McIlvaine et al. (1992) points out that:
Most HEPA filters are constructed so that the filter paper is arranged in a large number of parallel pleats. The pleats are very narrow and very deep. A typical 2 ft by 4 ft by 6 in. HEPA filter contains 140 to 190 such pleats. These pleats act to straighten the direction of gas as it flows through the filter. In addition, the resistance of HEPA filter paper to gas flow is reasonably uniform, which means that approximately equal volumes of gas move simultaneously through each pleat of the filter. These two factors, the uniform resistance of the filter paper and the large number of parallel pleats, cause the air to flow uniformly 12 in. downstream of the HEPA filter face. It is this uniformity of flow that initiates the phenomenon of laminar flow, causing small particles to be dragged along in the path of the flow. The use of a HEPA filter does not guarantee laminar flow. Some HEPA filters are constructed with V-banks of pleated media, and filters of this type are not intended for laminar flow applications. Incorporation of adsorption capabilities into HEPA filters is also accomplished during the pleating process. The media impregnated with carbon is pleated along with the microfiberglass media.
