Clogging of HEPA fibrous filters by solid and liquid aerosol particles: An experimental study

A fibrous filter from industrial of the

is a common gas streams.

filter

cleaning device A fundamental

performance

under

aerosol

time. The purpose of this paper in presenting our experiments evolution

of pressure

drop

deposit

P. Penicot, LSGC-CNRS

load

and

is to illuminate of solid and

of HEPA

structure

often used to remove question concerns the

filters

is analysed

D. Thomas,

the

prediction

particles evolution of its life

some of the issues liquid particle filtration.

is described

iin both

as precisely

involved, The

cases

and

the

as possible.

F?Contal, D. Leclerc, J. Vendeb

/ Universitk Henri Poincark, 1 rue Grandville-BP *CE Saclay- DPEI-SERAC,

b&.393,

91191

451, 54001

NANCY

Gif-sur-Yvette,

Cedex, FfUNCE

FENCE

or several years, environmental purification p”‘“‘%4 !‘r ‘I requirements have become more and more stringent. 9, j, i Among all the devices designed to remove small i;*we*A-(2 ” 9 particles from gas streams, fibrous filters are

F

economically

.&%%h they are both

and quite

of an entanglement according

of thin

to several

inertia.

If clean

of many

is still not

completely

importance with

other

are relatively

solid

d eposit

particles

aspect

behaviours

well-known

the filtration

experiments

in both

(8)

during clogging is of major

of the filter.

and liquid drop

TC

or

and have been

the life time

of pressure

is analysed

I: !: 3: I: 5:

carried

particles

on the

in relation

Air production Dryer Aerosol generator Aerosol introductior, Upstream aerosol samplina system

7: 8:

Differential pressure transducer Downstream aerosol sampling system Mass flowmeter Exhaust filter Electropneumatic valve

9: IO: 11:

to the

cases and a comparison

of both

set-up

1 shows a dryer,

diagram

generator,

of the experimental

a filter

a mass flowmeter

and downstream is dried

system. captured

Aerosol particles penetrating by a back-up absolute filter. at different upstream

of aerosol

concentration

can be changed is kept drop

Filtration

using constant across

by passing

points

a by-pass using

results system.

a flow

the filter

& Separation

the test filter Temperature in order

pressure

acquisition

data.

is measured

Diluting

The

air velocity system with

from

concentration

systems

particle

air can velocity inside

a high accuracy

(1 Og/l

particles

Differential a Condensation

entails

aerosol

Particlee Mobility Nucleus

from

impactor

of both

Sizer

Counter

the nebulizer

then

oversized

of dried

compressed

the generator

enables

size and aerosol

size distribution Particle

to the

a nebulizer

to eliminate

from

particle

generated.

according

with

whose

of both

concentration

stream

in order

aerosol

solution

was used

is generated

aerosol

the

modification

by the introduction

the second

modification

concentration.

an uranine

from

generator

impactors

and solidifies

air. Removing

came

and the aerosol

The

two

enables

of a soda fluorescein

44-011

at 1.8 bars. The through

another the

particles

A computer

or 1 OOg/l)

NFX

process.

operated

the influence

and the

solid

size distribution

following passes

transducer.

the atomizing

A standardised

are perfectly and pressure

to test

obtained

the test

air to the

and the filtration

regulator

It

a refrigeration

in the system.

on our

sampling

compressed

it through

of the filter

set-up.

containing

aerosol

The

generator

are controlled

holder

and two

the filter.

aerosol

be introduced

differential

Th e generated

a schematic

of an aerosol

upstream

pressure

interception

up

is performed.

consists

filter

are collected

diffusion,

on one hand

evolution

Experimental

filter,

particles

‘x2z3,, their behaviour understood3. This aspect paper

since

are made

in which like

technology

to use. They

studies

in this

hand. The

Figure

fibres

as it determines

We describe

interesting simple

mechanisms

filters

the subject

out

the most

effective

was measured (DMPS

(CNC

TSI

3071)

TSI 3020)

March

using

a

in line with and was found

1999

59

to be log-normal 0.3 1 nm each

with

and 0.4nm

the following with

mean

a geometric

diameters:

standard

O.l8nm,

deviation

the characterization,

of 1.8 in

case. particles

generated

measured

using

an Impactor around

section

density

The liquid aerosol

filter

two

from

Personal

urn. Table

a di-octyl

Particle

techniques

Marple

0.6

came

at 1.1 bars.

