Air Flow and Particle Movement into Flanged Circular Local Exhaust Hoods

Current practice for designing and dimensioning local exhaust hoods is largely based on empirical values and rules of thumb. Efforts to model the contaminant movement process have attracted increasing interest in recent years. This kind of modelling work was previously scarcely feasible because it needs large computational resources. However, rapid improvements in the performance and availability of personal computers have made the modelling of contaminant movement a practical reality. Local exhaust hoods are used to protect workers from hazardous materials. Increasing emphasis on health and safety means that hood performance is becoming more important. At the same time economic pressures make over design less acceptable. An accurate model to show if particles are captured should result in better protection and more economic local exhaust hood design. A review of the development of exhaust hood design from the pioneering work of Dalla Valle in (1930) is followed by an examination of the behavior of airborne particles. The objectives of the present study are then defined as the development of computer programs to predict particle trajectories and suitable for use on a personal computer and the validation of these programs by comparison with experimental trajectories. Six models for air flow fields outside exhaust hoods are presented, discussed and evaluated. They describe flow fields outside circular and rectangular flanged exhaust hoods in ideal stagnant air locations. Two models for flow into circular flanged hoods are selected for further work on the basis of probable validity, ease of computation and scope for experimental validation. Cross draughts are identified as having a significant effect on the flow fields. These are incorporated into the models by superimposition (vectorial addition). The motion of particles in the resultant flow fields is then calculated taking into account the effects of gravity, buoyancy, drag force and inertia. The program was written in BASIC and calculates the positions and velocities of a particle at specific time intervals and can be used for testing capture of particles under different conditions. The program is interactive and enables users to define and change conditions. Exhaust hood configuration and flow, particle size, density, initial position and velocity are the variable parameters. Soap bubbles were used to simulate airborne particles in a wind tunnel fitted with a circular exhaust. A combination of constant and stroboscopic lighting enabled bubble trace photographs with time marks to be obtained. This enabled a quantitive comparison to be made between corresponding calculated and experimental trajectories. Measurements from the photographs also allowed individual bubble diameters and densities to be obtained. These were used when predicting the trajectory of that bubble. The major difficulty for designers in using the program developed is in estimating appropriate cross draughts. Further work is suggested in this area while the need for in site testing is emphasized