Abstract:
At least one aspect of the present invention relates to a system for heating a gas using positive temperature coefficient (PTC) elements. More particularly, aspects of the present invention are directed towards PTC element configurations and orientations. In one aspect of the present invention, PTC elements are interposed between heat sinks such that they transmit current from one heat sink to another. The PTC elements may be arranged in a radial configuration, which may have more than one PTC element per radial flange. The heat sinks may have grilles which allow air to flow perpendicular to the heat sinks and between the PTC elements.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a heating apparatus. Specifically, this invention relates to heaters which incorporate a positive temperature coefficient (PTC) element.  
         BACKGROUND OF THE INVENTION  
         [0002]    Positive temperature coefficient (PTC) materials have been used in heating applications for many years. Upon application of an electric bias to a PTC material, it initially has a low electrical resistance and heats up quickly due to current flow through the material. However, once the PTC material reaches the Curie point, its resistance increases so that it maintains a substantially constant temperature and heat output. This self-regulating characteristic of PTC materials significantly decreases the potential for heating element burnout as well as the need for temperature-regulating electronics, and thus makes these materials attractive for use in heating elements such as those in space heaters, hair dryers, and other applications.  
           [0003]    U.S. Pat. No. 4,654,510 to Umeya et al. describes one type of PTC heating element. Holes formed through the PTC element, parallel to the direction of current flow, provide a pathway for air. The air is heated as it passes through the PTC element.  
           [0004]    U.S. Pat. No. 4,855,570 to Wang discloses another PTC element arrangement where the PTC elements are exposed directly to airflow. The heating unit described by Wang includes a plurality of PTC elements arranged radially between two cylindrical electrodes. The PTC elements are arranged so that their broad surfaces are parallel to air flow through the heating element.  
           [0005]    Other heater designs include heat sinks which receive heat from the PTC elements and transfer it to air passing by and/or through the heat sinks. To increase the convective heat transfer to the air, these heat sinks typically have many holes providing paths for air flow. One such configuration is described in U.S. Pat. No. 4,654,510 to Nakamura et al., in which the heat sinks provide a plurality of fluid pathways for air to flow through. By orienting the fluid pathways in the heat sinks parallel to the broad surfaces of the PTC elements, these heating devices do not require holes through the PTC elements themselves. Additionally, Nakamura utilizes the heat sinks as electrodes which may stabilize current spikes and reduce the likelihood of PTC element burnout.  
         SUMMARY OF THE INVENTION  
         [0006]    In at least one aspect of the present invention, a heating element utilizing positive temperature coefficient (PTC) elements has sufficient surface area for effective heat transfer as well as the capability to heat a large volume of air without creating a large internal air resistance.  
           [0007]    In another aspect of the present invention, an arrangement of PTC elements in a heating element can be configured to provide the desired heat output and desired heat distribution.  
           [0008]    In another aspect of the present invention, a PTC heating element may be provided where broad surfaces of the PTC element(s) are arranged substantially perpendicular to the direction of airflow.  
           [0009]    In another aspect of the present invention, a PTC heating element may include at least one heat sink and at least one PTC element, configured such that there is sufficient pressure between the PTC element and the heat sink to promote heat transfer and provide sufficient electrical and/or thermal contact between the PTC element and heat sink.  
           [0010]    In another aspect of the present invention, a heating element includes a first heat sink and at least one PTC element thermally coupled to the first heat sink, aligned such that a current direction of the PTC element (i.e., the direction in which current would flow if an electric bias were applied to the heating element) is substantially parallel to a fluid pathway formed by openings in the first heat sink.  
           [0011]    In another aspect of the present invention, a heating element includes a first heat sink and a PTC element thermally coupled to the first heat sink positioned substantially out of a fluid pathway formed by openings in the first heat sink so that a largest surface area of the PTC element is approximately perpendicular to the fluid pathway.  
           [0012]    In another aspect of the present invention, a heating element includes at least one heat sink and a PTC element thermally coupled to the heat sink such that at least 50% of the heat output by the PTC element is transferred to heat sink(s) coupled to the PTC element and arranged so that a largest surface of the PTC element is approximately perpendicular to a fluid pathway formed by opening(s) in the heat sink(s).  
