WO1997035066A1

WO1997035066A1 - Dewatering device for a paper machine

Info

Publication number: WO1997035066A1
Application number: PCT/FI1997/000182
Authority: WO
Inventors: Pekka Rantala
Original assignee: Ppr Consulting Ltd. Oy
Priority date: 1996-03-20
Filing date: 1997-03-20
Publication date: 1997-09-25
Also published as: EP0894162A1; AU2161697A; FI961273A0

Abstract

The invention concerns a dewatering device to be located between the wire section and the dryer section of a paper machine in connection with the felt and the wire. This device constitutes at least one porous absorbent web (2) arranged against and running with the wire or the felt.

Description

Dewatering device for a paper machine.

The innovation concerns raising the dry matter content of paper or other fibre webs without wet pressing. The innovation replaces the conventional felt presses used since the middle of the 18th century. During the last few decades all paper machine suppliers have developed their own press section solutions all based on the use of high nip pressures and the transfer of water by the use of pressure from the fibre web into the felt structure.

The purpose of a conventional press section is to remove water after forming so that the dryness of the fibre web is increased from about 20 % to the level of 40 - 50 %. The dry matter content after the dryer section is of great economic importance. For instance an increase from 40 % to 41 % means a saving of 4 % in the drying or corresponding improvement in the drying capacity. Because of this, the development of the press section technology has been very active.

When considering what really happens in the press section, the inventor's article published in 1970 (Tappi conference) can be used as an example and the drawing presented therein it can be stated that:

- the fibre web shifts over to the 1st press felt with a dry matter content of 20 % carrying along 350 l/min of water.

- the fiber web proceeds to the drying section having passed three so called nips with a dry matter content of 42 % carrying along 110 l/min of water.

- simultaneously in the three felts of the press 4400 l/min of water in total is recirculating,

- the total amount of water drainage from the fibre web is only 5,5 % of the total water flow.

The drainage efficiency of a conventional press section is thus very low. The main reasons for this inefficacy are as follows :

- the cellulose fibers are more absorbent than the synthetic fibers and wool used for the preparation of felts.

- the capillary forces in paper are stronger than in the press felt.

- pressures are, and the felts must be harder.

- the dry matter content of the fiber web as well as press felt is highest in the nip but due to the aforesaid reasons immediately after the nip has opened the water runs back to the fiber web (rewetting) .

Using for example 3-4 nips and high pressure the fiber web is compressed and loses its bulky character which is important especially for certain paper qualities (stiffness, opacity, porosity etc) . Also in drying the water is more easily drainage if the fiber structure is more open at the initial stage of the drying.

According to the solution of the invention it is possible to eliminate the difficulties in the press section described above. Characteristic for the invention is that a felt structure is developed utilizing the high absorbance of the fiber structure whereby no pressing power is needed.

The fiber structure should be open and the absorption fast. The water soaked up by the fibers should correspondingly be removed rapidly from the felt structure.

The fibre web is transferred by this so called suction felt directly from the wire/former onto the surface of the first dryer. Impurities like fillers and fine fiber material (o- fiber) stick on the felt surface. For the restoration of the felt before the next contact with the fiber web the suction/pressure scrubber of the invention is being used. No abrasive friction forces are applied on the fiber web having an essential effect on absorption and life time. The suction felt system according to the invention picks up the fiber web from the wire and convey it in closed manner directly to the surface of the first dryer. In some cases it is advantageous to use low pressure in applying the web on the surface of the dryer. Also in reconditioning of the felt it is in some cases possible to use moderate pressure, so-called wringer washer press.

The removal of water from fiber web to the felt and if required further to the suction device can be improved by using "Rotabelt" technic which earlier has been used in the wire sections of board machines but applied for the present purpose. Said "Rotabelt" concept consist of a perforated rubber belt running between two rolls (dia. 20-40 cm) as well as a suction box on the inside.

