BLOWBOX FOR USE IN A PLANT FOR DRYING A MATERIAL WEB

Abstract: A blow box (2) for use in a plant for drying a material web (1), comprises an essentially horizontal upper box wall (3) having formed therein circular orifices (5) and so-called eyelid perforations (6), each one of which consists of a slit (7) which extends in parallel with the centre line (C) of the upper box wall (3) and of a depression (8) formed in the upper box wall (3) adjacent one side of the slit. The eyelid perforations (6) are arranged in two rows in parallel with the centre line (C), and together they form a zigzag pattern about the centre line (6) which intersects the depressions (8) of the eyelid perforations (6) in both rows. The orifices (5) are disposed in a first and a second pair of rows (9a, 9b, and 10a, 10b, respectively) with both rows in each pair positioned on either side of and spaced equally from the centre line (C). The upper box wall (3) has a shallow continuous arcuate shape across its entire width.

Description: BLOW BOX FOR USE IN A PLANT FOR DRYING A MATERIAL WEB The subject invention relates to a blow box intend¬ ed to be used in a plant for drying a material web, such as a web of papermaking pulp being advanced through said plant, said blow box having an elongate, essentially right-angled parallelepipedon shape including an essen¬ tially horizontal upper box wall and being intended to be placed, jointly with a plurality of identical blow boxes, below the material web for the purpose of holding the material web floating, i.e. suspended in the air in a well-defined stable position above the upper blow box wall by means of air streams that are being blown out of the box while simultaneously drying the material web, said blow box being designed to be placed below the mate¬ rial web in a manner ensuring that the longitudinal cen- tre line of the upper box wall extends transversely of the direction of advancement of the web, the upper wall of said blow box being formed with a plurality of essen¬ tially circular nozzle orifices, and with a plurality of so-called eyelid perforations, each one of said perfora- tions consisting of a slit extending in parallel with said centre line and of a depression formed in the upper box wall adjacent the slit, at one side of the latter, said eyelid perforations being arranged in two rows extending in parallel with and on either side of the cen- tre line, said nozzle orifices being arranged in rows, likewise extending in parallel with and on either side of said centre line, the eyelid perforations in one row being so displaced in the longitudinal direction of the blow box relatively to the eyelid perforations of the other row that the eyelid perforations of both rows together for.n a zigzag pattern, the eyelid perforations of both rows being directed in such a manner as to cause air to exit essentially in parallel with the upper box wall, towards said centre line, and essentially perpendi¬ cularly thereto, and the nozzle orifices being directed in such a manner as to cause air to exit essentially at right angles to the upper box wall. The purpose of the subject invention is to provide a blow box of the kind outlined in the aforegoing and designed to provide improved heat transfer and thus improved drying efficiency compared with prior-art blow boxes of this kind, as also an increased floating height or higher level of suspension of the material web in the air, this being achieved under equal air efficiency and web fixation conditions. This purpose is achieved in accordance with the invention with the aid of a blow box of the kind defined in the introduction and characterised in that the slits of the eyelid perforations in both rows are equally spaced from said centre line, that said centre line intersects the depressions of the eyelid perforations in both rows, that the nozzle orifices are arranged in respectively a first and a second pair of rows, the two rows of each pair being positioned on either side of and spaced equally from said centre line, that the distance of the rows of the first pair to said centre line is smaller than the distance of the rows of the second pair to said line, that for each eyelid perforation there is a corresponding nozzle orifice in the row of the first pair of nozzle orifices that is positioned on the same side of said centre line as a slit of the associated eyelid per¬ foration, and also a corresponding nozzle orifice in the row of the second pair that is positioned on the opposite side of said centre line, each