Patent Publication Number: US-11650638-B2

Title: Dual redundant cooling system for a container

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. non-provisional patent application Ser. No. 16/441,402, filed on Jun. 14, 2019, the contents of which are incorporated herein by reference 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a cooling system and, more particularly, to a dual redundant cooling system for a container. 
     BACKGROUND 
     Certain goods, such as pharmaceuticals, require a temperature-controlled supply chain, also referred to as a cold chain. During storage and shipping, for example, cold chain goods must be held within a desired low-temperature range. This is particularly critical for pharmaceutical or biopharmaceutical products, as even a small temperature variation can lead to expensive and time-consuming evaluations of product liability and even complete product loss. Numerous pharmaceuticals do not arrive at their destination in usable condition. Even small temperature variations in the cold chain can cost hundreds of thousands of dollars in testing and wasted supplies. 
     SUMMARY 
     A dual redundant cooling system for a container is provided. The dual redundant cooling system includes a first cooling unit and a second cooling unit. The first cooling unit is positioned in a first cabinet attached to the container. The first cooling unit includes a first controller operating a first cooling loop to cool an interior of the container. The second cooling unit is positioned in a second cabinet attached to the container and adjacent the first cabinet. The second cooling unit includes a second controller operating a second cooling loop to cool the interior of the container. The first cooling unit and the first cooling loop are separate from the second cooling unit and the second cooling loop. The first controller and the second controller communicate a switch signal between each other so that either the first cooling unit is a primary cooling unit operating the first cooling loop or the second cooling unit is the primary cooling unit operating the second cooling loop. The switch signal switching the primary cooling unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example with reference to the accompanying Figures, of which: 
         FIG.  1    is a perspective view of a dual redundant cooling system according to an embodiment on a container; 
         FIG.  2    is an end view of the cooling system on the container; 
         FIG.  3    is a perspective view of a first compressor of a cooling unit of the cooling system; 
         FIG.  4    is a perspective view of a second compressor of the cooling unit; 
         FIG.  5    is a perspective view of an evaporator of the cooling unit; 
         FIG.  6    is a perspective view of an evaporator coil of the evaporator; 
         FIG.  7    is a front view of a unit control box of the cooling unit; 
         FIG.  8    is a front view of a system interface box of the cooling system; 
         FIG.  9    is a block diagram of the cooling system; and 
         FIG.  10    is a flowchart of a process of cooling an interior of the container using the cooling system. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT(S) 
     Embodiments of the present invention will be described hereinafter in detail with reference to the attached drawings, wherein like reference numerals refer to like elements. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that the disclosure will convey the concept of the invention to those skilled in the art. 
     A dual redundant cooling system  10  according to an embodiment is shown in  FIGS.  1  and  2   . The dual redundant cooling system  10  may also be referred to as a cooling system  10  throughout the specification. The cooling system  10  generally includes a first cabinet  110 , a second cabinet  120 , a first cooling unit  200  positioned in the first cabinet  110 , a second cooling unit  300  positioned in the second cabinet  120 , and a system interface box  400  connected to the cooling units  200 ,  300 . 
     As shown in  FIGS.  1  and  2   , the cooling system  10  is positioned on an end of a container  500 . The cooling system  10  is used control a temperature of an interior of the container  500 , as described in greater detail below. In an embodiment, the container  500  is an insulated shipping container. In other embodiments, the container  500  may not be insulated, and may be any type of container used for storing or shipping goods. 
     As shown in  FIGS.  1  and  2   , the first cabinet  110  and the second cabinet  120  are positioned on top of one another on the end of the container  500 . The first cabinet  110  and second cabinet  120  are secured in place on the end of the container  500  and respectively house the elements of the first cooling unit  200  and the second cooling unit  300 . 
     The first cooling unit  200  and the second cooling unit  300  are separate but each have identical components that function identically. As will become clear in the following description, identical components of the first cooling unit  200  and the second cooling unit  300  have similar reference numbers; reference numbers of identical components between the first cooling unit  200  and the second cooling unit  300  share the same last two digits. These identical components will be described and referenced together in some of the drawings and a portion of the description, in which multiple similar reference numbers reference the same component to indicate that the same component is present in the first cooling unit  200  and the second cooling unit  300 . Where the identical components of the first cooling unit  200  and the second cooling unit  300  are referenced separately, the component of the first cooling unit  200  may be labeled “first” and the component of the second cooling unit  300  may be labeled “second.” 
