Determine the per cent free area for the actual louvre being specified (and supplied). Generally, most louvres have a maximum of 50 % free area. Be cautious about controlling outdoor air quantity to maintain a CO2 level. It is better to define a specific amount of O.A. as minimum and have the system adjusted to provide the minimum. Always ensure that each and every occupied area will have at least the minimum ASHRAE rates of outdoor air ventilation (e.g. 20 CFM per person) under all operating conditions
For an approximate maximum flow, I use 50% of the nominal size to get free area then size the intake at 650 fpm to achieve max. flow. If you need the exact number, you need the manufactures tables based upon the exact louver and size. Free area vs. size, DP and water carrying velocity varies greatly with louver selection.
Noise level is another factor. Longer blade spans might be inclined to buzz at higher velocities; noise would be appreciably higher; pressure drop starting to get to the steep part of the curve; more rain ingress for intakes.
DrRTU clearly indicated 650fpm (3m/s) above as a max, i'd add that i would be more inclined to design at say 500 fpm (2.5m/s) as the limit for airflow.
