abstract translations

As a service to readers who understand German, French or Spanish better than English, the abstracts for the Research article in this issue follow in these languages.

The Influence of Pleat Geometry on the Pressure Drop in Deep-pleated Cassette

Filters Dr.-lng. Thomas Caesar & Drdng. Thomas Schroth

Der Einfluss von Faltengeometrie auf den Druckabfall bei Kassettenfiltern mit Tieffalten Dr.-lng. Thomas Caesar 6 DipI.-lng. Thomas Schroth

Im Bereich der Luftfiltration stellt der Druckabfall eines Filters bei einer definierten Abfangleistung einen wichtigen

Parameter dar. In dem vorliegenden Referat werden die Veranderlichen erortert, die sich auf den Druckabfall bei Luftfiltern

mit Tieffaltenmedien auswirken. Bei kassettenartigen Feinfiltern gem& EN 779 oder bei HEPAiULPA Filtern gemaR

EN 1822 sind die am weitesten verbreiteten Medien papierahnliche Stoffe einer Starke von weniger als 1 mm, die dem

durchfliefienden Luftstrom ziemlich hohen Widerstand entgegensetzen. Hersteller sind daher bemuht, ein HochstmaR an

Filtermediumfldche in einem moglichst kleinen Raum unterzubringen. Urn sicherzustellen, dass die bei Zuluft-, Abluft- und

Umluftfiltration ublichen Druckabfalle gewahrleistet sind, ist das Filtermedium daher in schmalen, tiefen Falten ausgelegt.

Insbesondere bei der Handhabung groi3er Luftmengen pro Filterelement ist es vorteilhaft, das Filtermedium in Tiefen von

1.50 mm bis 280 mm zu falten. Die art der Filtertechnik und die sich daraus ableitende Faltengeometrie uben einen

entscheidenden Einfluss auf den jeweiligen Druckabfall aus.

(5 sn., 8 figs., 0 tabs., 7 refs.)

Influence de la giomitrie du plissage sur la perte de charge des filtres-cassettes profonds Or-lng, Thomas Caesar et DipI.-lng. Thomas Schroth

8 plis

Dans les applications de filtration dair, la perte de charge pour une efficacite don&e de separation constitue un important

parametre du filtre. Cet article traite des variables influengant la perte de charge dans les filtres a air dont le media filtrant

est a plis profonds. Pour les filtres-cassettes fins (suivant EN 779)) ou pour les filtres HEPAPAIULPA (suivant EN 1822), les

media filtrants les plus couramment utilises sont les mattriaux semblables au papier avec une epaisseur de moins dun mm

offrant une resistance relativement importante a lair qui les traverse. Les fabricants sefforcent de loger un maximum de

surface filtrante dans un espace minimum. Afin de permettre des pertes de charge convenables dans les filtrations avec

aspiration, refoulement ou recirculation, le media filtrant est par consequent dispose en plis etroits et profonds. En

particulier, lorsque les debits dair par element filtrant sont elevts il est avantageux de plisser le media avec des profondeurs

de 150 a 280 mm. La technique de conversion et la geometric du plissage exercent une influence cruciale sur la perte de

charge.

(5 pags., 8 figs., 0 tabs., 7 refs.)

La lnfluencia de la Geometria Ondulada sobre la Caida de la Presion en Filtros de Cassette de Profunda Ondulaciun Dr.-lng. Thomas Caesar y Dipl.-lng. Thomas Schroth

En aplicaciones de filtration de aire, la caida de la presion de un filtro a una eficacia de captacion definida es un parametro

importante. Esta ponencia discute las variables que influencian la caida de la presion en filtros de aire con medios de

filtration de profunda ondulacion. Para 10s filtros finos de1 tipo cassette, de acuerdo con EN 779, o para filtras

