Patent Publication Number: US-7215879-B2

Title: Defrosting heater with concentric glass tubes separated by end plugs

Description:
This Application is a U.S. National Phase Application of PCT International Application PCT/JP02/07426. 
     1. Technical Field 
     The present invention relates to a defrost heater for removing frosts sticking to cooling device of a refrigeration cycle in which an inflammable refrigerant is used. 
     2. Background Art 
       FIG. 9  is a cross sectional view showing part of a conventional defrost heater used in a refrigerator which uses an inflammable refrigerant, as disclosed in the Japanese Laid-open Patent No. H11-257831. A heater  100  made of a resistive metal is housed in a first glass tube  101 , which tube is further covered with a second glass tube  102  and a third glass tube  103 ; thus, it is formed in a multiple structure. 
     The multiple tube consisting of the first glass tube  101 , the second glass tube  102  and the third glass tube  103  is sealed at both ends with a rubber plug  104  so as to prevent an inflammable refrigerant from sneaking into inside of the glass tube. The air inside the first glass tube  101  is evacuated so that temperature of the glass surface does not become too high. The multiple-structured glass tube prevents surface temperature of the third glass tube  103 , which glass tube may be exposed to an environment of inflammable refrigerant, from reaching a combustible temperature of inflammable refrigerant. 
     Since the third glass tube  103  has a larger outer diameter, dispersion in the dimensions is great, and it has a larger contact area with the plug  104 . As a result, when attaching the plug  104  to the multiple glass tube, a force needed for insertion disperses wide in relation to the third glass tube  103 . If it is designed so that a necessary fitting strength can be secured with the fitting force at its lowest dispersion, a very high insertion force will be needed at the highest dispersion. This deteriorates the overall efficiency of assembly operation, and may result in an incomplete plug insertion to the glass tube, or even a damaged glass tube. 
     DISCLOSURE OF THE INVENTION 
     The present invention addresses the above problems and aims to offer a defrost heater comprising a multiple glass tube that can be attached to a plug with ease at high operational efficiency. A defrost heater in the present invention is used for heating the cooling device of a refrigeration cycle which uses an inflammable refrigerant, for the purpose of removing frosts sticking thereto. 
     A defrost heater of the present invention comprises a first glass tube; a second glass tube which covers around the first glass tube; a heater wire housed in the first glass tube; a plug made of an elastic material for covering the opening at both ends of the first and the second glass tubes, the plug having a cylindrical protrusion, the inner circumferential wall of the cylindrical protrusion is making a sealing contact with the outer surface of first glass tube while the outer circumferential wall of the cylindrical protrusion is making a sealing contact with the inner surface of second glass tube; and a lead wire going through the plug to be connected to the heater wire at the end portion. Wherein, strength of the sealing contact between the second glass tube and the plug is specified to be weaker than that between the first glass tube and the plug. 
     With the above-described configuration, dispersion of the force needed for inserting a plug to the glass tube can be reduced, while keeping the withdrawal strength at a certain specified level high enough for preventing a plug from withdrawing. Thereby, the defrost heaters can be manufactured through a smooth and efficient assembly operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view showing key portion of a defrost heater in accordance with a first exemplary embodiment of the present invention. 
         FIG. 2  is an exploded perspective view of the defrost heater, used to describe the assembly operation. 
         FIG. 3  is a perspective view showing a plug and a lead wire of the defrost heater. 
         FIG. 4  is a perspective view showing a plug of the defrost heater. 
         FIG. 5  is a perspective view showing a plug of the defrost heater. 
         FIG. 6  is a perspective view showing a plug of defrost heater in accordance with a second exemplary embodiment of the present invention. 
         FIG. 7  is a cross sectional view showing key part of a defrost heater in accordance with a third exemplary embodiment of the present invention. 
         FIG. 8  is a perspective view used to describe a method of assembling the defrost heater. 
         FIG. 9  is a cross sectional view showing key part of a conventional defrost heater. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Now in the following, the defrost heater is described in accordance with exemplary embodiments of the present invention, referring to the drawings. 
     EMBODIMENT 1 
       FIG. 1  is a cross sectional view showing a defrost heater in accordance with a first exemplary embodiment of the present invention.  FIG. 2  is an exploded perspective view of the defrost heater, used to describe the assembly operation.  FIG. 3  is a perspective view showing a plug and a lead wire of the defrost heater.  FIG. 4  is a perspective view showing other plug of the defrost heater.  FIG. 5  is a perspective view showing a still other plug of the defrost heater. 
