Energy and Built Environment (Jan 2021)
Experimental characterization of the removal efficiency and energy effectiveness of central air cleaners
Abstract
This study assessed six commercially available in-duct air cleaning devices which are designed to be mounted in the central ventilation system of offices or commercial buildings. The selected devices use different air cleaning technologies: mechanical filtration, electrostatic precipitation, gas filtration, ionization / cold plasma, photocatalytic oxidation (PCO) and catalysis under UV light. They were tested against particles, a mixture of volatile organic compounds containing acetone, acetaldehyde, toluene, heptane and formaldehyde, and two bio-contaminants: Aspergillus brasiliensis (fungus) and Staphylococcus epidermidis (bacteria).Two different test rigs were used. The single pass efficiency of each device was determined for three airflow rates, corresponding to face velocities ranging from 0.9 to 2.7 m/s, and two sets of temperature and humidity that are representative of indoor air conditions in wintertime and summertime. The concentration of the challenge volatile organic compounds was also varied in the 30 to 100 µg/m3 range as a way to characterize their influence on efficiency at realistic concentration levels for non-industrial buildings. Measurements of ozone and formaldehyde concentration downstream of the air cleaners were carried out to determine the emission rate of by-products into the air stream. Finally, the energy issue was addressed by measuring the electric power drawn and pressure loss of the devices.The results showed that two devices, namely a radiant catalytic ionizer and a plasma ionizer, had a very low single pass efficiency against all the challenge pollutants. The association of the plasma ionizer and the electrostatic precipitator did not produce a synergetic effect between the two technologies either, contrary to what their manufacturer claims. Finally, three of the six devices tested were effective in terms of pollutant removal, but only two had an acceptable energy effectiveness in view of their use in low or zero energy buildings. Their energy effectiveness ranged from a few thousand m3/kWh for VOCs at the highest airflow rate (3600 m3/h), to more than 60 000 m3/kWh for particles and bio-contaminants at 1200 or 1600 m3/h. These results are at least one order of magnitude higher than the majority of stand-alone air cleaners. Moreover, they suggest that optimal IAQ and energy conditions can be achieved if variable air volume control methods are used to maintain indoor temperature and humidity.
