Will you help my question on fan sizing?

[ballsonwalls]

I'm new to the industry and I'm a bit confused on sizing supply and return fans.

It seems that supply fans are sized for the external static pressure losses at the outlet. However, a return fan is sized for the static pressure losses at the inlet. Why is this theoretically the case? How does this differ if you have a draw through as opposed to blow through system?

THANK YOU.

 

[quark]

Cost factor.

 

[willard3]

Draw-through or blow-through coils each have their proponents....I prefer draw-through for better flow distribution due to less kinetic effects from high local velocities.

 

[lilliput1]

The worst case duty for the supply fan is to draw in 100% OA (during economizer cycle) through the OA intake and discharge the air to the most remote/most friction loss supply duct run.

The worst case return fan duty is to pull in return through the most remote/most friction loss return duct run and discharge the air 100% to the relief air discharge during economizer cycle.