Patent Publication Number: US-2011049148-A1

Title: Water tank with sealing element

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority from Korean Patent Application Nos. 10-2009-80778, 10-2010-30517 and 10-2010-71065 filed on Aug. 28, 2009, Apr. 2, 2010 and Jul. 22, 2010, respectively in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates to a water tank with a sealing element. More particularly, the present invention relates to a water tank with a sealing element, which facilitates its assembling, improves a sealing force in diverse manners, and makes it possible to clean the interior of the water tank. 
     2. Description of the Prior Art 
     Typically, in a device which heats or cools water supplied from the outside and makes it possible to obtain water or beverages having a desired temperature, such as a hot/cold water dispenser, a water purifier, a vending machine, or the like, a water tank is provided, and it is general that the water tank is made of a metallic material, such as stainless steel, aluminum, or the like, for convenience in heat resistance, anti-corrosion, anti-shock performance, durability, and formation. 
     Such a water tank is generally composed of a body and a cover, and in order to secure sealing between the body and the cover, welding may be performed. However, a crack or water leak phenomenon may occur due to the vulnerability occurring along a welded region between the body and the cover, and materials that are harmful to a human body may occur due to the corrosion of the corresponding region in the case where the water tank is used for a long time. 
     In the case of using the water tank for a long time, a microorganism may propagate itself, and an alien substance such as fur may form in the interior of the water tank, so that it is required to clean the interior of the water tank. 
     Also, since the water tank is mainly made of a metallic material that has a high thermal conductivity, the heat is easily released to the outside. Accordingly, even if the water in the water tank is heated or cooled at a desired temperature, the temperature of the water in the water tank readily gets similar to the ambient temperature, and thus it is required that the water in the water tank is continuously heated or cooled to cause the energy consumption. 
     Also, in order to minimize a heat loss, a complicated work for wrapping the above-described water tank in a flammable and inflammable heat insulator that is separately provided on the outside of the water tank should be accompanied, and in this case, the effect of the heat insulator may be not up to the expectation. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art while advantages achieved by the prior art are maintained intact. 
     The present invention is related to the providing of a water tank having a sealing element, which achieves an accurate sealing. 
     Also, the present invention is related to the providing of a water tank having a sealing element, which facilitates the cleaning of the interior of the water tank and achieves an accurate sealing. 
     Also, the present invention is related to the providing of a water tank having a sealing element, which can minimize the energy consumption for heating or cooling water in the water tank through heightening of the heat insulation performance. 
     In one aspect of the present invention, there is provided a water tank, which includes a body which has one open side and accommodates water therein; a cover which finishes the body and has a temperature control member that heats or cools the water accommodated in the body; and a sealing portion for making the body and the cover mutually engaged with each other to seal up the interior of the body. 
     In a preferred of the present invention, the sealing portion has a structure that includes a first bent portion which is inserted into an inner wall of an expanded tube portion formed along an edge on an open side of the cylindrical body and is formed along an edge of the cover so as to form a valley that is parallel to the inner wall; and a second bent portion which extends from the first bent portion, is engaged with the expanded tube portion through a curling process, and is compressed along an outer wall of the body. 
     In a preferred embodiment of the present invention, the body includes an inner cylinder that accommodates the water and an outer cylinder that forms a space portion in association with an outer surface of the inner cylinder and surrounds the inner cylinder, the cover finishes the inner cylinder and the outer cylinder, and the sealing portion is mounted between the inner cylinder and the cover or between the outer cylinder and the cover to keep a sealing state between the inner cylinder and the outer cylinder. The outer cylinder is integrally engaged with the edge on one open side of the inner cylinder, and a vacuum state is maintained between an inner surface of the outer cylinder and an outer surface of the inner cylinder. 
     With the above-described construction, the water tank having a sealing element according to the present invention has the following advantages. 
     First, the sealing force can be improved by a curling process in a state where the cover and the body are firmly fixed during their processing without shaking. 
     Since the body has a double structure including an inner cylinder having a vacuum space portion formed therein and an outer cylinder, the heat insulation performance can be improved and thus the power consumption for heating or cooling the water can be reduced. 
     Since the cover and the body adopt a detachable sealing structure, the cleaning of the water tank can be easily performed. 
     Also, by adopting a structure in which a vacuum space portion is provided between the inner cylinder and the outer cylinder, it is not required to add a separate heat insulator, and thus the heat insulation performance can be improved with reduction of the cost for raw materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view illustrating the whole structure of a water tank with a sealing element according to an embodiment of the present invention; 
         FIG. 2  is a perspective view illustrating the whole structure of a water tank with a sealing element according to another embodiment of the present invention; 
         FIG. 3  is a conceptual sectional view of  FIG. 2 ; 
         FIGS. 4 and 5  are enlarged views of a portion A of  FIG. 3 ; 
         FIGS. 6 to 14  are conceptual views illustrating the whole structure of a water tank with a sealing element according to various embodiments of the present invention; 
         FIGS. 15 to 25  and  27  to  29  are conceptual views illustrating a sealing portion that is a main portion of a water tank with a sealing element according to various embodiments of the present invention; and 
         FIG. 26  is a conceptual view illustrating a use state of a water tank with a sealing portion according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the entire description of the present invention, the same drawing reference numerals are used for the same elements across various figures. 
     The matters defined in the description, such as the detailed construction and elements, are nothing but specific details provided to assist those of ordinary skill in the art in a comprehensive understanding of the invention, and the present invention is not limited thereto. 
     Also, in the drawings, sizes and shapes of the constituent elements as illustrated in the drawing may be exaggerated for clarity in explanation. The spatially defined wordings in consideration of the construction and effect of the present invention may differ in accordance with a user&#39;s or operator&#39;s intention or custom, and the definition of such wordings should be made based on the contents throughout the entire description of the present invention. 
       FIG. 1  is a perspective view illustrating the whole structure of a water tank with a sealing element according to an embodiment of the present invention,  FIG. 2  is a perspective view illustrating the whole structure of a water tank with a sealing element according to another embodiment of the present invention, and  FIG. 3  is a conceptual sectional view of  FIG. 2 . 
     A water tank with a sealing element according to an embodiment of the present invention has a structure that briefly includes a body  10  and a cover  300 , and the body  10  and the cover  300  are mutually engaged with each other by a sealing portion  400  to seal up the interior of the body  10 . 
     The body  10  is a portion one side of which is open to accommodate water therein and which provides a space where the cover  300  to be described later and the sealing portion  400  are installed. 
     In the body, an unexplained reference numeral “ 101 ” denotes a drain tube. 
     The cover  300  finishes the body  10 , and is a portion in which a temperature control member  301  that heats or cools the water accommodated in the body  10  by a heat source supplied from the outside, and a thermostat (not illustrated) that keeps the water temperature that is adjusted by the temperature control member  301  are installed. 
     The temperature control member  301  may .be a heater (see left drawing of  FIG. 26 ) that heats the water accommodated in the body  10  by power supply from the outside or a cooling tube (see right drawing of  FIG. 26 ) in which a refrigerant of a cooling cycle, which is supplied from the outside to circulate, flows. 
