US3384973A - Ventilating dryers - Google Patents

Description

May 28, 1968 Filed Sept. 23. 1966 AKE JOHANSSON VENTILATING DRYERS 2 Sheets-Sheet l WWW/AP Maia/ United States Patent a VENTILATING DRYER?) Alto Johansson, Klinten, Sweden, assignor to Aktichoiaget Svenslra Flaktfabriken, Stockhoim, Sweden Filed Sept. 23, 1966, Ser. No. 581,547 Claims priority, application Sweden, June 9, 1966, 7,885/66 5 Claims. (Cl. 3423) ABSTRACT OF THE DISCLOSURE In a cylinder dryer in which paper or other web material is dried by being pressed against the drying cylinders by a porous fabric, a ventilating duct operable to blow air against the outer surface of the fabric at a first position prior to its disengagement from the cylinder and to blow air against the fabric at a second position where the fabric is withdrawn from the cylinder so as to permit the air at that position to flow through the fabric into the pocket formed between the fabric and the web by such withdrawal. The duct leaves a space on the outside of the web at a third position free of air impingement so as to permit discharge of air outwardly through the fabric from the pockets prior to engagement of the fabric with the succeeding cylinder. Ventilating air may also be impinged obliquely against the outer surface of the fabric on the second cylinder at a fourth position beyond the third position to assist the aforesaid discharge outwardly through the fabric.

This invention refers to a method and an arrangement for the ventilation of a cylinder drying part wherein a material web is moved upwards and downwards between and around a plurality of heated rotating drying cylinders disposed in two rows, one above the other, and where the material web is pressed into contact with at least two of a row of subsequent drying cylinders by an endless dryer fabric which between the said two cylinders is guided by a guide roll.

The invention is primarily intended for us in connection with the drying of paper. For this reason, the description in the fol-lowing will deal with this technique.

According to the conventional method utilized since a long time ago the paper web is directed along a serpentine path in wrapping relation with heated rotating drying cylinders having a diameter of about 1.5 In. As a paper web in wet state has low strength, it is not possible to obtain a sufficiently intimate contact between the drying cylinders and the paper web by tensioning the paper web. The web, instead, usually is pressed against the cylinder surface by endless sheets of textile so-called dryer felts. This method, however, involves a great number of disadvantages, among which may be mentioned that special felt drying cylinders must be arranged, as the felts absorb water from the paper web, further that the felts impede the evaporation from the paper during the moments the paper web is pressed by the dryer felts against th drying cylinders, and that the felts together with the paper web form a great number of spaces, so-c-alled pockets, between the drying cylinders, which pockets are difficult to ventilate and wherein a very moist atmosphere is produced.

In recent years, therefore, the development has "brought about other members of a more permeable type, adapted to press the paper web against the drying cylinders and to replace the said dryer felts. These members, the so-called dryer fabrics, are in most cases a relatively loose fabric of threads of a synthetic material. Such a dryer fabric does not absorb water from the wet paper web and, therefore, need not be dried over special cylinders. The dryer fabrics Patented May 28, 1963 are sufficiently permeable for allowing evaporation from the paper web even during the moments the web is pressed by the dryer fabric against the drying cylinders, and it prevents by its permeability the production of too moisty atmosphere in the said pockets.

In spite of the exchange of conventional dryer felts against dryer fabrics with a permeability more than a hundred times greater than that of a dryer felt, it was found that the air condition in the pockets remains relatively moist, though being considerably less moist than in the case of dryer felts. As it is known from the use of dry felts to ventilate the pockets by pressing air into them through the dryer felts, it was obvious to utilize the same method also after the said exchange of the dryer felts against dryer fabrics.

it was found, however, that the dryer fabrics due to their rough surface exert a great frictional resistance to air and, consequently, take along a very thick boundary layer of air in their running direction. While running over a drying cylinder in contact with the paper web, the said boundary layer which stands still in relation to the paper web and dryer fabric impedes the evaporation from the paper web, because the evaporated water vapour must be transported by diffusion through the boundary layer. As a result of tests carried out with the aim to ventilate the pockets and increase the evaporation, it was found that a substantial capacity increase of a paper machine can be achieved at a reasonable cost by blowing air substantially perpendicularly to the dryer fabric across its entire width in at least two positions one of which is located before the point in which the dryer fabric runs off a drying cylinder while a second position is located between the point in which the dryer fabric runs off the same drying cylinder and the point in which the dryer fabric runs onto the next following guide roll. The arrangement according to the invention comprises means for blowing drying air against and through the dryer fabrics. The arrangement is characterized in that each of the said means for blowing drying air against and through the dryer fabrics comprises a duct extending along a drying cylinder and located substantially between the said cylinder and the next following drying cylinder in the same row of drying cylinders said duct being connected to the pressure side of a transport means for air and comprising at least two series of blowing nozzles disposed close to and directed towards the dryer fabric, one of the said series of blowing nozzles being adapted to blow drying air against the dryer fabric immediately befort the said fabric runs off a drying cylinder, the second of the said series of blowing nozzles being adapted to blow drying air against the dryer fabric when th said fabric has run off the same said drying cylinder, but before it has come into contact with the next guide roll.

