Patent Publication Number: US-2010107661-A1

Title: Method for operating transport refrigeration unit with remote evaporator

Description:
FIELD OF THE INVENTION 
     This invention relates generally to transport refrigeration and, more specifically, to facilitating defrost of the heat exchanger coil of a remote evaporator during the period when product is being loaded into a compartment cargo box of a truck trailer equipped with a refrigeration unit including multiple evaporators. 
     BACKGROUND OF THE INVENTION 
     The refrigerated cargo box of a truck trailer requires a refrigeration unit for maintaining a desired temperature environment within the interior volume of the cargo box. A wide variety of products, ranging for example from freshly picked produce to deep frozen seafood, are commonly shipped in refrigerated truck trailers and other refrigerated freight containers. To facilitate shipment of a variety of products under different temperature conditions, some truck trailer cargo boxes are compartmentalized into two or more separate cargo compartments each of which will typically have a door that opens directly to the exterior of the trailer. The cargo box may be compartmentalized into a pair of side-by-side axially extending compartments, or into two or more back-to-back compartments, or a combination thereof. 
     Conventional transport refrigeration units used in connection with compartmentalized refrigerated cargo boxes of truck trailers include a refrigerant compressor, a condenser, a main evaporator and one or more remote evaporators connected via appropriate refrigerant lines in a closed refrigerant flow circuit. The refrigeration unit includes a housing mountable to the exterior of the front wall of the cargo box of the truck trailer with the main evaporator disposed on the interior of the front wall such that the air or gas/air mixture or other gas within the interior volume of the forward most container may be circulated over the evaporator coil by means of an evaporator fan associated with the evaporator coil. The refrigeration unit may also be equipped with an economizer incorporated into the refrigerant circuit, if desired. The refrigeration unit must have sufficient refrigeration capacity to maintain the perishable product stored within the various compartments of the cargo box at the particular desired compartment temperatures over a wide range of outdoor ambient temperatures and load conditions. 
     In addition to the main afore-mentioned evaporator, one or more remote evaporators, typically one for each additional compartment aft of the forward most compartment, are provided to refrigerate the air or other gases within each of the separate aft compartments. The remote evaporators may be mounted to the ceiling of the respective compartments or one of the partition walls of the compartment, as desired. The remote evaporators are generally disposed in the refrigerant circulation circuit in parallel with the main evaporator. Typically, a solenoid operated shut off valve is disposed in the refrigerant circulation circuit upstream of each of the remote in operation with a system controller so that each remote evaporator may to independently and selectively open and closed to refrigerant flow in response to the cooling demand of the respective compartment with which the respective remote evaporator is operatively associated. For example, U.S. Pat. No. 5,065,587 discloses a refrigeration system for a compartmentalized truck trailer having a forward compartment and a rear remote compartment. 
     Product to be shipped is loaded into the cargo box of the truck trailer through one or more doors. In a compartmentalized cargo box, each of the respective cargo compartments generally has one or more associated doors through which product may be loaded directly into the various compartments from externally of the truck trailer. When loading product into a compartment of the truck trailer, it is customary to leave the door or doors to the compartment open to facilitate the loading operation. With the door or doors open, relatively warmer moisture bearing ambient air from externally of the truck trailer may enter the compartment, such as, for example, when fresh fruits and vegetables are loaded direct from the field. Simultaneously, relatively colder refrigerated air may pass out of the compartment. Additionally, when product is loaded into the container “hot”, that is at a temperature above the desired product storage temperature, such as for example directly from the field at ambient outdoor temperature, a substantial refrigeration load is imposed upon the transport refrigeration unit in pulling the temperature of the newly loaded product down from ambient outdoor temperature to the desired product storage temperature. The net effect being to raise the temperature of and to add moisture to the air resident within the compartment, particularly in regions with warm and humid ambient conditions. 
     In the compartmentalized transport refrigeration system disclosed in the aforementioned U.S. Pat. No. 5,065,587, the remote evaporator is automatically shut down whenever the door to the rear compartment is open. However, many operators prefer to run the transport refrigeration unit during the product loading operation to counter the impact of the infiltration of relatively warmer ambient air and the added heat load associated with the product, and to reduce the time required to pull down the cargo box when the loading operation has been completed. As a result of the increased moisture within the interior volume of the compartment or compartments being loaded, frost will build-up on the evaporator heat exchanger coil disposed within the compartment or compartments being loaded and will do so more quickly than under normal steady-state operating conditions. As frost builds up on the evaporator heat exchanger coil, cooling capacity and the pull down of the refrigeration unit decreases and air flow through the affected evaporator decreases. 
