Abstract:
A system for dispensing cryogenic liquid to a use point includes a bulk tank containing a supply of cryogenic liquid and a pressure builder that is in communication with the tank via a pressure building valve. The pressure builder uses heat exchangers to vaporize a portion of the cryogenic liquid as needed to pressurize the bulk tank. The pressurized cryogenic liquid is dispensed through a dispensing line running from the bottom of the tank. A vent valve also vents vapor from the tank to control pressure. Operation of the vent and pressure building valves is automated by a controller that receives data from sensors. The controller determines the required saturation pressure for the tank and varies the tank pressure to match and provide a generally constant outlet pressure depending on conditions of the cryogenic liquid.

Description:
CLAIM OF PRIORITY 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 13/216,666, filed Aug. 24, 2011, currently pending. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to systems for storing and dispensing fluids and, more particularly, to a bulk cryogenic liquid pressurized dispensing system and method. 
       BACKGROUND 
       [0003]    It is well known that cryogenic liquids, or liquids having similar properties, have found great use in industrial refrigeration and freezing, cryo-biological storage repository and lab test applications. Cryogenic liquids are typically stored in thermally insulated bulk tanks which consist of an inner vessel mounted inside, and thermally isolated from, an outer vessel. The liquid is then directed from the tank through thermally isolated pipes to a supply point where it is used for a variety of applications such as industrial, medical, or food processing. 
         [0004]    Prior art bulk tanks typically use a pressure regulator at the top of the bulk tank. Such a system is limited in its flexibility. When the tank is full there is a certain amount of liquid head pressure. This head pressure is added to the tank vapor pressure and this is the supply pressure out of the tank. For some applications it may be important to maintain a constant supply pressure. As the liquid level in the tank drops from usage the vapor pressure in the tank needs to increase to compensate for the decrease in head pressure. 
         [0005]    A mechanical pressure regulator is set to open when the pressure in the bulk tank drops below a set point and closes when it rises above the set point. The regulator is usually set to provide enough pressure inside the tank to operate at low liquid levels. This means that the supply pressure will be higher when the tank is full and drop off as the liquid level drops. As a result, a user may experience product losses or loss in efficiency near the bottom of the tank. This is not ideal for high flow rates where the condition of the supplied cryogenic liquid is important. 
         [0006]    Failure to install a properly designed system for storing and dispensing cryogenic liquid with consistent quality causes wasted energy in lost cooling power. The poor control of the liquid conditions allows the outlet pressure to fluctuate so wildly that many times customers cannot utilize the lower one-third of the tank&#39;s capacity. The primary culprit of this complaint stems from a reduction in tank outlet pressure (tank vapor+liquid head pressure) at the liquid withdrawal point. This leads to a reduction in liquid flow rate at the application and as a result, inconsistent cooling. 
         [0007]    In applications such as food freezing where the product is moving at a specified rate in the tunnel, it&#39;s critical that the quality of the cryogenic liquid being dispensed is consistent so the process can be tuned for maximum production throughput. If it becomes out of tune from liquid conditions changing at the application, the only recourse a plant manager has control over (other than slowing down production) is to call their liquid supplier and expedite the tank refill in order to restore the liquid to pre-tuned conditions. Not only is this an emergency delivery, but it&#39;s usually before the desired refill point so the tank can&#39;t take a full trailer load. The fresh liquid resolves the problem because it is usually colder and lowers the overall liquid saturation pressure, but more importantly, the pressure at the bottom of the tank is increased so the tuned liquid nitrogen flow rate is restored. A simple electrical analogy is like a voltage outage has just been restored. The cryogenic food freezer, like any electrical appliance wants to run on a constant supply pressure or voltage, so the liquid nitrogen flow rate or amperage draw remains constant. 
