how to define pressure drop with CFX post
 

Dear all, i have a very simple question. I would define with CFX post the pressure drop of an rotating installation. An centrifugal fan are going to be connected to this sytem to cool it. I would want to know what are going to be the operating point to choose the "good" fan. I modeled the system with a fan (not very adapted) but i don't know where i have to consider pressure difference. Also, what are the best practices to define (with cfx post) the pressure drop of the system ( plane coulored by pressure, iso volume of pressure etc...) . Please could you help me

Thanks