229502079-filter (1).ppt
TRANSCRIPT
Aerosol & Particulate Research Lab 110/03/2002
FILTRATION
Packing density/solidity
Fiber filter
Reading: Chap 6
porosity - 1 volumetotal
mefiber volu
For fiber filter, < 0.1For woven filter, ~ 0.3
Q: Do filters function simply as sieves (to collect particles larger than the sieve spacing)?
Aerosol & Particulate Research Lab 210/03/2002
Theodore & Buonicore, Air Pollution Control Equipment, CRC Press, 1988.
Aerosol & Particulate Research Lab 310/03/2002
Shaker Baghouse
Theodore & Buonicore, Air Pollution Control Equipment, CRC Press, 1988.
Frequency Several cycles/sMotion type Simple harmonic
or sinusoidalPeakacceleration
1-10 gravity
Amplitude Fraction to a fewinches
Mode Off-streamDuration 10-100 cycles, 30
s to a few minutesCommonbag diameter
5, 8, 12 in
Shaker Cleaning Parameters
Q: What are the common problems encountered?
Aerosol & Particulate Research Lab 410/03/2002
Reverse-Air
Q: Pros and Cons?Cleaning dust on baghouse walls by traditional sledge-hammering
Aerosol & Particulate Research Lab 510/03/2002
Reverse-Air Cleaning Parameters
Reverse-Jet
Aerosol & Particulate Research Lab 610/03/2002
Pulse-Jet
Q: How can the blown-away particles by the on-line cleaning process be collected?Q: Felted fabric or woven fabric?
Aerosol & Particulate Research Lab 710/03/2002
Air/Cloth Ratio
AQV
Filtration velocity(average velocity)
Q: If thicker fabric is needed to sustain the high force, is its operating cost higher?
Aerosol & Particulate Research Lab 810/03/2002
Filtration Mechanisms• Diffusion (Lee & Liu, 1982)
EKu
Pe
Ped U
D
Ku
D
f
2 581
12
34 4
2 3
0
2
.
ln
/
Peclet number
Kuwabara hydrodynamic factor
Lee, K.W. & Liu, B.Y.H., Aerosol Sci. Technol.,1:47-61, 1982
Q: How does efficiency change wrt dp?Q: How to increase efficiency by diffusion?
http://aerosol.ees.ufl.edu/respiratory/section04.html
Aerosol & Particulate Research Lab 910/03/2002
• Impaction (Yeh & Liu, 1974)
EStk JKu
StkUd
d C Ud
J R R R
I
f
p p c
f
( )
( . ) . .. .
2
18
29 6 28 27 5 0 4
2
02
0
0 62 2 2 8
for f
p
dd
R
Yeh. H.C. & Liu, B.Y.H., J. Aerosol Sci., 5:191-217, 1974
Q: How to increase impaction efficiency? Q: How does efficiency change wrt dp?
(J = 2 for R > 0.4)
Aerosol & Particulate Research Lab 1010/03/2002
• Interception (Krish & Stechkina, 1978)
2
2
)1(2
)2
1(1
11)1ln(221 R
RR
KuRER
Krish, A. A. & Stechkina, I. B., “The theory of Aerosol Filtration with Fibrous Filters”, in Fundamentals of Aerosol Science, Ed. Shaw, D. T., Wiley, 1978.
Q: How to increase interception efficiency?
Fat Man’s Misery, Mammoth Cave NP
Aerosol & Particulate Research Lab 1110/03/2002
• Gravitational Settling
• Total Single Fiber Efficiency
• Total Filter Efficiency
E G R
GVU
d C gU
G
TS p p c
( )1
180
2
0
for U and V in the same direction0 TS
GRID
GRID
EEEEEEEEE
)1)(1)(1)(1(1
ESdHE
PE ff
exp1)1(
4exp11
Sf: Solidarity factor
Q: How does the filter efficiency change wrt particle size?
Aerosol & Particulate Research Lab 1210/03/2002
H = 1mm = 0.05df = 2m
U0=10 cm/s
Q: Should we increase or decrease flow velocity in order to increase collection efficiency for (a) tobacco smoke, (b) cement dust?
Aerosol & Particulate Research Lab 1310/03/2002
• ProblemFilter is made of fibers with diameter of 10 um, density of
2.35 g/m3 and solidity of 5%Filter must collect 0.5 um particles with 90% eff.Air velocity is 10 ft/min
Question: How thick must the filter be?
Aerosol & Particulate Research Lab 1410/03/2002
Aerosol & Particulate Research Lab 1510/03/2002
Parallel Flow
Operation
Q: How do you determine when to clean?
Aerosol & Particulate Research Lab 1610/03/2002
Pressure Drop (Filter Drag Model) spf PPPP
VLVtKVK 21
Areal Dust Density LVtW
Filter drag
VPS
WKKS 21
K2
K1
K1 & K2 to be determined empiricallyPf: fabric pressure dropPf: particle layer pressure dropPs: structure pressure drop
Time (min) P, Pa0 1505 380
10 50520 61030 69060 990
Q: What is the pressure drop after 100 minutes of operation? L = 5 g/m3 and V = 0.9 m/min.
Aerosol & Particulate Research Lab 1710/03/2002
Time to clean
ccrf tttNt )(
NQQN
Flow rate
11 N
QQN
Filtering velocity
CC
NN NA
QAQV
CC
NN AN
QA
QV)1(
11
Q: What are the parameters that affect our decision on the number of compartments to be used?
Aerosol & Particulate Research Lab 1810/03/2002
Areal dust density
))(1( 1 cNrNj LtVLtVNW
Filter drag
jj WKKS 21
Actual filtering velocity
1 NNj VfVPressure drop
jjmj VSPP
N 1/ NfN VVf3 0 . 8 74 0 . 85 0 . 7 67 0 . 7 1
1 0 0 . 6 71 2 0 . 6 51 5 0 . 6 42 0 0 . 6 2
2 0
Aerosol & Particulate Research Lab 1910/03/2002
Ex. Calculate the max pressure drop that must be supplied for the following baghouse for a filtration time of 60 minutes: K1 = 1 inch H2O-min/ft, K2 = 0.003 inch H2O-min-ft/grain, tc = 4 min, 5 compartments, L = 10 grain/ft3, Q = 40000 ft3/min, Ac = 4000 ft2/compartment.
Other factors to be considered for selecting a filter?• Temperature & Humidity• Chemical nature of gas• Fire/Explosion• Bag/arrangement• Dust handling• Fan location
Aerosol & Particulate Research Lab 2010/03/2002
• Advantages:
• Disadvantages:
Advantages & Disadvantages of a Baghouse