         [0008]    A need therefore exists for a bulk cryogenic liquid pressurized dispensing system and method that addresses the above issues. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIGS. 1A-1C  are schematic views illustrating a liquid CO 2  tank filled, approximately half full and in need of refilling, respectively; 
           [0010]      FIG. 2  is a perspective view of an alternative embodiment of the baffle of the system of the present invention; 
           [0011]      FIG. 3  is a graph illustrating improvements in snow yield v. temperature possible with the system of  FIGS. 1A-1C ; 
           [0012]      FIG. 4  is a perspective view showing an alternative embodiment of the heat exchanger coil of the system and method of  FIGS. 1A-1C ; 
           [0013]      FIG. 5  is a side elevational view of the heat exchanger coil of  FIG. 4 ; 
           [0014]      FIG. 6  is a schematic view illustrating an embodiment of the system of the invention; 
           [0015]      FIG. 7  is a graph illustrating how the outlet pressure of the system of  FIG. 6  stays generally constant in accordance with an embodiment of the method of the invention; 
           [0016]      FIG. 8  is a flow chart illustrating the processing performed by the programmable logic controller of the system of  FIG. 6  in controlling the vent valve in accordance with an embodiment of the system and method of the invention; 
           [0017]      FIG. 9  is a flow chart illustrating the processing performed by the programmable logic controller of the system of  FIG. 6  in controlling the pressure building valve in accordance with an embodiment of the system and method of the invention: 
           [0018]      FIG. 10  is a schematic view illustrating an alternative embodiment of the system of the invention; 
           [0019]      FIG. 11  is a flow chart illustrating the processing performed by the programmable logic controller of the system of  FIG. 10  in controlling the vent valve in accordance with an embodiment of the system and method of the invention; 
           [0020]      FIG. 12  is a flow chart illustrating the processing performed by the programmable logic controller of the system of  FIG. 10  in controlling the pressure building valve in accordance with an embodiment of the system and method of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0021]    A system, indicated in general at  10  in  FIGS. 1A-1C  includes a bulk tank, indicated in general at  12 , that includes an inner tank  14  surrounded by outer jacket  16 . The tank preferably is vertically oriented, being sized so as to have a height that is greater than the width of the interior  17  of the inner tank  14 . Inner tank  14  is preferably sized to hold a reservoir of liquid having a depth of at least 6 feet. The annular insulation space  18  defined between the inner tank  14  and outer jacket  16  may be vacuum-insulated and/or at least partially filled with an insulation material so that inner tank  14  is insulated from the ambient environment. As an example only, the insulation material may include multiple layers of paper and foil that are preferably combined with the vacuum insulation in the annular insulation space. 
         [0022]    When used for food freezing and/or refrigeration processes, the inner tank  14  is preferably constructed of grade T304 stainless steel (food grade). Such an inner tank provides operating temperatures down to −320° F. at pressures of around 350 psig. Outer jacket  16  is preferably constructed of high grade carbon steel. Pre-existing tanks could be retrofitted with stainless steel inner tanks for use in food processing applications of the present invention. 
         [0023]    While the invention will be described below in terms of liquid carbon dioxide for use in food refrigeration and/or freezing processes, it should be understood that the invention may be used for other liquids useful in refrigeration and/or freezing related processes, including cryogenic liquids. 
         [0024]    As illustrated in  FIGS. 1A-1C , the inner tank  14  features a top portion  19  to which a fill vent line  20  is connected. In addition, a liquid fill line  22  is connected to a lower portion of the inner tank  14 , as will be described in greater detail below. The distal end of the fill vent line  20  is provided with a fill vent valve  24  while the distal end of the liquid fill line  22  is provided with liquid fill valve  26 , and both are adapted to be connected to a source of liquid, such as a tanker truck, for refilling the bulk tank. The fill vent line  20  provides a vapor balance during the refilling operation. 
         [0025]    A baffle  30  is positioned within the lower portion of the interior tank  14 . The baffle is preferably constructed of stainless steel and has a thickness of approximately 0.105 inches. The baffle features a shallow cone shape and is circumferentially secured to the interior surface of the inner tank  14 . The baffle features a number of openings  32  that permit passage of liquid. The functionality of the baffle will be explained below. 
         [0026]    An internal heat exchanger coil  34  is positioned in the bottom portion  35  of the tank and is connected by coil inlet line  36  to a refrigeration system  38 . A coil outlet line  42  joins the internal heat exchanger coil  34  to the refrigeration system  38  as well. Coil inlet line  36  optionally includes a coil inlet valve  44  while coil outlet line  42  optionally includes a coil outlet valve  46 . 
         [0027]    While a single coil heat exchanger is indicated at  34  in  FIGS. 1A-1C , the heat exchanger could alternatively feature a number of coils, connected either in series or in parallel or both. For example, an alternative embodiment of the heat exchanger coil  34  is indicated in general at  45  in  FIGS. 4 and 5 . As indicated in  FIGS. 4 and 5 , the heat exchanger  45  includes four coils  47   a ,  47   b ,  47   c  and  47   d  connected in parallel with an inlet  49  and an outlet  51 . Alternatively, coils  47   a - 47   d  could be connected in series. As another example, the heat exchanger coil may include two or more concentric coils connected in parallel or in series. 
