Abstract:
A method and system for controlling the environment of storage facilities, including produce and livestock storage facilities, and the like, is described. Movement of air within the facility is accomplished by air-handling units or fans. The speed of each fan is controlled by a variable-speed drive, allowing the fans to run at speeds below full capacity. Environmental parameters, such as temperature or humidity, are monitored to determine the existing state of the environment which is then compared to a desired state. The speed of the fans or air-handling units is adjusted to alter the existing environmental state, bringing it in alignment with the desired state. The fans or air-handling units are operated continuously, typically at reduced capacity. Other various facets are included with the system and method, including the control of the admittance of external air into the storage facility.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part of U.S. application Ser. No. 09/621,509, filed Jul. 21, 2000, now U.S. Pat. No. 6,467,695 B1, which issued Oct. 22, 2002. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to environmental control of storage buildings and facilities. More particularly, the present invention relates to the control of such parameters as temperature, humidity, and carbon dioxide (CO 2 ) within a storage facility wherein produce or like commodities are stored. 
   2. State of the Art 
   Produce providers often desire to store fruits and vegetables for extended periods of time. Produce is often stored to maintain adequate supplies during periods when a particular commodity is out of season. Processors of fruit and vegetables increasingly desire commercial growers to store their products for longer and longer periods of time. Indeed, processors require a year-round supply of produce while requiring that the quality of such produce remain high. 
   To store produce for extended periods of time without substantial degradation of quality, it becomes imperative to control the environment in which the produce is stored. Control of the storage facility environment may include the control of, for example, temperature, humidity, and air quality including carbon dioxide (CO 2 ) content. Typically, control of such parameters in a storage facility environment entails movement of air within the facility. Oftentimes, this includes introduction of air from outside the facility. Other times it may simply involve the circulation of existing air inside the storage facility. 
   One method of controlling the environment has been to place fans or air-handling units in the facility. The fans may be turned on when the temperature rises above a predetermined upper level and shut off when the temperature of the facility reaches a predetermined lower level. A system of this type is described in U.S. Pat. No. 3,801,888 to Faulkner. This type of system utilizes the fans at full power, allowing them to cool the facility at a relatively quick pace, but also allowing temperatures or other environmental parameters to change rapidly within a specified range. Rapid changes in temperature or temperature spikes may often cause a temperature-induced shock to the stored inventory, ultimately resulting in quality degradation. Similarly, rapid changes in other environmental parameters may degrade the quality of the stored commodity. 
   Some systems have sought to utilize multi-speed fans such as is described in U.S. Pat. No. 3,896,359 to Olander et al. Such a system is implemented with the desire of allowing temperature or other environmental changes to take place at a slower rate. However, even these systems do not allow the desired flexibility in controlling a chosen environmental parameter within the storage facility. Such systems employ low- and high-speed control of the fan or air-handling unit. While this allows for a stepped transition from one temperature to another, it simply reduces the magnitude of any temperature spike rather than providing a continuous control of temperature within the storage facility. This is because the high- and low-speed settings each correspond to a defined range of operability. Thus, for example, in controlling temperature, the fans will remain inoperative if the temperature of the facility is within a defined temperature range. The fans will then operate at a low-speed setting once the temperature increases into a second defined range. Finally, the fans will operate at a high-speed setting if the temperature increases into a third defined range. The process will reverse itself as the temperature decreases. However, the ranges cannot be defined too tightly, otherwise the fan will be constantly starting and stopping as the temperature fluctuates between the first and second range. On the other hand, the defined ranges may not be set too broadly. If the ranges are too broad, then the temperature will increase to the point where the fans will be operating at the high-speed setting for extended periods of time in an attempt to bring the temperature back to an acceptable value. Also, depending on the commodity being stored, broad parameter ranges may simply not be acceptable from a quality standpoint. 
   Another important consideration in the environmental control of a storage facility is the efficient use of power. With most systems relying on fans that are cycled between on and off positions, or those systems having high/low-speed settings, power consumption is of paramount concern to the facility operator. Storing commodities for extended periods of time requires a significant consumption of power with existing systems and methods. The cost of such power, while initially resting with the facility operator, ultimately gets passed along to the consumer in the form of higher prices at the market. Thus, an efficient and accurate environmental control system for storage facilities would be of benefit to more than just the facility operator. 
