The present invention relates generally to a method and apparatus for manufacturing webs of multilayer filter materials on the wire of a multichannel inclined wire former and is directed, more particularly, to a novel method and apparatus for producing filter paper for use in air and oil filters which have a high filtration capability.

It is known that to filter particles from gases or fluids, filter materials which are made of layers of different densities should be used in order to achieve a longer service life of the filter.

Methods and apparatuses for the manufacture of multilayer filter papers are known.

Multilayer filter papers can be made by forming the individual layers on the wires of individual web formers and couching them together.

U.S. patent No. 2,928,765 describes a process for the manufacture of a double-layered air filter paper on a web former using an inclined wire. The process comprises the steps of forming a coarse-pored layer by flowing a slurry of a first slurry of fibers onto the inclined wire and forming a fine-pored layer by immediately thereafter flowing a second slurry of fibers onto the wire.

DE-AS No. 1220716 describes an apparatus for the manufacture of multilayer fibrous webs wherein the stocks are supplied to the inclined wire of a web former through a plurality of horizontally arranged individual channels.

When the multilayer filter material is formed on an inclined wire and the slurries of fibers for the different layers are applied one immediately after the other, an

intermingling of the fibers at the interfaces between the layers is achieved. Due to the presence of the mixed zones, the filtration results of the filter material are positively influenced.

When the multilayer filter is produced by couching together individual layers formed on the wires of individual web formers, no such intermingling of the fibers between the layers is caused.

Depending on the use of the filter material, different filtration properties are required. The porosity grade and the basis weight are chosen so that, as regards the efficiency and the service life of the filter material, optimal values are achieved. -=

The object of the present invention is to provide a new and improved method and apparatus for the manufacture of webs of multilayered filter material, by means of which filter webs having better properties than webs hitherto formed, and which can more easily be adapted to meet a variety of requirements as regards the capacity, efficiency, bulkiness, lifetime, basis weight etc. of the filter material.

Other objects, features and advantages of the invention will be readily apparent from the following description of a representative embodiment thereof, taken in conjunction with the accompanying drawing although variations and modifications may be effected without departing from the spirit and scope of the novel concept embodied in the following detailed description of the invention.

The method of the present invention comprises the steps of delivering stock to a forming wire having an inclined wire portio and a substantially horizontal wire portion, and forming layers of fibers in consecutive forming zones in each of which a layer of fibers, superimposed on the

layer formed in the preceding forming zone, is to be formed. It is a characteristic feature of the method according to the invention that stocks of different grades are supplied to a plurality of stock chests and stock is fed -from one or more selected stock chests to one or more selected flow channels.

The present invention is also directed to an apparatus for manufacturing webs of multilayer filter material, comprising a web former provided with a forming wire having at least one inclined wire portion and a horizontal or substantially horizontal wire portion, and a multichannel headbox in which the flow channels are arranged one above the other in such a way that the upper wall of one channel forms the lower wall of the flow channel above it, said flow channels leading to subsequent forming zones in each of which a layer of fibers, superimposed on the layer formed in the preceding forming zone, is to be formed. It is a characteristic feature of the apparatus according to the invention that the means for supplying stock to the headbox comprises a conduit and valve system by means of which stock from any one of the stock chests can be delivered to any one of the flow channels of the headbox.

In the following description, by way of example, reference will be made to the accompanying drawings, in which

Fig. 1 is a schematic elevational sectional view showing a former for use in the practice of the present invention,

Fig. 2 is an enlarged fragmentary detail of one of the drainage zones shown in Fig. 1,

Fig. 3 is a longitudinal section of the drainage roll shown in Fig. 2, and

Fig. 4 is an enlarged fragmentary elevational sectional view showing the transition region between the inclined wire and the horizontal wire portion.

Fig. 1 illustrates a web former 2 adapted for producing a multilayered filter paper. It comprises a forming fabric such as a wire 4 running in an endless loop around a breast roll 6 and a plurality of return rolls, guide rolls and stretch rolls 8 in a manner known per se. The wire comprises an inclined wire portion 10 and a horizontal or substantially horizontal wire portion 12. A headbox 14 is by means of partition walls 16 _x - 16 _n divided into individual flow channels lδ _j -18 _n facing the inclined wire.

Each flow channel has an upper wall and a lower wall and the upper wall 16 of_one channel lβ _j -. _j forms the lower wall of the flow channel 18 _n on top of it.

