EXTENDED NIP PRESS APPARATUS

Abstract: The invention relates to an arrangement for impulse drying of a paper web, which arrangement comprises a shoe press arrangement (2) and a counter-roll (4), which together create an extended nip (6), said shoe press arrangement comprising a shoe (20), which displays a concave shoe mating surface which is turned toward the counter-roll (4), a support beam (12) which supports one or several pressing device(s) (16, 18) arranged to press the shoe (20) radially outwards against the counter-roll (4), a heating device (28) for heating the counter-roll surface (21), the support beam (12) displaying a central axis (36) and the counter-roll (4) displaying an axis of rotation (34) and the central axis for the support beam (12) together with the axis of rotation (34) for the counter-roll (4) defining a plane of the press (30), and the pressing device (16, 18) being arranged symmetrically in relation to the plane of the press (30) and the shoe being displaced a distance (Z) in relation to the plane of the press (30), so that the part of the shoe mating surface which is located downstream of the plane of the press is larger than the part of the shoe mating surface which is located upstream of the plane of the press (30), the shoe press arrangement consisting of a closed shoe press roll with two end walls (42) and an endless, flexible jacket (24), which is attached at the end walls with its edge parts and surrounding the support beam (12) and the shoe (2), and that the end walls (42) being rotatable about a centre of rotation (31), which is located downstream of the plane of the press (30).

Description: ARRANGEMENT FOR IMPULSE DRYING BACKGROUND OF THE INVENTION In connection with industrial production of paper or cardboard, a web of fibre material is dewatered in a roll nip in different steps. Such roll nips are usually formed by two rotating rolls arranged against each other, the contact surface between the rolls constituting the actual nip. The degree of dewatering in one step has large importance for the manufacturing capacity. Generally, it is stated that 1 % in increased dewatering means 4% in increased production. Consequently, it is a general endeavour within the field to try to increase the degree of dewatering in a step, but without waiving at the necessary quality requirements. Impulse drying is in that respect a very promising concept, which yet have not found commercial application. At impulse drying, the web is heated under high pressure in the nip to a temperature over evaporation level, so that formed steam brings free liquid out of the web, enabling high dewatering. From US-4,324,613 is known an early proposal for realisation of impulse drying, at which a part of the surface of one of two counter-rolls is continuously, considerably heated before contact with the fibre web in the nip, so that steam is to be formed locally when the hot surface of the heated roll comes into contact with the water-containing web of fibre material. The created steam shall thereby drive water out of the web and into a press felt, which runs together with the web through the nip. However, it has proved that the proposed method brings a risk for delamination, i.e. that the formed steam destroys the web directly after the roll nip when the external pressure rapidly decreases. From US 5,071,513 is known a method to try to counteract delamination by utilising an extended nip, i.e. a nip with larger contact length between the surfaces pressing against each other wherein a pressure profile is being simultaneously used with decreasing pressure through the nip, so-called reversed pressure profile. Consequently, with reversed. pressure profile is meant that the web initially, when it comes into the roll nip, is exposed to a relatively high pressure, after which, in conjunction with the web being on its way to pass out of the nip, is exposed to a lower pressure. This lower pressure is arranged so that essentially there is not any hydraulic pressure existing on the web, the pressure being lower than the steam pressure in the water in the web. By using a reversed pressure profile the aim is to avoid the problem that the web otherwise has a tendency to be damaged, in the form of delamination, when it passes out of the hot roll nip. In US 5,071,513 is especially suggested utilisation of two rows of hydrostatic, pressure generating elements in order to achieve the reversed pressure profile. However, it is realised that a double-row system brings additional costs both in view of construction and in view of maintenance. Moreover, in US 5,071,513 is also indicated use of a single row press system, that uses at least two pockets, placed along the shoe press surface, which are intended to be supplied with pressure medium in order to receive, in a hydrostatic way, a desired pressure profile in the nip. However, it is realised that this latter method is relatively complicated to monitor and adjust. By US 5,302,252 is suggested another solution for an extended roll nip for impulse drying with a reversed pressure profile. In the disclosed roll nip merely one row of pressure generating elements is used, which are equipped with only one pocket, which can be pressurised hydrostatically by means of a pressure medium, for achieving a locally increased pressure against the web. In this case as well, both adjusting of the pressure generating element and optimisation of the hydrostatic pressure in the pocket still must be