Steam Drying Device For Paper Making Machine

Abstract

Steam is applied to the paper web on a papermaking machine at a flow rate which not only causes the usual dewatering enhancement by reducing the viscosity of the water but also applies sufficient heat to remove water by evaporation. The steam is applied to the web through an unrestricted throat of large area. The steam is delivered to this throat through a series of pressure-reducing stages. Succeeding stages deliver the steam in substantially opposite directions and the final stage delivers the steam into a throat-defining housing in a direction directed away from the throat.

Description

BACKGROUND OF THE INVENTION

This invention relates to improved dewatering devices for dewatering wet fibrous webs such as paper webs. The dewatering of fibrous webs in papermaking machines has been the subject of substantial activity throughout the years and many mechanisms have been proposed and are utilized to enhance the natural drainage of water through the web and its support as the web moves from the headbox to the final drying apparatus.

It has long been known, for example, that the application of heat to the fibrous web enhances the dewatering action by reason of reducing the viscosity of the water. An example of this technique may be seen in U.S. Pat. No. 3,097,994 issued to Willaim A. Dickens, et al. As has been stated, such devices previously proposed operate to enhance dewatering by reason of elevating the temperature of the web and thereby decreasing the viscosity of the water. These previous techniques are successful, despite the fact that a slight amount of water is actually added to the web by the steam, by reason of the significant decrease in the water viscosity effected by the elevation in temperature.

While it is of course theoretically conceivable that the flow rate of steam could be increased in such devices to such an extent that the amount of heat added would elevate the temperature of the water in the web to a point at which the water actually would be removed by evaportion, such techniques have previously not been employed because the resultant steam velocity would be sufficiently high as to cause damage to the web. This is so despite the fact that the steam normally available at a papermaking site is of sufficiently high pressure and temperature as to enable the requisite flow rate to be applied to the web.

BRIEF SUMMARY OF THE INVENTION

It is of primary concern in connection with the present invention to provide a steam-applying system for papermaking machines in which a sufficiently high mass rate flow of steam can be applied to the web as will not only cause the conventional enhancement of dewatering by virtue of decreasing the viscosity of the water, but which will also effect evaporation of water directly from the web.

Generally speaking, the above objective of the present invention is achieved by virtue of applying the steam at a high mass rate flow through an unrestricted throat which is of sufficiently large cross-sectional area as to avoid high velocity impingement of the steam against the web thereby effectively avoiding damaging the web while achieving the requisite rate of heat transfer thereto as to effect evaporation of water from the web.

The introduction of steam to the throat is effected in a plurality of stages, the final stage of which directs the steam into a housing which defines the throat in a direction directed away from the throat. Each stage effects a pressure drop in the steam and the steam is delivered to the throat essentially at low pressures such as atmospheric pressure whereby the impingement velocity of the steam against the web is extremely low and is in any event well below that velocity which could cause damage to the web.

The successive stages of steam delivery or staging each effects a pressure drop in the steam and each is effected directionally such that successive stages deliver steam in relatively opposite directions. Thus, the steam changes directions a number of times before exiting through the throat as aforesaid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic elevational view illustrating the present invention and its environment in a papermaking machine;

FIG. 2 is an enlarged section taken substantially along the plane of section line 2-2 in FIG. 1 showning details of the present invention;

FIG. 3 is a plan view showing the first stage delivery chamber body in flattened condition clearly to show the steam delivery ports;

FIG. 4 is a view similar to FIG. 3 but showing the final delivery chamber body in flattened condition illustrating also its steam delivery ports;

FIG. 5 is an enlarged transverse section taken substantially along the plane of section line 5--5 in FIG. 2; and

FIG. 6 is a perspective view of the steam applicator according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference is had first to FIG. 1 although it will be understood that the papermaking machine diagrammatically illustrated therein is not intended to be limitive with respect to the present invention in any way and is illustrated herein in the manner shown strictly for the purpose of describing in general the environmental relationship with which the present invention is to be utilized. As shown in FIG. 1, there is provided the usual headbox 10 which receives the conventional slurry of cellulosic fiber in water and discharges same through the nozzle 12 onto the upper flight of an endless web-supporting means 14 which as is conventional will be a foraminous belt, wire or felt. The support means 14 passes over a breast roll 16 and then passes over a number of table rolls 18 and thereafter over the suction boxes 20 to reverse direction over the driven roll 22 and is supported along its path of movement back to the breast roll 16 by the conventional return rolls 24 as the web W of fibrous material passes from right to left in FIG. 1 from the nozzle which originates its formation. It is conventional first to allow dewatering of the web by gravity drainage through the foraminous support means 14, which water is collected in the receiving pan 26 from which it either will be returned to headbox or discarded. Further dewatering of the forming web is effected by the suction boxes 20 which, as is conventional, are connected to a suitable source of vaccum such as motor driven pumps and, ultimately, the web issues as indicated at 28 from the support means 14 to pass between press rolls 30 and ultimately to a dryer 32.

