Abstract:
A container for transporting a load in conditioned space air, where the container includes a number of walls defining a closed conditioned space with an ambient air intake opening and a conditioned space air exhaust opening provided on one of the container walls; a temperature control system for controlling the temperature of the conditioned space air; an ambient air exchange so system having a door movable relative to the exhaust and intake openings to thereby change the rate that conditioned space air is exchanged with ambient air, and a motor for repositioning the ambient air exchange door. The container also including a container controller in signal transmitting relation with the air exchange door repositioning motor. The controller sends signals to the repositioning motor to move the ambient air exchange door to a position which produces the required ambient air exchange rate to preserve the load.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/109,985, filed Nov. 25, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a system for controlling the ambient air exchange rate of a refrigerated container to control the composition of the conditioned space air, and more particularly the invention relates to a system and method for controlling the composition of the conditioned space air by automatically and precisely setting and controlling the ambient air exchange rate of refrigerated container by comparing a predetermined set point value for a container operating parameter with a sensed value for the operating parameter, and if the sensed value is outside the set point value, automatically adjusting the air exchange rate to obtain the conditioned space air concentration required to preserve the container cargo. 
     Containers are loaded with perishable cargo such as fruits, vegetables, and flowers for example, and the loaded containers are then placed on ships which bring the perishable cargo to its destination a number of days after the cargo has been loaded in the container. When transporting a temperature controlled load of perishable cargo, the cargo is subject to degradation due to varying concentration levels of common gasses in the conditioned space defined by the transport container. Fresh air exchange with the conditioned space air is used to help prolong the life of the cargo transported in the conditioned space. The commonly present conditioned space gasses include nitrogen, carbon dioxide, and ethylene. Some mixtures of these gasses are beneficial and promote longevity and freshness of the cargo, while other mixtures of these gasses are harmful to the cargo and can reduce freshness and spoil the cargo. 
     Some of the common conditioned space gasses are actively produced by the respiration of the cargo. Upon loading, the concentration of the conditioned space gasses is acceptable. However, over time while in transit, the cargo respirates and as the gasses are emitted by the cargo, the gas concentration levels change. Depending on the nature of the composition of the conditioned space air, the change in gas concentration could reduce the freshness of the cargo and ultimately result in cargo spoilage. As a result, the concentrations of the common gases in the container conditioned space must be closely monitored and controlled during shipment to prevent the production of harmful gas concentrations. 
     The typical method for controlling the container concentrations of common gasses is by manually adjusting the position of a fresh air exchange door which, when opened, permits outside ambient air to enter the container and supplant the conditioned space air. After determining the fresh air exchange rate required to maintain the desired concentration of the common gasses in the container, the exchange door is manually opened as required to the door position that will produce the requisite common gas concentration levels. In such systems, the exchange door is located near the container differential air pressure caused by the fan that blows temperature conditioned air over the cargo. This differential pressure draws the ambient air into the container. This conventional method of achieving air exchange in a container conditioned space does not take into account changes to the conditioned space air during shipment. Moreover, such conventional system cannot quickly adjust door position as a result of changes in the composition of the conditioned space air and as a result of changes in the ambient air composition. 
     Other problems do exist with the manually adjusted fresh air exchange doors. One common problem is human error. If the door is forgotten or misadjusted by the container operator, the perishable cargo will suffer degradation. The containers are frequently placed in difficult to access locations on the ships and as a result, are inspected infrequently by a container operator. By the time the operator realizes the load is degrading due to harmful gas concentration levels, it is too late to manually reposition the door to achieve the concentration level required to save the cargo. 
     Another problem with conventional manually adjusted doors is illustrated when warm perishable cargo is loaded in warm ambient conditions. In this situation the combination of the cargo&#39;s warm temperature and the warm ambient air entering the container through the open door results in a very long time period for temperature to pull down in the container. The consequence of the long container pull down period is cargo degradation. 
     The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, this is accomplished by providing an automatic fresh air control system and method that overcomes the shortcomings of conventional manually actuated air exchange doors by the improvements generally set forth below. 
     An effective improvement can be made to the manual fresh air exchange door by automating the opening of the door. The movement of the door may be controlled by a motor that in turn is controlled by a microprocessor. Such a system would be customer controlled and the customer would enter an air exchange set point in cubic feet per minute, percent door open, or cubic meters per hour into the container controller. The microprocessor system then directs the door to open based on the customer input or based on the default setting if the customer forgets to enter a value. Human error is eliminated. 
