Abstract:
A cryogenic temperature control apparatus comprises a storage tank housing a quantity of cryogenic fluid, a housing defining a conditioned space, and a heat exchanger in thermal communication with the conditioned space. A first flow path fluidly connects the storage tank and the heat exchanger. A first valve is positioned along the first flow path to selectively fluidly seal the first flow path between the storage tank and the heat exchanger. A second flow path fluidly connects the storage tank and the heat exchanger. A second valve is positioned along the second flow path to selectively fluidly seal the second flow path between the storage tank and the heat exchanger.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority under 35 U.S.C. §119 to a provisional patent application No. 60/302,918, filed on Jul. 3, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to air conditioning and refrigeration systems, and more specifically to a cryogenic temperature control apparatus and a method of operating a cryogenic temperature control apparatus to pull down and maintain the temperature in an air-conditioned space.  
         BACKGROUND OF THE INVENTION  
         [0003]    Air conditioning and refrigeration systems typical utilize a chloroflourocarbon (CFC) refrigerant in a mechanical refrigeration cycle. However, because of the suspected depleting effect of CFCs on stratospheric ozone (O 3 ), practical alternatives to the use of CFCs are being sought. One such practical alternative is a cryogenic temperature control system.  
           [0004]    Conventional cryogenic temperature control systems typically utilize a cryogen such as carbon dioxide, liquid nitrogen, etc. Typically, the cryogen is compressed to a cool liquid state and is stored in a pressurized storage tank. The cryogen is directed along a conduit from the storage tank to an evaporator coil that extends through a heat exchanger. Relatively warm air is passed across the evaporator coil and is cooled by contact with the evaporator coil. Contact with the warm air heats and vaporizes the cryogen in the evaporator coil. After the heat transfer has occurred, the vaporized cryogen is typically exhausted to the atmosphere. Alternatively, cryogenic temperature control systems can be closed and can condense the vaporized refrigerant before recycling the refrigerant through the temperature control system. The cooled air is then returned to an air-conditioned space.  
           [0005]    To allow cryogenic temperature control systems to operate in a heating mode or in a defrost mode, conventional cryogenic temperature control systems typically include a heating element. Conventional heating elements normally include a propane heater for superheating cryogen. During heating and defrost modes, the cryogen is heated by the propane heater. The heated cryogen gas is then directed through a set of electronically operated valves through the evaporator coil to either defrost the evaporator coil or to heat the air-conditioned space.  
           [0006]    Conventional cryogenic temperature control systems typically include a series of sensors distributed throughout the system to record temperature and pressure values in various locations throughout the system. The data collected by the sensors is transmitted to an elaborate fuzzy logic based controller, which periodically determines the rate of change of the temperature of the discharge air, as well as the acceleration or deceleration of this rate of change. The controller then manipulates the operating parameters of the system by manipulating valves distributed throughout the system to achieve and maintain the set point temperature.  
           [0007]    Different types of temperature control systems, including cryogenic systems, are currently used in mobile applications to control the temperature in a cargo compartment. Mobile temperature control systems are typically mounted on straight trucks, the trailer of a tractor-trailer combination, a refrigerated shipping container, a refrigerated railcar, and the like, to refrigerate air-conditioned spaces. It is generally desirable to maintain the temperature of an air-conditioned space within a relatively narrow range around a predetermined set point temperature. In this manner, temperature sensitive cargo can safely be stored and/or transported in the air-conditioned space. Refrigerated transport vehicles for frozen foods such as seafood, meat, ice, frozen deserts, and the like, must maintain the air-conditioned space at a set point temperature, which is normally below freezing. Similarly, refrigerated transport vehicles are also used to transport fresh foods and beverages, which must be maintained at a set point temperature that is normally above freezing. In this manner, the mobile temperature control system can be used to maintain the temperature of the cargo at or near the desired set point temperature during transportation so that the cargo is not damaged or spoiled during transportation.  
