Extract from
The American Society Of Mechanical Engineers

www.asme.org

COLLECTION METHODS
The three major mist elimination methods are impaction, interception and Brownian movement.

Impaction depends on the particle having a high enough kinetic energy (a combination of particle velocity and mass) to allow the particles to leave their own slip stream, move across other flow stream lines, and impact against a target where it can coalesce with other droplets already collected at the target. Surface tension maintains the droplet on the target where it continues to coalesce with other droplets until it is sufficiently large for other forces (such as gravity or vapor shear) to overcome the surface tension and displace it.

Interception occurs when the deflected vapor stream passes the target within a distance less than the radius of the particle.

Collection with Brownian movement depends on the natural random movement of molecule-size particles. These randomly-moving particles are deflected and coalesced as they collide with targets and other droplets. As a mist passes through a bed of targets, the statistical probability of a particle coalescing on the targets can be predicted based on the nature of the bed of targets and its thickness, the particle sizes, and the time the vapor is in the bed. These calculated statistical probabilities have been verified by laboratory and field measurements. With the Brownian movement mechanism, collection efficiency is enhanced with a slow superficial velocity trough the bed; whereas in the impaction method described above, removal efficiency is enhanced with a higher superficial velocity. Attempts to collect submicron mist particles predictably and efficiently using internal separation mist eliminators (mesh, zigzag baffle, and centrifugal cyclone separators) have been unsuccessful. The critical and practical limitations of velocity, differential pressure, and available kinetic energy required prevents achieving the high (99+ %) removal efficiency normally required in LOV applications.

All three mist removal mechanisms have inherent limitations on the maximum velocity that can be handled without allowing reentrainment. Interception and impaction devices also have turn down limitations, such that collection efficiency decreases significantly if the air velocity is reduced.

However, the Brownian diffusion devices have no turn down limitation. A slower superficial velocity gives the particles more time within the bed which results in a higher efficiency of collection and coalescing of the oil mist.

Cartridge filters and electrostatic precipitators (ESP) have been successfully used to remove submicron particles. Both, however, have onerous maintenance requirements.

Cartridge filters can be successfully interrupt and collect the submicron particles. Prior to becoming saturated with oil, cartridge filters can successfully eliminate the submicron mist from the vapor. For continued efficiency, however, the filters must be replaced before they get saturated with oil. The time period for saturation is fre-quently quite short. If a saturated filter is not replaced, a portion of the removed mist will be reentrained into the exhausted vapor, reducing the removal efficiency significantly.

Abstract  |  Particle Size  |  Collection Methods  |  Fiber Beds  |  Collection Efficiency  |  Benefits
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