Model development of area capture efficiency for flanged slot hoods.

摘要

Capture efficiency was measured in front of local exhaust hoods with different slot sizes under uniform crossdraft conditions in a wind tunnel. Sulfur hexafluoride (SF6) was released at a point in front of the hood and the concentration of SF6 was measured in the duct downstream of the hood. Capture efficiency was calculated by dividing the duct concentration when the source was placed at the point of interest by the duct concentration when the source was placed in the hood. For each experiment, capture efficiency was measured as the source was moved away from the hood face while holding all other variables (face velocity (FV), crossdraft velocity (CV), hood length (HL), and hood width (HW)) constant. Whenever these four conditions were held constant the contour of efficiency can be defined, e.g. the function, Efficiency = f(X/HL, Z/HL, CV/FV, HW/HL), can be defined.; Statistical methods were used to fit this function, they were: collinearity diagnostics, correlation analysis, the Guerrero and Johnson transformation of the dependent variable, and multiple linear regression.; The capture efficiency of an area source (usually an open-top-tank) was determined by integrating the statistical function developed in the laboratory experiments over the source area. All other variables in the model are treated as constants when the ventilation system and process are defined.; The final model was validated using capture efficiencies, of local exhaust hoods used to exhaust vapor degreasers, measured under operating conditions. The model predicted capture efficiency agreed well with field measured values, especially considering the wide variation of conditions found under operating conditions.