     The thermostat is typically mounted on a fixing frame  160  provided on the external surface of the body  10  as illustrated in  FIG. 2  to adjust the temperature of the water accommodated in the body  10 , and although being described in various embodiments to be described later, it is mounted preferably on the cover  300  because of the sealing of a space portion  900  provided in the body  10  to form the vacuum state. 
     An unexplained reference numeral “ 303 ” denotes a fixing bracket, “ 304 ” denotes a water inlet tube, “ 305 ” denotes a water outlet tube, and “ 306 ” denotes a temperature sensor, which are mounted on the cover  300 . 
     The sealing portion  400  serves to make the body  10  and the cover  300  mutually engaged with each other to seal up to interior of the body  10 , and embodiments of the present invention to which the sealing portion is applied in diverse manners will be described in more detail. 
     The present invention is applicable by the above-described embodiment, and for more detailed explanation, main portions of the present invention will be described in detail hereinafter. 
     First, the sealing portion  400  is to seal up the interior of the body  10  by making the body  10  and the cover  300  mutually engaged with each other, and as illustrated in  FIGS. 2 to 5 , it has a structure including a first bent portion  401  and a second bent portion  405 . 
     Here, the other sealed side and wall surface portion of the body are integrally formed by a drawing process, and this is because in the case of making them engaged with each other by a method such as welding or the like, leakage may occur in an engagement portion or drinking water may be polluted due to the corrosion of the engagement portion. 
     An expanded tube portion  140  is a portion that is formed by bending an edge of one open side of the body to the outside and expanding the edge, and is engaged with the second bent portion  405 . 
     The first bent portion  401  is inserted into an inner wall of the expanded tube portion  140  formed along the edge on the open side of the cylindrical body  10 , and is formed along an edge of the cover  300  so as to form a valley  307  that is parallel to the inner wall. The first bent portion  401  is a portion from which the second bent portion  405  extends. 
     The second bent portion  405  extends from the first bent portion  401 , is engaged with the expanded tube portion  140  through a curling process, and is compressed along an outer wall of the body  10  to maintain the sealing state of the body  10 . 
     Here, the first bent portion  401  may adopt a structure that includes the other side wall  402 , a bottom  403 , and one side wall  404  to form the valley  307 . The other side wall  402  is slantingly bent from the inner wall of the body along the edge of the cover  300 , the bottom  403  extends in parallel to the cover  300  from an end portion of the other side wall  402  to the inner wall of the body  10 , and the one side wall  404  extends from the edge of the bottom  403  and is bent to be in contact with the inner wall of the body  10 . 
     In this case, the second bent portion  405  extends from the one side wall  404  and is engaged with the expanded tube portion  140 . AS the second bent portion  405  is engaged with the expanded tube portion  140 , it forms a circular cross section along the outer wall of the body  10  as shown in  FIG. 4 , becomes in close contact with the outer wall of the body  10  as shown in  FIG. 3 , or surrounds the end portion of the expanded tube portion  140  as shown in  FIG. 5 . 
     Also, the first bent portion  401  is press-fit engaged with the inner wall of the body  10  by the above-described structure, and the other side wall  402  slantingly formed has a strong elastic repulsion force against a pressure applied from the edge of the cover  300 , i.e. applied from the edge of the outer wall of the body  10 . 
     Accordingly, once the one side wall  404  of the first bent portion  401  is press-fit engaged with the inner wall of the body  10 , the body  10  and the cover  300  are firmly fixed to each other, and thus the end portion of the first bent portion  401  and the expanded tube portion  140  can clearly and prominently form the sealing portion  400  including the second bent portion  405  without movement or misalignment during the curling process. 
     On the other hand, it is preferable that a packing material P intervenes along a portion in which the body  10  and the cover  300  become in contact with each other, i.e. along a direction in which the sealing portion is formed, and the packing material P seals even a minute gap that may occur between the body  10  and the cover  300  to prevent the leakage of water. 
     The packing material P may typically be a material such as silicon. For example, a worker can form the sealing portion  400  by fixing the body  10  and the cover  300  to each other, spreading silicon between the second bent portion  405  and the expanded tube portion  140 , and performing a curling process after the silicon is dried. 
     On the other hand, embodiments of the present invention are illustrated in  FIGS. 6 to 14 . 
     As illustrated, the body  10  according to this embodiment has a double wall structure composed of an inner cylinder  100  and an outer cylinder  200 , and the inner cylinder  100  and the outer cylinder  200  are made of a metallic material such as aluminum, stainless steel, or the like. 
     The inner cylinder  100 , which is briefly in the form of a cylinder, has one open side, and is provided with a water storage space formed therein. 
     The outer cylinder  200 , which is briefly in the form of a cylinder, has a diameter that is larger than that of the inner cylinder  100 , and is arranged to accommodate the inner cylinder  100  therein. 
     That is, the outer cylinder  200  is arranged to surround the outer side of the inner cylinder  100 , and a gap that corresponds to a difference in diameter between the inner cylinder  100  and the outer cylinder  200  is formed between the inner cylinder  100  and the outer cylinder  200 . 
     Open end portions of the inner cylinder  100  and the outer cylinder  200  are mutually bonded in a body, and thus an airtight space portion as wide as the gap is formed between the inner cylinder  100  and the outer cylinder  200 . 
     The bonding type of end portions of the inner cylinder  100  and the outer cylinder  200  may be welding. 
     The airtight space portion  900  functions as a heat insulation layer to keep the hot water stored in the inner cylinder  100  warm. 
     In order to perform the function as the heat insulation layer, an air layer may be formed in the space portion  900 , and in order to maximize the warmth keeping efficiency of the hot water stored in the hot water tank, the space portion  900  may be in a vacuum state or a heat insulation material may fill in the space portion  900 . 
     In order to make the space portion  900  in a vacuum state, diverse vacuum forming methods may be used. As an example, a hole for vacuum exhaust is formed on the outer cylinder  200 , and the inner cylinder  100  and the outer cylinder  200  bonded in a body is put in a vacuum heating furnace to be vacuum-heated. By sealing the vacuum exhaust hole when the vacuum state reaches a predetermined level, the vacuum state of the space portion  900  is formed. 
     As another example, a vacuum exhaust device is connected to the vacuum exhaust hole to perform the vacuum exhaust, and by sealing the vacuum exhaust hole when the vacuum state reaches a predetermined level, the vacuum state of the space portion  900  is formed. 
     A heat insulation material (not illustrated) may fill in the space portion  900 , and in this case, the heat insulation material may be a heat insulation material having an excellent heat insulation performance, such as Styrofoam, foam urethane, or the like, or a heat insulation material that considers even the flammability. 
