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 or by laminating.
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 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 or fibers for the different layers are applied one immediately after the other before drainage and consolidation, 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, can be produced.
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 an inclined wire portion and optionally to a horizontal or a substantially horizontal wire portion of a forming wire, 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 the layers are formed in forming zones the length of which gradually increases on the inclined wire portion in the direction of travel of the forming wire. Preferably the layers are formed in forming zones of gradually decreasing inclination.
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 length of the forming zones on the inclined wire increases gradually in the direction of travel of the forming wire. Preferably the inclination of the wire of the inclined wire portion decreases gradually.
According to a preferred embodiment of the invention, the inclined wire is supported by drainage rolls.
In the following description, by way of example, reference will be made to the accompanying drawings, in which
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 16₁ - 16n divided into individual flow channels 18₁ -18n facing the inclined wire.
Each flow channel has an upper wall and a lower wall and the upper wall or lip 16n of one channel 18n-1 forms the lower wall or lip of the flow channel 18n on top of it.
The inclined wire portion is supported by a plurality of drainage rolls 20₁- 20n, which define a plurality of flat forming zones 22₁ - 22n the inclination angle α 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 24₁ -24n of the wire on which fibers from a flow channel 18₁ -18n is deposited. Thus, each flow channel leads to a flat forming zone having a different inclination angle which can be changed if required.
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 under 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 in each of which a plurality of support elements 28 such as deflector or hydrofoil or foil blades may be disposed. The drainage rolls 20 are also disposed in the drainage boxes.
The flow to each flow channel is separately controlled in relation to the wire speed by means of a flow regulator circuit in order to achieve a desired ratio between stock flow and wire speed as shown in Fig. 5, which illustrates the control system for one of the flow channels. The drainage at each forming zone is regulated according to the flow of the corresponding flow channel by means of flowmeters controlling the flow through each drainage box acting on a vacuum control in the drainage box. The level in the drainage boxes is kept constant by means of a level regulating circuit. 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 flow channel 18n ₊ ₁ leading to the horizontal wire portion is controlled as a conventional headbox by controlling the pressure in such a way that a desired ratio between the stock flow and wire speed is achieved. Also the flow channel 18n must be controlled in accordance with the velocity of the outlet flow as a portion of this flow leads to the horizontal wire portion.
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 portion. 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, which has the highest density and which normally is on the exit side of the filter paper, is formed on the horizontal wire from stock delivered by a flow channel 18n+1 disposed on top of flow channel 18n. 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, as shown in Fig. 4, so that there is caused an intermingling of the fibers at the interface. In the transition region between the inclined wire 10 and the horizontal wire 12 there is still fiber suspension from the last inclined formation zone 22n.
The stock delivery system comprises a plurality of stock chests 52₁ - 52n. Each stock chest is connected to a stock pump 54₁ - 54n through lines 56₁ - 56n and 58₁ - 58n 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 pit 60₁. The white waters from the other forming zones 22₂ - 22n are collected in a second white water pit 60₂. Alternatively each forming zone could be provided with its own white water pit. Each flow channel is connected to a mixing pump 62₁ - 62n through a stock delivery line 64₁ - 64n. The mixing pump 62₁ is connected to the first white water pit through a white water return line 66₁ and the mixing pumps 62₂ - 62n are connected to the second white water pit through white water return lines 66₂ - 66n so as to form a plurality of flow circuits. The inlet of the first mixing pump 62₁ delivering stock to the first flow channel 18₁ is connected to each of the stock circuits through connecting lines 68₁ - 68n. The inlet of the second mixing pump 62₂ delivering stock to the second flow channel 18₂ 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 70₁-70n by means of which stock from one or more elected stock chests can be delivered to one or more selected flow channels. This provides for a high flexibility of the operation of the multilayer filter paper machine.
Fig. 6 shows another embodiment of the invention in which the inclination angle α is constant. The angle β between the upper wall or lip 16 and the wire 4 decreases gradually in the direction of travel A of the wire so as to increase the length of the formation zone 24 towards the end of the inclined wire portion without increasing the lip opening H.
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. It is clear that the method of the invention can be carried out by using a curving wire instead of an inclined wire comprising a plurality of flat sections as in the embodiment shown.