           [0013]    In another aspect of the present invention, a heating element includes first and second heat sinks with openings that form a fluid pathway and a plurality of PTC elements substantially aligned in a single plane such that the current direction of the PTC elements is substantially parallel to one another and the PTC elements are arranged radially inside a circle, where the first and second heat sinks are configured to act as electrodes for the plurality of PTC elements.  
           [0014]    In another aspect of the present invention, a heating element includes a heat sink and at least one PTC element in thermal communication with the heat sink, where a fluid pathway formed by at least one opening in the heat sink first passes either the heat sink or the PTC element and then passes the other.  
           [0015]    In another aspect of the present invention, a heater includes an air circulator to move air through a heating element, where the heating element includes a first heat sink thermally coupled to at least one PTC element, where the PTC element is aligned such that the current direction is substantially parallel to a fluid pathway formed by the first heat sink.  
           [0016]    In another aspect of the present invention, a heating element has a plurality of PTC elements radially arranged within a circle. The radial arrangement includes radial flanges, and at least one radial flange may include one or more PTC elements. In one embodiment, the heat sinks may shield the PTC elements from direct air flow. The heat sinks may also act as electrodes to the PTC elements. If the heat sinks function as electrodes, electrically non-conductive fasteners connecting the heat sinks and PTC elements may be used so as to avoid short circuiting the heat sinks when an electric bias is applied. The fasteners may additionally apply pressure between the plurality of PTC elements and heat sinks.  
           [0017]    These and other features of the present invention will be elucidated through the accompanying drawings and detailed description below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a perspective exploded view of one embodiment of a heating element in one aspect of the present invention;  
         [0019]    [0019]FIG. 2 is a side view of one embodiment of a heating element in one aspect of the present invention;  
         [0020]    [0020]FIGS. 3 through 11 are top views of different PTC element arrangements according to one aspect of the present invention; and  
         [0021]    [0021]FIG. 12 is a side view of a heater utilizing a PTC heating element in one aspect of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]    A heating element according to aspects of the present invention can be sized and configured for any suitable use. For example, a heating element according to aspects of the invention may be used to heat air in an electric portable space heater, hair dryer, heat gun, etc. Although embodiments are described below in connection with heating air, a heating element in accordance with at least one aspect of the invention may be used to heat any suitable material, whether a gas, liquid or solid. As used herein, the term fluid refers to both gases and liquids.  
         [0023]    [0023]FIG. 1 shows an illustrative embodiment of a heating element  100  that incorporates various aspects of the invention. In this embodiment, the heating element  100  includes a plurality of PTC elements  110  disposed between a pair of heat sinks  120 , although any number of PTC elements  110  and heat sinks  120 , such as one each, may be used. The heat sinks  120  are electrically and thermally coupled to the PTC elements  110  so that electric current and heat may be conducted between them. In this embodiment, the heat sinks  120  have solid portions which electrically and/or thermally contact the PTC elements  110 , as well as openings  140  between the solid portions which enable fluid flow (e.g., air flow) through the heat sinks  120 . Thus, when an electric bias is applied to the heat sinks  120  and/or other electrodes, the resulting current through the PTC elements  110  causes the PTC elements  110  and heat sinks  120  to heat up. In turn, the heat sinks  120  may transfer at least a portion of the heat to air passing through the openings  140  and/or around the heat sinks  120 .  
         [0024]    In accordance with one aspect of the invention, the openings  140  may form a fluid pathway through a heat sink  120  that is substantially perpendicular to a plane in which at least some of the PTC elements  110  are arranged. For example, air may pass through the heating element  100  in a direction approximately perpendicular to the first heat sink  120   a  and a plane of PTC elements  110  (e.g., the plane  210  shown in FIG. 2). As a result, the air may flow sequentially past the heat sinks  120  and PTC elements  110 , e.g., first past the first heat sink  120   a , then past a plane in which at least some of the PTC elements  110  are arranged, and then past the second heat sink  120   b . In the embodiment of FIG. 1, the two heat sinks  120  act as both heat conductors and electrodes for the PTC elements  110 , although such dual operation is not necessary. If a single heat sink  120  is used, the PTC elements  110  and heat sink  120  may be arranged in any suitable arrangement relative to air flow through the heating element  100 , e.g., either the first or second heat sinks  120   a  or  120   b  may be eliminated. When a single heat sink is used, a complete electric circuit may be constructed by connecting an electrode to the PTC elements  110  on the side opposite the single heat sink  120 .  
