Patent Publication Number: US-6982400-B1

Title: Electrical heater apparatus

Description:
FIELD OF THE INVENTION 
     This invention relates generally to electrical heaters and more particularly to electrical heaters for crankcase heaters used in compressor heat pump applications. 
     BACKGROUND OF THE INVENTION 
     Crankcase heaters are typically used in HVAC compressor applications. After prolonged off periods and before start up, refrigerant in compressors tend to mix with oil in the compressor forming a refrigerant/oil solution. A crankcase heater is used to drive out refrigerant from the refrigerant/oil solution in order to prevent damage to the compressor due to lack of lubrication and/or high solution volumes. 
     An effective crankcase heater which has become widely used is shown and described in U.S. Pat. No. 4,236,065. This heater device comprises a self-regulating positive temperature coefficient of resistivity (PTC) element potted inside a thermally conductive ceramic body, such as alumina or steatite. The ceramic body and PTC element are potted with grit (alumina powder and de-ionized water mixture) and then sealed with a sealant, such as RTV polymer, to seal the device from moisture. The ceramic body provides electrical isolation between the device and a metallic housing or well provided in the shell of a compressor which receives the heater device. Heat generated by the PTC element is conducted through the ceramic body and the metallic housing into the refrigerant/oil solution to boil off the refrigerant from the solution within the compressor. When heated, the PTC element increases in temperature to a preselected temperature based on the anomaly temperature or the Curie point of the PTC element. 
     However, due to tolerance stack-ups between the device and the inner well diameter, the ceramic body is required to be undersized so that it will fit into the metal well attached to the compressor housing. To augment heat transfer from the heater to the well, thermal joint compound such as thermal grease is used to fill the air pocket between the heater and the well. The inner diameter of the well determines the permissible outer diameter of the ceramic body which is to be fitted within the well. A heater device is installed by applying thermal grease on the device and in the well. The device is then inserted in the well and a retaining clip is inserted to secure the device in the well. 
     Although the above described heater device is reliable, long lasting and inexpensive, the installation procedure is more labor intensive and cumbersome than desired. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a reliable, long lasting and inexpensive heating device for crankcase heaters and the like which are more easily and inexpensively installed in a heater well of a compressor than prior art heaters. 
     Another object of the invention is the provision of an electric heater for use as a crankcase heater which does not require the use of thermal grease when installing the heater in the well of a compressor. 
     Yet another object of the invention is the provision of a crankcase heater which overcomes the limitations of the prior art noted above and provides a more cost effective solution. 
     Briefly, according to the invention, a heater assembly comprises first and second generally semi-cylindrical members or terminal blocks formed of suitable thermally conductive metallic material such as aluminum, which sandwich a PTC heating element closely received in an elongated tubular casing having closed and open opposite ends. The casing material is designed and controlled to tight tolerances to ensure proper fit and performance with no thermal interface material used for enhancement of heat transfer between the heater device and the well and is formed of relatively flexible, thermally conductive material such as silicone polymer. The terminal blocks are each formed with a groove formed in a surface of the terminal blocks facing the PTC element for receipt of a respective electrical spring contact. The terminal blocks are spaced from one another by means of opposing longitudinally extending ribs formed in the side wall of the casing. The longitudinal length of the terminal blocks is less than the longitudinal length of the casing thereby providing space for placement of an alumina-water (grit) compound. After grit curing, a sealant is applied to the top of the device to protect the internal components from moisture. 
