Shutter-
A gravity or motor operated shutter open to outside air that opens automatically when theexhaust fan(s) operate and large enough toreplace air removed by the exhaust fan(s)
Forced air-
A duct system open to outside air with a fan thatforces replacement air into the paint area toreplace the air removed by the exhaust fan(s)
Conditioned Forced air-
A duct system open to outside air with a fan andequipment to heat or cool or control humidity orperform any or all three elements in the paintarea. These systems may or may not recirculatethe paint area treated air.