Abstract:
In an induction heater, preheated, pressurized air is further heated in the heating cabinet and also drawn into the coil tube via a suction fan. The simultaneous pulling and pushing of the twice-heated air through the tube provides superior air flow to pick up more moisture from the can ends being dried. The tube ends rest on upwardly concave collars and are held in place by gravity, with a single screw acting as a stop above to prevent upward movement. Removal requires only removing the single screw at each end then lifting the tube straight up out of the cabinet, which is facilitated by providing a hinged cover on the cabinet.

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 13/568,448, filed Aug. 7, 2012, for INDUCTION DRYER, which is a divisional application of U.S. patent application Ser. No. 12/195,626, filed Aug. 21, 2008, for INDUCTION DRYER, now U.S. Pat. No. 8,258,445, which is a divisional of Ser. No. 10/527,860, filed Nov. 7, 2005, for INDUCTION DRYER, now U.S. Pat. No. 7,432,480, which is a national phase entry under 35 U.S.C. §371 and claims priority to International Application No. PCT/US03/38942, with an International Filing Date of Dec. 9, 2003, for INDUCTION DRYER, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/431,938 filed on Dec. 9, 2002, for LOW COST END DRYER, the entire disclosures of which are fully incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to apparatus and methods for heating and, thereby, drying, a plurality of plate-like metal objects such as metal can lids, also known as “closures” or “ends”. 
     Closures for metal beverage containers are generally of a circular shape with a flanged perimeter called a curl. The closures may also be of a rectangular shape. The closures are usually made of aluminum or steel, and the curl is used in attaching the closure to a can body through a seaming operation. To aid the integrity of the seal that is formed between the can body and the closure, it is a common practice to apply a bead of sealant or adhesive (“compound”) within the curl of the can end during manufacture of the closure. Different types of coatings are also selectively or generally applied to can closures and can bodies for various other purposes as well, for example, to repair damaged coatings. For the purposes of the present description, coatings, sealants and adhesives are all considered to be “liquids” applied to a workpiece. 
     It is necessary in this manufacturing operation to cure or dry such liquids. It is known to dry can closures by infrared radiation, convection heating, or induction heating. An induction dryer, for example, typically includes a cabinet that supports a tube extending generally horizontally across the cabinet from one end to the other. The tube is larger in diameter than the can ends. An induction coil is wrapped around the tube. The ends move through the tube in a stacked relationship, that is, with abutting face-to-face contact with each other (“in-stick”). When a suitable electric current is passed through the coil, the metal can ends are inductively heated. The heat is transferred to the compound on the can ends by conduction from the heated metal. The compound is heated and water is driven off from the compound into the surrounding air. 
     Because of the close proximity of one end to another in the stick, it is desirable to have as much warm air as possible contact the ends, while they are in the dryer, to remove the water from the area around the can ends. In one prior art induction dryer, air is heated with an ambient air heater that is mounted externally to the cabinet, for example, on top of the cabinet. The air flows from the heater along a flexible external duct and is directed into an air box secured on the inlet wall of the cabinet, surrounding the inlet opening into the tube. Some of the air flows from the air box to atmosphere through an opening in the air box that admits the moving can ends from an external source. The remainder of the heated air flows from the air box into the tube, flowing in the direction of the moving can ends. The air that is forced into the tube flows out the outlet end of the tube at the opposite end wall of the cabinet, under the force of the air being forced in at the inlet end. The flow of heated air through the tube helps to remove the moisture that is driven off from the heated can ends in the tube, and thus promotes drying of the ends. 
     In the prior art induction dryer, a thermocouple is located at the outlet end of the tube. The thermocouple is mounted in the end wall of the cabinet, at the circumferential top of the outlet opening. As the can ends pass through the outlet opening, the thermocouple registers the temperature of the can ends. The thermocouple provides an electric output that is used by a controller for the dryer to help control the current in the induction coil and/or other factors in the heating apparatus. 
