Abstract:
A defrosting heater in a refrigerator with a flammable refrigerant sealed therein, including a first glass tube ( 53 ) having a heater wire ( 52 ) inside thereof, a plug ( 58 ) covering both end opening parts of the first glass tube ( 53 ) and a second glass tube ( 54 ), a lead wire ( 55 ) piercing the plug ( 58 ) and connected to an end of the heater wire ( 52 ), and a positioning plate ( 57 ) disposed on a connection part of the lead wire ( 55 ) with the heater wire ( 52 ). Flame propagation can be prevented by setting the size of a space ( 58   b ) formed by the plug ( 58 ) and the positioning plate ( 59 ) according to the sealing quantity of the flammable refrigerant and the surface temperature of the heater wire ( 52 ), and an unstable state in a defrosting mode is prevented even when the flammable refrigerant leaks.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a defrosting heater in a refrigerator or the like for removing frost sticking and depositing on a cooler of refrigeration cycle packed with flammable refrigerant, and a refrigerator having such heater.  
         BACKGROUND ART  
         [0002]    [0002]FIG. 16 is a sectional view of a conventional refrigerator disclosed in Japanese Laid-open Patent No. H8-54172. A refrigerator main body  1  comprises a freezing compartment  2 , a refrigerating compartment  3 , and a cooling section  20 . The cooling section  20  incorporates an evaporator  10  cooled by circulation of refrigerant, and a defrosting heater  15  having a Nichrome wire coil covered with a glass tube.  
           [0003]    A fan  11  sucks air into the cooling section  20  from the freezing compartment  2  and refrigerating compartment  3  through a freezing compartment suction port  7  and a refrigerating compartment suction port  8  for cooling the air by exchanging heat with the evaporator  10 . The fan  11  sends the cooled air into the freezing compartment  2  through a diffusion port  9 . The cooled air is also distributed into the refrigerating compartment  3  from the freezing compartment  2  through the passage not shown. When the air sucked into the cooling section  20  exchanges heat with the evaporator  10 , the moisture in the air is frosted and sticks to the evaporator  10 .  
           [0004]    Before the frost deposit begins to lower the cooling capacity of the refrigerator, the frost is thawed by applying current to the Nichrome wire of the defrosting heater  15 . As the Nichrome wire is energized, heat rays are emitted from the Nichrome wire to the evaporator  10  and peripheral parts through the glass tube. Heat rays emitted to a bottom plate  17  are reflected to peripheral parts including the evaporator  10  and defrosting heater  15 . Heat rays thaw the frost deposits on the evaporator  10 , a gutter  13  and drain port  14 . A roof  16  is provided to protect the defrosting heater  15  from thawing water. The thawing water drops into the gutter  13 , and discharged outside of the refrigerator through the drain port  14 .  
           [0005]    In the conventional constitution, however, the surface temperature of the glass tube of the defrosting heater  15  is always very high temperature. Further, the bottom plate  17  is located near the defrosting heater  15 , and part of the heat rays radiated from the defrosting heater  15  are reflected again to the defrosting heater  15 , and hence the glass tube temperature rises abnormally high, possibly exceeding the ignition point of the flammable refrigerant.  
           [0006]    Hence, when the flammable refrigerant is used, it is an important problem that the defrosting heater  15  should never be source of ignition due to supply of power even if the flammable refrigerant should leak out from the evaporator or the piping installed in the portion communicating with the inside of the refrigerator.  
         DISCLOSURE OF THE INVENTION  
         [0007]    In the light of the above problems, it is hence an object of the invention to present a defrosting heater of high safety even in the case of defrosting in an environment of the flammable refrigerant leaking into the atmosphere of installation of the defrosting heater.  
           [0008]    The defrosting heater of the invention is a defrosting heater for heating and removing frost deposits on the cooler in the refrigeration cycle packed with a flammable refrigerant, comprising a glass tube, a heater wire of metal resistance element installed in the glass tube, a plug covering both end openings of the glass tube, a lead wire penetrating through the plug and connected to the end of the heater wire, and a positioning plate disposed at the junction of the heater wire and lead wire and held by the plug for preventing the junction from moving, in which the size of the gap formed between the plug and the positioning plate is set depending on the packing amount of the flammable refrigerant, and therefore if the flammable refrigerant passes through the gap formed between the plug and the positioning plate and invades into the heater wire side and is ignited, the gap formed between the plug and the positioning plate is set in a size not to allow the flame to propagate, so that the safety is guaranteed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is an essential sectional view of a defrosting heater in a first embodiment of the invention.  
