Assessing intake contamination from atmospheric dispersion of building exhaust.

The possibility of unacceptable internal air pollution levels can cause concern at the design stage given the potential for cross contamination between building exhausts and ventilation intakes is there. The complexity of airflows around buildings, however, makes it extremely difficult to predict the contamination levels at the intake locations. This paper reports a wind tunnel technique using a model of a proposed building to determine the pollutant levels expected at various inlet locations due to the re-ingestion of noxious emissions from its two stacks. Tests were carried out in the BRE environmental wind tunnel on a 1 in 200 scale model of the proposed building with the approach wind simulated to correspond to the flow over a suburban terrain. Two tracer gases, sulphur hexafluoride and nitrous oxide, were injected separately, and at known concentrations, from the stacks at an efflux velocity corresponding directly to that required at full scale. Tests were carried out over a range of wind directions and speeds expected to occur for over 95% of the time. Air samples were taken at various locations on the model surface through brass tubes fitted from the inside. The concentration of the sampled air was measured using infrared gas analysers and the results presented as pollutant fractions in grams of pollutant measured to a kilogram of emitted pollutant. Comparison of the measured maximum concentration levels with those predicted from an ASHRAE procedure showed in general good agreement. However, the wind tunnel test procedure was able to provide detailed information on the contaminant levels that would be expected at these intake locations for the range of wind speeds and directions anticipated at the site.