Abstract:
A method of temperature control in a cryogenic temperature control apparatus comprises providing a heat exchanger in thermal communication with an air-conditioned space. The heat exchanger includes an air inlet and an evaporator coil having an outlet. The method further comprises providing a first temperature sensor being operatively coupled to a controller, measuring the temperature in the outlet and sending the temperature in the outlet to the controller, providing a second temperature sensor being operatively coupled to the controller, measuring the temperature in the air inlet, and sending the temperature in the air inlet to the controller, and providing a plurality of temperature control values. The flow of cryogen from a storage tank to the evaporator coil is altered each time the temperature in the outlet passes one of a first plurality of temperature control values and each time the temperature in the air inlet passes one of a second plurality of temperature control values.

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.  
         BACKGROUND OF THE INVENTION  
         [0003]    Conventional cryogenic temperature control systems typically store a compressed cryogen such as carbon dioxide, liquid nitrogen, etc. 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 the evaporator coil. The cooled air is returned to cargo compartment to pull down the temperature of the cargo compartment to a predetermined set point temperature. 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.  
           [0004]    Conventional cryogenic temperature control systems typically include a series of sensors which record temperature and pressure values in various locations throughout the system. The sensors generally supply the temperature and pressure data to a controller, which uses an elaborate fuzzy logic scheme to control the operating parameters of the system based upon the data provided by the sensors. In order to achieve and maintain the set point temperature, the controller 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. Based upon these and other calculations, the controller increments the flow of cryogen from the storage tank to the evaporator coil by activating and deactivating an electronically controlled valve. Generally, the fuzzy logic schemes are relatively complicated to program and to operate.  
           [0005]    The controllers used to operate conventional cryogenic temperature control apparatuses are generally relatively complex. These systems generally require substantial computing power and programming skill to properly implement and operate. Additionally, the system complexity generally limits the flexibility of conventional cryogenic temperature control apparatuses. Also, they generally consume relatively large quantities of cryogen. This is particularly problematic on vehicle mounted cryogenic temperature control apparatuses. Cryogenic temperature control systems are currently used in mobile applications to control the temperature in a cargo compartment and are typically mounted on straight trucks, the trailer of a tractor-trailer combination, a refrigerated shipping container, a refrigerated railcar, and the like. For obvious reasons, it is generally desirable to reduce the weight and size of the cryogenic temperature control system. Often conventional storage tanks can weigh 1400 pounds or more when filled. It is therefore generally desirable to minimize the amount of cryogen that is carried in the storage tank and to reduce the rate at which the cryogen is consumed while ensuring that the air-conditioned space temperature is maintained at or near the set point. Additionally, cryogen may not always be readily available for refilling the storage tank so it is important, particularly during long hauls, to regulate the consumption of cryogen.  
         SUMMARY OF THE INVENTION  
         [0006]    According to the present invention, a method of temperature control in a cryogenic temperature control apparatus comprising providing a heat exchanger in thermal communication with an air-conditioned space is provided. The heat exchanger includes an air inlet and an evaporator coil having an outlet. A first temperature sensor is operatively coupled to a controller, measures the temperature in the outlet, and sends the temperature in the outlet to the controller. A second temperature sensor is operatively coupled to the controller, measures the temperature in the air inlet, and sends the temperature in the air inlet to the controller. The invention further comprises providing a first plurality of temperature control values and a second plurality of temperature control values. The flow of cryogen from a storage tank to the evaporator coil is altered each time the temperature in the outlet passes the first plurality of temperature control values each time the temperature in air inlet passes the second plurality of temperature control values.  
           [0007]    In preferred embodiments, the method of temperature control includes providing a first cooling mode corresponding to a first flow rate of cryogen from the storage tank to the evaporator coil, providing a second cooling mode corresponding to a second flow rate of cryogen from the storage tank to the evaporator coil, providing a third cooling mode corresponding to a third flow rate of cryogen from the storage tank to the evaporator coil, and providing a fourth cooling mode corresponding to a fourth flow rate of cryogen from the storage tank to the evaporator coil. Altering the flow of cryogen from the storage tank to the evaporator coil when the temperature in the outlet and the temperature in the air inlet are beyond the plurality of temperature control values includes switching between the first cooling mode, the second cooling mode, the third cooling mode, and the fourth cooling mode.  