     The filling of the heat insulation material means the filling of the heat insulation material in the space portion in a manner that a heat insulation material that tightly surrounds the outer surface of the inner cylinder  100  is formed, the inner cylinder and the heat insulation material in this state are accommodated in the outer cylinder  200  so that the outer surface of the heat insulation material is closely surrounded by the inner surface of the outer cylinder  200 , and the end portions of the inner cylinder  100  and the outer cylinder  200  are bonded in a body. 
     The cover  300  is also made of a metallic material such as aluminum, stainless steel, or the like, and one open side of the body  10 , and specifically one open side of the inner cylinder  100 , is covered by the cover  300  to seal the inner water storage space of the inner cylinder  100 . 
     The cover  300  may be engaged with the body  10  by making the edge of the cover engaged with the edge of the open side of the body  10  using a curling process or welding. 
       FIG. 6  illustrates an example of a curling process. Specifically, the edge region of the open side of the body  10 , i.e. the region in which the outer cylinder  200  and the inner cylinder  100  are mutually bonded in a body, is bent outwardly of the body  10 , and the edge region of the cover  300  pressingly surrounds the bent edge region of the body  10 . 
     The curling process is not limited to the above-described examples, and can be curling-processed in diverse shapes as illustrated in  FIGS. 7 to 10 . 
     Specifically,  FIG. 7  shows an example of a curling process in which the edge region of the cover  300  is curled up on the edge region of the open side of the body  10 , so that the vertical cross section of the mutually engaged region of the body  10  and the cover  300  is briefly in the form of a circle. 
       FIG. 8  illustrates an example of a curling process in which the mutually engaged region of the cover  300  and the body  10  is bent downward once more and becomes in close contact with the outer surface of the outer cylinder  200  in the example of the curling process as illustrated in  FIG. 6 . 
     On the other hand, the cover  300  may be engaged with the body  10  as the edge region of the cover  300  is welded onto the edge region of the open side of the body  10 , and diverse welding types such as examples in  FIGS. 9 and 10  may be presented. 
     As an example, in  FIG. 9 , the edge on the open side of the body  10 , specifically, the region in which the outer cylinder  200  and the inner cylinder  100  are bonded in a body, is formed to be bent outwardly of the body  10 , and the edge region of the cover  300  is bent to be in close contact with an inner periphery of the upper end of the inner cylinder  100  and the upper surface of the bent portion, so that the cover  300  covers the open side of the body  10  as if the cover  300  is inserted into the interior of the body  10 , the end of the edge of the cover  300  and the end of the edge on the open side of the body  10  are neighbored to be in contact with each other, and these regions are engaged with each other in welding. 
     As another example, in  FIG. 10 , in the same manner as in  FIG. 9 , an outwardly bent region is formed on the edge region on the open side of the body  10 , and the cover  300  covers the open side of the body  10  as if the cover  300  is put on the open side of the body  10 , so that the end of the edge of the cover  300  and the end of the edge on the open side of the body  10  are neighbored to be in contact with each other, and these regions are engaged with each other in welding. 
     As described above, a packing P is interposed between the engaged portions of the cover  300  and the body  10  mutually engaged with each other. 
     The packing P is included to secure the airtight of the water storage space in the hot water tank, and is made of synthetic rubber or a silicon material. 
     The above-described curling process or welding is performed in a state where the packing P is interposed between the mutually engaged regions of the body  10  and the cover  300 . 
     On the other hand, diverse heating means for heating the water stored in the body  10  may be provided, and  FIGS. 6 to 10  illustrate an example of a temperature control member  301  such as a pipe heater that is penetratingly engaged with one side of the cover  300  so as to be connected to the water storage space of the body  10 , specifically, the water storage space in the inner cylinder  100 . 
     In addition, a temperature sensor  306  for measuring the temperature of the water stored in the body  10  is provided, and as illustrated, the temperature sensor  306  is provided on the surface of the cover  300 . 
     However, the positions where the temperature control member  301  and the temperature sensor  306  are installed in the water tank including the body  10  and the cover  300  are not limited to the examples as illustrated, and the temperature control member  301  and the temperature sensor  306  may be installed in diverse positions. 
     Also, the water tank having the above-described water tank may be fixed by a fixing bracket  303  in the interior of the hot/cold water dispenser, water purifier, vending machine, or the like. 
     In an example as illustrated, the fixing bracket  303  is provided in the cover  300  in a state where the one open side of the body  10  is directed upward. 
     However, the installation position of the fixing bracket  303  and the direction of the open side of the body  10  are not limited to those in the illustrated examples. The body  10  may be engaged with the cover  300  in a state where the open side of the body  10  is directed downward, and the fixing bracket  303  may be installed in diverse positions as needed. 
     In the foregoing description, the unexplained reference numerals “ 304 ” and “ 305 ” denote a water inlet tube and a water outlet tube, respectively, provided in a general hot water tank, and “ 101 ” denotes a drain tube. 
     On the other hand, the present invention is applicable by the embodiment as illustrated in  FIG. 11 . 
     That is, in this embodiment of the present invention, unlike the above-described embodiments, the hot water tank does not have a structure in which the body  10  and the cover  300  are not engaged with each other, but has a structure in which an upper body  10   u  and a lower body  10   d  are engaged with each other. 
     Accordingly, explanation will now be made in due consideration of the structures of the upper and lower bodies in a state where unexplained reference numerals in the drawing are substituted by the explanation in the above-described embodiments. 
     In this embodiment, a cylindrical upper body  10   u  having an open lower side and a cylindrical lower body  10   d  which has an open upper side and which is engaged with the lower edge of the upper body  10   u  in welding are included. Also, a packing P is interposed between mutual engagement regions of the upper body  10   u  and the lower body  10   d.    
     In particular, the upper body  10   u  and the lower body  10   d  in this embodiment have a double wall structure in the same manner as the body  10  in the above-described embodiments. 
     That is, the upper body  10   u  and the lower body  10   d  include cylindrical inner cylinders  100   u  and  100   d  having open sides (in the case of the inner cylinder  100   u  included on one side (the upper body  10   u ), they mean lower sides, while in the case of the inner cylinder  100   d  included on the lower body  10   d,  they mean upper sides) and cylindrical outer cylinders  200   u  and  200   d  arranged as if they surround the inner cylinders  100   u  and  100   d.    
     Here, the respective end portions of the inner cylinders  100   u  and  100   d  and the outer cylinders  200   u  and  200   d  are mutually bonded in a body, and an airtight space portion  900  is formed between the inner cylinders  100   u  and  100   d  and the outer cylinders  200   u  and  200   d.    
     The airtight space portion  900  may be in a vacuum state or may be filled with a heat insulation material for an efficient heat insulation of the hot water tank in the same manner as the first embodiment. 
       FIG. 11  shows a state where edges of the upper body  10   u  and the lower body  10   d  are engaged with each other in welding. However, the engagement type is not limited thereto, and in the same manner as the above-described examples, the upper body  10   u  and the lower body  10   d  may be engaged with each other by a curling process. 
     On the other hand, the present invention is applicable by the embodiments as illustrated in  FIGS. 12 and 13 . 
     In this embodiment of the present invention, unlike the above-described embodiments, the body  10  and the cover  300  are separably engaged with each other. 