     Further improvements arc realized by noting the differential air pressure source varies based on the varying speed of the fan that blows temperature controlled air. The microprocessor system can utilize fan speed information of this variable speed fan to more accurately calculate the required door position to provide the desired fresh air exchange rate. 
     Further improvements are realized by noting that when warm cargo material is loaded, field heat may be more rapidly removed from the load if the fresh air exchange door remains closed during the temperature pull down to prevent introducing high ambient temperature air into the container. A closed door time limit must be implemented to prevent the door from remaining closed for too long of a period which might allow gas concentration levels to damage the load. The time limit is selectable by the customer and also has a default setting if the customer does not enter a time limit. 
     Further improvements are realized by incorporating a carbon dioxide sensor to monitor carbon dioxide levels within the container. The carbon dioxide levels are compared to a carbon dioxide set point to allow the microprocessor system to incrementally open or close the fresh air exchange door as needed to prevent the carbon dioxide levels from exceeding set point. This effectively limits the maximum concentration of carbon dioxide levels within the container. An oxygen sensor for monitoring oxygen levels may also be used. 
     Further improvements can be realized by utilizing the temperature set point of the load to automatically modify the carbon dioxide set point. The algorithm is a conversion from temperature set point to carbon dioxide set point as a percentage of gas concentration. The formula is a polynomial equation where the carbon dioxide set point equals various orders of the temperature set point with individual coefficients. 
     Further improvements can be realized by incorporating customer overrides for carbon dioxide set point and fresh air exchange door position. These overrides are useful when introducing a known concentration of gasses to the container, when hauling fresh fish or meats or other non-plant perishable produce, and when performing pre-trip functions to verify the health of the system that controls the temperature and atmosphere of the conditioned space. 
    
    
     The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures. 
     DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is a perspective view of a container that includes a front panel comprised of a temperature control system that includes the automatic fresh air exchange system of the present invention; 
     FIG. 2 is a front plan view of the container temperature control system which substantially comprises the front panel of the container of FIG. 1; 
     FIG. 3 is a rear isometric view of the temperature control system of FIG. 2; 
     FIG. 4 is a partial view of the fresh air exchange system as mounted along the front of the container temperature control system; 
     FIG. 5 is an isometric view of the fresh air exchange system of the present invention; 
     FIG. 6 is an isometric view of the fresh air exchange system of the present invention with the door in the closed position; 
     FIG. 7 is an isometric view of the fresh air exchange system of FIG. 6 with the door in the open position; 
     FIG. 8 is a schematic representation of the container, container temperature control system and fresh air exchange system; 
     FIG. 9 is a flow chart representation of the portion of the fresh air exchange control logic that positions the door based on load temperature; 
     FIG. 10 is a flow chart representation of the portion of the fresh air exchange control logic that keeps the fresh air exchange door closed for a period of time during pull down; 
     FIG. 11 is a flow chart representation of the portion of the fresh air exchange control logic that positions the door based on CO 2  concentration in the conditioned space air; and 
     FIG. 12 is a flow chart representation of the portion of the fresh air exchange control logic that positions the door based on O 2  concentration in the conditioned space air. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now turning to the drawing Figures, FIGS. 1-4 and  8  illustrate the container  10  that includes the fresh air exchange system of the present invention  12 ; and FIGS. 5-7 specifically illustrate the operation of the system  12 . 
     The container includes a conditioned space  14  that is defined by top panel  16 , bottom panel  18 , longitudinal side panels  20  and  22 , and lateral side panels  24  and  26  that join the other top, bottom and longitudinal panels. The front lateral panel  24  is comprised of a temperature control system  28 , as shown in FIGS. 1 and 2. With the exception of automatic fresh air exchange unit  12 , the temperature control system  28  is of conventional, design well known to one skilled in the relevant art, and as a result, further detailed description of the system  28  is not required. 
