Abstract:
Low voltage, high current electrical energy is supplied in a closed conductive loop. Preferably, the current is induced into the loop by a transformer. The loop has, at least in part, one, or a series of, conductive elements of a structure to be warmed. The amount of current is sufficient in relation to the inherent resistivity of the elements to cause the generation of heat within the elements. Preferably the current is an alternating current of a frequency which causes a majority of the current to travel at or near the skin of the elements in order to increase the effective resistivity of the elements and thereby increase the generation of heat therein, primarily around the skin of the elements.

Description:
This is a Continuation of application Ser. No. 08/158,931, filed Nov. 29, 1993 now abandoned which is a continuation-in-part of Ser. No. 07/544,319, filed Jun. 27, 1990 now U.S. Pat. No. 5,266,773 by Bertil S. M. Granborg. The entire disclosure therein is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to devices for using the inherent resistivity of electrically conductive elements of a structure to generate heat within the structure to keep them from becoming inoperable or damaged due to a cold and/or freezing environment. 
     SUMMARY OF THE INVENTION 
     This invention presents a device for generating heat in an electrically conductive element of a structure comprising a means for causing an alternating current through said element, the current being sufficient in relation to an inherent resistivity of said element to generate a desired amount of heat. The desired amount of heat will be that amount sufficient to prevent damage or icing under the circumstances. The frequency of the current is preferably high enough to cause at least a majority of the current carriers to travel on and within a skin portion of said element for more efficient surface heating. The alternating current can be induced and a means for inducing the current can be a source of alternating voltage which is transformed into the alternating current, the alternating voltage being applied to a primary winding of a transformer and the element or elements being serially within an electric current loop, i.e. a closed circuit, of a secondary winding of the transformer. 
     An object of this invention is to provide a means for generating heat within one or more electrically conductive elements of a structure and thereby heat the structure without the use of any dedicated heating elements, that is, elements whose only function is to generate heat such as heating coils and the like. 
     Further objects of this invention will be discussed and/or will be readily discernable from a reading of the specification and claims herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the application of this invention to an engine block, an oil pan, and a carburetor. 
     FIG. 2 illustrates the application of this invention to a conductive casing for a battery. 
     FIG. 3 illustrates the application of this invention to a vehicle&#39;s transmission and gear box, and the vehicle&#39;s drive train differential. 
     FIGS. 4,  5  and  9  illustrate the application of this invention to towers having conductive structural members. 
     FIG. 6 illustrates the application of this invention to an oil drilling platform. 
     FIGS. 7 and 10 illustrate application of this invention to a ship&#39;s bulwark. 
     FIG. 8 illustrates application of this invention to the wing frames of an aircraft. 
     FIGS. 11-13 illustrate application of this invention to pipes, such as oil carrying pipes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The basic principle is to provide heat to structures to prevent them from becoming inoperable or destroyed due to temperatures below zero degree Centigrade or due to ice accretion. In the preferred embodiments described herein, alternating electrical energy is applied to the primary of a stepdown transformer in which a secondary winding produces high current and low voltage in a circuit made up of structural elements or sequences of them serially connected. The high current preferably alternates at a frequency high enough to generate heat by the resistive losses close to the surface of the conductive elements due to skin effect, which concentrates the current at or near the surface, i.e. “skin.” 
     FIG. 1 illustrates this invention applied as an engine block heater, oil pan heater and carburetor heater. A source of alternating voltage  2  energizes a primary winding  4  of a transformer, generally designated  6 . The secondary comprises basically one conductor  8  having one end electrically connected to a first side of an engine block  10  at connection  12 . The other end of the conductor is electrically connected to the second and opposite side of engine block at connection  14 . The engine block being typically either cast iron or cast aluminum is electrically conductive and so a current induced in the secondary conductor  8  will flow from the first to the second side of the engine block, and if the frequency of the current is sufficiently high, skin effect will take place effectively increasing the inherent resistivity of the engine block. The inherent resistivity will generate heat within the engine block warming it. 