     Specifically, the body  10  included in this embodiment has a double wall structure in the same manner as the above-described embodiments with reference to  FIGS. 6 to 10 . 
     In this case, a heat insulation material having an excellent heat insulation performance, such as Styrofoam, foam urethane, or the like, or a heat insulation material that considers even the flammability may fill in the space portion  900  between the inner cylinder  100  and the outer cylinder  200 . 
     However, a body engagement portion  10   c  that is formed on an outer periphery on the open side of the body  10  along a circumferential direction is further included, and on the outer periphery of the body engagement portion  10   c,  a thread  100   s  is formed. 
     Also, the cover  300  that is engaged to cover the open side of the body  10  further includes a cover engagement portion  300   c  which is formed on the outer periphery along the circumferential direction and which can be screw-engaged with the body engagement portion  10   c,  and on an inner periphery of the cover engagement portion  300   c,  a screw groove  300   s  which can be screw-engaged with the thread  100   s  of the body engagement portion  10   c  is formed. 
     As described above, since the body engagement portion  10   c  and the cover engagement portion  300   c  are formed on the body  10  and the cover  300 , respectively, to be screw-engaged with each other, the body  10  and the cover  300  can be separably engaged with each other. 
     Also, during the mutual engagement, a packing P may be interposed between the edge on the open side of the body  10  and the edge of the cover  300  to secure the airtight in the body  10 . 
     In this case, a bent portion that is bent outwardly in the form of an arc may be formed on the respective edge portions of the body  10  and the cover to widen the area that presses the packing P. 
     Also, the packing P, as shown in  FIG. 12 , has a plurality of protrusions  300   p  formed on the outer periphery of the packing P in a circumferential direction, and thus the airtight performance between the body  10  and the cover  300  can be further improved. 
     Also, the packing P has a locking piece  300   h,  which is twice bent downward at right angles in the circumferential direction, formed thereon to be locked in the edge of the end portion of the body  10 , and thus the engagement and disengagement of the packing P is facilitated during the assembling and disassembling of the packing. 
     In this embodiment, since the separable structure further includes the body engagement portion  10   c  and the cover engagement portion  300   c,  the body  10  and the cover  300  are not required to be engaged with each other by the mutual curling process or welding as in the embodiments illustrated in  FIGS. 6 to 11 , and thus the material is not limited to a metallic material. 
     That is, the body  10  and the cover  300  may be made of a metallic material or a ceramic material. 
     If the body engagement portion  10   c  and the cover engagement portion  300   c  are made of a plastic material in a state where the body  10  and the cover  300  are made of a metallic material, the body engagement portion  10   c  and the cover engagement portion  300   c  may be engaged with the body  10  and the cover  300 , respectively, by an insert injection or a press-fit engagement method. 
     In this case, in order for the body engagement portion  10   c  and the cover engagement portion  300   c  to be firmly engaged with the body  10  and the cover  300 , respectively, without movement, protrusion shaped structures may be provided on the outer peripheries of the body  10  and the cover  300 , respectively. 
     Although not illustrated in the drawing, in the case where the body  10  and the cover  300  are made of a ceramic material, the body engagement portion  10   c  and the cover engagement portion  300   c  may also be made of a ceramic material to be formed in a body. 
     Also, the inner cylinder  100  and the outer cylinder  200  of the body  10 , as the embodiments illustrated in  FIGS. 6 to 10 , are not bonded in a body by welding or the like, but are formed in a body so that the space portion  900  is formed between the inner cylinder  100  and the outer cylinder  200 . 
     Also, in this embodiment, as illustrated in  FIG. 13 , although not illustrated in  FIG. 12 , a flammable or inflammable heat insulation material  300   i  having a high flammability and a high heat insulation performance, which is covered by an upper portion of the cover  300  to minimize a loss of the heat energy of the water accommodated in the body  10  through the cover  300 , may be further included. 
     On the other hand, the present invention can adopt the structure as illustrated in  FIG. 14 . 
     That is, in this embodiment, in the same manner as the embodiments as illustrated in  FIGS. 12 and 13 , the hot water tank has a structure in which the cover  300  and the body  10  are separably engaged with each other by a screw engagement method. 
     The structure of the cover  300  is the same as that in the third embodiment, but the structure of the body  10  differs from that in the third embodiment. In the following description, the structure of the body  10 , which is different from that according to the third embodiment, will be described in a state where unexplained reference numerals in the drawing are substituted by the explanation in the above-described embodiments. 
     In this embodiment, the body  10 , in the same manner as the embodiments as illustrated in  FIGS. 6 to 13 , has a double wall structure that includes the inner cylinder  100  and the outer cylinder  200 . However, end portions of the inner cylinder  100  and the outer cylinder  200  are not bonded in a body, but the outer cylinder  200  is fit-engaged with the body engagement portion  10   c  formed on the outer periphery of the inner periphery. 
     Specifically, on the outer periphery on the open side of the inner cylinder  100 , the body engagement portion  10   c  having the same function as that in the embodiments as illustrated in  FIGS. 12 and 13  is formed. 
     In this case, on one side of the body engagement portion  10   c,  for example, on the lower end portion of the outer periphery of the body engagement portion  10   c,  a fitting portion  100   sb  extending downward at right angles is formed along the circumferential direction of the body engagement portion  10   c.    
     Also, the outer cylinder  200  is fit-engaged with the fitting portion  100   sb  as the outer periphery of the fitting portion  100   sb  becomes in surface contact with the inner periphery of the end portion on the open side of the outer cylinder  200 . 
     In this case, on the outer periphery of the fitting portion  100   sb , at least one fitting protrusion  100   sbp  is formed to project (in  FIG. 14 , an example in which two fitting protrusions  100   sbp  that are spaced apart in upward and downward directions are formed on the outer periphery of the fitting portion  100   sb ), and corresponding to the fitting protrusion, fitting grooves  100   g  are formed on the inner periphery of the end portion of the outer cylinder  200 . 
     The outer cylinder  200  and the fitting portion  100   sb  can be firmly and stably fit-engaged with each other as the outer cylinder  200  easily finds its position by the fitting grooves  100   g  and the fitting protrusions  100   sbp  inserted into the fitting grooves  100   g,  respectively. 
     The space portion  900  between the outer cylinder  200  and the inner cylinder  100  is formed as wide as the gap formed between the fitting portion  100   sp  and the inner cylinder  100 , and is in a vacuum state or is filled with a heat insulation material. 
     On the other hand, the packing P is interposed along the border of the open side of the inner cylinder  100  and the edge that is bent outwardly in the form of an arc from the edge of the cover  300 . 
     Also, the body  10  may adopt a double structure to improve the heat insulation performance as shown in  FIGS. 15 to 26 , and specifically, may adopt a structure that includes the inner cylinder  100  and the outer cylinder  200 . 
