It has also been observed that if the particles are charged before they enter the filter, they will be collected with a higher efficiency than if they am not. This has led to the ESP-baghouse combination, in which an old eSP that does not meet new emission standards has a baghouse attached to its downstream side. The particles passing from the ESP to the baghouse are mostly the smallest of the particles that entered the ESP, and many of them are charged. The measured performance of this combination is often better than one would predict for an ESP plus a baghouse treating uncharged particles 9. 2.3 Filter media Whether a filter behaves as a surface or a depth filter depends on the type of filter medium used For shake-deflate baghouses(Fig. 9. 4)the filter bags are made of tightly woven fibers, much like those in a pair of jeans. (The reader is invited to look at the sun through a single layer of such fabric, seeing that it has some pinholes, allowing light to come through, and to blow into such a fabric, observing that one can breathe in and out through one. )Pulse-jet baghouses( Fig. 9.5)use high-strength felted fabrics, so that they act partly as depth filters and partly as surface filters This allows them to operate at superficial velocities(air-to-cloth ratios) two to four times those of shake-deflate baghouses; in recent years this higher capacity per unit size has allowed them to take market share away from the previously dominant shake-deflate type baghouses Filter fabrics are made of cotton, wool, glass fibers, and a variety of synthetic fibers. The choice depends on price and suitability for the expected service. Cotton and wool cannot be used above 180 and 200F, respectively, without rapid deterioration, whereas glass can be used to 500-F(and short-term excursions to 550.F). The synthetics have intermediate service temperatures. In addition the fibers must be resistant to acids or alkalis if these are present in the gas stream or the particles as well as to flexing wear caused by the repeated cleaning. Typical bag service life is 3 to 5 years. Generally fibers that have many small microfibers sticking out their sides form better cakes than those that do not. The student should examine under a microscope a thread of cotton, hich has such microfibers, and one of monofilament fishing line which does not 9.2.4 Scrubbers for particulate Control Just as filters work by separating the flow of particle-laden gas into many small streams, so also scrubbers effectively divide the flow of particle-laden gas by sending many small drops through it In air pollution control engineering the term scrubber originally meant a device for collecting fine particles on liquid drops. Then when liquid drops were used to collect sulfur dioxide, the devices that did that were also called scrubbers. Recently, alas, some other types of devices have been marketed as dry scrubbers. In this chapter, we will use the original meaning of the term: a scrubber is a device that collects particles by contacting the dirty gas stream with liquid drops Most fine les will drop if contactor Mixed gas separatorClean gas they contact it. So if we and liquid an make the drop and the (scrubber) (cyclone) particle touch each other, the particle will be caught Dirty liquid on the drop Particles 50 H and larger are easily Clean liquidLiquid-solid separator our problem is to collect a clones will not work at Collected solid all. However if we were to introduce a large Fig. 9-6 Component parts of a number of 50-u diameter scrubber installation drops of a liquid (normally water)into the gas stream to collect the fine particles, then we could pass the stream through a cheap, simple cyclone and collect the drops and the fine particles stuck on them. This idea is the basis of almost all scrubbers for particulate contro A complete scrubber has several parts, as sketched in Fig. 9.6. Most often, the gas-liquid separator is a simple cyclone of the type discussed in Sec. 9.1.2; water drops of the size encountered in most scrubbers pose few difficulties for such cyclones. The liquid-solid separator can be of many kinds although gravity settlers seem to be the most common. If possible, the engineer should try to sav money by finding a place where the contaminated water stream can be recycled inside the plant9-7 It has also been observed that if the particles are charged before they enter the filter, they will be collected with a higher efficiency than if they am not. This has led to the ESP-baghouse combination, in which an old ESP that does not meet new emission standards has a baghouse attached to its downstream side. The particles passing from the ESP to the baghouse are mostly the smallest of the particles that entered the ESP, and many of them are charged. The measured performance of this combination is often better than one would predict for an ESP plus a baghouse treating uncharged particles. 9.2.3 Filter Media Whether a filter behaves as a surface or a depth filter depends on the type of filter medium used. For shake-deflate baghouses (Fig. 9.4) the filter bags are made of tightly woven fibers, much like those in a pair of jeans. (The reader is invited to look at the sun through a single layer of such fabric, seeing that it has some pinholes, allowing light to come through, and to blow into such a fabric, observing that one can breathe in and out through one.) Pulse-jet baghouses (Fig. 9.5) use high-strength felted fabrics, so that they act partly as depth filters and partly as surface filters. This allows them to operate at superficial velocities (air-to-cloth ratios) two to four times those of shake-deflate baghouses; in recent years this higher capacity per unit size has allowed them to take market share away from the previously dominant shake-deflate type baghouses. Filter fabrics are made of cotton, wool, glass fibers, and a variety of synthetic fibers. The choice depends on price and suitability for the expected service. Cotton and wool cannot be used above 180 and 200~F, respectively, without rapid deterioration, whereas glass can be used to 500~F (and short-term excursions to 550 。 F). The synthetics have intermediate service temperatures. In addition the fibers must be resistant to acids or alkalis if these are present in the gas stream or the particles as well as to flexing wear caused by the repeated cleaning. Typical bag service life is 3 to 5 years. Generally fibers that have many small microfibers sticking out their sides form better cakes than those that do not. The student should examine under a microscope a thread of cotton, which has such microfibers, and one of monofilament fishing line, which does not. 9.2.4 Scrubbers for Particulate Control Just as filters work by separating the flow of particle-laden gas into many small streams, so also scrubbers effectively divide the flow of particle-laden gas by sending many small drops through it. In air pollution control engineering, the term scrubber originally meant a device for collecting fine particles on liquid drops. Then when liquid drops were used to collect sulfur dioxide, the devices that did that were also called scrubbers. Recently, alas, some other types of devices have been marketed as dry scrubbers. In this chapter, we will use the original meaning of the term: a scrubber is a device that collects particles by contacting the dirty gas stream with liquid drops. Most fine particles will adhere to a liquid drop if they contact it. So if we can make the drop and the particle touch each other, the particle will be caught on the drop. Particles 50 μ and larger are easily collected in cyclones. If our problem is to collect a set of 0.5-μ particles, cyclones will not work at all. However, if we were to introduce a large number of 50-μ diameter drops of a liquid (normally water) into the gas stream to collect the fine particles, then we could pass the stream through a cheap, simple cyclone and collect the drops and the fine particles stuck on them. This idea is the basis of almost all scrubbers for particulate control. A complete scrubber has several parts, as sketched in Fig. 9.6. Most often, the gas-liquid separator is a simple cyclone of the type discussed in Sec. 9.1.2; water drops of the size encountered in most scrubbers pose few difficulties for such cyclones. The liquid-solid separator can be of many kinds although gravity settlers seem to be the most common. If possible, the engineer should try to save money by finding a place where the contaminated water stream can be recycled inside the plant Fig. 9-6 Component parts of a scrubber installation