The steam-drying arrangement according to the present invention is indicated generally by the reference character 34 and will be seen preferably to be located in the suction box region of the papermaking machine. As can be seen in FIG. 2, the mechanism 34 is in the form of a housing which defines a throat T of relatively large cross-sectional area and which is unrestricted having a transverse dimension completely or substantially completely to encompass the width of the web W. The housing proper may be formed by an inner wall structure which, as is shown in FIG. 5, includes the opposite end walls 36 and 38 joined by the downwardly facing arched roof portion 40 and having the planar opposite side walls 42. The lower edges 44 and 46 of the end wall structures are positioned in close proximity to and above the web W whereas the opposite side wall structures have their lower edges 48 and 50 disposed in somewhat elevated position as is shown. Suitable support bracket portions 52 are provided on the housing structure facilitating its mounting on the frame portions 54 of the papermaking machine, as will be evident.

The housing structure preferably includes an outer wall structure 56 spaced from the inner wall structure and defining a space with the latter within which is received suitable insulating material 58 to avoid unnecessary heat loss.

A final steam delivery chamber body 60 is located within a housing means and preferably is of cylindrical form extending between the opposite side walls 42 so as to be coextensive or substantially coextensive in length with the arched roof 40. It will also be noted that the body member 60 is displaced within the housing so as to be spaced more closely to the arched roof 40 and relatively remote from the unrestricted throat portion T. It is also preferred that the arched roof portion 40 be coaxial or substantially coaxial with respect to the cylindrical body member 60 as is shown in FIG. 5.

Located within the body 60 is a further body 62 also of cylindrical form and again coextensive or substantially coextensive in length both with the body member 60 and the arched roof 40. In the form of the invention shown, the body member 62 is eccentrically disposed within the body 60 so as to be displaced in the direction of the arched roof 40.

The further body 62 is provided with a flange 64 at that portion thereof which projects outwardly of the housing as is shown in FIG. 2 and is adapted to be connected to a source of relatively high pressure steam. In operation of the device, the high pressure steam is introduced into the interior of the body 60 and issues therefrom through a series of openings 66, 68 and 70 which direct the steam into the interior of the body 60 while effecting a first stage pressure drop. The issuing steam impinges against and is directed toward the lower inner wall region 72 of the body 60 where it reverses direction and flows upwardly around the body 62 to issue into the interior of the housing through the series of openings 74, 76 and 78 which, as shown, are directed generally away from the direction of the throat I so that the issuing steam is directed toward and impinges against the arched roof 40 whereafter it changes direction to flow downwardly around the body 60 ultimately to issue through the throat T at a low velocity, high mass rate of flow in the blanket or cloud at relatively low pressure.

The flattened body 62 is illustrated in FIG. 3 whereas the flattened body 60 is illustrated in FIG. 4. In FIG. 3, it will be seen that the lines of openings 66, 68 and 70 are distributed through a minor circumferential extent of the body 62 and that, from the entrance end 80 of the body toward the opposite or blind end thereof, the lines of openings 66, 68 and 70 are of progressively diminishing spacing. The purpose of this arrangement is to effect a uniform flow of steam along the length of the body 62 so as to introduce the steam at substantially uniform pressure into the interior of the body 60. It will be understood that the arrangment need not be exactly as is shown in FIG. 3 since other arrangements would be effective to produce the same result. For example, the lines of openings 66, 68 and 70 could be such that the spacing between the individual openings are equidistant with the openings being of progressively greater diameter from the end 80 to the blind end or any other suitable arrangement could be utilized.

Since the introduction of the steam is of substantially uniform pressure along the length of the body 60, its openings as illustrated in FIG. 4 can be of uniform spacing and sizes as indicated.