         [0010]    [0010]FIG. 2 is a perspective sectional view of the defrosting heater.  
         [0011]    [0011]FIG. 3 is a schematic diagram of a refrigerating system of a refrigerator using the defrosting heater.  
         [0012]    [0012]FIG. 4 is a perspective view showing an example of cylindrical protrusion of a plug of the defrosting heater.  
         [0013]    [0013]FIG. 5 is an essential sectional view showing an example of a positioning plate of the defrosting heater.  
         [0014]    [0014]FIG. 6 is an essential perspective sectional view of the defrosting heater.  
         [0015]    [0015]FIG. 7 is a perspective view showing the groove shape of a cylindrical protrusion of the defrosting heater.  
         [0016]    [0016]FIG. 8 is an essential sectional view of a defrosting heater in a second embodiment of the invention.  
         [0017]    [0017]FIG. 9 is a sectional view showing a state of using a plug of other shape in the defrosting heater.  
         [0018]    [0018]FIG. 10 is a perspective view showing a different shape of the plug of the defrosting heater.  
         [0019]    [0019]FIG. 11 is a perspective view showing a different shape of the plug of the defrosting heater.  
         [0020]    [0020]FIG. 12 is a perspective view showing a different shape of the plug of the defrosting heater.  
         [0021]    [0021]FIG. 13 is a perspective view showing a different shape of the plug of the defrosting heater.  
         [0022]    [0022]FIG. 14 is an essential sectional view of a defrosting heater in a third embodiment of the invention.  
         [0023]    [0023]FIG. 15 is an essential perspective view of the defrosting heater.  
         [0024]    [0024]FIG. 16 is a schematic sectional view of a refrigerator having a conventional defrosting heater. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    Referring now to the drawings, preferred embodiments of the invention are described specifically below.  
         [0026]    (First Embodiment)  
         [0027]    [0027]FIG. 1 is an essential sectional view of a defrosting heater in a first embodiment of the invention, and FIG. 2 is a perspective sectional view of the defrosting heater.  
         [0028]    In FIG. 1 and FIG. 2, reference numeral  51  is a defrosting heater for heating, thawing and removing frost deposits on an evaporator  10 , and numeral  52  is a heater wire of resistance wire formed in a coil, having a connection end  52   a  folding and twisting the heater wire by a specified length, instead of coil shape, near the both ends of the heater wire  52 . Reference numeral  53  is a first glass tube covering the heater wire  52 , having a cylindrical shape of 10.5 mm in outside diameter and 8.5 mm in inside diameter, with both ends being opened.  
         [0029]    Reference numeral  54  is a second glass tube covering the first glass tube  53 , having a cylindrical shape of 20 mm in outside diameter and 17 mm in inside diameter, with both ends being opened. The overall length of the first glass tube  53  is longer than the overall length of the second glass tube by 17 mm, and when laid down by matching the middle point of each overall length, the end face of the first glass tube  53  projects from the end face of the second glass tube  54  by 8.5 mm.  
         [0030]    Reference numeral  55  is a lead wire connected to the heater wire  52 , and numeral  56  is a conductive coupling pipe coupling the heater wire  52  and lead wire  55 .  
         [0031]    Reference numeral  57  is a circular positioning plate, having a central hole  57   a  for inserting the coupling pipe  56 , and three air vents  57   b  (1.5 mm in diameter) are disposed around the hole  57   a  at intervals of 120 degrees of central angle from the center of the hole  57   a . The outside diameter of the positioning plate  57  is same as or slightly smaller than the outside diameter of the first glass tube  53 .  
         [0032]    To connect the heater wire  52  and lead wire  55 , first, the heater wire  52  is inserted into the first glass tube  53 , and the coupling pipe  56  is put into the hole  57   a  of the positioning plate  57 , and inserted until the positioning plate  57  comes to the central position of the coupling pipe  56 . The end portion of the heater wire  52  is inserted from one opening end of the coupling pipe  56 , and the end portion of the lead wire  55  is inserted from other opening end of the coupling pipe  56 , and the both ends of the coupling pipe  56  are crimped by a tool with care not to deform the positioning plate  57 . As a result, the end portion of the heater wire  52  and end portion of the lead wire  55  are coupled together by the coupling pipe  56 , and the positioning plate  57  does not slip out by deformation of the both ends of the coupling pipe  56 .  