   In view of the shortcomings in the art, it would be advantageous to provide an environmental control system for a storage facility which effectively controls specified environmental parameters while consuming a reduced amount of energy. Such a system or method should be simple to employ in existing as well as new storage facilities. 
   BRIEF SUMMARY OF THE INVENTION 
   In accordance with one aspect of the invention, a method is provided for controlling the internal environment of a storage facility, such as a storage bin for produce. The method includes the steps of providing a fan, or a plurality of fans, for moving the internal air of the storage facility. The fans are continuously operated within the storage facility. The fans may be operated continuously at a speed which is below their full capacity for continuous parameter control and reduced power consumption. The system monitors a parameter indicative of the internal environment of the storage facility. For example, a temperature sensor may be employed to monitor the internal temperature of the storage facility. Once the temperature has been monitored, the speed of the fans may be altered accordingly. For example, if the internal temperature needs to be reduced, then the fans may be operated at a higher rotational speed, increasing the air movement within the storage facility. Likewise, if the air temperature needs to be increased, the fan speeds will again be altered to accomplish this requirement. The same method may be applied in monitoring other parameters and changing the rate of air flow to obtain a desired value for the given parameter. 
   Additionally, environmental parameters outside of the storage facility may be monitored to assist in the regulation of airflow inside the storage facility. For example, outside air temperature may be monitored and compared to the desired facility temperature to determine whether outside air should be admitted into the facility via a ventilation inlet. Various restrictions may be placed on the admittance of outside air, such as prohibiting outside air into the facility if the outside temperature is above a specified maximum or below a specified minimum. 
   In accordance with another aspect of the present invention, a system is provided for controlling the internal environment of a storage facility. The system includes a fan or multiple fans which may be adapted to operate continuously. The fans may be operated continuously at a speed which is below their operational capacity. The fans are placed to move the internal air of the storage facility during operation. Each fan is coupled to a variable speed drive for controlling the operational speed of the fans. At least one sensor is employed to monitor one or more internal environmental parameters of the storage facility such as temperature, humidity, gas levels, or chemical levels. The sensor is coupled to an electronic control unit which is also coupled to the variable speed drive. The sensor provides a signal to the electronic control unit, the signal representing a measured value of an internal environmental parameter. The electronic control unit then provides a signal to the variable speed drive based upon the sensed parameter causing the associated fan to vary in speed accordingly. 
   Additional elements may be configured with the system to render greater control and flexibility. For example, sensors monitoring an external environment may be coupled to the electronic control unit to assist in determining fan speed. Ventilation inlets or outlets may also be coupled to the electronic control unit for controlling flow of air into and out of the facility, respectively. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
       FIG. 1  is a plan view of a storage facility in accordance with certain aspects of the present invention; 
       FIG. 2  is an elevational view of the storage facility of  FIG. 1  taken along the section line  2 — 2 ; 
       FIG. 3  is a plan view of a storage facility according to another aspect of the present invention; 
       FIG. 4  is a schematic representation of an environmental control system in accordance with certain aspects of the present invention; 
       FIG. 5  is a block diagram illustrative of the logic employed in one embodiment of the invention; and 
       FIG. 6  is a block diagram illustrative of the logic implemented according to another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a storage facility  10  implementing an environmental control system according to a particular embodiment of the invention is depicted in plan view. The storage facility includes exterior walls  12  which separate the storage facility from an external environment. A fan  14 , which may be a simple industrial sized fan or any other type of air-handling unit suitable for use in such a facility, is housed at one end of a main air duct  16  or plenum. An interior wall  18  serves as a barrier between the main air duct  16  and a storage area  20 , also referred to herein as the storage bin  20 . A series of secondary or lateral air ducts  22  pass through the interior wall  18  from the main air duct  16  to the storage bin  20 . Each lateral air duct  22  includes a plurality of vents or openings  24  which allow for distribution of air throughout the storage bin  20 . 