In the present invention Rotabelt is utilized diverging from the original scope of application as follows:

- instead of the rubber belt depending on the use also a plastic wire can be used which roughness applied directly on the fiber web is essentially lower than in reconditioning the felt,

- inside the wire there are in some cases vacuum and for some purposes narrow hot air jets for more effectively driving the water into the belt and/or transferring the water in the belt to the suction device,

- said Rotabelt may be used also for drying the fiber web, whereby the fiber web is directly or via the wire fabric ion contact with it. The fiber web may also be run between two opposite Rotabelt units applying suction or hot air jet. Here a so called through drying is concerned and the device of the invention may be used also on the wire/formation section.

The invention is suitable for use in all paper machines. The invention may be utilized also in already existing press devices by running the device without pressure at the actual rounds of the felt. With the suction felt according to the invention the fiber web may be led in a closed manner through a so called pressure section directly to a first dryer. It is especially suitable to be combined with the invention of the patent application 960102 as a part of a total solution.

There are several benefits to be achieve by the invention:

- the construction of the press section is shrunk to 20 % by

- the building does not need to have a cellar which reduces building volume. The establishment expenses are lower.

- the need for vacuum decreases drastically

- paper technical advantages:

-the fiber structure remains open resulting in improved stiffness, opacity and porosity -the drying is easier -the bulk of the paper may be increased if required during drying, etc. -the felt is not closed, it can be easily installed in correct position

-because the fiber web does not run through any press nip the "quick seam" of the felt does not cause any felt markings on the fiber web.

The invention may be summarized as follows:

- to develop a new type of a felt for a paper machine

- a mechanical framework including additional equipment for running and servicing the felt

- device for reconditioning the suction felt

- a so called Rotabelt type device for servicing the suction felt and improving drainage - is also suitable for drying the fiber web drainage of the wire section.

The principe of the invention has been presented in the accompanying drawings where

Figure 1 shows in cross section a suction box used in the invention, Figure 2 shows an application wherein three successive suction boxes are being used,

Figures 3 and 4 shows a device according to the invention arranged between the winding and drying sections of a paper machine,

Figure 5 shows an embodiment of the suction box according to this invention.

In the suction box solution according to the invention a porous felt web 2 is running guided by two guiding wheels 1. Between upper and lower segment of the web track a housing 3 for the suction box is arranged provided with connections 6 for suction and/or elevated pressure. The wall 5 of the housing 3 facing the inner surface of the felt web 2 is perforated or equipped with slots for directing suction to blow on the felt web 2. The wall surface 5 is preferably made of an abrasive resistant material such as ceramics, plastic material, e.g. teflon or alike.

The perforations 5 or slots 5 are arranged transversely to the running direction of the web or obliquely to the running direction, an air velocity as high as possibly is preferred.

The felt web 2 follows the felt carrying the paper web and in contact with it whereby water is sucked up by the felt web which is of a high absorbent material and essentially dryer than the fiber web, preferably with a dry matter content of 50 - 60 %. The reduced pressure further improves this transfer of water from the paper web to the felt web 2. The elevated pressure applied on the fiber web further improves the water transfer from the fiber web to the surface of the felt web or its structures. Figure 2 illustrates an application corresponding to that disclosed in figure 1 but between the guiding rolls 2 three successive suction and/or blower boxes 3 have been arranged. Of these boxes for example the one in the middle may be a blowing device whereby a suction and a blowing effect is applied to the paper web. This impact effect is effectively releasing water entrapped in the web and eventual impurities.

Figures 3 and 4 show how the suction system 7 according to the invention is associated to a paper machine. In figure 3 the wire section is presented by reference numeral 8 from which the paper web is transferred assisted by guiding rolls to the dryer section 9. Between these are the suction and/or blowing devices 7 according to the invention located on both sides of the felt transporting the paper web, the lower device 7 intensifying the transfer of water from the paper web to the felt and the device above 7 is further removing water from transport web or transfers it to the structure or removes it from the web. A dewatering device 7 according to the invention has further been attached to the dryer section before winding the paper web or between the dryers.

