The fan velocity pressure, [P.sub.vf], is the dynamic pressure based on the average velocity at the fan discharge.
For each candidate fan, the fan velocity pressure would be calculated from the required flow rate and fan outlet area.
([parallel]) The resulting pressure drop, for a large plenum chamber, is identical to the fan velocity pressure. Therefore, the correct fan total pressure for selection purposes must be the system pressure drop up to the plenum, plus the fan velocity pressure.
Of course the fan velocity pressure is not known in advance because we have not selected the fan yet.