         [0033]    Reference numeral  58  is a silicone rubber plug covering the opening end of the first glass tube  53  and second glass tube  54 . The plug  58  has a lead wire insertion hole  58   a  for inserting the lead wire  55 , and preferably the lead wire  55  should be inserted into the plug  58  before the end portion of the lead wire  55  is crimped by the coupling pipe  56 . Reference numeral  58   b  is a gap formed between the positioning plate  57  and plug  58 .  
         [0034]    The plug  58  has a cylindrical protrusion  59 , and the diameter of its inner circumference  59   a  is smaller than the outside diameter of the first glass tube  53  by about 1 mm, and the diameter of the outer circumference  59   b  is same as the inside diameter of the second glass tube  54 . Accordingly, when fitting the plug  58  into the opening end of the first glass tube  53  and second glass tube  54 , the first glass tube  53  is slightly press-fitted into the inner circumference  59   a , and the outer circumference  59   b  is slightly expanded, and the outer circumference  59   b  is slightly press-fitted into the second glass tube  54 .  
         [0035]    The positioning plate  57  is interposed between the end face of the first glass tube  53  and inner wall  59   c  of the cylindrical protrusion  59 , and the outer peripheral edge of the positioning plate  57  contacts tightly with the inner circumference  59   a  of the cylindrical protrusion  59 . The outside diameter of the positioning plate  57  is same as or slightly smaller than the outside diameter of the first glass tube  53 , and therefore the positioning plate  57  will not get inside of the first glass tube  53 .  
         [0036]    A lead wire insertion hole  58   a  of the plug  58  penetrates through the inner wall  59   c  of the cylindrical protrusion  59 , and gas can come in and out from the gap between the lead wire  55  and lead wire insertion hole  58   a  to the inner wall  59   c  of the cylindrical protrusion  59 .  
         [0037]    The gas coming into the inner wall  59   c  of the cylindrical protrusion  59  passes through the air vents  57   b  of the positioning plate  57 , and invades into the first glass tube  53 , and contacts with the heater wire  52 . The sectional area at an arbitrary position of the clearance between the lead wire insertion hole  58   a  formed in the plug to cover the both end openings of the glass tube  53  and the outside diameter of lead wire  55  passing through this insertion hole  58   a  is 7.1 square millimeters or less.  
         [0038]    In the lead wire insertion hole  58   a , the lead wire  55  and the coupling pipe (junction) are disposed in a total length of at least 6 mm along the insertion hole  58   a.    
         [0039]    [0039]FIG. 3 is a schematic diagram of a refrigerating system of a refrigerator using the defrosting heater of the first embodiment of the invention, and in FIG. 3, reference numeral  60  is a compressor,  61  is a condenser, and numeral  62  is a pressure reducing mechanism, and the compressor  60 , condenser  61 , pressure reducing mechanism  62 , and evaporator  10  are functionally connected to form a refrigeration cycle, which is packed with flammable refrigerant.  
         [0040]    By operation of the compressor  60 , the evaporator  10  of the refrigeration cycle is cooled, and by the fan  11  operating simultaneously with the operation of the compressor  60 , the compartment air of the refrigerator passes through the cooled evaporator  10 , and cold air exchanged of heat with the evaporator  10  is diffused into the compartment. After a specific time of operation of the compressor  60 , the operation of the compressor  60  is stopped. At the same time, power is supplied to the heater wire  52  through the lead wire  55 , the heater  52  is heated.  
         [0041]    As the heater wire  52  generates heat, part of radiant heat ray directly passes to outside, but the remainder is transferred to the first glass tube  53  and second glass tube  54 , and the surface of the second glass tube  54  rises to a temperature less than the ignition point of the flammable refrigerant, thereby defrosting the peripheral parts.  
         [0042]    In the inner space of the first glass tube  53 , at this time, the gas is expanded by temperature rise, and flows outside from the gap between the lead wire  55  and lead wire insertion hole  58   a  through the air vents  57   a  of the positioning plate  57 .  