           [0008]    The above-described conventional mobile temperature control apparatus must rapidly achieve the desired set point temperature within the air-conditioned space with only a minimum amount of cryogen, since the amount of cryogen that can be carried in such a system is limited. Further, the controllers used to operate conventional mobile temperature control apparatuses are generally relatively complex. These systems generally require substantial computing power and programming skill to properly implement and operate. The system complexity generally limits the flexibility of the system. Therefore, a cryogenic temperature control apparatus and method that efficiently utilizes the cryogen would be welcomed by users of such systems.  
         SUMMARY OF THE INVENTION  
         [0009]    According to the present invention, a cryogenic temperature control apparatus includes a storage tank housing a quantity of cryogen, a housing defining a conditioned space, and a heat exchanger in thermal communication with the conditioned space. A first flow path fluidly connects the storage tank and the heat exchanger. A first valve is positioned along the first flow path between the storage tank and the heat exchanger. The first valve has a first open position and a first closed position. In the first open position, the first valve and the first flow path fluidly connect the storage tank and the heat exchanger. In the first closed position, the first valve fluidly seals the first flow path between the storage tank and the heat exchanger. A second flow path fluidly connects the storage tank and the heat exchanger. A second valve is positioned along the second flow path between the storage tank and the heat exchanger and has a second open position and a second closed position. In the second open position, the second valve and the second flow path fluidly connect the storage tank and the heat exchanger. In the second closed position, the second valve fluidly seals the second flow path between the storage tank and the heat exchanger.  
           [0010]    In preferred embodiments, the present invention includes a controller. The controller is operable to move the first valve between the first open position and the first closed position and to move the second valve between the second open position and the second closed position.  
           [0011]    A vehicle supports the cryogenic temperature control apparatus and includes an engine and an engine cooling system. The engine cooling system circulates an engine coolant through the engine. A heating coil is in fluid communication with the cooling system and extends through the heat exchanger. The heat exchanger is operable in a heating mode and includes a third valve. The third valve is operable to fluidly connect and to fluidly disconnect the heating coil and the engine cooling cycle. In the heating mode, the third valve selectively fluidly connects the heating coil and the engine cooling cycle.  
           [0012]    The cryogenic temperature control apparatus is operable in a first cooling mode. In the first cooling mode, the first valve is in the first open position and the second valve is in the second closed position. The cryogenic temperature control apparatus is also operable in a second cooling mode and the first and second valves have different portings. In the second cooling mode the second valve is in the second open position and the first valve is in the first closed position.  
           [0013]    The cryogenic temperature control apparatus is also operable in a third cooling mode. In the third cooling mode, the first valve is in the first open position and the second valve is in the second open position. The cryogenic control apparatus is operable in a fourth cooling mode. In the fourth cooling mode, the first valve is in the first closed position and the second valve is in the second closed position.  
           [0014]    The cryogenic temperature control apparatus includes a fan, which has an operable condition and an idle condition. In the operable condition, the fan is operable to move a quantity of air across the heat exchanger. The controller is operable to shift the fan between the operable condition and the idle condition.  
           [0015]    A temperature sensor is preferably positioned within the conditioned space. The temperature sensor is operable to shift the fan between the operable condition and the idle condition. The cryogenic temperature control apparatus also includes a second fan, which is operable to move air across the heat exchanger.  
           [0016]    The cryogenic temperature control apparatus includes a conduit extending between the storage tank and the heat exchanger. The conduit has a first branch and a second branch. The first flow path follows the first branch of the conduit and the second flow path follows the second branch of the conduit.  
           [0017]    According to the present invention, a method of controlling the temperature of a conditioned space with a heat exchanger includes providing a heat exchanger which is in thermal communication with a conditioned space. The heat exchanger includes a storage tank, which houses a cryogen, a first valve operable to fluidly connect the storage tank and the heat exchanger along a first fluid path, and a second valve operable to fluidly connect the storage tank and the heat exchanger along a second fluid path. The method further includes sensing the temperature in the conditioned space and comparing the temperature to a desired temperature. The first valve is opened to introduce cryogen into the heat exchanger through the first fluid path and air is moved from the conditioned space through the heat exchanger. The second valve is also preferably opened to introduce cryogen into the heat exchanger through the second valve along the second path. Preferably, the temperature in the conditioned space is sensed and compared to the desired temperature. The first and second valves are then closed, thereby preventing fluid from flowing along the first and second paths between the storage tank and the heat exchanger.  
