| Popis: |
A typical ventilation exhaust stack has been studied for gas mixing and uniformity of flow. A test gas, sulfur hexafluoride (SF6), was introduced into a rectangular duct entering the 30.5-cm (12 inch) interior diameter stack at 45°. Cross sectional air samples were drawn through a multipoint sampling probe from two perpendicular directions at three stack levels. A photoacoustic infrared spectrometer measured SF6 concentrations. Gas distributions were quite nonuniform at 1.5 stack diameters from the last disturbance. When introduced at duct center, the gas was well mixed (relative standard deviation ≤0.05) by 10 stack diameters from the last disturbance. This agrees with Environmental Protection Agency and American National Standards Institute “rules of thumb.” Comparisons with another experimental study using SF6 indicate that the angle of entry of the air into a stack or duct determines the distance required for mixing. Mixing was reduced at off-center injection, but apparently unchanged at halved air flow. |
