The DBF 4XL features our FX4XL fan with a housing made from galvanized steel, suitable for most commercial applications | The fans can handle lengths of duct up to 60 feet and six elbows | The backward inclined blades allow lint to pass through the fan | The patented control automatically operates the booster fan whenever the dryer is running | When the dryer turns on, the fan is activated by the automatic presure sensitive switch so warm, moist air is exhausted out of the house quickly | Suitable for dryer runs with up to a maximum 60 linear feet of 4" rigid duct with six elbows