Patent Publication Number: US-2015068235-A1

Title: Auxiliary ambient air refrigeration system for cooling and controlling humidity in an enclosure

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/199,744 filed Sep. 7, 2011 which hereby claims the benefit of and priority thereto under 35 U.S.C. §§119, 120, 363, 365, and 37 C.F.R. §1.55 and §1.78, which is incorporated herein by this reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an auxiliary ambient air refrigeration system for cooling and controlling humidity of air in an enclosure. 
     BACKGROUND OF THE INVENTION 
     There have been a number of auxiliary ambient (outside) air refrigeration systems proposed. Some of these systems, such as those described in U.S. Pat. Nos. 4,175,401 and 4,023,947, employ a control system having a “changeover” thermostat that senses the outside temperature and de-energizes the conventional refrigeration system and energizes an outside air refrigeration system whenever the outside temperature falls below a predetermined temperature. Another control strategy for outside air systems is to have no electrical interconnection between the conventional refrigeration system and the outside air system. This type of “independent” system is found in U.S. Pat. Nos. 4,250,716, 4,178,770, 4,147,038, 4,619,114, 4,244,193, and 4,358,934. The operation of each of these outside air systems is controlled by two thermostats, one sensing the outside temperature and one sensing the temperature inside the enclosure. The thermostat controlling the operation of the conventional refrigeration system is set at a higher operating range than the thermostat sensing the enclosure temperature for the outside air system. The conventional refrigeration system does not operate as long as the outside air system can adequately cool the enclosure. The outside air thermostat is set at a predetermined cut-in temperature such that the outside air system will only be used when the outside air is cold enough to always be at least as efficient as the conventional refrigeration system. A differential thermostat senses the temperature of both the air inside the enclosure and the outside or ambient air, compares them, and actuates at least one fan or blower to circulate cool outside air so as to cool the inside of the enclosure. As long as the outside or ambient temperature is at least a pre-selected number of degrees cooler than the temperature inside the enclosure and this inside temperature is above a pre-selected cut-in temperature for the outside air refrigeration system, the outside air fan, or fans, circulate cool outside air until the temperature inside the enclosure falls to a pre-selected cut-out setting for the outside air system or until the enclosure temperature is cooler than a pre-selected number of degrees warmer than the outside air temperature, at which time the outside air fan, or fans turn off. See U.S. Pat. No. 5,239,834. However, there remains yet another issue: humidity. Too little can affect the quality of the contents, especially food products, for example. Too much humidity can also be a problem for such goods as well as other types of goods and may increase the cooling load, resulting in lower efficiency and can damage equipment. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide an improved, auxiliary ambient air refrigeration system for cooling and controlling humidity in an enclosure. 
     It is a further object of this invention to provide such an improved auxiliary ambient air refrigeration system which prevents the refrigerated air from being too dry or too humid while preserving and improving the efficiency of the cooling burden. 
     The invention results from the realization that an improved auxiliary ambient air refrigeration system for cooling and controlling humidity in an enclosure which prevents too much or too little humidity and improves efficiency can be achieved using a controller responsive to an inside the enclosure sensor unit and an ambient air sensor unit and their indicated dew points for enabling an auxiliary refrigeration unit to provide cool ambient air to the enclosure when the temperature inside the enclosure is above a first predetermined temperature, the ambient temperature is less than the enclosure temperature by a predetermined differential temperature and the dew point of the ambient air is matched to the dew point range of the air in the enclosure. 
     The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
     This invention features an auxiliary ambient air refrigeration system for cooling and controlling humidity in an enclosure including a conventional refrigeration unit for providing cool refrigerated air to the enclosure, an auxiliary refrigeration unit for providing cool ambient air to the enclosure, a first sensor unit for sensing air temperature and humidity inside the enclosure, a second sensor unit for sensing ambient air temperature and humidity; and a controller responsive to the sensor units and their indicated dew points of the enclosure air and the ambient air for enabling the auxiliary refrigeration unit to provide cool ambient air to the enclosure when temperature inside the enclosure is above a first predetermined temperature, the ambient temperature is less than the enclosure temperature by a predetermined differential temperature and the dew point of the ambient air is matched to the dew point range of the air in the enclosure. 