     The thermocouple is adjusted to touch the can ends. This engagement of the thermocouple with the can ends can create a jam point if the ends are not in perfect stick form. Also, the thermocouple bracket is subject to deformation which would move the thermocouple away from the stick, which would register a temperature fault, shutting down the system. 
     The stick is, preferably, constantly moving. However, jams may occur, or some other occurrence may prevent the can ends from moving smoothly through the dryer. The prior art dryer includes a wheel that is mounted at the inlet end of the dryer and that contacts the upper edges of the moving can ends. If the stick stops moving, the wheel stops rotating, and an appropriate output signal is provided to the controller for the dryer, alerting it that the stick is not moving. 
     At times the induction coil tube needs to be removed from the cabinet, for example, for maintenance or to replace the tube with a different diameter tube more suitable for drying can ends of a different diameter. In the prior art dryer, the tube ends are held in place in the cabinet end walls with split collar hubs. Each upper hub is loosened by removing four screws. The upper hub can then be lifted upward a little and the tube can be pulled out of the cabinet through one end wall or the other of the cabinet. This process requires clearing away any equipment, such as an upstacker or a separator, from the end of the cabinet, to clear space for pulling out the entire tube, which may be four to eight feet in length. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a heater for heating workpieces, such as can ends, to drive off moisture from a compound on the can ends. The invention is directed towards improving the design of induction heaters and to solve the problems described above. 
     A first aspect of this invention is to pressurize the air in the heating cabinet, and draw this air directly into the induction coil tube via a suction fan at the outlet end of the tube that draws the air in through the inlet end. The simultaneous pulling and pushing of the air through the tube provides superior air flow to pick up more moisture from the can ends being dried. 
     A second aspect of this invention is to preheat the air in the heating cabinet, preferably by using it to draw heat from power and control circuitry of the dryer. The preheated air is then heated again with an open coil heater than is located inside the heating cabinet adjacent to the inlet end of the tube. This double heating of the air helps to pick up more moisture from the can ends being dried. 
     A third aspect of the invention involves the relocation of a temperature-sensing thermocouple, at the outlet end of the tube, from the top of the tube to the bottom of the tube, where the moving can ends will ride directly over the thermocouple. This arrangement provides superior temperature sensing for controlling the heating process, in that the new location insures that the can ends ride centered on the sensor with pre-set tension. 
     A further aspect of the invention relates to replacing the rotary wheel motion sensor at the inlet end of the tube, used to sense whether the stick is moving or not, with a laser sensor. The laser sensor is more accurate and is less prone to jamming because it is non-contact (not touching the can ends) and h as no moving parts to wear or jam. 
     Yet another aspect of the invention relates to a new supporting system for the tube. The tube ends rest on upwardly concave collars and are held in place by gravity, with a single screw acting as a stop above to prevent upward movement. Removal requires only removing the single screw at each end then lifting the tube straight up out of the cabinet, which is facilitated by providing a hinged cover on the cabinet. This new mounting and retention mechanism provides for substantially easier removal of the tube, as is periodically needed during use and maintenance of the tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the present invention will become apparent to one skilled in the art to which the present invention relates upon consideration of the following description of the invention with reference to the accompanying drawings, in which: 
         FIG. 1  is a pictorial view of a dryer that is one embodiment of the invention; 
         FIG. 2  is a schematic, longitudinal sectional view of the dryer of  FIG. 1 ; 
         FIG. 3  is an enlarged sectional view of an inlet end of the dryer of  FIG. 1 ; 
         FIG. 4  is an interior elevational view of the inlet end of the dryer of  FIG. 1 ; 
         FIG. 5  is an enlarged sectional view of an outlet end of the dryer of  FIG. 1 ; and 
         FIG. 6  is an interior elevational view of the outlet end of the dryer of  FIG. 1 ; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to apparatus and methods for drying plate-like metal objects such as metal can lids or “ends”. The invention is applicable to various apparatus and methods for drying such objects. AS representative of the invention,  FIG. 1  illustrates a dryer  10  constructed in accordance with a first embodiment of the invention. 
