Abstract:
A device for selectively controlling the diameter of sections of a calender roll. The device comprises a plurality of nozzles which direct jets of superheated steam against sections of the calender roll. Thermal expansion, resulting from localized heating by the steam jets, corrects local non-uniformities in the gap between adjacent cooperating calender rolls. Moisture which condenses from the steam onto the calender roll surface is removed by a flow of air past the roll surface.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of calenders, and more particularly to devices for controlling the diameter of rolls used in calenders or analogous machines. 
     Pressing a material between two calender rolls can change the physical characteristics of the material. For example, calendering paper can change its density, thickness and surface features. Thus, the calendering process is frequently used in the manufacture of paper and other sheet materials where it is often desirable to change the density, thickness or surface features of the material. 
     A common problem associated with calendering is an uneven thickness of the calendered material, or &#34;web&#34;. Localized variations in a variety of parameters affect the diameter of individual calendar rolls and create variations in the spacing or &#34;nip&#34; between cooperating rolls. Variations in the nip across the width of a pair of calender rolls produces a web having non-uniform thickness. Thus, a more uniform thickness could be obtained if the local diameters of the rolls could be controlled. 
     If the rolls are made of a material that responds to changes in temperature, one may control local roll diameters by varying the temperature of selected cylindrical sections of the calender roll. Previous devices have used this principle by directing jets of hot or cold air against sections of a rotating calender roll to control its local diameters. Many of these devices blow hot air from a supply plenum against sections of the calender roll to increase the diameter of the roll and thus decrease the thickness of the web. Alternatively, when these devices release cold air from a second supply plenum against selected cylindrical sections of the calender roll, those sections contract. This decreases the local roll diameter and increases the thickness of the web. Examples of such devices are shown in U.S. Pat. No. 4,114,528 to Walker and U.S. Pat. No. 3,770,578 to Spurrell. 
     These previously known devices, however, are subject to certain limitations and inefficenicies. For example, the nip control range is determined by the maximum and minimum temperatures of the air jets. The air in the hot air plenum is usually pressurized by a blower and heated by steam from the facility power plant. Typically, however, steam supplied by such a power plant is waste steam, having a maximum temperature of about 350° F. Inefficiencies in the heat exchange process further limit the maximum temperature of such steam heated air to about 325° F. 
     The calender roll control device of the present invention has a number of features which overcome many of the disadvantages of many air jet control devices heretofore known. For example, the present invention uses jets of steam to heat the calender roll. The direct use of steam avoids the inefficiencies in the air heating process. Additionally, since the invention uses steam jets rather than steam-heated air, the higher temperature provides a greater control range then conventional hot air devices. Furthermore, the invention does not require a blower to pressurize an air plenum. Instead, the steam plenum used with the present invention is pressurized directly by the thermal energy of the steam. 
     Another type of prior calender roll control device uses magnetic fields to heat the calender roll, for example, as shown in U.S. Pat. No. 4,384,514 to Larive et al. In this type of device, the roll is made of a conducting material and magnets are positioned close to the roll surface. As the rotating roll passes under the magnets, cylindrical sections of the roll are heated by magnetic induction. The magnetic fields induce currents in the calender roll which dissipate their energy heating the roll. However, because ordinary 50/60 Hz electromagnets have high magnetic forces which may bend the roll, 25 Khz alternating current electromagnets are generally used. Thus, effective magnetic induction calender roll control devices require a special alternating current power supply. 
     Furthermore, to achieve the greatest heating effect, the magnets should be positioned within about 1/8&#34; of the roll surface. However, placing the magnets this close to the calender roll may lead to damage when the web breaks. The broken web can wrap around the roll a sufficient number of times to build up a thick layer of calendered material on the roll. Once the layer becomes more than 1/8&#34; thick, the rotating calender roll can drive the paper into the magnets with sufficient force to damage both the magnets and their supporting structure. 
     The device of the present invention also provides a number of advantages over magnetic induction calender roll control devices. For example, the invention does not require a special alternating current power supply to energize electromagnets. Instead, the invention heats the calender rolls with steam which is generally a less costly form of energy then electricity. Electric power is a relatively expensive energy source since the steam to electric power conversion process is usually only about 44% efficient. The direct use of steam to heat the calender roll is more economical. Furthermore, depending upon the particular application, the steam nozzles used in the present invention to direct steam jets against the calender roll are usually positioned approximately two inches from the roll surface. This two inch gap between the nozzles and the calender roll greatly decreases the possibility of damage to the nozzle by contact with the calendered material. 
     The present invention thus provides a number of advantages over prior art calender roll control devices. These and other advantages will become apparent in the description which follows. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward a controller for controlling local calender roll diameters by selectively directing jets of steam against sections of the calender roll. The roll is made of a material having at least one dimension which responds to changes in temperature. Therefore, thermal expansion, resulting from localized heating by the steam jets, corrects non-uniformities in the nip formed between cooperating calender rolls. Moisture that condenses from the steam onto the roll surface is removed before it wets the calendered material. 
