Air filter performance under variable air volume conditions.

Samples of 31 different air filters, representative of the designs and efficiency range of those used in general ventilation, were tested at constant airflow. The complete array of ASHRAE Standard 52.1 tests was run on these filters, plus particle-size efficiency tests and tests measuring the reentrainment of dust from the filters when subjected to ultra-clean air flow. Additional samples of the same filters were then subjected to the same test array, except that VAV flow was simulated. No significant difference was noted between the results using constant flow and VAV flow; low efficiency filters did, however, show substantial reentrainment for both cases. Additional studies of the filter media resistance showed that media resistance as a function of media airflow velocity can be predicted from media geometry more accurately than by existing theories if media compression under airflow is taken into account. Expressions for overall filter resistance as a function of filter flow and means for obtaining their coefficients are given.