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Will you help my question on fan sizing?

Will you help my question on fan sizing?

Will you help my question on fan sizing?

(OP)
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.

RE: Will you help my question on fan sizing?

Cost factor.

RE: Will you help my question on fan sizing?

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.

RE: Will you help my question on fan sizing?

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.

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