     Referring again to FIG. 1, a source of alternating voltage  16  energizes the primary winding  18  of a second transformer, generally designated  20 . The secondary of the transformer is a single conductor  22  which has its opposite ends connected to opposite sides of an oil pan  24  at connection points,  26  and  28 . Typically oil pans are made from steel, or other conductive alloy, and are therefore conductive and have an inherent resistivity. Current induced in the secondary conductor  22  will flow from one side of the oil pan to the other and generate heat therein due to the pan&#39;s inherent resistivity. One transformer could be used for heater both the block and the oil pan if proper electrical connections are provided between them. 
     Referring again to FIG. 1, also illustrated is a carburetor heater, intended to prevent moisture condensation. A source of alternating voltage  30  induces a current in single conductor  32  by means of a transformer, generally designated  34 . The ends of the conductor  32  are connected to opposite points of a carburetor  36 . Carburetors are typically made from cast iron, steel or aluminum and are therefore conductive, and therefore have inherent resistivity which can be used to generate heat in the manner previously discussed. 
     FIG. 2 illustrates this invention applied as a battery  38  heater. In this case the battery has an outside insulation case  40  and an inside partial metal case  42  connected across the secondary of a transformer  43 . The metal case is heated by resistive losses, particularly in the skin, according to the principles of this invention as explained above. 
     FIG. 3 illustrates this invention applied to heating the casings of a transmission/gear box  44  and a differential  46  via transformers  47 A and  47 B, respectively, according to the principles of this invention. The secondary connections are made on opposite sides of the casings. 
     The conductive structural members of an antenna tower  48  is illustrated in FIG. 4 as being heated via transformers,  49 A and  49 B, according to the principles discussed above. FIG. 5 illustrates a transmission line tower  50  heated via transformer  51 , and oil rig  52  in FIG. 6 is illustrated to have structural members being heated via transformers  53 A and  53 B according to this invention. There are several different variations of these structures and the applications of the heaters must therefore be custom made in each case. 
     FIG. 7 illustrates this invention applied to heat a bulwark  54  of a boat or ship. Transformers  55 A and  55 B are used to induce alternating high current in respective sides of the vessel. Similar arrangements can be made for the deck, auxiliary equipment or superstructure of the vessel. 
     FIG. 8 illustrates one configuration for heating the conductive wing frames  56 A and  56 B of an aircraft via transformers  57 A and  57 B respectively. In this case struts  58 A and  58 B have current induced directly into them, and each causes the current to flow through connected frame loops. Similar arrangements can be made for the wing and tail surfaces, conductive control surfaces, conductive engine components, conductive landing gear components or other auxiliary equipment of the air vessel. 
     Referring to FIG. 9, an alternative configuration for heating an antenna tower  48  is illustrated. Contrary to the configuration of FIG. 4, only one transformer  60  is used to induce a current in a single conductor  62 . The conductor  62  is connected at one end to the apex  64  of the tower, and at an opposite end to all four tower support legs at their base. In the configuration illustrated in FIG. 4, each transformer induced current in conductors connected to diagonally opposite pairs of legs. 
     Referring to FIGS. 7 and 10, the configurations in FIG. 10 require less current than the configurations illustrated in FIG.  7 . On one side of a ship&#39;s deck  66  a bulwark  68  is heated by having a segment  70  of it have current induced therein by having a core  72  of a transformer enclose the segment by means of holes  74 A and  74 B defined by the bulwark through which the core extends. An alternating voltage source  76  creates varying magnetic flux in the core which cuts the bulwark segment  70  and thereby induces current therein. This current then heats the conductive bulwark through resistive losses preferably in the skin of the bulwark. 
     Referring again to FIG. 10, on the opposite side of the deck  66  is a second bulwark  78  which defines only one hole  80  through which the core  82  of a transformer extends. As already noted, similar arrangements can be made for the deck, auxiliary equipment or superstructure of the vessel. 