     Operation of the temperature control system including fresh air exchange system  12  is controlled by microprocessor based controller  30 . The controller is in signal receiving relation with conventional oxygen, carbon dioxide and load temperature sensors  32 ,  34 , and  36  respectively that are located at the required sensing position in the conditioned space  14 . The positioning of the sensors is shown generally in FIG. 8 however it should be understood that this is for illustrative purposes and the sensors  32 ,  34 , and  36  may be located at any suitable location in the condition space  14 . Temperature sensors  32  are located in the container controlled atmosphere chamber. The sensors  34  are spaced throughout the chamber to ensure an accurate chamber temperature is obtained. Sensors for sensing the relative percentage of carbon dioxide in the controlled atmosphere chamber are located at the front of the chamber and are preferably mounted on the inside of the front container wall illustrated in FIG. 1 and 2. The controller  30  is also connected to the fresh air exchange system in signal transmitting relation to system  12 . The controller  30  controls the operation of the conventional components of system  28 , and includes a conventional means for inputting set point values for operating parameters such as oxygen and carbon dioxide concentration levels, load temperature, and set point temperature, for example. The input means may be a tactile keypad. 
     The temperature control system  28  is supported by a frame  33  having a plurality of connected outer support members  38 ,  39 ,  40 , and  41 . The frame  33  includes a pair of inner support members  42  and  43  with one end connected to member  40  and an opposite end connected to respective members  41  and  39 . As shown in FIG. 1, the inner support members converge to substantially from a V-shape. As illustrated in FIG. 3, system  28  is further supported by panel  44  having a substantially planar upper portion  46  and a rectangular lower portion  48 . The lower portion  48  and members  39 ,  40 , and  43  define an air exchange pocket  50  which is substantially triangle shaped. 
     A conventional fan  52  is located on the front panel and the fan provides the pressure differential that draws ambient fluid into the conditioned space  14  through intake opening  54  and displaces conditioned space air out the exhaust opening  56 . The fan speed is controlled by controller unit  30 . 
     The fresh air exchange system  12  of the present invention will now be described. Turning now to FIGS. 5,  6 , and  7 , system  12  is located in chamber  50  and includes a rigid tray  60  that is conventionialy and removably connected to lower portion  48  of wall  44  by upwardly turned flanges  61  and  63 . The tray  60  is oriented substantially horizontally and supports repositioning motor  62 . 
     Hinge  64  is provided along a tray free outer edge and a filter door  66  is adapted to be hingeably connected to tray hinge  64 . As shown in FIG. 5 the door  66  includes a plurality of spaced apart rectangular apertures which permit ambient air to pass through the door  66  and prevent debris from being drawn into the conditioned space through openings  54  and  56 . The door  66  may be locked by a conventional locking device that is passed through opening  70 . 
     The repositioning motor  62  is preferably a twelve volt DC motor of conventional design. The repositioning motor  62  is in signal receiving relation with the system microprocessor  30 . Door limit member  72  is provided on one side of the motor housing and limits the maximum amount that the door  74  can be moved when opened. 
     The door  74  is hingeably connected to portion  48  by conventional hinge  76 , and in this way is moveable between closed and open positions as shown in FIGS. 6 and 7 respectively. 
     The door  74  is connected to motor  62  by links  78  and  80 . One end of the first link  78  is rotatably connected to the motor  62  by a pin connection. The opposite end of the first link  78  is rotatably connected to the first end of the second link  80 . The second end of the second link  80  is connected to a bracket  82  that is welded or otherwise conventionally attached to the underside of the door  74 . In this way, movement of the pin moves the links  78  and  80  and thereby causes the door  74  to be repositioned toward or away from openings  54  and  56 . 
     When it ii determined that the door  74  needs to be repositioned, the microprocessor  30  sends a signal to the motor  62 , energizing the motor  62 . The signal contains data that indicates the direction the motor  62  is to turn. The motor  62  turns in the required direction and moves the link  78 . The movement of the link  78  in turn moves the link  80  and thereby moves the door  74 . The motor  62  is energized until the door  74  is relocated at the desired position to obtain the requisite air exchange. 
     During,operation of the container unit, the microprocessor  30  obtains readings of temperature and carbon dioxide levels from the sensors  36 ,  34  and compares the values to desired set point values or ranges for these operating parameters. If one or both of these parameters is outside the desired set point value or range, the microprocessor  30  determines the direction and amount the door  74  must be repositioned to obtain the desired value or values for the operating parameters. Once the direction and distance have been determined, the microprocessor  30  sends the signal to the motor  62  actuating the motor  62  and repositioning the door  74 . 