   A ventilation inlet  26  is located in an exterior wall  12  near the fan  14 . The ventilation inlet  26  allows for external air to be introduced into the main air duct  16  when desirable. An outside sensor  28  is located external to the facility  10  to monitor a defined environmental parameter. For example, the temperature or humidity of the external air may be monitored to determine the suitability of external air and the desirability of admitting such air. It is contemplated that one or more sensor(s) may be used in such a facility to monitor various external environmental parameters. 
   Generally, airflow is induced by the fan  14  and travels down the main air duct  16  as indicated by directional arrows  30 . Airflow then continues into the lateral air ducts  22  as indicated by directional arrows  32  and into the storage bin  20  through the ventilation openings  24  of the lateral air ducts  22 . The air may then be exhausted through ventilation outlets or returned to the main air duct  16  as more fully described below. The airflow provided by the fan  14  is used to control the internal environment of the storage bin  20 . The circulation of air, including the recirculation of internal air or the introduction of external air and exhausting of internal air, when necessary, can be controlled to manipulate various internal environmental parameters. Such parameters may include, for example, temperature, humidity or CO 2  content of the facility. 
   Referring now to  FIG. 2 , an elevational view of the facility  10  is depicted as indicated by sectional line  2 — 2  of FIG.  1 . The ventilation inlet  26  is shown to be adjusted by an actuator  34 . The ventilation inlet  26  is shown to be a hinged door or hatch actuated by a hydraulic or pneumatic cylinder. The configuration as shown, with the inlet  26  swinging inwardly, allows the inlet  26 , when closed, to permit only the recirculation of air from within the facility, as depicted by directional arrow  40 . When the inlet is open only external air is introduced into the main air duct  16  as indicated by directional arrow  42 . When the inlet  26  is positioned at an intermediate position, a mixture of recirculated air and external air is introduced into the main air duct  16 . While the above-described embodiment is simple and effective for the purpose of introducing external air into the storage facility, it is to be understood that any method of creating and actuating a ventilation inlet known in the art is considered to be within the scope of the disclosed invention. 
   A ventilation opening  36  is formed within the interior wall  18 . Through the ventilation opening  36 , the upper limit of a produce pile  38  may be seen. While not shown in  FIG. 1 , the produce pile is located in the storage bin  20  over the lateral air ducts  22 . In addition to allowing one to view the inside of the storage bin  20 , the ventilation opening  36  also allows air to return from the storage bin  20  and back into the main air duct  16 . Thus, when the ventilation inlet  26  is closed, air is circulated through the main air duct  16  as indicated at  30 , through the lateral air ducts  22  as indicated at  32 , up through the produce pile  38 , and through the ventilation opening  36  back to the main air duct  16  as indicated by directional arrows  40 . Alternatively, adjustable vents/louvers may be placed over the ventilation opening  36  to help control recirculation. 
   When the ventilation inlet  26  is fully opened, external air only is allowed into the main air duct  16  as indicated by directional arrow  42 . When the door is in the midway position, the external air combines with the recirculated air to create a mixed flow. During external air or mixed flow operation, it may be necessary to exhaust some of the air due to a positive pressure experienced in the storage bin  20 . While not shown in either  FIG. 1  or  2 , an exhaust vent may be placed in an exterior wall  12  or in the ceiling of the storage bin  20  to accommodate such exhaust. While various types of vents may be utilized, an exhaust vent with gravity louvers is often sufficient. This type of vent allows air to exhaust to an external environment when a positive pressure is present on the interior of the building, while prohibiting air flow when the interior of the building experiences a negative or equilibrated pressure. The louvers thus open when an internal positive pressure is experienced and close, due to gravity, in the absence of a positive pressure. 
   Additional sensors  44  and  46  are shown in  FIG. 2. A  supply air sensor  44  is located in the main air duct  16  and allows for the monitoring of a chosen parameter of the supply air prior to its introduction into the storage bin  20 . A return air sensor  46  is located near the ventilation opening  36  to similarly monitor the air as it returns from the storage bin  20 . Thus, the air is monitored at various locations to assist in determining whether any adjustments need to be made. Adjustments would typically include changing the rate at which air is circulated and/or adjusting the amount of external air being introduced into the facility  10 . These adjustments, and the logic of making such adjustments, will be discussed in greater detail below. 