     The inner cylinder  100  provides a space for accommodating the water, and the outer cylinder  200  has a structure in which the space portion  900  is formed between the outer cylinder  200  and the outer surface of the inner cylinder  100  as surrounding the inner cylinder  100 . The cover  300  to be described later finishes the inner cylinder  100  and the outer cylinder  200 , and the sealing portion  400  is installed between the inner cylinder  100  and the cover  300  or between the outer cylinder  200  and the cover  300  to keep the sealing state of the inner cylinder  100  and the outer cylinder  200 . 
     Here, the outer cylinder  200  is engaged with the edge of one open side of the inner cylinder  100 , and it is preferable that a gap between the inner surface of the outer cylinder  200  and the outer surface of the inner cylinder  100  is kept in a vacuum state to improve the heat insulation performance. 
     In this case, the inner cylinder  100  and the outer cylinder  200  may be made of a metallic material such as stainless steel, aluminum, or the like in consideration of the convenience in forming, durability, and the like. 
     Also, the inner cylinder  100  and the outer cylinder  200  may be made of diverse materials such as glass, ceramics, enamel, and the like. 
     Also, since the space portion  900  is kept in the vacuum state, the outer surface of the outer cylinder  200  has the lowest thermal conductivity, and thus the water accommodated in the inner cylinder  100  can be kept hot or cool. 
     On the other hand, the sealing portion  400  may adopt a structure that includes a first engagement member  406 , a second engagement member  407 , and a packing  408 , as shown in  FIGS. 15 and 16 , in the body  10  that has the double structure including the inner cylinder  100  and the outer cylinder  200 . 
     That is, the first engagement member  406  is engaged with the outer side of the edge of the outer cylinder  200  and has a screw portion  406   s  formed along the outer periphery thereof. The second engagement member  407  is engaged with the outer surface of the cover  300  and has a screw portion  407   s  formed along the inner periphery thereof to be screw-engaged with the first engagement member  406 . The packing  408  is interposed between the end portion of the body  10  and the edge on the inner side of the cover  300  and serves to make the interior of the body  10  airtight. 
     Here, a structure may be adopted, in which support protrusions  200   p  are formed to protrude along the outer periphery of the outer cylinder  200  to support one side of the first engagement member  406 , and support protrusions  300   p  are formed to protrude along the outer periphery of the cover  300  to support one side of the second engagement member  407 . 
     In this case, the second engagement member  407 , when the cover  300  and the body  100  are engaged with each other, surrounds the first engagement member  406  so that the screw portions  406   s  and  407   s  are screw-engaged with each other. 
     Also, the packing  408  is made of a rubber or silicon material, and has diverse shapes including an O-ring of which the vertical cross section is “O”-shaped. 
     At the end portion of the body  10 , a third bent portion  15  that is bent to the outside to be engaged with the packing  408  may be formed, and as shown in  FIG. 16 , end portions on the open sides of the inner cylinder  100  and the outer cylinder  200  are bent in a body to form the third bent portion  15 . 
     It is preferable that a fourth bent portion  35  that is bent to face the third bent portion  15  so that the packing  408  is pressingly fixed along the edge of the cover  300  is formed, and on one side of either or both of the third bent portion  15  and the fourth bent portion  35 , a projection portion  15 ′ projecting to the packing  408  is further included.  FIG. 16  shows the structure of the projection portion  15 ′ projecting from the third bent portion  15 . 
     The packing  408  has a vertical cross section briefly in a “C”-shaped so that the lower side of the packing  408  is engaged with the end portion of the outer side of the third bent portion  15 . Since the packing  408  is fit-engaged to cover the upper surface of the third bent portion  15 , it can be easily detached when the body  10  is cleaned. 
     Also, the packing  408  has at least one ring-shaped protrusion flange  409  formed along a surface opposite to the third bent portion  15  or the fourth bent portion  35  toward the third bent portion  15  or the fourth bent portion  35 , and a plurality of small protrusions  408 ′ formed apart from one another on the interior or exterior of the protrusion flange  409  along the surface opposite to the third bent portion  15  or the fourth bent portion  35 , so that the fastening force between the body  10  and the cover  300  can be strengthened. 
     Also, it is preferable that an extension portion  409 ′ that extends in upward and downward directions is integrally formed on the end portion of the interior of the packing  408 , being in contact with the edges of the interiors of the body  10  and the cover  300 , to secure the sealing more accurately. 
     Accordingly, as the pressed area of the fourth bent portion  35  against the packing  408  is widened, the airtight performance of the body  10  can be reinforced. 
     On the other hand, the sealing portion  400  may adopt a structure that includes a locking portion  406 ′, a fastening portion  407 ′, and the packing  408  as shown in  FIGS. 17 and 18 . 
     That is, the locking portion  406 ′ is engaged with the exterior of the edge of the outer cylinder  200 , and a locking protrusion  406 ′ p  is formed to protrude on one side thereof. The fastening portion  407 ′ includes a fixing member  407 ′ a  and a wing  407 ′ b  having a locking piece  407 ′ h  formed thereon, and is engaged with the locking portion  406 ′. The packing  408  is interposed between the end portion of the body  10  and the edge on the interior of the cover  300 , and serves to make the interior of the body  10  airtight. 
     Among them, the fixing member  407 ′ a  of the fastening portion  407 ′ is fixed to the outer surface of the cover  300 , and the wing  407 ′ b  rotatably extends along the edge of the fixing member  407 ′ a , and the locking piece  407 ′ h  that is engaged with the locking protrusion  406 ′ p  projects toward the outer side of the outer cylinder  200 . 
     The fixing member  407 ′ a  and the wing  407 ′ b  are connected by diverse connection methods such as hinge engagement (not illustrated) by a hinge pin. 
     However, in the drawing, the fastening portion  407 ′ is made of a material such as plastic, and the connection region of the fixing member  407 ′ a  and the wing  407 ′ b  is made to be thin, so that the connection region has a rotatable integral structure in upward and downward directions. 
     As the wing  407 ′ b  moves toward the body  10 , the locking piece  407 ′ h  is locked in the locking protrusion  406 ′ p  to achieve the engagement, and when the wing  407 ′ b  becomes far from the outer surface of the body  10 , more particularly, from the exterior of the outer cylinder  200 , the engagement is released. 
     At this time, the straight distance from the connection point of the fixing member  407 ′ a  and the wing  407 ′ b  to the locking piece  407 ′ h  is somewhat shorter than the distance to the lower end of the locking protrusion  406 ′ p  formed on the outer side of the bottom portion of the locking portion  406 ′ on the horizontal line at the same position, and thus the locking piece  223  is locked as it passes through the lower end of the locking protrusion  406 ′ p.    
     Once the locking piece  407 ′ h  is locked in the locking protrusion  406 ′ p , the cover  200  is pressingly engaged with the body  10 . 
     Here, it is preferable that at least two fastening portions  407 ′ are formed along the edge of the cover  300 , and another fastening portion  407 ′ is provided in a position opposite to one fastening portion  407 ′ so that the convenience is given to the rotation of the wing  407 ′ and the compression force by the fastening portion  407 ′ is uniformly given along the outer surface of the outer cylinder  200 . 
