Post on 04-Jan-2016
Aerosol & Particulate Research Lab 109/22/2008
Fabric Filters
• Filtration• Fabric Selection• Fabric Cleaning• Air/Cloth Ratio, Filtration Velocity• Pressure Drop and Design
Consideration • Nanofiber Filter• Filtration Mechanisms
Reading: Chap. 6
http://exportdeck.com
Aerosol & Particulate Research Lab 209/22/2008
Types of Filter
Fibrous filter Felted filter
Aerosol & Particulate Research Lab 309/22/2008
Types of Filter
Packing density/solidity porosity - 1 volumetotal
mefiber volu
For fiber filter, a < 0.1For woven filter, a ~ 0.3
Do filters function simply as sieves (to collect particles larger than the sieve spacing)?
Aerosol & Particulate Research Lab 409/22/2008
Theodore & Buonicore, Air Pollution Control Equipment, CRC Press, 1988.
Aerosol & Particulate Research Lab 509/22/2008
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
What are the common problems encountered?
Aerosol & Particulate Research Lab 609/22/2008
Reverse-Air
Aerosol & Particulate Research Lab 709/22/2008
Frequency Clean a compartment at a time,sequencing 1 compartmentafter another; continuous orinitiated by a max.-pressure-drop switch
Motion Gentle collapse of bag(concave inward) upondeflation; slowly repressurize acompartment after completionof a backflush
Mode Off streamDuration 1-2 min, incl. valve opening,
closing & dust settling periods;reverse-air flow itself normally10-30 s
Bag diameter 8, 12 inch; length 22, 30 ftBag tension 50-75 lb
Reverse-Air Cleaning Parameters
Cleaning dust on baghouse walls by traditional sledge-hammering
Pros and Cons?
Aerosol & Particulate Research Lab 809/22/2008
Pulse-Jet Frequency A row of bags at a time; sequenced 1 row after another; can sequence such that no adjacent rows clean one after another; initiation of cleaning can be triggered by max-pressure-drop switch or may be continuous
Motion Shock wave passes down bag; bag distends from cage momentarily
Mode On-stream; in difficult-to-clean applications such as coal-fired boilers, off-stream compartment cleaning being studied
Duration Compressed air (100 psi) pulse duration 0.1 s; bag row effectively off-line
Bag diameter
5-6 in
How can the blown-away particles by the on-line cleaning process be collected? Felted fabric or woven fabric?
Aerosol & Particulate Research Lab 909/22/2008
Air/Cloth Ratio
A
QV
Filtration velocity(average velocity)
If thicker fabric is needed to sustain the high force, is its operating cost higher?
Aerosol & Particulate Research Lab 1009/22/2008
Parallel Flow
Operation
How do you determine when to clean?
Aerosol & Particulate Research Lab 1109/22/2008
Pressure Drop (Filter Drag Model)
spf PPPP VLVtKVK 21
Areal Dust Density LVtW
Filter drag
V
PS
WKKS 21
K2
K1
K1 & K2 to be determined empirically (resistance factor)DPf: fabric pressure dropDPf: particle layer pressure dropDPs: structure pressure drop
Time (min) DP, Pa
0 150
5 380
10 505
20 610
30 690
60 990
What is the pressure drop after 100 minutes of operation? L = 5 g/m3 and V = 0.9 m/min.
Aerosol & Particulate Research Lab 1209/22/2008
tr
tc
tf
tj
Time
Pre
ssu
re
Dpm
Time to clean
ccrf tttNt )(N
QQN
Flow rate
11 N
QQN
Filtering velocity
CC
NN NA
Q
A
QV
CC
NN AN
Q
A
QV
)1(1
1
What are the parameters that affect our decision on the number of compartments to be used?
Aerosol & Particulate Research Lab 1309/22/2008
Areal dust density
))(1( 1 cNrNj LtVLtVNW
Filter drag
jj WKKS 21
Actual filtering velocity
1 NNj VfVPressure drop
jjmj VSPP
N1/ NfN VVf
3 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 1409/22/2008
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.
Nanofiber Filter Nano-alumina on microglass
Argonide
E-spun PAN nanofiber
Dr. Wolfgang Sigmund, MSE. UF
Aerosol & Particulate Research Lab 1509/22/2008
Filtration Mechanisms• Diffusion (Lee & Liu, 1982)
factor ichydrodynam Kuwabara 44
3ln
2
1
numberPeclet
158.2
2
0
3/2
Ku
D
UdPe
PeKu
f
D
Lee, K.W. & Liu, B.Y.H., Aerosol Sci. Technol.,1:47-61, 1982
http://aerosol.ees.ufl.edu/respiratory/section04.html
How does efficiency change wrt dp?How to increase efficiency by diffusion?
Aerosol & Particulate Research Lab 1609/22/2008
• Impaction (Yeh & Liu, 1974)
4.0for 5.27)286.29(
18
2
)(
8.2262.0
02
0
2
RRRJ
d
UCd
d
UStk
Ku
JStk
f
cpp
f
I
f
p
d
dR
Yeh. H.C. & Liu, B.Y.H., J. Aerosol Sci., 5:191-217, 1974
(J = 2 for R > 0.4)
How to increase impaction efficiency? How does efficiency change wrt dp?
Aerosol & Particulate Research Lab 1709/22/2008
• Interception (Krish & Stechkina, 1978)
2
2
)1(2
)2
1(1
11)1ln(2
2
1R
RR
Ku
RR
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.
Fat Man’s Misery, Mammoth Cave NP
How to increase interception efficiency?
Aerosol & Particulate Research Lab 1809/22/2008
• Gravitational Settling
• Total Single Fiber Efficiency
• Total Filter Efficiency
0
2
0
TS0
18
direction same in the V and for U )1(
U
gCd
U
VG
RG
cppTS
G
GRID
GRID
)1)(1)(1)(1(1
ff
Sd
HP exp1
)1(
4exp11
Sf: Solidarity factor
How does the filter efficiency change wrt particle size?
Aerosol & Particulate Research Lab 1909/22/2008
H = 1mma = 0.05
df = 2mm
U0=10 cm/s
Should we increase or decrease flow velocity in order to increase collection efficiency for (a) tobacco smoke, (b) cement dust?
Filter efficiency for individual mechanism and combined mechanisms
dp (m)0.01 0.1 1 10
Eff
icie
ncy
0.0
0.2
0.4
0.6
0.8
1.0
InterceptionImpactionDiffusionGravitationTotal
Aerosol & Particulate Research Lab 2009/22/2008
Minimum Efficiency Reporting Value (MERV) for HVAC (Heating, Ventilating and Air Conditioning) Filters
http://www.mechreps.com/PDF/Merv_Rating_Chart.pdf
Aerosol & Particulate Research Lab 2109/22/2008
Quick Reflection