           [0008]    In preferred embodiments, a system for incorporating the method includes a first valve and a second positioned between the storage tank and the evaporator coil for altering the flow of cryogen from the storage tank to the evaporator coil. The first valve has a first position and a second position and the second valve has a third position and a fourth position. The first valve is moved into the first position and the second valve is moved into the third position to provide a first mass flow rate of cryogen from the storage tank to the evaporator coil. The first valve is moved into the first position and the second valve is moved into the fourth position to provide a second mass flow rate of cryogen from the storage tank to the evaporator coil. The first valve is moved into the second position and the second valve is moved into the third position to provide a third mass flow rate of cryogen from the storage tank to the evaporator coil. The first valve is moved into the second position and the second valve is moved into the fourth position to provide a fourth mass flow rate of cryogen from the storage tank to the evaporator coil.  
           [0009]    The heat exchanger includes a heating element. The flow of cryogen from the storage tank to the evaporator coil is discontinued each time the temperature in the outlet passes at least one of a third plurality of temperature control values and each time the temperature in the air inlet passes at least one of a fourth plurality of temperature control values. Air in the heat exchanger is heated with the heating element each time the temperature in the outlet passes at least one of the third plurality of temperature control values and each time the temperature in the air inlet passes at least one of the fourth plurality of temperature control values.  
           [0010]    Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    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.  
         [0012]    In the drawings, wherein like reference numerals indicate like parts:  
         [0013]    [0013]FIG. 1 is a side view of a truck including a preferred apparatus for implementing the present invention;  
         [0014]    [0014]FIG. 2 is a schematic drawing of the cryogenic temperature control apparatus in accordance with the present invention;  
         [0015]    [0015]FIG. 3 is a diagram detailing operation in the Fresh Cooling Range; and  
         [0016]    [0016]FIG. 4 is a diagram detailing operation in the Frozen Cooling Range. 
     
    
     DETAILED DESCRIPTION  
       [0017]    [0017]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, the air-conditioned space  14  is the cargo compartment in a truck  16 . In other applications, the cryogenic temperature control apparatus  12  can alternatively be used on other vehicles, such as a tractor-trailer combination, a container, and the like. Similarly, the cryogenic temperature control apparatus  12  can be used to control the temperature in the passenger space of a vehicle, such as for example, a bus or the passenger compartment of a truck. 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.  
         [0018]    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”).  
         [0019]    As shown in FIGS. 1 and 2, the air-conditioned space  14  has an outer wall  18 , which includes one or more doors  19  that 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 . The cryogenic temperature control apparatus  12  also includes a storage tank  20 , which houses a cryogen under pressure. 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.  
         [0020]    A conduit  22  is connected to the underside of the storage tank  20  and includes a filter  23 , a first branch  24 , and a second branch  25 . The conduit  22 , including the first branch  24 , defines a first flow path  28 . Similarly, the conduit  22 , including the second branch  25 , defines a second flow path  30 . 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 .  
         [0021]    With reference to FIG. 2, the first branch  24  includes a first control valve  26 . The first control valve  26  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  26  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.  
         [0022]    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.  
         [0023]    Additionally, as shown and described herein, the first and second control valves  26 ,  32  are preferably two position 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  26 ,  32  can be modulation valves, pulse valves, expansion valves, or the like. In these embodiments, the cryogenic temperature control apparatus  12  can provide a greater variety of available mass flow rates between the storage tank  20  and an evaporator coil  42  (described below). Similarly, in other embodiments (not shown), the flow path  22  can include three or more conduits, which extend between the storage tank  20  and the evaporator coil  42 . Each of these conduits can include a control valve (not shown) for regulating the mass flow rate of cryogen out of the storage tank  20 .  
         [0024]    The first and second control valves  26 ,  32  are controlled by a microprocessor controller  34 . As explained in more detail below, the controller  34  preferably uses ladder logic to control the flow of cryogen out of the storage tank  20 . The controller  34  is preferably powered by the truck&#39;s engine  36  or by an alternator (not shown) positioned within the engine  36 . In alternative embodiments, the controller  34  can also or alternatively be powered by a battery, a fuel cell, a generator, or the like. In other embodiments, a stationary power source (not shown), for example an outlet located on a building, can supply power to the controller  34 .  
