Particle thermal precipitation

Other lesser mechanisms that result in aerosol removal by filters are (1) gravitational settling due to the difference in mass of the aerosol and the carrying gas, (2) thermal precipitation due to the temperature gradient between a hot gas stream and the cooler filter medium which causes the particles to be bombarded more vigorously by the gas molecules on the side away from the filter element, and (3) Brownian deposition as the particles are bombarded with gas molecules that may cause enough movement to permit the aerosol to come in contact with the filter element. Browruan motion may also cause some of the particles to miss the filter element because they are moved away from it as they pass by. For practical purposes, only the three mechanisms shown in Fig. 29-1 are normally considered for removal of aerosols from a gas stream. 

Collection of particles is based on filtration, gravitational and centrifugal sedimentation, inertial impaction and impingement, diffusion, interception, or electrostatic or thermal precipitation (e.g., see Spurny, 1986, Chapter 3). The choice of method depends on a number of parameters such as the composition and size of the particles, the purpose of the sample, and acceptable sampling rates. Table 11.10 summarizes some of the commonly used methods and the size ranges over which they are effective. 

THERMAL PRECIPITATION. Sinclair (13D, Chap. 8) describes an apparatus developed to test the theory of thermal precipitation. An aerosol particle will move in a temperature gradient from a hot body toward a colder body with a velocity proportional to the temperature gradient. 

Thus, at any instant, the gas should contain several different generations of particles, each of a characteristic size. Microscopic observations of aerosol samples deposited by thermal precipitation indicate that this is the case. 

The limit to the effectiveness of a gas cleaning device is its efficiency ip the removal of aerosols. Neither electrical nor thermal precipitation has proven practical for the rapid removal of aerosols, and filtration seems to be the best method of removal. Aerosol filters consist of loosely aggregated fibers, and in order to avoid excessive resistance to flow of gas, the mesh of the fiber must be larger than the size of the particle to be removed. There is therefore no screening action the removal of the particle depends entirely upon a chance collision of the particle with a fiber of the filter. Once having collided, the particle adheres by adsorptive forces. 

The smaller aerosol particles can be captured from the air for subsequent counting and size measurement by means of so-called thermal precipitators. In these instruments, metal wires are heated to produce a temperature gradient. Aerosol particles move away from the wire in the direction of a cold surface, since the impact of more energetic gas molecules from the heated side gives them a net motion in that direction. The particles captured are studied with an electron microscope. Another possible way to measure Aitken particles is by charging them electrically under well-defined conditions. The charged particles are passed through an electric field and are captured as a result of their electrical mobility (see equation [4.6]). Since size and electrical mobility are related, the size distribution of particles can be deduced. These devices are called electrical mobility analyzers. 

More sophisticated collectors include electrostatic precipitators and thermal precipitators, which will collect particles down to 0.001 to 0.01 /tm. 

Particle velocity, flow velocity, and configuration are used to optimize the design geometry of thermal precipitators (14, 15). 

Thermal Precipitation. Particles in a thermal gradient medium move in the direction of negative gradient, that is, from hotter to colder regions (7, 8). Based on this principle, the instrument typically consists of two parallel round microscopic plates and a heated wire in between as shown in Figure 36. The sample is drawn between the plates, and... 

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