Velocity at air terminal

Greetings,

I would be so greatful if you can advice me on the velocity issue at a air terminal.
I have one exhaust system for the pump room. The fan is 20000 CMH @ 300 pa. The duct is 1200x620 and it got 3 wiremesh grille (700x700). I am worry that the last grille cant extract to the desire pump dissipated heat cos we cant extend the duct anymore.
So kindly advise me the mathematical formula that is related b/w flow,velocity and head.
Is (p = 0.0981 h SG ) can use in this application? If so, the pressure is the fan total pressure -(friction+fitting loss).
Thanks for your time.

5.56 cubic metres per second and face velocity at intake at 11.33 Ln metres per second, will not work. Fan will cavitate and be extremely noisy. Larger discharge duct and bigger intake grille required

Hi Magoo,

Thanks for your advise.
The fan is at the end in the fan house, which got the louver @ 1050x1000.
The wire mesh (700x700) with 3 nos. So I believe that will not be 11.337 m/sec.
Thanks for your time

Now you mention a design difference.

Sorry Magoo for my poor language skills and presentation.

Get the fan suppliers fan curve graph chart, establish the total fan static with a pitot tube or differential gauge and read off the chart. Then establish actual volume flow. Google engineering toolbox for all formulas and calculations