         [0025]    As shown in FIG. 1, a heat exchanger  37  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  37  through the air intake  38  and is exhausted through the air outlet  39 . As shown in FIG. 1, the air outlet  39  preferably includes a damper  40  for altering airflow through the heat exchanger  37 . Alternatively, in some embodiments (not shown), the heat exchanger  37  does not include a damper  40  and fans or blowers independently control airflow through the heat exchanger  37 .  
         [0026]    The first and second flow paths  28 ,  30  are fluidly connected to an inlet of an evaporator coil  42  located in the heat exchanger  37 . During cooling operations, cryogen from the storage tank  20  flows along the flow path  22  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 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 an outlet  43  and is exhausted to the atmosphere.  
         [0027]    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 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 slightly above 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  above 60.43 psig.  
         [0028]    The cryogenic temperature control apparatus  12  also includes three sensors. The first sensor or return air sensor  45  is located between the evaporator coil  42  and the inlet  38  and records the return air temperature (“RA”), which is the temperature of the air returning to the heat exchanger  37  from the air-conditioned space  14 . The second sensor or evaporator coil outlet temperature sensor  46  is positioned adjacent the outlet  43  and records the temperature of cryogen vapor (“ECOT”) exiting the evaporator coil  42 . The third sensor or defrost termination switch  48  is positioned on the heat exchanger  37  and signals the controller  34  when the temperature of the heat exchanger  37  reaches a predetermined defrost termination temperature (“DTS”).  
         [0029]    As shown in FIGS. 1 and 2, a first fan  50  and a second fan  52  are positioned within the heat exchanger  37  and are operable to draw air from the air-conditioned space  14  through the heat exchanger  37 , which includes a heating element  53 . In other applications, the heat exchanger  37  may include one, three, or more fans  50 . As shown in FIG. 2, a heating element  53  is located in the heat exchanger  37  and includes a heating coil  54  and a fluid conduit  55 , which extends between the heating coil  54  and a coolant cycle (not shown) located in the truck&#39;s engine  36 . 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  36  heats the coolant in the coolant cycle. When heating is required, the third valve  58  is opened and coolant is directed through the heating element  53  to heat air in the heat exchanger  37 . In other embodiments, other fluids can be heated and can be directed through the heating element  53  to heat air in the heat exchanger  37 . In still other embodiments, other heating elements  53 , such as for example, electrical heaters (not shown) can also or alternatively be used to heat air in the heat exchanger  37 .  
         [0030]    To begin operation of the cryogenic temperature control apparatus  12 , the user is prompted to enter operating parameters into the controller  34 , including the set point temperature SP. Other operating parameters are described below and may be entered at startup by the user or may be preprogrammed by a system administrator. The cryogenic temperature control apparatus  12  is preferably operable in either a Fresh Cooling Range or a Frozen Cooling Range. During startup, the user preferably directs the controller  34  to operate the cryogenic temperature control apparatus  12  in either the Fresh Cooling Range or in the Frozen Cooling Range by selecting the set point temperature SP. If the user enters a set point temperature SP that is equal to or below 15° F., the unit will operate in the Frozen Cooling Range. Conversely, if the user enters a set point temperature SP that is greater than 15° F., the unit will operate in the Fresh Cooling Range.  
         [0031]    Once the set point temperature SP and the other operating parameters are entered, the first and second fans  50 ,  52  preferably cycle on for a predetermined time period (e.g., 30 seconds) to circulate air in the air-conditioned space  14 . The controller  34  then begins operation in either the Fresh Cooling Range or the Frozen Cooling Range.  
         [0032]    Referring first to FIG. 3, the Fresh Cooling Range includes six modes of operation, including a First Fresh Cooling Mode, a Second Fresh Cooling Mode, a Third Fresh Cooling Mode, a Null Fresh Mode, a Heating Mode, and a Defrost Mode. If the Fresh Cooling Range is selected, the controller  34  directs the cryogenic temperature control apparatus  12  to begin operation in one of these modes based upon data supplied by the return air temperature sensor  45 , the evaporator coil outlet sensor  46 , and the defrost termination switch  48 .  