         [0028]    A liquid dispensing or feed line  52  exits the bottom  53  of the inner tank  14  and is provided with liquid feed valve  54  and liquid feed check valve  56 . 
         [0029]    A pressure builder inlet line  60  also exits the bottom portion of the inner tank  14  and connects to the inlet of pressure builder  62 . The pressure builder inlet line  60  is provided with a pressure builder inlet valve  64 , and automated pressure builder valve  66  and a pressure builder check valve  68 . A pressure builder outlet line  72  exits that pressure builder  62  and travels to the top of the inner tank  14 . The pressure builder outlet line  72  is provided with a pressure switch  74  and a pressure builder outlet valve  76 . As will be explained in greater detail below, the pressure switch  74  is connected to the automated pressure builder valve  66 . 
         [0030]    In operation, with reference to  FIG. 1A , after the tank  12  has been filled, the inner tank  14  contains a supply of liquid CO 2    80  with a headspace  82  defined above. Fill valves  24  and  26 , feed valve  54  and automated pressure builder valve  66  are closed, while coil inlet and outlet valves  44  and  46  and pressure builder inlet and outlet valves  64  and  76  are open. While the description below assumes that the feed valve  54  is closed, it may be open in alternative modes of operation, also described below. As an example only, the refill transport provides the liquid CO 2  at a pressure of approximately 270 psig and a temperature of approximately −10° F. 
         [0031]    The pressure switch  74  senses the pressure in headspace  82  via pressure builder outline line  72 . If the pressure is below the target pressure of 300 psig, the pressure switch  74  opens automated pressure builder valve  66  so that liquid CO 2  flows to the pressure builder  62 . The liquid CO 2  is vaporized in the pressure builder and the resulting gas travels through line  72  to the headspace  82  so that the pressure in inner tank  14  is increased. Pressure builder check valve  68  prevents burp backs through the pressure builder inlet line  60  and into the bottom of the tank that could cause undesirable mixing between the liquid CO 2  below the baffle and the remaining liquid CO 2  above the baffle. Pressure building continues until pressure switch  74  detects the target pressure of 300 psig in the inner tank  14 . When the pressure switch detects the pressure of 300 psig, it will close the automated pressure builder valve  66  so that pressure building is discontinued. At this pressure, the liquid CO 2    80  will have an equilibrium temperature of approximately 0° F. 
         [0032]    The bottom portion of the tank is provided with a temperature sensor  90 , such as a thermocouple, that communicates electronically with a temperature controller  92 . Sensor  90  can alternatively be a pressure sensor or a saturation bulb. The temperature controller  92  controls operation of the refrigeration system  38  and may be a microprocessor or any other electronic control device known in the art. When the temperature controller detects, via the temperature sensor, a temperature that is higher than the desired or target temperature, it activates the refrigeration system  38 . Continuing with the present example, the temperature sensor detects the 0° F. temperature of the liquid CO 2  in the inner tank and activates the refrigeration system  38 . A refrigerant fluid in liquid form then travels through line  36  to the internal heat exchanger coil  34  and is vaporized so as to subcool the liquid CO 2  in the bottom portion of inner tank  14 . The vaporized refrigerant fluid travels back to the refrigeration system  38  via line  46  for regeneration. More specifically, the refrigeration system  38  includes a condenser for re-liquefying the refrigerant fluid. As an example only, the refrigerant fluid is preferably R-404A/R-507. 
         [0033]    The refrigeration system and internal heat exchanger coil continue to subcool the liquid CO 2  in the bottom portion of the inner tank until the target temperature, −40° F. for example, is reached. The temperature controller  92  senses that the target temperature has been reached, via the temperature sensor  90 , and shuts down the refrigeration system  38 . 
