Particle Control Technologies Lecture notes adapted from Prof. Dr. Benoit Cushman-Roisin Thayer School of Engineering at Dartmouth
Design Criteria Design of a system which remove solid or liquid particulate matter from a gaseous medium Chacacteristics of gaseous medium and particulate matter to consider in the design: Size T,P,Q, Chem. Composition Chem. Composition Resistance Pressure Drop PM Control Device
Collection Efficiency Considering the wide range of size of particulates, efficiency will be different for each size. The overall efficiency (h) can be calculated on a basis of total number (or mass) of particles Generally regulations are written based on mass, and efficiencies are calculated on mass basis.
Collection Efficiency Efficiencies calculated on mass basis: h: overall collection efficiency (fraction) Mi: total mass input rate (g/s or equivalent) Me: total mass emission rate (g/s or equivalent) Li: particulate loading in the inlet gas to the device (g/m3) Le:particulate loading in the exit gas stream, (g/m3)
Collection Efficiency When the particulate size distribution is known, and the efficiency of the device is known as a function of particle size, the overall collection efficiency can be calculate: where hj: collection efficiency for the jth size mj: mass percent of particles in the jth size
Example 3.1 from the book
PM Control Devices Gravity Settler Cyclones ESP Filters and Baghouses Wet Scrubbers
Settling Chamber Efficient for particles with diameter of 10-50 mm (depending on its density) Velocity through chamber < 0.3-3 m/s (to prevent reentrainment) V H L
Settling Chamber Settling time < transit time through chamber v H L Settling time < transit time through chamber t = H/vt = L/v Settling chambers are cheap to build and operate but not preferred due to their large space requirement
Settling Chamber Dp (um) Vt (m/s) Required Area 0.1 8.6 (10)-7 3 km2 Assuming unit density sphere at STP, vt and chamber Lw are tabulated below: Assumed flow rate Q = 150 m3/min Dp (um) Vt (m/s) Required Area 0.1 8.6 (10)-7 3 km2 0.5 1.0 (10)-5 0.25 km2 1.0 3.5 (10)-5 71000 m2 5 7.8 (10)-4 3200 m2 10 3.1 (10)-3 810 m2
Settling Chamber Baffled Settling Chamber Large particles can not make sudden direction change and settle into dead space of chamber Baffle chambers are used as precleaners
Cyclones :
Cyclones :
Cyclone Geometry
Cyclone Geometry
Cyclone Theory
Cyclone Theory
Cyclone Theory
Cyclone Theory
Collection Efficiency
Collection Efficiency (i) increase Vt (expensive, since DP a Vt2, as we will see in the next slides
Collection Efficiency
Collection Efficiency
Pressure Drop K: a constant depends on cyclone configuration and operating conditions. Theoretically K can vary considerably but for air pollution work with standard tangential-entry cyclones values of K are in the range of 12 to 18 Cyclone pressure drops range from about 0.5 to 10 velocity heads (250 to 4000 Pa)
Cyclone Analysis
Example
Example Conventional Type (No:3) N=(1/H) (Lb+Lc/2) = (1/0.5)(2+2/2)=6 Vi=Q/WH =150 /(0.25*0.5) =1200 m/min 20 Vi=20 m/s
Example Calculate efficiency for each size range witch dpc = 5.79 um:
Example 4.5
Example 4.5
Example 4.5
ESP
ESP
ESP Geometry
ESP Theory
Corona Power vrs Efficiency
ESP Theory
ESP THEORY
ESP Theory
ESP Theory
ESP Theory
ESP Theory
ESP Theory
ESP Theory
ESP Theory
ESP Theory
ESP Theory
Efficiency
Efficiency
Effect of Resistivity
Resistivity Resistivity (P) is resistance to electrical conduction and can vary widely P of a material is determined experimentally by establishing a current flow through a slab (of known geometry) of the material P = (RA/L)=(V/i)(A/L) [ohm-cm] R:resistance, ohm A: area normal to the current flow, cm2 L:path length in the direction of current flow, cm, V: voltage, i: current, A
Resistivity
Resistivity
Sparking
Internal Configuration Internal configuration design is more art than science The even distribution of gas flow through the ducts is very important to the proper operation of an ESP The number of ducts (Nd) is equal to one less than the number of plates (n-1) Nd = Q/uDH (eq 5.15) u: linear gas velocity (m/min) D Channel width (plate separation), m H: plate height, m
Internal Configuration At the start of the design, use 5.15 to estimate Nd by assuming a value for H and choosing representative values of u and D
Typical Values for the Fly-Ash ESP Parameter Range of Values Drift velocity 1-10 m/min Channel (Duct) Width, D 15-40 cm Specific Collection Area Plate area/Gas Flow 0.25-2.1 m2/(m3/min) Gas velocity u 1.2-2.5 m/s Aspect Ratio (R) Duct Length/Plate Height 0.5-1.5 Corona Power Ratio Pc/Q 1.75-17.5 W/(m3/min) Corona Current Ratio (Ic/A) 50-750 mA/m2 Plate area per electrical set As 460-7400 m2 Number of electrical sections 2-6 Table 5.1
Internal Configuration The overall length of the precipitator (Lo) Lo=NsLp + (Ns-1)Ls + Len +Lex Lp:: length of plate Ls: spacing between electrical sections (0.5-1.0 m) Len: entrance section in length (several meters) Lex:exit section in length (several meters) Ns: number of mechanical fields Ns ranges between 2 and 6. Ns=RH/Lp R is the aspect ratio
Internal Configuration When the numbers of ducts and sections have been specified, the actual collection area (Aa) can be calculated as: Aa=2HLpNsNd During the design process several plate sizes and numbers of ducts are tried until one combination is found such that Aa is equal to the required collection area.
Collection Efficiency vrs Particle Diameter
An Example
Example
Example
POWER REQUIREMENT
POWER REQUIREMENT k: an adjustable constant in the range of 0.5-0.7 for we in ft/sec and Pc/A in W/ft2
Problem 5.10 Provide a reasonable design for a 99.4% efficient ESP treating 30,000 m3/min of gas. The dust has a resistivity of 7.1 (10)10 ohm-cm. Specify the total plate area, channel width, number and size of plates, number of electrical sections (total and in the direction of flow), and total corona power to be supplied, and estimate the overall dimensions.
SOLUTION
SOLUTION
SOLUTION
SOLUTION
Video Demonstration on Electrostatic Precipitation http://www.youtube.com/watch?v=y5w0IGuLR3A http://www.youtube.com/watch?v=9kauc7OmmLQ http://www.youtube.com/watch?v=x5YFK8mmeRQ http://www.youtube.com/watch?v=BdRk3op2zpE http://www.youtube.com/watch?v=iUXHzYLgrB0