Abstract:
A method and apparatus for dielectric drying is disclosed wherein a load to be heated positioned on a carrier the bottom of which provides a secondary electrode is moved with the carrier a dielectric heating chamber having a primary electrode positioned above load when said load on said carrier is moved into an operative position in the chamber. The carrier and load are elevated into heating position and this movement simultaneously connects the secondary electrode to ground, so that when high frequency power is applied to the primary electrode the circuit is made to dielectrically heat the load.

Description:
FIELD OF INVENTION 
     The present invention relates to an improved dielectric heating system having a simplified contact system to automatically connect the bottom electrode to ground. 
     BACKGROUND OF THE INVENTION 
     Uses of dielectric heating/drying systems are known and are currently in use or have been proposed for use in agriculture, polymer manufacture, pharmaceuticals, bulk powder, food processing, wood products, panel manufacture, and other industries. One of the key industries using these dielectric heating/drying systems is the wood products industry and the present invention will be described particularly with respect to the wood products industry, although the invention, with suitable modifications where required, may be applied in the other industries in which dielectric heating/drying is to be performed. 
     In dielectric drying or heating systems particularly those used for drying wood, it is the conventional practice to load the material to be dried onto a wheeled cart and to roll the loaded cart into the kiln which is provided with rails to receive the wheels of the cart. See for example, U.S. Pat. No. 3,986,268 issued Oct. 19, 1976 to Koppelman and U.S. Pat. No. 4,472,618 issued Sep. 18, 1984 to Cloer. In these systems, the carts serve as both a conveyor and electrode. Clearly the cart, which is the electrode, is moveable and thus the cart-electrode must be moved into the kiln and connected electrically before the kiln chamber is closed and the drying process proceeds. 
     As above indicated, all of these cart systems require manually connecting the grounding system to the cart loaded with material to be dried and positioned in the kiln before the drying cycle may be started and disconnecting the grounding system after drying and before the loaded cart may be moved from the kiln. This loading and unloading, connecting and disconnecting etc., necessitates the use of professionally trained personnel both for safety and operating procedures to better ensure there are no major problems or accidents. These limitations imposed primarily by the use of carts have given the process of dielectric drying a reputation as being non-robust in that it requires flimsy attachments, which lead those in the lumber industry to imply that the technique is still in the research or experimental stage, and has not yet been developed for commercial industrial purposes. 
     U.S. Pat. No. 3,986,268 issued Oct. 19, 1976 to Koppelman recognized the problem of carts and in one embodiment employs vertical electrodes and uses a conveyor (roller conveyor) to deliver the load to be dried into position between the vertical electrodes and then after drying to convey the dried load from between the electrodes. This system could permit computer-controlled operation, however it was found that uniform contact of the vertical electrodes with the sides of the load was difficult and could not be consistently made whereby the effectiveness of the system was compromised. 
     U.S. Pat. No. 6,080,978 issued Jun. 27, 2000 teaches the use of a conveyor system with the conveyor portion within the kiln directly connected to ground to provide a continuous system where attachment of grounding straps to a cart is eliminated entirely. This system is more suited to applications requiring both an infeed and outfeed door where the load does not need to be reversed. 
     BRIEF DESCRIPTION OF THE PRESENT INVENTION 
     It is the main object of this invention to provide a cart conveying system for a dielectric drying system wherein the grounding of the cart is automatically accomplished by movement of the cart to operative position. 
     Broadly the present invention relates to a method and apparatus for dielectric drying of a load comprising positioning the load on a carrier incorporating a secondary electrode, moving the load on said carrier substantially horizontally into a chamber having a primary electrode positioned above said load when said load on said carrier is moved into an operative position in the chamber, elevating said carrier and said load to vertically move said load into heating position while simultaneously connecting said carrier and thereby said secondary electrode to ground, applying high frequency power to said primary electrode to dielectrically heat said load. 
     Preferably said high frequency is radio frequency (RF). 
     Preferably said elevating moves said load into intimate contact with said primary electrode. 
