WO1993023614A1

WO1993023614A1 - A shoe type press

Info

Publication number: WO1993023614A1
Application number: PCT/SE1993/000390
Authority: WO
Inventors: Antti Isakki Ilmarinen; Jorma Tapio Laapotti
Original assignee: Valmet-Karlstad Ab
Priority date: 1992-05-12
Filing date: 1993-05-05
Publication date: 1993-11-25
Also published as: SE9201497D0; SE470235B; SE9201497L

Abstract

In a shoe type press used in a wet press for the dewatering of a fibrous web (W), the press shoe (2) is exposed to forces tending to bend the shoe (2) in a plane substantially parallel to the direction of the movement of the web (W) through the extended nip. According to the invention, means (12) are provided for at least substantially outbalancing such forces. These means (12) may include means (13) for sensing a difference in temperature between the leading edge (8, temperature TA) and the trailing edge (9, temperature TB) of the shoe (2), and means (14) for cooling the trailing edge (9) and/or means (20) for heating the leading edge (8), so as to minimize the temperature difference (TB - TA). The cooling and/or the heating may be carried out be providing a conduit (18 and 21) in the respective edge (9 and 8) and passing a cooling or a heating agent, respectively, through the conduit in heat exchanging relationship with the shoe (2).

Description

A SHOE TYPE PRESS

TECHNICAL FIELD The present invention relates to a shoe type press of the kind including a liquid impermeable press belt movable in an endless loop, an elongated press shoe located inside the belt loop and extending substantially over the width of the belt, an elongated counter member located parallel to the press shoe outside the belt loop, and means for hydraulically pressing the shoe against the belt, so as to form an extended press nip between the belt and the counter member for pressing a fibrous web fed through the extended nip, said press shoe having a leading edge and a trailing edge with respect to the direction of movement of the web.

BACKGROUND OF THE INVEMTION Such shoe type presses are well known and are used in wet presses for paper- making and boardmaking machines for raising the dry solids content of the paper or board web. In recent years, the importance of the shape of the pressure profile curve in the machine direction (MD) has been paid attention to, and to get closer to an ideal shape of the pressure profile curve it has been suggested to control the tilt of the press shoe. As disclosed in U.S. Patents No. 4,917,767 (Ilmarinen et al.), 4,917,768 (Ilmarinen), and 5,084,137 (Ilmarinen et al.), for example, all of which relate to shoe type presses having hydrostatic press shoes, tilt control may be achieved by using two separately controllable rows of hydraulic jacks, one row for the leading edge of the press shoe and one for the trailing edge. Tilt control may be achieved also with the shoe type press design disclosed in Finnish Patent Publication No. FI-B- 84,740. By applying a larger press force at the trailing edge than at the leading edge, the press shoe will be tilted and, with a hydrostatic press shoe, the originally substantially plane center portion of the pressure profile curve in the machine direction will get a hump at the trailing edge of the press shoe, thereby bringing the actual shape of the pressure profile curve closer to a desired substantially triangular shape. However, when the tilt and the machine speed are large, the press shoe will be exposed to forces that tend to bend the press shoe in a plane substantially parallel to a main direction of movement o the web through the extended nip, and this problem becomes more pronounced with increasing width of the machine. Even a moderate bending of the press shoe will result in a machine direction pressure profile curve that in the middle of the shoe is different from the one at the ends of the press shoe. Another problem, namely the one that due to the high press loads used in a shoe type press having a hydrodynamic press shoe, the press belt supporting surface of the hydrodynamic press shoe is exposed to high wear, is discussed in German Published Patent Application DE-A1- 32 22 932. This wear causes the actual pressure profile curve of the press nip to deviate from the desired one. To avoid a dismantling of the . press and a substitution of a new hydrodynamic press shoe for the worn one, the DE-A1- 32 22 932 proposes the provision of means for locally changing the temperature of the hydrodynamic press shoe, so as to locally change the shape of the press belt supporting surface of the press shoe to an extent that is sufficient for compensating for the wear of the press belt supporting surface.