     On the other hand, the third bent portion  15  and the fourth bent portion  35  may adopts a structure as shown in  FIG. 10 . 
     That is, grooves  15 ″ and  35 ″ are formed on the third bent portion  15  and the fourth bent portion  35 , respectively, and both edges of the grooves  15 ″ and  35 ″ have the shape of projection regions. 
     As the third bent portion  15  and the fourth bent portion  35  have the above-described shapes, the packing  408  can be concentrically pressed by the projection regions on both sides of the grooves  15 ″ and  35 ″ when the packing  408  is interposed, and thus the airtight performance can be further improved. 
     On the other hand, in addition to the above-described embodiments, the sealing portion  400 , as illustrated in  FIGS. 20 to 26 , is mounted between the inner cylinder  100  and the cover  300  or between the outer cylinder  200  and the cover  300 , and includes a finishing surface and a sealing member. 
     Hereinafter, diverse embodiments of the present invention to be described relate to the sealing portion  400  that includes a finishing surface and a sealing member. The finishing surface extends along the edge of the cover  300  and is engaged with the inner cylinder  100  or the outer cylinder  200 , and the sealing member is mounted on the finishing surface to serve to keep the sealing state as being in contact with the outer cylinder  200 . 
     For reference, the definition of the terms to be described hereinafter will be briefly described hereinafter. 
     First, the terms “edge outer side” and “edge inner side” define an inner side and an outer side neighboring the above-described edges on the basis of the edges of the inner cylinder  100  and the outer cylinder  200  welded in a body, and indicate regions which are in contact with the sealing member to be described later. 
     Also, the terms “end side inner surface” and “end side outer surface” indicate an inner surface and an outer surface which are engaged to face the finishing surface to be described later in the inner surface and the outer surface of the inner cylinder  100  and the outer cylinder  200  welded in a body. 
     First, the finishing surface  40  of the cover  300  is screw-engaged with the outer cylinder  200  as shown in  FIG. 20 , and the sealing member  420  is in contact with the edge of the exterior of the outer cylinder  200 . 
     That is, the cover  300  includes a shielding plate  310  for sealing up one open side of the inner cylinder  100 , and a contact groove  320  formed between the shielding plate  310  and the finishing surface  410  along the edge of the shielding plate  310  to face the edge of the interior of the inner cylinder  100 . 
     The contact groove  320  is formed for the purpose of reinforcement that properly disperses the load and stress due to the temperature control member  301  and the thermostat  302  mounted on the cover  300 . 
     Here, the finishing surface  410  is screw-engaged with the end side outer surface of the outer cylinder  200 , and the sealing member  420  is interposed between the contact groove  320  and the finishing surface  410  to be in close contact with the edge of the inner cylinder  100  and the edge outer side of the outer cylinder  200 , so that the sealing becomes possible. 
     On the other hand, in the present invention, the finishing surface of the cover  300  is screw-engaged with the inner cylinder  100 , and the sealing member becomes in contact with the inner side of the inner cylinder  100 . This structure will be described with reference to  FIGS. 21 and 22 . 
     First, the cover  300  may be prepared in the form that includes a shielding plate  310  that seals up one open side of the inner cylinder  100  as shown in  FIG. 21 , a contact groove  330  formed between the shielding plate  310  and the finishing surface  430  along the edge of the shielding plate  310  to face the edge of the inner side of the inner cylinder  100 , and a finishing sleeve  340  extending to be bent from the end portion of the finishing surface  430 , having a sealing member  440  mounted thereon, and surrounding the edges of the outer cylinder  200  and the inner cylinder  100 . 
     The contact groove  330  is formed for the purpose of reinforcement that properly disperses the load and stress due to the temperature control member  301  and the thermostat  302  mounted on the cover  300 , and the finishing sleeve  340  is prepared to provide a space in which the sealing member  440  is installed. 
     Here, the finishing surface  430  extends from the edge of the contact groove  330  and is mutually screw-engaged with the end side inner surface of the inner cylinder  100 , and the sealing member  440  has one side that is in close contact with the edges of the inner cylinder  100  and the outer cylinder  200 , and the other side that is in close contact with the edge inner side of the inner cylinder  100 , so that the sealing becomes possible. 
     Also, the cover  300  may be prepared in the form that includes a shielding plate  310  that seals up one open side of the inner cylinder  100  as shown in  FIG. 22 , and a finishing sleeve  350  extending to be bent from the end portion of the finishing surface, having a sealing member  460  mounted thereon, and surrounding the edges of the outer cylinder  200  and the inner cylinder  100 . 
     The finishing sleeve  350  is prepared to provide a space in which the sealing member  460  is installed. 
     Here, the finishing surface  450  is mutually screw-engaged to face the end side inner surface of the inner cylinder  100 , and the sealing member  460  has one side that is in close contact with the edges of the inner cylinder  100  and the outer cylinder  200 , and the other side that is in close contact with the edge inner side of the inner cylinder  100 , so that the sealing becomes possible. 
     Also, the cover  300  may be prepared in the form that includes a shielding plate  310  that seals up one open side of the inner cylinder  100  as shown in  FIG. 23 , a finishing sleeve  360  extending to be bent from the end portion of the finishing surface  450  and surrounding the edges of the outer cylinder  200  and the inner cylinder  100 , and an engagement valley  370  formed along the outer surface of the finishing surface  470  and having a ring-shaped sealing member  480  mounted thereon. 
     The finishing sleeve  360  is prepared to protect a welded region in which the edges of the inner cylinder  100  and the outer cylinder  200  are welded in a body from external physical and chemical impacts. 
     The engagement valley  370  is prepared to provide a space in which the sealing member  480  is installed. 
     Here, the finishing surface  470  is mutually screw-engaged to face the end side inner surface of the inner cylinder  100 , and the sealing member  460  is mounted on the engagement valley  370  to be in close contact with the end side inner surface of the inner cylinder  100 , so that the sealing becomes possible. 
     Also, the cover  300  may be prepared in the form that includes a shielding plate  310  that seals up one open side of the inner cylinder  100  as shown in  FIG. 24 , a finishing sleeve  360  extending to be bent from the end portion of the finishing surface  470  and surrounding the edges of the outer cylinder  200  and the inner cylinder  100 , and an engagement valley  370  formed along the outer surface of the finishing surface  470 . 
     The finishing sleeve  360  and the engagement valley  370  are prepared to provide a space in which the sealing member  480  is installed. 
     Here, the finishing surface  470  is mutually screw-engaged with the end side inner surface of the inner cylinder  100 , and the sealing member  440  is in close contact with the edges of the inner cylinder  100  and the outer cylinder  200 , and is in close contact with the end side inner surface of the inner cylinder  100 , so that the sealing becomes possible. 
     At this time, the sealing member  480  may adopts a structure that includes a first sealing member  482  mounted on the finishing sleeve  360  and having one side that is in close contact with the edges of the inner cylinder  100  and the outer cylinder  200  and the other side that is in close contact with the edge inner side of the inner cylinder  100 , and a ring-shaped second sealing member  484  mounted on the engagement valley  370  to be in close contact with the end side inner surface of the inner cylinder  100 . 
