Abstract:
A system and a method for dewatering an acoustical tile basemat comprising a continuously traveling wire, a device for depositing a dilute water slurry of solids including fiber and binder on the wire, a vacuum box below the wire for separating water from the solids deposited on the wire to form a basemat, a vacuum source, and a valve for cyclically connecting the vacuum source to the vacuum box at a cyclic rate that limits vacuum exposure to a length of the basemat newly arriving at the vacuum box to less than four inches per cycle.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method and apparatus for the manufacture of wet felted acoustical ceiling tile. 
     PRIOR ART 
     Acoustical ceiling tile is commonly made by a wet felting process on a Fourdrinier or Oliver mat forming machine. A dilute water slurry of mineral and/or other fibers, binder and other minor constituents is deposited on a travelling metal screen, known as a wire. Water is separated from the solids through the wire openings by gravity, the application of vacuum to the underside of the wire, and by pressing the solids with a roll or rolls situated above the wire some of which can be supplied with a vacuum. 
     Progressively higher vacuum level stations are used as the solids forming the mat progress along the path taken by the wire. 
     When as much water as practical has been removed by vacuum and, optionally, pressing, the mat is carried into a drying oven to remove excessive moisture and rigidify the mat for use in an acoustical tile. 
     By way of example, the slurry/mat can start with a thickness of 3 inches and be reduced to a thickness of ½ inch. The more water drawn from the mat prior to entry into a drying oven, the less energy is used in evaporating the excess moisture and the faster a production line can run. 
     Vacuum removal of water from the wire supported mat is limited by a tendency for the mat to crack along lines transverse to the wire movement. When a vacuum box operating at a high vacuum relative to the water content of the mat, excessive shrinkage can occur locally in the direction of wire travel. The result can be a transverse crack developed across the mat that renders it defective for use as a finished tile. This problem has existed for decades and attempts to solve it have had limited success. A commonly used technique is a slot or hole pattern at a vacuum box cover that applies suction to areas that depart from a plain transverse line so that cracking on such a natural line is less likely. Despite these efforts, cracking due to vacuum induced shrinkage has remained a limitation on line speed and, therefore, production capacity. 
     SUMMARY OF THE INVENTION 
     The invention involves the control of the application of dewatering vacuum in the production of a water felted mat for an acoustical tile. The control can modulate or pulsate the vacuum applied to the mat at a particular vacuum box or boxes so that the full vacuum level is applied in short time cycles. The effect of the modulated or pulsed vacuum is to avoid an abrupt application of suction over a relatively large area that otherwise would induce cracking of the mat through large scale shrinkage of the mat or cake due to evacuation of water. 
     The rate of vacuum modulation in relation to the conveying speed of the wire is high so that the mat is treated in small steps along the conveying length in a manner that may be compared to a jiggle or vibratory action. The modulated vacuum is less prone to produce cracking with the same vacuum level used in a traditional manner applied constantly to a vacuum box. The overall effect of the inventive process is the removal of more water from the mat prior in to entry in a dryer with the result of less time and energy expended in the dryer and a higher production rate. 
     As disclosed, the vacuum can be modulated by a constantly rotated valve that is ported to apply a cutoff vacuum to one or more vacuum boxes. The cycle of applied vacuum is determined by the rate of rotation of the valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic representation of a water felting line for producing mats for acoustical tile in accordance with the invention; and 
         FIG. 2  is a diagrammatic exploded view of an exemplary vacuum control valve in association with a vacuum pump used in the mat production line of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to  FIG. 1 , in a wet felting system  10 , a dilute water slurry of mineral and/or other fiber, binder and minor quantities of other solid constituents are delivered from a mixing tank  11  to a travelling metal screen or wire  12  through a head box  15 . Water, drawn by gravity, is removed from the constituents at a first section  13 . The mass of solids is initially loosely distributed on the wire  12 , for example, at a thickness of 3 inches. One or more rollers  14  may be employed to compact and unify the mat, designated  16 , as it is being formed and conveyed by the wire  12  from right to left. 