     Each of the first cooling unit  200  and the second cooling unit  300 , as shown in  FIGS.  1 ,  2 ,  5 , and  6   , has a condenser  210 ,  310 , a high compressor  220 ,  320  and a low compressor  230 ,  330 , an evaporator  240 ,  340  connected to the condenser  210 ,  310 , and a unit control box  260 ,  360  connected to and controlling the condenser  210 ,  310 , the high compressor  220 ,  320 , the low compressor  230 ,  330 , and the evaporator  240 ,  340 . 
     Each condenser  210 ,  310 , as shown in  FIGS.  2 - 4   , has a condenser coil  212 ,  312 , a condenser fan  214 ,  314  adapted to blow air over the condenser coil  212 ,  312 , and a condenser motor  216 ,  316  adapted to drive the condenser fan  214 ,  314 . The condenser coil  212 ,  312  is connected to and receives a refrigerant from the high compressor  220 ,  320  and the low compressor  230 ,  330 . The refrigerant received at the condenser coil  212 ,  312  is in a vapor state with a high temperature and a high pressure. The condenser fan  214 ,  314  blows air over the condenser coil  212 ,  312 , cooling the refrigerant in the condenser coil  212 ,  312  and turning the refrigerant into a liquid state. 
     As shown in  FIG.  2   , the condenser  210  of the first cooling unit  200  is positioned approximately centrally in the first cabinet  110  and the condenser  310  of the second cooling unit  200  is positioned approximately centrally in the second cabinet  120 . The condenser fan  214 ,  314  faces away from the container  500  and blows air to an area exterior of the cooling system  10 . 
     As shown in  FIGS.  3  and  4   , each condenser  210 ,  310  includes a plurality of condenser coils  212 ,  312  separately receiving a first refrigerant from the high compressor  220 ,  320  and a second refrigerant from the low compressor  230 ,  330 . The condenser fan  214 ,  314  blows air over all of the condenser coils  212 ,  312  and cools the first refrigerant and the second refrigerant into the liquid state. In an embodiment, the first refrigerant is an R134a refrigerant and the second refrigerant is an R23 refrigerant. 
     Each high compressor  220 ,  320 , as shown in  FIGS.  2  and  3   , has a pair of high compressor units  222 ,  322 , a pair of high compressor sensors  224 ,  324  disposed at the high compressor units  222 ,  322 , a high pressure transducer  226 ,  326  connected to the high compressor units  222 ,  322 , a low pressure transducer  227 ,  327  connected to the high compressor units  222 ,  322 , and an ambient temperature sensor  228 ,  328 . Each of the high compressor units  222 ,  322  receives the first refrigerant from the evaporator  240 ,  340  in the vapor state and compresses the first refrigerant, raising the temperature and the pressure of the first refrigerant. The first refrigerant is output from the high compressor units  222 ,  322  in the vapor state. The high pressure transducer  226 ,  326  controls a high pressure of the first refrigerant in the high compressor units  222 ,  322  and the low pressure transducer  227 ,  327  controls a low pressure of the first refrigerant in the high compressor units  222 ,  322 . The high compressor sensors  224 ,  324  respectively sense a temperature of each of the high compressor units  222 ,  322 . The ambient temperature sensor  228 ,  328  senses an ambient temperature in the portion of the cabinet  110 ,  120  containing the high compressor  220 ,  320 . 
     Each low compressor  230 ,  330 , as shown in  FIGS.  2  and  4   , has a low compressor unit  232 ,  332 , a low compressor sensor  234 ,  334  disposed at the low compressor unit  232 ,  332 , a high pressure transducer  236 ,  336  connected to the low compressor unit  232 ,  332 , a low pressure transducer  237 ,  337  connected to the low compressor unit  232 ,  332 , and a hot gas solenoid valve  238 ,  338  connected to the low compressor unit  232 ,  332 . As shown in  FIG.  1   , a compressor door  231  is attached by a hinge to a side of the first cabinet  110  and is rotatable about the hinge to cover or expose the first low compressor  230  of the first cooling unit  200 . 