     The casing is formed with a self retaining clip feature comprising opposed projections extending radially outwardly from the casing wall at the open end thereof. The projections are received in an annular recess formed either directly in the well or between the side wall of the well and the opening in the compressor shell which communicates with the well for locking retention in the well upon insertion of the heater device. Preferably, the projections are formed with an inclined wall on the bottom side to facilitate insertion of the heater device into the well and a generally radially extending surface on the top side to impede removal of the heater device from the well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and features of the invention will become apparent by reference to the following detailed description of preferred embodiments when considered in connection with the accompanying drawings wherein: 
         FIG. 1  is a perspective view of a heater device made in accordance with a preferred embodiment of the invention; 
         FIG. 2  is a longitudinal cross sectional view of the  FIG. 1  device; 
         FIG. 3  is an enlarged top plan view of the  FIG. 1  heater device; 
         FIG. 4  is a cross sectional view taken on line A—A of  FIG. 3 ; 
         FIG. 5  is a cross sectional view taken on line B—B of  FIG. 3 ; 
         FIG. 5A  is an enlarged broken away portion of  FIG. 5 ; 
         FIG. 6  is an enlarged perspective view of one of two like terminal blocks employed in the  FIG. 1  heater device; 
         FIG. 7  is a longitudinal cross section of one type of well for receiving the heater device shown for the purpose of illustrating the retaining feature of the casing; and 
         FIG. 8  is similar to  FIG. 7  but shows another type of well for receiving the heater device. 
       Corresponding reference characters indicate corresponding parts through the several views of the drawings. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With particular reference to  FIGS. 1–6 , numeral  10  indicates a heater device made in accordance with the preferred embodiment of the present invention. Heater device  10  comprises a tubular, generally cylindrical casing  12  formed of relatively flexible, electrically insulative and thermally conductive material such as silicone elastomer. Casing  12  has an elongated side wall  12   a , a bottom end wall  12   b  and an open end  12   c . Diametrically opposed retainer projections  12   d  are integrally formed at open end  12   c  and will be discussed below. 
     A heater assembly comprising first and second like terminal blocks  14  sandwiching an electrical resistance heating element  16  with electrical lead wires and spring contacts  18  are closely received in the casing in good thermal conductive relationship therewith. 
     With particular reference to  FIG. 6 , each terminal block  14  is a generally semi-cylindrical solid of suitable heat conductive material, such as aluminum. The length of terminal blocks  14  is selected to be less than that of side wall  12   a  of casing  12  to leave a cavity for the purpose of placing potting and sealing material, to be discussed. 
     Preferably, and as shown, terminal blocks  14  are each formed with a shallow seating recess  14   a  extending the length of the block which serves to seat heater element  16 . Casing  12  is formed with two opposing, longitudinally extending ribs  12   e  effectively dividing the chamber within the casing into two sections with each section receiving a respective terminal block. The semi-cylindrical side wall of the terminal blocks at the extremity of the curved peripheral portion is shortened at  14   b  essentially half the width of the ribs  12   e  so that the mating surfaces of the terminal blocks and the inside surface of side wall  12   b  will closely match, see  FIG. 3 . 
     A spring contact receiving groove  14   c  is formed in heater element seat  14   a  having an opening at end  14   d  of the terminal block and a closed end  14   e  intermediate to terminal block ends  14   d ,  14   h . Preferably, an enlarged recess  14   f  in communication with groove  14   c  is also formed at end  14   d  to accommodate the end portion of the insulation sleeve on wire lead  18  to be received therein. 
     Another shallow groove  14   g  is preferably formed in seat  14   a  along the longitudinal axis of the semi-cylindrical terminal blocks along its entire length to facilitate the potting procedure. Potting of heater element  16  and terminal blocks  14  in casing  12  requires alumina-water (grit) compound  20  to be first deposited in casing  12  after which terminal blocks  14  are inserted in casing  12  along longitudinally extending ribs  12   e , followed by insertion of the heater element  16  into casing  12  between the shallow recessed faces  14   a  of terminal blocks  14 . The embodiment is then filled with grit to just cover the top surface of heater element  16 . The shallow grooves  14   g  along the entire length of terminal blocks  14  allow for easy insertion of heater element  16  by permitting grit to flow upwards during insertion of heater element  16  and downwards during the filling operation to ensure total surface area coverage of heater element  16 . 
