Correct evaporator air flow... 2 fans or 3 ?

[lana]

Is there a formula for choosing the correct air flow for an evaporator?

This is a late response, But I give it anyway.

For air conditioning application try to keep the air velocity around 500 FPM. If you increase the velocity you will get higher outlet temperature from the coil. If you decrease it you will get lower temp.
For air cooled condenser where noise is not an issue then you can increase the velocity up to 1200FPM.
For blast coolers (dry-coolers with huge fans) the velocity must go higher.
Hope this helps.
Cheers