eyelid perforation and its two corresponding nozzle orifices being essentially in alignment in the crosswise direction of the blow box, and in that the upper box wall has a shallow continuous arcuate shape across its entire width. In accordance with a preferred embodiment the height of the arc H representing the arcuate shape of the upper box wall is related to the degree of perforation P as follows: H = k • P - 5.88, wherein k is a constant which is between 6.2 and 6.9 when H is denoted in mm and P in %. The product of the distance I2, in mm, between the centre line and the edge of the slit closest to said line, and the width L of the blow box, in mm, advanta¬ geously is 2,800-4,100, preferably approximately 3,440. The ratio of the length 1^ of the depression to the distance I2 between said centre line and the edge of the slit closest to that line advantageously is 1.2-1.6, pre¬ ferably 1.4. The invention will be described in closer detail in the following with reference to the accompanying draw- ings, wherein: Fig. 1 is a perspective view, schematically illu¬ strating a plant for drying a material web with the aid of blow boxes in accordance with the invention; Fig. 2 is a view showing a part of a blow box in accordance with the invention; Fig. 3 illustrates the blow box in a sectional view along line III-III in Fig. 2; Fig. 4 illustrates the blow box in a sectional view along line IV-IV of Fig. 2; Fig. 5 is a sectional view along line V-V in Fig. 2 of a part of the blow box; and Fig. 6 illustrates an eyelid perforation in a sec¬ tional view along line VI-VI in Fig. 2. Fig. 1 illustrates a material web 1, for example a web of papermaking pulp, which is being advanced through a plant for drying the web. The material web 1 is advanc¬ ed horizontally through the plant, and its direction of travel is illustrated in Fig. 1 by means of arrows Pl. A plurality of identical blow boxes 2, each one being shaped as an elongate, right-angle parallelepiped including an essentially horizontal upper box wall 3, are positioned underneath the material web 1 with the upper surfaces of the box walls 3 positioned in the same hori¬ zontal plane. The blow boxes 2 have an extension at right angles to the direction of travel Pl of the material web 1 and are arranged side by side, separated by a predeter- mined gap 4. The upper wall 3 of each blow box 2 is formed with a plurality of air-exit apertures 5 and 6 to be described in closer detail further on. Fan means (not shown) force air into the blow boxes 2. The air exits from the blow boxes 2 by way of the apertures 5 and 6. The exiting air carries the material web 1, thus sustaining the latter floating or suspended in the air at a predetermined floating height or level of suspension above the blow boxes 2. The exiting air also transfers heat to the material web 1 for the purpose of drying the latter. The air blown out of the blow boxes 2 is caused to exit by way of the gaps 4 intermediate the blow boxes 2. The air-exit apertures 5 and 6 consist of a plurali¬ ty of circular nozzle orifices 5 and a plurality of so- called eyelid perforations 6. Each eyelid perforation 6 consists of a slit 7 extending in parallel with the lengthwise centre line C of the upper box wall 3, and of a depression 8 formed adjacent the slit 7 at one side of the latter. The depression 8 is produced from the upper face of the upper box wall 3. The depression 8 has an essentially parabola-shaped peripheral outline and a depth that decreases in a direction perpendicularly away from the slit 7 (see Fig. 6). The cross-sectional con¬ figuration of the depression 8 as seen in parallel with the slit 7 is arcuate (see Fig. 5). Thus, the air-exit aperture 6 proper consists of one upwardly open part 6a delimited by the slit 7 and causing the exiting air to flow essentially perpendicularly to the upper box wall 3, and one part 6b (Fig. 6) which is open in the transverse direction of the blow box 2 and which is defined between the upper box wall 3 and the edge of the depression 8 that is closest to the slit 7, said part 6b serving to cause the exiting air to flow in a direction essentially in parallel with the upper box wall 3. The eyelid perforations 6 are arranged in two rows extending in parallel with and positioned on either side of the centre line C. The slits 7 of the eyelid perfora¬ tions 6 in the two rows are equally spaced from the cen¬ tre line 6. The eyelid perforations 6 in one row are dis¬ placed in the lengthwise direction of the blow box 2 in relation to the eyelid