     Referring to  FIG. 1  and  FIG. 2 , a heater wire  11  is coiled in the middle portion, accompanied by straight ends  11   a ,  11   b  having a certain specific length. A plug  12  is made of a silicone rubber or the like material that is superior in the heat resisting property and elasticity. It is provided with a cylindrical protrusion  12   a  for fixing the glass tube; diameter at the inner wall  12   b  is 9.6 mm, that at the outer wall  12   c  is 16.7 mm. A first glass tube  13  is a glass cylinder with the outer diameter 10.5 mm, which contains the heater wire  11  within inside. The first glass tube  13  is fitted with the plug along the inner wall  12   b . A second glass tube  14  is a glass cylinder with the inner diameter 17 mm, which houses the first glass tube  13  and fitted with the plug along the outer wall  12   c.    
     The first glass tube  13  has a longer overall length than the second glass tube  14 . Lead wire  15  is provided through the plug  12  at a lead wire hole  12   k  for making electrical connection with the heater  11 . A conductive connection terminal  16 , which is consisting of a caulking section  16   a  and a stopper section  16   b  which being an extension of the caulking section  16   a , is used for connecting the heater wire  11  and the lead wire  15 . The caulking section  16   a  electrically connects the heater wire  11  with the lead wire  15 ; while the stopper section  16   b , whose size is identical to or slightly smaller than the outer diameter of first glass tube  13 , sets a right positioning for the heater  11 . 
     The above-configured defrost heater is assembled in the following steps:
     (a) As illustrated in  FIG. 3 , the lead wire  15  is provided through the plug  12  for making a lead wire assembly  17 . Depending on needs, the lead wire  15  may be provided with an input connector and a protection tube.   (b) The lead wire assembly  17  is attached to the heater wire  11  at one end  11   a  (ref  FIG. 2 ), using a connection terminal  16  (not shown in  FIG. 2 ).   (c) The heater wire  11  is inserted in the first glass tube  13 , and the first glass tube  13  is fixed to the plug  12  in the right ( FIG. 2 ).   (d) The second glass tube  14  is fixed to the right plug  12  ( FIG. 2 ).   (e) The other end  11   b  of heater wire  11  is drawn out of first glass tube  13  to be connected to a lead wire assembly  17  via connection terminal  16  ( FIG. 2 ).   (f) Finally, a plug  12  for the left is attached to the first glass tube  13  and the second glass tube  14 , simultaneously.   

     Since inner diameter of the inner wall  12   b  of cylindrical protrusion  12   a  is 9.6 mm against the 10.5 mm outer diameter of the first glass tube  13 , it is fitted to the first glass tube  13  with a compression for 0.9 mm. Diameter of the outer wall  12   c , the original size of which being 16.7 mm, has been enlarged to 17.3 mm as a result of insertion of the first glass tube  13 ; so, it is fitted to the second glass tube  14 , whose inner diameter is 17 mm, with a compression for 0.3 mm. 
     The plug  12  needs to withstand a pulling force of approximately 50N so that it does not fall off a defrost heater during handling. The 50N pulling strength is secured by the first glass tube  13  which has been fitted to the plug with a higher compression, while the pulling strength provided by the second glass tube  14 , which has been fitted to the plug with a less compression, is approximately 10N. The pulling strength required for preventing the plug from falling off may be considered to be substantially identical to a strength needed for inserting a plug. 
     The tolerance allowed for the inner diameter of second glass tube  14  is ±0.2 mm; accordingly, the compression quantity may disperse in a range from 0.1 mm to 0.5 mm. It has been confirmed through experiments that it provides an insertion force of approximately 25N, when the compression quantity is 0.5 mm. It has also been confirmed that the insertion strength of second glass tube  14  reaches approximately 100N, when the compression quantity is approximately 1.0 mm. This indicates that the insertion strength per unit compression quantity becomes high as the result of an increasing compression quantity. 
     Therefore, it is known that a shift in the insertion strength is less when the compression quantity is in a low level. For example, when the compression quantity is in a level of 0.3 mm, the insertion strength shifts by approximately 5N at each change of 0.1 mm; whereas, when the compression quantity is in a level of 1.0 mm, the insertion strength shifts by approximately 20N at each change of 0.1 mm. 
     Thus, the dispersion in the insertion strength of plug  12  can be made smaller by specifying the compression quantity in relation to the second glass tube  14  to be smaller than that in relation to the first glass tube  13 . The reduced dispersion improves the efficiency of assembly operation. Namely, the operation efficiency improves when the strength of sealing contact between plug  12  and second glass tube  14  is smaller than that between plug  12  and first glass tube  13 . 
     The second glass tube  14  has greater dimensions, which implies a greater dimensional dispersion. In the same token, the outer wall  12   c  of plug  12  has greater dimensions, which means a greater dimensional dispersion. Therefore, dispersion in the compression quantity in relation to a second glass tube  14  is much greater than that in relation to a first glass tube  13 . In a case where it is designed to secure a minimum required pulling strength with a compression quantity at its smallest dispersion, the efficiency of assembly operation is impaired when the compression quantity increased. 