     The low compressor unit  232 ,  332  receives the second refrigerant from the evaporator  240 ,  340  in the vapor state and compresses the second refrigerant, raising the temperature and the pressure of the second refrigerant. The second refrigerant is output from the low compressor unit  232 ,  332  in the vapor state. The high pressure transducer  236 ,  336  controls a high pressure of the second refrigerant in the low compressor unit  232 ,  332  and the low pressure transducer  237 ,  337  controls a low pressure of the second refrigerant in the low compressor unit  232 ,  332 . The low compressor sensor  234 ,  334  senses a temperature of the low compressor unit  232 ,  332 . The hot gas solenoid valve  238 ,  338  is adapted to controllably divert a hot gas generated by the low compressor unit  232 ,  332 . In an embodiment, the high compressor  220 ,  320  is adapted to remove heat from the low compressor  230 ,  330 . 
     Each evaporator  240 ,  340 , as shown in  FIGS.  5  and  6   , has an evaporator housing  242 ,  342 , an evaporator fan  244 ,  344  disposed in the evaporator housing  242 ,  342 , an evaporator motor  246 ,  346  adapted to drive the evaporator fan  244 ,  344 , and an evaporator coil  248 ,  348 . Each evaporator  240 ,  340  is positioned within the interior of the container  500  shown in  FIGS.  1  and  2    and connected to an output of the condenser  210 ,  310 . The evaporator coil  248 ,  348  shown in  FIG.  6    is positioned within the evaporator housing  242 ,  342  shown in  FIG.  5   . The evaporator coil  248 ,  348  receives the first refrigerant and the second refrigerant from the condenser  210 ,  310  in the liquid state having a low temperature and a low pressure. The first refrigerant and the second refrigerant absorbs heat from the interior of the container  500  and vaporizes in the evaporator coil  248 ,  348 . The evaporator fan  244 ,  344  blows air over the evaporator coil  248 ,  348  and aids in the absorption of heat. 
     Each evaporator  240 ,  340 , as shown in  FIGS.  5  and  6   , has a plurality of sensors adapted to detect temperatures within and around the evaporator  240 ,  340 . An evaporator motor sensor  250 ,  350  is disposed at the evaporator motor  246 ,  346  and adapted to detect a temperature of the evaporator motor  246 ,  346 . A supply air sensor  252 ,  352  is disposed at an outlet of the evaporator fan  244 ,  344  and is adapted to detect a temperature of a supply air output from the evaporator  240 ,  340  into the interior of the container  500 . A return air sensor  254 ,  354  is disposed at an inlet of the evaporator  240 ,  340  and is adapted to detect a temperature of a return air from the interior of the container  500  into the evaporator  240 ,  340 . 
     The evaporator coil  248 ,  348  is connected to an output of the hot gas solenoid valve  238 ,  338  of the low compressor  230 ,  330 . The hot gas solenoid valve  238 ,  338  can be controlled to divert the hot gas generated by the low compressor unit  232 ,  332  to the evaporator coil  248 ,  348  to heat to the evaporator coil  248 ,  348 , such as to defrost the evaporator coil  248 ,  348 . Each evaporator  240 ,  340  has a defrost sensor  256 ,  356 , as shown in  FIG.  6   , disposed at and adapted to detect a temperature of the evaporator coil  248 ,  348 . 
     Each unit control box  260 ,  360 , as shown in  FIGS.  1  and  2   , is attached by a hinge to a side of the cabinet  110 ,  120  and is rotatable about the hinge to cover or expose the respective high compressor  220 ,  320 . Each unit control box  260 ,  360  has a box door  261 ,  361  rotatable about a hinge to cover or expose a controller  262 ,  362  and a plurality of electrical components  268 ,  368  contained within the unit control box  260 ,  360 , shown in  FIG.  7   . 
     The system interface box  400 , as shown in  FIGS.  1  and  2   , is attached by a hinge to a side of the second cabinet  120  and is rotatable about the hinge to cover or expose the low compressor  330  of the second cooling unit  300 . The system interface box  400 , as shown in  FIG.  8   , has an interface corresponding to each of the first cooling unit  200  and the second cooling unit  300 . 
     For the first cooling unit  200 , as shown in  FIGS.  8  and  9   , the system interface box  400  has a first user interface  410  connected to the first controller  262  of the first cooling unit  200 , a first alarm  430  connected to the first user interface  410 , and a first switch  450  adapted to power on or power off the first cooling unit  200 . The first user interface  410  has a first display  412  and a first input  414 . In the shown embodiment, the first input  414  is a keypad. In other embodiments, the first input  414  may be any other type of computer input. 