         [0025]    In another aspect of the invention, PTC elements  110  may be arranged so that a current direction of the PTC elements  110 , or direction that the current would flow when an electric bias is applied, is substantially parallel to a fluid pathway through a heating element  100 , or a portion of the heating element  100 . For example, although the PTC elements  110  may take any suitable shape, size or other feature, in the FIG. 1 embodiment, the PTC elements  110  each have a pair of opposing, broad-surfaces  180  with a relatively large surface area that are configured to transmit current to and from the heat sinks  120  when an electric bias is applied. In this embodiment, current will flow from one heat sink  120  to another through the PTC elements  110  in a direction approximately parallel to an air path through the heating element  100 .  
         [0026]    In another aspect of the invention, the broad surfaces  180 , which may be the surfaces with the largest surface area of the PTC element  110 , may be approximately perpendicular to the fluid pathway. For example, the broad opposing surfaces  180  may be aligned such that at least some of the PTC elements  110  are arranged in one or more planes, such as the plane  210  shown in FIG. 2. In this embodiment, the fluid flow direction through the heating element  100  is approximately perpendicular to the surfaces  180  of the PTC elements  110 . The approximately perpendicular direction of fluid flow through the heating element  100  need not require that all individual flow paths in an overall fluid flow or all molecules in a fluid flow follow a perpendicular path through the heating element, but rather that the overall direction of movement of air is approximately perpendicular to the heating element. For example, water flow in a river is said to generally be in a particular direction, i.e., the overall flow direction of the river, even though particular parts of the river may have currents, eddies and other flows that are not necessarily aligned with the overall flow of the river. A similar situation may exist in the fluid flow through the heating element, and thus fluid flow direction may refer either to particular localized flow or the overall flow of fluid through the element.  
         [0027]    In another aspect of the invention, the PTC elements may be arranged in a radial arrangement in a way similar to spokes in a bicycle wheel. For example, as shown in FIG. 1, the PTC elements  110  may have a radial arrangement such that the PTC elements  110  are arranged within a circle  150 . As seen in FIG. 1, the radial arrangement may include any suitable number of radial spokes, or flanges  160 , and any number of PTC elements in any one of the flanges  160 . A radial arrangement of PTC elements  110  within a circle  150  may provide an even heat distribution in the heating element  100 , e.g., when a standard radial fan is used to move fluid through and/or around the heating element  100 . Of course, the PTC elements  110  may be arranged in any suitable way, such as in a linear array, a concentric circular pattern, and so on.  
         [0028]    In another aspect of the invention, since the heat sinks  120  may be thermally conductive, the PTC elements  110  may be thermally coupled to the heat sinks  120  such that they transfer at least a portion of the heat they generate to at least one heat sink  120 . For example, the PTC elements  110  may transfer at least 50% of the heat they generate to one or more heat sinks  120 . Preferably, the PTC elements  110  transfer at least 70% of the heat they generate to the heat sinks  120 . More preferably, the PTC elements  110  transmit at least 80% of the heat they generate to the heat sinks  120 . Because the heat from the PTC elements  110  may be transferred to the heat sinks  120  by conduction, the contact surface area between the heat sinks  120  and the PTC elements  110  may be relatively large. Although in the FIG. 1 embodiment the contact area between the heat sinks  120  and the PTC elements  110  is flat, the contact area may have any suitable surface features, such as corrugations, grooves, recesses, etc., to enhance thermal and/or electrical contact between the heat sinks  120  and the PTC elements  110 .  
         [0029]    In another aspect of the invention and as discussed above, the heat sinks  120  may act as electrodes for the PTC elements  110 . When used as electrodes, the heat sinks  120  may stabilize current spikes and thus protect the PTC elements  110 . Therefore, the heat sinks  120  may be in electrical contact with the PTC elements  110  and may include an electrically conductive material such as a metal. The heat sinks  120  may also include a thermally and electrically conductive material such as copper, stainless steel, or steel. In this embodiment, the heat sinks  120  are formed from a plate or sheet of metal, such as aluminum, and the openings are stamped, machined or otherwise produced in the sheet. However, aspects of the invention are not limited to heat sinks  120  that are formed as flat plates, but instead may have any suitable arrangement, whether for functioning as an electrode or a heat transfer mechanism. For example, the heat sinks  120  may have fins, corrugations, or other features to enhance heat transfer. In addition, the heat sinks  120  need not be made from a single material or as a single piece. Instead, the heat sinks  120  may be made in multiple parts and/or from two or more different materials. Furthermore, the heat sink materials may include insulators, conductors and/or semiconductors in any suitable arrangement. If desired, a conductive grease can also be used between the PTC elements  110  and the heat sinks  120  to improve the electrical and/or thermal contact between these elements. With the heating element  100  configured in this way, one of the heat sinks  120  can be positively electrically charged and the second can be electrically neutral as shown in FIG. 2.  
