CBRN filters are designed to filter known chemical and biological warfare agents. The NATO standard No.1 composite filter, which combines UHE (Ultra High Efficiency) elements and charcoal beds in matching forms, is a highly efficient (99.997%) radial-flow unit consisting of the following two elements:
1. Charcoal filter - made from a high carbon content raw material. It is ‘activated’ with chemicals to open up the pores to produce a microscopic sponge-like material with a vast internal surface area. This makes the surface more active and receptive to known chemical warfare agents through a process of adsorption and chemisorbtion.
2. HEPA filters - composed of a mat of randomly arranged fibres. The fibres are typically composed of fibreglass of diameters between 0.5 and 2.0 microns. Key factors affecting function are fibre diameter, filter thickness and face velocity. The air space between HEPA filter fibres is much greater than 0.3 microns. The common assumption, that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through, is incorrect. HEPA filters are designed to target much smaller particles. These particles are trapped (by sticking to a fibre) through a combination of the following three mechanisms:
i. Interception, where particles following a line of flow in the air stream come within one radius of a fibre and adhere to it.
ii. Impaction, where larger particles are unable to avoid fibres by following the curving contours of the air stream and are forced to embed in one of them directly; this effect increases with diminishing fibre separation and higher airflow velocity.
iii. Diffusion, an enhancing mechanism is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 µm in diameter. These are thereby impeded and delayed in their path through the filter; this behaviour is similar to Brownian motion and raises the probability that a particle will be stopped by either of the two mechanisms above; it becomes dominant at lower airflow velocities.
Cyclone filters are often used as a pre-filter to protect and prolong the life of CBRN filters. Inertial separators use cyclonic action to separate dust particles from the air stream. In a typical cyclone, the dust airstream enters at an angle and is spun rapidly. The centrifugal force created by the circular flow throws the dust particles toward the wall of the cyclone. After striking the wall, these particles fall into either a hopper located underneath or are removed by a secondary cyclone scavenge fan. Cyclones can achieve efficiencies as high as 97% when tested using BS 1701 coarse dust.