     For the second cooling unit  300 , as shown in  FIGS.  8  and  9   , the system interface box  400  has a second user interface  420  connected to the second controller  362  of the second cooling unit  300 , a second alarm  440  connected to the second user interface  420 , and a second switch  460  adapted to power on or power off the second cooling unit  300 . The second user interface  420  has a second display  422  and a second input  424 . In the shown embodiment, the second input  424  is a keypad. In other embodiments, the second input  424  may be any other type of computer input. 
     The controller  262 ,  362  of each cooling unit  200 ,  300 , as shown in  FIG.  9   , has a processor  264 ,  364  and a memory  266 ,  366  connected to the processor  264 ,  364 . The memory  266 ,  366  is a non-transitory computer readable medium capable of storing data and instructions thereon that are executable by the processor  264 ,  364  to perform the functions of the controller  262 ,  362  described herein. In various embodiments, the memory  266 ,  366  may be a read-only memory, a random access memory, a database, or any other type of non-transitory computer readable medium known to those with ordinary skill in the art. 
     As shown in  FIG.  9   , the first controller  262  of the first cooling unit  200  communicates with the first user interface  410  by execution of the first processor  264 . The second controller  362  of the second cooling unit  300  likewise communicates with the second user interface  420  by execution of the second processor  364 . The controllers  262 ,  362  exchange data and control instructions with the user interfaces  410 ,  420  as described in greater detail below. 
     As shown in  FIG.  9   , the controller  262 ,  362  of each cooling unit  200 ,  300  is connected with the condenser  210 ,  310 , the evaporator  240 ,  340 , the high compressor  220 ,  320 , and the low compressor  230 ,  330  of the respective cooling unit  200 ,  300  and exchanges data and control instructions with these elements. The controller  262 ,  362  is connected to and exchanges data and control instructions with the condenser coil  212 ,  312  and the condenser motor  216 ,  316  of the condenser  210 ,  310 . The controller  262 ,  362  is connected to and exchanges data and control instructions with the evaporator motor  246 ,  346 , the evaporator coil  248 ,  348 , the evaporator motor sensor  250 ,  350 , the supply air sensor  252 ,  352 , the return air sensor  254 ,  354 , and the defrost sensor  256 ,  356  of the evaporator  240 ,  340 . The controller  262 ,  362  is connected to and exchanges data and control instructions with the high compressor units  222 ,  322 , the high compressor sensors  224 ,  324 , the high pressure transducer  226 ,  326 , the low pressure transducer  227 ,  327 , and the ambient temperature sensor  228 ,  328  of the high compressor  220 ,  320 . The controller  262 ,  362  is connected to and exchanges data and control instructions with the low compressor unit  232 ,  332 , the low compressor sensor  234 ,  334 , the high pressure transducer  236 ,  336 , the low pressure transducer  237 ,  337 , and the hot gas solenoid valve  238 ,  338  of the low compressor  230 ,  330 . 
     A process  600  of using the dual redundant cooling system  10  to cool the interior of the container  500  will now be described in greater detail primarily with reference to  FIG.  10   . 
     In a step  610  of the process  600 , the first cooling unit  200  and the second cooling unit  300  are started. To start the cooling units  200 ,  300 , a user switches the first switch  450  and the second switch  460  from an off state to an on state. The first switch  450  activates power to the first cooling unit  200  and the second switch  460  activates power to the second cooling unit  300 . 
     A primary cooling unit of the first cooling unit  200  and the second cooling unit  300  is determined in a next step  620  of the process  600 . The user interfaces  410 ,  420  receive an activation signal from the switches  450 ,  460  and determine when power is activated to the first cooling unit  200  and the second cooling unit  300 . In an embodiment, the primary cooling unit is determined by a relative time of turning on the first cooling unit  200  and the second cooling unit  300 . In an embodiment, if the activation signals from the switches  450 ,  460  indicate that the first cooling unit  200  and the second cooling unit  300  were turned on within 1.5 minutes of each other, the first cooling unit  200  is determined to be the primary cooling unit and the second cooling unit  300  is determined to be a secondary cooling unit. If the first cooling unit  200  was turned on more than 1.5 minutes after the second cooling unit  300 , then the second cooling unit  300  is determined to be the primary cooling unit and the first cooling unit  200  is determined to be the secondary cooling unit. In another embodiment, the system interface box  400  may have a primary switch by which the user may select which of the first cooling unit  200  and the second cooling unit  300  is the primary cooling unit. 