         [0034]    Due to stratification in the inner tank and the baffle  30 , even though the liquid CO 2  below the baffle has been subcooled, the pressure remains at  300  psig for pushing the liquid CO 2  from the tank during dispensing. The headspace  82  preferably operates at  300  psig to allow direct replacement of older systems so as not to alter the food freezing equipment set up for 300 psig. More specifically, stratification occurs throughout the liquid CO 2    80  between the CO 2  gas in the headspace  82  of the inner tank and the subcooled liquid CO 2  in the bottom portion of the tank. The baffle assists in the stratification by creating a cold zone in the bottom of the tank that is mostly insulated from the remaining liquid CO 2  above the baffle. This improves the efficiency of the internal heat exchanger coil in subcooling the liquid beneath the baffle and inhibits migration of the subcooled liquid into the warmer liquid above the baffle. As a result, the tank holds an inventory of high pressure equilibrium liquid CO 2  in the region above the baffle, similar to that available from a conventional high pressure storage vessel, and an inventory of high pressure, subcooled liquid CO 2  in the region or zone below the baffle. 
         [0035]    As an example only, for a tank having an inner tank height of 29 feet, and an inner tank width of 8 feet, the baffle  30  would ideally be positioned 7 feet from the bottom of the tank. In general, the baffle  30  is preferably positioned approximately 24% of the total height of the inner tank from the bottom of the inner tank or at a level where approximately 30% of the tank volume is below the baffle. 
         [0036]    When the tank target pressure and target subcooled liquid temperature have been reached, the liquid feed valve  54  may be opened so that the subcooled liquid CO 2  may be dispensed through feed line  52  and expanded at atmospheric pressure to make snow or otherwise used for a food freezing or refrigeration process. In an alternative mode of operation, the liquid feed valve  54  may be left open during filling for operation of the system during filling or prior to full refrigeration at a reduced efficiency. Check valve  56  prevents burp backs through the feed line  52  and into the bottom of the tank that could cause undesirable mixing between the subcooled liquid CO 2  and the remaining liquid CO 2  above the baffle. 
         [0037]    As illustrated in  FIG. 1A , the liquid feed line  52  is provided with a pressure relief check valve  94  that communicates with fill vent line  20  via liquid feed vent line  95 . In the event that the pressure within the feed line  52  rises above a predetermined level, the pressure relief valve  94  automatically opens so that pressure is vented through line  20 . 
         [0038]    As illustrated in  FIG. 1B , the level of the liquid CO 2    80  drops as liquid CO 2  is dispensed through feed line  52 . As this occurs, liquid CO 2  travels from the region above the baffle  30 , through the openings  32  of the baffle, and into the zone below the baffle. Temperature sensor  90  constantly monitors the temperature of the liquid CO 2  in the zone below baffle  32  and pressure switch  74  constantly monitors the pressure within the head space  82  above the liquid CO 2 . The pressure switch opens the automated pressure building valve  66  as is necessary to maintain and hold the tank operating pressure at approximately 300 psig via the pressure builder  62 . Temperature sensor  90  and temperature controller  92  similarly activate refrigeration system  38  as is necessary to maintain the temperature of the liquid CO 2  in the zone below the baffle at approximately −40° F. via the internal heat exchanger coil  34 . 
         [0039]    It should be noted that alternative automated control arrangements known in the art may be substituted for the temperature sensor and controller  90  and  92  and/or the pressure switch and automated pressure building valve  74  and  66 . For example, in an alternative embodiment of the system, a single system programmable logic controller (PLC) is connected to a pressure sensor in the head space  82  of the tank and the temperature sensor  90  so as to control operation of the refrigeration system  38  and the pressure builder  62 . 
         [0040]    With reference to  FIG. 1C , when the level of liquid CO 2  reaches 25% above the baffle  30 , dispensing of liquid CO 2  through feed line  52  may be halted by closing feed valve  54 . In the PLC embodiment, feed valve  54  is automated and a liquid level detector, which is in communication with the PLC, is positioned in the tank. The liquid level detector signals the PLC when the liquid level in the tank reaches the 20% above baffle  30  level, and the PLC then automatically shuts the feed valve  54  and provides a notification to the user, such as an illuminated light or audible warning. 
         [0041]    It should be noted that liquid may be dispensed to levels lower than 25% above the baffle, but the heat exchanger coil  34  may become less efficient as the liquid level drops lower than the coil. 
         [0042]    A tanker truck, or other liquid CO 2  delivery source, is connected to the fill vent line  20  and the liquid fill line  22  via fill connections  102 . Fill vent valve  24  and liquid fill valve  26  are opened so that the inner tank  14  is refilled with liquid CO 2 . 