         [0033]    If the return air temperature RA is passes the sum of the set point temperature SP and a first switch point temperature (“FS 1 ”) (e.g. 6° F.), the controller  34  is programmed to operate the cryogenic temperature control apparatus  12  in the First Fresh Cooling Mode. In the First Fresh Cooling Mode, the first and second control valves  26 ,  32  are opened to allow a maximum flow rate of cryogen through the evaporator coil  42 , thereby providing a rapid temperature pull down. The first and second fans  50 ,  52  are turned on and the damper  40  is opened to provide airflow across the evaporator coil  42 . Additionally, the third valve  58  is closed to ensure that no coolant enters the heating element  53 .  
         [0034]    If the return air temperature RA is less than or equal to the sum of the first switch point temperature FS  1  and the set point temperature SP at startup, the controller  34  is programmed to begin operation in the Second Fresh Cooling Mode. Similarly, if after operating in the First Fresh Cooling Mode, the return air temperature RA drops below or becomes equal to the sum of the first switch point temperature FS 1  and the set point temperature SP, the controller  34  shifts the cryogenic temperature control apparatus  12  into the Second Fresh Cooling Mode.  
         [0035]    In the Second Fresh Cooling Mode, the first valve  26  is opened and the second valve  32  is closed to provide a second flow rate of cryogen through the evaporator coil  42 , thereby providing a relatively rapid temperature pull down and simultaneously conserving cryogen. The first and second fans  50 ,  52  are turned on and the damper  40  is opened to provide airflow across the evaporator coil  42 . Additionally, the third valve  58  is closed to ensure that no coolant enters the heating element  53 .  
         [0036]    The controller  34  is also programmed to shift the cryogenic temperature control apparatus  12  into the Second Fresh Cooling Mode from the First Fresh Cooling Mode if the sensors determine that liquid cryogen is about to exit the evaporator coil  42  and enter the outlet  43 . In some cases, particularly when the mass flow rate of cryogen through the evaporator coil  42  is relatively high, some or all of the cryogen may not be completely vaporized in the evaporator coil  42 . In these cases, the cryogenic temperature control apparatus  12  is not operating in the most efficient manner. Additionally, if flooding is left unchecked, some or all of the cryogen may solidify in the evaporator coil  42 , rendering the cryogenic temperature control apparatus  12  inoperable. Therefore, if the difference between the return air temperature RA and the evaporator coil outlet temperature ECOT is greater than a flood point differential (“FPD”) (e.g., 15° F.), the controller  34  is programmed to shift from the First Fresh Cooling Mode to the Second Fresh Cooling Mode. Similarly, if the evaporator outlet coil temperature ECOT drops below −40° F., the controller  34  is programmed to shift the cryogenic temperature control apparatus  12  from the First Fresh Cooling Mode into the Second Fresh Cooling Mode.  
         [0037]    The cryogenic temperature control apparatus  12  continues to operate in the Second Fresh Cooling Mode until either of two conditions is achieved. First, if the return air temperature RA rises above the sum of the set point temperature SP, the first switch point temperature FS 1  and a fresh switch offset (“FSO”) (e.g., 2° F.), the cryogenic temperature control apparatus  12  shifts into the First Fresh Cooling Mode. Second, if the return air temperature RA drops below or becomes equal to the sum of the set point temperature SP and a second switch point temperature (“FS 2 ”) (e.g., 3° F.), the cryogenic temperature control apparatus  12  shifts into the Third Fresh Cooling Mode.  
         [0038]    Additionally, in some applications flooding can occur during operation in the Second Fresh Cooling Mode. Therefore, the controller  34  is preferably programmed to shift the cryogenic temperature control apparatus  12  into the Third Fresh Cooling Mode if the difference between the return air temperature RA and the evaporator coil outlet temperature ECOT is greater than the flood point differential FPD or if the evaporator coil outlet temperature ECOT drops below −40° F. The cryogenic temperature control apparatus  12  can also begin operation in the Third Fresh Cooling Mode at startup if the return air temperature RA is less than or equal to the sum of first switch point temperature FS 2  and the set point temperature SP and if the return air temperature RA is greater than the sum of the set point temperature SP and the second switch point temperature FS 2 .  
         [0039]    In the Third Fresh Cooling Mode, the first control valve  26  is closed and the second control valve  32  is opened to provide a lower mass flow rate of cryogen through the evaporator coil  42 . Additionally, the first and second fans  50 ,  52  are turned on and the damper  40  is opened to improve airflow through the heat exchanger  37  and the third valve  48  is closed to prevent heating.  
