Abstract:
A rectangular double walled cryogenic freezer has a vacuum space filled with layers of a reflective material. The support material is an open-celled three dimensional geometric grid that provides structural support for the freezer walls to prevent wall deformation when a vacuum is drawn.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to cryogenic freezers, and, more particularly, to a vacuum insulated cryogenic freezer that provides increased storage capacity. 
     Cryogenic freezers have a wide variety of industrial applications, including but not limited to, storing biological materials such as blood, bone marrow, and micro-organic cultures. These biological materials must be maintained at low temperatures in order to be stored for an extended period without deteriorating. 
     Cryogenic freezers are double walled, vacuum insulated containers partially filled with a cryogenic liquid such as liquid nitrogen for establishing an extremely cold storage environment. Liquid nitrogen has a low boiling point, 77.4 K (−320.4° F). Since cryogenic liquids have a low boiling point and, thus, a low heat of vaporization, heat inflow from the ambient can cause significant losses of cryogen due to the evaporation. 
     In order to minimize the amount of cryogen lost due to evaporation, the cryogenic freezer requires thermal and radiant barriers such as insulation and a high vacuum between the container walls. The vacuum space can also be filled with multiple layers of insulation to reduce heat transfer. 
     An example of multi-layered insulation is a low conductive sheet material comprised of fibers for reducing heat transfer by conduction. Also, the insulation can comprise radiation layers that are combined with the fiber layers. The radiation layer reduces the transmission of radiant heat in the freezer see for example U.S. Pat. No. 5,542,255 to Preston et al. and U.S. Pat. No. 5,404,918 to Gustafson. 
     The insulation and vacuum chamber of prior cryogenic freezers addresses the heat transfer problems due to the low boiling point of the cryogen. But, the characteristics of the insulation materials pose limitations to the physical design of the cryogenic freezers. 
     Containers have been designed with the vacuum space capable of maintaining a low pressure of 0.1 microns when the container is holding a cryogen. The shape of these containers has been restricted to a round, oval, or cylindrical structure. These structure provide the strength required by the walls of the container when such a high vacuum is drawn. If the cryogenic freezer were rectangular, the walls would collapse or deform when the vacuum is drawn due to insufficient structural support. Typically, the insulation materials disposed in the vacuum space of flat panel freezers fail to provide enough structural support for the container walls. Thus, the shape of the container is limited to cylindrical shapes. 
     Accordingly, it is desirable to provide a cryogenic freezer with optimum storage capacity such as a cube or rectangular enclosure which enables the walls of the freezer to maintain their shape when a high vacuum is drawn. 
     It is an object of the present invention to provide a cryogenic freezer that offers maximum storage capability at a low cost with flat interior and exterior walls. 
     It is another object of this invention to provide a cryogenic freezer with minimal thermal conductivity. 
     It is another object of the invention to provide a cryogenic freezer with reduced radiant energy transfer. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a cryogenic freezer for storing materials such as biological products. The cryogenic freezer is rectangularly shaped to provide the freezer with additional storage capacity. The cryogenic freezer includes an inner container with four walls and a bottom surface. The inner container is surrounded by an outer container also with four walls and a bottom surface. The inner and outer containers are secured together at the top edge such that there is a vacuum space defined therebetween. Alternate layers of a reflective insulating material and a support material comprised of a three dimensional geometric grid (geodesic structure) are placed in the vacuum space. The cryogenic freezer also includes a top which covers the inside of the freezer. A vacuum is drawn in the vacuum space creating the thermally insulated cryogenic freezer. The support grid prevents deflection or collapse of the walls due to the pressure differential. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 . is a side elevation view showing a section of the cryogenic freezer of the present invention. 
     FIG. 2 is a sectional view of the support material and the reflective material that are inserted between the inner and outer container of the cryogenic freezer as seen in FIG.  1 . 
     FIG. 3 is a perspective view of the support material that is inserted between the inner and outer container of the cryogenic freezer as seen in FIG.  1 . 
     FIG. 4 is a top view of the support material. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, the cryogenic freezer constructed in accordance with the present invention is indicated generally at  10 . The cryogenic freezer  10  features an inner container  12 , an outer container  14 , and a vacuum space  16  therebetween. The inner container  12  and outer container  14  are preferably constructed from stainless steel. The vacuum space  16  varies depending on the size of the freezer. Typical freezer dimensions are 27″×27″×35″(L×W×H). 