perforations 6 of the other row in such a manner that together the eyelid perforations of the two rows form a zigzag-shaped pattern. The orientation of the eyelid perforations 6 in both rows is such that the exiting air caused to flow essen¬ tially in parallel with the upper box wall 3 is directed towards the centre line C, essentially at right angles thereto. The spacing between the slits 7 and the centre line C is such that the centre line C intersects the depressions 8 (see Fig. 2). The circular nozzle orifices 5 forcing the air to exit essentially at right angles to the upper box wall 3 are arranged in a first and a second pair of rows 9a, 9b and 10a, 10b, respectively, extending in parallel with the centre line C. The two rows 9a, 9b and 10a, 10b, respectively, of each pair are positioned on either side of and spaced equally from the centre line C. The spacing of rows 9a, 9b of the first pair from the centre line C is shorter than the spacing of the rows 10a, 10b of the second pair from that line. For eacn eyelid perforation 6 two corresponding nozzle orifices 5 are provided, viz. one nozzle orifice 5 in the row 9a or 9b of the first pair that is positioned on the same side of the centre line C as the slit 7 of the associated eyelid perforation 6 and one nozzle ori¬ fice 5 in the row 10a or 10b of the other pair that is positioned on the opposite side of the centre line C. Each eyelid perforation 6 and its two corresponding nozzle orifices 5 are positioned in mutual alignment relationship or at least in essential mutual alignment relationship in the transverse direction of the blow box 2. The upper box wall 3 has a shallow continuous arcuate shape (see Figs 3 and 4) across its entire width, viz. also in the area occupied by the eyelid perforations 6. The arcuate shape in this area is achieved in that the punching and stamping tools used to produce the eyelid perforations 6 have a shape corresponding to the arc cur- vature. The box 2 in accordance with the illustrated prefer¬ red embodiment, which has been tested and found to func¬ tion in a highly satisfactory manner, has a width L of 215 mm, and the length and width of the slits 7 are 15 mm and 1.5 mm respectively, the length 1^ of the depressions 8 perpendicularly to the centre line C is 22.5 mm, the angle of depression α of the depressions 8 (see Fig. 6) is approximately 10", the distance I2 from the centre line C to the slit edge positioned closest thereto is 16 mm, the mutual spacing d between the eyelid perfora¬ tions 6 in both rows is 20 mm, the diameter of the nozzle orifices 5 is 3.7 mm, the distances I3 and I4 between the centre line C and respectively the rows 9a, 9b and 10a, 10b in the first and second pairs of orifice rows are 70 mm and 95 mm, respectively, and the radius of curva¬ ture of the arcuate shape of the upper box wall 3 is 1,927 mm with a consequential height of the arc H of 3.0 mm. As shown, the product of I2 • L is 16 • 215 = 3,440, when the values are expressed in mm. This product is of essential importance to web fixation and preferably lies in the interval 2,800 to 4,100. As further evidenced, the ratio lχ/12 = 22.5/16 = 1.4. This ratio is essential to the floating height or level of air suspension of the web and preferably falls within the interval 1.2 to 1.6. In addition, in accordance with the embodiment illu¬ strated each nozzle orifice 5 has an area A^ of 10.8 mm and the upwards open part 6a (slit 7) and the part 6b open in the cross-sectional direction of the box 2 of each eyelid perforation 6 have areas A2 and A3 of respec¬ tively 22.5 mm and 23.6 mm . As shown, 2A « A2 ~ A3, providing satisfactory heat transfer and a sufficient floating height or level of air suspension of the web. By "degree of perforation" P should in this case be understood the ratio between areas 2A^ + A2 + A3 and (L + G) • d, wherein G designates the width of the gap 4 which in accordance with the embodiment illustrated preferably is 35 mm. Thus, in accordance with the embo¬ diment illustrated the degree of perforation P = (2 • 10.8 + 22.5 + 23.6) / (215 + 35) • 20 * 0.0135, i.e. 1.35%. Tests have been performed using different blow boxes wherein the spacing d varied between 20 mm and 28 mm. It was found that highly satisfactory opera¬ tional conditions were obtained at the occurrence of a specific ratio between the height of the arc H and the degree of perforation P. The relation may be expressed as H = k • P - 5.88, wherein k is a constant of between 6.2 and 6.9 when H is denoted in mm and P in %.