     The dryer  10  includes a heating cabinet  12 , through which workpieces such as can ends  14  pass to be heated and dried, and a power and control cabinet  16 . The power and control cabinet  16  serves as a base for and supports the heating cabinet  12 . The power and control cabinet  16  includes power and control circuitry indicated schematically at  18  which may include, for example, one or more transformers. 
     As described below in detail, the heating cabinet  12  supports a nonconductive tube  20  around which an induction coil  22  extends. The induction coil  22  is electrically connected with the power and control circuitry  18  by wires  24 . Operation of the power and control circuitry  18  generates an electric current that flows through the induction coil  22  to heat any conductive material located within the tube  20 . Thus, steel or aluminum ends can be heated. 
     Operation of the power and control circuitry  18  also generates heat which flows upwardly through one or more vents openings  28  into the heating cabinet  12 . A fan  30  in the power and control cabinet  16  pulls ambient air into the power and control cabinet to cool the equipment  18  therein. The heated air flows over a heat sink then, with some residual heat still in it, exits the power and control cabinet  16  into the heating cabinet  12 , through one or more of the vent openings  28 . 
     The heating cabinet  12  includes a cabinet base  34  and a lid  36 . The lid  36  is movable relative to the base  34 . The lid  36  is preferably hinged to the base  34  at the back edge of the lid, so that it may be lifted open. When the lid  36  is lifted open or removed, the interior of the heating cabinet  12  is accessible from above, to enable removal of the tube  20 , as described below. 
     The cabinet base  34  includes a plurality of walls that define a heating chamber  40  in the cabinet. The walls include a bottom wall  42  ( FIG. 2 ); a front wall  44  ( FIG. 1 ); an opposite back wall (not shown); an inlet end wall  48 , and an outlet end wall  50  ( FIGS. 2-6 ). When the dryer  10  is in operation as described below, can ends  14  move into the dryer through an opening  52  in the inlet end wall  48 , and exit the dryer through an opening  54  in the outlet end wall  50 . 
     The inlet end wall  48  of the cabinet  12  supports an inlet hub  66 . The inlet hub  60  in the illustrated embodiment is a molded plastic member having a cylindrical main body portion  62 . The outer diameter of the main body portion  62  is selected to fit within the opening  52  in the inlet end wall  48  of the heating cabinet  12 . The main body portion  62  has a cylindrical inner surface  66  that defines a cylindrical passage  68  extending through the hub  60 . The size of the passage  68  is selected to accommodate can ends  14  to be dried in the dryer  10 . 
     An annular mounting flange  70  of the inlet hub  60  extends radially outward from the main body portion  62 . The mounting flange  70  is secured by fasteners shown schematically at  72  to the inlet end wall  48  of the cabinet  12 . As a result, the inlet hub  60  is secured to the cabinet  12 , with the main body portion  62  projecting into the interior of the cabinet  12  through the opening  52  in the inlet end wall  48  of the cabinet. 
     The inlet hub  60  includes a support ring  74 . The support ring  74  extends inward from the main body portion  62  of the inlet hub  60 . The support ring  74  has an arcuate configuration and is formed as a continuation of a lower circumferential sector of the main body portion  62 . The inner diameter of the support ring  74  is substantially equal to the outer diameter of the tube  20 . As a result, an inlet end  76  of the tube  20  can be supported on the support ring  74  so that the cylindrical inner surface  78  of the tube forms a continuation of the cylindrical inner surface  66  of the main body portion  62  of the inlet hub  60 . Therefore, when a stick of can ends  14  moves into the dryer  10 , it can slide smoothly from the main body portion  62  of the inlet hub  60  into the tube  20 . 
     The main body portion  62  of the inlet hub  60  has a heater inlet opening  80  at or near the top. In addition, the main body portion  62  has an opening  82  for receiving a retainer or stop member  84 , in the form of a stop screw, directly above the support ring  74 . 