executed simultaneously, which brings along increased complexity. An attempt to simplify an extended roll nip for impulse drying with a reversed pressure profile can be seen in US 4,973,384, the pressure profile being achieved in accordance with the same principle as in US 5,302,252 or by using a pressure generating plate, which is provided with a number of grooves on the underside, so that the plate can be arranged to be pivoted around an optional groove. The generated pressure profile depends on which groove that is selected for the pivoting. The method is shown only in connection with an open shoe press, which, among others, is not preferable because of environmental and safety reasons. From the patent it cannot be seen that the proposed solution could be implemented on a closed shoe press that has not the same ability as an open shoe press to move a shoe. SOLUTION AND ADVANTAGES The present invention has the object to eliminate or at least minimise the problems mentioned above, by presenting a cost efficient solution for obtaining a suitable pressure profile at impulse drying, which according to the invention is achieved with an arrangement at impulse drying of a paper web, comprising a shoe press roll and a counter-roll, between which exists an extended nip. Said shoe press roll comprises a support beam, at which is arranged a single row pressing device for a shoe, a pressing mechanism for moving of the shoe in a radial direction, an endless, flexible jacket, which constitutes the outer periphery of the shoe press, a heating apparatus arranged nearby the surface of said counter-roll. The single-row pressing mechanism presses the shoe along a line that is displaced a certain distance from the shoe centre in a direction towards the inlet opening of the nip. The arrangement is characterised by said jacket surrounding a space in a sealing manner and by said shoe and press mechanism is arranged within said sealed space. By means of an arrangement according to the invention, it has been shown possible to achieve a cost efficient and relatively easily adjustable arrangement for impulse drying. BRIEF DESCRIPTION OF THE DRAWINGS In the following, reference will be made to the annexed figures, in which Fig. 1 shows a shoe press roll with a counter-press roll according to the principles of the invention, and Fig. 2 shows the pressure curve for a shoe press according to the invention. DETAILED DESCRIPTION OF THE INVENTION In Fig. 1 is shown in cross-section a closed shoe press 2, which is co-operating with a counter-press roll 4, so that an extended nip 6 is formed between them. A web 8 and a press felt 10 run through the nip 6. The shoe press 2 comprises a support beam 12 A, 12B, 12C in the form of an I-beam consisting of a waist 12A and a lower T-part 12B, and an upper T-part 12C. In the upper surface 14 of the upper T-part 12C is arranged a cavity 16. Within the cavity 16 is arranged a lower pressing device 18, and a shoe 20, the shoe 20 being fixed on top of the lower pressing device 18. The shoe 20 is arranged with a concave surface 22 directed outwards. This concave surface 22 is co-operating with a flexible jacket 24, which is attached at end walls 42 and which is surrounding the shoe press in a sealing manner. Above the shoe press roll 2 and co-operating with it, a counter-roll 4 is arranged, the surface 26 of which is manufactured in a material with high capacity of heat transmission, preferably steel. Further is shown, that the counter- roll 4 is provided with a heating device 28, which in a suitable manner, for instance radiation and/or flames, is heating the outer surface 26 of the counter-roll 4. The heater 28 extends along the entire width of the roll 4 but surrounds only a minor part of its circumference and is arranged somewhere in the area of its down-going roll side. As can be seen from the figure, the pressing device 18 for the shoe 20 is a single row. At impulse drying, a reversed pressure curve is desired in order to achieve desired results, i.e. a higher pressure is created across the web near its entry in to the shoe press than at its exit. This is reached by means of the invention by applying the plane of the press 30 eccentrically, B, in relation to the centre C of the concave edge line 22 of the shoe, so that a so-called tilting, T, of the single row pressed shoe is reached. At which: (1) T= F1/F2, Fl and F2, respectively, are fictitious forces (in exchange of the single-row actual force) which act in each end part of the shoe, at which Fl specifies the force at the end part where the paper web leaves the shoe and thus F2 specifies the force at the inlet part. The plane of the press 30 lies on an imagined line between the counter-roll centre of rotation 34 and the centre 36 of the I-beam of the shoe press roll. The jacket 24 of the shoe press roll and the sealing end walls onto which it is attached are rotating around an axis 31 , which is displaced, in relation to the plane of the press 30, a distance B towards the dry end of the paper machine. Suitably, the beam centre (36) is displaced along an arc (38) in relation to the centre of rotation (31) of the end walls and that the centre for said arc (38) is arranged in or adjacent to the centre of rotation (34) of the counter-roll (4). Preferably, the radius (R2) of said arc (38) is the sum Rl + H (0.9-1.1) 12, in which Rl is the radius of the counter-roll and H is the height of the support beam (12). Furthermore, it is important that the