Particulate Collection Theory

If the gas flow through the precipitator corresponds to viscous flow, near 100% collection efficiency can be achieved in a precipitator of finite length. Unfortunately, this does not happen because the gas is in turbulent flow. As a result, the collection efficiency is an exponential term influenced by several variables. The following is a general collection efficiency relationship for ESPs based upon the Deutsch-Anderson equation [4]  

A = collecting electrodes cross-sectional area w = particle drift velocity Q = gas flow rate 

This equation shows that for a given collection efficiency, the precipitator size is inversely proportional to particle drift velocity and directly proportional to gas flow rate. Increasing the gas density (migration velocity is a function of gas viscosity) by reducing its temperature or increasing the pressure will reduce the precipitator size. However, theory does not account for gas velocity. This is a variable that influences particle re-entrainment and the drift velocity. This typically requires an ESP design at lower velocities than predicted in theory. 

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Contact power theory is an empirical approach relating particulate collection efficiency and pressure drop in wet scrubber systems. The concept is an outgrowth of the observation that particulate collection efficiency in spray-type scrubbers is mainly determined by pressure drop for the gas plus any power expended in atomizing the liquid. Contact power theory assumes that the particulate collection efficiency in a scrubber is solely a fimction of the total power loss for the unit. The total power loss, is assumed to be composed of two parts the power loss of the gas passing through the scrubber, q, and the power loss of the spray liquid during atomization, The gas term can be estimated by... 

An ESP is a particulate control device that uses electrical forces to move particles entrained within an exhaust stream onto collection surfaces. The basic theory has already been described under dry ESPs, but a brief summary here is included, with... 

The installation of a venturi scrubber is proposed to reduce the discharge of particulates from an open-hearth steel ftimace operation. Preliminary design information suggests water and gas pressure drops across the rubber of S.Opsia and 36.0 in. of H2O, respectively. A liquid-to-gas ratio of 6.0gpm/1000acfm is usually employed with this industry. Estimate the collection efficiency of the proposed venturi scrubber. Assume contact power theory to apply with a and P given by 1.26 and 0.57, respectively. Recalculate the collection efficiency if the power requirement on the liquid side is neglected. 

The two-fluid granular flow model is formulated applying the classical Eulerian continuum concept for the continuous phase, while the governing equations of the particle phase are developed in accordance with the principles of kinetic theory. In this theory it is postulated that the particulate system can be represented considering a collection of identical, smooth, rigid spheres, adapting a Boltzmann type of equation. This microscopic balance describes the rate of change of the distribution function with respect to position and time. 

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