It has also been proposed, cf. U.S. Patent No. 4,643,802 (Schiel), for example, to substantially avoid impermissible deformation of the press shoe as a result of the action of heat and/or to compensate for a deviation in the shape of the slide path of the press belt on the press belt supporting surface of the press shoe by insulating a supported upper part of the press shoe, which is in contact with the press belt, from a supporting lower part of the press shoe. Such insulating may be accomplished by installing an insulating layer between the upper and lower parts of the press shoe and strongly reduces heat transfer to the lower part of the press shoe. As a result, the press shoe is subjected only to a slight and therefore controllable deformation. DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a shoe type press, in which the bending of the press shoe may be counteracted to an extent sufficient for avoiding detrimental differences in the machine direction pressure profile curve at different locations along the cross machine length of the press shoe. In accordance with the present invention, this object is achieved by providing a shoe type press of the kind described in the first paragraph above with means for at least substantially outbalancing a force that during operation of the press tends to bend the press shoe in a plane substantially parallel to a main direction of movement of the web through the extended nip. By at least substantially outbalancing the force that during operation of the press tends to bend the press shoe in a plane substantially parallel to a main direction of movement of the web through the extended nip, the MD pressure profile curve will remain substantially constant at different locations along the cross machine direction (CD) of the length of the press shoe.

We have found that a major cause of the bending of the press shoe in said plane is a difference in temperature between the leading edge and the trailing edge of the press shoe. Friction forces in a lubricant film provided between the press belt and the . press shoe generate heat, part of which is absorbed by the press shoe. The higher the load, the thinner the lubricant film is, and the more heat will be generated and absorbed by the press shoe. Especially with a higher load at the trailing edge than at the leading edge of the press shoe, the trailing edge will absorb more heat than the leading edge and, consequently, be exposed to a larger thermal expansion, which tends to bend the press shoe in a plane substantially parallel to the main direction of movement of the web through the extended nip.

For example, in a shoe type press operating in a boardmaking machine at a normal load for obtaining a desired dry solids content of the web, and at a tilt ratio (i. e. the ratio of the load at the trailing edge to the load at the leading edge) of 1.8 and a machine speed of 700 meters/minute, the difference in temperature between the leading edge and the trailing edge may be 20 °C. In a 10 meter wide machine, for example, this temperature difference may cause a deflection (designated b in Fig. 3) of the press shoe in the MD direction on the order of 7 millimeters. This will cause the thickness of the lubricant film and also the shoe temperature profile curve to vary over the CD length of the press shoe, which results in a varying MD pressure profile curve over the CD length of the presf shoe. In addition, the temperature difference may also have a deleterious effect on the stability or shape and/or the life of the press belt. To minimize the MD deflection of the press shoe, the at least substantially outbalancing means suitably include means for cooling the trailing edge of the press shoe and/or means for heating the leading edge of the press shoe. Preferably, said at least substantially outbalancing means include means for sensing a difference in temperature between the leading edge and the trailing edge of the press shoe, and means for cooling the trailing edge of the press shoe and/or means for heating the leading edge of the press shoe, so as to minimize the temperature difference. Then it is preferred that the cooling means and the heating means include a conduit provided in the trailing edge and the leading edge, respectively, of the press shoe, and means for passing a cooling agent and a heating agent, respectively, through the conduit in heat exchanging relationship with the press shoe.

Since the present invention is especially useful in shoe type presses of the kind where the tilt of the press shoe is controllable, it is preferred that said means for hydraulically pressing the press shoe against the belt include means for separately controlling hydraulic press forces applied at the leading edge and at the trailing edge of the press shoe.

Further, when the shoe type press is used in a wet press of a papermaking or boardmaking machine, it is preferred that at least one press felt for absorbing moisture from the web is arranged to run in an endless loop through the extended press nip together with the web, and that said press belt is located inside the loop formed by said at least one press felt.

