System Effect in Effect After a neighbor complained about the noise generated by this backward-inclined fan, which exhausted fumes from a tank at a rate of 50,000 cu ft (3,750 lb) per minute, the owner sought to redirect the noise by reversing the flow of air so that the air came back toward the fan. It was a self-defeating proposition: The air turbulence caused by the 180-degree turn at the outlet and the speed at which the fan had to run to overcome the system effect actually increased the noise. A fan and collector purchased at a salvage auction. The owner used a 50-gal. drum This process fan exhausting to as part of the duct system atmosphere has to overcome between the fan and collec- system effect resulting from the tor. In addition to a base that use of a cutoff sheet in con- is unstable, there is no length junction with a rectangular duct of ductwork at the fan outlet and a 90-degree-elbow turn to establish uniform airflow. without any run of ductwork. Instead of a rectangular-to-round transition from Pressure blowers used in a combustion-air sys- the fan inlet, a transition plate is being used in tem. The 90-degree elbows are turning air in the this application, resulting in the bottom-loading opposite direction it exits the fans. The system of air on the fan wheel. effect could have been avoided with upblast fans. w w w. a m c a .o r g 2020 A M CA i n m o t i o n 17http://www.amca.org