     In the illustrated embodiments, the invention comprises a plurality of nozzles which direct jets of steam at a rotating calender roll. The nozzles are dispersed at intervals along the length of the roll so that steam escaping from each nozzle affects the diameter of a particular section of the roll. A valve associated with each nozzle controls the volume of steam discharged by the nozzle. When a valve opens, steam escapes from the associated nozzle and heats the adjacent section of the calender roll. This steam heating causes the adjacent section of calender roll to expand, thereby contracting the nip formed between cooperating calender rolls. As a result, the narrowed section of nip produces a thinner web. 
     The steam is preferrably superheated to minimize condensation of the roll. Condensation will wet the calendered material, which may be adversely affected by water. Any condensation which does occur, however, may be sucked off the surface of the roll by a vacuum plenum having an inlet port near the surface of the roll. Furthermore, the steam jets are preferrably directed against the side of the calender roll moving away from the nip. As the hot calender roll rotates, the exposed area of the roll travelling from the steam jets back to the nip allows any remaining condensate to evaporate before reaching the nip. 
     To maintain a uniform thickness of calendered material, a valve control device controls the valves and hence the heating of each section of the calender roll. In the illustrated embodiments, a web thickness sensor measures the thickness of the web at intervals along its width and generates signals corresponding to the measured thicknesses. The signals from the sensor are fed to a valve control device which maintains a uniform web thickness by adjusting the steam valves to thereby control the diameter of particular sections of the temperature sensitive calender roll. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of one embodiment of the present invention having two rows of steam jets directed against the lowermost calender roll and a vacuum plenum for removing moisture from the roll. 
     FIG. 2 is a front view of the steam jet and plenum structure taken along the line 2--2 of FIG. 1. 
     FIG. 3 illustrates another embodiment of the present invention having a single row of steam jets directed against an intermediate calender roll and a shroud for preventing cold air entrainment. 
     FIG. 4 illustrates still another embodiment of the present invention supported by an over-center support mechanism. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The first embodiment of the present invention is illustrated in FIG. 1. The invention comprises a steam plenum 5 containing superheated steam at 500° F. and 10 psig. A plurality of hollow cylinders 7 communiate with the interior of the plenum 5 so that steam from the plenum 5 enters the cylinders 7. A nozzle 9 is positioned at the exterior end of each cylinder 7 to direct a jet of steam against adjacent sections of the calender roll 11 to control its local diameters. 
     A valve 13 associated with each hollow cylinder 7 controls the flow of steam from the plenum 5 to each nozzle 9. Each valve 13 comprises a plug 15 which gradually opens and closes an orifice 17 located in the wall of the cylinder 7. A rod 19 controls the plug 15 which in turn is controlled by air pressure acting on a diaphragm 21. The air pressure works against a spring 23 which holds the plug 15 in the closed position. Increasing the air pressure displaces the plug 15 away from the orifice 17, thereby allowing more steam to escape through the nozzle 9. 
     When a valve 13 opens, steam escapes from the plenum 5 through the hollow cylinder 7 and its associated nozzle 9. The steam jet heats the section of calender roll 11 which is adjacent to the nozzle 9. As the temperature of the heated section of calender roll 11 increases, the thermally expanding roll 11 decreases the size of the nip 25 formed between the heated section of calender roll 11 and the adjacent cooperating roll 27. Thus, the heated section of calender roll 11 produces a thinner section of calendered material 29. 
     The steam in the plenum 5 is superheated by a steam superheating device 30 (see FIG. 2) to minimize condensation on the calender roll 11 which may otherwise wet the calendered material 29. However, any steam which does condense on the roll 11 is sucked off the roll surface 31 by a vacuum plenum 33. 
     Four shims provide a means for substantially enclosing the volume bonded by the steam plenum 5, the calender roll 11, and the vacuum plenum 33. Two of the shims 35, 37 are illustrated in FIG. 1. The two remaining shims 39, 41 are illustrated in FIG. 2, which is a front view of the device along the line 2--2 of FIG. 1. The first shim 35 extends between the top of the steam plenum 5 and the calender roll 11. The second shim 37 is positioned between the vacuum plenum 33 and the calender roll 11. The two remaining shims 39, 41, illustrated in FIG. 2, are positioned at either end of the plenums and complete the enclosure. Therefore, air sucked into the vacuum plenum 33 passes through the narrow gap between the shims 35, 37, 39, 41 and the calender roll 11. The flow of air rushing through this gap removes condensate from the calender roll surface 31 and is sucked into the vacuum plenum 33. 