     The inner container  12  and the outer container l 4  each have four side walls and a bottom surface. A top  26  is pivotally connected to the top edge of the inner and outer containers  12 ,  14 . The rectangular freezer takes up the same amount of floor space as cylindrical shaped cryogenic freezers commonly known in the art. But, the larger volume of the rectangular design provides additional storage space in the freezer. 
     As seen in FIG. 2, the vacuum space  16  is filled with alternate layers of a reflective material  18  and a support material  22 . 
     The vacuum space includes a molecular sieve  24 . The molecular sieve  24 , can be, but is not limited to, a carbon or ceramic based material. The molecular sieve  24  is laid on the inside bottom surface of the outer container  14  during assembly. The molecular sieve  24  addresses the problem of out-gassing and chemically absorbs gas remaining after a vacuum is drawn. 
     Alternatively, getters, commonly known in the art, can be placed at the bottom of the freezer in the vacuum space. The getters also address the problem of out-gassing. The getters chemically absorb the gas remaining after a vacuum is drawn. 
     The reflective material  18  is comprised of pieces of reflective foil surrounding an insulating material, such as Supergel™ foam manufactured by Cabot Corporation. At least one piece of reflective foil is placed on either side of the insulating material. The air between the reflective foil and the insulating material is evacuated as the pieces of the reflective foil are sealed together. The reflective foil reduces the radiant energy that is transmitted through the vacuum space  16  between the inner container  12  and the outer container  14 . The insulating material  20  provides a thermal barrier between each layer of reflective foil. 
     FIG. 3 illustrates a perspective view of the three dimensional (geodesic) support material  22 . The support material  22  may be, but is not limited to, a composite, plastic, or a ceramic grid structure. The support  22  should be selected to limit the thermal conductivity and control out-gassing in the vacuum space. For example, the support material may be, but is not limited to, polyurethane, Ryton R4, Vectra LCP, Vectra E130, Noryl GFN- 3-801 , Ultem 2300, Valox 420, profax PP701N, Polypropylene Amoco, and Nylon 66. 
     The support material  22  provides physical support to the walls  12  and  14  so that when a vacuum is drawn, they do not collapse. The support material  22  can withstand the maximum pressure at full vacuum because of its lattice structure. The support material  22  uniformly distributes the load on the inner and outer walls  12 ,  14 . Thus, the thickness of the inner and outer container  12 ,  14  can be reduced. The yield strength of the support panel is greater than 15 psi. One source of such material is Molecular Geodesics, Inc. of Boston, Mass. 
     FIG. 4 illustrates a top view of the support material  22 . The support material  22  is configured with an open-cell structure with a minimal thermal transmission path to allow air to be evacuated out of the vacuum space  16  to form the vacuum. The open cell grid structure enables the molecular sieve  24  to absorb residual moisture and gas in the vacuum space to insure long vacuum life. 
     The low heat transfer coefficient (K≦0.001 w/   mk ) of the support material  22  will minimize the heat conducted from the outer wall  14  to the inner wall  12 . The support material  22  also reduces heat conductivity by maximizing the open space and minimizing direct contact between the support material  22  and the walls  12 ,  14 . 
     The cryogenic freezer  10  is assembled by placing the molecular sieve  24  on the inside bottom surface of the outer container  14 . Alternate layers of the reflective material  18  (and insulation) and the support material  22  are layered in the vacuum space such that the first and last layer placed are reflective material  18 . The inner container  12  is inserted into the outer container  14  so the final layer of reflective material  18  abuts against the outside of wall  12 . After the inner container  12  is positioned, the inner container  12  and the outer container  14  are welded together at their tips to seal the space  16  therebetween. 
     A vacuum is drawn in space  16  to increase the insulation value of the freezer. The cryogenic freezer  10  includes a port  28  in the wall  14  for that purpose. The port  28 -may be located at the rim of the top or on the bottom of the freezer. A vacuum pump, well known in the art, is connected to the port  28  to evacuate the air in the vacuum space  16 . Thereafter the port is sealed. 
     While the preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.