         [0043]    In this state, by stopping power supply to the heater wire  52 , when cooling is started again, the inside of the first glass tube  53  is reduced in pressure by temperature decline, and the external air surrounding the defrosting heater  51  passes through the gap between the lead wire  55  and lead wire insertion hole  58   a , and flows into the first glass tube  53  through the air vents  57   a  of the positioning plate  57 .  
         [0044]    In this situation, in the event of flammable refrigerant existing around the defrosting heater  51 , the flammable refrigerant may flow into the inner space of the first glass tube  53 , and the flammable refrigerant may be ignited by heat generation of the heater wire  52  upon start of defrosting.  
         [0045]    However, if the flammable refrigerant flowing into the first glass tube  53  is ignited, there is no problem in safety so far as the flame does not propagate by passing over the air vents  57   b  of the positioning plate  57 , and therefore, in this embodiment, the area of the air vents  57   b  of the positioning plate  57  is defined in a size not allowing the flame to propagate. More specifically, it has been confirmed that there is no danger although the surface temperature of the heater wire  52  reaches up to 590° C. in the atmosphere of the flammable refrigerant existing by 3.0 percent by volume, on condition that the both ends of the first glass tube  53  are closed with the plugs  58  in the normal state, but the positioning plates  57  are removed from both ends of the first glass tube  53  so that the heater is set in open state (opening area being about 57 square millimeters), and 110 V is applied to both ends of the heater wire  52 .  
         [0046]    Therefore, even if the gas moves through the air vents  57   b  of the positioning plate  57 , the sum of three areas of the air vents  57   b  of 1.5 mm in diameter is about 5.3 square millimeters, and there is no risk of explosion. In this specification, it has been confirmed that there is no risk even 170 V is applied to both ends of the heater wire  52  and the surface temperature of the heater wire  52  is raised up to 613° C.  
         [0047]    Further, if the air vents  57   b  are assembled into one and the diameter is expanded to 3 mm (an area of 7.1 square millimeters), freedom from risk is confirmed.  
         [0048]    Hence, even if there is flammable refrigerant around the defrosting heater  51 , accidents due to propagation of flame can be prevented.  
         [0049]    In this embodiment, air vents  57   a  are formed in the positioning plate  57 , but not limited to this example, for example, without forming air vents  57   a , the air vents  57   a  may be replaced by a gap formed between the outer peripheral edge of the positioning plate  57  and the inner circumference  59   a  of the cylindrical protrusion  59 .  
         [0050]    Also in the embodiment, the outer circumference of the cylindrical protrusion  59  is circular, but it may be also formed in a corrugated shape, for example, as shown in FIG. 4. In FIG. 4, reference numeral  63  is a plug having a same function as the plug  58 , numeral  64  is a cylindrical protrusion provided in the plug  63 , an inner circumference  64   a  is slightly press-fitted into the outer circumference of the first glass tube  53 , and an outer circumference  64   b  is also slightly press-fitted into the inner circumference of the second glass tube  54 . At this time, since the outer circumference  64   b  is formed on corrugation, the compressed top  64   c  moves to the bottom  64   d  to be fitted well, and it is easy to assemble and the working efficiency is enhanced.  
         [0051]    Further, when the top  64   c  of the outer circumference  64   b  is compressed and moved to the bottom  64   d , if a gap is formed between the bottom  64   d  and inner circumference of the second glass tube  54 , as far as the size of the gap is set to such an extent not to allow propagation of the flame preliminarily depending on the packed amount of the flammable refrigerant, if power is supplied to the heater wire  52  in order to defrost in an atmosphere filled with leaking flammable refrigerant, the flammable refrigerant invading from the gap between the bottom  64   d  of the outer circumference  64   b  and the inner circumference of the second glass tube  54  is not ignited to propagate the flame to outside, so that the safety is guaranteed.  
         [0052]    In the defrosting heater  51  in which the sectional area is 7.1 square millimeters at an arbitrary position in the clearance between the lead wire insertion hole  58   a  formed in the plug covering the both end openings of the glass tube  53  and the outside diameter of lead wire  55  passing through the insertion hole  58   a . Even if the flammable gas flows into the glass tube  53  and is ignited in the glass tube  53  when the heater is energized, by defining the sectional area of the clearance of the lead wire insertion hole  58   a  at less than a specified value, ignition outside of the glass tube  53  and propagation of flame can be prevented, and the defrosting heater  51  of high safety is realized.  