     Referring to FIGS. 11-13, illustrated are pipes, such as oil pipes, which can be heated according to this invention. FIG. 11 illustrates a conductive pipe  84  having its opposite ends connected via a single conductor  86 . This conductor is in the secondary of a transformer  88  whose primary is energized by alternating electrical energy source  90 . The pipe is a segment in the secondary of the circuit of the transformer and therefore carries current induced into the secondary. The current according to the principles of this invention, as discussed above, generates heat within the pipe length. 
     Referring to FIG. 12, it is the same pipe  84  but in this configuration its opposite ends are connected to a conductive medium such as ground. A core  92  of a transformer surrounds the pipe and when alternating electrical energy source  94  energizes primary windings wrapped around the core, the core will induce a current in the conductive pipe, thereby heating it according to the principles discussed above. 
     Referring to FIG. 13, the pipe  84  is in the secondary circuit of a transformer  92 , as previously discussed with respect to FIG. 12, but in this case the opposite ends of the pipe are not connected to a conductive medium but rather to a second conductive pipe  96  which completes the secondary loop. In this way, both pipes are heated by a single transformer according to the principles as discussed above. 
     It should be realized that the pipes illustrated in FIGS. 11-13 would necessarily be covered by electrical insulation material. It should further be realized that the conductive medium referenced with respect to FIG. 12 can also be a conductive medium such as sea water or any other conductive liquid medium as well as any conductive solid medium. 
     In operation, a voltage is applied to the primary of the transformer causing current through its primary windings. This induces a current into the secondary winding, which is a high-current low-voltage circuit, by means of well known transformer induction principles. Since it is a stepdown transformer, a current gain is felt in the secondary. By proper selection of the primary winding count, the core material of the transformer, and the voltage levels, the current gain can be on the order of hundreds of amperes, enough to generate heat when opposed by the inherent resistivity of a structure&#39;s element or elements in the circuit of the transformer&#39;s secondary winding, preferably one turn. The high alternating current will generate heat by the resistive losses close to the surface of the conductive elements due to the skin effect, which concentrates the current at or near the surface. 
     For the purpose of system design the electrical impedance of a cylindrical metal bar is calculated by means by Bessel-type differential equations. The electrical impedance of a solid cylindrical bar is as follows:        Z   =       R   +     jω                 L       =       Hl     2      π                 α            ϱ        (     1   +   j     )                     ohm                 where                 H     =         0.5        ωμμ   o       ϱ                     α   =   radius           ω   =     2      π                   f   (     f   =     frequency                 in                 Hz       )                   l   =   length           μ   =     relative                 permeability                 ϱ   =     specific                 resistivity               μ   o     =     4      π   ×     10     -   7                                      
     This formula is valid for large H×α. 
     For comparison the DC resistance is:          R   o     =       l     πα   2          ϱ                 ohm                            
     A solid iron bar was theoretically analyzed and tested as follows: 
     α=0.9525 cm 
     l=0.67 m 
     q=0.119×10 −6  ohm m 
     f=60 hz 
     μ=815 
     which yields: 
     Z=2.399677×10 −3  ohm, and 
     R o =0.02797×10 −3  ohm. 
     The measured data was: 
     I=250 A (amperes) 
     V=0.6 V (volts) 
     S=VI=150 VA (complex power) 
     P=S COS Θ=150(0.707)=106 W 
     The bar could maintain an estimated 75° C.temperature and showed a considerable heat capacity. 
     The alternating voltage sources as described herein can be any alternating voltage source of sufficient capacity as “sufficient” is defined in this specification. 
     It is necessary to have flexible connections between any moving parts in order to have good electrical contact between the metal parts and to avoid them being welded together. (A few hundred amperes are to be expected.) In order to optimize the operating cost, a control system of conventional design can be used with temperature sensors and switches operating such that heating takes place only below freezing temperatures, or if so desired, only during intermittent periods. The transformers can also be designed such that they have minimum leakage. 
     The foregoing description and drawings were given for illustrative purposes only, it being understood that the invention is not limited to the embodiments disclosed, but is intended to embrace any and all alternatives, equivalents, modifications and rearrangements of elements falling within the scope of the invention as defined by the following claims.