Fan and Duct Network

In some instances the fan has a free discharge. Typical is the axial fan installed in a wall opening (wall fan). In most cases the fan is connected to a duct run in this instance the total pressure difference and volume flow are determined from both the fan and duct network characteristics. 

The term Ap = plj T ds is the entropy generation given as pressure loss. 

In a straight duct where Zj = Z2 and the cross-sectional area is uniform, then Cl = C2- This gives Ap = pi -p2 0 P is the duct pressure loss, which is a result of entropy generation. Entropy generation is due to the flow friction, which is the reason for pressure loss. 

Consider the pressure loss in a duct with straight, uniform cross-sectional area. The pressure loss is caused by friction. When different air sheets move against each other, friction is generated. The velocity and thermodynamic properties of air influence the friction. The duct wall has an overall roughness, which causes vortices to be formed with resulting friction in gas. The velocity has a pronounced effect in flow with low velocity, the vortices are small. Eor a straight duct the pressure loss Ap can be determined from 

L is the duct length D is the inner diameter / is the flow friction factor 

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