fabric filters

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 FilterFibrous filter Felted filter


Types of Filter

Packing density/solidity porosity - 1 volumetotal

mefiber volu

For fiber filter, < 0.1For woven filter, ~ 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

AQV

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

VPS

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, Pa0 1505 38010 50520 61030 69060 990

What is the pressure drop after 100 minutes of operation? L = 5 g/m3 and V = 0.9 m/min.


tr

tc

tftj

Time

Pres

sure

pm

Time to clean

ccrf tttNt )(NQQN

Flow rate

11 N

QQN

Filtering velocity

CC

NN NA

QAQV

CC

NN AN

QA

QV)1(

11

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

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


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

3ln21

numberPeclet

158.2

2

0

3/2

Ku

DUd

Pe

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

dUCd

dUStk

KuJStk

f

cpp

f

I

f

p

dd

R

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(221 R

RR

KuR

R

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(

UgCd

UVG

RG

cppTS

G

GRID

GRID

)1)(1)(1)(1(1

ff

SdH

P exp1)1(

4exp11

Sf: Solidarity factor

How does the filter efficiency change wrt particle size?


H = 1mm = 0.05df = 2mU0=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

Effic

ienc

y

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

fabric filters

Documents

theory of aerosol filtration

types of filter fibrous

woven filter

fabric pressure dropdpf

max pressure

filtration velocitypressure

filtration time

woven fabric