         [0040]    The cryogenic temperature control apparatus  12  continues to operate in the Third Fresh Cooling Mode until either of two conditions is met. First, if the return air temperature RA drops below the sum of the set point temperature SP and a cool-to-null temperature (“CTN”) (e.g., 0.9° F.), the cryogenic temperature control apparatus  12  switches to operation in the Null Fresh Mode. Second, if the return air temperature RA rises above the sum of the set point temperature SP, the second switch point temperature FS 2 , and the fresh switch offset FSO, the cryogenic temperature control apparatus  12  shifts from the Third Fresh Cooling Mode to the Second Fresh Cooling Mode.  
         [0041]    As explained above, the cryogenic temperature control apparatus  12  can shift from operation in the Third Fresh Cooling Mode to operation in the Null Fresh Mode. The cryogenic temperature control apparatus  12  can also begin operation in the Null Fresh Mode if the return air temperature RA is within a control band differential (“CBD”) (e.g., 4° F.) surrounding the set point temperature SP. Generally, the control band differential CBD is determined to be the preferred operating temperature range for a particular cargo and is therefore preferably user adjustable, but may also or alternatively be entered by the system administrator. If the return air temperature RA rises above the sum of the control band differential CBD and the set point temperature SP, the controller  34  is programmed to shift the cryogenic temperature control apparatus  12  from operation in the Null Fresh Mode to operation in the First Fresh Cooling Mode.  
         [0042]    In the Null Fresh Mode, the first and second control valves  26 ,  32  are closed to prevent cryogen from flowing through the evaporator coil  42  and the third valve  48  is closed to prevent coolant from entering the heating element  53 . Additionally, the first and second fans  50 ,  52  are turned off to conserve power and to prevent the fans  50 ,  52  from heating the air-conditioned space  14 . However, in some applications, the first and second fans  50 ,  52  can remain on during the Null Fresh Mode to maintain airflow in the air-conditioned space  14 .  
         [0043]    When the cryogenic temperature control apparatus  12  is switching from operation in the Third Fresh Cooling Mode to operation in the Null Fresh Mode, the first and second control valves  26 ,  32  are closed, as explained above. However, some residual cryogen still remains in the evaporator coil  42  after the first and second control valves  26 ,  32  are closed. This residual cryogen provides additional cooling to the air-conditioned space  14  to pull down the temperature of the air-conditioned space  14  after the flow of cryogen has been stopped. Additionally, the cooling capacity of the residual cryogen in the evaporator coil  42  is approximately equal to the cool-to-null temperature CTN. Therefore, when the cryogenic temperature control apparatus  12  is shifted from the Third Fresh Cooling Mode to the Null Fresh Mode, the residual cryogen pulls the temperature of the air-conditioned space  14  down to the set point temperature SP.  
         [0044]    The controller  34  is also programmed to accommodate failure of the sensors. More particularly, if during the First, Second, or Third Fresh Cooling Modes either the return air temperature sensor  45  or the evaporator coil outlet temperature sensor  46  record temperature values which are outside a predetermined value, indicating that the sensors are damaged or defective, the controller  34  is programmed to disregard the data supplied by that sensor. If a sensor fails, the cryogenic temperature control apparatus  12  activates an alarm (not shown) and continues to operate in the appropriate mode. If both the return air temperature sensor  45  and the evaporator coil outlet temperature sensor  46  fail, the cryogenic temperature control apparatus  12  operates in the Third Fresh Cooling Mode for a predetermined time period (e.g., two minutes) before shutting down.  
         [0045]    If the controller  34  determines that either the return air temperature sensor  45  or the evaporator coil outlet temperature sensor  46  has failed during operation in the First or Second Fresh Cooling Modes, the controller  34  is preferably programmed to shift the cryogenic temperature control apparatus  12  into the Third Fresh Cooling Mode. If the return air temperature sensor  45  fails, the cryogenic temperature control apparatus  12  operates in the Third Fresh Cooling Mode until the evaporator coil outlet temperature ECOT drops below the sum of the set point temperature SP, the cool-to-null temperature CTN, and −5° F., at which time the cryogenic temperature control apparatus  12  shifts to the Null Fresh Mode. If the return air temperature sensor  45  fails and the evaporator coil outlet temperature ECOT rises above the sum of the set point temperature SP and the control band differential CBD, the controller  34  shifts from operation in the Null Fresh Mode to operation in the Third Fresh Cooling Mode.  
