Abstract:
A cartridge heater and method of manufacturing same. In one embodiment, the cartridge includes a sheath having a first end and a second end. The first end of the sheath forms a seat. An elastomeric bushing is swaged against the seat such that it forms a mechanically bonded seal substantially preventing moisture egress into the cartridge heater. A heating element is also disposed in the sheath and is connected to leads protruding from the bushing. The heater includes crushable insulation material disposed within the sheath.

Description:
BACKGROUND OF THE INVENTION 
     Various configurations of electric cartridge heaters are known in the prior art. A typical cartridge heater includes a metal sheath around a resistance-wire heating element coiled around a core of insulating material. An insulating filler material with appropriate thermal conductivity and electrical insulating properties is used to fill the space between the coil and the sheath. Granulated magnesium oxide is typically used as the insulating filler material. After the sheath is filled, the sheath is subjected to compression forces, for example, by swaging. Compression compacts the granulated magnesium oxide and improves its dielectric and thermal conductivity properties. Lead wires may be attached to the coil before or after filling the sheath and may be held in place with an end plug made of materials such as Teflon, mica and silicone rubber. The lead wires become secured within the plug after swaging. The lead wires may then be potted with sealants to provide moisture resistance. Depending upon the intended application, cartridge heaters of varying sizes and voltage ratings may be required. U.S. Pat. No. 6,172,345, for example, discloses a high voltage cartridge heater which includes a core sleeve of pre-compacted insulating material. 
     With current manufacturing technology, it has proven to be a challenge to reliably produce high-voltage cartridge heaters for high moisture environments. Heaters in operation in high moisture environments are prone to dielectric breakdown and current leakage problems caused by the egress of moisture and water into the dielectric insulating material. In high moisture environments, dielectric integrity and current leakage must be kept within predetermined limits in order for the cartridge to meet certain industry standards, such as those standards established by Underwriters Laboratories, for example, the UL 471 standards. 
     One apparent reason for such problems is that the potting sealants and sealant methods used to seal the lead wire end of the cartridge do not provide adequate bonding with the lead wires and the sheath. Sealant materials, such as epoxy and silicone, for example, do not bond adequately with the stainless steel used for the construction of the sheath or with the silicone-coated lead wires. As a result, high-voltage cartridge heaters are traditionally only offered with sealants that do not qualify for certification for high moisture environments under the applicable industry standards. 
     SUMMARY 
     One embodiment of the invention provides a cartridge heater. The cartridge includes a sheath having a first end and a second end. The first end of the sheath forms a seat. An elastomeric bushing is swaged against the seat such that it forms a mechanically bonded seal substantially preventing moisture egress into the cartridge heater. A heating element is also disposed in the sheath and is connected to leads protruding from the bushing. The heater may include crushable insulation material disposed within the sheath. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying Figures, there are shown present embodiments of the invention wherein like reference numerals are employed to designate like parts and wherein: 
     FIG. 1 is a sectional view of an embodiment of cartridge heater according to the present invention; 
     FIG. 2 a  is a rear view of an embodiment of a sheath for the cartridge heater of FIG. 1; 
     FIG. 2 b  is a side view of the sheath of FIG. 2 a;    
     FIG. 2 c  is a front view of the sheath of FIG. 2 a;    
     FIG. 3 a  is a side view of one embodiment of a seal bushing for the cartridge heater of FIG. 1; and 
     FIG. 3 b  is a side view of one embodiment of a seal bushing for the cartridge heater of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings for the purpose of illustrating the invention and not for the purpose of limiting the same, it is to be understood that standard components or features that are within the purview of an artisan of ordinary skill and do not contribute to the understanding of the various embodiments of the invention are omitted from the drawings to enhance clarity. In addition, it will be appreciated that the characterizations of various components and orientations described herein as being “vertical” or “horizontal”, “right” or “left”, “side”, “top” or “bottom”, are relative characterizations only based upon the particular position or orientation of a given component for a particular application. 
     FIG. 1 shows a side cross-sectional view of a cartridge heater  10  in accordance with one embodiment of the invention. In the embodiment of FIG. 1, the heater  10  may include an elongate heater element wind core  12  about which a resistive heating element wire  14  may be coiled, in an essentially conventional configuration. The wind core  12  may be made of magnesium oxide, and is substantially cylindrical. The wind core  12  and the coiled wire  14  are disposed within an outer sheath  16  made of, for example, stainless steel or the like. Interposed between the inner diameter of the sheath  16  and the heating element  14  is an electrically insulating, thermally conducting material  18  (hereinafter “insulating material”). The insulating material  18  may be composed of loose-fill or pre-compacted magnesium oxide. 