     In a preferred embodiment the controller may be responsive to the sensor units for enabling the conventional refrigeration unit when the enclosure temperature is at a second predetermined temperature higher than the first predetermined temperature. The dew point range may include a minimum humidity dew point and the auxiliary refrigeration unit may not be enabled if the dew point of the enclosure air is at or below that minimum dew point and if the dew point of the ambient air is below the dew point of the enclosure air. The dew point range may include a maximum humidity dew point and the auxiliary refrigeration unit may not be enabled if the dew point of the enclosure air is at or above that maximum dew point and if the dew point of the ambient air is above the dew point of the enclosure air. There may be further included a humidifier and the controller may be responsive to a humidity below the minimum humidity dew point to enable the humidifier. The sensor unit may include a temperature sensor and a dew point sensor. The dew point range may include a minimum humidity dew point and the auxiliary refrigeration unit may not be enabled if the dew point of the ambient air is below that minimum dew point. The dew point range may include a maximum humidity dew point and the auxiliary refrigeration unit may not be enabled if the dew point of the ambient air is above that maximum dew point. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
         FIG. 1  is a simplified diagram of an embodiment of the auxiliary ambient air refrigeration system of this invention; 
         FIG. 2  is a more detailed partial, cross sectional view with portions removed of an enclosure served by one embodiment of the auxiliary ambient air refrigeration system of this invention; 
         FIG. 3  is a schematic electrical diagram of the system of  FIG. 2 ; 
         FIG. 4  is a logic block diagram illustrating humidity control using ambient air dew point control for minimum humidity criteria; 
         FIG. 5  is a logic block diagram illustrating humidity control using ambient air dew point control for maximum humidity criteria; and 
         FIG. 6  is a logic block diagram illustrating an alternative approach for humidity control combining maximum and minimum humidity logic. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
     There is shown in  FIG. 1  one embodiment of an auxiliary ambient air refrigeration system  10  according to this invention for cooling and controlling humidity in an enclosure  12 . The auxiliary ambient air refrigeration system  10  includes a conventional refrigerator unit  14  and an auxiliary ambient air refrigerator unit  16 . There is an inside temperature/humidity (dew point) sensor unit  18  inside enclosure  12  and an outside or ambient air temperature/humidity (dew point) sensor unit  20  outside of enclosure  12 . Controller  22  is responsive to both inside and ambient sensor units  18  and  20  to control the operation of conventional refrigerator unit  14  and auxiliary ambient air refrigerator unit  16 . There may also be a humidifier  24  that may be controlled by controller  22  to keep the humidity within enclosure  12  within a desired range. Controller  22  responds to the sensor units and the indicated dew points of the enclosure and the ambient air and enables the auxiliary refrigerator unit to provide cool ambient air to the enclosure when the temperature inside the enclosure is above a first predetermined temperature, the ambient temperature is less than the temperature inside the enclosure by a predetermined differential temperature and the dew point of the ambient air is matched to the dew point range of the air in the enclosure. Controller  22  also responds to the sensor units  18  and  20  to enable the conventional refrigeration unit when the temperature is at above a second predetermined temperature that is higher than the first predetermined temperature. The dew point range may include a minimum humidity dew point for the enclosure, in which case the auxiliary refrigeration unit will not be enabled if the dew point of the air inside the enclosure is below that minimum dew point and the dew point of the ambient air is not higher than the dew point of the air inside the enclosure. The dew point range may include a maximum humidity dew point and the auxiliary refrigeration unit in that case will not be enabled if the dew point of the air inside the enclosure is above that maximum dew point and the dew point of the ambient air is not lower than the dew point of the air inside the enclosure. Sensor units  18  and  20  may include a temperature sensor and a humidity sensor from which controller  22  calculates the dew point. Or the temperature humidity sensors  18  and  20  may actually include a dew point meter to directly provide the dew point to controller  22 . 
     One source defines a dew point as the temperature to which a given parcel of humid air must be cooled at a constant barometric pressure for water vapor to condense into water. The dew point is actually a saturation temperature. Dew point is closely associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates that the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases relative humidity will decrease. Dew point meters are used to measure dew point over a wide range of temperatures. One version of such meters can consist of a polished metal mirror which is cooled as air is passed over it. The temperature is monitored and the temperature at which the dew forms is by definition the dew point. In fact, these devices are often used to calibrate other types of humidity sensors. If controller  22  receives a dew point reading directly from sensor units  18  and  22  it is unnecessary to calculate the dew point. But if controller  22  receives temperature and relative humidity it may calculate the dew point according to a well known approximation which calculates the dew point T d  given the relative humidity RH and the actual temperature T of the air. That approximation is as follows: 
     