Claims: CLAIMS 1. A blow box intended to be used in a plant for drying a material web ( 1 ) , such as a web of papermaking pulp being advanced through said plant, said blow box having an elongate, essentially right-angled parallel- epipedon shape including an essentially horizontal upper box wall (3) and being intended to be placed, jointly with a plurality of identical blow boxes, below the mate¬ rial web ( 1 ) for the purpose of holding the material web floating, i.e. suspended in the air, in a well-defined position above the upper blow box wall ( 3 ) by means of air streams that are being blown out of the box while simultaneously drying the material web, said blow box being designed to be placed below the material web ( 1 ) in a manner ensuring that the longitudinal centre line (C) of the upper box wall (3) extends transversely of the direction of advancement (Pl ) of the web (1), the upper wall (3) of said blow box being formed with a plu¬ rality of essentially circular nozzle orifices (5), and with a plurality of so-called eyelid perforations ( 6) , each one of said perforations consisting of a slit (7) extending in parallel with said centre line (C) and of a depression (8) formed in the upper box wall (3) adjacent the slit, at one side of the latter, said eyelid perfora¬ tions (6) being arranged in two rows extending in paral¬ lel with and on either side of the centre line (C), said nozzle orifices being arranged in rows (9a, 9b, 10a, 10b), likewise extending in parallel with and on either side of said centre line (C), the eyelid perforations in one row being so displaced in the longitudinal direction of the blow box relatively to the eyelid perforations of the other row that the eyelid perforations of both rows together form a zigzag pattern, the eyelid perforations ( 6) of both rows being oriented in such a manner as to cause air to exit essentially in parallel with the upper box wall (3), towards said centre line (C), and essen¬ tially perpendicularly thereto, and the nozzle orifices ( 5) being directed in such a manner as to cause air to exit essentially at right angles to the upper box wall (3), c h a r a c t e r i s e d in that the slits (7) of the eyelid perforations (6) in both rows are equally spaced from said centre line (C) , that said centre line (C) intersects the depressions ( 8) of the eyelid perfora¬ tions (6) in both rows, that the nozzle orifices (5) are arranged in respectively a first and a second pair of rows (9a, 9b and 10a, 10b, respectively), the two rows of each pair being positioned on either side of and spaced equally from said centre line (C), that the distance of the rows (9a, 9b) of the first pair to said centre line (C) is smaller than the distance of the rows (10a, 10b) of the second pair to said line, that for each eyelid perforation (6) there is a corresponding nozzle orifice (5) in the row (9a or 9b) of the first pair of nozzle orifices that is positioned on the same side of said cen- tre line (C) as a slit (7) of the associated eyelid per¬ foration (6), and also a corresponding nozzle orifice (5) in the row (10a, 10b) of the second pair that is posi¬ tioned on the opposite side of said centre line (C), each eyelid perforation ( 6) and its two corresponding nozzle orifices (5) being essentially in alignment in the cross¬ wise direction of the blow box, and in that the upper box wall (3) has a shallow continuous arcuate shape across its entire width. 2. A blow box as claimed in claim 1, c h a r a c - t e r i s e d in that the height of the arc H of the arcuate shape of the upper box wall (3) is related to the degree of perforation P as follows: H = k • P - 5.88, wherein k is a constant which is between 6.2 and 6.9 when H is denoted in mm and P in %. 3. A blow box as claimed in claim 1 or 2, c h a r ¬ a c t e r i s e d in that the product of the distance 12, in mm, between said centre line (C) and the edge of the slit (7) closest to said line, and the width L of said blow box (2), in mm, is 2,800-4,100, preferably about 3,440. 4. A blow box as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the ratio of the length l l of the depression (8) to the distance I2 between said centre line (C) and the edge of the slit positioned closest to that line is 1.2-1.6, preferably 1.4.

Inventors: HALLDIN CLAES, KARLSSON INGEMAR, NILSSON LARS, SIGVANT ROGER, HALLDIN, CLAES, KARLSSON, INGEMAR, NILSSON, LARS, SIGVANT, ROGER
Applicants: FLAEKT AB, HALLDIN CLAES, KARLSSON INGEMAR, NILSSON LARS, SIGVANT ROGER, ABB FLAEKT AB, HALLDIN, CLAES, KARLSSON, INGEMAR, NILSSON, LARS, SIGVANT, ROGER
Application year: 1996
Main classification: D21F5
IPC codes: B65H20, B65H23, D21F5, F26B13