     During the product loading process, it is conventional practice for the operator to manually initiate an evaporator coil defrost cycle whenever the operator deems appropriate. However, some operators may forget to manually initiate, the defrost cycle until the evaporator heat exchanger coil has become excessively burdened with frost build up and the refrigeration capacity and performance of the evaporator heat exchanger coil has substantially deteriorated. Accordingly, it would be desirable to provide a transport refrigeration unit wherein a defrost cycle is automatically initiated during the product loading procedure whenever the refrigeration performance of the refrigeration unit has deteriorated below an acceptable level. 
     SUMMARY OF THE INVENTION 
     The operation of a transport refrigeration unit is controlled during loading of product into a cargo compartment equipped with a remote evaporator to automatically defrost the remote evaporator when a selected parameter indicative of the performance of the refrigeration unit has transgressed a limit threshold indicative of deteriorating performance. 
     A method is provided for operating a transport refrigeration unit during loading of product into a cargo box, for example in an embodiment a truck trailer, having at least a first and a second compartment. The refrigeration unit has a refrigerant flow circuit including a refrigerant compressor having a suction inlet, a main evaporator disposed in a first compartment and a remote evaporator disposed in a second compartment, connected in refrigerant flow communication. The method includes the steps of: determining a degree of superheat (CSS) in the refrigerant at the suction inlet to the compressor, comparing the degree of suction superheat (CSS) to a lower limit for the degree of superheat (LSS) at the suction inlet to the compressor, continuing operation of the refrigerant unit in a refrigeration mode if the degree of suction superheat (CSS) is greater than the lower limit for the degree of superheat (LSS) and terminating operation of the refrigerant unit in the refrigeration mode if the degree of suction superheat (CSS) is not greater than the lower limit for the degree of superheat (LSS) and initiating a defrost of the remote evaporator. 
     In an embodiment, the step of determining a degree of superheat (CSS) in the refrigerant at the suction inlet to the compressor comprises the steps of sensing a refrigerant temperature (CST) indicative of the refrigerant temperature at the suction inlet to the compressor, sensing a refrigerant pressure (CSP) indicative of the refrigerant pressure at the suction inlet to the compressor, determining a suction saturation temperature (SST) based upon the sensed refrigerant pressure (CSP) indicative of the refrigerant pressure at the suction inlet to the compressor, and subtracting the suction saturation temperature (SST) from the sensed refrigerant temperature (CST) indicative of the refrigerant temperature at the suction inlet to the compressor. 
     In an embodiment, the step of initiating a defrost of the main evaporator if product is being loaded into the first compartment and initiating a defrost of the remote evaporator if product is being loaded into the second compartment. In an embodiment, the step of initiating a defrost of at least one of the evaporators comprises the steps of: determining whether the door to a compartment is open, and initiating a defrost of the evaporator associated with that compartment if the door to that compartment is determined to be open. In an embodiment, the step of initiating the defrost of an evaporator comprises initiating an electric defrost of the evaporator. In an embodiment, the step of initiating the defrost of an evaporator comprises initiating an electric defrost of the evaporator or a predetermined time period. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a further understanding of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawing, where: 
         FIG. 1  is a perspective view, partly in section, of a refrigerated truck trailer having a compartmentalized cargo box and equipped with a transport refrigeration unit having multiple evaporators; 
         FIG. 2  is a schematic representation of an exemplary embodiment of the multiple evaporator transport refrigeration unit of the invention; 
         FIG. 3  is a block diagram illustrating an exemplary embodiment of the method for operation of the transport refrigeration unit of  FIG. 2  during loading of the refrigerated perishable product container of  FIG. 1 ; and 
         FIG. 4  is a block diagram illustrating an exemplary embodiment of the step of initiating a defrost mode during loading of the refrigerated perishable product container of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 1 and 2 , there is shown a truck trailer  100  having a refrigerated cargo box  110  subdivided, i.e. compartmentalized, by internal partition walls  104 ,  106  into a forward cargo compartment  112 , a central cargo compartment  114  and an aft cargo compartment  116 . The cargo compartments  112 ,  114  and  116  have access doors  113 ,  115  and  117 , respectively, which open directly to the exterior of the truck trailer to facilitate loading of product into the respective cargo compartments  112 ,  114  and  116 . The truck trailer  100  is equipped with a transport refrigeration system  10  for regulating and maintaining within each of the respective cargo compartments  112 ,  114  and  116  a desired storage temperature range selected for the perishable product being shipped therein. Although the invention will be described herein with reference to the three compartment, refrigerated cargo box, illustrated in  FIG. 1 , it is to be understood that the invention may also be used in connection with truck trailers having compartmentalized cargo boxes with the cargo compartments arranged otherwise, and also in connection with other refrigerated transport vessels, including for example refrigerated box of a truck, or a refrigerated freight container of compartmentalized design for transporting perishable product by ship, rail and/or road transport. 