     Preferably after said heating is completed, said carrier and said load are lowered to said operative position and then said carrier and said load are removed from said chamber. 
     Preferably during drying, said load is separated from said primary electrode and weighed. 
     Broadly, the present invention relates to an apparatus for dielectric heating of a load comprising a carrier, means forming a secondary electrode on said carrier, a first set of electrical contacts along the outer periphery of said carrier and connected to said secondary electrode, a heating chamber, means defining a pathway for said carrier into said chamber, a primary electrode in said chamber positioned above said pathway, elevating means positioned along said pathway so that said carrier when positioned in operative position in said pathway may be lifted by said elevating means into heating position, a cooperating set of electrical contacts secured along said pathway, contacts of said set of cooperating contacts being in position to make electrical contact with adjacent contacts of said first set of contacts on said carrier and form mating connections between contacts of said first set of contacts and said cooperating set of contacts when said elevating means has raised said carrier to said heating position from said operative position, and means connecting said primary electrode to a source of high frequency power. 
     Preferably said mating connections are formed by a flexible contact and a plate contact. 
     Preferably said flexible contact is one of said set of cooperating contacts. 
     Preferably the chamber is provided with at least one door, and further electrical contact means are provided on said door, means to move said door to a closed position and to a sealing position wherein said door is sealed and said further electrical contact means are positioned to make electrical contact with an adjacent contact of said first set of electrical contacts when the carrier and load are elevated from operative position to heating position. 
     Preferably said flexible electrical contacts are made from fingerstock having fingers extending from a connecting band and formed into an open triangular cross section with a pair of spaced bearing areas forming one incomplete side of said triangular cross section, said connecting band forming one of said pair of bearing areas and free ends of said fingers remote from said connecting band forming the other of said pair of bearing areas. 
     Preferably said bearing area formed by the band is fixed to its supporting structure and the bearing area formed by the free end bears against but is free to move relative to the supporting structure. 
     Preferably contacts of said set of cooperating electrical contacts are provided on all sides and end of said pathway and cooperate with adjacent contacts of the first set of electrical contacts. 
     Preferably said carrier is formed by a plurality of carts in end to end relationship and with mating connection being formed between adjacent ends of adjacent of said carts. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
     Further features, objects and advantages will be evident from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which; 
     FIG. 1 is an isometric illustration of the present invention applied to a heating chamber in the form of a vacuum kiln for drying wood and showing a carrier or cart in position to enter the kiln. 
     FIG. 2 is a section across the chamber or kiln illustrating the inside of the kiln and the position of the cart and load as it enters the kiln and is moved horizontally to operative position. 
     FIG. 3A is a longitudinal section through the kiln showing the cart in heating position in the kiln with the door closed and showing the end contact. 
     FIG. 3B is a view similar to FIG. 3A but showing a pair of carts in end to end relationship in heating position to heat individual loads positioned one on each of the carts. 
     FIG. 4 is a section similar to FIG. 2 but showing the cart and load in elevated heating position. 
     FIG. 5 is a schematic cross section through the kiln showing the relative position of the cart and the flexible contact when the cart is in operative position in the kiln 
     FIG. 6 is a schematic cross sections similar to FIG. 5 but showing the relative position of the cart and the flexible contact when the cart is in elevated heating position in the kiln. 
     FIG. 7 is a flatted plan view of the fingerstock used to provide the flexible connector. 
     FIG. 8 is an isometric view of the formed fingerstock used to make the flexible connection. 
     FIG. 9 is a section along the line  9 — 9  of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1 a carrier in the form of a cart  10  is provided. The carrier  10  has an upper deck  12  that functions to support the load  14  (see FIG. 2) and as a secondary electrode for applying power to the load  14  during the dielectric heating process as will be described below. A first set of electrical contacts is provided around the periphery of the carrier or cart  10  and generally includes a pair of contacts  16  one at each side of the carrier  10  each extending the full length of the carrier  10  and a pair of end contacts  32  and  34  for making electrical contact with an adjacent electrical contact of a set of cooperating contacts which includes the contacts  18  (see FIGS. 1 and 3) mounted in fixed relationship to a connection table  20  which defines a path  22  along which the carrier  10  and load are moved into and out of the heating chamber  24  of the heater or dryer or kiln  26  and  36  and  38  at the end and on the door of the chamber  24 . The contacts of the set of cooperating contacts  18  are fixed to the side or peripheral walls  28  and  30  of the passage or path  22  in a manner to be described below and are positioned along the upper portion of the path  22  facing in and projecting toward the center of the path  22 . 