         [0043]    As an alternative to shutting feed valve  54 , when the level of liquid CO 2  in the tank reaches the level 20% above the baffle,  32 , the tanker truck, or other CO 2  liquid delivery source, may be connected to fill connections  102 , and the dispensing of liquid CO 2  may continue uninterrupted. The pressure builder  62  and refrigeration system  38  and coil  34  operate under the direction of the pressure switch  74  and automated pressure building valve  66  and the temperature sensor  90  and temperature controller  92  as described above to maintain the approximate 300 psig pressure and −40° F. temperature (below baffle  30 ) within inner tank  14 . As a result, the system permits the delivery of subcooled liquid CO 2  to continue uninterrupted. 
         [0044]    As noted previously, the baffle  30  helps separate the liquid underneath the baffle from the liquid above so that the liquid below is not disturbed. This increases the efficiency in creating and maintaining the subcooled state of the liquid CO 2  below the baffle. Positioning the fill line opening  104  of the liquid fill line  22  above the baffle helps prevent the incoming liquid CO 2  from disturbing the subcooled liquid CO 2  under the baffle, which further aids in increasing efficiency in creating and maintaining the subcooled state of the liquid CO 2  below the baffle. 
         [0045]    An example of a suitable pressure builder  62  is the sidearm CO 2  vaporizer available from Thermax Inc. of South Dartmouth, Mass. An example of a suitable refrigeration system  38  is the Climate Control model no. CCU1030ABEX6D2 condensing unit available from Heatcraft Refrigeration Products, LLC of Stone Mountain, Ga. 
         [0046]    While the baffle of  FIGS. 1A-1C  is shown to be cone shaped, the baffle alternatively could be provided with a disk shape, as illustrated at  130  in  FIG. 2 . The baffle  130  is also preferably constructed from stainless steel that is approximately 0.105 inches thick and includes openings  132  and  134  to permit liquid CO 2  to travel from the upper region of inner tank  114  to the zone or region below the baffle. 
         [0047]    As yet another alternative embodiment of the baffle, the baffle takes the form of a plurality of glass or STYROFOAM insulation beads, indicated in phantom at  138  in  FIG. 1B , that float between upper and lower screens  140  and  142 , respectively. The screens may be mounted to ring-like frames that are circumferentially attached to the interior surface of inner tank  13 . The bead material is chosen so that the beads have a density which allows them to float on the denser subcooled liquid CO 2  up to the level of upper screen  140 . The beads are large enough in both size and number that the cross section of the inner tank  14  is generally covered. As a result, the beads form a floating baffle arrangement that creates an insulation layer between the subcooled liquid CO 2  below and the remaining liquid CO 2  above. In this regard, reference is made to U.S. Pat. No. RE35,874, the contents of which are hereby incorporated by reference. 
         [0048]    By dispensing subcooled liquid CO 2 , the present invention improves snow yield when the liquid is expanded to ambient pressure, as illustrated in  FIG. 3 . More specifically, by subcooling the liquid CO 2  in the region or zone below the baffle, the snow yield rises from slightly over 42% for liquid CO 2  at equilibrium temperature for 0° F. to over 52% at equilibrium temperature for −43° F. This equates to an increase in refrigeration capacity of the subcooled liquid CO 2 , which permits faster food throughput in food freezing operations. An example of suitable snow making equipment (snowhorn), which was used to create the data of  FIG. 3 , is available from Gray Tech Carbonic, Inc. 
         [0049]    The increase in snow yield and refrigeration capacity of the above system results in less carbon dioxide consumption. As a result, there is less CO 2  gas delivered to the environment, which makes the system and method of the invention a “green” technology. In addition, the baffle of the system increases the efficiency of the refrigeration system in subcooling the liquid CO 2  below the baffle. This permits smaller, and thus more efficient, compressors to be used in the refrigeration system. 
         [0050]    An embodiment of the system of the invention is indicated in general at  200  in  FIG. 6 . Similar to the system  10  of  FIGS. 1A-1C , the system  200  includes a bulk tank, indicated in general at  212 , that includes an inner tank  214  surrounded by outer jacket  216 . The tank preferably is vertically oriented, being sized so as to have a height that is greater than the width of the interior  217  of the inner tank  214 . The annular insulation space  218  defined between the inner tank  214  and outer jacket  216  may be vacuum-insulated and/or at least partially filled with an insulation material so that inner tank  214  is insulated from the ambient environment. As an example only, the insulation material may include multiple layers of paper and foil that are preferably combined with the vacuum insulation in the annular insulation space. 