           [0018]    Preferably, the method of controlling the temperature in a conditioned space with a heat exchanger also includes sensing the temperature in the conditioned space and blowing air across the heat exchanger.  
           [0019]    Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The present invention is further described with reference to the accompanying drawings, which show preferred embodiments of the present invention. However, it should be noted that the invention as disclosed in the accompanying drawings is illustrated by way of example only. The various elements and combinations of elements described below and illustrated in the drawings can be arranged and organized differently to result in embodiments which are still within the spirit and scope of the present invention.  
         [0021]    In the drawings, wherein like reference numerals indicate like parts:  
         [0022]    [0022]FIG. 1 is a side view of a truck including one embodiment of a cryogenic temperature control apparatus in accordance with the present invention; and  
         [0023]    [0023]FIG. 2 is a schematic drawing of the cryogenic temperature control apparatus of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0024]    [0024]FIGS. 1 and 2 illustrate a cryogenic temperature control apparatus  12  in accordance with the present invention. The cryogenic temperature control apparatus  12  is operable to control the temperature of an air-conditioned space  14 , as shown in FIG. 1, enclosed in a truck  16 . The cryogenic temperature control apparatus  12  can alternatively be used on other vehicles, such as a tractor-trailer combination, a container, and the like. Also, the cryogenic temperature control apparatus  12  can be used to condition air in the passenger space of a vehicle (e.g., a bus or a truck cab) for passenger comfort or the cryogenic temperature control apparatus  12  can be used to condition air in a cargo space. In some aspects, the cryogenic temperature control apparatus  12  can be used to condition air in the cargo space of a vehicle and can be used to condition air in the passenger space of the vehicle.  
         [0025]    Alternatively, the cryogenic temperature control apparatus  12  can be operable in stationary applications. For example, the temperature control apparatus  12  can be operable to control the temperature of buildings, areas of buildings, storage containers, refrigerated display cases, and the like. In all cases, the air-conditioned space  14  preferably has an outer wall  18 , which includes one or more doors  19 , which open into the air-conditioned space  14  so that an operator can insert a product into and remove the product from the air-conditioned space  14 .  
         [0026]    As used herein and in the claims, the term “air-conditioned space” includes any space to be temperature and/or humidity controlled, including transport and stationary applications for the preservation of foods, beverages, and other perishables, maintenance of a proper atmosphere for the shipment of industrial products, space conditioning for human comfort, and the like. The cryogenic temperature control apparatus  12  is operable to control the temperature of the air-conditioned space  14  to a predetermined set point temperature (“SP”). It should be understood that in some applications, the temperature of the air-conditioned space  14  is controlled to a predetermined band adjacent the selected set point temperature SP.  
         [0027]    As shown in FIG. 1, a fluid storage tank  20  containing a cryogen is mounted on the outer wall  18  of the truck  16 . In other applications (not shown), the storage tank  20  can be mounted or affixed to any other location on the truck  16 , including the truck frame, the bottom of the body of the truck  16 , or any other place on the interior or exterior of the truck  16 . The storage tank  20  stores the cryogen under pressure in a liquid state. However, it should be appreciated that in some applications, a portion of the cryogen in the storage tank  20  may be in a vapor state. More particularly, the storage tank  20  preferably maintains the cryogen under pressure at a level significantly above atmospheric pressure. In this manner, the pressure within the storage tank  20  supplies the motive force for moving the cryogen through the cryogenic temperature control apparatus  12 .  
         [0028]    The cryogen is preferably carbon dioxide (CO 2 ). However, it will be readily understood by one of ordinary skill in the art that other cryogens, such as LN 2  and LNG can also or alternately be used. However, cryogens that are environmentally friendly and are non-reactive are highly desirable for obvious reasons.  
