[flybouy]

Air, like water, electricity, non- motivated people, will take the path of least resistance. So placing a fan in a screen may end up with the air "looping around the sides of the fan. If the fan is placed so the exhaust is directly inline with the fins the air exiting the fan will go across the fins. Trying to pull the air across the fins without a shroud (like the radiator in your truck) may not be very efficient.