         [0046]    If the evaporator coil outlet temperature sensor  46  fails during operation in the Null Fresh Cooling Mode, the cryogenic temperature control apparatus  12  continues to operate in the Null Fresh Mode until the return air temperature RA rises above the sum of the control band differential CBD and the set point temperature SP, at which time the controller  34  shifts to operation in the Third Fresh Cooling Mode.  
         [0047]    In some applications, such as when the ambient temperature is below the set point temperature SP, it may be desirable to heat the air-conditioned space  14 . Therefore, during operation in the Fresh Range, the cryogenic temperature control apparatus  12  can operate in a Heating Mode if the return air temperature RA drops below or becomes equal to the sum of the set point temperature SP and the control band differential CBD. Once the return air temperature RA reaches the set point temperature SP, the cryogenic temperature control apparatus  12  shifts into the Null Fresh Mode.  
         [0048]    Occasionally, 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 during operation in either the Fresh Range or the Frozen Range.  
         [0049]    When the cryogenic temperature control apparatus  12  operates in the Defrost Mode, the first and second control valves  26 ,  32  are closed so that cryogen does not enter the evaporator coil  42 . The third control valve  58  is opened to allow coolant to enter the heating element  53  and the damper  40  is closed to prevent warm air from entering the air-conditioned space  14 . Preferably, the first and second fans  50 ,  52  are deactivated.  
         [0050]    The cryogenic temperature control apparatus  12  can shift into the Defrost Mode in four different ways. First, the operator can manually direct the controller  34  to shift the cryogenic temperature control apparatus  12  into the Defrost Mode. However, to prevent the operator from unnecessarily initiating the Defrost Mode, the controller  34  is preferably programmed to prevent manual initiation unless either the evaporator coil outlet temperature ECOT is less than or equal to 35° F. or the set point temperature SP is less than or equal to 50° F.  
         [0051]    Second, the Defrost Mode is initiated at predetermined time intervals (e.g., two hours) which are preferably programmed by the system administrator. However, unless the evaporator coil outlet temperature ECOT is less than or equal to 35° F. or the set point temperature SP is less than or equal to 50° F., the Defrost Mode will not be initiated at the predetermined time intervals.  
         [0052]    Third, the Defrost Mode is initiated based upon demand when the controller  34  determines that specific requirements have been met. Specifically, the Defrost Mode is initiated if the evaporator coil outlet temperature ECOT is less than or equal to 35° F. and the mass flow rate of cryogen moving through the cryogenic temperature control apparatus  12  is above a predetermined mass flow rate (“M”) (e.g., during operation in the Third Cooling Mode when the first control valve  26  is closed and the second control valve  32  is open). Alternatively, the Defrost Mode is initiated when the return air temperature RA minus the evaporator coil outlet temperature ECOT is above a predetermined amount (e.g., 8° F.), which is preferably adjustable and may be programmed by the system administrator. The predetermined mass flow rate M is a function of the operating environment, including expected ambient humidity levels and evaporator sizes and therefore is preferably determined by the system administrator or may be entered by the operator during startup.  
         [0053]    Fourth, the Defrost Mode is automatically initiated when the evaporator coil outlet temperature ECOT is equal to or less than −40° F. and the mass flow rate of cryogen moving through the cryogenic temperature control apparatus  12  is above the predetermined mass flow rate M.  
         [0054]    Once the Defrost Mode is initiated, defrosting continues until the air temperature around the defrost termination switch  48  is equal to the defrost termination temperature DTS (e.g., 45° F.) or the evaporator coil outlet temperature ECOT reaches 59° F. Additionally, in some applications, the controller  34  is programmed to terminate the Defrost Mode after a predetermined time.  
         [0055]    Referring to FIG. 4, the Frozen Cooling Range includes five modes of operation, including a First Frozen Cooling Mode, a Second Frozen Cooling Mode, a Third Frozen Cooling Mode, a Null Frozen Mode, and a Defrost Mode. If the Frozen Cooling Range is selected (i.e., the set point temperature SP is less than 15° F.), the controller  34  directs the cryogenic temperature control apparatus  12  to begin operation in one of these modes based upon data supplied by the return air temperature sensor  45 , the evaporator coil outlet sensor  46 , and the defrost termination switch  48 .  