     In order to provide a favorable assembly efficiency, the fitting between the second glass tube  14  and the plug  12  may be designed within a small compression range, where the shift of insertion strength is less sensitive to a change in compression quantity. 
     Since the first glass tube  13  is longer than the second glass tube  14 , it is easy to insert a plug  12  in the left ( FIG. 2 ) to the first glass tube  13  and then to the second glass tube  14  consecutively. Providing a ring protrusion  12   d  on the outer surface  12   c , as illustrated in  FIG. 4 , is an effective measure for preventing a dispersion in the insertion strength from becoming wild, even when the compression quantity in relation to the second glass tube  14  dispersed. The ring protrusion  12   d  is compressed to make a contact sealing with the second glass tube  14 . Since the area of compression is small, dispersion in the insertion strength remains reasonable even when the second glass tube  14  has a substantially great dimensional dispersion. If the height h of ring protrusion  12   d  is specified to be greater than a tolerance in the inner diameter of second glass tube  14 , a contact sealing with the second glass tube  14  can be accomplished by a compression in the ring protrusion  12   d  alone, without the cylindrical protrusion  12   a  being compressed. Thus a dispersion in the insertion strength can be reduced. 
     It is not essential for the ring protrusion  12   d  to be disposed to cover the entire circumference of cylindrical protrusion  12   a ; it may take a shape of partial ring, or it may be provided in a plurality, for generating the same effect. 
     Furthermore, a dispersion in the insertion strength in relation to the second glass tube  14  may be reduced also by means of a hollow or a hole  12   e  provided in the end-face of cylindrical protrusion  12   a , as shown in  FIG. 5 . The hole  12   e  disposed at a location close to the outer circumference  12   c  makes the surface to have more elasticity. 
     As a result, it contributes to weaken the insertion strength of the second glass tube  14 , and dispersion of the insertion strength becomes smaller. The hollow  12   e  can either be a circular groove or a partial groove. 
     EMBODIMENT 2 
       FIG. 6  is a perspective view showing a plug  12  of defrost heater in accordance with a second exemplary embodiment of the present invention. The plug  12  has two cylindrical protrusions  12   f ,  12   g  disposed concentric, as shown in  FIG. 6 . The plug  12  supports the first glass tube  13  and the second glass tube  14  at the inner circumference  12   h  and  12   j , respectively. 
     Since each of the respective glass tubes is supported by an independent cylindrical protrusion, an inserted first glass tube  13  does not influence a force needed to insert a second glass tube  14 . Thus, dispersion in the force needed for inserting the second glass tube  14  is reduced, and the assembly efficiency is improved. 
     EMBODIMENT 3 
       FIG. 7  is a cross sectional view showing part of a defrost heater in accordance with a third exemplary embodiment of the present invention.  FIG. 8  is a perspective view used to describe a method of assembling the defrost heater. In  FIG. 7  and  FIG. 8 , those constituent parts identical to those of the foregoing embodiments are represented by using the same symbols, and detailed description of which is eliminated. 
     A first plug  18  is made of a silicone rubber or the like material superior in the heat withstanding property and the elasticity. The first plug  18  supports the first glass tube  13  with the cylindrical protrusion  18   a . A second plug  19  is made of said silicone rubber or a heat-resistive plastic material, and supports the second glass tube  14  with a cylindrical protrusion  19   a . The second plug  19  has a slit  19   d  provided from the outer circumference  19   b  towards the central portion  19   c , which slit  19   d  allows a lead wire  15  to go through when it is attached to the first plug  18 . 
     The above-configured defrost heater is assembled through the same process steps as described in the embodiment  1 , excluding the second glass tube  14  and the second plug  19 . Thereafter, a second glass tube  14  is attached to the second plug  19  to complete a finished defrost heater, as shown in  FIG. 8 . 
     The second plug  19  is attached to the first plug  18  at a section  18   c , which is a place irrelevant to mounting of the first glass tube  13 . Therefore, an already mounted first glass tube  13  does not ill-affect the operation of mounting a second glass tube  14 . So, efficiency of the assembly operation is improved. 
     Furthermore, since resistance value and electrical conduction of the heater wire can be inspected before a second glass tube  14  is mounted, there will be a greater freedom in the manufacturing process flow. Defrost heaters for use in conventional refrigerators, which refrigerators do not use inflammable refrigerant, employ only the first glass tube alone. 
     The defrost heaters in the present embodiment can be manufactured on an assembly line for conventional defrost heaters, by just adding an operation step for mounting the second glass tube. 
     INDUSTRIAL APPLICABILITY 
     The present invention offers defrost heaters for use in the refrigerator that employs an inflammable refrigerant, which defrost heaters can be assembled with ease at high efficiency.