Patent Publication Number: US-2002000443-A1

Title: Enclosure thermal shield

Description:
[0001] This application claims the benefit of U.S. Provisional Application No. 60/215,713 filed Jul. 3, 2000. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] This invention relates to a thermally insulated container, and more particularly to a thermally insulated container having a thermal shield designed to conduct thermal energy to or from a heat reservoir to maintain a uniform temperature within the container.  
       [0004] 2. Description of Prior Art  
       [0005] Prior insulated containers rely on the thermal resistivity of the material comprising the container and convection currents and a heat reservoir within the container chamber to maintain a desired thermal environment within the container. A typical prior art container designed to maintain cool temperatures is a polystyrene plastic box with ice or a frozen gelpack inside the box&#39;s payload region. A significant problem with this approach is the heat flux through the box walls. Depending on the thermal resistivity of the insulation and the ambient temperature outside the box, the heat leak into the box can be significant. The resulting heat load must be convectively carried to the heat reservoir to maintain constant temperature within the box.  
       [0006] Note a similar problem exists in reverse if a hot product is the payload and a heat source such as a hot brick is the heat reservoir. Everything stated below will be limited to the cold payload situation, but the present invention is not limited to that.  
       [0007] Prior art insulated containers have proved unsuitable for products that require tight temperature tolerances. Excessive heat gain can exhaust the heat reservoir, causing the temperature to rise rapidly with additional heat gain. Temperature variation can exceed tolerances because the heat reservoir may absorb too much heat from the product itself, lowering its temperature to an unacceptable level. The temperature gradient within the payload volume may be unacceptably large because the warmer air that accumulates near the top of the container is somewhat removed from the colder air surrounding the heat reservoir. Depending on the extent of temperature gradient, a payload could conceivably be too cold at the lower end and too warm on the upper end.  
       SUMMARY OF THE INVENTION  
       [0008] The present invention uses an innovative design to produce an enclosure thermal shield having a thermally insulated open container, a thermally insulated closure member, a thermally conductive liner along the container&#39;s inner surface and along the inner surface of the closure member that forms a thermal circuit when the closure member closes the container, and a heat reservoir in thermal contact with the thermal circuit. The heat reservoir can be placed within the container or incorporated into the closure member. If incorporated into the closure member, the heat reservoir can be placed in direct thermal contact with the thermal circuit or connected to the thermal circuit via a thermal conduit. The thermal shield can further comprise a layer of insulating material lining the interior surface of the conductive liner to further inhibit heat transfer into or out of the interior chamber of the container. The thermal shield and method for directing heat flow regulate the thermal environment of the chamber. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] So that the manner in which the described features, advantages and objects of the invention, as well as others which will become apparent, are attained and can be understood in detail, more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof that are illustrated in the drawings, which drawings form a part of this specification. It is to be noted, however, that the appended drawings illustrate only typical preferred embodiments of the invention and are therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.  
     [0010] In the drawings:  
     [0011]FIG. 1 is a cross section of an elevation view of a first embodiment of an enclosure thermal shield constructed in accordance with the present invention.  
     [0012]FIG. 2 is a cross section of an elevation view of a second embodiment of an enclosure thermal shield constructed in accordance with the present invention.  
     [0013]FIG. 3 is a cross section of an elevation view of a third embodiment of an enclosure thermal shield constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION  
     [0014] Referring to FIG. 1, enclosure thermal shield  10  comprises an open container  12  and closure member  14 , both of which are constructed using a highly thermally resistive material such as polystyrene plastic or vacuum insulation panels. Thermally conductive liner  16  lines the interior surface of container  12  and the lower surface of closure member  14 . Container  12  and closure member  14  each have a shoulder  15  which abut when closure member  14  closes container  12 . Closure member  14  fits snugly in container  12  to form an airtight seal and, when shoulders  15  are in abutting contact, thermally conductive liner  16  is also in abutting contact to complete a thermal circuit for conductive liner  16 . Heat reservoir  18  is placed in container  12  in thermal contact with liner  16 .  