In any event, it will be apparent that a number of pressure drop stages are provided and they may be of any number required. For example, it is desirable that the pressure drop stages be sufficiently gradual as will not cause the propagation of any undue sound or noise whereat, the same time, the pressure drops should be such as cumulatively effects substantially no pressure, i.e., atmospheric pressure, of the steam at the throat T. Such an arrangement is entirely satisfactory when the invention is utilized in association with suction boxes as is shown in FIG. 1 since the negative pressure created by the suction boxes will be more than adequate to cause the low pressure steam to pass through the fibrous web material

Most importantly, the overall arrangement should be such that the mas rate of flow of the steam is sufficiently high as to create a dewatering action by evaporation of the water from the foraminous web material. By following the above described relationships, the velocity of the steam issuing through the throat I will be insufficient to cause any damage to the web.

Steam available at papermaking machinery sites normally is of relatively high pressure, i.e., at least several atmospheres and is otherwise suitable for producing a sufficiently high mass rate of flow of steam as to achieve the objectives according to the present invention. As an example of a practical embodiment according to the present invention, steam at 50 psig and 350.degree.F is introduced into the body 62, the openings 66, 68 and 70 are sufficient to achieve a steam pressure of 20 psig within the interior of the body 60 at a temperature of 347.degree.F and the openings 74, 76 and 78 are sized to achieve substantially atmospheric pressure steam at 345.degree.F issuing through the throat T with the rate of steam flow being approximately 45,000 pounds per hour.

Claims

What is claimed is:

1. In a papermaking machine having travelling web-supporting means, headbox means for depositing stock onto said web-supporting means, and suction means for removing water from the stock downstream of said headbox means to form a progressively dewatered paper web, the improvement which comprises:

means positioned opposite said suction means for applying a high mass rate flow blanket of steam substantially at atmospheric pressure to said web substantially uniformly throughout a plan-view area extending across said web and an area sufficient to dewater the web by evaporation without damaging the web by steam impingement thereagainst, said means for applying a high mass rate flow blanket of steam comprising a housing having a substantially uniform transverse dimension substantially to encompass the width of said web and presenting an unrestricted discharged throat of sufficient plan view area to deliver steam at said high mass flow rate without damaging said web, a final steam delivery chamber having openings facing away from said throat, and delivering steam into said housing, at least one further steam delivery chamber within said final delivery chamber having openings facing toward said throat, said openings in said final delivery chamber and said further chamber being sized to reduce the pressure of said steam in stages to provide substantially atmospheric pressure steam at a sufficient mass flow rate to dewater said web by evaporation.

2. In a papermaking machine as defined in claim 1 wherein said housing means includes an arched roof facing said throat, said final delivery chamber is in the form of a cylinder extending along the length of and spaced closely to said roof, and said further chamber is in the form of a cylinder and substantially coextensive in length with said cylinder.

3. In a papermaking machine as defined in claim 2 wherein said further chamber is eccentrically disposed within said final chamber in the direction of said throat.

4. In a paper making machine as defined in claim 1 and further characterized in that the openings in the further steam delivery chamber are of a constant size and of a spacing which progressively diminishes away from said means for supplying steam.

5. In a paper making machine as defined in claim 1 further characterized in that the openings in the further steam delivery chamber are of equidistant spacing and which progressively increase in a size from said means for supplying steam.

6. In a paper making machine including travelling web-supporting means having a partially dewatered paper web thereon, and drying means for receiving said web, the combination of:

supply means for supplying steam at a pressure sufficiently high to deliver steam to said web, prior to introduction to said drying means, at a flow rate high enough to evaporate water from said web; and

steam applicator means disposed adjacent said web-supporting means and connected to said supply means for reducing the pressure of said steam in stages substantially to atmospheric pressure and applying it at said high mass rate of flow to said web whereby to remove water from the web by evaporation, said steam applicator means comprising a housing having a substantially uniform transverse dimension substantially to encompass the width of said web and presenting an unrestricted discharge throat of sufficient plan veiw area to deliver steam at said high mass flow rate without damaging said web, a final steam delivery chamber having openings facing away from said throat and delivering steam into said housing, at least one further steam delivery chamber within sid final delivery chamber having openings facing toward said throat, said openings in said final delivery chamber and said further chamber being sized to reduce the pressure of said steam in stages to provide substantially atmospheric pressure steam at a sufficient mass flow rate to dewater said web by evaporation.