Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 92

The coarse filter is used to remove larger particles from the sample gas. Because the majority of gas analyzers require almost complete removal of particles larger than 0.5 μm, additional filtration is necessary. This is accomplished by incorporating a fine filter before the analyzer inlet. The location of the fine filter is an important consideration in the CEM system design and is dependent upon the susceptibility of the other system components to the effects of fine particles. There are two types of fine filters: (i) surface filters and (ii) depth filters.

A surface filter can be simply a filter paper that excludes particles of a certain size. The filter material is porous to the moving gas, but the pores are of such a size that they prevent penetration of fine particulate matter. A filter cake can also build up on the filter, further reducing the size of particles passing into the gas stream. Because of the filter cake and developed electrostatic charges, surface filters can remove particles smaller than the actual filter pore size.

Depth filters collect particles within the bulk of a filter material. The filter may consist of loosely packed fibers of quartz wool or of filter material wrapped to a depth sufficient to remove fine particles. Such filters work particularly well for dry solids and moist gas streams containing aerosols. A depth filter can also be used as a probe filter, a technique that is used in some systems.