         [0051]    As an example only, bulk tank  212  may range in size from 11,000 gallons to 16,000 gallons and may have a pressure capacity of 175 psig. Examples of tank size include 114 inches in diameter with a height ranging from 450 inches to 600 inches. When used for food freezing and/or refrigeration processes, the inner tank  214  is preferably constructed of grade T304 stainless steel (food grade). Outer jacket  216  is preferably constructed of high grade carbon steel. 
         [0052]    While the invention will be described below in terms of liquid nitrogen, it should be understood that the invention may be used for other cryogenic liquids useful in refrigeration and/or freezing related processes, such as industrial, medical or food processing. 
         [0053]    As illustrated in  FIG. 6 , the inner tank  214  features a top portion  219  to which a fill vent line  220  is connected. In addition, a liquid fill line  220  is connected to a lower portion of the inner tank  214 . The distal end of the fill vent line  220  is provided with a fill vent valve while the distal end of the liquid fill line  22  is provided with liquid fill valve, and both are adapted to be connected to a source of liquid, such as a tanker truck, for refilling the bulk tank. The fill vent line  220  provides a vapor balance during the refilling operation. 
         [0054]    A liquid dispensing or feed line  252  exits the bottom  253  of the inner tank  214  and is provided with liquid feed valve  254  and liquid feed check valve  256 . The dispensing line is also provided with vacuum insulation  257 . The dispensing line  252  is constructed to attach directly to a vacuum jacketed house line for delivery of the cryogenic liquid inside the plant. 
         [0055]    A pressure builder inlet line  260  also exits the bottom portion of the inner tank  214  and connects to the inlet of a high performance pressure builder, indicated in general at  262 . As illustrated in  FIG. 6 , a first stage of the pressure builder features a number of parallel heat exchangers  261 . The outlet of the first stage of the pressure builder communicates with the inlet of a second stage of the pressure builder  262  which includes a number of series heat exchangers  263 . As an example only, the high performance pressure builder may take the form of the pressure building system disclosed in commonly owned U.S. Pat. No. 6,799,429, the contents of which are hereby incorporated by reference. 
         [0056]    The first stage of the pressure builder  262  preferably supports withdrawal rates up to 20 GPM while the second stage of the pressure builder preferably supports demands up to 40 GPM. To support these flow rates, the dispensing line  252  preferably is either 1½″ or 2″ in diameter. 
         [0057]    The pressure builder inlet line  260  is provided with an automated pressure builder valve  266  and a pressure builder check valve  268 . A pressure builder outlet line  272  exits pressure builder  262  and travels to the top of the inner tank  214 . The pressure builder outlet line is provided with a vent line  242  which includes an automated vent valve  244 . 
         [0058]    With reference to  FIG. 6 , after the tank  212  has been filled, the inner tank  214  contains a supply of liquid nitrogen  281  with a headspace  282  defined above. 
         [0059]    To promote stable liquid withdrawal during a product refill, the system incorporates a low-mounted internal horizontal baffle  230  with a side wall bottom fill designed to direct the incoming liquid up the side of the vessel during bottom filling. The baffle is circumferentially secured to the interior surface of the inner tank  214  by spaced braces. In addition to the spaces between the baffle braces, the baffle features a central opening  232  that permits passage of liquid. The baffle also aides in deflecting unwanted heat from the vessel bottom supports and piping penetrations up the sides of the tank to promote liquid stratification, which keeps the liquid colder at the tank bottom to feed the application. 
         [0060]    As illustrated in  FIG. 6 , the system  200  includes a liquid level sensor preferably in the form of a differential pressure gauge  280 , which communicates with the head space of the tank interior via low phase line  282  and the bottom of the tank interior via high phase line  284 . In addition, a vapor pressure sensor  286  communicates with the headspace of the tank via low phase line  282 . 
         [0061]    In addition, the dispensing line  252  is provided with a liquid outlet temperature sensor  288  while the bottom of the tank interior is provided with a tank liquid temperature sensor that is preferably a saturation pressure sensor  292  that communicates with a pressure bulb  294 . The pressure bulb  294  is a capped pipe inside the bottom of the tank surrounded by liquid. Inside the pipe is gaseous nitrogen. The liquid cools the pipe and condenses the gas inside. The pressure inside the pipe is the saturation pressure of the liquid. The pressure sensor  292  is in communication with the interior of the pipe. As will be explained below, the tank liquid temperature may be calculated from the saturation pressure detected by the pressure sensor  292 . 