     Turning to FIGS. 9-12 that represent the controller logic, the operation of system  12  will be described in greater detail. 
     Microprocessor Controls the Fresh Air Exchange Rate 
     The fresh air exchange rate is programmed through the refrigerated container&#39;s microprocessor  30  in either cubic feet per minute, percent fresh air exchange door open, or cubic meters per hour. Once the exchange rate is set, the microprocessor  30  positions the fresh air exchange door  74  at the location required to provide the desired air exchange rate. 
     Fresh Air Exchange Based on Load Temperature 
     The fresh air exchange rate may be adjusted based on the load temperature. Turning now to FIG. 9, which generally represents the segment of controller logic where the fresh air exchange rate is adjusted based on load temperature. The logic is identified generally at  500 . 
     In Step  502 , the load temperature is obtained by sensor  36  and after reading the load set point temperature in Step  503 , the load temperature is compared with the load set point temperature in Step  504 . 
     In Step  505  if the load temperature is not equal to the set point temperature for the load, a new door position is calculated using the following equation: 
     
       
         New Door Position=( A   1 * e   A2 )+( A   3 * 1 n( A   4 ))+ A   5 +( A   6 * A   7 )+( A   8 * A   9   2 )+( A   10 * A   11   3 )+( A   12 * A   13   4 ) . . .  
       
     
     In the polynomial, the polynomial equation consists of various orders of the temperature differential computed in Step  505  with individual coefficients. 
     Once the new position is calculated, the microprocessor  30  sends a signal corresponding to the new position to the motor  62  to change the door  74  position the distance required to located the door  74  in the position which produces the desired fresh air exchange. See Step  506 . 
     Programmable Delayed Opening of Fresh Air Exchange Door 
     From time to time, the cargo loaded in container  10  may have a temperature that is at or above the predetermined desired load set point temperature. In such situations, the field heat may be more rapidly removed from the load by keeping the fresh air exchange door  74  closed for a period of time and then automatically opening the door  74  when the closed door time limit has expired. At the expiration of the closed door time limit, the load temperature will be reduced to the load set point. 
     The closed door time limit is the operator determined period of time required to reduce the temperature of the load. In the present invention, the operator may manually enter a closed door time limit value in the microprocessor  30 . The closed door time limit may be set in one hour increments for up to a total of seventy-two hours. Conversely, the operator can accept a default value of zero hours if no value is entered. 
     Turning now to FIG. 10 which is a flow chart representation of Routine  600  for delayed opening of fresh air exchange door  74 . The logic of routine  600  represents a portion of the logic routine executed by microprocessor controller  30 . In Step  602  the processor reads the closed door time limit which may be either the value inputted by the operator or the system default value. In Steps  604  and  606  the system obtains the load set point temperature and actual load temperature respectively. 
     In Step  608  the processor determines if the load temperature is greater than or equal to the set point temperature for the load. If the load temperature is greater than or equal to the set point temperature, the fresh air exchange door  74  is kept closed until the closed door time limit expires in timer. See Step  612 . 
     After the time limit expires, in Step  614  a signal is sent from the microprocessor  30  to the motor  62  to energize the motor  62  and open fresh air exchange door  74  the required amount. 
     This feature helps ensure that the container&#39;s interior temperature is brought down to the desired set point temperature before initiating fresh air exchange. 
     Because the door  74  can be programmed to remain closed for up to seventy-two hours, temperature pull down to the desired set point can be reached in half the time it would otherwise require with the door  74  open. As a result of the invention, container operators are no longer required to manually open fresh air exchange door  74  when they believe the load temperature is at the set point. 
     Frozen Lockout 
     When the load set point is programmed for frozen cargo, the microprocessor  30  automatically locks out the fresh air exchange by keeping the fresh air exchange door  66  closed during shipment. For the present invention frozen cargo is defined as a load with a temperature of five or more degrees below freezing temperature (e.g. 32° F. or 0° C.). If the load is frozen, it is not respirating and therefore there is not a need to provide fresh air to the load thereby permitting the door  74  to be closed. 