     On the other hand, the present invention may adopt a structure in which a sealing member  490 ′ is interposed between the edges of the inner cylinder  100  and the outer cylinder  200  and the finishing surface  490 . 
     Also, the cover  300  may be prepared in the form that includes a shielding plate  310  that seals up one open side of the inner cylinder  100  and a contact groove  380  formed between the shielding plate  310  and the finishing surface  490  along the edge of the shielding plate  310  to face the edge of the inner side of the inner cylinder  100 . 
     Here, the finishing sleeve  490  extends in parallel to the shielding plate  310  from the edge of the contact groove  380 , and the sealing member  490 ′ extends from the edges of the inner cylinder  100  and the outer cylinder  200  bonded in welding in a body and is in close contact between the finishing surface  490  and the bent surface  490 ″ facing the finishing surface  490 , so that the sealing becomes possible. 
     At this time, the cover  300 , the inner cylinder  100 , and the outer cylinder  200  are sealed by a seal ring block  600  having one side that is engaged with the contact groove  380  and the finishing surface  490  and the other side that is engaged between the bent surface  490 ″ and the outer surface of the outer cylinder  200 . The seal ring block  600  briefly includes a first ring block  610  and a second ring block  620 . 
     The first ring block  610  is in a ring shape as a whole, and has a structure in which a ring protrusion  612  having a shape corresponding to the contact groove  380  is prepared and a finishing groove  614  extending from the ring protrusion  612  and accommodating the finishing surface  490 . 
     The second ring block  620  is in a ring shape as a whole, and has a structure in which a stepped portion  622  for safe placement of the bent surface thereon is prepared and is engaged with the first ring block  610 . 
     It is preferable that the first ring block  610  and the second ring block  620  are penetratingly engaged with each other by a plurality of fixing bolts  630  mounted apart from one another. 
     On the other hand, it is preferable that on one side of the outer cylinder  200 , a vacuum retaining portion  500  for discharging remaining air between the inner cylinder  100  and the outer cylinder  200  is further included. The vacuum retaining portion  500  is necessary to make the space  900  in a vacuum state after the edges of the inner cylinder  100  and the outer cylinder  200  are welded in a body, and is further necessary for the later maintenance and inspection. 
     The vacuum retaining portion  500  includes an air discharge tube  510  provided on one side of the outer cylinder  200 , and a protection cover  520  engaged with one side of the outer cylinder  200  to protect the air discharge tube  510 . 
     Also, as illustrated in  FIGS. 1 and 27  to  29 , the present invention may be applicable by an embodiment in which a pressure member is formed on the outer surface of the outer cylinder  200 , and the sealing portion  400  is formed by a curling process, welding, or filler P. 
     The pressure member that is formed on the outer surface of the outer cylinder  200  keeps the balance of pressure on the inner and outer sides of the space portion  900  and keeps the shape of the inner cylinder  100  and the outer cylinder  200  when the remaining air is discharged through the air discharge tube  510  as illustrated. 
     The pressure member is to keep the balance of pressure on the inner and outer sides of the space portion  900  and to keep the shape of the inner and outer cylinders  100  and  200 , and includes a first protrusion ring  210  and a second protrusion ring  534 . 
     The pressure member is provided to improve the entire structural strength of the water tank in addition to the purpose of preventing the inner and outer cylinders  100  and  200  from being deformed due to the pressure unbalance on the inner and outer sides of the space portion  900  in the process of discharging the remaining air to keep the vacuum state of the space portion  900  through the vacuum retaining portion  500  to be described later. 
     The first protrusion ring  210  projects from the outer surface of the outer cylinder  200  in a ring shape along at least one outer periphery in upward and downward directions. The second protrusion ring  210  is provided on a bottom surface  530  of the outer cylinder  200  through which a drain tube  101  that communicates with the center of the bottom surface of the inner cylinder  100  passes. At least one second protrusion ring  210  projects in the form of a concentric circle around the drain tube  101 . 
     The first and second protrusion rings  210  and  534  are provided on the outer periphery and the bottom surface of the outer cylinder  200 , respectively, which are caused by the fact that a panel of which the actual area has been increased due to its projecting or wrinkled structure has a structural strength that is higher than that of a smooth and flat panel that does not have the projecting or wrinkled structure. 
     On the other hand, the cover  300  finishes the body  10  as described above. As shown in  FIGS. 1 ,  27 , and  28 , a valley  307  that is recessed in the body  10  is formed in a ring shape along the edge, and the sealing portion  400  extends from the edge of the valley  307  and firmly keeps the sealing state in a method such as curling process, filling of packing material P, welding, and the like after the valley  307  is fit-engaged with the edge of the body  10 . 
     The valley  307  is firmly fastened in the initial process of making the cover  300  engaged with the body  10 , and efficiently disperses and supports the load of the respective members mounted on the cover such as the pressure member and so on, and improves the structural strength. The valley  307  includes one side wall  307 ′, a bottom  307 ″, and the other side wall  307 ″. 
     The one side wall  307 ′ extends from the edge of the cover  300 , and bent to the interior of the body  10  at right angles to the cover  300 . The bottom  307 ″ extends from the end portion of the one side wall  307 ′ and is bent in parallel to the cover. The other side wall  307 ″&#39; extends from the bottom  307 ″ and is bent in parallel to the one side wall  307 ′. 
     Here, the sealing portion  400  to be described later extends from the edge of the other side wall  307 ″ and an outer surface of the other side wall  307 ″ becomes in contact with the inner surface of the inner cylinder  100 . 
     In this case, it is preferable that a steam bent tube  308  penetrates the cover  300 , and the steam bent tube  308  is provided to compulsorily discharge steam to the upper side in order to prevent the steam, which is generated when the water is heated in the case where the body  10  is used as a hot water tank, from flowing into the water outlet tube or flowing backward. 
     On the other hand, the sealing portion  400 , as described above, is provided to keep the sealing state between the body  100  and the cover  300 , and as shown in  FIG. 27 , includes a fifth bent portion  45 , a sixth bent portion  65 , and a welding engagement portion W and a packing material P. 
     The fifth bent portion  45  is a portion prepared by bending the engagement surface, which is obtained by bonding the edge of the open side of the inner cylinder  100  and the edge of the open side of the outer cylinder  200 , toward the outer side of the outer cylinder  200  in the form of a flange. 
     The sixth bent portion  65  is a portion which extends from the edge of the cover, i.e. the edge of the other side wall  307 ″ of the valley  307 , surrounds the upper and lower portions of the engagement surface in a “c⊂” or “D⊃”-shaped, and thus is kept in an actual sealing state. 
     The welding engagement portion W is a portion prepared by performing welding along the region that is in contact with the end portion of the fifth bent portion  45 , i.e. the end portions of the inner cylinder  100  and the outer cylinder  200 , and prevents external air from flowing into the space portion  900 . 