         [0053]    It is a further feature of the defrosting heater  51  that the lead wire  55  and the coupling pipe (junction)  56  are disposed in a total length of at least 6 mm along the insertion hole  58   a  in the lead wire insertion hole  58   a . Even if the flammable gas flows into the glass tube  53  and is ignited in the glass tube  53  when the heater is energized, by defining the total length of junction  56  of connecting the lead wire  55  and heater wire  52  at more than a specific length, ignition outside of the glass tube  53  and propagation of flame can be prevented, and the defrosting heater  51  of high safety is realized.  
         [0054]    In the embodiment, the positioning plate  57  has air vents  57   b , but it may be also provided with a sleeve having air vents as shown in FIG. 5. In FIG. 5, reference numeral  70  is a positioning plate having a same function as the positioning plate  57 , and a sleeve  71  penetrating through the positioning plate  70  has air vents  71   a . By properly defining the position of the sleeve  71 , the sleeve  71  is slightly press-fitted into the inner circumference of the first glass tube  53 , and it is easier to hold the positioning plate  70 , so that the working efficiency is enhanced. Moreover, even if the leaking flammable refrigerant is ignited by the heater wire  52 , since the air vents  71   a  passing through the sleeve  71  are long in creeping distance, and flame cannot propagate through the air vents  71   a , so that the safety is guaranteed.  
         [0055]    By adjusting the sleeve length and pore diameter, characteristic of flame propagation can be changed easily. The positioning plate may be formed in a wire mesh structure as shown in FIG. 6. In FIG. 6, reference numeral  80  is a positioning plate having a same function as the positioning plate  57 , and is formed of at least 20 meshes of wire in order to prevent flame propagation.  
         [0056]    Having a central hole  82  for inserting a coupling pipe  81 , the outside of the positioning plate  80  is same as or slightly smaller than the outside diameter of the first glass tube  53 . Reference numeral  53   a  is one end face of the first glass tube  53 . Reference numeral  52  is a heater wire forming a resistance wire in a coil, having a connection end  52   a  folding and twisting the heater wire by a specified length, instead of coil shape, near the both ends of the heater wire  52 . Reference  53  is a first glass tube covering the heater wire  52 , having a cylindrical shape of 10.5 mm in outside diameter and 8.5 mm in inside diameter, with both ends being opened. The glass tube end face  53   a  and the coil heater wire  52  keeps a distance of at least 20 mm by way of the connection end  52   a . As a result, the heater wire  52  as heat source may be set apart from the positioning plate  80 , and since the positioning plate  80  is formed of at least 20 meshes of wire, if the heater wire  52  is energized for defrosting in the atmosphere of leaking flammable refrigerant, the invading flammable refrigerant is not ignited to propagate the flame to outside, so that there is no problem in safety.  
         [0057]    Still more, since the positioning plate  80  is a wire mesh structure of at least 20 meshes or more, the exhaust resistance when the moisture invading into the glass tube  53  is evaporated and discharged is smaller than the case of air vent structure, and it can be discharged efficiently, so that rusting of heater wire due to stagnant moisture can be prevented.  
         [0058]    In the embodiment, the outer circumference of the cylindrical protrusion  59  is circular, but it may be grooved as shown in FIG. 7. In FIG. 7, reference numeral  90  is a plug having a same function as the plug  58 , numeral  91  is a cylindrical protrusion provided in the plug  90 , and an inner circumference  91   a  is slightly press-fitted into the outer circumference of the first glass tube  53 , and an outer circumference  91   b  is slightly press-fitted into the inner circumference of the second glass tube  54 . At this time, a groove  92  is formed in the outer circumference  91   b , and hence its flexibility is enhanced, and it is easier to assemble and the working efficiency is enhanced.  
         [0059]    The sectional area of the groove  92  is 7.1 square millimeters or less, and if a gap equivalent to the sectional area is produced against the inner circumference of the second glass tube  54 , when the heater wire  52  is energized for defrosting in an atmosphere of invading flammable refrigerant, the invading flammable refrigerant is not ignited to propagate flame to outside, so that there is no problem in safety.  
         [0060]    In the embodiment, the glass tube covering the heater wire  52  of the defrosting heater  51  is a double structure of first glass tube  53  and second glass tube  54 , but it may be formed in a single glass tube, and the resistance value of the heater wire and the watt density per unit may be adjusted so that the surface temperature of the glass tube may be less than the ignition temperature of the flammable refrigerant. In the case of single glass tube, the cost can reduced as compared with the double structure.  