Figure 4 illustrates an embodiment corresponding to figure 3 wherein, however, the wire section 8 and the dryer section 9 have been differently arranged.

Figure 5 presents a suction hood 3 used in the invention essentially intended to be located above the felt web as a suction box. In this embodiment the hood 3 is curved and its lower surface 4 facing the felt 2 is provided with oblique canals 5 through which suction and/or blowing may be applied on the felt 2. As a result of the curved wall portion 10 the humid air stream in including water droplets and eventual impurities entering the hood collides with the wall and forced to flow along it. Upon colliding with the wall the humidity is condensed and the accumulated droplets are flowing along the wall 10 to a collecting chamber 11 at the rear edge in view of the running direction. In the embodiment of figure 5 the suction connection 6 is located above the collecting chamber 11 and above the rear edge of surface 10.' Thus the humidity released from the felt can be recovered. The air departing through the connection 6 is additionally conveyed to a moisture trap where the remaining moisture may be removed.

Claims

1. Dewatering device to be located between the wire section ₍8₎ and the dryer section (9) of a paper machine in connection with the felt and the wire, c h a r a c t e r i z e d in that it constitutes of at least one porous absorbent web ⁽2⁾ ar¬ ranged against and running with the wire or the web.

2. Dewatering device according to claim 1, c h a r a c t e r i z e d in that on opposite side of the surface the porous absorbent web (2) arranged against the wire or the web a suction and/or blowing device is provided.

3. Dewatering device according to claim 1, c h a r a c t e r i z e d in that the suction and/or blower device (3) is formed of a housing provided with vacuum connec¬ tions (6) and the web side surface (4) of which is perforated

(5) or provided with slots (5) for creating reduced pressure between the wall surface (4) of the housing and the web (2) for sucking up water from the felt.

4. Dewatering device according to claim 1, c h a r a c t e r i z e d in that the suction and/or blower device ₍3₎ is formed of a housing provided with pressure connections (6) and the web side surface ⁽4⁾ of which is perforated (5) or provided with slots (5⁾ for creating pres^¬ sure between the wall surface (4) of the housing and the web ₍2₎ for blowing the water out of the pores of the felt or the fiber web into the felt structure.

5. Dewatering device according to claim 1, c h a r a c t e r i z e d in that the absorbent felt web (2⁾ contains absorbent fiber compositions, e.g. 70 - 80 % viscose and 20 - 30 % wool or polyester fibers.

6. Dewatering device according to any of claims 1 - 4, c h a r a c t e r i z e d in that the suction and/or blower device (3) is provided with suction and blowing sections in the running direction of the felt.

7. Dewatering device according to any of claims 1 - 6, c h a r a c t e r i z e d in that the several suction and/or blower devices (3) have been located at the same side of the felt and/or face to face on opposite sides of the suction felt.

8. Dewatering device according to claim 7, c h a r a c t e r i z e d in that separate suction and/or blowing devices functions either as suction devices or blowing devices.

9. Dewatering device according to any of claims 1 - 7 c h a r a c t e r i z e d in that the narrow (l - 5 mm) slots ⁽5⁾ of the suction and/or blower device (3) enables under both suction and elevated pressure high velocities of suction air or high pressure blowing through hot air.

10. Dewatering device according to any of claims 1 - 7 c h a r a c t e r i z e d in that the surface material (5₎ is made od ceramic or plastic material enabling low abrasion friction.

11. Dewatering device according to any of claims 1 - 7 c h a r a c t e r i z e d in that between the rolls (φ 2₀ -

40 mm⁾ a so called plastic wire is running preferably having a length of 3 - 6 meters.

12. Dewatering device according to claims 1 - 7, c h a r a c t e r i z e d in that the device may be provided with an own drive or it driving effect is supplied indirectly.