then you may use "Equal Friction Method"
2- Please note that in "Static Regain Method" calculations are a bit complicated and sizes of ducts will be greater than the previous method. Additionally the initial costs will be more respectively. However system will be balanced easier and the operation costs will be lower.
Regards
1-equal friction (very common )
we have the CFM for the AC , and from SMACNA get the air velocity in (fpm) according to the usage of the area
then with ( the CFM and the velocity ) we go to mody chart and get the friction
by this fixed friction from the chart and the CFM we can determine the size for every duct box.
2-equal velocity :
from SMACNA get the velocity
with the velocity and the CFM we determine the duct size by ASHRAE table or ductsizer software
3-Trunk duct design/ static regain
is very popular with open plan offices, factories, large chillers and freezer stores, basically the trunk duct is selected for minimal duct velocity, and each take-off or outlet supply is regulated at take-off point with a damper.