         [0056]    If the return air temperature RA is greater than the set point temperature SP, the cryogenic temperature control apparatus  12  begins operating in the First Frozen Cooling Mode. In the First Frozen Cooling Mode, the first and second control valves  26 ,  32  are opened to allow a maximum flow rate of cryogen through the evaporator coil  42 , thereby providing a rapid temperature pull down. The first and second fans  50 ,  52  are turned on and the damper  40  is opened to provide airflow across the evaporator coil  42 . Additionally, the third valve  58  is closed to ensure that no coolant enters the heating element  53 . Once, the return air temperature RA becomes equal to or drops below the set point temperature SP, the cryogenic temperature control apparatus  12  is shifted from the First Frozen Cooling Mode to the Null Frozen Mode (described in more detail below).  
         [0057]    As explained above with respect to the Fresh Cooling Range, some or all of the cryogen in the evaporator coil  42  may not evaporate during cooling operations and the evaporator coil  42  may begin to fill with liquid cryogen. If the flooding occurs, the cryogen may solidify in the evaporator coil  42  and may damage the cryogenic temperature control apparatus  12 . Therefore, to prevent flooding, the cryogenic temperature control apparatus  12  shifts from the First Frozen Cooling Mode into the Second Frozen Cooling Mode if one of two conditions is met. First, if the difference between the return air temperature RA and the evaporator coil outlet temperature ECOT drops below the flood point differential FPD (e.g., 15° F.), the cryogenic temperature control apparatus  12  shifts into the Second Frozen Cooling Mode. Second, if the evaporator coil outlet temperature ECOT drops below −40° F., the cryogenic temperature control apparatus  12  shifts into the Second Frozen Cooling Mode.  
         [0058]    In the second Frozen Cooling Mode, the first valve  26  is opened and the second valve  32  is closed to provide a second flow rate of cryogen through the evaporator coil  42 , thereby providing a relatively rapid temperature pull down and simultaneously conserving cryogen. The first and second fans  50 ,  52  are turned on and the damper  40  remains opened to allow airflow across the evaporator coil  42 . Additionally, the third valve  48  is closed to prevent heating.  
         [0059]    The cryogenic temperature control apparatus  12  continues to operate in the Second Frozen Cooling Mode as long at the return air temperature RA remains above the set point temperature SP and until one of three conditions is achieved. First, if the difference between the return air temperature RA and the evaporator coil outlet temperature ECOT drops below the flood point differential FPD, the cryogenic temperature control apparatus  12  shifts into the Third Frozen Mode. Second, if the evaporator coil outlet temperature ECOT drops below −40° F., the cryogenic temperature control apparatus  12  shifts into the Third Frozen Mode. Third, if the return air temperature RA becomes equal to or drops below the set point temperature SP, the cryogenic temperature control apparatus  12  is shifted from operation in the Second Frozen Cooling Mode to operation in the Null Frozen Cooling Mode.  
         [0060]    In the Third Frozen Cooling Mode, the first control valve  26  is closed and the second control valve  32  is opened to provide a relatively low mass flow rate of cryogen through the evaporator coil  42 . Additionally, the first and second fans  50 ,  52  are turned on and the damper  40  remains opened to allow airflow through the heat exchanger  37  and the third valve  48  is closed to prevent heating.  
         [0061]    If the return air temperature RA drops below or becomes equal to the set point temperature SP, the cryogenic temperature control apparatus  12  shifts from the Third Frozen Cooling Mode to the Null Frozen Mode. In the Null Frozen Mode, the first and second control valves  26 ,  32  are closed and the first and second fans  50 ,  52  remain on for a predetermined time (e.g., 30 seconds) and then shut off.  
         [0062]    The cryogenic temperature control apparatus  12  continues to operate in the Null Frozen Mode as long as cooling is required and the return air temperature RA is less than or equal to the sum of the set point temperature SP and a predetermined control band differential CBD (e.g., 4° F.). If the return air temperature RA rises above the sum of the control band differential CBD and the set point temperature SP and if the return air temperature RA is greater than a null flood prevent temperature (“NFP”) (e.g., 15° F.), the cryogenic temperature control apparatus  12  shifts to the First Frozen Cooling Mode. Conversely, if the return air temperature RA rises above the sum of the control band differential CBD and the set point temperature SP and the return air temperature RA is less than or equal to the null flood prevent temperature NFP (e.g., 15° F.), the cryogenic temperature control apparatus  12  shifts into the Second Frozen Cooling Mode.  