     A plurality of vacuum boxes  17 ,  18  and  19  are situated below the upper run of the wire  12 . The number and position of the vacuum boxes can vary depending on the design of the system  10 . The system  10  may have, for example, three vacuum levels developed by three separate vacuum pumps. The vacuum levels may be, by way of example, 2.5, 7 to 9 and 14 to 15 inches of mercury. One or more vacuum boxes may be assigned to each vacuum level; in the illustrated system  10 , there are three vacuum boxes associated with each vacuum level. The vacuum boxes  17  upstream of the screen conveying direction are at a low vacuum level, the boxes  18  are at the intermediate vacuum level, and the downstream boxes  19  are at the high vacuum level. It should be understood that more or less levels of vacuum may be used and more or less boxes for each vacuum level may be provided. Vacuum is delivered to the sets of vacuum boxes  17 - 19  through large ducts  21 ,  22  and  23 , respectively, that run parallel alongside the wire  12 . 
     Vacuum is developed by large pumps, one for each vacuum level, that operate continuously when the system  10  is operated. 
     The invention contemplates rapid modulation of the vacuum applied to an individual vacuum box  17 - 19  to draw water from the mat while reducing the tendency of the mat to crack from water removal induced shrinkage. Vacuum is modulated by a separate valve  26  for each vacuum level diagrammatically represented in  FIG. 2  inserted in a line between a duct  21 - 23  and respective vacuum boxes  17 - 19 . Each duct  21 - 23  serves as both a manifold and, due to its volume, an accumulator of vacuum storage. A valve  26  operates cyclically by rotation developed by a motor  27  that preferably is speed adjustable. The illustrated valve  26  has three inlet ports, one for each vacuum box  17   a, b, c , or  18   a, b, c , or  19   a, b, c  with which it is associated. An outlet of a valve  26  is connected to one of the vacuum ducts  21 - 23 . The valve  26  shown in  FIG. 2  is connected to a low level duct  21  and three corresponding vacuum boxes  17   a ,  17   b  and  17   c  arranged in a series along the wire  12 . A rotatable valve element  28  within the valve  26  successively connects and disconnects an inlet port  29  and an outlet port to the vacuum source or pump  24  through a duct. 
     A typical line speed of the wire  12  can range, for example, between 24 and 32 feet per minute. The valve can be rotated at, for example, between 60 and 120 rpm. This means that the valve element  28  will pulse or modulate 1 to 2 times per second, and the mat  16  will move between 4.8 and 6.4 inches in one second. It is envisioned that the valve  26  will have at least one inlet port  29  that will be open not more than 50 percent of a revolution. It can be shown that at 32 feet per minute, the mat is moving at 6.4 inches per second; if the valve is rotating at 60 rpm and open 50 percent of the time, the mat will move 3.2 inches while the valve applies vacuum to an advancing length of the mat. Based on geometrical considerations and general observations, application of a vacuum to an area of the mat corresponding to this incremental advance could be considered optimal for avoiding cracking due to excessive local shrinkage. That is to say, this advance of the mat under the vacuum at a box being influenced for the first time at that box, would not appear to result in excessive shrinkage. Where the wire  12  is running at less than 32 feet per minute and/or the valve had more than one inlet port open for its respective fraction of a revolution and/or the valve rotated at a rate higher than 60 rpm, the mat would advance a proportionately shorter distance than 3.2 inches. More specifically, it can be been that these factors of wire speed, port number, and valve rotational speed each have a multiplier effect. Thus, the valve  26  can influence a much shorter increment of advancing mat and thereby reduce the effect of any shrinkage produced by the vacuum across the increment of advancing web length. The result is that the mat can be subjected to a vacuum level of some conventional magnitude but the risk of shrinking the mat by water depletion through the degree that it cracks is greatly diminished. 
     Improved water removal reduces time and energy required in a dryer shown at  36 . This can permit the production line to be operated at a higher speed and acoustical tiles to be produced at lower cost. 
     The same vacuum modulating valve  26  may be employed, as shown at the other sets of vacuum boxes  18  and  19 . It is conventional that the downstream vacuum boxes  18 ,  19  are operated at higher vacuum levels than the preceding vacuum boxes  17 ,  18 . 
     The valve  26  diagrammatically illustrated in  FIG. 2  is but one of a variety of constructions that can be envisioned to practice the invention. For example, the valve  26  can have one or more inlet ports, multiple ports can be actuated at irregular times in a cycle, some ports can be open longer than others, and multiple ports can be open at the same time. 
     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.