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The Recirculation of Industrial Exhaust Air

The three tests are not directly comparable; however, Figure 4-17 shows the general relationship. Table 4-6 compares several important characteristics of commonly used air filters. Considerable life extension of an expensive final filter can be obtained by the use of one or more cheaper, less efficient, prefilters. For example, the life of a HEPA filter can be increased 25% with a throwaway prefilter. If the throwaway filter is followed by a 90% efficient extended surface filter, the life ofthe HEP A filter can be extended nearly 900%. Th is concept of “progressive filtration” allows the final filters in clean rooms to remain in place for ten years or more. The European Committee on the Construction of Air Handling Equipment has developed a method for testing air filters in general ventilation. Although their method, called Eurovent 4/5, is based directly on ASHRAE Standard 52-76, some wording and definitions have been amended to suit the needs of Eurovent.