     For the purposes of clarity in the following description with reference to  FIG.  10   , the first cooling unit  200  will be considered to be the initial primary cooling unit and the second cooling unit  300  will be considered the initial secondary cooling unit determined at step  620 . However, as would be understood by those with ordinary skill in the art, the same description with reference to  FIG.  10    will similarly apply if the second cooling unit  300  is the initial primary cooling unit and the first cooling unit  200  is the initial secondary cooling unit. 
     In a next step  630 , as shown in  FIG.  10   , a function test is performed on the primary cooling unit  200 . The function test determines whether an operating ampere data from a number of different elements of the primary cooling unit  200  falls within an appropriate range. The first controller  262  compares the operating ampere data received from each of the elements to an operating ampere data range stored for each element on the first memory  266 . If the received operating ampere data for each of the elements falls within the stored operating ampere data range, the first controller  262  determines that the primary cooling unit  200  has passed the function test. If the received operating ampere data for any of the elements falls outside of the corresponding stored operating ampere data range, the first controller  262  determines that the primary cooling unit  200  has failed the function test. The first controller  262  sends a test passed or a test failed message to the first user interface  410  to be output at the first display  412 . 
     In an embodiment, as shown in  FIG.  9   , the first controller  262  of the primary cooling unit  200  receives an operating ampere data from each of the condenser motor  216 , the evaporator motor  246 , the high compressor units  222 , the low compressor unit  232 , and the hot gas solenoid valve  238  in the step  630  and compares each of these to a corresponding stored operating ampere data range. In an embodiment, the stored operating ampere data range can range from 0.5 A to 12 A and can be different for each of the elements. 
     If the first controller  262  determines that the primary cooling unit  200  has failed the function test, the process proceeds to an alarm condition as shown in step  640  in  FIG.  10   . In the alarm condition, the first controller  262  of the primary cooling unit  200  sends an alarm signal to the first user interface  410 . The alarm signal includes an alarm message and an alarm code stored on the first memory  266  that corresponds to the determined condition. For example, the alarm signal may include an alarm message of “auto test error, amps too low” and an alarm code of a corresponding letter, number, or series of letters and numbers if the received operating ampere data for any of the elements falls below the corresponding stored operating ampere data range. One with ordinary skill in the art would understand that the alarm message and the corresponding alarm code would vary based on the determined condition. 
     In the alarm condition step  640 , first the user interface  410  receives the alarm signal and displays the alarm message and/or the alarm code on the first display  412  for the user. The first user interface  410  also outputs the alarm signal at the first alarm  430 . In the shown embodiment, the alarm  430  is a lamp that is lit at the alarm condition. The user can use the input  414  to acknowledge the alarm signal on the display  412 . In an embodiment, the alarm  430  remains lit until the determined condition is resolved. A step  660  that follows the alarm condition step  640  will be described in greater detail below. 
     If the first controller  262  determines that the primary cooling unit  200  has passed the function test, the process proceeds with normal operation of the primary cooling unit  200  to cool the interior of the container  500  in a step  650  shown in  FIG.  10   . In normal operation, the primary cooling unit  200  cools the interior of the container  500  in a first cooling loop  270  shown in  FIG.  9   . 
     The first controller  262  controls the first refrigerant and the second refrigerant to flow through the first cooling loop  270 . At the condenser  210 , the first controller  262  controls the condenser motor  216  to blow air over the condenser coil  212  to an area exterior of the cooling system  10 , turning the refrigerant received from the compressors  220 ,  230  into a liquid state with a lower temperature and expelling hot air to the exterior the cooling system  10 . 
     The first refrigerant and second refrigerant then enter the evaporator  240 , where the first controller  262  controls the evaporator motor  246  to blow air received from the interior of the container  500  over the evaporator coil  248  that contains the liquid refrigerant with the lower temperature. The liquid refrigerant in the evaporator coil  248  absorbs heat from the passing air and the evaporator motor  246  blows colder air back into the interior of the container  500 , cooling the interior of the container  500 . Liquid refrigerant in the evaporator coil  248 , as described above, vaporizes in the evaporator coil  248  as it absorbs heat. 