         [0029]    A conduit  22  is connected to the underside of the storage tank  20  and includes a first branch  24  and a second branch  25 . The conduit  22 , including the first branch  24 , defines a first flow path  26 . Similarly, the conduit  22 , including the second branch  25 , defines a second flow path  28 . As shown in FIG. 1, the first and second branches  24 ,  25  are fluidly connected to the storage tank  20  and converge at a junction located downstream from the storage tank  20 .  
         [0030]    With reference to FIG. 2, the first branch  24  includes a first control valve  30 . The first control valve  30  has a first porting and controls the mass flow rate of cryogen through the first branch  24  during heating and cooling cycles. The first control valve  30  is preferably moved between a first open position and a first closed position by an electrically controlled solenoid (not shown). However, in other applications, other valves and actuators can also or alternatively be used.  
         [0031]    The second branch  25  also extends from a low point of the storage tank  20  and includes a second control valve  32 . The second control valve  32  has a second porting, which is preferably smaller than the first porting. However, in some embodiments of the present invention, the first and second control valves  30 ,  32  can have the same porting. The second control valve  32  is preferably an electrically operated valve and controls the mass flow rate of cryogen through the second branch  25  during heating and cooling cycles. Preferably, the second control valve  32  is operated by an electrically controlled solenoid (not shown), which moves the second control valve  32  between a second open position and a second closed position. However, as explained above with respect to the first control valve  30 , other valves and actuators can also or alternatively be used.  
         [0032]    Additionally, as shown and described herein, the first and second control valves  30 ,  32  are preferably relatively simple on-off valves. However, one of ordinary skill in the art will appreciate that in other applications, one or both of the first and second control valves  30 ,  32  can be modulation valves, pulse valves, expansion valves, or the like. In these embodiments, the cryogenic temperature control apparatus  12  can include a greater variety of available mass flow rates between the storage tank  20  and an evaporator coil  42  (further described below). Similarly, other embodiments (not shown) include three or more branches. Each of these branches can include a control valve (not shown) for regulating the mass flow rate of cryogen out of the storage tank  20 . In still other embodiments (not shown), the first and second branches  24 ,  25  can extend between the storage tank  20  and the evaporator coil  42  without converging at the junction.  
         [0033]    The first and second control valves  30 ,  32  are controlled by a microprocessor controller  34 . The controller  34  is preferably powered by the truck&#39;s engine  35  or by an alternator (not shown) positioned within the engine  35 . In alternative embodiments, the controller  34  can also or alternatively be powered by a battery, a fuel cell, a generator, or the like. In still other embodiments (not shown), an external power supply, for example a wall socket on a building, can supply power to the controller  34 .  
         [0034]    As shown in FIG. 1, a heat exchanger  36  is positioned within the air-conditioned space  14  and includes an air intake  38  and an air outlet  39 . In operation, air from the air-conditioned space  14  enters the heat exchanger  36  through the air intake  38  and is exhausted through the air outlet  39 . As shown in FIG. 2, the air outlet  39  includes a damper  40 , which is adjustable between a number of positions to open and close the air outlet  39 .  
         [0035]    The conduit  22  is fluidly connected to an inlet of an evaporator coil  42  located in the heat exchanger  36 . During cooling operations, cryogen from the storage tank  20  flows along the first and/or second flow paths  26 ,  28  in a liquid or mostly liquid state into the evaporator coil  42 . Air from the air-conditioned space  14  travels across the evaporator coil  42  and is cooled by contact with the relatively cold evaporator coil  42 . At the same time, the cryogen in the evaporator coil  42  is vaporized by contact with the relatively warm air. The cooled air is returned to the air-conditioned space  14  through the air outlet  39  to cool the air-conditioned space  14  and the vaporized cryogen flows out of the evaporator coil  42  through the outlet  43  and is exhausted to the atmosphere.  