(DOP) was

(DMPS

associated

Sampler

1 sums

phtalate

size distribution 290)

up the main

with

features

A HEPA

(High

Efficiency

for

all experiments.

of the

Air

Several

Filter)

techniques

glass fibre were

used

the

Packing

the following

equation

(1):

(1)

G and Z are respectively

filter

filter

the grammage

and pf is the fibre fibre

law’(see Particulate

using

observing

micrograph.

to be

The

was used

calculated

Pf z

the filter

characteristics

was measured

electron

G

and

particles.

Filter

thickness

a scanning

was then a=--

CNC

and was found

The

using

mean

diameter

equation whose

(2))

packing

for

was first

predicting density

AP -=16p Z

estimated

using

the pressure

is cx , fibre

of

Davies’

drop

radius

of a fibrous

rf and thickness

Z:

CX”~ (1 + 56~‘) z f

U,

dDavles

and the thickness

density.

=2

J+lhereKis

We define the slope drop

0

0.5

WT 1 Collected

Mass

1.5 (g/m*)

2

numerical

0.056

and dDaVies=

the effect

the generated constant

was weighed

several

different

filtration

aspect 1 r

2500

0 .

tf=Zl mn tf=46 mn

filtration

0

Table

I ” 0

50

100 150 Mass Collected

200 (g/m*)

w CLOG 250 300

filtration

1999

pressure

drop,

the face velocity

the tested

filter

mass. This

the same

operating

so that we could electron

was

a linear

up all experiments

of the filtration

at various linking

carried

out.

evolution of liquid

Excellent

reproducibility

as can be seen from times

tf, are situated

both

of the

parameters.

of pressure loss or solid aerosol

that

on the same

Filtration

with

states against

of the experiments

the fact

was

the deposit

mass was plotted equation

the filter

process

conditions

observe

micrographs

the collected

to obtain

IX =

pm.

on the filter keeping

the collected

time

through urn,

At the end of the experiment,

under

scanning

0.14)

passes (f50)

In Figure 2 and 3, we can see the evolution of the pressure during clogging respectively in the case of liquid and solid attained

March

time 2 sums

(t

loss across

clogging.

Moreover,

Comparison during the particles

particles.

60

taking

are: Z = 575

1.14

was filtered

times

the pressure

Experiments

to determine

repeated

clogging. g -b

aerosol

during

Us)

of the loading

and the pressure

measured

AP is

values

of the

To describe

2.5

AP to

a flow

the medium.The (? 0.006)

Us.

of air (velocity

line linking when

Description

2

, ooo

of straight

of the filter

all plots

drop

was for different

curve.

& Separation

a

the evolution mass but

of pressure

suddenly

We can easily (for

both

solid

collected

particle

takes

place

chainlike

analysis

Plotting for

filtration

pressure

As illustrated of two

collected

becoming

quite

give rise

consists

a given

two

from

drop.

particles

The

expected processes

mass

a linear clogging

parts.

different

Filtration

steps

function

Nevertheless, from

by Figure

(here

by liquid

intuition

2, the clogging

wr),

the behaviour the solid

one.

& Separation

is firstly

it is more

of the loading particles

‘7 6. The

to the

appearing

as loading

increase

that

on the same of solid

firstly

drop

the filter

surface.

to form

are filled

in, corresponding

the filter

surface.

graph

(Figure aerosol,

entails

a far higher

We

on the bridges

at the

all to the

6), the increase we become

is

of this film.

deposited

together

and solid

particles

as droplets

At the end of clogging,

layer

covering

liquid

droplets

The

of pressure

on the thickness

and join fibres.

on the first of a film

covering

particles.

that in the early

are deposited

of a film

grow

show

of pressure

aware

pressure

that the drop

than

and previous

to an increase

: the increase called

particles

of several

both

This

work4,

out for the liquid

exponential

we can assume

interstices

of

4A).

attributed

5 clearly

any information

progressively

presence drop

The

by the presence

intersections

(Figure previous

thickness

in Figure

liquid

not obtain

fibres

firstly

the formation

can be easily

was carried

presented

the fibres.

