Method of making multi-pocket filter

A filter element, as in a filter bag, comprises a pocket having partitions between the side walls to form pocket portions. The margins of the partitions are welded ultrasonically to the inner surface of the layers. The inlet end of the partitions may be split into flaps to ease connection of the bag to a header frame. A simple yet effective method and apparatus may be utilizable to make the filter element, and a bag including it.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates to the filtration of gases, typically of air to 
remove dust particles. The particles may range from about 0.3 to about 30 
micron in size. 
It is known to make a gas filter bag from gas permeable natural material in 
the form of generally parallel pockets. The pockets have an open upstream 
inlet and a closed bottom end, the intention being that the dust particles 
will be trapped in the closed end of the pockets as the gas flow passes 
therethrough. The pockets may be defined by longitudinal seams formed by 
stitching. Usually a hot melt adhesive is applied over the stitched holes 
to seal them to avoid escape of particles. This is a labour intensive 
manufacturing method and the gas entrapment properties are not very good. 
It is one object of the invention to provide a gas filter of improved 
performance. 
According to the invention in one aspect there is provided a gas filter 
element comprising 
two side walls forms of gas permeable material joined along three of their 
sides to define a pocket having an upstream open inlet and a downstream 
closed end, characterised in that generally parallel partitions bridge the 
opposite side walls to define individual pocket portions extending from 
the inlet end to the closed end, marginal portions of the partitions being 
secured to the inside of the side walls, whereby in use the air filter 
element exhibits a low pressure drop and a high degree of dust efficiency 
and dust holding capacity. 
The walls may be made of any suitable material which may be wholly or 
mainly synthetic fibres. Because the partitions are joined to the walls by 
a welding process, the material should be fusible (or at least thereof 
should be). 
Preferably the partition is a thin generally planar strip of fusible 
material which in use presents a narrow edge to the flow of air. 
Preferably the marginal portions are secured to the inner surface of the 
side walls by ultrasonic welding. 
Preferably a short length portion of one edge of the partition adjacent the 
open inlet end is not joined to the inner surface of the adjacent wall to 
permit flexure of the front end of the pocket so that it may easily be 
mounted in say a filter header frame. Alternatively and more preferably an 
elongate slit is formed in the upstream end portion of the partition to 
provide two flaps which enable the pocket to be opened in the manner of a 
jaw to engage the frame. The length and shape of the slit will be 
determined according to each filter element and frame, which typically is 
a header frame. 
In another aspect the invention provides a continuous method of making a 
succession of pockets for a gas filter element, the method comprising: 
advancing two elongate webs of synthetic fibre material to a work station 
to lie on opposite sides of an ultrasonic welding unit; 
advancing parallel strips of fusible material to extend generally parallel 
to each other and to be positioned between the webs and substantially at 
right angles to the major axis of the webs; 
passing the strips through a former to fold up the longitudinal marginal 
portions so that the marginal portions lie on the inner surface of the 
respective webs; 
passing the webs and the marginal portions through the ultrasonic welding 
unit so as to ultrasonically weld the marginal portions to the respective 
web and so connect each side of the strip to the webs to form bridging 
portions in between the webs; 
sealing the side portions of the webs; 
cutting across the sealing the side sealed webs to form a closed end of the 
gas filter element and separate it from the webs.

DETAILED DESCRIPTION OF THE DRAWINGS 
The same reference numerals are used where convenient in describing the 
different embodiments. 
The filter bag shown in FIGS. 1 to 4 comprises two layers L1,L2 of gas 
permeable of synthetic fibrous material secured together to provide a bag 
having elongate filter elements, i.e. cells or pockets P which are shown 
in FIG. 2. The side walls may be formed of any suitable synthetic fibre 
material or blend of materials. One example is polypropylene. The material 
may be treated, e.g. with an antimicrobial agent to resist microbial mould 
growth and build up of moulds or mildews. The two layers are sealed 
together along three sides by double seals S1, as a result of which the 
pockets are closed at their bottom or distal ends Pb but open at their 
mouths Pm. As shown in FIG. 1 the bag is generally rectangular as seen in 
plan. The pockets have a generally rectangular open inlet or mouth Pm, 
(except for the end ones Pe where they tend to be more triangular because 
of the way the two layers L1,L2 are joined together). 
According to the invention individual pockets contain partitions 5 to form 
pocket portions. The partitions 5 are made of webs of fusible natural or 
synthetic sheet material such as polypropylene. The marginal portions 
5a,5b are sealed to the inside surface of the layers L1,L2 respectively. 
The partitions 5 extend the full distance from the front to the rear of 
the bag so that as shown in FIG. 1 there are parallel elongate seams S2. 
The bag is made in the following general way and as shown in FIG. 5. 