         [0036]    The outlet  43  includes a back pressure regulator  44 . The back pressure regulator  44  may automatically regulate the cryogen vapor pressure above a predetermined value (e.g., the triple point of the cryogen) or the back pressure regulator  44  may be electrically operated and controlled by the controller  34 . Alternatively, a mechanical type, automatic back pressure regulating valve can be used. The back pressure regulator  44  maintains the pressure within the evaporator coil  42  at a desired pressure. Preferably, the desired pressure is equal to or greater than the triple point of the cryogen. For example, in applications in which the cryogen is carbon dioxide, the back pressure regulator  44  maintains the pressure in the evaporator coil  42  at 60.43 psig.  
         [0037]    A return air temperature sensor  45  is located in the air inlet  38  to record the temperature of the air (“RA”) as the air enters the heat exchanger  36 . The return air temperature sensor  45  is preferably an analog sensor with an operating range from −50° C. to 70° C. (−58° F. to 158° F.) and is in electrical communication with the controller  34 .  
         [0038]    An evaporator coil outlet temperature sensor  48  is positioned adjacent the outlet  43 . The evaporator coil outlet temperature sensor  48  records the temperature of cryogen vapor (“ECOT”) exiting the heat exchanger  36 . The evaporator coil outlet temperature sensor  48  is preferably an analog sensor with an operating range from −50° C. to 70° C. (−58° F. to 158° F.) and is in electrical communication with the controller  34 .  
         [0039]    A first fan  50  and a second fan  52  are positioned within the heat exchanger  36  and are operable to draw air from the air-conditioned space  14  through the heat exchanger  36 . As shown in the figures, the first fan  50  can be positioned above the second fan  52 . Alternatively, the first and second fans  50 ,  52  can be arranged side-by-side or in any other configuration as dictated by space concerns.  
         [0040]    As shown in FIG. 2, a heating element  53  is located in the heat exchanger  36  and includes a heating coil  54  and a fluid conduit  55 , which extends between the heating coil  54  and a coolant cycle  56  located in the truck&#39;s engine  35 . A third valve  58  is positioned along the fluid conduit  55  for controlling the flow of engine coolant from the cooling cycle to the heating coil  54 . During operation, the engine  35  heats the coolant in the coolant cycle  56 . When heating is required, the third valve  58  is opened and coolant  56  is directed through the heating element  53  to heat air in the heat exchanger  36 . In other embodiments, other fluids can be heated and can be directed through the heating coil  54  to heat air in the heat exchanger  36 . In still other embodiments, other heating elements  53 , for example electrical heaters (not shown), can also or alternatively be used to heat air in the heat exchanger  36 .  
         [0041]    As shown in FIG. 2, the heating element  53  is positioned between the fans  50 ,  52  and the evaporator coil  42 . However, in other arrangements, the evaporator coil  42  can be positioned between the fans  50 ,  52  and the heating element  53 . Additionally, in some embodiments, the heating element  53  and the evaporator coil  42  can be combined to conserve space.  
         [0042]    The controller  34  is preferably programmed to operate the cryogenic temperature control apparatus  12  in at least six different modes, including a First Cooling Mode, a Second Cooling Mode, a Third Cooling Mode, a Fourth Cooling Mode, a Heating Mode, and a Defrost Mode. In the First Cooling Mode or super cooling mode, the first and second valves  30 ,  32  are open to provide a maximum mass flow rate of cryogen from the storage tank  20  to the evaporator coil  42 . In this manner, the cryogenic temperature control apparatus  12  can rapidly pull down the temperature of the air-conditioned space  14 . The controller  34  is programmed to operate the cryogenic temperature control apparatus  12  in the First Cooling Mode for a relatively short time after loading or when the temperature of the air-conditioned space  14  is significantly above the set point temperature SP.  
         [0043]    When less cooling is required or when the temperature of the air-conditioned space  14  is relatively close to the set point temperature SP, the controller  34  preferably shifts the cryogenic temperature control apparatus  12  into the Second Cooling Mode to conserve cryogen. In the Second Cooling Mode, the first control valve  30  remains open and the second control valve  32  is closed, thereby reducing the mass flow rate of cryogen from the storage tank  20  through the evaporator coil  42  to a second lower mass flow rate.  