However

that the filtration

other

are

of the

4B).

of filtration,

could

filtration

drop

curve

micrographs

with

dendrites with

(Figure

explained

have been

called

of a cake of increasing

around

both

show

increases

stage

particles

surfaces of the filter

of pressure

same

drop

electron

presence The

of the pressure

Scanning

filter

of the loading

to the way that

the filter.

loaded

observations

As might

each part

is in agreement

rise

function

exponential.

filtrations)

agglomerates

linear

studies,

attribute

in the depth

observation

is a linear

and liquid

inside

solid

drop

the rise becomes

in by solid

slow

and for

rapid,

mass.

(Figure

3) is also made

of the liquid In the early

stage

aerosol

up of is

of filtration,

March

1999

61

between

the transition

had also observed

z

6000

this

agreement

between

correlation

wy

particles

with

quickly

entailing

formation

point

and the particle

linear

relationship.

our

experimental

~3.975

results

dp. We may

a higher

specific

a greater

of the cake

Renaudin7

that

form

effect.

reached

8 shows smaller

dendrites

The

for

good

and a simple

suppose

area will

clogging

is then

size. Figure

more

point

a smaller

of amount

of

particles. In the case of liquid clogging

point

covering

the filter

the increase 100

150

Mass

200

Collected

250

300

The

(g/m”)

influence

tendency

is a smaller

suppose

better

that

collection

we want solid

DOP

WT

1 1.5 Mass Collected

particles.

In other

words,

h’ ig h er amount

of liquid

than

and then from

before

of uranine, new

dendrites

collecting

Points

of filter

particles

entailing

deposited

they

around

is greater.

velocity.

We

entails

of the filter.

a

Thus,

in the case of a

mass needed

Further

Figure

general

velocity

may be deeper

the collected

studied. The

a higher

layers

was velocity.

to form

experiments

a

would

be

this hypothesis.

to be able to predict

structure

is of major

the behaviour

importance

of the filters

if

under

loading.

explanation

can easily flow

a

its destruction comes

in the filter.

a higher

of the first

surface

wCLoG

3

can collect

before

A possible

are collected

are created:

fibres

case of droplets

In the case

be considered

resistance

than

as in the

I

0

0.1

0.2

0.3

0.4

0.5

the fibres.

of Interest

As described filter

particles

2.5

with

film

for which

of the filtration

of the filtration

of the deposit

and liquid

exponential.

air velocity.

point

the

(g/m*)

this kind solid

its replacement.

the way that

2

efficiency

to confirm

Characterization

0.5

clogging

an increase

and then

necessary

becomes

mass

size has not yet been

of the aerosol

on the filter

wCLOG of an entire

is the collected

of w CLOG with

smaller

velocity

studied

as a function

the penetration film

0

drop

of the particle

the evolution

may

wCLoG

determined

9 shows

we have

to the formation

surface.

of pressure

experimentally 50

particles,

corresponding

above,

in two

particular

steps. points

both

solid

It seems

and liquid

particles

to be interesting

corresponding

clog

to study

to a transition

the HEPA those

between

both

parts. In the case of solid transition which

point depth

begins, point

wr

found

filtration

comes

tangents

that

for

determined

the collected

to an end

studied

way whatever

62

the velocity

of fibres.

by the intersection 2. Let us firstly

particles

sizes,

of the filtration

March

at which

Let us also stress

1999

for

formation

by Figure velocity

particles the linear

wr was (on the range

3 < U, < 50 cm/s) as illustrated by Figure 7.This that (on the range studied), the aerosol is collected vicinity

wT the mass value

and cake

determined

as illustrated

the three

to be independent

we have

words,

was experimentally

of both

emphasize

particles,

or in other

result means in the same

are arriving

0

5

10 U, (cm/s)

15

in the

relationship

Filtration

& Separation

20

8000

z-

/

I

/

!

