Comparison of Three Dimensional Velocity Models for Flanged Rectangular Hoods

The purpose of a local exhaust hood is to capture an airborne contaminant as close to the point of generation as possible and prevent it from entering the worker's breathing zone. Presently, local exhaust hood designs are based on capture velocity and practical experience. This process involves several defiencies, the most significant being the inability to determine the contaminant concentration in the worker's breathing zone as well as the inability to incorporate crossdrafts and source momentum characteristics. Three-dimensional velocity field models which predict the magnitude and direction of the velocity vector at any point in the field of flanged, rectangular exhaust hoods have been developed recently. This is a first step in determining the contaminant concentration in the worker's breathing zone and can, by vector addition, predict how crossdrafts and source momentum will effect contaminant capture. The purpose is to determine which of the available velocity field equations is the most accurate predictor of the velocity vector. This should help to determine optimal hood size, position and flow, resulting in more efficient and economical exhaust designs for industry. The theoretical basis for models in potential theory is presented and the limitations for industrial settings are discussed. The accuracy of the models was evaluated by measuring two-dimensional velocity vectors in the x-z and y-z planes of local exhaust hoods having aspect ratios of 1, 2, and 5. Measurements were made from 5% to 80% of the maximum hoodface velocity with an anemometer modified to permit rotation about its axis. The X, Y, and Z velocity components of each point were statistically compared to calcualted data. The ability to implement the models in computer programs is also discussed. Much work remains. The analysis can be expanded over a larger range of hood aspect ratios, and known crossdraft and source momentum vectors can be included.Master of Science in Public Healt