The apparatus comprises two rolls of webs W1,W2 which are located one on 
each side of two ultrasonic welding units U each made up of a shoe 6 and a 
horn unit 7 and are fed to extend parallel to each other one on each side 
of the units U. Strips or ribbons R of polypropylene are fed between the 
webs to the units U and when the assembly is fed passed the ultrasonic 
welding unit fusion takes place to form the partitions 5. While one unit U 
is arranged to make a continuous join the other has an interruption to 
provide a short length portion of one edge of the partition to be formed 
adjacent the intended open inlet so that the formed pocket P can flex at 
its front end for mounting in a filter header frame. The sides of the bag 
precursor are sealed by other ultrasonic welding units U. The formed bag 
is fed passed a cutting unit C to cut an individual bag, the rear end of 
which is then sealed, preferably heat sealed. This process is continuous, 
and the parameters may easily be adjusted according to the nature of the 
webs, fusion temperature and time, and dimensions. 
In a trial, a filter bag of the invention was tested in the filtration of 
dust laden air. The bag was mounted in a frame. It was easy to inflate. 
There was a high level of dust collection and a very low overall pressure 
drop. The filter resistance was controlled. The partitions 5 hold the 
pockets P in controlled aerodynamic configuration to ensure a uniform 
shape independent of the air flow. The ultrasonic bonding ensures a strong 
connection without holes. This eliminates leakage or bypass of the 
particles through such holes. It also avoids the need to seal 
needlepunched holes using a hot melt sealant, giving more useful 
filtration area. While we do not intend the invention to be limited by the 
following theory, our investigations show that the flow of air through the 
pocket is aerodynamically streamlined or laminar, so drawing more 
particles therein, and because each pocket is a sealed container very few 
if any of the particles escape. Evaluations have shown that a filter 
element according to the invention remains intact and dimensionally stable 
up to pressures exceeding 750 pa. 
In the embodiment shown in FIGS. 6 to 9, the inlet end of the partition 5 
is slit to form two general parallel side flaps F1,F2 sides separated by a 
gap. The slit extends a short distance in from the end, sufficient to give 
the necessary flexibility according to the strength of the materials 
forming the layers L1,L2 and the partition 5. The flaps are secured to the 
layers by the double seals 6A,6B which are ultrasonic seals. As a result, 
the mouth of the pocket can be open in the manner of jaws, and the 
engagement of the partition and the walls will not be broken. 
As shown in FIG. 9, the bag is connected to a metal header or frame 10 of 
generally rectangular section. The front portions of adjacent pockets are 
crimped or otherwise secured by metal clamps to the edge of the frame. The 
header or holding frame may be of any suitable type, e.g. for front, rear 
or side withdrawal mounting. The frame may be made of metal or plastics. 
The bag can be made on a continuous basis in the following way shown in 
FIGS. 10 and 11. 
The apparatus comprises parallel rod-shaped ultrasonic welding units U1, U2 
and U3 each made up of a shoe and horn or anvil unit and two rolls of 
synthetic fibrous material W1,W2 which are located on opposite side of U1 
and U2 and are fed to extend parallel to each other on opposite sides of 
U1 and U2 (see FIG. 10). Strips or ribbons R of polypropylene are fed 
between the webs W1, W2 and the units U1 and U2 to fuse both marginal 
edges of R to the web. The sides of the precursor are sealed by other 
ultrasonic welding units U3. The partly formed bag is fed passed a cutting 
unit C to cut an individual bag, the rear end of which is then sealed, 
preferably heat sealed. A marginal 5a,5b portion of each partition 5 near 
the upstream end of the element is then cut to introduce the longitudinal 
slit and form the flaps F1,F2. 
As shown in FIG. 11 each strip R is wound off a reel 11 over a roller 12 
and fed into a former 20 the top edge of which is curved to upturn the two 
margins 5a,5b so that the strip is generally channel shaped. The folded up 
strip is then passed through the unit at the lower end of which is an 
ultrasonic shoe 6 in the form of a wheel having a patterned surface. The 
wheel 6 is on a shaft which is rotated by means of a belt drive 21. Behind 
the wheel is a web W1,W2 and behind that is the horn 7 of the ultrasonic 
welding unit in line with the wheel. As the margin 5a of the strip and the 
web W1 meet the shoe 6 and the horn 7 ultrasonic energy is applied to fuse 
them together. (In practice the web comprises an inner skin and an outer 
carrier surface with filter media in between; the margin 5a is fused to 
the inner skin). Pressure is applied by the arm 31 of a rocker 30 under 
pneumatic control to urge the margin 5a onto the web so that there is good 
contact in between. The welding must be sufficient to secure the margin 5a 
and the web W1 together but not so strong that either material will be 
torn in use or be burned by the welding. The margin 5b will be welded to 
its web W2 using another unit further down the line. 
In a preferred embodiment, not shown, the side seals S1 are made a short 
distance in from the longitudinal edge from the webs and a slitter, not 
shown, is used to cut away the excess material which is then disposed of. 
The invention also includes a method of filtration, comprising passing gas 
under pressure through a filter element as defined and causing or allowing 
the particles to collect in the filter pockets. The invention also 
includes apparatus for making the filter elements. 
The invention is useful wherever bag filters are suitable. The invention 
ensures that heavyweights of dust are trapped in the filter elements over 
extended maintenance periods; can protect equipment in general air 
conditioning plant, e.g. in hospitals, communal buildings, office suites, 
retail outlets and the like; and reduces energy consumption because of the 
low initial resistance to air flow.