     Mounted in the exit opening  54  ( FIG. 5 ) of the outlet end wall  50  of the heating cabinet  12  is an outlet hub  90  of the dryer  10 . The outlet hub  90  is similar in configuration to the inlet hub  60 . The outlet hub  90  is a molded plastic member having a cylindrical main body portion  92 . The outer diameter of the main body portion  92  is selected to fit within the opening  54  in the outlet end wall  50  of the cabinet  12 . The main body portion  92  has a cylindrical inner surface  96  that defines a cylindrical exit passage  98  extending through the hub  90 . The size of the exit passage  98  is selected to accommodate can ends  14  to be dried in the dryer  10 . 
     An annular mounting flange  100  of the outlet hub  90  extends radially outward from the main body portion  92 . The mounting flange  100  is secured by fasteners shown schematically at  102  to the outlet end wall  50  of the cabinet  12 . As a result, the outlet hub  90  is secured to the cabinet  12 , with the main body portion  92  projecting into the interior of the cabinet through the opening  54  in the outlet end wall  50  of the cabinet. 
     The outlet hub  13  includes a support ring  104 . The support ring  104  extends inward from the main body portion  92  of the outlet hub  90 . The support ring  104  has an arcuate configuration and is formed as a continuation of a lower circumferential sector of the main body portion  92 . The inner diameter of the support ring  104  is substantially equal to the outer diameter of the tube  20 . As a result, an outlet end  106  of the tube  20  can be supported on the support ring  104  so that the cylindrical inner surface  28  of the tube forms a continuation of the cylindrical inner surface  96  of the main body portion  92  of the outlet hub  90 . Therefore, when a stick of can ends  14  moves through the dryer  10 , it can slide smoothly from the tube  20  onto the main body portion  92  of the outlet hub  90 . 
     The main body portion  92  of the outlet hub  90  has an exhaust opening  108  at or near the top. In addition, the main body portion  92  has an opening  110  for receiving a retainer or stop member  112  in the form of a stop screw, directly above the support ring  104 . 
     The inlet end wall  48  of the cabinet  12  supports a sensor  120 , at a location above the inlet hub  60 . The sensor  120  is operative to sense the presence or absence of movement of a stick of can ends  14  through the inlet hub  60 . 
     In the illustrated embodiment, the sensor  120  is a non-contact sensor, preferably a laser sensor. The laser sensor  120  emits a laser beam, shown schematically at  122 , that is directed toward the inlet opening of the inlet hub  60 . The output of the laser sensor  120 , in response, is used in controlling operation of the dryer  10 , as described below. 
     The dryer  10  also includes a heater  130 . The heater  130  is located inside the heating cabinet  12  and is supported on the inlet hub  60 . The heater  130  is an electrically powered, open coil heater including a tubular main wall  132  within which are exposed electrical heating coils  134 . The coils  134  are connected by lead wires  136  with a controllable source of electric current, such as the power and control circuitry  18 . 
     The main wall  132  of the heater  130  is connected with an outlet wall  138  extending perpendicular to the main wall to form an L-shaped configuration for the heater. The outlet wall  138  is secured to the main body portion  62  of the inlet hub  60  in a manner that the heater interior communicates with the heater inlet opening  80  in the inlet hub. 
     The dryer  10  includes an exhaust blower or exhaust fan  140 . The exhaust fan  140  is preferably located inside the heating cabinet  12  and, in the illustrated embodiment, is supported on the bottom wall  42  of the heating cabinet exhausting to an opening (not shown) in the back wall of the cabinet. A flexible duct  144  extends between the exhaust fan  140  and the exhaust opening  108  in the outlet hub  90 . The duct  144  is connected with the outlet hub  90  by a rigid connector tube  146 . The exhaust fan  140  is an electrically powered device that is operative to draw air from the interior of the outlet hub  90  and deliver it through the duct  144  to the opening in the back wall and thence to atmosphere, in a manner as described below. 
