Development of filter-free particle filtration unit utilizing condensational growth: With special emphasis on high-concentration of ultrafine particles

The public health impact of particulate matter in ambient air with a size of 2.5 μm or smaller (PM2.5) has been of great concern. It is well known that PM2.5 is much more harmful to human health than coarse particles. To remove PM2.5, most air purifiers on the market have been equipped with a high efficiency particulate air (HEPA) filter. Under the circumstances that generate PM2.5 aerosols at high concentrations such as during indoor cooking or in work places, HEPA-grade filters are neither durable nor applicable because of their high replacement cost. Thus, a large number of cooks and workers are exposed to intensive emissions of PM2.5 without proper filtration. In this paper, we introduce a novel concept to remove PM2.5 without HEPA filters. A key idea is to use the condensational growth of particles. Once the particles have grown to a few micron, they are much easier to remove because of their increased inertia. Based on this, we developed the first prototype of a filter-free particle filtration unit consisting of an air saturator (equipped with water spray nozzles), a condenser in which humid air is cooled down to a supersaturation state and thereby allows particles to grow by condensation, and a multi-nozzle-impactor assembly for collecting the grown particles downstream of the condenser. We started with a small-scale model, and then demonstrated that a large-scale prototype could remove organic, inorganic, and metallic ultrafine particles with a collection efficiency of larger than 80% at a volume flow rate of 50–92 L/s.