     To further insure that no condensate wets the calendered material 29, the steam jets are located on the side of the calender roll 11 which moves away from the nip 25. The roll is typically maintained at an average temperature of about 190° F. Therefore, the exposed area of the hot calender roll 11 extending from the second shim 37 to the nip 25 evaporates any remaining condensate before it reaches the nip 25. 
     A doctor blade 43 extends along the length of the calender roll 11 and is positioned above the calender roll control device to protect the device from pieces of the calendered material which may break off of the calendered sheet 29. The vacuum plenum 33 is also protected by a filter 45. The filter 45 covers the inlet port 47 of the vacuum plenum 33 and prevents particles of the calendered material or other foreign matter from entering the vacuum plenum 33. 
     As shown in FIG. 2, a plurality of nozzles 9 are positioned along the length of the steam plenum 5. The nozzles 9 are disposed in two rows and dispersed at intervals along the length of the plenum 5 corresponding to sections of the calender roll 11 whose diameters are to be controlled. Typically, each section of calender roll or &#34;slice&#34; is about six inches wide. However, depending upon the particular situation, each slice may be wider or narrower. Additional nozzles 49 are located near the ends of the plenum 5 to compensate for the increased cooling tendency of the calender roll ends. 
     As shown in FIG. 1, a computerized valve control device 50 controls the heating of each section of the calender roll 11 to maintain a uniform thickness of calendered material 29. A web thickness sensor 51 senses the thickness of the calendered material 29 at various locations along its width and sends signals, which correspond to the thicknesses of the material, to the control device 50. Depending on the degree of deviation of the calendered material 29 from the desired thickness, the valve control device 50 selectively directs air pressure against certain diaphragms 21 which in turn adjust the associated valves 13 so that the valves discharge a greater or lesser amount of steam from each nozzle 9. 
     If the sensor 51 detects a thick section of calendered material 29, the control device 50 adjusts the valve 13 adjacent to that section of the calender roll 11 and allows more steam to heat that section of the roll 11. The additional steam heating the section of the calender roll 11 causes it to expand. The expanding section of the calendar roll 11 decreases the corresponding section of nip 25, thus decreasing the thickness of the calendered material 29 produced by the heated section of the roll 11. 
     Alternatively, when the sensor 51 detects a thin section of calendered material 29, the control device 50 adjusts the valve 13 adjacent to that section of the calender roll 11 to allow less steam to heat the roll 11. Since less steam is directed at that section of the roll 11, it cools and contracts. This increases the nip 25 formed between the cooperating calender rolls 11, 27 and results in a thicker section of calendered material 29. 
     FIG. 3 illustrates a second preferred embodiment of the present invention. It operates in essentially the same manner as the first embodiment. However, the plenum 105 is supported by an arm 152 and positioned so that a single row of nozzles 109 direct steam against an intermediate calender roll 111. Furthermore, although a vacuum plenum could be used with this device, FIG. 3 illustrates operation without a vacuum plenum. 
     The steam jet nozzles 109 are shown protruding from a concave shroud 160 having approximately the same curvature as the surface of the calender roll 111. The shroud 160 acts to constrain the steam emitted from the nozzles 109 to remain in contact with the calender roll 111, thus enhancing the efficiency of the heat transfer to the roll 111. The shroud 160 also prevents cold ambient air from being entrained by the steam jets. The cold air would reduce the effective temperature of the jets. Of course, a similar shroud could be used with the embodiment of the invention illustrated in FIG. 1. 
     The support member of arm 152 is mounted on the drive shaft 154 of a motor 156. When the motor 156 is activated, the drive shaft 154 and supporting arm 152 pivot the plenum 105, nozzles 109, valves 113 and shroud 160 away from the calender roll 111 for convenient inspection, repair or replacement of the device. 
     Alternatively, the calender roll control device may be supported by an over-center support mechanism, as shown in FIG. 4. In this embodiment, a rigid pivotable support member on arm 252 is disposed at either end of the steam plenum 205. These arms 252 support the plenum 205 so that the plenum 205 and shroud 260 are pivotable toward or away from the calender roll 211. 
     An extendible air cylinder 264 is associated with each pivotable arm 252. Pressurizing the cylinders 264 with air causes them to expand, thus rocking the plenum 205 and shroud 260 away from the calender roll 211. 
     In the operating position, each air cylinder 264 is pressurized so that the calender roll control device leans slightly toward the calender roll 211. In this metastable position, if the web 229 breaks and wraps around the roll 211, a slight forceful contact between the web 229 and the shroud 260 is sufficient to rock the device back away from the calender roll 211 and thus avoid damage to the device. 
     Three embodiments of the present invention have been described. Nevertheless, it is understood that one may make various modifications without departing from the spirit and scope of the invention. Thus, the invention is not limited to the embodiments described herein.