In addition, it is also preferred that said movable liquid impermeable press belt is a rotatable flexible jacket for a shoe type press roll incorporating said press shoe, that said counter member is a rotatable controlled deflection roll, and that said press shoe has a concave surface portion adapted to the shape of the counter roll. Thereby, several advantages are obtained, e. g. the lubricant is contained for certain inside the shoe type press roll and can not pollute the fibrous web.

Additional features mat characterize the invention and what is achieved by means of these features will be described below.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional end view of a single-felted wet press having an extended press nip formed between a shoe type press roll and a counter roll, the shoe type press roll being provided with a first preferred embodiment of the means for at least substantially outbalancing a force that during operation of the press tends to bend the press shoe in a plane substantially parallel to the main direction of movement of the web through the extended nip.

Fig. 2 is an example of an MD pressure profile curve obtained when tilting the press shoe in the press shown in Fig. 1. Fig. 3 is a longitudinal elevational top view of the press shoe shown in Fig. 1 when the press shoe is exposed to a deflection in a plane substantially parallel to the main direction of movement of the web through the extended nip, said deflection being greatly exaggerated with respect to the dimensions of the press shoe. Fig. 4 is a schematic cross-sectional end view similar to Fig. 1 and shows a second preferred embodiment of the means for at least substantially outbalancing a force that during operation of the press tends to bend the press shoe in a plane substantially parallel to the main direction of movement of the web through the extended nip.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The single-felted wet press illustrated in Figs. 1 and 3 includes a liquid imperme¬ able press belt 1 movable in an endless loop. In the shown embodiment the press belt is a rotatable flexible jacket 1 for a shoe type press roll of the kind described in U.S. Patent No. 5,084,137 (Ilmarinen et al.), herewith incorporated by reference. The shoe type press roll has an elongated press shoe 2, which is located inside the belt loop, i. e. the jacket 1, and extends substantially over the width of the belt, i. e. the axial length of the jacket 1.

Further, the single-felted press includes an elongated counter member 3 extending parallel to the press shoe 2. In the shown embodiment, the counter member is a rotatable counter roll 3, preferably a controlled deflection roll, and means 4 and 5 are provided for hydraulically pressing the press shoe 2, which has a concave surface portion adapted to the shape of the counter roll 3, against the flexible jacket 1, so as to form an extended press nip between the flexible jacket 1 and the counter roll 3 for the pressing of a wet fibrous web W fed through the extended nip together with a water absorbing press felt F. The press felt forms an endless loop, inside of which the shoe type press roll is located.

In the illustrated embodiment there are provided a plurality of hydrostatic pressure pockets 6 in the concave surface portion of the press shoe 2. These pockets 6 are located side by side to form a row extending along the CD length B of the press shoe 2 as shown in Fig. 3. Although the hydrostatic pressure pockets 6 may be of a substantially rectangular cross sectional shape in the machine direction, it is preferred, as shown in Figs. 1 and 4, that at least a downstream portion of each pocket 6 has a gradually diminishing depth in the direction of movement of the web W. A hydrostatic press shoe having pressure pockets of such shape offers a plurality of advantages and i the subject of Swedish Patent Application No. 9103823-2. An alternative advantageous pocket shape is the one disclosed in U.K. Patent Publication GB-A- 2,239,268, where the pocket bottom is formed with a radius somewhat smaller than the radius of the concave surface portion of the press shoe, and the pocket is very shallow, no more than 0.75 πiillimeters deep.

Further, the press shoe 2 is mounted on a carrier 11 by means of screws, not shown, to form a press shoe assembly, which is movable towards and away from the counter roll 3 by the means 4 and 5 for hydraulically pressing the press shoe 2 against the jacket 1. For each hydraulic pressure pocket 6 there is provided a separate conduit 7 extending through the press shoe assembly for feeding a pressurized lubricant, usually oil, to the pocket 6.