,

/

i ,

1-t

7000

F DOP filtration

6000

; : p

Clean air circulation I

3 a a e 75 E ? $ &

5000

both

‘1

I

. .

I I

.

stages

*

*

was reached.

The

step

between 1

cm/s).

the filter

second

.

(U,=l.8

experiment,

passing

of the experiment

varied

Each

(corresponding

velocity

As illustrated drop

until

As shown IO

0

20

30

40

Filtration

50

60

70

(small

time (mn1 (mn)

The

air velocity

(last

particles

We tried

to give

particle

a better

loading.

according

We first

to several

experimentally

observed

with

m/s

surface.

the linear pressure of a filter radius

Z,,

rp corresponding

Liquid

The

of the slope

be compared

value

an equilibrium

process phenomena:

Clean

air going

loading

effect

paper Both

collected that both

of such

through

or weak drop

during

loading).

is not due to a re-

in the filter. the film

formed

the fibres,

on the

had not initially

that an internal to be as minor

as

the air circulation. particles

loading,

the filter

and to bring

filter.

of particles

It also seems

during

filtration

This

the filter

no presence

for the resistance

opposed

phenomenon

for a higher

downstream

around

state.

of the liquid, place

as in the first

occur

omission

the effect

observed

be depicted

as

and relocation.

has enabled

to the fore

has also been

can then

drainage

of the

of relocation.

by Walsh

et al.“.

to the pressure

density for

of

that the drop

size. Then

a given

according

6000

process

up all results

we can also compare

was obtained previous

between

solid

the effect and liquid

of loading particles

with

corresponding

to values

issued

all methods

experimental

7

L

set of operating

of the filtration

3 sums

outlines

particles

were

on a HEPA

studied.

More

ap = 1 - E, and

E,.

Table

from

and all droplets

This

clearly

for a heavy

initially

several

cake

papers. values

(either

we can suppose

takes

Pa.

experiment

we can also see the

that the fall of pressure

of particles

possible,

of

4700

Zc the cake

We assumed

to the particle

that

agreement

with

area,

calculation curves.

simulations

of dendrites.

Good

with

estimate

to 0.3 1 urn particles other

of 10 %. The

of filter

of packing

we performed

formation

was

showed

Consequently,

relocation

cake thickness

an accuracy

drop

of the cake could

we could

Finally,

The

the aerosol

entrainement

attained was

around

of the experiment).

the filter

surface

to a decrease

if it is not so obvious especially

decrease

means

rise

density.

was made

law and the slope

conditions,

section

mass per unit

of thickness

to Davies’

of particles

the filter

by:

in the pressure

drop

amount

Then,

drop

result

filter

time

-1 PZC

estimate part

a sufficient

m/s

and the particle

A second

was

of the cake

a huge

with

calculated

E,

stage

through

That

this parameter

out during

microscope.

the deposited

thickness

and as

-z a g % E z t? 6

5000

1

4000

I ;

2000

L

/

1

Perte de charge

l

I

/

(Pa)

U. 0-M

. I li‘

’

1’

j

0.09

. . . . . . . . . *.,e

l ‘)** DOF’

l~

a9 * a..

0

Clean air circulation

j 0.04 0.03

studies.

particles

In this case, we tried stability

point

the loading coalescence

is much

of liquid

particles.

we carried to a high

in which a flow

was loaded (presence

Filtration

filter. until

field solid

more

variable

out two and a small

amount DOP

In the case of the first a pressure

of a film),

The

drop air velocity

& Separation

aerosol,

of 7000

from

0

its

20

40 60 Filtration time $I!?)

100

120

was suggested the nature

of

due to the possible

experiments

process

air (without

structure

of study

with

the filtration

of clean

the deposit

particular

behaviour

corresponding

the clogged

This

in comparison

Accordingly, particles)

to characterize

of view.

by the fact that,

where

was carried

passing

by solid

the cake porosity with,

the estimation

to be estimated

was then Ep =l-(

well

with

an electron

enough

porosity

from

To begin

to obtain

on the filter

fibre

to estimate

in order

deposited

with

tried

An experiment

of exposure

of the cake formed

techniques.

calculated

thickness.

great

description

phenomena

sampling

the pressure

of pressure

of deposited

to be attained

particles), even

and

15 min.