         [0061]    (Second Embodiment)  
         [0062]    [0062]FIG. 8 is an essential sectional view of a defrosting heater in a second embodiment of the invention. Same parts as in the first embodiment are identified with same reference numerals and detailed description is omitted.  
         [0063]    In FIG. 8, reference numeral  100  is a plug having same function as the plug  58  in the first embodiment, which comprises a plug main body  101  and a cylindrical protrusion  102  provided in the plug main body  101 , and the inner circumference  102   a  of the cylindrical protrusion  102  is slightly press-fitted into the outer circumference of the first glass tube  53 , and the outer circumference  102   b  is also slightly press-fitted into the inner circumference of the second glass tube  54 .  
         [0064]    Reference numeral  103  is a passage penetrating through the plug main body  101  in the longitudinal direction of the cylindrical protrusion  102 . Reference numeral  104  is a space formed by the first glass tube  53 , second glass tube  54 , and plug  100 .  
         [0065]    In the defrosting heater having such constitution and the refrigerator having this defrosting heater, the operation is described below. By operation of the compressor  60 , the evaporator  10  of the refrigeration cycle is cooled, and by the fan  11  operating simultaneously with the operation of the compressor  60 , the compartment air of the refrigerator passes through the cooled evaporator  10 , and cold air exchanged of heat with the evaporator  10  is diffused into the compartment. After a specific time of operation of the compressor  60 , the operation of the compressor  60  is stopped. At the same time, power is supplied to the heater wire  52  through the lead wire  55 , and the heater  52  is heated.  
         [0066]    As the heater wire  52  generates heat, part of radiant heat ray directly passes to outside, but the remainder is transferred to the first glass tube  53  and second glass tube  54 , and the surface of the second glass tube  54  rises to a temperature less than the ignition point of the flammable refrigerant, thereby defrosting the peripheral parts.  
         [0067]    In the space  104  formed by the first glass tube  53 , second glass tube  54 , and plug  100 , at this time, the gas is expanded by temperature rise, and flows outside from the passage  103 .  
         [0068]    In this state, by stopping power supply to the heater wire  52 , when cooling is started again, the space  104  is reduced in pressure by temperature decline, and the surrounding external air containing moisture flows into the space  104  through the passage  103 .  
         [0069]    By supplying power again to the heater wire  52  to heat the heater wire  52 , the space  104  is raised in temperature and the moisture is evaporated, and the pressure in the space  104  begins to rise again by the steam. However, since part of the steam flows outside through the passage  103 , the pressure rise in the space  104  is alleviated.  
         [0070]    By this action, breakage of the first glass tube  53  and second glass tube  54  by pressure rise due to steam evaporation can be prevented, and safety is assured.  
         [0071]    If the flammable refrigerant leaks into the refrigerator compartment and the flammable refrigerant flows into the space  104 , as explained in the first embodiment, as far as the sectional area of the passage for circulation of the flammable refrigerant is not more than 7.1 square millimeters, if the flammable refrigerant is ignited, the flame does not propagate and explosion does not take place, and hence explosion is prevented by setting the maximum sectional area of the passage  103  at 7.1 square millimeters or less.  
         [0072]    In this embodiment, the passage  103  is a full tubular form, but it may be formed like a groove as shown in FIG. 9. In FIG. 9, reference numeral  200  is a plug having same function as the plug  100 , which comprises a plug main body  201  and a cylindrical protrusion  202 , and the inner circumference  202   a  of the cylindrical protrusion  202  is slightly press-fitted into the outer circumference of the first glass tube  53 , and the outer circumference  202   b  is also slightly press-fitted into the inner circumference of the second glass tube  54 . The end face of the second glass tube  54  is stopped at a position about 1 mm apart from the plug main body  201 . The outer circumference  202   b  of the cylindrical protrusion  202  has a groove  203  extending in the longitudinal direction from the root to the tip, and a passage  204  is formed by the second glass tube  54  and the groove  203 .  
         [0073]    Or the end face of the second glass tube  54  may be stopped at a specified position by forming positioning means as shown in FIG. 10 and FIG. 11.  
