Gravitational Settling Chambers Simple gravity separation is the basis of one of the oldest collection devices—the settling chamber. A settling chamber is a long, enlarged section in the exhaust system. As the cross-sectional area increases, there is a corresponding decrease in the exhaust gas velocity, which per permits mits coar coarse serr part partic icle les s to fall fall from from the the air air su susp spen ensi sion on.. Sett Settli ling ng cham chambe bers rs oer oer the the adva advant ntag ages es of mode modest st cons constr truc ucti tion on,, simp simple le operation, dry collection, and small pressure drops. Structural space requi require remen ments ts are are a signi signica cant nt dis disadv advant antage age and are are genera generally lly the limi limiti ting ng fact factor or in us use e of sett settli ling ng cham chambe bers rs as prim primar ary y coll collec ecti tion on methods. !he design of a settling chamber is based on the settling velocity of the particles to be removed. "enerally, chambers will have #ow-th #ow-thro rough ugh veloci velocitie ties s less less than than $ m%s and will will eecti eectivel vely y remov remove e particles greater than &' (m ( m in si)e. *ry collectors, such as scrappers and screw conveyors, are used to collect the settled particles.
Gravimetri Gravimetric c Efciency Efciency : +t is dened as the ration of the weight of the material collected or retained by the collector to the total amount entering the collector. G E (%) = 100 x
Wc We
here, " is "ravimetric ciency, c is weight of material collected and e is the weight of material entering the collector.
Wet Scrubb Scrubbers ers : /sing a variety of methods, wet scrubbers are collection devices that wet particles in order to remove them from the gas stream. !hey utili)e inertial impaction and%or 0rownian diusion as the particle collection mechanism, and droplets, sheets, and 1ets as the
liquid collection mechanism. et scrubbers typically use water as the cleaning liquid. ater usage and wastewater disposal requirements are important factors in the evaluation of a scrubber alternative, and both are in#uenced by the quantity of particles collected, the si)e distribution of the particles, and the presence of dissolved contaminants in the wastewater. 2any applications recirculate the scrubber liquid, which reduces consumption demand but will lead to higher particulate and dissolved solids concentrations, requiring treatment and disposal.
Advantages and Disadvantages o Wet Scrubbers Advantages
Disadvantages
Absorbs gas phase emissions suggests pollution 3ompact si)e cient through wide loading range temperature gases +nsensitive to moisture content
3ondensate plume
5ow capital cost handling required 5ow operating and maintenance cost 4e-entrainment rare 6ersatility for ha)ardous emissions
3orrosion +necient with high4equires high power input aste scrubber liquid
Fabric Filters 7ne of the most ecient devices for removal of particulates is the fabric lter collector. 8abric lters have the capability of maintaining collection eciencies above 99: for particle si)e down to '.$ (m. !he basic features of a fabric lter unit consist of woven or felted fabric, usually in the form of tubes ;bags< that are suspended in a housing structure ;baghouse<. !he emission stream is distributed by means of specially designed entry and exit plenum chambers, providing equal gas #ow through the ltration medium. !he particle collection mechanisms for fabric lters include inertial impaction, 0rownian diusion, interception, gravity settling, and electrostatic attraction. !he particles are collected in dry form either on a dust ca=e supported by the fabric ;most ecient< or on the fabric itself. !he process occurs with a relatively low pressure drop requirement. >eriodically, most of the dust ca=e is removed for disposal? the residual dust serves as an initial ltration aid while the new dust ca=e develops.
@As particles adhere to the fabric, the removal eciency increases, but so does the pressure drop. !he pressure drop is the sum of the pressure drop owing to the fabric and the drop owing to the ca=ed or adhered particles, and is expressed as
x x ∆P = υµ f + p K f K p where
∆P
υ
µ
x C
B B B B B
overall pressure drop in meters of water, supercial gas velocity through the fabric ;in m%s<, gas viscosity ;in ><, thic=ness of lter ;%< and particle layer ;p< ;in m<, and lter ;f< and particle layer ;p< permeability.
Electrostatic Preciitators Electrostatic Precipitation Systems. !he classication of electrostatic precipitators may be as dry or wet systems and%or as single- or two-stage systems. Dry Systems. !he dry precipitator with plate-type collection electrodes is the predominant type of system in industrial applications and is the process most discussed in this section. Wet Systems. !he primary dierence in wet and dry systems is the method by which collected particles are cleaned from the collection electrode. !hree common wet precipitator congurations are the plate type with hori)ontal gas #ow, the concentric plate type with vertical gas #ow, and the conventional pipe type with reversing vertical #ow. lectrostatic precipitators use electrical energy to charge and collect particles. !hey have high removal eciencies? in addition, they are eective for a variety of source categories and emission gas characteristics.
$!he collection eciency of an electrostatic precipitator may be estimated by an empirical equation
- Aυ d Q
R = 1 - exp
here DA B total area of the collecting surface of the collection electrodes ;m<, E B #ow rate of gas through the pipe ;m $%s<, υd
B drift velocity &; velocity of the particles toward the collecting electrode< B '.& d ;m%s< * B diameter of the particle ; µ m<
Advantages and Disadvantages o Electrostatic Preciitation Advantages: @. Fighly eective collection with eciencies exceeding 99: in some cases. . 5ow power requirements and associated low operating power costs. $. 3apable of removing very small particles, even those not removable by other treatment schemes. D. *ry dust collection often used, which may be useful for product and by-product recovery. &. >ressure drops are small.
G. !emperature changes are small with dry systems? most impacted in spray-wet systems. H. 5ow maintenance requirements due to few or no moving parts. I. System tolerant to high temperatures, with no special provisions below &$&J3 ;@'''J8<. 9. 3ollection eciency and system capacity can be easily expanded with larger or additional units. @'. Some pollutants, such as acid and tar mists, are eectively collected by other schemes.
Disadvantages: @. . $. D. &. G. H.
3apital investment is high. Space requirements are relatively large. 6ery low or high resistivity particles are dicult to remove. 4elatively constant operating conditions are needed for ecient collection. "aseous wastes are not aected. Safety is a concern due to high voltage. et systems produce sludges that may require dewatering before disposal.
