Aerosol & Particulate Research Lab 110/03/2002

Fabric Filters

• Filtration• Fabric Selection• Fabric Cleaning• Air/Cloth Ratio, Filtration Velocity• Filtration Mechanisms• Pressure Drop and Design Consideration

Reading: Chap. 6

Aerosol & Particulate Research Lab 210/03/2002

Filtration

Packing density/solidity

Fiber filter

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 310/03/2002

Theodore & Buonicore, Air Pollution Control Equipment, CRC Press, 1988.

Aerosol & Particulate Research Lab 410/03/2002

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 510/03/2002

Reverse-Air

Q: Pros and Cons?Cleaning dust on baghouse walls by traditional sledge-hammering

Aerosol & Particulate Research Lab 610/03/2002

Reverse-Air Cleaning Parameters

Reverse-Jet

Aerosol & Particulate Research Lab 710/03/2002

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 810/03/2002

Air/Cloth Ratio

A

QV

Filtration velocity(average velocity)

Q: If thicker fabric is needed to sustain the high force, is its operating cost higher?

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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

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 1010/03/2002

• 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

Q: How to increase impaction efficiency? Q: How does efficiency change wrt dp?

(J = 2 for R > 0.4)

Aerosol & Particulate Research Lab 1110/03/2002

• 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.

Q: How to increase interception efficiency?

Fat Man’s Misery, Mammoth Cave NP

Aerosol & Particulate Research Lab 1210/03/2002

• 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

Q: How does the filter efficiency change wrt particle size?

Aerosol & Particulate Research Lab 1310/03/2002

H = 1mm = 0.05

df = 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?

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Parallel Flow

Operation

Q: How do you determine when to clean?

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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)Pf: fabric pressure dropPf: particle layer pressure dropPs: structure pressure drop

Time (min) P, Pa

0 150

5 380

10 505

20 610

30 690

60 990

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 1610/03/2002

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

Q: What are the parameters that affect our decision on the number of compartments to be used?

Aerosol & Particulate Research Lab 1710/03/2002

Areal dust density

))(1( 1 cNrNj LtVLtVNW

Filter drag

jj WKKS 21

Actual filtering velocity

1 NNj VfVPressure drop

jjmj VSPP

N 1/ NfN 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 1810/03/2002

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.

Aerosol & Particulate Research Lab 1910/03/2002

Quick Reflection