The biggest advantage to recirculation is energy savings attained by re-heating/cooling/humidifing already conditioned air. This savings can be as much as 50% depending on your system and usage.
The air for recirculation can be obtained from the spray area exhaust or the spray area itself.
Recirculation can not be used during spraying as the overspray could cause fires, explosions and caking.
Air make up systems can be ordered from the manufacturer with recirculation as an option and will come with all the controls and equipment needed to operate.
Ducting will have be designed to accomodate recirculation.