       
         
           
             
               T 
               d 
             
             = 
             
               
                 b 
                  
                 
                     
                 
                  
                 
                   γ 
                    
                   
                     ( 
                     
                       T 
                       , 
                       RH 
                     
                     ) 
                   
                 
               
               
                 a 
                 - 
                 
                   γ 
                    
                   
                     ( 
                     
                       T 
                       , 
                       RH 
                     
                     ) 
                   
                 
               
             
           
         
       
       
         
           where 
         
       
       
         
           
             
               γ 
                
               
                 ( 
                 
                   T 
                   , 
                   RH 
                 
                 ) 
               
             
             = 
             
               
                 aT 
                 
                   b 
                   + 
                   T 
                 
               
               + 
               
                 ln 
                  
                 
                   ( 
                   
                     RH 
                     / 
                     100 
                   
                   ) 
                 
               
             
           
         
       
     
     where the temperatures are in degrees Celsius and “ln” refers to the natural logarithm. The constants are: 
       a=17.271 
       b=237.7° C.
 
     There is also a very simple approximation that allows conversion between the dew point, the dry-bulb temperature and the relative humidity. This approach will be accurate to within about ±1° C. as long as the relative humidity is above 50%. The equation is: 
     
       
         
           
             
               T 
               d 
             
             = 
             
               T 
               - 
               
                 
                   100 
                   - 
                   RH 
                 
                 5 
               
             
           
         
       
       
         
           or 
         
       
       
         
           
             RH 
             = 
             
               100 
               - 
               
                 5 
                  
                 
                   
                     ( 
                     
                       T 
                       - 
                       
                         T 
                         d 
                       
                     
                     ) 
                   
                   . 
                 
               
             
           
         
       
     