11, in the case of the second drop

Pa the

stages

velocity)

the evolution

air gives

seems

of 1400 during

to several

every

amount

of clean value

of deposited

Moreover,

Solid

(large

of pressure

experiment.

was applied

10 showing

by Figure

amount

decrease

according

experiment

a stable

drop

the filter

to the filtration

stage

the circulation

pressure

a pressure through

by Figure

for the first

particles),

until

air velocity

1.8 cm/s

8.5 cm/s.

In the case of the second

was loaded

(respectively of deposited

was followed particles)

by a step passed

experiment,

through

the filter

. i(,..:_ . Previous

inveestigations

[8],

[9]

From

85 to 92

Pa was reached

was kept

constant

during

March

1999

63

particularly,

the evolution

described.

Solid particles

of the filter pressure

before

stage

a cake

a linear with

0.3 1 urn. The

value

collected

in the depth

surface:

the

of the deposited by several

previous

influence

was precisely

on the filter

function

the cake porosity

are all in agreement for

drop

forming

loss becomes

determined

of pressure are in a first

works

around

of the particle

Cake

porosity

CL

Fluid

viscosity

Fibre

density

Pf

mass. We

techniques

&P

which

the value

P (T

Aerosol

AP

Filter

C-1 (Pa.i) (kg/m3)

particles

Standard

density

(kg/m3)

deviation

C-1

of 85%

size has not yet been

pressure

drop

Pa)

analysed. Liquid

particles

to form

displayed

droplets

on, droplets

around

coalesce

intersections.

The

filter

which

surface

pressure

drop.

the liquid

inside

of the film

theoretical between process

of filtration

evaluation

for

situated

may take

in a film

on the

increase that

Davies,

of

relocation

Characterization

it would

be of a great

of the solid

interest

the transition

and liquid

particle

C. N. (1973)

of

New-York.

of the

Kuwabara,

S. (1959)

distributed

parallel

in progress.

of wT and wCLOG

stages

seem carries

at the fibre

shown

place.

they

loading

results

have

is currently study,

since

When

the exponential

experiments

the filter this

both

bridges

stage

accounts

thickness

behaviour

surface.

to build

final

Several

To complete

different

the filter

to give

flow

a

points

at small

filtration

Penicot,

Reynolds

Company

for

supply

to express

gratitude

to Bernard

DUMAS

diameter

df

diameter

dP G

Particle

m/s

Collected

Tf

Fibre

l-P

tf

wT

grammage mass per

unit

of area

radius

Particle

radius

Filtration

Uo wcLocClogging

with

diameter

Filter

Fluid

estimated

time

velocity

Transition filtration

point

for liquid

point

from

to surface

filtration

Davies’law

media

Hinds,

(kg/m*)

Loading

(kg/m*)

Aerosol

(ml

Renaudin,

(4

k-4

fibres. DEA de 1’INPG Schmidt, E., Loffler, F. (1991)

The Analysis

(m/s)

Structures.

System

(kg/m2)

109.

(kg/m*)

Experimental of fibrous

(4

10

Walsh, The

64

March

density

of deposited

1999

particles

Recent

Behaviour

Advances under

in the Understanding

Solid

Particle

N. P. (1997)

on Penetration

and Resistance

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and khnology

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fibrous

filter

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Filtration

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Stenhouse, of solid material

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J.I.T,

Liu,

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B.Y.H.

Scurrah,

K.L., particle

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Cake 8, 105.

clogging

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a

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J.I.T.,

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162-173.

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D.C.,

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W. C., Kadrichu,

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HEPA

Air Cleaning

782-798

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thickness

Packing

Nuclear

(ml

Cake

aP

San Diego,

I? G. (1990)

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Zlst DOE/NRC

material.

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characteristics

W

Zc

packing

loading

B., Abrahamson,

P R., Ellison,

(m)

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thickness

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Filter

Filter

particles

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a

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Gases, 243-255

C., Richardson, filter

ofthe

P, Leclerc, drop

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98,4th

J., Higgins,

Efficiency

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The

The circular

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Academic

Airfiltration,

& Separation