Cost and waste volume reduction in HEPA filter trains by effective pre-filtration

Data published elsewhere (Moore, et el 1992; Bergman et al 1997) suggests that the then costs of disposable type Glass Fibre HEPA filtration trains to the DOE was USD 55 million per year (based on an average usage of HEPA panels of 11,748 pieces per year between 1987 and 1990), USD 50 million of which was attributable to installation, testing, removal and disposal - although the life cycle costs are themselves based on estimates dating from 1987-1990. The same authors suggest that by 1995 the number of HEPA panels being used had dropped to an estimated 4000 pieces per year due to the ending of the Cold War. The yearly cost to the DOE of 4000 units per year was estimated to be USD 29.5 million using the same parameters that suggested the previously stated USD 55 million for the larger quantity. Within that cost estimate, USD 300 was the value given to the filter and USD 4,450 was given to peripheral activity per filter. Clearly, if the USD 4,450 component could be reduced, tremendous saving could result, in addition to a significant reduction in the legacy burden of waste volumes. This same cost is applied to both the 11,748 and 4000 usage figures. The work up to now has focussed on the development of a low cost, long life (cleanable) direct replacement of the traditional filter train, but this paper will review an alternative strategy, that of preventing the contaminating dust from reaching and blinding the HEPA filters, and thereby removing the need to replace them. What has become clear is that 'low cost' and 'stainless HEPA' are not compatible terms. The original Bergman et al work suggested that 1000 ft3/min stainless HEPAs could be commercially available for USD 5000 each after development (although the USD 70,000 development unit may be somewhat exaggerated - the authors have estimated that development units able to be retro-fitted into strengthened standard housings would be available for perhaps USD 30,000). The likely true cost of such an item produced industrially may be closer to USD 15,000 each. That being the case, the economics for replacing glass fibre HEPAs by a metallic, cleanable alternative are unjustifiable except on ethical grounds. By proposing the protection of the traditional Glass Fibre HEPA from its blinding contamination, a means is presented to reduce their life costs and reduce ultimate waste volumes. An examination of the case for self-cleaning HEPA protection also suggests that, even when the mechanical life limit of the HEPA train is reached, the degree of contamination could be reduced to such an extent that its means/classification of final disposal may be modified to further reduce cost. Pulsed jet filtration using metallic filter media is a practical and industrially proven means by which solids can be prevented from reaching the HEPA train and returned to the operator for disposal, whilst not interrupting the process flow through the system. Field experience and data to prove the contention is available. There are clearly benefits with regard to disposal in returning to the user the small quantities of dust that would otherwise lead to the contamination and blinding of the large volume of the filter train. A cost benefit analysis shows that this radical solution to HEPA cost amelioration can work. Presenting a review of the technology and its application to other areas illustrates that where gross dust removal or recovery is necessary, or where extreme conditions make traditional HEPA technologies impractical, metallic filtration systems can (and do) also offer economic and industrially real solutions. (authors)