   A chosen parameter may be monitored in the main air duct  16  to prevent the introduction of air having a far greater or lesser temperature (or other chosen environmental parameter) than that of the storage bin,  20 . A sudden change in the temperature or other property of the air (e.g., humidity or CO 2  concentration) surrounding the produce may cause the quality of the produce to unacceptably deteriorate. Thus, in conjunction with monitoring the chosen parameter, the speed of fan  14  can be varied until the properties of the air in the main air duct  16  more closely match, or are within a specified range relative to, the properties of the air in storage bin  20 . The ratio of external air to recirculated air may be altered using the ventilation inlet  26  to modify the environment in chamber  16 . Additionally, in some applications, heating or cooling coils, or some other conditions apparatus, may be used to further adjust a selected parameter of the air in the main air duct  16  prior to its introduction into the storage bin  20 . 
   Turning now to  FIG. 3 , a sectional plan view of the storage bin  20  is shown wherein additional components are shown and described. The produce pile  38 , as described previously, sits atop the lateral air ducts  22 . Air flow is directed through the ventilation openings  24  (as shown in  FIG. 1 ) and through the produce pile as generally indicated by directional arrows  48 . As described above, circulation of the air typically causes the air to return to the main air duct  16  for recirculation. However, in certain circumstances, it may be desirable to create an exchange of air by exhausting air at a more rapid pace. Such a technique would be desirable, for example, to remove air having a higher content of CO 2  than is desired. 
   An auxiliary fan  50  is placed at the upper end of the storage bin  20  near an exhaust vent  52  such as a louvered gravity vent. An auxiliary ventilation inlet  54  is located in an exterior wall  12  opposite the fan  50  and exhaust vent  52 . The ventilation inlet  54  may be operated by an actuator  56 , as shown, or by other suitable means, such as, for example, gravity louvers. When in operation, the auxiliary fan  50  draws external air through the ventilation inlet  54 , across the storage bin  20 , and out through the exhaust vent  52  as indicated by directional arrows  58 . A sensor  60  is located in the storage bin to monitor a desired parameter, such as the CO 2 . It is understood that the actual physical location of the fan  50  and associated vents  52  and  54 , while typically located toward the vertical extremes of the facility, will depend on the actual layout of the storage facility in which they are employed and may be arranged in various configurations to accomplish the same or similar results. 
   An auxiliary system, such as that depicted in  FIG. 3 , assists in maintaining various internal environmental parameters when control of the main system is limited by the external environmental parameters, for example, during an extended period of time the external temperature (as measured by sensor  62 ) may be either too warm or too cold to open the main inlet door  26  for the introduction of fresh air. In such a case, it is still desirable to control the oxygen, carbon dioxide or other gas levels within the storage bin  20 . The auxiliary system shown in  FIG. 3  may be utilized to introduce just enough external air to control the gas level without unduly influencing other internal environmental parameters such as temperature or humidity. The auxiliary fan  50  and ventilation inlet  54  may be controlled simultaneously to introduce the proper amount of external air in such a situation. 
   Alternatively, fan  50  may be used as the primary control of the environment within the storage facility. Fan  14  may operate at a constant speed to maintain air circulation within the storage facility. Additionally, inlet  26  ( FIGS. 1 and 2 ) need not be present, making ventilation inlet  54  ( FIG. 3 ) the primary or sole source of external air. The speed of the fan  50  and/or the amount of external air being introduced into the storage bin  20  through ventilation inlet  54  may be adjusted to control the environment. In such a configuration, fan  14  ( FIGS. 1 and 2 ) may be used to simply circulate the air from the storage bin  20  throughout the produce pile  38 . Such a system enables control of parameters such as temperature, humidity, and carbon dioxide concentration. Determination of the needed adjustments may include consideration of external environmental parameters measured by sensor  62  and/or environmental parameters within the storage bin measured, for example, by sensor  60 . 