         [0030]    The openings  140  in the heat sinks  120  may be sized and configured to provide both a large surface area for effective heat convection and heat and/or electrical conduction as well as large vents to promote relatively unhindered air flow through the heating element  100 . As known by those of skill in the art, the configuration of openings  140  can be designed and configured for the specific fan blade size, volume of unheated fluid moving through the heating element  100 , and amount of expansion of the fluid due to heating occurring within the heating element  100 .  
         [0031]    Although not necessary, the openings  140  in the heat sinks  120  may be aligned to create a substantially straight fluid pathway through the heating element  100  and thus reduce resistance to fluid flow. The fluid pathway created by the openings  140  may be substantially parallel to the current direction through the PTC elements  110  when an electric bias is applied and/or substantially perpendicular to the opposing broad sides of the PTC elements  110 .  
         [0032]    In one aspect of the invention, the heat sinks  120  may substantially shield the PTC elements  110  from the fluid pathway. For example, as shown in FIG. 1, if the PTC elements  110  are aligned under solid portions of the heat sinks  120 , the heat sinks  120  may substantially shield the PTC elements  110  from direct fluid flow and furthermore may provide a larger conductive and convective heat transfer surface for the fluid and PTC elements  110 . In one embodiment, the heat sinks  120  substantially shield the PTC elements  110  from the fluid pathway such that the fluid pathway may be adjacent to less than 50% of the PTC elements&#39;  110  surface area. In other words, the fluid pathway does not contact the majority of the PTC elements&#39;  110  surface area. The fluid pathway may preferably be adjacent to less than 30% of the PTC elements&#39;  110  surface area. More preferably, the fluid pathway may be adjacent to less than 20% of the PTC elements&#39;  110  surface area. However, it should be understood that the PTC elements  110  may be partially or wholly exposed to fluid flow by the openings  140  in the heat sinks  120 , and the PTC elements  110  may include openings through which fluid flows as well.  
         [0033]    As shown in FIGS. 1 and 2, the heating element  100  may include fasteners  130 , such as rivets, bolts, screws, etc., which hold the PTC elements  110  firmly between the heat sinks  120 . If one heat sink is used, fasteners  130  may be employed to hold the PTC elements  110  to the heat sink  120 . Additionally, if the heat sinks  120  are used as electrodes, the fasteners  130  may be electrically non-conductive, e.g., at least partially composed of plastics, ceramics, and non-conductive metals. Using non-conductive materials for the fasteners  130  may prevent the heating unit  100  from electrically short circuiting when an electric bias is applied. However, other means, such as interposing an insulating material between the fasteners  130  and the heat sinks  120 , are available to prevent short circuiting as known by those of skill in the art. Of course, the heating element  100  may be held together by other means, such as one or more clamps, adhesives, etc. or a combination of fasteners, clamps, adhesives, etc.  
         [0034]    The fasteners  130  may be sized and configured to generate pressure between the PTC elements  110  and the heat sinks  120 , thereby creating sufficient contact between the heat sinks  120  and the PTC elements  110 . Sufficient pressure between the PTC elements  110  and heat sinks  120  may help secure the PTC elements  110  in place and/or may improve the electrical and/or thermal contact between the heat sinks  120  and the PTC elements  110 , thereby potentially making the heating element  100  more efficient. The fasteners  130  may be placed around or through the PTC elements  110  such that they generate pressure directly on the PTC elements  110 .  