HEPAIULPA, de acuerdo con EN 1822,los medios mas comunmente usados son 10s materiales de1 tipo papel con un

espesor de menos de 1 mm, que ofrecen una relativamente alta resistencia al aire que fluye a traves de 10s mismos. Los

fabricantes, por lo tanto, procuran facilitar un maxim0 de area de1 medio de filtration en un pequeno espacio. Para

permitir la seguridad de la caida de presion normal en la filracion de1 aire de entrada, de exhaustacion y recirculada, el

medio de filtracibn se dispone, por lo tanto, en ondulaciones estrechas y profundas. Especialmente cuando se esta tratando

con grandes cantidades de aire por element0 de filtration, es ventajoso ondular el medio de filracion en profundidades de

150 mm a 280 mm. La tecnica de conversion y la geometria ondulada resultante ejercen una influencia crucial sobre la

caida de presion en cuestion.

(5 pigs., 8 figs., 0 tabs., 7 refs.)

48 November 2002 www.filtsep.com

article

In air filtering applications, a filters pressure drop at a defined collection efficiency constitutes an

important parameter. This paper discusses the variables influencing the pressure drop in air filters

featuring deep-pleated filter media. For cassette-type fine filters in accordance with EN 779

or for HEPA/ULPA filters in accordance with EN 1822, the most commonly used media are

paper-like materials with a thickness of less than 1 mm, which offer a relatively high resistance to the

air flowing through them. manufacturers accordingly endeavour to accommodate a max;mum of filter

medium area in a small space. To enable the pressure drops customary in intake, exhaust

and re-circulated air filtration to be assured, the filter medium is therefore arranged in narrow,

deep pleats. Particularly when large quantities of air are being handled per filter element, it is

advantageous to pleat the filter medium in depths of 150 mm to 280 mm. The conversion technique

and the resultant pleat geometry exert a crucial influence on the pressure drop concerned,

Dr.-lng. Thomas Caesar & DipA-lng. ~bo~as ~c~rotb Freudenberg Vliesstoffe KG, D-69465 Weinheim, Germany.

Tel: +49 6202 806264; Fax: +49 6202 886299.

Corresponding author (E-mail: thomas.caesar@freudenberg.de)

; . he fundamentals for calculating the pressure drop when a gas flows through a zig-zag shaped pleated filter medium

are derived below. It is here assumed that the filter

mediums rigidity is sufficiently high, i.e. that the air flow does

not affect the specified pleat geometry, and therefore that the

pleats can be regarded as completely rigid. This applies in good

approximation, particularly for paper-like media at the face

velocities customarily employed for air filters in indoor climate

control systems.

Figure 1 shows a V-shaped pleat geometry in diagrammatic

form. The air flows out of the surroundings onro the pleat

system at the velocity W,, and enters the pleat at z = 0, causing

the air to accelerate because the flow cross-sectional area

narrows at a ratio of b(z = 0) i F,. Inside the pleat, the air flows

in z-direction at a velocity w(z), passes through the filter medium

at a point z at the velocity v,(z~, and exits again from the pleat

system at z = F,. The flow velocity decreases here, due to the

widening of the cross-sectional area in the ratio F,/b(z = 0).

Three components can be differentiated for the total pressure

drop of such a deep-pleated filter element: l pressure difference Ap, inside the pleat due to friction losses

and dynamic pressure gain; l pressure drop due to contraction ApE and expansion Ap,

when entering and leaving the pleat system; and . pressure drop ApM while flowing through the filter medium.

For these three components approxil~ation equations are

derived below, which can be solved (analytically or numerically)

to calculate the total pressure drop.

Due to the flow conditions inside a pleat and the resultant

friction losses and dynamic pressure gain, there is a pressure

difference between the pleat entry and the point at which the air

flows through the filter medium. If we assume a stationary flow

field inside the pleat, the pressure difference in the main

direction of flow can be described by the Navier-Stokes equation

below:

where u, v and w are the velocity components in the x, y and z

b :Free -prt h d directions, p the air density and 1-1 the viscosity. Equation 1 is a

&:DkOMQd+?W *)-_.__ ._ ,_~. ..__._- __.) Fr.pfestdepm I

three-dimensional partial second-order differential equation, not

amenable to Drecise analvtical solution. For the geometrv under Figure 1: Pleat geometry and specification of d . , 1