     Adjacent pairs of contacts, one contact from the first set of electrical contacts ( 16 ,  32  and  34  and the other from the cooperating set of contacts  18 ,  36  and  38  making mating electrical contact are formed by one flexible contact and one plate like contact. In the illustrated arrangement, contacts of the cooperating set of contacts  18 ,  36  and  38  (fixed to the kiln  26  are shown as flexible and contacts of the first set of contacts  16 ,  32  and  34  (on the cart  10 ) have been shown as plate like contacts, but some or all may be reversed if desired. 
     The carrier or cart  10 , as above indicated, is provided with a pair of end contact plates  32  and  34  each of which extends across the full width of the cart  10  at the front and back respectively of the cart or carrier  10  respectively. These contact plates  32  and  34  are positioned to cooperate with (make mating electrical contact with) the flexible contacts  36  and  38  respectively as will be described below. The contact  36  is fixed to the end wall  40  of the path  22  (also the end wall of the kiln  26  in the illustrated arrangement and the contact  38  is fixed to the door  42  for closing the chamber  24  (see FIGS.  1  and  3 ). In the arrangement as constructed, the contacts on the cart contacts  32  and  34  are flexible contacts while the  36  &amp;  38  attached to the end wall and door are plate contacts, but the arrangement as illustrated will operate equally well. The contact  38  is only in operative position when the door  42  is in closed position. 
     The contacts of the first set of contacts  16 ,  32  and  34  are all electrically connected to the bottom or secondary electrode  12  of the cart  10 . 
     These flexible contacts are preferably formed from “fingerstock” (as will be described below in conjunction with FIGS. 7,  8  and  9 ). Generally any gaps in the contacts along the lengths of and between the first set of contacts  16 ,  32  and  34  and their respective adjacent contacts of the cooperating set of contacts  18 ,  36 , and  38  will not exceed 12 inches (30 cm) and normally will be less than 12 inches (30 cm). The term gaps in contacts is intended to mean longitudinal spacing between adjacent contact areas along any side or end of the cart  10 . In other areas where contact is formed the flexible contacts  18 ,  36  and  38  in the illustrated arrangement will be firmly pressed against their respective solid or plate contacts  16 ,  32  and  34 . As described above the contacts are on all 4 sides of the cart  10  i.e. around the fully perimeter of the cart  10 . If a gap is made too long, detrimental effects such as arcing, high circulating currents, field non-uniforrnity, etc. will likely be encountered. 
     The cart  10  is preferably mounted on wheels  44  which roll along tracks  46  (see FIGS. 2,  3  and  4 ) when it is moved between the loading position  48  (see FIG. 1) and it operative position  50  (see FIG.  3 A). 
     As above indicated and shown in FIG. 1 and 3, the kiln  26  is provided with a closable door  42  that is moveable from the open position shown in FIG. 1 to a closed position (FIG. 3A) by suitable means as schematically represented by the arrows  68  and  70 . In the constructed system, the door is on overhead rollers and is manually opened or closed by horizontally pushing the door. Generally, the door is manually opened/closed but if desired a hydraulic cylinder system represented by arrow  70  may be provided to pull the door  24  toward or push the door away from the kiln  22  to seal the opening into the chamber  24 —movement of the door to sealing position as indicated by the arrow  70  compresses a sealing gasket (not shown) extending around the circumference of the opening to seal the kiln and positions the flexible contact  38  in an operative position to contact it plate contact  34  as the cart and load are moved from the operative position  50  into a heating position  56  as will be described below. 
