CHARACTERIZATION OF GAS MIXING IN AN EXHAUST STACK

A typical ventilation exhaust stack has been studied for gas mixing an d uniformity of flow. A rest gas, sulfur hexafluoride (SF6), was intro duced into a rectangular duct entering the 30.5-cm (12 inch) interior diameter stack at 45 degrees. Cross sectional air samples were drawn t hrough a multipoint sampling probe from two perpendicular directions a t three stack levels. A photoacoustic infrared spectrometer measured S F, concentrations. Gas distributions were quite nonuniform at 1.5 stac k diameters from the last disturbance. When introduced at duct center, the gas,vas well mixed (relative standard deviation less than or equa l to 0.05) by 10 stack diameters from the last disturbance. This agree s with Environmental Protection Agency and American National Standards Institute ''rules of thumb.'' Comparisons with another experimental s tudy using SF6 indicate that the angle of entry of the air into a stac k or duct determines the distance required for mixing. Miring was redu ced at off-center injection, bur apparently unchanged at halved airflo w.