The problem of air intake contamination from nearby exhaust vents has developed increasingimportance in recent years in light of the emphasis now placed on environmental protection andindustrial safety. Also, energy recovery from exhaust air by regenerative heat exchangers ispresently a common practice for medical facilities and, as fuel costs continue to rise, it isprobable that this form of energy recovery will become economically justifiable in othersituations as well. The need for passing both inlet and exhaust air through a single heatexchanger can dictate a shorter separation between inlet and exhaust vent locations, and thisin turn can only serve to aggravate inlet air contamination problems.

Three areas of special concern caused by exhaust gas recirculation are health hazards, undesirable odors, and accelerated corrosion and degradation.

The present practice in estimating dispersion near buildings is to measure experimentallythe dilution of tracer gases around a few typical building configurations under controlled windtunnel conditions, and to apply the general physical principles of turbulent mixing togeneralize the results and extrapolate them to other situations of interest. The purpose ofthis paper is to present the results of such a generalized experimental program in a formwhich will allow a designer to determine the concentration on a building surface from a nearbyroof exhaust vent. With this information, the designer will be able to estimate if a problemdoes indeed exist and, if so, to adjust either the allowable emission rate from the vent orits location in such a way as to reduce vent gas concentration at the inlet to acceptablelevels.

One important problem which this paper does not discuss is how the characteristics ofthe vent itself can be modified, for example by a change in height of the vent stack or exitvelocity, to alter the concentrations at a receptor point on the buildigg. Some aspects ofthis vent problem are discussed by Halitsky and Clarke, and by Wilson in the final reportof ASHRAE RP-136, of which the data presented here form a part.