         [0063]    The controller  34  is also preferably programmed to accommodate failure of one or both of the return air sensor  45  and/or the evaporator coil outlet temperature sensor  46  during operation in the Frozen Cooling Range. As explained above with respect to operation in the Fresh Cooling Range, the controller  34  determines whether or not the return air temperature sensor  45  and the evaporator coil outlet temperature sensor  46  are damaged or defective by comparing the data supplied by the sensors to predetermined expected ranges. If the return air temperature sensor  45  or the evaporator coil outlet temperature sensor  46  record values outside these expected ranges, the controller  34  disregards the data supplied by that sensor and relies on the data supplied by the other sensor.  
         [0064]    Specifically, if the cryogenic temperature control apparatus  12  is operating in either the First Frozen Cooling Mode or the Second Frozen Cooling Mode and the controller  34  determines that the return air temperature sensor  45  or the evaporator coil temperature sensor  46  has failed, the cryogenic temperature control apparatus  12  is shifted into the Third Frozen Cooling Mode.  
         [0065]    Similarly, the cryogenic temperature control apparatus  12  is shifted from the Null Frozen Mode to the Third Frozen Cooling Mode if the controller  34  determines that the return air temperature sensor  45  has failed and the evaporator coil outlet temperature ECOT is greater than the sum of the set point temperature SP and the control band differential CBD. Alternatively, the cryogenic temperature control apparatus  12  is shifted from the Null Frozen Mode to the Third Cooling Mode if the controller  34  determines that the evaporator coil outlet temperature sensor  46  has failed and the return air temperature RA is greater than the sum of the set point temperature SP and the control band differential CBD. Also, the cryogenic temperature control apparatus  12  shifts from the Third Frozen Cooling Mode to the Null Frozen Mode if the controller  34  determines that the return air temperature sensor  45  has failed and the evaporator coil outlet temperature ECOT is greater than or equal to the sum of the set point temperature SP, the cool-to-null temperature CTN and −8° F.  
         [0066]    As explained above, the cryogenic temperature control apparatus  12  operates in a Defrost Mode during operation in the Frozen Range. However, operation of the Defrost Mode during the Frozen Range is substantially similar to operation of the Defrost Mode during the Fresh Range and is therefore not described further herein.  
         [0067]    During operation in either the Fresh Range or the Frozen Range, the controller  34  is preferably programmed to include time delays when shifting between the various modes of operation. This ensures that a temperature spike does not shift the cryogenic temperature control apparatus  12  into an inappropriate mode. In different applications, the delays can be any length but are preferably between one second and twenty seconds.  
         [0068]    The cryogenic temperature control apparatus  12  includes a door sensor  62 , which is operable to determine if the doors  19  are open or closed. Preferably, the cryogenic temperature control apparatus  12  shuts down operation when the doors  19  are opened and does not resume normal operation until the doors  19  are closed. Alternatively, in some embodiments, the system administrator can program the controller  34  to resume normal operation if the doors  19  remain open for an extended time.  
         [0069]    The embodiments described above and illustrated in the drawings are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. As such, it will be appreciated by one having ordinary skill in the art, that various changes in the elements and their configuration and arrangement are possible without departing from the spirit and scope of the present invention as set forth in the appended claims.  
         [0070]    For example, the present invention is described herein as being used to pull down and maintain the temperature in a truck  16  having a single air-conditioned space  14 . However, one having ordinary skill in the art will appreciate that the present invention could also be used in trucks or trailers having multiple air-conditioned spaces  14 . Similarly, the present invention can also be used to pull down and maintain the temperature in buildings, containers, and the like.  
         [0071]    Similarly, the present invention is described herein as including a first control valve  26  with a first relatively large orifice and a second control valve  32  with a second smaller orifice. This arrangement preferably provides four distinct mass flow rates. One having ordinary skill in the art will appreciate that in other applications additional valves can be used to provide additional flow rates. Also, one having ordinary skill in the art will appreciate that an adjustable valve, a pulse valve, an expansion valve, or the like could be used to provide additional mass flow rates and additional modes of operation.  
         [0072]    As such, the functions of the various elements and assemblies of the present invention can be changed to a significant degree without departing from the spirit and scope of the present invention.