Y lscusslon, however, it is possible to find a one-dimensional

coordinate system. differential equation for the pressure difference using appropriate

Filtration+Separation November 2002 49

; : . . -::article

0

3 4 5 6 7 8 9 10

Pleatdi&anceinmm

Figure 2: V-shaped pleat geometry: dependence of the total pressure drop and the components entailed by the medium and the pleat geometry on the pleat

distance (F,) for a filter medium with $, = 9350 m/s (pleat depth = 290 mm).

approximations and simplifications. It is amenable to numerical

solution through a suitable computer programme on normal

PCs, with a comparatively small amount of calculation work.

The following simplifying assumptions can be made:

a) In the x and y directions, the pressure p is almost constant.

We thus obtain:

dp, 3~ -=F dz &

b) The Navier-Stokes equation (Equation 1) is considered only

along the axis of symmetry (y = 0), where u = v = 0

c) The velocity profile of w(z, y, x) can be approximated with

sufficient accuracy in the x and y directions by a parabolic area.

We thus obtain:

P $+ =-12.pLw(z)lbZ(z) i I JY

where W(z) is the flow velocity in z direction averaged in x and y

directions.

Using these three assumptions, the Navier-Stokes equation

(Equation 1) reduces to:

dp,_ dz

__2,2517.wdM/-12~wlb2+1,5~dZ (2) dz dz2

Together with the continuity equation, from whose

numerical solution we obtain the mean velocities W(Z) as

discrete values along a calculation lattice, Equation 2

can be solved numerically

When the air enters and leaves the filter, a pressure drop

is generated by the alterations in cross-sectional area as

the air flows into the pleat and flows out of the pleat.

With W_ as the face velocity, and the air density p, these

contraction and expansion pressure drops can be

calculated using:

Ap, =fp.k,(K_-Z(z=O))*

b :MldmMOlp0n

Figure 3: Diagrammatic representation of a rectangular pleat geometry with separators

as spacers. The fundamentals of the calculation described in the

section above have been converted into a computer

50 November 2002 www.filtsep.com

Using

F ii;(z=O)=W,.A

b(z = 0)

it follows that:

APE.4 =ApPE+ApPA=+&+kA) (3)

where the constants k, and k, will depend on the sharpness of

the entry and exit edges.

For filter media customarily used in ventilation and air-

conditioning systems, and the operating conditions

predominantly encountered, the validity of Darcys Law

can be stipulated. Thus, the pressure drop while flowing through

the filter medium is proportional to the flow velocity vM:

APP, =~P&,.v~z) (4)

where

article

programme, which was used to calculate the results

presented here. The influence of V-shaped and

rectangular pleat geometries on the pressure drop in

deep-pleated filters, plus the influence of different pleat

depths and pleat distances, are discussed below. A

distinction is drawn here between two components of

the pressure drop: the pressure drop ApM caused by the

filter medium and the pressure drop attributable to flow

through the pleat structure Apt-,,. The pressure drop

caused by the filter medium has been calculated using

Equation 4 approximated with the mean flow velocity _ vhf, which follows from the assumption that the volume

flow is distributed evenly over the entire filter area. Thus,

the media pressure drop depends only on the face

velocity, the filter area and the material constant 5,. The

geometrical component is obtained from the difference

between the total pressure drop calculated using the

above-mentioned computer programme and the media

pressure drop: ApGCO = ApTot - Ap,.