         [0035]    Several aspects of the present invention have been described. However, many modifications to the described embodiment can be made within the scope of the present invention. For example, the PTC elements  110  may have a rectangular, sheet-like shape, as shown in FIG. 1. However, as known to those of skill in the art, any suitable shape may be used. As also shown in FIG. 1, multiple PCT elements  110  can be placed on a single radial flange, e.g.,  110   a  and  110   b  on  160   d . Although the PTC element arrangement in FIG. 1 has sixteen PTC elements  110  arranged with two PTC elements  110  per radial flange  160 , many alternative radial PTC element arrangements are possible as shown in FIGS. 3 through 6. More or fewer than two PTC elements  110  can be placed per flange  160 , and the number of flanges  160  can likewise vary. Because each PTC element  110  may have a power limit, positioning multiple PTC elements  110  on a single radial flange  160  may allow the heating element  100  to produce more heat per flange without damaging the PTC elements  110 . Additionally, the number, sizes, and shapes of PTC elements  110  included in each flange  160  do not have to be uniform between radial flanges  160 , as shown in FIGS. 5 and 6. Thus, the arrangement of PTC elements  110  can be sized and configured to provide the desired power output while maintaining a desired heat distribution within the heating element  100 . That is, the heater  100  may be configured to have an uneven internal heat distribution, although in many cases an even internal heat distribution may be desirable.  
         [0036]    In another embodiment of the present invention, the PTC elements  110  can be placed in a grid-like pattern. As shown in FIGS.  7 - 9 , the perimeter of this grid-like pattern can be square, rectangular, or any other shape with any number and shape of PTC elements  110  aligned in the grid. Again, the PTC element grid can be sized and configured to optimize the heating element  100  for its desired use.  
         [0037]    Many other PTC element configurations are possible. Two alternative configurations are shown in FIGS. 10 and 11. Notably, for all of the PTC element configurations of FIGS. 3 through 11, the PTC elements  110  are arranged such that the broad opposing surfaces  180  of the PTC elements are aligned in one or more planes. As noted above, to increase the conductive surface area with the heat sinks  120 , the broad opposing surfaces  180  also may be the sides with the largest surface area.  
         [0038]    Although in FIG. 1 the openings  140  in the heat sinks  120  have an arcuate shape, the openings  140  can have any suitable configuration, size and/or shape. Specifically, the openings  140  can be rectangular slots that are radially oriented, rectangular slots that are arranged like chords, triangular holes, circular holes, or any other configuration of shapes and sizes. The shape of the heat sink  120  and placement of the solid portions and openings  140  of the heat sink can be altered to accommodate different PTC element configurations. For example, the heat sinks  120  can be configured such that their solid portions substantially shield the PTC elements  110  from direct airflow regardless of the PTC element configuration chosen.  
         [0039]    Although the fasteners  130  are located at the ends of the radial flanges  160  in FIG. 1, the fasteners  130  may be placed in many different locations in the heating element  100 . For example, the fasteners  130  alternatively or additionally can be placed along the sides of the radial flanges  160  or between the radial flanges  160 . The fasteners  130  may also be configured to hold a clamping mechanism or brace instead of contacting the heat sinks  120  directly. If it includes an electrically insulating material, a clamping mechanism or brace could additionally be used to prevent short circuiting when the heat sinks  120  are used as electrodes.  
         [0040]    The fasteners  130  may be any of various types as well. Rivets are depicted in FIG. 1, but as known to those of skill in the art, many types of fasteners such as screws, bolts, press fit fittings, and clamps can also be used. Additionally, welds, epoxy, or other adhesives can be used to fasten the heat sinks  120  together and hold the PTC elements  110  firmly in place with sufficient electrical and thermal contact between the heat sinks  120  and the PTC elements  110 . If an adhesive such as epoxy is used, fasteners  130  may be unnecessary.  
         [0041]    As shown in FIG. 12, a heating element  100  can be used in a portable heater  440  which is sized and configured to allow a single human to carry it without mechanical assistance. In a portable air heater  440 , at least one heating element  100  may be placed in front of air movement means such as a fan  400  inside a housing  410 . The fan  400  may direct air substantially perpendicular to the heating element  100  as shown by the arrows  420 . As the air passes by and/or through the heating element  100 , at least a portion of it is heated by the heat sinks  120  and/or the PTC elements  110 . At least part of the heated air is then vented out of the housing  410  as shown by the arrows  430 .  
         [0042]    Although aspects of the present invention have been fully described by way of example, modifications to the designs can be made within the scope of the invention as known to those of skill in the art. Therefore, the examples used herein should not be construed as limiting, but merely intended to completely describe an illustrative embodiment of the invention.