The means for hydraulically pressing the press shoe 2 against the jacket 1 include means 4 and 5 for separately controlling hydraulic press forces applied at a leading edge 8 and a trailing edge 9 of the press shoe 2. More precisely, the means for separately controlling hydraulic press forces applied at the leading edge 8 of the press shoe 2 include a first separately controllable row of hydraulic jacks 4, and the means for separately controlling hydraulic press forces applied at the trailing edge 9 of the press shoe 2 include a second separately controllable row of hydraulic jacks 5. By applying a higher load at the trailing edge 9 than at d e leading edge 8 of the press shoe 2, which causes a certain tilting of the press shoe 2, it is possible to achieve an MD pressure profile curve of the advantageous general shape illustrated in Fig. 2, where P designates the pressure and L designates the MD width of the press shoe 2. The originally substantially plane center portion of the MD pressure profile curve will get a hump at the trailing edge 9 of the press shoe 2, thereby bringing the actual shape of the MD pressure profile curve closer to a desired substantially triangular shape. To achieve a controllable CD pressure profile curve it is preferred, but not shown, that the hydraulic jacks 4 and 5 in each of the two rows are divided into separately controllable groups, and that each group is supplied with hydraulic agent of an adequate, separately controlled pressure.

The two rows of hydraulic jacks 4 and 5, which carry the press shoe assembly, are shown mounted on a stationary non-rotatable support beam 10 but, if desired, they might as well be inversely mounted on the press shoe carrier 11 as disclosed in Finnish Patent No. 84,740. The support beam 10 extends axially through the flexible jacket 1 and has a stub shaft, not shown, at each axial end. A self-aligning bearing, likewise not shown, is mounted on each of the two stub shafts for permitting the support beam 10 to deflect elastically when loaded, while the press shoe 2 remains straight. At its ends, the shoe type press roll has end walls, not shown, preferably rotatably mounted on the stub sfc-.__s, and means for sealingly connecting the axial ends of the flexible jacket 1 to the end walls, thereby preventing the lubricant supplied to the hydrostatic pockets 6 from leaking out of the shoe type press roll. On the support beam 10 there is also mounted an elongated support bearing, not shown in order not to unnecessarily crowd the drawing, which support bearing is disposed downstream of the press shoe 2 and extends parallel to the trailing edge 9 thereof and is positioned for engaging the press shoe assembly to absorb horizontal forces acting on the press shoe 2 during operation, substantially as disclosed in U.S. Patent No. 4,917,768 (Ilmarinen), herewith incorporated by reference.

As indicated above, when the tilt of the press shoe 2 and the machine speed are large, the press shoe 2 will be exposed to forces that tend to deflect the press shoe 2 a distance b (Fig. 3) in a plane substantially parallel to the main direction of movement of the web W through the extended nip, and this problem becomes more pronounced with increasing width of the machine and, thus, with increasing CD length B (Fig. 3) o the press shoe 2. Ev=_=. a moderate deflection of the press shoe 2 will result in an MD pressure profile curve that in the middle of the press shoe 2 is different from the one at the ends of the press shoe 2. This disadvantage is overcome, in accordance with the present invention, by providing the shoe type press with means, generally designated 12, for at least substantially outbalancing a force that during operation of the press tends to bend the press shoe 2 in a plane substantially parallel to the main direction of movement of the web W through the extended nip.

