Correct method for return air sizing???

[bundysnake]

Does anyone know how to calculate return air grille size properly not using a rule of thumb method?

I know how to size duct etc but not return air grille size.

Let's say I have 2 x 18kw fcu's from different manufacturers both with the same airflow but one has a much higher static pressure, disregarding return air noise how does the FCU with the higher static pressure provide me with more flexibility when sizing my return air grille?

Usually I base my return size on an airflow velocity of 1.5m/s, if I increase this velocity at which point will it actually reduce airflow?

If anyone can help explain this I would much appreciate it

[RSTC]

The pressure drop will depend on the grill type, design, etc. You will have to get that from the manufacturer as every one will be different.

If noise is a concern ... what are you looking to be flexible with?

Here is a selection chart for an extract typical grill
http://catalogue.systemair.com/file/...unIGoWvWD.aspx
If you have, say, 200l/s extract then for the 160 the pressure drop is 150pa or the 400 it's 10pa