The Influence of Pleat Geometry on the Pressure Drop in Deep-pleated Cassette Filters
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Transcript of The Influence of Pleat Geometry on the Pressure Drop in Deep-pleated Cassette Filters
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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
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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: [email protected])
; . 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
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; : . . -::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
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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~~ ,
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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
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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