     Projecting from the floor  52  of the chamber  24  and of the path  22  is a vertical movement system such as plurality of hydraulic pushers  54  illustrated (only 2 shown but there will be a sufficient number strategically positions to lift and hold the cart  10  with the load thereon stabilized in elevated heating position  56  (see FIG.  3 A). For some applications, a hydraulic or electrical scissor hoist system (or some other means) may be the preferred vertical movement system. 
     A primary electrode  58  is suspended from the roof of the kiln  26  on isolators  60  which hold the primary electrode preferably in fixed position in the chamber  24 . RF power is delivered to the primary electrode  58  via a connection  62  connected to a suitable source of power  64 . For convenience, this connection  62  is shown in FIG. 2 as extending upward at an angle whereas in the actual construction the connection  62  is connected at the center of the electrode  58  and projects out of the chamber at the top. The primary electrode  58  may be fixed since with the present invention the load  14  is moved into contact with the electrode  58  whereas in conventional systems the electrode is lowered onto the surface of the load to make contact 
     The chamber  24  may be placed under vacuum conditions when the door  42  is closed and the chamber  24  is connected to a source of negative pressure (vacuum) as schematically illustrated by the arrow  66 . Preferably the vacuum system will be based on the system described in Applicants co-pending US patent application Ser. No. 09/691,148 the teaching of which is incorporated herein by reference. In operation the cart  10  is loaded in loading position  48  and then the cart  10  carrying load  14  is rolled into the chamber  24  into operative position  50  the door  42  of the chamber  24  is closed and hydraulically pulled tight against the chamber door frame to seal the chamber and if vacuum is to be applied, to create a vacuum tight seal. At the same time the door is pushed inward the flexible connecter  38  is positioned in its operative position. The cart is now raised by activating the hydraulic lifting/lowering cylinders until the wood comes into contact with the electrode. As the cart  10  and load  14  are raised by the cylinders  54  into the heating or drying position  56  intimate electrical contact is achieved along each side of the cart  10  between the plate contacts  16 ,  32  and  34  and their respective flexible contacts  18 ,  36  and  38  and maintained as the load  14  (wood) is brought into contact with the RF principal electrode  58 . The hydraulic system  54  is also used to provide as specified compressive loading on the load  14  throughout the drying process by pushing the cart and its load against the fixed electrode  58 . 
     As shown in FIG. 5 when the cart is in the operative position the flexible contacts  18 ,  36  and  38  are in expanded or rest position and are positioned above the cart  10  particularly the contact plates  16 ,  32  and  34  of the cart  10  to provide a suitable clearance C c  between the flexible contacts  18 ,  36  and  38  and the adjacent rounded edge which will have a radius r c . The clearance C c  will generally be at least ½″ and the radius r c  will generally be at least 2.25″. Obviously a suitable clearance C will be provided between the adjacent side and ends of the cart  10  and path  20  is moved from operative  50  position to the heating position  56 . 
     When the drying or heating process is completed, the power is cut to the system and the vacuum, if applied, is brought back to atmospheric pressure. The carrier  10  and the load  14  lowered to operative position, the door  42  opened and the dried load and carrier  10  are moved to the position  48  which also may function as the unloading position. 
     It is also possible, as shown in FIG. 3B, for the carrier  10  to take the form of a plurality of individual carts  10 A,  10 B etc (only  2  carts shown in FIG. 3B) which are positioned in end-to-end relationship along the path  22 . In this arrangement, individual electrodes (one for each cart  10 A,  10 B, etc.) as indicated at  58 A,  58 B, etc, will be provided and individually supplied with power via connectors (not shown—shown at  62  for the FIG. 1,  2 ,  3 A and  4  embodiment). Electrical connectors are preferably provided to connect adjacent end of adjacent carts  10 A,  10 B, etc. a mating pair of which are schematically indicated at  100  in FIG.  3 B. In some cases it may be desirable to design a system to accommodate individual carts  10 A,  10 B etc. that contain loads  14 A,  14   b  etc. of different heights. In this system, electrical connectors  100  will be designed to accommodate relative movement and each cart  10 A,  10 B etc. will be provided with and independent lifting system  54 A,  54 B etc. to elevate each cart. 