     This can be expressed as a simple rule of thumb: For every 1° C. difference in the dew point and dry bulb temperatures, the relative humidity decreases by 5%, starting with RH=100% when the dew point equals the dry bulb temperature and where in this case RH is in percent, and T and T d  are in degrees Celsius. 
     Controller  22  may be a hard wired apparatus as shown in one specific embodiment in  FIGS. 2 and 3  or it may be a properly configured microprocessor such as a Freeaire® Cooler Controller™ model 2100. 
     In one mode of operation there may be set a maximum allowable relative humidity within an enclosure  12  (MAX humidity) and a minimum allowable relative humidity (MIN humidity). Controller  22  monitors and compares the temperature and the relative humidity or dew point of both the enclosure (inside) and the ambient (outside) air at all times. The outside or ambient air system operates based on temperature until either the MAX or MIN humidity is reached. After that ambient air is only brought into the space if the relative humidity of the outside air is such that it would improve or match the relative humidity in the enclosure once it warms to the temperature inside the enclosure  12 . Controller  22  thus either calculates the dew point or is delivered a sensed dew point. If the inside space is already too dry, the controller  22  would not bring in the ambient air if the ambient air dew point is lower that the enclosure air dew point. If the air inside enclosure  12  is already too humid, controller  22  would not bring in outside air if its dew point is higher than the dew point of the enclosure air. Generally, if there is a range of acceptable humidity with a minimum humidity for a given enclosure temperature and the enclosure air is already too dry then no ambient air with a dew point temperature lower than that of the enclosure air can be used even if it is at 100% relative humidity. Contrastingly, if there is a maximum acceptable humidity for a given enclosure temperature and the enclosure air is already too humid then all ambient air with a dew point temperature lower than the enclosure air can be used if it is cold enough and even if it is at 100% relative humidity. To protect the minimum humidity range when the ambient air dew point is lower than the minimum humidity, controller  22  may activate a humidifier  24  to add moisture to the space or the humidifier could be provided with its own controls so the two systems are completely separate. With the humidifier  24  the outside air could be used even if it contained low humidity. 
     Referring to  FIG. 2 , there is shown an insulated refrigerated enclosure  101  with an outside wall  102  which separates the enclosure  101  from the outside atmosphere, and an inside wall  103  that separates the enclosure  101  from a mechanical room  104 . The present invention is not limited to the specific conditions herein described; there are many different situations in which the present invention would work well, including the case in which the enclosure is separated from the ambient or outside atmosphere by another room and the case in which the mechanical “room” is in the outside atmosphere. What is herein described is a typical situation in which a refrigerated enclosure such as a walk-in cooler or storage room is located in a building such as a grocery store or restaurant and is in a climate where the outside air temperature is cold enough to be used for refrigeration for a significant portion of the year. 
     In  FIG. 2  there is also shown a conventional refrigeration system including an evaporator  105 , with three identical evaporator fans  106  and evaporator coils (not shown), a refrigerant liquid line  108 , a liquid line solenoid valve  109 , an expansion valve  110 , and a refrigerant suction line  111  inside the enclosure  101 , and a compressor  112 , a condenser  113 , a condenser fan  114 , and a low pressure control  115  inside the mechanical room  104 . The conventional refrigeration system also includes the addition of a circulating fan  116  which is attached to the inside wall  103  by bracket  117 . 
     The auxiliary outside or ambient air refrigeration system includes an intake damper housing  118  that has a pair of insulated dampers  120 , a gasket  121 , and a damper closure spring (not shown), mounted on the inside surface of the outside wall  102 , in line with a first airflow passage  123  through the outside wall  102 . On the outside surface of the outside wall  102 , in line with the airflow passage  123 , is mounted the outside air fan  124 , which is contained in an outside air fan housing  125 . The outside air fan housing  125  also houses a filter  126 , which is removable by sliding the filter  126  along the filter track  127 . Elsewhere on the outside surface of the outside wall  102 , in line with a second airflow passage  130 , is an enclosure air fan  128 , that has a finger guard (not shown) mounted on its face. In line with the second airflow passage  130 , on the outside surface of the outside wall  102  is an outside wallcap  131  that surrounds the enclosure fan  128 , that is mounted to the end of an exhaust damper housing  132  extending through the wall  102  to its inside face to which it is mounted and which ends with a pair of insulated outward-opening dampers  133 , a gasket  134 , and a damper closure spring  135 . 
     The controller  136  is mounted on the inside surface of the outside wall  102  and is connected to a source of power through four electrical conductors,  137 ,  138 ,  139 , and  140 . The controller  136  is also connected electrically to the outside air fan  124  by an electrical conductor  141 , to the enclosure air fan  128  by the electrical conductor  142 , to the liquid line solenoid valve  109  by the electrical conductor  143 , to the evaporator fans  106  by the electrical conductor  144 , and to the circulating fan  116  by electrical conductor  145 . Also, the controller  136  is electronically connected to an inside temperature and dew point sensor unit  146  mounted on the inside surface of the wall  102  near the controller  136 , by a low voltage conductor  147 , and to an outside ambient temperature and dew point sensor unit  148 , mounted on the outside surface of the outside wall  102 , by a low voltage conductor  149  which passes through a hole  150  in the outside wall  102 . 