$ 500 0 400 ij g 300

@

t! 200

100

Qf 6? d R? p i$ I 3 !:i i> 3 * A- i*$ $_~y&rqp~~ ,

52

80%

60%

3 4 5 6 7 8 9 10

Pleat distance in mm

Figure 6: Component of the pressure drop caused by the pleat geometry in % (relative geometrical

component) as a function of the pleat distance (Fe] for V-shaped pleats, with different pleat depths

(100 mm, 200 mm and 260 mm) at a face velocity of 1 m/s. And for a 260 mm pleat depth at a face velocity of 2.9 m/s and 10 m/s for a filter medium

with

80%

60%

20%

0%

3 4 5 6 7 8 9 10

Pleat distance in mm

Figure 8: Proportion of pressure drop caused by the pleat geometry in % (relative geometrical component) as a function of the pleat distance (Fe) for V-shaped

pleats with a pleat depth of 280 mm. Two filter media with different air permeability at a face

velocity of 2.8 m/s.

exhibits a significantly weaker dependence on the air-

permeability of the medium concerned. Accordingly, the

percentage of the total pressure drop attributable to the

geometry increases with diminishing $. In the example shown in

Figure 8, the proportion of the total pressure drop attributable to

the pleat geometry is 22 % for 6, = 9350 m/s for a pleat distance

of 6 mm. Media with this air-permeability are mostly used in the

field of HEPA filtration, with Classes HlO to H14 in accordance

with EN 1822. For media with $, = 935 m/s, a typical value for a

filter medium in the field of fine filtration with Classes FS to F9

in accordance with EN 779 - the geometrical component is 76%

for a pleat distance of 6 mm.

The results discussed show that the pleat geometry, and thus

the conversion technique used for deep-pleated cassette filters,

exerts a significant influence on the pressure drop of the filter

elements concerned. In the case of deep pleats in filter media of

high air-permeability, the pleat geometry is a particularly

dominant influence on the filter elements pressure drop. The

V-shape created when using the patented thermal embossing

process, for example, is to be regarded as the optimum pleat

geometry. Every deviation from this ideal geometry leads to an

increase in the pressure drop overall. With the separator and

mini-pleat designs, a V-shaped pleat geometry is impossible to

implement in any satisfactory form. But even production-

entailed deviations from the ideal pleat geometry, such as a

curvature of the pleats in the main direction of flow, will have a

seriously adverse effect on the pressure drop.

An air filters pressure drop at a defined collection efficiency

constitutes an important parameter. The basis for calculating the

pressure drop in a deep-pleated cassette filter is the Navier-

Stokes equation for stationary flows. Using appropriate

assumptions, a one-dimensional differential equation system can

be found, amenable to numerical solution with comparatively

little computation work. The results it supplies show that the

pleat geometry exerts a crucial influence on the pressure drop of

the entire filter, particularly when large pleat depths and small

pleat distances are involved. The V-shaped pleat geometry, as can

be achieved, for example, using the patented thermal embossing

process, can be regarded as ideal. Deviations from this ideal

pleat geometry have a seriously adverse (i.e. increasing) effect on

the pressure drop. In the case of fine filters with large pleat

depths, particularly, the pleat geometry is the dominant factor in

the pressure drop. The choice of conversion technique for

producing the pleat structure is thus more important for these

filters than the air-permeability of the medium employed. If an

unsuitable conversion technique is chosen, no significant

reduction in the filters pressure drop can be achieved even if the

filter medium itself is optimized. l

1.

2.

3.

4.

5.

6.

7.

EN 779: Particulate air filters for general ventilation. 1994.

Beuth Verlag GmbH, Berlin, Germany,

EN 1822: High efficiency particulate air filters (HEPA and

ULPA). 199812001. Part l-5, Beuth Verlag GmbH, Berlin,

Germany.

Forster B. 1999. Ein neues Zellenmodell zur Bestimmung

von Abscheidegrad und Druckverlust der in der

Klimatechnik verwendeten Filtermedien. Dissertation

Universitiit GHS Essen, Germany

Loffler F. 1988. Staubabscheiden. Georg Thieme Verlag,

Stuttgart, Germany

Nietzold I. 1979. Luftfiltration. Reihe Luft- und

Kaltetechnik, VEB Verlag, Berlin, Germany.

Schroth T. 1996. New HEPAiULPA Filters for Clean-Room

Technology Filtration+Separation, 33 (3),

p.245-250.

Zierep J. 1997. Grundziige der Stromungslehre. Springer

Verlag, Berlin, Germany.

54 November 2002 www.filtsep.com