displacement, Z, is held within certain values, in order to receive the desired pressure profile, as is shown in Fig 2. According to the invention, Z shall be approximately 0.1-0.3 x L, and most preferably around 0.2. In order to obtain this desirable displacing/tilting a real positioning of the support beam centre 36 must be done with reference to the centre of rotation 30 of the belt and the extension of the shoe L, at which: (1) Z = L x 0.1-0.3 and at which following approximate formula applies for the eccentricity B; B= Z (R2/R1) where accordingly L is the length of the shoe in a cross-section, which runs across the longitudinal direction of the shoe press roll (see Fig. 1), Rl is the radius of the counter- roll 4 and R2 is the distance from of counter-roll centre 34 to the centre 36 of the shoe press roll. Z is the distance in the machine direction between the centre line C (see Fig. 2) of the shoe and the plane of the press 30, in which the resultant force in the roll nip acts. Examples: L= 200 mm Rl= 600 mm R2 = 1400 mm Z = 0.167 L Which gives that: B = 78 mm Through the invention, an arrangement for impulse drying is disclosed which is utilising an extended roll nip with a reversed pressure curve and which through its relatively inexpensive construction also is cost efficient. By using the parameters defined above in the shoe press, a reversed pressure curve is easily obtained, which implies that the fibre web 8, when it enters the nip 6, is exposed to an initially high pressure while being heavily heated by the hot surface of the counter-roll. Thereby a very quick heating of the fluid content in the fibre web occurs, which is partly evaporated. The created steam moves itself out of the fibre bed towards the press felt 10 and brings thereby non- evaporated liquid, whereby a very effective dewatering is achieved. Thereafter the pressure is gradually lowered in the nip 6, which leads to an increased evaporation that removes additional liquid out of the web. The invention is not limited to the above description, but can be varied within the scope of the claims. Consequently, it is realised that also other shapes of single-row presses than the one shown can be used, for instance a non-divided, united shoe element, for instance in the manner shown in US 5,688,375 or US 5,676,799.

Claims: CLAIMS 1. An arrangement for impulse drying of a paper web, which arrangement comprises a shoe press arrangement (2) and a counter-roll (4), which together creates an extended nip (6), said shoe press arrangement comprising a shoe (20), which displays a concave shoe mating surface which is turned toward the counter-roll (4), a support beam (12) which supports one or several pressing device/s (16, 18) arranged to press the shoe (20) radially outwards against the counter-roll (4), a heating device (28) for heating the counter-roll surface (21), the support beam (12) displaying a central axis (36) and the counter-roll (4) displaying an axis of rotation (34), and the central axis for the support beam (12) together with the axis of rotation (34) for the counter-roll (4) defining a plane of the press (30), and the pressing device (16, 18) being arranged symmetrically in relation to the plane of the press (30) and the shoe being displaced a distance (Z) in relation to the plane of the press (30), so that the part of the shoe mating surface which is located downstream of the plane of the press is larger than the part of the shoe mating surface which is located upstream of the plane of the press (30), characterised in that the shoe press arrangement consists of a closed shoe press roll with two end walls (42) and an endless, flexible jacket (24), which is attached at the end walls with its edge parts and surrounding the support beam (12) and the shoe (2), and that the end walls (42) are rotatable about a centre of rotation (31), which is located downstream of the plane of the press (30). 2. An arrangement as claimed in claim 1, characterised in that said support beam (12) consists of an I-beam with an upper T-part (12C) and a lower T-part (12A). 3. An arrangement as claimed in claim 2, characterised in that the centre (36) of the- beam is displaced along an arc (38) in relation to the centre of rotation (31) of the end walls. 4. An arrangement as claimed in claim 3, characterised in that the centre for said arc (38) is arranged in or nearby of the centre of rotation (34) of the counter-roll (4). 5. An arrangement as claimed in claim 4, characterised in that the radius (R2) of said arc (38) is the sum Rl + H (0.9-1.1) 12, in which Rl is the radius of the counter-roll and H is the height of the support beam (12). 6. An arrangement as claimed in claim 1, characterised in that said distance (Z) is adjusted so that Z = 0.1-0.3 x L, preferably 0.15-0.25 x L, and more preferred appr. 0.2 x L, in which L relates to the length of the shoe (20). 7. An arrangement at impulse drying as claimed in claim 6 characterised in that the centre (36) of the support beam is displaced a distance (B) in relation to the centre of rotation (30) of the end walls, which is calculated according to B = Z R2/R1, in which Rl is the radius of the counter-roll (4) and R2 is the distance from the counter-roll centre (34) to the centre (36) of the shoe press roll.

Inventors: ANDREASSON IVAR, ANDREASSON, IVAR
Applicants: ANDREASSON IVAR, VALMET KARLSTAD AKTIEBOLAG, ANDREASSON, IVAR, VALMET-KARLSTAD AKTIEBOLAG
Application year: 2000
Main classification: D21F3
IPC codes: D21F3