     The construction of the flexible electrical grounding contacts will now be described in more detail. As above indicated, these flexible contacts are preferably made from “fingerstock” as known in the trade which for the present invention preferably consist of thin compressible metal strips of heat treated berilium copper with tin plating. The berilium copper provides a high conductance of electrical current while the tin plating provides corrosion protection. Heat treating of this ‘fingerstock’ permits the metal strips to maintain its elasticity without permanently deforming and to generate sufficient force to ensure that a positive contact is established and maintained between the contacts forming the mating contacts between the contacts of the first and the cooperating sets of contacts ( 18 ,  36  and  38  and their mating contacts  16 ,  32  and  34 ). 
     The form of a typical fingerstock strips is illustrated in FIGS. 7,  8  and  9 . The strips  100  as shown in flattened condition in FIG. 7 is composed of a plurality of side by side finger elements  102  interconnected at one end by a connecting band  104 . Typically the fingers are about 1 ¾ inches (4.5 cm) wide and extend from the band  104  to provide a finger length of about 5 ½ inches (14 cm). The band  104  will normally be about ½ inches wide (1.25 cm) so that the total width of the fingerstock is about 6 inches (15 cm) and the length of the gap measured between the fingers  102  is normally at least about {fraction (1/16)} th  inches (0.16 cm) but preferably ⅛ th  inches (0.31 cm). 
     At spaced intervals along the length of the fingerstock  100  suitable mounting holes  108  are provided through which screws or the like may be passed to secure the fingerstock to the sides of the passage or path  22 . A section  110  is cut from the finger stock adjacent to each hole  108  to provide access to the screw for mounting. 
     As shown in FIGS. 8 and 9 the finger stock is formed into a substantially open triangular cross sectional shape with one side of the triangle being incomplete and being formed by one side the connecting band  104  being folded inward and on the other side the free ends of the fingers bend inward as shown in FIG.  9 . These bent in portions forming the incomplete side of the open triangle form a pair of bearing surfaces  112  and  114 . The bearing surface  112  formed by the band  104  is fixed to the wall of the path  22  and preferable in the illustrated arrangement is located at the bottom of the connectors  18 ,  36  and  38  while the bearing surfaces  114  formed by the bent over free ends of the fingers  102  forms a sliding contact with the wall of the path  22  to facilitate flexing and compression of the triangular shape when the system (cart  10  and load  12  are in heating position  56  with the contacts being made between the plates  16 ,  32  and  34  and the flexible contacts  18 ,  36  and  38  side projecting. 
     The entire loading process may be computer controlled as indicated by the computer schematically at  200  in FIG. 1 so that a single push button station initiates movement of the cart into the kiln and then following manual closure of the door, automated control is then used for elevation to heating position, application of power and vacuum if desired and at the end of the cycle reversing the operation as described above. 
     If desired the hydraulic lifters  54  may be provided with load sensors  202  that determine the weight of the load  14  and the change in weight of the load  14  and that are connected to the computer  200  via suitable connects (not shown) to facilitate control of the process. Preferably the load sensors include monitoring the fluid pressure in the hydraulic system lifting and holding the load  14  in heating position. This is of particular benefit in drying lumber. This provides the capability of being able to monitor the cart  10  &amp; load  14  weight by measuring the hydraulic fluid pressure. 
     An automated system may also be incorporated to weight the load  14  at any time during the drying process. To weight the load  14  the cart  10  and its load  14  must be lowered from the electrode  58  (i.e. remove compressive loading). After weighing the load, the cart and its load are raised back into position against the electrode  58  and the specified compressive loading reapplied. This entire ‘weigh load’ operation can be completely automated through the Process Control System programmed into the computer  200 . 
     One of the other main advantages of this invention is that there is no manual connecting of grounding connections. 
     Having described the invention, modifications will be evident to those skilled in the art without departing from the scope of the invention as defined in the appended claims.