   Referring now to  FIG. 4 , a schematic of the environmental control system  100  of the present technique is depicted. A first fan  102  is shown which may be taken to represent the main fan  14  located in the main air duct  16 . A second fan  104  is also shown, and may be taken to represent the auxiliary fan  50  shown in FIG.  3 . Each fan  102  and  104  is connected to a variable-speed drive  106  and  108 , respectively. There are numerous types of variable-speed drives commercially available, each having various benefits and features. It is contemplated that the present system and method may be practiced utilizing different types of variable-speed drives for varying the rotational speed of the fans  102  and  104 . For example, a variable-speed drive of the type employing a magnetic clutch would be suitable for use in the present technique. Such a drive varies the current supplied to the clutch causing the magnetic force to vary between the clutch and the shaft. This allows a certain amount of slip to occur between the shaft and the clutch. Ultimately, the rotational speed is varied by varying the amount of slip allowed in the magnetic clutch. While such a drive, and numerous others, may be suitable for use in practicing the present technique, the drives utilized in the presently disclosed embodiment are variable-frequency drives, sometimes referred to as inverter drives. 
   As known by those skilled in the art, a variable-frequency drive (VFD) is an electronic controller that adjusts the speed of an electric motor by modulating the power being delivered. More specifically, the speed of the electric motor is controlled by modulating the frequency of the power being supplied. The standard frequency of AC power in the United States is 60 Hz. A standard electric motor constructed for use in the United States is designed to be operated with a 60 Hz power supply. A decrease in the frequency of the power supply will result in a corresponding decrease in motor speed. For example, an electric motor that rotates at 100 rpm with a 60 Hz power supply would run at 50 rpm when the power supply is reduced to 30 Hz. 
   Referring still to  FIG. 4 , a set of actuators  110  and  112  represent the actuators  34  and  56  depicted in  FIGS. 2 and 3 , respectively. A plurality of sensors  114 ,  116 ,  118  and  120  are also shown and represent the various sensors disclosed and discussed above. Each of the VFDs  106  and  108 , the actuators  110  and  112 , and the sensors  114 ,  116 ,  118  and  120  are connected to a control unit  122  by way of electrical wiring  124  such as a dedicated harness. Alternatively, the electrical wiring could be a common bus such as in a controller area network. The control unit  122  receives signals from the various sensors  114 ,  116 ,  118 , and  120 , processes the information it receives, and then sends out command signals to the VFDs  106  and  108  and the actuators  110  and  112 . The VFDs  106  and  108  then interpret the command signals and send corresponding drive signals to the fans  102  and  104 , respectively. In the above described embodiment, a drive signal includes a signal from a power supply having an appropriately modulated frequency. 
   Through proper programming of either control unit  122 , VFDs  106  and  108 , or both, maximum speed settings may be established for the fans  102  and  104 . Likewise, minimum speed settings may be set. Furthermore, parameter set points may be established for the overall operation and logic of the system. For example, a temperature value at which the storage bin is to be maintained may be defined. Having a defined temperature value and sensing air temperature at various points in the stream of air flow, the system will operate to adjust fan speed and/or adjust the mix of air flow to alter an existing environmental parameter. The logic of controlling the environment with such a system will be discussed in greater detail below. 
   It is noted that with such a system, greater flexibility is realized through the use of variable-speed drives. By using VFDs or some other variable-speed drive, more gradual changes to the environment may be achieved. The possibility of reduced power consumption is also seen in the practice of the present technique. This is because the relationship between power consumption and fan speed is nonlinear. For example, it has been established that in a system similar to that described herein, a twenty percent reduction in fan speed results in a fifty percent reduction in power consumption. Knowing that the rate of air flow varies linearly with fan speed, a simple calculation may be performed to compare air flow and power consumption for a system operating at full speed with a system operating at a reduced fan speed of eighty percent. A system operating at full power may circulate air, for example, at 100,000 cfm (cubic feet per minute). This system will circulate 6,000,000 cubic feet of air in a given hour. The reduced-speed system, however, will circulate air at a rate of 80,000 cfm requiring an hour and fifteen minutes to circulate 6,000,000 cubic feet of air. However, even with the additional fifteen minutes of operating time, the reduced-speed system only consumes sixty-two and a half percent as much power as the full-speed system. Indeed, operating the fan at even slower speeds nets even larger savings in power. 
