Characteristics, Pressure Drop and Capture Efficiency of Heavily Loaded New and Repeatedly Washed HEPA Filters

Abstract

An extensive literature review revealed few published reports on the wet cleaning and reusing industrial HEPA filters, except for those on metallic and glass fiber filters. Accordingly, the effects of particulate loading effect on pressure drop as well as the capture efficiency of new custom fabricated high-efficiency particulate air (HEPA) filters were determined and the findings were compared with those of the same filters after being wet cleaned and reused multiple times. A set of five samples from three different types of HEPA rated filtration media, made of polypropylene (Puritrate®), Teflon and glass fiber filters, were fabricated in cylindrical shape. Each filter was mounted in a specially designed filter testing unit and gradually loaded with airborne particles of cadmium telluride (CdTe) in 10-gram increments up to a total of 100 grams. During the loading, the face velocity of each filter was kept constant at 17. 8 m/s (3500 ft/min). Four filters (two Puritrate^® and two Teflon) were fully loaded 4-10 times and each time they were wet cleaned in dilute (< 4%) nitric acid soaked for 24 hours, rinsed with deionized water and gradually dried at ambient temperature under a laboratory hood until the filter gained its original weight.  The glass fiber was used as a reference medium; it was loaded and tested only once and was not wet cleaned or reused. The pressure drop across all filters (new or reused) increased by cubic model expression as the filters were gradually loaded. Baseline pressure drop on new (unused) filters ranged from 45 Pa (Puritrate^®) to 115 Pa (Teflon).  As the filter-loading progressed, the pressure drop ranged from 146 Pa (Puritrate^®) to 306 Pa (Teflon).  After each wet cleaning and drying cycle, the filters' pressure drop returned almost to their original baselines. All filters, new or reused, performed well, with particulate capture efficiencies exceeding 99.97% at 0.3 µm. The results suggested that certain custom fabricated HEPA filters can be effectively wet cleaned and reused.