     The packing material P fills between the upper surface of the engagement surface and the sixth bent portion  65 , and typically is silicon and so on. More specifically, if the silicon spread in a direction of forming the sixth bent portion  65  and the engagement surface is solidified, the cover  300  and the body  10  are mutually engaged with each other, and the sixth bent portion  65  is formed through a method such as a curling process and so on. 
     Then, the sealing portion  400  may be applicable by an embodiment of the present invention, in which a separate reinforcement piece  55  is engaged with the first engagement portion  46  as shown in  FIG. 28 , welding and filling of the packing material P is performed, and then the sixth bent portion  65  is formed. 
     That is, the first engagement portion  46  is a portion that is formed by the engagement surface in which the edge on the open side of the inner cylinder  100  and the edge on the open side of the outer cylinder  200  are engaged with each other. The reinforcement piece  55  and the welding engagement portion W are formed in the first engagement portion  46 . 
     The reinforcement piece  55  is in the form of a ring that is engaged along the edge of the exterior of the engagement surface, and it is preferable that is made of a material of the same series as that of the inner cylinder  100  and the outer cylinder  200 . 
     The welding engagement portion W is a portion prepared by performing welding in the form of a ring along the regions where the first engagement portion  46  and the reinforcement piece  55  are in contact with each other. The welding engagement portion W is formed to intercept the inflow of external air into the space portion  900  in the direction of forming the first engagement portion  46  and to make the engagement of the reinforcement piece  55  that provides a space in which the sixth bent portion  65  is formed. 
     The sixth bent portion  65  extends from the edge of the cover  300  that is in contact with the inner side surface of the engagement surface, i.e. from the edge of the other side wall  307 ′″ of the valley  307 , and surrounds the end portion of the engagement surface and the upper and lower portions of the reinforcement piece  55 . The packing material P fills between the upper portion of the reinforcement piece  55  and the second bent portion  65 . 
     Here, it is preferable that the upper surface of the second bent portion  65  as illustrated in  FIG. 27  (see imaginary line of dotted portion extending from the upper surface of the cover  300 ) and  FIG. 28  is formed to be equal to or lower than the upper surface of the cover  300  on which electric devices such as a temperature sensor  306  or the like is mounted so that the electronic devices mounted on the upper surface of the cover  330  do not touch the water even if the water leaks through the valley  307 . 
     Also, in an embodiment of the present invention, the sealing portion  400  may be formed by making the reinforcement piece  55  engaged with the first engagement portion  46  as shown in  FIG. 29  and performing welding of the reinforcement piece  55  and the edge of the cover  300 . 
     That is, the first engagement portion  46  is a portion formed by the engagement surface that is obtained by mutually contacting the edge of the open side of the inner cylinder with the edge of the open side of the outer cylinder  200 . The reinforcement piece  55  and the welding engagement portion W to be described later are formed on the first engagement portion  46 . 
     The reinforcement piece  55  is in the form of a ring that is engaged along the edge on the outer side of the engagement surface, and it is preferable that the reinforcement piece  55  is made of a material of the same series as that of the inner cylinder  100  and the outer cylinder  200  so that the welding can be performed. 
     The second engagement portion  66  is a portion that is formed through the surface contact between the edge of the cover  300  and the reinforcement piece  55 . The first welding engagement portion W 1  is prepared by performing welding in the form of ring along the region where the first engagement portion  46  and the reinforcement piece  55  contact each other, and the second welding engagement portion W 2  is prepared by performing welding in the form of a ring along the end portions of the first engagement portion  46  and the second engagement portion  65 . 
     Here, the first welding engagement portion W 1  is formed to intercept the inflow of an external air into the space portion  900  and to make the engagement of the reinforcement piece  55  that provides a space in which a sixth bent portion  65  to be described later is formed. 
     At this time, the second welding engagement portion W 2  is formed to keep the sealing state by finally sealing the body  10  and the cover  300 . 
     Also, on the upper surface of the cover  300 , a structure for forming a reinforcement ring protrusion  308  that protrudes along the edge on the inner side of the reinforcement piece  55  on which the second engagement portion  66  is formed may be provided, and the reinforcement ring protrusion  308  is prepared for the purpose of improving the structural strength such as the valley  307 . 
     On the other hand, in order to keep the continuous heat insulation effect as described above, it furnishes a key to maximally lower the thermal conductivity of the body  10 , and for this, the space portion  900  is required to be kept nearly in vacuum state. The vacuum retaining portion  500  is prepared for this. 
     As shown in  FIGS. 1 and 27 , it is preferable that the protection cover  520  is configured to have a following structure, in which the remaining air in the space portion  900  is discharged through an air discharge tube  510 , the air discharge tube  510  is sealed by a separate finishing member (not illustrated), and then the air discharge tube  510  is surrounded by the protection cover  520 . 
     That is, the protection cover  520  may be configured to include a cylindrical wall  521  that is detachably engaged with the outer surface on the lower portion of the outer cylinder  200 , a bottom surface  521 ′ which extends from the edge of the lower end portion of the cylindrical wall  521  to a drain tube  101  communicating with the center of the bottom surface of the inner cylinder  100  and the center of which is penetrated, and a contact horn  523  which extends from the edge of the penetrated center of the bottom surface  521 ′ to a bottom surface  530  of the outer cylinder  200  and which is in close contact with the bottom surface  530  of the outer cylinder  200 . 
     That is, the protection cover  520  has a detachable structure which can be detached when it is required to discharge the remaining air in the space portion  900  through the air discharge tube  510  using a vacuum pump or the like in the case where the vacuum state of the space portion  900  becomes weak due to its long-time use. 
     Here, it is preferable that the edge of the end portion of the contact horn  523  is fixed in close contact with a contact ring protrusion  532  that projects in the form of a ring around the drain tube  101  from the bottom surface  530  of the outer cylinder  200 . 
     Specifically, the protection cover  520  is detachably engaged with a stepped engagement surface  220  having a stepped portion along the outer surface on the lower portion of the outer cylinder  200 . 
     In  FIG. 27 , the unexplained reference numeral “ 230 ” denotes a finishing sleeve that is prepared to make the bottom surface  530  of the outer cylinder  200 , on which a second protrusion ring  534  is formed, engaged with the outer periphery of the outer cylinder  200 . 
     On the cylindrical wall  521  of the protection cover  520 , a first locking ring groove  522  that is recessed along the outer periphery toward the stepped engagement surface  220  is provided, and on the stepped engagement surface  220 , a second locking ring groove  222  that is recessed toward the inner cylinder  100  in a position corresponding to the first locking ring groove  522  is provided, so that the first and second locking ring grooves  522  and  222  are engaged to provide a sealing structure. 
     As described above, the water tank having a sealing element according to the present invention facilitates the cleaning of the interior of the water tank and achieves an accurate sealing. Also, the water tank according to the present invention minimizes the energy consumption for heating or cooling water in the water tank through heightening of the heat insulation performance. 
     Although preferred embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.