     A thermocouple  150  is located on the outlet hub  90 . The thermocouple  150  has a body portion  156  disposed in an opening in the outlet hub  90 . The thermocouple  150  has a sensor portion  156  that projects upward from the body portion  152 , through a slot in the outlet hub  90 , into the central passage  98  of the outlet hub. The sensor portion  156  of the thermocouple  150  is in the path of movement of the can ends  14  as they are pushed through the outlet hub  90  in a generally horizontal direction. 
     The tube  20  defines a generally enclosed space  160  in the heating cabinet  12 , through which can ends  14  travel as they move through the dryer  12 . The inlet end  76  of the tube  20  is supported on the inlet hub  50  for receiving workpieces. The inlet end  76  of the tube  20  enables air to flow into the enclosed space  160  inside the tube, from the interior of the heating cabinet  12 . 
     The inlet end  76  of the tube  20  rests by gravity on the support ring  74  of the inlet hub  60 . The retainer or stop member  84  is connected with the inlet hub  60 , at a location opposite the support ring  74 . In the illustrated embodiment, the retainer or stop member  84  is a nylon screw that is screwed into the opening  82  in the main body portion  62  of the inlet hub  60 , at a location diametrically opposite the support ring  74  and at the top of the inlet end  76  of the tube  20 . A different type of retainer or stop member  84  could be used. 
     When the screw  84  is in the opening  82 , the screw blocks upward movement of the inlet end  76  of the tube  20  off the support ring  74  of the inlet hub  60 . When the screw  84  is out of the opening  82 , upward movement of the inlet end  76  of the tube  20 , off the support ring  74  of the inlet hub  60 , is not blocked, and the inlet end of the tube can be lifted upward. 
     In a similar manner, the outlet end  106  of the tube  20  rests by gravity on the support ring  104  of the outlet hub  90 . The retainer or stop member  112  is connected with the outlet hub  90 , at a location opposite the support ring  104 . In the illustrated embodiment, the retainer or stop member  112  is a nylon screw that is screwed into the opening  110  in the main body portion  92  of the outlet hub  90 , at a location diametrically opposite the support ring  104  and at above the outlet end  106  of the tube  20 . A different type of retainer or stop member  112  could be used. 
     When the screw  112  is in the opening  110 , the screw blocks upward movement of the outlet end  106  of the tube  20  off the support ring  104  of the outlet hub  90 . When the screw  112  is out of the opening  110 , upward movement of the outlet end  106  of the tube  20 , off the support ring  104  of the outlet hub  90 , is not blocked, and the outlet end of the tube can be lifted upward. As a result, removal of the tube  20  for maintenance and changing of tube sizes is very easy. 
     Can ends  14  to be dried are conveyed into the inlet passage  68  of the inlet hub  60  and thence into the inlet end  76  of the tube  20 . The can ends  14  as they move through the tube  20  are acted upon by an alternating magnetic field generated by the induction coil  22 . The can ends  14  are heated as a result, and this heat is conducted into the compound on the can ends. As the compound is heated, water is driven out of the compound into the surrounding air within the enclosed space  160  of the tube  20 . This water is removed from the tube  20  as follows, to enable more can ends  14  to be dried within the tube. 
     The heater cabinet  12  is pressurized (above atmospheric) with heated air from the power and control cabinet  16 . The fan  30  in the power and control cabinet  16  forces heated air from the power and control cabinet upward through the vent opening  28  in the bottom wall  42  of the heating cabinet  12 . As a result, the air in the heating chamber  40  of the heating cabinet  12 , surrounding the tube  20 , is pressurized and heated to some extent. 
     The exhaust fan  140  in the heating cabinet  12  draws air from the outlet end  106  of the tube  20 . This suction creates a flow of air through the tube  20  in a direction from the inlet end  76  of the tube to the outlet end  106  of the tube. As a result, air is drawn into the inlet end  76  of the tube  20 , through the heater inlet opening  80 , from the interior of the heating cabinet  12 . 
