Passive filtration of airborne particles from buildings ventilated by natural convection: Design procedures and a case study at the Buddhist cave temples at Yungang, China

Air exchange between interior spaces and the outdoor atmosphere can occur d ue to a variety of processes, including wind-driven hows and natural connec tion-driven flows. As air is exchanged with the outdoors, airborne particle s can be brought inside. Depending on the use of the indoor space, the pres ence of particles in indoor air could be a nuisance to the occupants or cou ld be damaging to materials kept indoors. While one obvious solution to suc h problems is to install a mechanical air filtration system, that is not al ways practical. In particular the character of some historical houses and s ome archaeological sites would be degraded by the presence of a mechanical air distribution system, and in some parts of the world the reliable electr ical power supply needed for such a filtration system may not be available. In the present paper we consider principles for the design of passive filtr ation systems in which air motion through the filter material is induced by a natural convection flow rather than by a mechanical fan. A fluid mechani cal model first is described for predicting the air flow through an interio r space that acts as a thermal siphon. The effect of placing filter materia l in the path of such air flows is examined nest. The indoor-outdoor air qu ality model of Nazaroff and Cass (1989a) is matched to the natural connecti on air exchange model, and calculations are performed to determine the rela tionship between the outdoor particle size distribution and indoor particle size distributions and particle deposition rates given a passive filtratio n system. Example calculations are worked for the case of a passive particle filtrati on system that could be installed to protect the interior of the Buddhist c ave temples at Yungang, China. These are a collection of manmade cave templ es dating from the 5th century AD, now situated in the middle of one of Chi na's largest coal-mining regions with its accompanying air pollution proble ms.