         [0074]    In FIG. 10, reference numeral  300  is a plug having same function as the plug  100 , which comprises a plug main body  301  and a cylindrical protrusion  302 , and the inner circumference  302   a  of the cylindrical protrusion  302  is slightly press-fitted into the outer circumference of the first glass tube  53 , and the outer circumference  302   b  is also slightly press-fitted into the inner circumference of the second glass tube  54 .  
         [0075]    Bumps  302   c  are provided at the root of the cylindrical protrusion  302 , and the bumps  302   c  are disposed at intervals of 90 degrees around the central axis of the cylindrical protrusion  302 , and project from the root of the cylindrical protrusion  302  by 1 mm in the longitudinal direction. Since the end face of the second glass tube  54  is positioned by the bumps  302   c , the end face of the second glass tube  54  is stopped at a position apart from the plug main body  301  by about 1 mm.  
         [0076]    The outer circumference  302   b  of the cylindrical protrusion  302  has a groove  303  extending in the longitudinal direction from the root to the tip, and a passage  304  is formed by the second glass tube  54  and the groove  303 . In FIG. 11, reference numeral  400  is a plug having same function as the plug  100 , which comprises a plug main body  401  and a cylindrical protrusion  402 , and the inner circumference  402   a  of the cylindrical protrusion  402  is slightly press-fitted into the outer circumference of the first glass tube  53 , and the outer circumference  402   b  (second outer circumference) is also slightly press-fitted into the inner circumference of the second glass tube  54 . The range of 1 mm in the longitudinal direction from the root of the cylindrical protrusion  402  is formed in an outer circumference  402   c  (first outer circumference) larger in diameter than the inside diameter of the second glass tube  54 , and the end face of the second glass tube  54  is positioned by a step portion formed between the outer circumference  402   b  and outer circumference  402   c , and hence the end face of the second glass tube  54  is stopped at a position about 1 mm apart from the plug main body  401 .  
         [0077]    The outer circumference  402   b  and outer circumference  402   c  of the cylindrical protrusion  402  have a groove  403  extending in the longitudinal direction from the root to the tip, and a passage  404  is formed by the second glass tube  54  and the groove  403 .  
         [0078]    Or, as shown in FIG. 12, a groove may be formed in the plug main body. In FIG. 12, reference numeral  500  is a plug having same function as the plug  100 , which comprises a plug main body  501  and a cylindrical protrusion  502 , and the inner circumference  502   a  of the cylindrical protrusion  502  is slightly press-fitted into the outer circumference of the first glass tube  53 , and the outer circumference  502   b  is also slightly press-fitted into the inner circumference of the second glass tube  54 .  
         [0079]    The outer circumference  502   b  of the cylindrical protrusion  502  has a groove  503  extending in the longitudinal direction from the root to the tip, the plug main body  501  has a groove  504  extending in the perpendicular direction, crossing with the groove  503 , and a passage  505  is formed by the second glass tube  54  and the groove  503  and groove  504 .  
         [0080]    Thus, by forming grooves in the plug, in the space  104  formed by the first glass tube  53 , second glass tube  54 , and plug, if the air in the space  104  is expanded by heat generation of the heater wire  52  and the pressure is elevated, the gas flows out through the groove, and the pressure elevation in the space  104  is lessened, and hence rupture of the first glass tube  53  and second glass tube  54  is prevented.  
         [0081]    Or, as shown in FIG. 13, a groove may be formed in the inner circumference of the cylindrical protrusion. In FIG. 13, reference numeral  600  is a plug having same function as the plug  100 , which comprises a plug main body  601  and a cylindrical protrusion  602 , and the inner circumference  602   a  of the cylindrical protrusion  602  is slightly press-fitted into the outer circumference of the first glass tube  53 , and the outer circumference  602   b  is also slightly press-fitted into the inner circumference of the second glass tube  54 . The inner circumference  602   a  of the cylindrical protrusion  602  has a groove  603  extending in the longitudinal direction from the root to the tip.  
         [0082]    The groove  603  is coupled to a lead wire insertion hole  601   a , and a passage  604  is formed by the first glass tube  54 , lead wire insertion hole  601   a  and the groove  603 . A plurality of grooves  603  may be also provided in the inner circumference  602   a . In this configuration, the gas in the spacer  104  can be moved by way of the lead wire insertion hole  601   a  and passage  604 , and further since the gas flow inlet of the passage  113  is not visible from outside of the plug main body, and it is preferred from the viewpoint of the design.  