By the said method it is achieved that the boundary layer impeding the evaporation from the paper web is removed from the last portion of the cylinder, i.e. from the portion where the boundary layer has grown to its greatest thickness and where at the same time the temperature of the paper has increased so much that the evaporation is intensive, and that ventilating air is supplied to the pocket in the place where the dryer fabric itself pumps air into the pocket, due to its removal from the cylinder surface. According to an embodiment of the invention, the said pump effect can be increased additionally by blowing air through a third series of nozzles, in such a manner, that at the same time air is ejected from the pocket.

The invention is described in the following in a greater detail, reference being made to the accompanying drawings whereon FIG. 1 and FIG. 2 each shows a fragmentary longitudinal section through a cylinder drying part in a paper machine, the said drying part being provided with 3 arrangements according to two embodiments of the invention, and FIGS. 3A and 3B show a longitudinal section through the arrangement according to FIG. 2.

In FIG. 1 a paper web 1 runs upwards and downwards between and around a plurality of hollow drying cylinders 2 heated from the inside with steam. The said cylinders 2 are mounted rotatably and driven by motors (not shown). For pressing the paper web 1 into intimate contact with the heated cylinders 2, two endless dryer fabrics 3, 4 are provided which are highly permeable and woven of synthetic material. Each of these fabrics operates only on the cylinders in one row of cylinders and, therefore, must be turned over guide rolls 5 between the cylinders 2. As the dryer fabrics 3, 4 are endless, they also comprise a return part (not shown) which runs below and above the cylinders respectively. The said return part is provided with an arrangement for stretching the dryer fabric, in such a manner, that the required pressing of the paper web 1 against the drying cylinders 2 is obtained. Between two subsequent cylinders 2 in one row of cylinders a duct 6 is arranged in such a manner, that it extends across the entire width of the dryer fabric. The duct is supplied with hot air from a fan, and within the duct two hole series 7 and 8 are provided through which air flows against the dryer fabric.

When the machine is in operation and the dryer fabrics run at a speed equal to that of the machine, i.e. 300-700 m./minute (1000-2500 feet/see), the fabrics take along considerable amounts of air. According to studies made of the air flow, the dryer fabric during their contact with the cylinder surface build up a thick boundary layer which follows the fabrics at their speed and strongly prevents the water vapour evaporated from the paper web from diffusing from the vicinity of the paper. As a result thereof, the evaporation speed is kept low.

When a fabric 3, 4 with its boundary layer starts running off a drying cylinder 2, a vacuum is produced in the gap between the fabric 3, 4 and the cylinder surface, which vacuum sucks the boundary layer through the fabric 3, 4 which in this Way acts as a pump pumping air into the so-callcd pockets 9 formed by the paper web 1, a dryer fabric 3, 4 and the surface of a cylinder 2. When thereafter the fabric runs on the guide roll 5, in a corresponding manner an overpressure is produced in the gap between the dryer fabric running onto the said roll and the surface of said guide roll. By the overpressure additional air is pressed through the fabric into the pocket. When the fabric runs off the guide roll 5, the procedure is opposite to that decsribed above. The fabric now pumps air through itself in the opposite direction, i.e. out of the pocket. The effect is increased when the fabric runs onto the next drying cylinder 2 whereby in the gap between the fabric and the cylinder surface an overpressure is produced which presses air out of the pocket 9.

This self-pumping effect of the fabric, which is due to its rough surface and its high permeability, contributes, of course, to the ventilation of the pocket. However, this also involves the disadvantage that the air primarily pumped into the pocket by the fabric is its own, very moist boundary layer.

This disadvantage is avoided by the invention, in that first the boundary layer is blown away while the fabric still is on the cylinder surface, whereafter large amounts of dry air are supplied right in the zone wherein the fabric pumps air into the pocket. The blowing away of the boundary layer in itself should preferably be carried out by jets directed against the direction of travel of the fabric, but doing so a jet pump effect would be created which counteracts the pump efiect of the fabric. For this reason, it has proved most suitable to avoid the jet pump effect by blowing perpendicularly to the wire whereby the boundary layer, nevertheless, is blown away sufiiciently effectively.