     The first controller  262  controls the high compressor units  222 , the high pressure transducer  226 , the low pressure transducer  227 , the low compressor unit  232 , the high pressure transducer  236 , and the low pressure transducer  237  to compress the vaporized liquid refrigerant received from the evaporator  240 . The high compressor  220  and the low compressor  230  are controlled by the first controller  262  to output the first refrigerant and the second refrigerant in the vapor state with a higher temperature and a higher pressure. This output is received at the condenser  210 , restarting the first cooling loop  270 . 
     The first controller  262  operates the first cooling loop  270  in the step  650  to cool the interior of the container  500  to a predetermined set point temperature. The set point temperature may be set by the user using the input  414  at the user interface  410 . The first controller  262  receives the temperature of the return air from the interior of the container  500  from the return air sensor  254 . In normal operation, the first controller  262  operates the first cooling loop  270  if the return air temperature at the return air sensor  254  is 1° C. or more above the set point temperature. In an embodiment, the first controller  262  continues to run the first cooling loop  270  for the longer of a fifteen minute period and when the return air temperature reaches the set point temperature. If the return air temperature at the return air sensor  254  is 2° C. or more below the set point temperature, the first controller  262  stops running the first cooling loop  270  and, in an embodiment, waits a minimum of ten minutes before running the first cooling loop  270  again. In an embodiment, the first cooling loop  270  is capable of maintaining a temperature in the interior of the container  500  and a range of 0 to −65° C. 
     During the normal operation in step  650 , the first controller  262  monitors for additional alarm conditions stored in the first memory  266 . The alarm conditions include, for example, a detected temperature of a temperature sensor falling outside of a corresponding temperature range stored in the first memory  266  and a detected pressure of a pressure transducer falling outside of a corresponding pressure range stored in the first memory  266 . 
     In an embodiment, during normal operation in step  650 , the first controller  262  receives a detected temperature from the high compressor sensor  224 , the ambient temperature sensor  228 , the low compressor sensor  234 , the evaporator motor sensor  250 , the supply sensor  252 , the return air sensor  254 , and the defrost sensor  256 . The first controller  262  compares the detected temperature from each of the sensors to a first corresponding stored range to determine if the sensor is working properly. The first controller  262  also compares the detected temperature from the high compressor sensor  224 , the low compressor sensor  234 , and the evaporator motor sensor  250  to a second corresponding stored range to determine if the sensed element is overheating. In an embodiment, the first controller  262  also receives a detected pressure from the high pressure transducer  226 , the low pressure transducer  227 , the high pressure transducer  236 , and the low pressure transducer  237 . The first controller  262  compares the detected pressure from each of the sensors to a corresponding first stored range to determine if the sensor is working properly and to a corresponding second stored range to determine if the sensed element is working properly. One with ordinary skill in the art would understand that additional and/or other types of sensors could be used in the cooling system  10  to determine other alarm conditions. 
     If an alarm condition is determined in step  650 , the process proceeds to step  640  in  FIG.  10   . As described above, in the alarm condition step  640 , an alarm signal is sent to the first user interface  410 , the first user interface  410  displays the alarm message and/or the alarm code on the first display  412  and lights the first alarm  430 . 
     If no alarm condition is determined during normal operation in step  650 , the process proceeds to an initial defrost in a step  660  shown in  FIG.  10   . A defrost operation of the primary cooling unit  200  is required after a period of normal operation to ensure that the first cooling loop  270  is properly cooling the interior of the container  500 . In the defrost operation, the first controller  262  controls the hot gas solenoid valve  238  to divert a hot gas generated by the low compressor unit  232  to the evaporator coil  248 . The hot gas diverted to the evaporator coil  248  heats the evaporator coil  248 , defrosting the evaporator coil  248  by melting any frost accumulated on the evaporator coil  248  that could impair the heat absorption by the evaporator coil  248  and correspondingly impair the cooling of the interior of the container  500 . The first controller  262  continues to divert the hot gas to the evaporator coil  248  until a defrost temperature received from the defrost sensor  256  rises by a preset limit. In an embodiment, the preset limit of the defrost temperature is an increase of 5-30° C. 
     A user sets a defrost timer at the first input  414  of the first user interface  410  and the defrost timer is transmitted to and stored on the memory  266 . For the initial defrost in step  660 , or the first defrost after the primary cooling unit  200  was turned on, the first controller  262  performs the defrost operation after a predetermined portion of the defrost timer has elapsed during operation of the first cooling loop  270 . In an embodiment, the predetermined portion is one quarter of the period set in the defrost timer; the defrost timer is set to 24 hours and normal operation of the first cooling loop  270  is run for 6 hours before the initial defrost. The primary cooling unit  200  and the secondary cooling unit  300  remain the same and are not switched during the initial defrost of the primary cooling unit  200 . 