     In  FIG. 3 , there is shown a schematic wiring diagram of the auxiliary ambient air refrigeration system in combination with the conventional refrigeration system. Components of the conventional refrigeration system include the compressor  112  and the condenser fan  114  both of which are in series with the low pressure control  115 . The controller  136  is powered by electricity through electrical conductor  137  and is controlled by an on/off switch  151 . A “power on” light  152  is in series with the switch  151 . Also in series with the switch  151  is a circuit connecting a differential thermostat  153 , an inside thermostat  154  for the ambient air refrigeration system, and the coil  157  of an ambient air refrigeration system relay  156 . The circuit made by the electrical conductors  138  and  141  and the outside air fan  124  and the circuit made by the electrical conductors  138  and  142  and the enclosure air fan  128  are both controlled by the normally open contacts  158  of the relay  156 . Another component in series with the switch  151 , and in parallel to the outside air refrigeration system control circuit, is the inside thermostat  155  for the conventional refrigeration system. The inside temperature and dew point sensor unit  146  supplies the temperature information about the air temperature inside the enclosure to the inside thermostat  155  for the conventional refrigeration system as well as for the differential thermostat  153  and the inside thermostat  154  for the outside air refrigeration system as well as dew point information. The outside or ambient temperature and dew point sensor unit  148  supplies temperature information to the differential thermostat  153  as well as dew point information. The coil  160  of the conventional refrigeration system relay  159  and the coil  163  of the time-delay relay  162  are in series with the thermostat  155  and switch  151 , but are in parallel with each other. The circuit made by electrical conductors  139  and  143  and the liquid line solenoid valve  109  is controlled by the normally open contacts  161  of the relay  159 . The circuit made by electrical conductors  140  and  144  and the evaporator fans  166  is controlled by the normally open contacts  164  of the time-delay relay  162 . The circuit made by the electrical conductors  140  and  145  and the circulating fan  116  is controlled by the normally closed contacts  165  of the time-delay relay  162 . 
     The components of the conventional refrigeration system are arranged so as to extract heat from the enclosure  101  and transfer it to the mechanical room  104 . The On/off switch  151  must be in the “on” (closed) position. The inside thermostat  155  in the controller  136  replaces the thermostat which would normally control the operation of the conventional refrigeration system. When the inside temperature and dew point sensor unit  146  senses that the temperature of the air is at or above the predetermined cut-in temperature setting for the conventional refrigeration system (typically 38 degrees F.), the inside thermostat  155  closes, energizing the coil  160  of the relay  159  which closes the normally open contacts  161  making an electrical circuit through the electrical conductor  139  and  143  which energizes the liquid line solenoid valve  109 . This allows liquid refrigerant to move through the refrigerant liquid line  108  and the expansion valve  110  to enter the evaporator coils and evaporate. The evaporation of the refrigerant inside the evaporator coils extracts heat from the enclosure air flowing past the evaporator coils as a result of the operation of the evaporator fans  106 . 
     The process continues until the enclosure  101  is sufficiently cooled that the inside temperature and dew point sensor unit  146  senses that the air temperature has dropped to the predetermined temperature representing the cut-out temperature setting for the conventional refrigeration system (typically 36 degrees F.) This, in turn, causes the inside thermostat  155  to open, which de-energizes the coil  160  of the conventional refrigeration system relay  159 , which causes the normally open contacts  161  to open, which de-energizes the liquid line solenoid valve  109 , causing it to close. As the compressor  112  continues to operate the evaporated refrigerant is pumped out of the refrigerant suction line  111 , which causes the pressure in it to drop until it reaches a predetermined pressure representing the cutout pressure setting for the compressor. This causes the low-pressure control  115  to de-energize the compressor  112  and condenser fan  114 . 
     When the inside sensor  46  senses the temperature inside the enclosure  101  has risen to the predetermined cut-in temperature setting for the conventional refrigeration system (typically 38 degrees F.), causing the inside thermostat  155  to close, the coil  163  of the time-delay relay  162  is energized. This causes the normally open contacts  164  to close, thereby energizing the evaporator fans  106 , and the normally closed contacts  165  to open, thereby de-energizing the circulating fan  116 . When the enclosure temperature drops to the predetermined cut-out temperature setting of the conventional refrigeration system (typically 36 degrees F.), the inside thermostat  155  opens, the coil  163  of the time-delay relay  162  is de-energized. After a predetermined delay, the normally open contacts  164  open, de-energizing the evaporator fans  106 , and the normally closed contacts  165  close, energizing the circulating fan  116 . The predetermined delay in the operation of the contacts  164  and  165  of the time-delay relay  162  is user-adjustable to allow for shortening the period of time the evaporator fans  106  operate and extending the period of time the circulating fans  116  operate in order to reduce energy use, and for extending the period of time the evaporator fans  106  operate in order to allow more heat transfer to the evaporator coils. 
