Thermal Precipitators

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. 

Also, the figures contain nephelometric curves for the product t (that is, for p + s, in the proportions formed in the synthesis) and the fraction o, which was also a thermally precipitating product and deposited onto the walls of the reaction vessel in the course of the copolymerization of NVC1 with NVIAz at their initial molar ratio of 85 15 (Table 1). One can see that the precipitation behaviour of the total product t differs, although the amount of the s-fraction is almost the same at 31-33%. Obviously, this depends on the properties of the s-fraction. For instance, the heat-induced precipitation of the sample t formed from the feed with a comonomer molar ratio of 90 10 (Fig. 3b) is suppressed by the presence of its own s-fraction to a markedly lesser extent when compared to the product t obtained at the comonomer molar ratio of 85 15 (Fig. 3a). Most likely, such differences reflect the divergent influence of the s-fractions on the coagulation processes in the thermo-precipitating fractions of the total product t. These differences, for example different surface... 

Table 2 Molecular parameters of the thermally precipitating and non-precipitating fractions of poly(NVCl-co-NVIAz) in aqueous solutions at 20 and 50 °C (the data from [42])... 

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. 

Burdett G. 1998. A comparison of historic asbestos measurements using a thermal precipitator with the membrane filter-phase contrast microscopy method. Arm Occup Hyg 42 21-31. 

The earliest of these graticules by Patterson and Cawood has 10 globes and circles ranging in diameter from 0.6 to 2.5 pm when used with a +2 mm lOOx objective-eyepiece combination and is suitable for thermal precipitator work [69]. 

There are a number of different techniques belonging to the category of phase inversion solvent evaporation, precipitation by controlled evaporation, precipitation from the vapor phase, thermal precipitation, and immersion precipitation (13,34—36). The most commercially available membranes are prepared by the last method. 

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