The inclined wire portion is supported by a plurality of drainage rolls 20 _x - 20 _n , which define a plurality of flat forming zones 22 _x - 22 _n the inclination angle *• of which decreases in the traveling direction A of the wire. The location and length of each forming zone corresponds generally to the location and length of the portion 24 _x - 24 _n of the wire on which fibers from a flow channel 18 _x - 18 _n is deposited. Thus, each flow channel leads to a flat forming zone having a different inclination angle.

The drainage roll 20 is shown more in detail in Figs 2 and 3. The roll comprises a rotatable shaft 30 on which a plurality of discs 32 are disposed a distance apart from each other. A plurality of longitudinal rods 34 are attached to the rims of the discs spaced apart from each other so as to form a roll having a very low flow resistance. In operation, the roll allows the wire to move towards the roll center every time the gap between a successive pair of rods passes inder the wire, which causes oscillation of

the wire, whereby the formation and dewatering of the fiber layers on the wire are positively influenced.

Under the wire, there is a plurality of vacuum or suction drainage boxes 26 and in each drainage box, there is a plurality of drainage elements 28 such as deflector or hydrofoil or foil blades. The drainage rolls are also disposed in the drainage boxes.

The flow to each flow channel is separately controlled by controlling the pressure in the respective drainage boxes 26 as shown in Fig. 4, which illustrates the control system for one of the flow channels. The flow of stock through line 36 to flow channel 18 is measured by a flow meter 38. The flow of white water from the drainage boxes 26 is measured by flow meters 40. The level of the white water in the drainage boxes is controlled by level controller 42 and valves 44. A vacuum pump 46 is connected to the drainage boxes. A pressure controller 48 is connected to the drainage box. A flow controller 50 is connected to vacuum controller 48 and flow controller 40. An equilibrium is maintained between the volume flow of stock to the flow channel and the volume flow of white water from the drainage boxes, i.e. the quantity of liquid which is passing through the wire. The quantity of stock which can be applied is limited by the drainage resistance of the fiber mat on the wire.

The filter paper is built up of a plurality of layers. Each layer is superimposed on the layer formed in the preceding forming zone. As the drainage resistance of the fiber mat increases from one forming zone to the next, the dewatering pressure has to increase correspondingly. However, as the inclination of the wire decreases in the traveling direction of the wire, the length of the forming zones increase towards the end of the inclined wire. This means that the available drainage time increases which compensates for the increasing drainage resistance.

At least a substantial part of the sheet will be formed on the inclined wire. The uppermost layer is formed on the horizontal wire from stock delivered by a flow channel 18 _{n+ 1} disposed on top of flow channel 18 _n . The stock for the last layer, as well as for all the other layers, is deposited on a sheet which is not yet fully formed so that there is caused an intermingling of the fibers at the interface.

The stock delivery system comprises a plurality of stock chests 52. - 52„ . Each stock chest is connected to a stock pump 54 _x _=- 54 _n through lines 56 _x - 56 _n and δδ _j - 58 _n which form stock circuits by means of which at least a portion of the stock withdrawn from each stock chest can be returned to it.

The white water from forming zone 22, is collected in a first white water saveall 60 ₁ . The white waters from the other forming zones 22 ₂ - 22 _n are collected in a second white water saveall 60 ₂ . Each flow channel is connected to a mixing pump 62 _x - 62 _n through a stock delivery line 64 _x - 64~. The mixing pump 62 _t is connected to the first white water saveall through a white water return line 66 ₁ and the mixing pumps 62 ₂ - 62 _n are connected to the second white water through white water return lines 62 ₂ - 62 _n so as to form a plurality of flow circuits. The inlet of the first mixing pump 62 _x delivering stock to the first flow channel 18 ₁ is connected to each of the stock circuits through connecting lines δβ _j - 68 _n . The inlet of the second mixing pump 62 ₂ delivering stock to the second flow channel 18 ₂ is in a corresponding way connected to each of the stock circuits through lines (not shown) etc. In each of the connecting lines, there is a valve 70 _x - 70 _n by means of which stock from one or more elected stock chests can be delivered to one or more elected flow channels. This provides for a high flexibility of the operation of the multilayer filter paper machine.

The invention is typically put into practice in the production of 3-layered filter paper, two layers of which are formed on the inclined wire portion and the third on the horizontal portion of the forming web. Alternatively, all layers can be formed on the inclined wire. The invention can be utilized in filter paper machines having an inclined wire portion the inclination angle of which is the same in all forming zones or a curved wire.