We have found that a major cause of the press shoe bending force is a difference in temperature between the leading edge 8 and the trailing edge 9 of the press shoe 2. Therefore, as is evident from the most preferred embodiment illustrated in Fig. 1, the means 12 for at least substantially outbalancing the press shoe bending force include means 13 for sensing a difference ΔT in temperature between the leading edge 8 (temperature T_A) and the trailing edge 9 (temperature T_B) of the press shoe 2, and means 14 for cooling the trailing edge 9 of the press shoe 2, so as to minimize the temperature difference ΔT. The temperature difference sensing means 13 may include a first temperature sensor 15 for sensing the temperature T_A of the leading edge 8 of the press shoe 2, a second temperature sensor 16 for sensing the temperature T_B of the trailing edge 9 of the press shoe 2, and means 17 for checking the size of the temperature difference ΔT by subtracting T_A from T_B. The temperature sensors 15 and 16 are located in the material of the press shoe 2, close to the shoe surface pressed towards the press belt or flexible jacket 1. The cooling means 14 may include a conduit 18 provided in the trailing edge 9 of the press shoe 2 and extending from one shoe end to the other at a larger distance from said shoe surface than that of the second temperature sensor 16, and means 19 for passing a cooling agent through the conduit 18 in heat exchanging relationship with the trailing edge 9 of the press shoe 2. The cooling agent may be water or oil, or any other liquid or gaseous cooling agent, of a suitable temperature, and the size of the temperature difference ΔT is used for controlling the cooling agent passing means 19, to effect control of either the flow rate of the cooling medium or the temperature thereof. Of course, if desired, both the flow rate and the temperature may be controlled. For the sake of clarity we can mention that since the press shoe assembly support bearing (not shown) prevents the CD center portion of the press shoe 2 from moving in the direction of transport of the web W, the heat induced bending of the press shoe 2 will result in that the shoe ends move in the opposite direction.

Also in the embodiment illustrated in Fig. 4, the means 12 for at least substan¬ tially outbalancing the press shoe bending force include means 13 for sensing a difference ΔT in temperature between the leading edge 8 (temperature T_A) and the trailing edge 9 (temperature T_B) of the press shoe 2, but in Fig. 4 the means 14 for cooling the trailing edge 9 of the press shoe 2, so as to minimize the temperature difference ΔT, are replaced by means 20 for heating the leading edge 8 of the press shoe 2. Like in Fig. 1 the temperature difference sensing means 13 may include a first temperature sensor 15 for sensing the temperature T_A of the leading edge 8 of the press shoe 2, a second temperature sensor 16 for sensing the temperature T_B of the trailing edge 9 of the press shoe 2, and means 17 for checking the size of the temperature difference ΔT by subtracting T_A from T_B. The temperature sensors 15 and 16 are located in the material of the press shoe 2, close to the shoe surface pressed towards the press belt or flexible jacket 1. The heating means 20 may include a conduit 21 provided in the leading edge 8 of the press shoe 2 and extending from one shoe end to the other at a greater distance from said shoe surface than that of the first temperature sensor 15, and means 22 for passing a heating agent through the conduit 21 in heat exchanging relationship with the leading edge 8 of the press shoe 2. Like in the embodiment shown in Fig. 1 the heating agent may be water or oil, or any other liquid or gaseous heating agent, of a suitable temperature, and the size of the temperature difference ΔT is used for controlling the heating agent passing means 22, to effect control of either the flow rate of the heating medium or the temperature thereof. Also, if desired, both the flow rate and the temperature may be controlled.

Naturally, it is possible, if desired, both to cool the trailing edge 9 and to heat the leading edge 8 of the press shoe 2, but as a rule this would just make the press shoe deflection outbalancing means 12 more complicated without giving some additional advantage. Further, since in the embodiment illustrated in Fig. 1 the cooling of the trailing edge 9 of the press shoe 2 also effects a cooling of the lubricant, which accompanies the press belt or jacket 1 when the latter leaves the extended nip, and since the cooled lubricant effects a cooling of the press belt or jacket 1, thereby avoiding a deterioration of the stability or shape thereof and/or prolonging the life thereof, the embodiment shown in Fig. 1 is preferred over the one shown in Fig. 4. While the present invention above has been described with reference to the drawings, which show two preferred embodiments, several obvious modifications there¬ of are possible within the scope of the appended claims. As an illustrative example, it would be possible to dispense with the sensing of the temperatures T_A and T_B in the leading edge 8 and the trailing edge 9, respectively, of the press shoe 2 and, consequently, also with the checking of the size of the temperature difference ΔT, and instead control the cooling and/or heating of the press shoe 2 guided by an empirically derived functional relation between the cooling and/or heating required, and the tilt, the machine speed and the press load used. However, since the development of such an empirically derived functional relation is believed to require averaging of data over extended periods of time, such a control method is less preferred. Further, if a heating of the leading edge 8 of the press shoe 2 is desired, the heating also can be carried out by means of an electric heater (not shown), such as a resistor element or an induction heating device, for example. Still further, the support bearing, which is disposed downstream of the press shoe 2 and positioned for engaging the press shoe assembly to absorb horizontal forces acting on the press shoe 2 during operation, does not have to be provided for the proper operation of the present invention. Finally, even though the press shoe 2 in the preferred embodiments described above is of hydrostatic type, the present invention, of course, is applicable also to shoe type presses of the kind where the press shoe is of hydrodynamic type.