     [0015] As stated above, heat reservoir  18  can be hot or cold, depending on the application. An ideal heat reservoir remains at a constant temperature independent of the amount of heat put onto or withdrawn from it. Thus, a heat reservoir is useful as a thermostatic device because it will maintain a constant temperature for the environment in thermal contact with it. Heat reservoir  18  approximates an ideal heat reservoir, but actually is more like a heat sink or source in the sense it generally either absorbs or delivers heat, depending on the application. We choose the term “heat reservoir” because the thermal mass of the material being used as a heat reservoir will generally be large relative to the anticipated heat load, such that the temperature of the heat reservoir will not change appreciably during its expected period of use. “Heat reservoir” also conveys the idea that it can absorb or deliver heat, although as a practical matter it generally is intended to do one or the other. For ease of discussion, the description below shall be limited to the cold temperature/heat sink scenario.  
     [0016] In such a situation, it is anticipated that the enclosure thermal shield  10  will be placed in an ambient environment that is warmer than the desired temperature of a payload. Thus, there will be a net flux of heat toward the container&#39;s interior chamber  20 . Ordinarily, heat  22  (represented by squiggly arrows in figures) would pass through the thermally resistive material comprising container  12  and closure member  14 . Without conductive liner  16 , heat  22  would enter chamber  20 . However, conductive liner  16  absorbs heat  22  and directs it to heat reservoir  18 . Heat reservoir  18  absorbs the infiltrated heat  22  and traps it within the reservoir  18 . Thus, the infiltrated heat  22  is intercepted and transported away from the container&#39;s interior chamber. The embodiment of FIG. 1 relies on convection to minimize the thermal gradient in chamber  20 .  
     [0017] While the vast majority of heat  22  will be conducted into heat reservoir  18 , it is possible that some of heat  22  will radiate or conduct from conductive liner  16  and enter chamber  20  as heat  24  (represented by small squiggly arrows in FIGS. 2 and 3). The embodiments of FIGS. 2 and 3 add insulation layer  26  onto the interior surface of conductive liner  16 . Insulation layer  26  reduces heat transfer from liner  16  into chamber  20 . Thus, very nearly all of infiltrated heat  22  is conducted into heat reservoir  18 , minimizing the amount of heat  24  that actually enters chamber  20 .  
     [0018]FIGS. 2 and 3 show heat reservoir  18  in closure member  14  instead of within chamber  20  as was done in the embodiment of FIG. 1. In FIG. 2, heat reservoir  18  is placed in direct thermal contact with the outer surface of liner  16 . Placing heat reservoir  18  in closure member  14  allows for greater payload capacity and allows one to chill heat reservoir  18  and closure member  14  as a unit in anticipation of enclosure thermal shield&#39;s  10  next application. Having beat reservoir  18  on top also increases the convection efficiency when used to cool chamber  20  and minimizes the temperature gradient within chamber  20 .  
     [0019] In FIG. 3, heat reservoir  18  is within closure member  14 , but separated from liner  16  by the insulation material of closure member  14 . Heat reservoir  18  is thermally linked to liner  16  by thermal conduit  28 . Conduit  28  allows one to control the rate of heat transfer into heat reservoir  18 . For example, conduit  28  can be a thermal conductor sized according to expected heat loads and the desired temperature range within chamber  20  to regulate heat transfer. Thermal conduit  28  can also comprise a thermally resistive material. Additional alternative embodiments for conduit  28  include an air passage, a material that switches state, a thermoelectric device, or a thermal switch.  
     [0020] The present invention offers many advantages over the prior art. The temperature gradient within a container using the thermal shield varies less than in prior art containers. By placing less demand on convection for heat transfer, the temperature within the container is better regulated. Using a thermal conduit allows use of a subcooled heat reservoir without risk of excess heat transfer, thus precluding the possibility of a product being destroyed as a result of excess chilling.  
     [0021] While the invention has been particularly shown and described with reference to a preferred and alternative embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.