     The ambient air refrigeration cycle begins,  FIGS. 2 and 3 , when the outside sensor unit  48  senses that the temperature of the outside atmospheric air is cooler than a pre-selected number of degrees cooler than the temperature of the air inside the enclosure  101 , sensed by the inside temperature and dew point sensor unit  146 , which represents the cut-in temperature differential for the outside air refrigeration system (typically 6 degrees F.). This causes the differential thermostat  153  to close. When the inside temperature and dew point sensor unit  146  also senses that the temperature inside the enclosure  101  is at or above the cut-in temperature setting for the outside air refrigeration system (typically 36 degrees F.), this causes the inside thermostat  54  for the outside air refrigeration system to also close. Since both the thermostats  153  and  154  and the switch  151  are in series, when they are all in a closed position they cause the coil  157  of the outside refrigeration system relay  156  to be energized. This, in turn, causes the normally open contacts  158  to close, which energizes the outside air fan  124  (through electrical conductors  138  and  141 ) and the enclosure air fan  128  (through electrical conductors  138  and  142 ). 
     When the outside air fan  124  is energized it draws outside atmospheric air through the filter  126  into the outside air fan housing  125 . The air is then forced through the first airflow passage  123  and the inside wall-cap base  119  where the force exerted by the incoming air overcomes the force exerted by the damper closure spring  122  and opens the damper  120  allowing the outside air to pass through the inside wallcap  118  and enter the enclosure  101 . When the enclosure air fan  128  is energized, it draws air from the enclosure  101 , through the finger guard  129  and forces the air into the second airflow passage  130  and through the outside wall cap base  132  where the force exerted by the air overcomes the force exerted by the damper closure spring  135  and opens the damper  133  allowing the enclosure air to flow through the outside wallcap  131  into the outside atmosphere. 
     The simultaneous operation of the two fans  124  and  125  results in a gradual lowering of the air temperature within the enclosure. When the inside temperature and dew point sensor unit  146  senses that the air temperature within the enclosure has reached the predetermined cut-out temperature setting for the outside air refrigeration system (typically 34 degrees F.), the inside thermostat  154  opens, which de-energizes the coil  157  of the relay  156 , which opens the normally open contacts  158 , which, in turn, de-energizes the fans  124  and  128 , stopping the flow of outside air into the enclosure  101 . The operation of the two fans  124  and  128  is also stopped when the outside temperature and dew point sensor unit  148  senses that the outside temperature has risen (or the inside temperature has dropped) so as to make the outside temperature warmer than a predetermined number of degrees cooler than the inside temperature, as sensed by the inside temperature and dew point sensor unit  146 , which represents the cut-out temperature differential setting for the outside air refrigeration system (typically 4 degrees F.), which causes the differential thermostat  153  to open, de-energizing the coil  158  of the relay  156 , causing the contacts  158  to open and thereby de-energizing the fans  124  and  128 . 
     The cut-out temperature differential setting for the outside air refrigeration system is selected so as to cause the operation of the fans  124  and  128  when the amount of cooling provided by those fans is greater than the amount of cooling provided by the conventional refrigeration system while consuming an equal amount of electrical energy. The “breakeven point” at which the outside air refrigeration system is equally as energy efficient as the conventional refrigeration system is typically reached when the outside air temperature is about 4 degrees F. cooler than the temperature inside the enclosure. Therefore, a differential of about 4 degrees is the smallest differential that should be used in order to minimize the use of energy. 
     The cut-in temperature differential is selected so as to maximize the operation of the outside air refrigeration system without causing unacceptable short-cycling of the fans  124  and  128 . A relatively small hysteresis, or difference between the cut-in and cut-out temperature differential settings, typically about 2 degrees F., is all that is needed. A larger hysteresis leads to unnecessary loss of operation of the outside air system and a smaller hysteresis can result in the fans  124  and  128  cycling on and off too frequently. 
     Because when the outside air refrigeration system operates it is more efficient than the conventional refrigeration system, to minimize energy use it is necessary to operate the conventional system only when the outside air system cannot maintain a cool enough temperature inside the enclosure. This is accomplished by having the inside thermostat  155  for the conventional refrigeration system have a higher operating temperature range than the inside thermostat  154  for the outside air refrigeration system. Typically, for inside thermostat  155  for the conventional system, the cut-in temperature setting is 38 degrees F. and the cut-out setting is 36 degrees F., and for the inside thermostat  154  for the outside air system, the cut-in temperature setting is 36 degrees F. and the cut-out setting is 34 degrees F. As long as the outside air system can keep the temperature inside the enclosure from rising to 38 degrees F. the conventional system will not operate. 
