Environmental Properties and Installation Requirements of HEPA Filters

The ability of HEPA (i.e. 'absolute') filters to withstand severe environmental conditions is often critical to reliable operation of a contaminated exhaust system. This paper discusses findings related to the installation, mechanical strength, and the heat, moisture, and corrosion resistance of HEPA filters determined during the course of the filter development program of the USAEC Division of Operational Safety. As a result of these findings, wood- cased filter units are recommended in preference to steel-cased filter units wherever possible, wood-cased filter units should have rabbeted casing joints, not butt joints. The limiting temperature for continuous exposure of HEPA filters to hot air is 400°F, above this temperature the tensile strength of the media drops sharply, to as little as 50% of the room-temperature tensile strength at 500°F. Fibre fracture and fibre loss occur at temperatures in excess of about 550°F. The limiting temperature of the assembled filter element is actually determined by the sealant used between the casing and the frame and, for wood- cased units, the casing material. Plywood is good for continuous exposure at temperatures up to 300°F for periods up to 48 hours, and up to 200°F for indefinite exposure (i.e. weeks or months). Steel is good for any temperature within the useful operating range of the media, although damage may be caused by differential expansion between the casing and the filter pack. Volatilization and reduction in shear strength of adhesives limits continuous operating temperatures to 250-350°F for periods up to 48 hours, and 200-275°F for indefinite exposure (according to adhesive used in manufacture). Steel-cased units with special sealants may withstand substantially higher continuous operating temperatures but in most cases are unable to meet the requirements of Underwriter's Laboratories Standard UL-586 for fire and hot (750°F) air. Resistance of the filter element to flexure, vibration damage, 'working' of casing joints, gasket relaxation, and casing damage under abnormal pressure loadings is in large part a function of the mounting frame or fixture in which the filter is installed. Critical aspects of mounting frame and fixture design are structural rigidity, the method of joining individual frame members and the method of sealing the frame or fixture into the filter housing (all-weld construction and sealing preferred), the tolerances on dimensions, flatness, alignment, and surface finish of gasket seating surfaces, rigidity of filter clamping devices, and uniformity and degree of compression of filter gaskets in the frame or fixture. Recommendations on each of these factors are presented. (author)