     Heater element  16  is preferably a self-regulating positive temperature coefficient of resistivity (PTC) element of the type disclosed in U.S. Pat. No. 4,236,065, referenced above, the disclosure of which is incorporated herein by this reference. PTC element  16  is preferably formed generally in the configuration of a parallelepiped and of a size to fit in close thermally conductive relation with seating surface  14   a  of the terminal blocks spaced apart by ribs  12   e . A suitable electrically conductive coating  16   a  is applied to opposite face surfaces of the element in a known manner, one coating of which is shown in  FIG. 5 . The particular anomaly or Curie temperature of the PTC element is selected based on the particular application. For example, for certain compressor applications, a 120° C. Curie temperature element is used as in the referenced patent while in another compressor application a 160° C. Curie temperature element is employed. 
     As best seen in  FIG. 4 , first and second lead wires  18  having a respective spring contact  18   a  (one spring contact being shown) are inserted into respective grooves  14   c  of the terminal blocks with the spring contacts biased into electrical engagement with the facing conductive coatings of the PTC element. 
     In assembling the heater device, the terminal blocks, PTC element and lead attached spring contacts, i.e., the heater assembly, are inserted into the chamber of casing  12  and potted with an alumina-water (grit) compound  20  potting material. This assembly is then placed in a fixture and allowed to cure. After grit curing, the remaining space at the opening of the chamber is infilled with a suitable RTV polymer  22  to protect the heater assembly from moisture. 
     As noted above, casing  12  is formed with opposed retainer projections  12   d  at open end  12   c . The projections extend radially outwardly beyond side wall  12   a  of the casing a distance “a” (see  FIG. 5A ) selected so that the projections will extend into a recess formed in the well in which heater device  10  is to be disposed in use, as will be discussed further, below. 
     Preferably, the bottom surface  12   f  is inclined in an upward, outward direction to facilitate insertion of the casing through an opening to the well with a projection receiving recess portion formed adjacent to the opening and intermediate to the opening and a smooth tubular portion of the well which receives side wall  12   a  of casing  12 . In effect, the projections are compressed and cammed inwardly as the projections of the casing are inserted past the opening and into the recessed portion. Surface  12   f  is shown in the drawings as a curved surface but it will be appreciated that at least a portion thereof could extend in a straight direction as well. Also, as shown in the drawings (see FIG.  5 ), projections  12   d  extend longitudinally beyond the remainder of the opening at end  12   c . This enhances flexibility of the projections when inserting the projections through the opening into the well. Preferably, the top surface  12   g  of the retainer projections lie in a plane generally normal to the longitudinal axis of casing  12  or even slope upwardly in an outward direction from side wall  12   a  in order to lock casing  12  into the well. 
     Two projections  12   d  are shown in the drawings but it is within the purview of the invention to use a single annular projection or more than two projections, as desired. 
       FIGS. 7 and 8  show two examples of wells with which heater device  10  can be used. For purposes of illustration, only casing  12  of heater device  10  is shown to illustrate the locking action of projections  12   d . In  FIG. 7 , well  24  formed of suitable material such as copper or steel has an annular recess  24   a  adjacent to opening  24   b  and a straight cylindrical side wall portion  24   c  which receives side wall  12   b  of the casing in close heat conductive relationship therewith. Well  24  is suitably attached to the compressor housing by resistance welding, brazing or the like. The heater device is inserted into the well with projections  12   d  being bent and compressed inwardly as they pass opening  24   b  during insertion and then are received in recess  24   a  when the heater device is fully seated. 
     In  FIG. 8 , well  26  has an outwardly flared flange portion  26   a  at its open end which is suitably attached to the compressor housing in alignment with opening  28   b  of compressor housing wall  28   a . The diameter of opening  28   b  has a size generally corresponding to the inside diameter of side wall  26   c  which receives the side wall  12   a  of the casing. Thus, the space  26   d  between flange portion  26   a  and compressor housing  28   a  forms an annular projection receiving recess. Projections  12   d  are received in recess  26   d  by being bent and compressed as they pass opening  28   b  in the same manner as in the  FIG. 7  well. 
     Thus, in accordance with the invention, installation of heater device  10  into a compressor housing well is accomplished in a one step operation in which the device is inserted into the well without thermal grease and without a disparate retaining clip. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous objects attained. 
     Although the invention has been described with regard to a certain preferred embodiment thereof, variations and modifications will become apparent to those of ordinary skill in the art. It is therefore, the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.