   With reduced-fan speed consuming considerably less energy than does full-speed operation, a fan can be operated continuously to maintain the storage facility environment within a tightly defined parameter range. For example, if the storage facility is desired to be maintained at a temperature of 50° F., the fans can be operated continuously at a reduced speed to maintain the temperature within a few degrees. Furthermore, with proper fan speed control, in conjunction with proper inlet ventilation control, temperature can be maintained within a range of approximately 1° F. Thus, large temperature spikes may be eliminated from the storage environment with reduced power consumption. 
   It is noted that while the schematic of  FIG. 4  shows a single control unit  122 , it is possible that multiple controllers be employed in operation of the system  100 . For example, the overall system  100  could be subdivided into subsystems wherein the main fan  102  and drive  106  were considered an individual subsystem. Similarly, the control of the auxiliary fan  104 , drive  108  and auxiliary actuator may be taken together as a subsystem. Indeed, a subsystem may simply include a controlling actuator-associated main ventilation inlet. 
   Turning now to  FIG. 5 , and with reference to  FIG. 4 , the logic employed according to one aspect of the present technique is discussed. First, a parameter set point  142  is defined. The parameter set point is the value at which the storage facility environment should be maintained. For example, it may be a value concerning temperature, humidity, CO 2  or some other environmental parameter. For sake of clarity, and not by way of limitation, the use of temperature will be maintained as the specific environmental parameter throughout the following discussion. 
   Maximum and minimum fan speeds are defined, as shown at step  144 , and are programmed into either the control unit  122  or the VFD  106  (illustrated in FIG.  4 ). Alternatively, maximum and minimum power consumption rates may be defined for the fans. An environmental parameter is then sensed  146  and an appropriate data signal is communicated to the control unit  122 . The control unit  122  then determines if the sensed temperature is greater than the defined set point as indicated at  148 . If the result is affirmative, then the control unit  122  determines whether the current fan speed is less than the defined maximum as shown at step  150 . If this inquiry is affirmative, then the control unit  122  will increase the speed of the fan  102  as indicated at step  152 . Following the increase of fan speed, the temperature is again sensed as shown at step  146 , with the process ready to repeat itself. If the inquiry at step  154  is answered negatively, then the fan speed is maintained at the maximum speed and the process returns to step  146 . 
   If, however, the inquiry at  148  yields a negative response, the control unit  122  then will inquire whether the sensed temperature is less than the defined set point as shown at  156 . If the result is affirmative, a second inquiry is made as to whether the fan speed is greater than the minimum setting as indicated at step  158 . If the result to this inquiry is affirmative, then the fan speed is reduced as shown at  160 , and the process returns to step  146 . If the inquiry at step  158  yields a negative response, then the fan speed is maintained at the minimum speed as shown at  162 , and the process returns to step  146 . Finally, if the inquiry at step  156  yields a negative result, the process likewise returns to step  146 . 
   Thus, using the logic described above, the fan is operated continuously and, if the maximum setting is less than full power, it is operated continuously at a reduced speed. In the example above, the present technique allows for the continuous control of fan speed to maintain the storage facility environment at a defined temperature. It is noted that the chosen parameter need not be temperature. It is also noted that the above logic is in reference solely to fan speed and that the control unit may contemporaneously control the ventilation inlet  26  (shown in  FIGS. 1 and 2 ) to influence the environment as well. Additionally, it is noted that, in certain circumstances, fan speed may be decreased if the sensed parameter is greater than the defined set point and increased if the sensed parameter is less than the defined set point. The fan speed may also be increased or decreased in response to the deviation of the sensed parameter from the set point or the difference between two sensed parameters. 
   Turning now to FIG.  6  and referring to  FIG. 4 , the operational logic regarding the operation of the auxiliary system of  FIG. 3  is described. First, parameter set points are defined as shown at step  172 . Both an internal set point and an external set point are defined. The internal set point is a parameter value at which the storage facility environment should be maintained. For example, it may be a value concerning temperature, humidity, CO 2  or some other environmental parameter. For sake of clarity, the following example will focus on the control of CO 2  as the internal parameter to be maintained. The external set point is a parameter value which is used to override the system in specific instances. For this discussion, the external set point is defined in terms of temperature. 