     In  FIG. 2  and temperature and dew point sensor units  146  and  148  provide inputs to the controller  136  to see that the auxiliary ambient air refrigeration unit operates only when the temperature inside the enclosure is above a first predetermined temperature, the ambient temperature is less than the enclosure temperature by a predetermined differential temperature and the dew point of the ambient air is matched to the dew point range of the air in the enclosure. For more details see U.S. Pat. No. 5,239,834 incorporated herein in its entirety by this reference. 
     Controller  136  operates to control humidity in the enclosure when supplying ambient (outside) air as shown in  FIGS. 4 and 5 . This humidity control is in addition to and simultaneous with the temperature control just described. The ambient air fans will operate only when both the humidity control and the temperature control allow it.  FIG. 4  is the logic diagram for providing ambient air whose humidity is matched to that of the range of the humidity in the enclosure when there is a minimum humidity control.  FIG. 5  does the same when there is a maximum humidity control. In  FIGS. 4 and 5  the following acronyms are used: RH=Relative Humidity, IH—Inside (relative) Humidity, IHMIN=Inside Humidity Minimum setting (default: 0%), IHMAX=Inside Humidity Maximum setting (default: 100%), IDP=Inside Dew Point temperature, and ODP=Outside Dew Point temperature. In  FIG. 4  the cycle begins with the outside air fans off,  200 . The inside humidity or dew point is read,  202 , and a determination is made as to whether it is moist enough inside (IH&gt;IHMIN+1% RH),  204 . If it is, the outside fans are allowed to turn on,  206 ; if it is not the inside and outside humidity or dew points are read and the IDP and ODP are calculated,  208 . Then it is determined whether it is moist enough outside (ODP&gt;IDP+1° F.),  210 . If it is not, then the outside fans are left off,  212 , and the cycle returns to read the inside humidity or dew point at  202 ; if it is moist enough outside, in condition  210 , then the outside air fans are allowed to turn on,  214 . With the fans on the inside humidity or dew point is determined,  216 ; if it is moist enough inside (IH&gt;IHMIN),  218 , the outside fans are left on,  220 . If it is not moist enough inside the IDP and ODP are calculated,  222 , and then inquiry is made again as to whether is it moist enough outside (ODP&gt;IDP),  224 . If it is not, the outside air fans are turned off,  226 ; if it is, the outside fans are left on,  228 . The maximum humidity control with outside air cycle begins in  FIG. 5  with the outside air fans off,  240 . The inside humidity or dew point sensor is read,  242 ; if it is dry enough inside (IH&lt;IHMAX−1% RH),  244 , the outside air fans are allowed to turn on,  246 . If it is not, the inside and outside humidity or dew points are read and the IDP and ODP are calculated,  248 . If it is dry enough outside (ODP&lt;IDP−1° F.),  250 , the outside air fans are allowed to turn on,  252 ; if it is not, the outside air fans are left off,  254 . When the outside air fans are on the inside humidity or dew point is read,  256 ; if it is dry enough inside (IH&lt;IHMAX),  258 , the outside air fans are left on,  260 . If it is not, the IDP and ODP are calculated again,  262 , then the inquiry is made again, if is it dry enough outside (ODP&lt;IDP),  264 . If it is not, the outside air fans are turned off,  266 ; if it is dry enough outside the outside air fans are left on,  268 . 
     An alternative approach for humidity control combining both maximum and minimum humidity logic is shown in  FIG. 6 . Initially the dry-bulb temperature logic calls for outside air  300 . Inquiry is then made as to whether the outside air fans are on  302 . If the response is yes the next decision point is: is inside humidity&lt;minimum humidity set point,  304 . If the response is yes, inquiry is made as to whether the outside dew point is &lt;inside dew point,  306 . If the response here is no or the response in step  304  is no then inquiry is made as to whether the inside humidity is &gt;maximum humidity set point,  308 . If the response is yes, inquiry is made as to whether the outside dew point&gt;the inside dew point,  310 . If the response is no or if the response was no to step  308  the outside fans are turned on  312 . If in step  302  it is noted that the outside fans are not on, then inquiry is made as to whether the inside humidity is &lt;(minimum humidity set point+ΔH), where ΔH is the hysteresis effect step  314 . If the response is yes, inquiry is made as to whether the outside dew point is &lt;the inside dew point+ΔDP where ΔDP is again a factor of hysteresis  316 . If the response here is no, or the response to step  314  was no inquiry is made as to whether the inside humidity is &gt;(maximum humidity set point−ΔH),  318 . If the answer is yes, inquiry is made as to whether the outside dew point&gt;(inside dew point−ΔDP),  320 . If the response is no, or the response to step  318  was no, the outside air fans are turned on  322 . If the response in either step  306  or  316  is yes or the response step  310  or  320  is yes, the outside air fans are turned off  324 . Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. 
     In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended. 
     Other embodiments will occur to those skilled in the art and are within the following claims.