     After the initial defrost in step  660 , the process proceeds to normal operation in step  670 . The normal operation of step  670  is the same as the normal operation of step  650 ; the first cooling loop  270  and the determination of alarm conditions occurs as described under step  650  above. 
     If no alarm condition is determined during the normal operation of step  670 , the process proceeds to a second defrost in a step  680  shown in  FIG.  10   . The second defrost is initiated by the first controller  262  by at least one of the end of the defrost timer set at the first input  414 , a manual initiation of a defrost initiated by the user at the input  414 , and a determination of a relative temperature. In an embodiment of the determination of the relative temperature, the first controller  262  compares a supply air temperature sensed by the supply air sensor  252  and a return air temperature sensed by the return air sensor  254 ; the first controller  262  initiates the second defrost if the supply air temperature is more than 10° C. lower than the return air temperature. In another embodiment of the determination of relative temperature, the first controller  262  compares the return air temperature and the defrost temperature sensed by the defrost sensor  256 ; the first controller  262  initiates the second defrost if the defrost temperature is more than 15° C. lower than the return air temperature. The defrost operation of the second defrost in step  680  is the same as the defrost operation in the initial defrost of step  660  described above. 
     As shown in  FIG.  10   , after the alarm condition in step  640  or after the initiation of the second defrost in step  680 , the process proceeds to a step  690 . In the step  690 , the primary cooling unit, described by way of example with reference to the first cooling unit  200  above, is switched to the secondary cooling unit and the secondary cooling unit, described by way of example with reference to the second cooling unit  300  above, is switched to the primary cooling unit. In the step  690 , the first controller  262  of the previously primary first cooling unit  200  sends a switch signal to the second controller  362  of the previously secondary second cooling unit  300  either at the determination of the alarm condition or at the initiation of the second defrost. The first cooling unit  200  then enters a standby state from the normal operation state. The second cooling unit  300  switches from the standby state to the normal operation state and, looping back to step  620 , the second cooling unit  300  becomes the primary cooling unit  300  and the first cooling unit  200  becomes the secondary cooling unit  200 . 
     The process shown in  FIG.  10    then executes the same steps  630 - 690  described above with the second cooling unit  300  as the primary cooling unit  300 . The function test is performed on the primary cooling unit  300  in step  630 ; the alarm condition is determined in step  640  if the primary cooling unit  300  fails the function test, and the primary cooling unit  300  enters normal operation in step  650  if the primary cooling unit  300  passes the function test. The same operations apply to the second cooling unit  300  as the primary cooling unit as described with respect to the first cooling unit  200  as the primary cooling unit above, with the similar reference numbers and elements of the second cooling unit  300  performing the same functions as the counterparts in the first cooling unit  200 . The second cooling unit  300  runs a second cooling loop  370  shown in  FIG.  9    during normal operation to cool the interior of the container  500 . The second cooling unit  300  as the primary cooling unit  300  engages in normal operation in step  650 , undergoes an initial defrost in step  660 , and again executes normal operation in step  670 . In the standby state, the first cooling unit  200  serving as the secondary cooling unit  200  does not run the first cooling loop  270 . Either the first cooling loop  270  or the second cooling loop  370  is operating at a given time. 
     When the second cooling unit  300  as the primary cooling unit  300  reaches the alarm condition in step  640  or the second defrost in step  680 , the process again switches the primary and secondary cooling units provided any alarm in the secondary cooling unit  200  has been resolved. 
     The process shown in  FIG.  10    continues to loop, switching the primary cooling unit that is responsible for cooling the interior of the container  500  between the first cooling unit  200  and the second cooling unit  300  based on the presence of an alarm condition or a non-initial defrost. The dual redundant cooling system  10  thereby avoids using a malfunctioning cooling unit  200 ,  300  to cool the interior of the container  500 . The dual redundant cooling system  10  thereby also avoids the heat generated during the non-initial defrost operations from raising the temperature in the interior of the container  500  by using the other cooling unit the cool the interior of the container  500  while the first cooling unit is defrosting. The dual redundant cooling system  10  thus avoids even small temperature variations during storing or shipping while maintaining the interior of the container  500  at the predetermined set point temperature. 
     The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalent.