Patent Publication Number: US-8534079-B2

Title: Freezer with liquid cryogen refrigerant and method

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to freezers and, more particularly, to freezers that use liquid cryogen as a refrigerant. 
     BACKGROUND 
     Freezers for storing biological specimens, samples, materials, products and the like often use cryogenic liquids as a refrigerant. Such freezers typically feature a reservoir of a liquid cryogen, such as liquid nitrogen, in the bottom of the freezer storage chamber with the product stored above the reservoir or partly submerged with in the cryogenic liquid. The freezers typically also feature a double-walled, vacuum insulated construction so that the storage chamber is well insulated. Such freezers provide storage temperatures ranging from approximately −90° C. to −195° C. 
     A disadvantage of prior art liquid cryogen freezers is that the temperature cannot be directly controlled. The temperature is controlled by maintaining the amount of cryogenic liquid in the reservoir. The temperature of the freezer storage compartment thus varies dependent upon the amount of liquid cryogen in the freezer. 
     A further disadvantage of prior art liquid cryogen freezers is that there is some concern that submerging biological specimens in the cryogenic liquid presents a risk of cross-contamination between specimen containers. Even when the stored specimen containers are placed in the cold vapor above the cryogenic liquid reservoir, there is still the potential for the specimen containers to come into contact with, or be submerged within, the cryogenic liquid if the freezer is overfilled with the cryogenic liquid. 
     Also available are freezers that use mechanical refrigeration systems in place of a liquid cryogen reservoir. The mechanical refrigeration systems typically include a compressor, an evaporator, a condenser and a fan. Air is circulated through the storage chamber and across a cooling coil to maintain the desired temperature in the freezer storage chamber. The freezers normally do not feature vacuum insulation and employ materials such as foam and/or fiberglass insulation to insulate the storage chamber. Such freezers typically provide storage temperatures in the −40° C. to −80° C. range. 
     A disadvantage of the mechanical freezer is that the mechanical refrigeration system requires a significant amount of electrical power to maintain the desired temperature within the freezer storage chamber. Furthermore, mechanical refrigeration systems remove heat from the storage chamber and reject it to the environment around the freezer. This adds significant heat to the room within which the freezer is stored so that additional air conditioning capacity is required for the room. This adds additional electrical power requirements to the facility. In addition, in the event of a power failure, the storage chamber will warm rapidly, which could result in the loss of the stored biological materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of an embodiment of the freezer with liquid cryogen refrigerant of the present invention; 
         FIG. 2  is a flow chart showing the processing performed by the controller of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment of the freezer with liquid cryogen refrigerant of the invention is indicated in general at  10  in  FIG. 1 . The freezer includes an inner vessel  12  which defines storage chamber  14 . An outer jacket  16  generally surrounds the vessel  12  so that an insulation space  18  is defined between the inner vessel  12  and the outer jacket  16 . A vacuum is preferably drawn on the insulation space  18  so that the storage chamber  14  is insulated. In an alternative embodiment, the vacuum insulation space  18  may be supplemented, or replaced, by insulation materials known in the art including, but not limited to, foam or fiberglass. 
     An insulated plug or lid  20  is removably positioned within an offset access opening  22  of the freezer which permits access to the storage chamber  14 . The lid  20  is preferably mounted to the remaining portion of the freezer by hinged bracket  24 . A rotating tray  26  is positioned within the storage chamber  14  and holds the items being stored while also providing access through offset access opening  22  when the lid  20  is open. 
     The storage chamber  14  of the freezer, and thus the items stored therein, are cooled by a heat exchanger positioned within a top portion of the storage chamber. The heat exchanger preferably takes the form of a cooling coil  28 , but alternative heat exchanger components or structures could be used instead. 
     A storage container  29  containing a supply of liquid cryogen refrigerant is in communication with the inlet  30  of feed line  32 . Feed line  32  communicates with the inlet of cooling coil  28 . While liquid nitrogen is discussed below as the liquid cryogen refrigerant, it should be understood that alternative cryogenic liquids could be substituted for the liquid nitrogen. The liquid nitrogen is pressurized for transfer to the inlet  30  of the feed line  32  such as by a pump  33 . Alternatively, the liquid nitrogen could be stored under pressure in storage container  29  so that no pump is needed. Other alternatives for supplying cryogenic liquid under pressure are known in the art and may be used as well. 
     With regard to operation of the freezer of  FIG. 1 , all of the valves of the freezer initially are closed. When cooling of the storage chamber  14  is desired, the operator initiates the cooling cycle via electronic controller  34 . Controller  34  may be a microprocessor or any other electronic control device known in the art. As illustrated by block  43  of  FIG. 2 , the controller  34  of  FIG. 1  opens the automated bypass valve  42  so that liquid nitrogen flows through the inlet  30  of feed line  32 . 
     There will initially be gas in the transfer line connecting the inlet  30  of the feed line with the source of pressurized liquid nitrogen. This gas normally will be warmer than the storage chamber of the freezer. To prevent this gas from entering the heat exchanger, a bypass line  38  having an outlet  40  also communicates with a portion of the feed line  32  positioned between the inlet of the cooling coil  28  and the inlet  30  of the feed line. When the controller opens bypass valve  42 , the warm gas that enters through inlet  30  is vented through the bypass line  38  and outlet  40 . 
     The temperature of the gas entering the feed line  32  is monitored by feed temperature sensor  44 , which also communicates with controller  34 . When the temperature of the incoming gas (indicated as T G  in decision block  45  of  FIG. 2 ) has cooled to a temperature below that of the freezer storage chamber  14  (indicated as T CH  in decision block  45  of  FIG. 2 ), the controller closes bypass valve  42  and a purge gas valve  46  is opened, as indicated at  48  and  50 , respectively, in  FIG. 2 . 
     As a result, liquid nitrogen refrigerant flows through the cooling coil  28 . The liquid nitrogen flowing through the cooling coil is colder than the gas inside of storage chamber  14  so that it absorbs heat from inside of the chamber. As the liquid nitrogen absorbs the heat, it is vaporized and exits the heat exchanger taking the absorbed heat with it. 
     As illustrated by arrows  51   a  and  51   b  in  FIG. 1 , the resulting cold gas surrounding the heat exchanger inside the storage chamber circulates throughout the chamber via natural convection. More specifically, the higher density cold gas from the top portion of the chamber within which the cooling coil is positioned descends (arrows  51   a ) thus forcing warmer lower density gas to rise (arrows  51   b ) to be cooled by the cooling coil. 
     As illustrated in  FIG. 1 , the open purge gas valve  46  is positioned on the outlet side of the heat exchanger. The vaporized nitrogen refrigerant exits the outlet of the heat exchanger through exit line  52  and travels into purge line  54 , since exhaust valve  56  is in a closed condition. Purge line  54  is provided with purge outlets  62  positioned adjacent to and over the cooling coil so that the nitrogen gas exits the purge line as a purge gas and provides additional cooling to the storage chamber  14 . 
     In addition, ice formation on the exterior surface of the cooling coil  28  can insulate it from the storage chamber of the freezer and reduce the coil&#39;s cooling effectiveness. The nitrogen purge gas exiting the purge outlets  62  above the cooling coil  28  is a dry gas. This dry nitrogen purge gas displaces ambient air (which could contain water) from the space around the exterior surface of the cooling coil to reduce the possibility of ice forming on the coil. Furthermore, when the process of  FIG. 2  is performed, the purge typically continues until a sufficient amount of dry nitrogen purge gas is introduced to the chamber to displace any moist air in the chamber. 
     To prevent purge gas that is substantially colder than the desired storage chamber temperature of the freezer from discharging into the chamber  14 , the controller  34  monitors the temperature of the purge gas via a purge gas temperature sensor  64 . When the temperature of the purge gas (indicated as T P  in decision block  66  of  FIG. 2 ) traveling through purge line  54  is cooled to the minimum desired temperature of the storage chamber of the freezer (indicated as T Dmin  in decision block  66  of  FIG. 2 ), the purge gas valve  46  is closed by the controller  34 , as indicated at  72  in  FIG. 2 . 
     When the purge gas valve  46  is closed, the cooling gas exhaust valve  56  is opened by the controller  34 , as indicated at  73  in  FIG. 2 , to vent nitrogen gas from the cooling coil external to the freezer via the exhaust line  74  and exhaust vent  76 . As long as the cooling coil  28  is at a temperature less than that of the gas inside of the storage chamber  14 , convection cooling will occur. 
     The controller  34  monitors the exhaust gas temperature via an exhaust gas temperature sensor  82 . When the temperature of the nitrogen exhaust gas flowing through exit line  52  and exhaust line  74  (indicated as T E  in decision block  78  of  FIG. 2 ) cools to a temperature approximately 10° C. to 20° C. below the minimum desired storage chamber temperature of the storage chamber (indicated as T Dmin −X in decision block  78  of  FIG. 2 ), the exhaust valve  56  is closed by the controller, as indicated at  84  in  FIG. 2 , so that the flow of liquid nitrogen into the cooling coil is paused. The nitrogen (liquid or gaseous) in the cooling coil then absorbs heat from the chamber and expands or evaporates so that no-flow cooling is accomplished. While the predetermined amount X above and in decision block  78  of  FIG. 2  is preferably approximately 10° C. to 20° C., alternative temperature amounts may be used instead. 
     The exhaust gas temperature sensor  82  is positioned external to the freezer. As a result, it is warmed by ambient external air while there is no flow through the cooling coil  28 . Once the exhaust gas temperature sensor detects that the gas within line  52  has warmed above the maximum desired storage chamber temperature (indicated as T Dmax  in decision block  86  of  FIG. 2 ), the exhaust valve  56  is again opened by the controller. 
     As indicated by decision block  90  of  FIG. 2 , the exhaust valve  56  is cycled in accordance with the above until the freezer storage chamber  14  cools to the minimum desired temperature as measured by a chamber temperature sensor  92 . At that time, as indicated at decision block  94 , all valves are closed and the controller simply monitors the storage chamber temperature. 
     As indicated by decision block  96 , when the storage chamber temperature of the storage chamber again warms to the maximum desired temperature, as measured by the chamber temperature sensor  92 , the bypass valve  42  is again opened by the controller and the process of  FIG. 2  begins again. 
     The freezer of  FIGS. 1 and 2  therefore removes heat from the storage chamber by vaporizing the liquid nitrogen in the cooling coil and then venting the gas outside of the freezer, and outside of the room within which the freezer is located, if desired. The gas created by vaporizing the liquid nitrogen can only be warmed to the temperature of the freezer storage chamber instead of above ambient as is the case with the refrigerant of a typical prior art mechanical freezer. As a result, no heat is added to the room within which the freezer is located to increase the air conditioning required for the room. 
     The freezer of  FIGS. 1 and 2  also allows for control of the freezer temperature, not possible with typical prior art liquid cryogen freezers, without the disadvantages of a mechanical freezer. In addition, the freezer of  FIGS. 1 and 2  prevents the stored product from making contact with and/or being submerged within the liquid cryogen by removing the liquid cryogen from the storage chamber of the freezer. 
     The freezer of  FIGS. 1 and 2  also eliminates the mechanical refrigeration components used by typical prior art mechanical freezers and thus the associated large electrical power requirements. Minimal power is required by the freezer of  FIGS. 1 and 2  to operate the controller that monitors and controls the freezer and the associated solenoid valves required for operation. 
     Furthermore, in the event of a power failure, the freezer of  FIGS. 1 and 2  is not immediately effected. Since the freezer incorporates a vacuum-insulated storage chamber, the storage chamber temperature is maintained over a longer period of time, thus requiring infrequent cooling cycles as opposed to the continuous cooling required by typical prior art mechanical freezers. This provides sufficient time to address power failure issues before the storage temperature inside the freezer is effected. 
     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.