Energy Efficiency and Degree of Filtration
The degree of air filtration needed is determined primarily by the process that the air stream serves and is typically stipulated by codes or researcher requirements. In a typical laboratory, high-efficiency filtration is not normally required. Filters with 30 percent ASHRAE efficiency (atmospheric dust spot test method) provide adequate filtration for a reasonable first cost if maintenance is provided at appropriate intervals. The type of laboratory isolation required, e.g., hazardous or protective (see Chapter 2), will also determine the degree of filtration necessary. In a laboratory isolated for hazardous research, the exhaust air steam may need to be filtered with High Efficiency Particulate Air (HEPA) and activated-carbon filters. A research laboratory that is protectively isolated may also require HEPA filtration of the supply air, as in the case of a cleanroom. For energy efficiency, the filter system should be "underrated." [McIlvaine, 1992; NAFA Guide..., 1993; Bas, 1995]
Underrating a filter system means passing less air through it than its rated capacity allows, that is, less volume of air per unit time than the clean filter can manage at a specified pressure drop. Because underrating means a lower pressure drop and increased dust holding capacity compared to operation at rated capacity, the filter will have a longer life and a lower energy consumption during its life. The NAFA Guide to Air Filtration (1993) points out that underrating means, "The time required for a pressure drop increase due to captured dust will be extended."