         [0083]    Thus, by forming grooves in the plug, in the space  104  formed by the first glass tube  53 , second glass tube  54 , and plug, if the air in the space  104  is expanded by heat generation of the heater wire  52  and the pressure is elevated, the gas flows out through the groove, and the pressure elevation in the space  104  is lessened, and hence rupture of the first glass tube  53  and second glass tube  54  does not take place, and moreover since the surface of the heater wire  52  and first glass tube  53  is not exposed to the atmosphere, even if the flammable refrigerant leaks in the refrigeration cycle packed with the flammable refrigerant, flame propagation leading to exposure does not take place, and the safety is guaranteed.  
         [0084]    (Third Embodiment)  
         [0085]    [0085]FIG. 14 is an essential sectional view of a defrosting heater in a third embodiment of the invention, and FIG. 15 is an essential perspective view of the defrosting heater of the embodiment. Same parts as in the foregoing embodiments are identified with same reference numerals and detailed description is omitted.  
         [0086]    In FIG. 14 and FIG. 15, reference numeral  700  is a plug having same function as the plug  100  in the second embodiment, which comprises a plug main body  701  and a cylindrical protrusion  702  provided in the plug main body  701 .  
         [0087]    Reference numeral  703  is a passage penetrating through the plug main body  701  in the longitudinal direction of the cylindrical protrusion  702 . Reference numeral  104  is a space formed by the first glass tube  53 , second glass tube  54 , and plug  700 .  
         [0088]    Reference numeral  705  is a shade held on the plug main body  701  of the plug  700  positioned above in the perpendicular direction of the second glass tube  54 , and it prevents water drops falling from the evaporator from hitting directly the surface of the second glass tube  54 .  
         [0089]    Near the both ends  705   a  of the shade  705 , there is a holding part  705   b  formed in a convex shape in a smaller width than in other regions. The holding part  705   b  is inserted into a holding hole  704  provided in the top of the plug main body  701 .  
         [0090]    At the edge of the shade  705 , a draining wall  705   c  is provided along the longitudinal direction, and water dropping from the evaporator is prevented from flowing into the inside of the shade  705 .  
         [0091]    When the height H of the draining wall  705   c  of the shade  705  is too high, gas is likely to stay between the shade  705  and the second glass tube  54 , and the surface temperature of the second glass tube  54  is raised due to temperature rise of the stagnant gas at the time of heat generation of the heater wire  52 .  
         [0092]    In particular, in the refrigeration cycle packed with flammable refrigerant, in order to assure safety even if the flammable refrigerant leaks, it is preferred to set the surface temperature of the second glass tube  54  at less than the ignition temperature of the flammable refrigerant. Accordingly, the height H of the draining wall  705   c  should be as low as possible so that gas may hardly stay between the shade  705  and second glass tube  54 .  
         [0093]    In this embodiment, the height H of the draining wall  705   c  is set at 0.5 mm or more to 5 mm or less, and stagnant gas is suppressed, and excessive temperature rise of the surface of the second glass tube  54  is prevented.  
         [0094]    Thus, setting the height of the draining wall  705   c  of the shade  705  disposed above in the perpendicular direction of the second glass tube  54 , depending on the refrigerant packed in the refrigeration cycle, temperature rise of the surface of the second glass tube  54  can be controlled, and in particular when packed with flammable refrigerant, by setting the height of the draining wall  705   c  at 0.5 mm or more to 5 mm or less, and gas hardly stays between the second glass tube  54  and shade  705 , and excessive temperature rise of the surface of the second glass tube  54  is prevented.  
         [0095]    Further, since the excessive temperature rise of the surface temperature of the second glass tube  54  can be suppressed, excessive temperature rise in the compartment in defrosting operation can be suppressed, and cooling may be started efficiently after defrosting, so that the energy may be saved.  
         [0096]    In the foregoing embodiments, the refrigerator is explained as an example of applying the defrosting heater, but not limited to this, it can be applied in any so-called cold storage having an evaporator, and it can be widely applied in refrigerated show case or automatic vending machine having refrigeration cycle packed with flammable refrigerant.  
       INDUSTRIAL APPLICABILITY  
       [0097]    The defrosting heater of the invention can safely heat and remove frost deposits collected and adhered on the cooler of the refrigeration cycle packed with flammable refrigerant.