Air Pollution Control Systems for Boiler and Incinerators.Unique control problems_4

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Air Pollution Control Systems for Boiler and Incinerators.Unique control problems_4TM 5-815-1/AFR 19-6Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.comefficient because the cyclone exhibits an increased col- They can also be used for collection of unburnedlection efficiency during high gas flow and dust loading particulate for re-injection into the furnace.conditions, while the precipitator shows and increase in c. Fine particles. Where particularly fine sticky dustcollection efficiency during decreased gas flow and must be collected, cyclones more than 4 to 5 feet indust loading. The characteristics of each type of diameter do not perform well. The use of small diame-equipment compensate for the other, maintaining good ter multicyclones produces better results but may beefficiency over a wide range of operating flows and subject to fouling. In this type of application, it isdust loads. Cyclones are also used as pre-cleaners usually better to employ two large diameter cyclones inwhen large dust loads and coarse abrasive particles series.may affect the performance of a secondary collector. d. Coarse particles. when cyclones handle coarse6-10Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 5-815-1/AFR 19-6 TM 6-11TM 5-815-1/AFR 19-6Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.comparticles, they are usually designed for low inlet of changing the dimensions of an 8 inch diametervelocities 5-10 feet per second (ft/sec). This is done to cyclones is shown in figure 6-11. The effects ofminimize erosion on the cyclone walls and to minimize changing gas inlet velocity, grain loading, particlebreakdown of coarser particles that would normally be specific gravity, gas viscosity, and particle sizeseparated, into particles too fine for collection. distribution on a 50 inch diameter cyclone are shown e. Limited space. In cases where cyclones must be in figures 6-12 and 6-13. These figures illustrate theerected in limited space, smaller diameter multi- dependence of cyclone collection efficiency on thosecyclones have an obvious space advantage over larger variables and the importance of maintaining proper gasdiameter units. Small cyclones also have the advantage inlet conditions.of increased efficiency over a single unit handling the b. Field performance. The actual in-field perfor-same gas capacity, although this advantage is some- mance of cyclone units will vary because of changes intimes lost by uneven gas distribution to each unit with operating conditions such as dust load and gas flow.resultant fouling of some elements. Table 6-2 illustrates the optimum expected perform- ance of cyclone units for particulate removal6-6. Cyclone performance application in combustion processes. a. Collection efficiency and pressure drop. For anygiven cyclone it is desirable to have as high a collectionefficiency and as low a pressure drop as possible.Unfortunately, changes in design or operating variableswhich tend to increase collection efficiency also tend toincrease pressure drop at a greater rate than the collec-tion efficiency. Efficiency will increase with an increasein particle size, particle density, gas inlet velocity,cyclone body or cone length, and the ratio of bodydiameter to gas outlet diameter. Decreased efficiencyis caused by an increase in gas viscosity, gas density,cyclone diameter; gas outlet diameter; and inlet widthsor area. The effect on theoretical collection efficiency6-12Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 5-815-1/AFR 19-6 TM6-7. Cyclone operation region must be maintained in order to eliminate a high circulating dust load and resultant erosion. The cyclone a. Erosion. Erosion in cyclones is caused by area most subject to erosion is opposite the gas inletimpingement and rubbing of dust on the cyclone walls. where large incoming dust particles are thrown againstErosion becomes increasingly worse with high dust the wall, and in the lower areas of the cone. Erosion inloading, high inlet velocities, larger particle size, and this area may be minimized by use of abrasion resistantmore abrasive dust particles. Any defect in cyclone metal. Often provisions are made from removable lin-design or operation which tends to concentrate dust ings which are mounted flush with the inside surface ofmoving at high velocity will accelerate erosion. The the shell. Erosion resistant linings of troweled or castareas most subject to erosive wear are opposite the refractory are also used. Dust particles below the 5 toinlet, along lateral or longitudinal weld seams on the 10 micron range do not cause appreciable erosioncyclone walls, near the cone bottom where gases ...