Claims

CLAIMS:

1. A shoe type press of the kind including a liquid impermeable press belt (1) movable in an endless loop, an elongated press shoe (2) located inside the belt loop and extending substantially over the width of the belt (1), an elongated counter member (3) located parallel to the press shoe (2) outside the belt loop, and means (4, 5) for hydraulically pressing the press shoe (2) against the belt (1), so as to form an extended press nip between the belt (1) and the counter member (3) for pressing a fibrous web (W) fed through the extended nip, said press shoe (2) having a leading edge (8) and a trailing edge (9) with respect to the direction of movement of the web (W), characterized by: means (12) for at least substantially outbalancing a force that during operation of the press tends to bend the press shoe (2) in a plane substantially parallel to a main direction of movement of the web (W) through the extended nip.

2. A shoe type press as claimed in claim 1, characterized in that said at least substantially outbalancing means (12) comprise means (14) for cooling the trailing edge (9) of the press shoe (2).

3. A shoe type press as claimed in claim ! - characterized in that said at least substantially outbalancing means (12) comprise "leans (13) for sensing a difference in temperature between the leading edge (8, temperature T_A) and the trailing edge (9, temperature T_B) of the press shoe (2), and means _4) for cooling the trailing edge (9) of the press shoe (2), so as to minimize the temperature difference (T_B - T_A).

4. A shoe type press as claimed in claim 2 or 3, wherein the elongated press shoe (2) has two ends, characterized in that the cooling means (14) comprise a conduit (18) provided in the trailing edge (9) of the press shoe (2) and extending from one shoe end to the other, and means (19) for passing a cooling agent through the conduit (18) in heat exchanging relationship with the press shoe (2).

5. A shoe type press as claimed in claim 1, characterized in that said at least substantially outbalancing means (12) comprise means (20) for heating the leading edge (8) of the press shoe (2).

6. A shoe type press as claimed in claim 1, characterized in that said at least substantially outbalancing means (12) comprise means (13) for sensing a difference in temperature between the leading edge (8, temperature T_A) and the trailing edge (9, temperature T_B) of the press shoe (2), and means (20) for heating the leading edge (8) of the press shoe (2), so as to minimize the temperature difference (T_B - T_A).

7. A shoe type press as claimed in claim 5 or 6, wherein the elongated press shoe (2) has two ends, characterized in that the heating means (20) comprise a conduit (21) provided in the leading edge (8) of the press shoe (2) and extending from one shoe end to the other, and means (22) for passing a heating agent through the conduit (21) in heat exchanging relationship with the press shoe (2).

8. A shoe type press as claimed in any one of claims 1 to 7, characterized in that said means (4, 5) for hydraulically pressing the press shoe (2) against the belt (1) include means (4,5) for separately controlling hydraulic press forces applied at the leading edge (8) and at the trailing edge (9) of the press shoe (2).

9. A shoe type press as claimed in any one of claims 1 to 8, characterized in that at least one press felt (F) for absorbing moisture from the web (W) is arranged to run in an endless loop through the extended press nip together with the web (W), and said press belt (1) is located inside the loop formed by said at least one press felt (F).

10. A shoe type press as claimed in any one of claims 1 to 9, characterized in that said movable liquid impermeable press belt (1) is a rotatable flexible jacket (1) for a shoe type press roll incorporating said press shoe (2), said counter member (3) is a rotatable controlled deflection roll (3), and said press shoe (2) has a concave surface portion adapted to the shape of the counter roll (3).