     The sheath  16  may be a tube that has a first end  32  and a second end  34 . An annular seat  26  is formed at the second end  34  of the sheath  16  prior to assembly by spin over or other conventional forming means. See FIGS. 2 a - 2   c . The seat  26  extends from the second end  34  of the sheath  16  and is curved 90° relative to the sheath  16  through a curved portion  27  to form an annular planar surface  29 . The inner radius of the curved portion  27  of the seat  26  may be, for example, {fraction (1/32)} of an inch and the outer radius {fraction (1/16)} of an inch. The corresponding thickness of the sheath may be 0.028 inches. 
     Operating power is supplied to cartridge heater  10  by means of two supply (lead) wires  20 . The wires  20  may 18-gauge, silicone rubber-coated wire rated to conduct on the order of 600 volts. The wires  20  enter the second end  34  of heater  10  through a seal bushing  22  and a mica disk  24  each having appropriately sized through-holes formed therein. The seal bushing  22  may be made of elastomeric or rubber-like material. For example, a fluorocarbon elastomer, such as the commercially available Viton® with Shore A durometer in the range of 70-80 may be used. A nitrile elastomer, such as BUNA N with Shore A durometer in the range of 65-75 may be also used. The seal bushing  22  may have a rounded edge  38  conforming to the curved portion  27  of the seat  34 , as shown in FIGS. 1 and 3 a,  or it may have straight edges as shown in FIG. 3 b.    
     In the embodiment of FIG. 1, the sheath  16  may be approximately four inches long, and may have an outer diameter of one-half inch or less. The core  14  may have a length of approximately three and one half inches. The seal bushing  22  has a diameter which is equal to the inner diameter of the sheath  16  before swaging, approximately {fraction (7/16)} of an inch. The axial dimension (length) of the seal bushing may be approximately half an inch or less. 
     The cartridge heater  10  may assembled as follows: The sheath  16  is cut to length with allowance for the material that will become the seat  26 . The seat  26  is mechanically formed in the sheath  16  by conventional methods such as spin over, lathe machining, peening, or die forming, etc. The various components of the heater  10  are inserted into the sheath  16  from its second end  34 . Once all of the components are assembled within sheath  16 , granular magnesium oxide is introduced into the second end  34  of sheath  16 , in order to fill all remaining voids therein to the extent possible. Next, an end cap  30  is welded over the second end  34 . Finally, the entire assembly is swaged, for example at a pressure of approximately 200,000 lbs per linear inch, to compress and reduce the overall diameter of the sheath  16 . This swaging process compacts the magnesium oxide, thereby enhancing the dielectric and thermal conductive properties of the heater  10 . Swaging also compresses the radius of the seal bushing  22  and compresses the seal bushing  22  into the formed seat  26 . 
     The swaged seal bushing  22  forms a mechanical bond with the seat  26  and the lead wires  20  such that moisture is substantially prevented from entering into the cartridge heater  10 , when the heater  10  operates in moist locations. Such moisture prevention is achieved through the swaging of the elastomeric seal bushing  22  against the seat  26  without the need to use of any chemical sealants, such as epoxy, silicone or other cementing material, which could limit the versatility of the heater  10  by restricting operability of the heater at certain temperature. 
     Tests conducted by the independent Underwriters Laboratories (UL) showed that the heater  10  meets the standards established for Commercial Refrigerators and Freezers, UL 471, 8 th  Edition for moist locations. A moist location is defined as a location in which the heater is exposed to moisture but is not subject to more than occasional contact with water in a refrigerator. The test is conducted by operating the heater for 1000 cycles at a rate of 1½ minutes on 13½ minutes off in an atmosphere of not less than 98% humidity at any convenient temperature above 0° C. (32° F.). A seal for a cartridge heater that demonstrably passes this test is defined herein as a seal that substantially prevents moisture egress into the heater. The heater  10  was also certified by UL for operation up to 190° C. temperature in the bushing. On the contrary, prior art epoxy seals are limited to 90° C. and Teflon seals are limited to temperatures of 150° C. 
     Whereas particular embodiments of the invention have been described herein for the purpose of illustrating the invention and not for the purpose of limiting the same, it will be appreciated by those of ordinary skill in the art that numerous variations of the details, materials and arrangement of parts may be made within the principle and scope of the invention without departing from the spirit of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather the scope of the invention is to be determined only by the appended claims and their equivalents.