A test-based method for estimating the service life of adsorptive portable air cleaners in removing indoor formaldehyde

Portable air cleaners (PACs) with activated carbon filters are frequently used to remove gaseous contaminants. However, activated carbon has a limited adsorption capacity for small polar molecules, such as formaldehyde. Formaldehyde is a ubiquitous air contaminant with hazardous effects to human health. Thus, characterizing carbon filter performance decay and being able to predict the service life of PACs is of high importance. This study proposes a testing method which enables the continuous measurement of the clean air delivery rate (CADR). Vaporized formaldehyde was injected at a constant rate, i.e. 14 mg/h or 45 mg/h, into an air-tight chamber where the PAC was operating. The CADR was calculated from the steady-state concentration in the chamber according to the mass-balance equation. An adsorptive PAC with a pleated composite filter of activated carbon and high efficiency particulate air (HEPA) was tested until the CADR decreased to ca. 30% of its initial value. The functional relationship between the CADR and the cumulative adsorbed mass (CAM) of formaldehyde was established. Then, formaldehyde emission rates and ventilation rates in real Chinese residences were estimated by literature research. The obtained data were used in simulation to investigate PAC service life under a range of different situations. Test results showed that the CADR tended to decay exponentially with the CAM. According to the simulation, the CADR declined by 90% after a duration of 130-190 h under the most strenuous condition. However, PAC service life could be prolonged to more than 2000 h under realistic usage conditions.