         [0044]    As the temperature in the air-conditioned space  14  continues to drop, the controller  34  is preferably programmed to shift the cryogenic temperature control apparatus  12  into the Third Cooling Mode, corresponding to a third lower flow rate. In the Third Cooling Mode, the first control valve  30  is closed and the second control valve  32  is opened to provide a third mass flow rate.  
         [0045]    Once the temperature of the air-conditioned space  14  is at or relatively near the set point temperature SP, the controller  34  preferably shifts the cryogenic temperature control apparatus  12  into the Fourth Cooling Mode or the Null Mode. In the Fourth Cooling Mode, the first and second control valves  30 ,  32  are both closed to provide a forth mass flow rate in which cryogen does not flow from the storage tank  20  to the evaporator coil  42 .  
         [0046]    When the cryogenic temperature control apparatus  12  is operating in the First, Second, Third, and Fourth Cooling Modes, the first and second fans  50 ,  52  are in operation. Additionally, the damper  40  is in the open position so that air can flow through the heat exchanger  36 . Alternatively, a person of ordinary skill in the art will appreciate that the first and/or second fan  50 ,  52  can be cycled on and off during the First, Second, Third, and Fourth Cooling Modes by a timer, (not shown), upon an operator&#39;s command, or by the controller  34 .  
         [0047]    When the set point temperature SP is below the ambient temperature, such as for example, in relatively cold climates, the controller  34  is preferably operable to shift the cryogenic temperature control apparatus  12  into the Heating Mode. When the cryogenic temperature control apparatus  12  is shifted into the Heating Mode, the first and second control valves  30 ,  32  are closed to prevent cryogen from entering the evaporator coil  42  and the third valve  58  is opened to allow relatively warm engine coolant to enter the heating element  53 . The first and second fans  50 ,  52  are turned on to direct air from the air-conditioned space  14  past the heating element  53  to absorb heat from the heating element  53  and to direct the air through the air outlet  39  back into the air-conditioned space  14 .  
         [0048]    In some instances water vapor from the air-conditioned space  14  can be separated from the air and can condense on the evaporator coil  42 , forming frost. To minimize the formation of frost on the evaporator coil  42  and to remove frost from the evaporator coil  42 , the controller  34  is programmed to operate the temperature control apparatus  12  in the Defrost Mode.  
         [0049]    In the Defrost Mode, the controller  34  is programmed to close both the first and second valves  30 ,  32 , close the damper  40 , and open the third valve  58 . In this manner, the heating element  53  heats the air within the heat exchanger  36  until the evaporator coil  42  is defrosted and the damper  40  prevents the heated air from entering the air-conditioned space  14 . Additionally, in some embodiments, the first and second fans  50 ,  52  are turned off during the Defrost Mode to prevent unnecessary heating of the air-conditioned space  14 .  
         [0050]    The cryogenic temperature control apparatus  12  can be shifted into the Defrost Mode in four different manners. First, the controller  34  is programmed to shift the temperature control apparatus  12  into the Defrost Mode based upon data supplied by the air return sensor  45  and the evaporator coil exit temperature sensor  48 . For example, the controller  34  may be programmed to shift the cryogenic temperature control apparatus  12  into the Defrost Mode if the evaporator coil outlet temperature sensor  48  records an evaporator coil outlet temperature ECOT that is below a predetermined value (e.g., −40° C.).  
         [0051]    Second, the controller  34  is programmed to periodically cycle through the Defrost Mode at predetermined time intervals, such as for example, every hour. Third, the controller  34  includes a user interface (not shown), which allows an operator to manually initiate the Defrost Cycle. Fourth, the controller  34  is programmed to shift the cryogenic temperature control apparatus  12  into the Defrost Mode if the difference between return air temperature RA and the evaporator coil exit temperature ECOT is greater than a predetermined value (i.e., 8° C.).  
         [0052]    Although particular embodiments of the present invention have been shown and described, other alternative embodiments will be apparent to those skilled in the art and are within the intended scope of the present invention.