   While not shown specifically in  FIG. 6 , maximum and minimum fan speeds may be defined according to the description in reference to FIG.  5 . An internal environmental parameter is then sensed as shown at step  174 , and an appropriate data signal is communicated to the control unit  122 . Again, for this discussion the sensed internal parameter will be the CO 2  level in the storage facility. An external parameter is also sensed as shown at  176 . For this discussion, the external parameter will be the ambient temperature outside the storage facility. The control unit  122  then determines if the sensed CO 2  is less than the defined set point as indicated at  178 . If the result is affirmative, then the control unit  122  will decrease the speed of the auxiliary fan  104  as indicated at  180 . Following the decrease in fan speed, the process returns to step  174 . If the inquiry at step  178  is answered negatively, then the control unit  122  determines whether the sensed CO 2  level is greater than the defined level as indicated at  182 . If the result is negative, then the speed is maintained as shown at  184 , and the process returns to step  176 . If, however, the result is affirmative, the control unit  122  further determines if the external temperature is less than the external set point as seen at step  186 . If the result to the inquiry at  186  is affirmative, then the fan speed is increased as shown at step  188  and the process returns to step  176 . If the result to the inquiry at  186  is negative, the control unit  122  determines whether the sensed external temperature is greater than the external set point as shown at step  190 . Again, if the result to this inquiry is negative, then the fan speed is maintained as shown at step  184 , and the process returns to step  176 . If, however, the result to the inquiry at step  190  is affirmative, then the fan speed is decreased as shown at step  192  and the process returns to step  176 . 
   Thus, the inquiries shown at steps  186  and  190  work as a check on the external environment. This allows an override function to be in place such that the admittance of external air by the auxiliary system does not interfere with the maintenance of one or more other environmental values. For example, if the main fan  102  is being utilized to control temperature and the auxiliary fan  104  is being utilized to control CO 2 , the use of external air to sweep out CO 2  may impair the system&#39;s ability to control temperature, depending on the temperature of the external air. Thus, the main fan  102  is given priority in the example above, such that control of temperature overrides the control of CO 2 . Of course the main and auxiliary systems could each control parameters different than those attributed in the above example with similar logic employed and similar results achieved. 
   It should be understood that while the logic discussed in connection with  FIGS. 5 and 6  related to a particular system, the logic may be applied to the other systems or subsystems disclosed herein. For example, the logic of  FIG. 5  may be easily adapted for use with the auxiliary system if so desired. Similarly, the logic discussed in connection with  FIG. 6  may equally be applied to operation of the main fan or possibly the control of the main ventilation inlet. It is noted that, as disclosed previously herein, the auxiliary fan may operate as the primary control of the internal environment if so desired. 
   While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims. For example, it is contemplated that while the embodiments and techniques described above have been shown to be combined into a single system, they may operate as individual systems or as subsystems. For example, what has been described as the auxiliary system, i.e.,  FIG. 3 , need not be connected to the same control unit as the systems described in  FIGS. 1 and 2 . As noted above, multiple controllers may be employed to operate the system in a similar manner. 
   It is further contemplated that a single control unit may interact with individual components of the system on an independent basis. For example,  FIG. 4  illustrates a system with a single control unit  122  networked with multiple components. Such a control unit  122  may be configured to receive information or data from a first sensor  114  and use that information to control the speed of the first fan  102 . The control unit  122  may then receive a signal from a second sensor  116  for use in controlling the second fan  104 . However, the first fan  102  may be operated at a speed independent of the speed of the second fan  104 . Likewise, contemporaneous and independent control may be exerted over the ventilation inlets. 
   Of course, additional components may be introduced into the system for added control and benefit. Such components may include, by way of example, heating equipment, cooling equipment, humidifiers, dehumidifiers, actuated exhaust controls, or fogging equipment for the introduction of desired chemicals into the environment.