     This effect is enhanced by the fact that the air in the heating cabinet  12  is already pressurized, to some extent, by the air flow from the fan  30  in the power and control cabinet  16 . Thus, the air flowing into the inlet end  76  of the tube  20 , and thence through the tube, is both pushed through the tube and pulled through the tube. This promotes a smoother and more effective flow of air through the tube  20 . 
     The air that flows from the interior of the heating cabinet  12  into the tube  20  flows through the heater  130 . As a result, this air passes over the exposed coils  134  of the heater  130 . This second heating of the air provides an increased ability to draw moisture from the tube  20  as the heated air passes through the tube, as compared to the prior art dryer. 
     For example, air in the prior art dryer described above is typically heated to 40 degrees Celsius, while air with the present dryer  10  is heated to about 60 degrees Celsius. With the present invention, heating the preheated air from the interior of the heating cabinet  12  also produces hotter air than does the heating of ambient air. Because the air is heated twice, and to a higher temperature, it is able to absorb more of the moisture in the enclosed space  160  that is driven off from the heated can ends  14 . Although 60 degrees is the presently preferred temperature, it is possible to achieve some of the benefits of the heated air, at a reduced level, by heating the air to a temperature of at least 50 degrees Celsius. It is normally preferred that temperatures above about 65 degrees Celsius not be used because they can cause the compound on the can ends  14  to skin over, trapping water within the compound. 
     Because the heater  130  is located inside the cabinet  12 , adjacent the inlet end  76  of the tube  20 , the heated air from the heater is ducted directly into the inlet hub  60  and thence into the inlet end of the tube. This configuration minimizes the opportunity for heat loss that might otherwise occur through extensive ductwork or external ductwork or boxes, as in the prior art dryer. 
     It has also been found that the suction created by the exhaust blower  130 , drawing the air through the tube  20 , is preferable to forcing air in at the inlet end  76 . Especially in combination with the flow of pressurized air into the heating cabinet  12  from the power and control cabinet  16 , improved moisture removal is accomplished with the suction fan  130  as compared to the prior art dryer. 
     The can ends  14  slide along the inner surface  96  of the outlet hub  90  and engage the sensor portion  156  of the thermocouple  150  as they do so. The sensor portion  156  resiliently or deforms bends from the contact by the can ends  14 . This direct contact of the can ends  14  with the thermocouple sensor  156  provides improved temperature sensing of the can ends, which always contact the thermocouple by gravity and provide a constant pressure due to design placement, as compared to the overhead sensing that was provided with the prior art dryer in which the thermocouple was subject to installation adjustment and product jams which alter sensitivity. The output of the thermocouple  150  is directed to the power and control circuitry  18  and can be used to help control the current flow to the induction coil  22 . 
     It is desirable to be able to keep track of movement of the stick of can ends  14  through the dryer  10 . If the can ends  14  are not moving, power to the induction coil  22  can be reduced or turned off completely. If the can ends  14  are moving, the induction coil  22  can be operated to heat and dry the can ends. 
     The laser sensor  120  is operative to sense the presence or absence of movement of a stick of can ends  14  through the inlet hub  60 . The output of the sensor  120  is directed to the power and control circuitry  18 . If the sensor  120  senses that the can ends  14  are moving into the dryer  10 , the induction coil  22  can be operated to heat and dry the can ends. If, on the other hand, the sensor  120  senses that the stick of can ends is slowed or stopped, for example by a jam or by simply a lack of workpieces coming into the dryer  10 , then the induction coil  22  can be controlled to reduce or eliminate current flow through the induction coil. Because the laser sensor  120  is a non-contact sensor, it is not affected by jams or out of position can ends  14  in a stick. In comparison to the prior art rotating wheel sensor, therefore, the laser sensor  120  of the present dryer  10  is a significant improvement.