Aerosol & Particulate Research Lab109/22/2008 Fabric Filters Filtration Fabric Selection Fabric Cleaning Air/Cloth Ratio, Filtration Velocity Pressure

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

http://exportdeck.com/


Types of Filter

Fibrous filter Felted filter


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)?


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


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?


Reverse-Air


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?

http://www.bha.com/SuccessStories/K6MK1035.pdf

http://www.bha.com/SuccessStories/K6MK1035.pdf


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?


Air/Cloth Ratio

A

QV

Filtration velocity(average velocity)

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


Parallel Flow

Operation

How do you determine when to clean?


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.


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?


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


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


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?

http://aerosol.ees.ufl.edu/respiratory/section04.html


• 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?


• 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?

http://www.geocities.com/wrique2002/ParentsVisit/FatmansMisery2.jpg


• 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?


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


Minimum Efficiency Reporting Value (MERV) for HVAC (Heating, Ventilating and Air Conditioning) Filters

http://www.mechreps.com/PDF/Merv_Rating_Chart.pdf




Quick Reflection

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Aerosol & Particulate Research Lab109/22/2008 Fabric Filters Filtration Fabric Selection Fabric Cleaning Air/Cloth Ratio, Filtration Velocity Pressure