         [0062]    Liquid level gauge  280 , vapor pressure sensor  286 , liquid outlet temperature sensor  288  and saturation pressure sensor  292  each communicate with a controller, such as programmable logic controller (“PLC”)  300  in  FIG. 6 . The PLC also communicates with, and controls operation of, automated pressure building valve  266  and automated vent valve  244 . An example of a suitable PLC is the Allen-Bradley MicroLogix 830 available from Rockwell Automation, Inc. of Milwaukee, Wis. It should be noted that devices other than a PLC, including, but not limited to, pressure switches, may be used as the controller  300 . 
         [0063]    The PLC performs with the system  200  as a dynamic pressure builder to maintain a constant pressure for the liquid nitrogen flowing through dispensing line  252  by varying the vapor pressure in the tank via the pressure building valve  266  and the vent valve  244 . The PLC takes sensor inputs for the liquid level (from differential pressure gauge  280 ), tank vapor pressure (from vapor pressure sensor  286 ), and tank temperature (from saturation pressure sensor  292 ) to calculate when to operate the pressure builder. In addition, the PLC calculates the necessary vapor pressure in order to deliver saturated liquid at the usage point using the liquid outlet temperature detected by sensor  288 , in combination with the other data inputs noted above. 
         [0064]    With regard to tank temperature, the PLC calculates the tank liquid temperature using the saturation pressure from saturation pressure sensor  292 . 
         [0065]    The PLC uses the tank liquid temperature and level of the liquid as well as the pressure of the vapor to calculate the pressure at the bottom of the tank (vapor pressure+liquid head=pressure at the bottom of the tank). 
         [0066]    Using the liquid outlet temperature detected by sensor  288  in the liquid dispensing line, the PLC  300  determines the required saturation pressure at the outlet and compares it with the pressure at the bottom of the tank calculated above. If the pressure at the bottom of the tank is too low (lower than the required outlet saturation pressure), the PLC will automatically open pressure building valve  266  so that the pressure builder  262  receives liquid from the bottom of the tank and vaporizes it. The vapor travels to the top of the tank via line  272  so as to pressurize it. As described above, stratification of the liquid in the tank and the baffle  230  help isolate the liquid at the bottom of the tank from temperature increases. Conversely, if the pressure at the bottom of the tank is too high (higher than the required outlet saturation pressure), the PLC  300  will open the vent valve  244  to vent vapor from the tank headspace through lines  272  and  242  to the atmosphere to lower the pressure in the tank. 
         [0067]    In view of the above, the PLC  300  enables the customer to set their requirements using input device  302  (which may be, for example, a computer keyboard or control panel) with very tight parameters (such as +/−2 psi) to operate these two valves. For example, in a typical food freezing application, the pressure builder can be set to 25 psig and the vent at 35 psig. These pressure set points are at the bottom of the tank, not at the traditional top vapor space. Not only is the band tighter in comparison to traditional regulators, but the system precisely controls the outlet pressure regardless of the tank liquid level. 
         [0068]    As illustrated in  FIG. 7 , the PLC program makes real-time adjustments so as the liquid level falls in normal use, the set point to turn on the pressure builder valve increases to compensate for the loss in liquid head pressure. The result is a generally consistent outlet pressure through the dispensing line  252  to the application regardless of tank liquid level. 
         [0069]    Flowcharts illustrating examples of the processing performed by the PLC  300  of  FIG. 6  are provided in  FIGS. 8 and 9 , where  FIG. 8  illustrates processing performed with regard to control of the vent valve  244  and  FIG. 9  illustrates processing performed with regard to the pressure building valve  266 . 
         [0070]    The system  200  is designed to run in two different modes, “Optimized” and “Basic.” In Optimized mode, which is described above, the PLC  300  does all of the necessary calculations to deliver saturated liquid to the delivery point. The Basic mode is used if the liquid outlet/dispensing line temperature sensor  288  experiences a failure. It is a fall back mode to continue operation with simplified programming. The Basic mode is designed to deliver liquid at a constant outlet pressure (which may not necessarily be saturation pressure) from the tank. Both of these modes operate with the dynamic pressure builder. 