     Low Ambient Temperature Frozen Load Protection 
     Occasionally, the ambient temperature will fall well below the set point temperature for the load. On these occasions when the ambient temperature is far below the set point temperature the fresh air exchange door  74  may be closed to protect cargo from freezing since freezing can destroy the cargo. 
     If a high volume of very cold fresh air exchange causes the load temperature to drop below the set point, then the fresh air exchange door  74  must be incrementally closed to reduce the quantity of cold air delivered to the cargo. 
     The microprocessor  30  compares the load temperature to the load set point temperature, and if after the comparison the sensed load temperature value is deemed to be below the load set point temperature and the system is producing as much heat as it can, the microprocessor  30  sends a signal to the motor  62  which actuates the motor  62  and thereby incrementally closes the fresh air exchange door  74 . 
     When the fresh air exchange door  74  is closed, the container becomes a closed system and the air in the container is recirculated through the container conditioned space  14  around the load. 
     The door  74  remains closed until the ambient temperature warms to a temperature that is not far below the set point temperature. At that point, the microprocessor  30  sends a signal to the motor  62  which causes the motor  62  to open the fresh air exchange door  74  the distance necessary to obtain the desired fresh air exchange rate. 
     Oxygen, Carbon Dioxide Monitoring and Control 
     The fresh air exchange system  12  can also be programmed based on the desired minimum oxygen and maximum carbon dioxide concentration levels. Sensors  32 ,  34  are used to measure the levels of the two gases. When the levels go out of a prescribed range, the microprocessor  30  will automatically adjust the fresh air exchange rate to keep the oxygen and carbon dioxide properly balanced for the container cargo. 
     Depending on the cargo, the container may include an oxygen sensor  32  only, a carbon dioxide sensor  34  only, carbon dioxide and oxygen sensors  32 ,  34  in combination, and in some instances the container may not include either O 2  or CO 2  sensors  32 ,  34 . Whether or not to include carbon dioxide and oxygen sensors  32 ,  34  is dependent on the load to be transported in the container. 
     The control algorithm for container  10  includes logic generally represented in FIGS. 11 and 12. FIG. 11 represents the logic for carbon dioxide concentration monitoring and FIG. 12 represents the logic for monitoring the levels of Oxygen concentration. The logic of FIGS. 11 and 12 is the same so the logic for sensing carbon dioxide and oxygen concentration levels will be described together. The operator enters a desired concentration value as a percentage of air for carbon dioxide and also for oxygen. The concentration level may be set at a value between zero and twenty-one percent. The set point carbon dioxide and oxygen concentration levels for the cargo are set at the input keypad of the microprocessor  30 . The set point values are read in Steps  702  and  802 . 
     If the concentration levels are not inputted they may be calculated based on load set point temperature using the following polynomial equation in the general form: 
     
       
         Concentration Level=( A   1 * e   A2 )+( A   3 * 1 n( A   4 ))+ A   5 +( A   6 * A   7 )+( A   8 * A   9   2 )+( A   10 * A   11   3 )+( A   12 * A   13   4 ) . . .  
       
     
     The polynomial algorithm is a conversion from temperature set point to concentration level where the concentration level equals various orders of the temperature set point with individual coefficients. The calculated concentration level for the gases is then saved in processor memory. 
     During operation of the container the carbon dioxide and oxygen sensors read the actual concentration levels in the container. See Steps  704  and  804 . 
     In Steps  706  and  806  the sensed concentration levels are compared to the set point concentration levels. If the concentration levels are not equal to the set point concentration levels the microprocessor  30  calculates a new door position required to obtain the required concentration levels. If the actual concentration levels are equal or at least within an acceptable tolerance value of the desired set point concentration level, the routine is exited in Steps  710  and  810 . 
     If a new door position is calculated by the microprocessor  30 , in Steps  708  and  808  a signal is sent to the motor  62  changing the door position. 
     Record Fresh Air Exchange Data 
     User selected exchange rates, the load set point temperature, closed door time delay intervals, O 2  and CO 2  levels, and any changes made to these settings are automatically stored in microprocessor memory where it can be retrieved for analysis along with pertinent temperature and unit operation data. 
     While we have illustrated and described a preferred embodiment of our invention, it is understood that this is capable of modification, and we therefore do not wish to be limited to the precise details set forth, but desire to avail ourselves of such changes and alterations as fall within the purview of the following claims.