In order to utilize the sacrificed fan effect in the best possible way, in the embodiment according to FIG. 2 a partition wall 10 has been provided in the duct 6a, which walls divide the duct into two chambers 11, t2 whereof the first chamber 11 effects the blowing against the fabric whilst on the cylinder surface, and the second chamber 12 blows against the fabric in the draw between a cylinder 2 and a guide roll 5. By means of a swivel damper 13 as shown in FIG. 3A, the amount of air can be adjusted between the said two chambers. A modified duct 6/) is shown in FIG. 3B in which a partition 1% separates the duct into two chambers 11b and 12b, and the amount of air is adjusted by swivel dampers 13a and 13b.

FIG. 2 further shows the blowing of air against the fabric in a position 14 obliquely in its direction of travel after the fabric has run onto the next drying cylinder. This blowing results in two effects. Firstly, the boundary layer of the fabric is disturbed in a phase prior to position 7 and, secondly, the jets in position 14 assist in ventilating the pockets 9, because they are directed with the flow direction of the air leaving the pocket, and by jet pump action suck out the same.

FIG. 3A shows how the chambers 11, 12 are supplied with air from a fan 15 via a heater 16 and swivel dampers 13, and FIG. 3B shows the supply to chambers 11b and 12b from a fan 15b and a heater 161).

In a machine with conventional dryer felts the partial pressure of the water vapour in a pocket 9 amounts to above 400 mm. Hg. Subsequent to their replacement by dryer fabrics, 300 mm. Hg were measured, and with an arrangement according to the invention being connected, only mm. Hg was measured. From this the importance of the inventiOn for an increased production and a uniform drying across the width of the paper web is easily understood.

What I claim is:

1. In a method for ventilating a cylinder dryer wherein a material web is moved upwards and downwards between and around a plurality of rotating drying cylinders arranged in two tiers, one above the other, and the material web is pressed into contact with at least two sequential drying cylinders in one row by means of an endless permeable dryer fabric guided by a guide roll between the two cylinders; the steps of blowing air substantially perpendicular to the dryer fabric across its entire width in at least two positions, one of said positions being located confronting the fabric before the point at which the dryer fabric runs off the first of said drying cylinders to thereby blow away the boundary layer of air carried along by the fabric, and the second position being located confronting the fabric between the point at which the dryer fabric runs off said first cylinder and the point at which it runs onto the following guide roll to thereby supply ventilating air to flow inwardly through the fabric at said position; and preventing blowing of air against the fabric in a third position located between the point at which the dryer fabric runs off the guiding roll and the point at which it runs onto the succeeding drying cylinder.

2. A method according to claim 1 including the step of blowing air against the dryer fabric in a fourth position located after the point at which the dryer fabric runs onto the next drying cylinder, said air at the fourth position being given an oblique blow direction directed in the direction of travel of the dryer fabric to provide a jet pump action sticking air away from said third position.

3. In apparatus for drying web material comprising an upper tier of heated and rotatably mounted drying cylinders, a lower tier of heated and rotatably mounted drying cylinders, at least one endless dryer fabric adapted to press the web material against at least two sequential drying cylinders in one tier, guide rolls intermediate said two drying cylinders for guiding the fabric away from the first of said cylinders and against the second of said cylinders, and supply means for supplying dry ventilating medium to blow against and through the dryer fabric; the improvement wherein said supply means comprises a duct extending along the length of the drying cylinders and located substantially between the said two drying cylinders in the one tier, said duct having at least two series of blow nozzles disposed close to and confronting the dryer fabric, one of said series of blow nozzles being adapted to blow dry medium against the fabric at a first position immediately before the fabric runs off the first drying cylinder to blow away the boundary layer of air carried by the fabric at said position, the second of said series being adapted to blow dry medium against the dryer fabric at a second position where the fabric has run off the first drying cylinder but before it has come into contact with the guide roll to supply ventilating medium to flow inwardly through the fabric at said position, said duct being imperforate at a third position located between the guide roll and the second drying cylinder to afford free flow of medium outwardly through said fabric at said third position.

4. Apparatus according to claim 3 including in said duct an internal longitudinal partition wall dividing the duct into two chambers, each extending along the length of the duct, and a swivel damper to proportion the medium supply to said two chambers, the first series of blow nozzles communicating with one of said chambers, and the other series of blow nozzles communicating with the other chamber.

5. Apparatus according to claim 3 including a third series of blow nozzles in said duct positioned to blow medium against the dryer fabric at a fourth position after the point at which the dryer fabric has run onto the subsequent drying cylinder, said blow nozzles being directed obliquely to the dryer fabric in the direction of travel of said fabric when in operation, so as to provide a jet pump action sucking medium away from said third position.

References Cited UNITED STATES PATENTS 1,199,394 9/1916 Liebeck 34-111 1,660,640 2/1928 Asten 34-111 XR 3,320,677 5/1967 Reitzel 34l11XR KENNETH W. SPRAGUE, Primaly Examiner.

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