         [0071]    In Optimized mode, the system has the option to incorporate a “black out” period. In many food freezing applications, a cryogenic liquid supply system will operate for 16 hours and then have an 8 hour period of non-use. This time is used to clean and disinfect the freezing chambers. This time is referred to as the black out period. During the black out period the operator has the opportunity to lower the saturation pressure of the stored liquid if it is necessary. That is, the system incorporates another key feature in its design, the automatic liquid de-saturation cycle. If the user has blackout (non-use) time periods programmed into the PLC  300 , the vent valve can automatically be directed to open and blow down the tank to conditions to or even below the desired outlet pressure. Once the vent valve closes, the pressure builder can turn on and create the desired amount of sub-cool (the difference between the vapor pressure and the saturation pressure of the liquid). This feature is desirable in applications with erratic usage patterns that cause the liquid to take on heat (from being idle) and for those where consistent liquid quality is critical for the application. This feature is primarily driven by the PLC input from the actual liquid nitrogen temperature in the bottom of the tank (from the saturation pressure sensor  292 ). 
         [0072]    To control the outlet pressure at the bottom of the tank during the refill process (which uses vent and refill lines  220  and  222 ), the driver still follows their normal procedure of adjusting the top and bottom fill valves to hit the “instructed fill target pressure” by monitoring the tank pressure gauge. However, the tank pressure gauge shows the liquid pressure at the bottom of the tank (vapor pressure+liquid head), not the traditional low-phase line vapor pressure. Thus, unknowingly, the driver reduces the vapor pressure as the tank is filling, holding the outlet pressure stable without changing their filling procedure. This also keeps the application on-line and unaffected by a tank refill process. 
         [0073]    The system of  FIGS. 6-9  described above therefore is well suited to users who consume large amounts of liquid nitrogen at high flow rates or simply want better control of their liquid supply. The system offers is an excellent alternative to a modified standard bulk tank and provides a more productive solution for such users. 
         [0074]    An alternative embodiment of the system is illustrated in  FIGS. 10-12 . The system, indicated in general at  400  in  FIG. 10 , features a construction identical to the system of  FIG. 6  with the exceptions described below. As illustrated in  FIG. 10 , the system  400  includes a tank storage pressure sensor preferably in the form of a pressure sensor  402  which communicates with the liquid space of the tank interior via high phase line  404 , which leads from the pressure sensor  402  to the bottom of the tank interior. As a result, the pressure sensor  402  provides the storage pressure of the liquid nitrogen at the bottom portion of the tank (P bottom ). 
         [0075]    in addition, the bottom of the tank interior is provided with a saturation pressure sensor  406  that communicates with a pressure bulb  408 . The pressure bulb  408  may be a capped pipe inside the bottom of the tank surrounded by liquid. Inside the pipe is gaseous nitrogen. The liquid cools the pipe and condenses the gas inside. The pressure inside the pipe is the saturation pressure of the liquid. The pressure sensor  406  is in communication with the interior of the pipe, and thus provides the saturation pressure of the liquid nitrogen (P sat ). 
         [0076]    Storage pressure sensor  402  and saturation pressure sensor  406  each communicate with a controller, such as programmable logic controller (“PLC”)  410  in  FIG. 10 . The PLC also communicates with, and controls operation of, automated pressure building valve  412  and automated vent valve  414 . An example of a suitable PLC is the Allen-Bradley MicroLogix 830 available from Rockwell Automation, Inc. of Milwaukee, Wis. It should be noted that devices other than a PLC, including, but not limited to, pressure switches, may be used as the controller  410 . 
         [0077]    The PLC performs with the system  400  as a dynamic pressure builder to maintain a constant pressure for the liquid nitrogen flowing through dispensing line  416  by varying the vapor pressure in the tank via the pressure building valve  412  and the vent valve  414 . The PLC  410  takes sensor inputs from the storage pressure sensor  402  and the saturation pressure sensor  406  and compares P button  with P sat  to determine when to operate the pressure builder. For example, if P bottom  is below P sat , the PLC  410  may open the pressure building valve  412  so that the liquid nitrogen at the bottom of the tank will become subcooled. Alternatively, if the P bottom  rises above P sat , the PLC  410  may open vent valve  414 . 
         [0078]    Flowcharts illustrating examples of the processing performed by the PLC  410  of  FIG. 10  are provided in  FIGS. 8 and 9 , where  FIG. 8  illustrates processing performed with regard to control of the vent valve  414  and  FIG. 9  illustrates processing performed with regard to the pressure building valve  412 . 
         [0079]    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.