     The transport refrigeration system  10  includes a refrigeration unit  12  and a power source  14 . The refrigeration unit  12  includes compressor  20 , a condenser  30  and associated condenser fan(s)  34 , a main evaporator  40  and associated evaporator fan(s)  44 , remote evaporators  50  and  60  and associated fans  54  and  64 , respectively, and evaporator expansion devices  46 ,  56  and  66  arranged in a conventional refrigeration cycle and connected in a refrigerant circulation circuit including refrigerant lines  2 ,  4 ,  6   a,    6   b ,  6   c  and  8 . Each of the evaporators  40 ,  50  and  60  may comprise a conventional finned tube coil heat exchanger. The transport refrigeration system  10  is mounted as in conventional practice to an exterior wall of the truck trailer  100 , for example the front wall  102  thereof, with the compressor  20  and the condenser  30  its associated condenser fan(s)  34  disposed externally of the refrigerated cargo box  110  in a housing  16 . 
     The main evaporator  40  is disposed on the wall  102  of the forward compartment  112  of the refrigerated cargo box  110 . The expansion device  46 , which as in the depicted embodiment may be an electronic expansion valve, is disposed in refrigerant line  6   a  upstream with respect to refrigerant flow of the main evaporator  40  for metering the flow of refrigerant through the evaporator in response to the degree of superheat in the refrigerant at the outlet of the evaporator  40 , as in conventional practice. A temperature sensor  41  and a pressure sensor  43  are mounted on the refrigerant line  6   a  closely downstream of the outlet of the evaporator  40  for monitoring the temperature and pressure of the refrigerant at the evaporator outlet. The electronic expansion valve  46  receives signals indicative of the evaporator outlet temperature and pressure from the sensors  41  and  43 , respectively, and determines therefrom the degree of superheat in the refrigerant at the evaporator outlet as in conventional practice. 
     The remote evaporator  50  is disposed on the partition wall  104  of the refrigerated cargo box  110  within the central compartment  114 . A solenoid operated flow shut-off valve  58  is disposed in refrigerant line  6   b  upstream with respect to refrigerant flow of the expansion valve  56 , which, as in the depicted exemplary embodiment, may be a thermostatic static expansion valve operatively associated with an evaporator outlet temperature sensing element  57 , for example a temperature sensing bulb, mounted on refrigerant line  6   b  downstream with respect to refrigerant flow of the evaporator  50  for sensing the refrigerant temperature at the evaporator outlet which reflects the degree of superheat of the refrigerant at the evaporator outlet. As in conventional practice, the expansion valve  56  meters the flow of refrigerant through the remote evaporator  50 . An external equalizer line  59  may also be provided in association with the expansion valve  56 . 
     The remote evaporator  60  is disposed on the ceiling of the cargo box  110  within the aft compartment  116 . A solenoid operated flow shut-off valve  68  is disposed in refrigerant line  6   c  upstream with respect to refrigerant flow of the expansion valve  66 , which, as in the depicted exemplary embodiment, may be a thermostatic static expansion valve operatively associated with an evaporator outlet temperature sensing element  67 , for example a temperature sensing bulb, mounted on refrigerant line  6   c  downstream with respect to refrigerant flow of the evaporator  60  for sensing the refrigerant temperature at the evaporator outlet which reflects the degree of superheat of the refrigerant at the evaporator outlet. As in conventional practice, the expansion valve  66  meters the flow of refrigerant through the remote evaporator  60 . An external equalizer line  69  may also be provided in association with the expansion valve  66 . 
     As in conventional practice, when the refrigeration unit  10  is in operation, low temperature, low pressure refrigerant vapor is compressed by the compressor  20  to a high pressure, high temperature refrigerant vapor and passed from the discharge outlet of the compressor  20  into refrigerant line  2 . The refrigerant circulates through the refrigerant circuit via refrigerant line  2  to and through the heat exchanger coil of the condenser  30 , wherein the refrigerant vapor condenses to a liquid, and the subcooler  32 , thence through refrigerant line  4  through the refrigerant-to-refrigerant heat exchanger  35 , thence through refrigerant line  6   a  through the main evaporator  40 , and through refrigerant lines  6   b  and  6   c  and the remote respective evaporator  50  and  60 , if the associated solenoid operated flow shut-off valves  58  and  68  are open. The refrigerant evaporates as it passes through the evaporators in heat exchange relationship the air within the respective cargo compartments. The refrigerant vapor leaving the evaporators thence passes through refrigerant line  8  through the refrigerant-to-refrigerant heat exchanger  35  and the suction modulation valve  22  to return to the suction inlet of the compressor  20 . In the refrigerant-to-refrigerant heat exchanger  35 , the hot, high pressure liquid refrigerant passing through refrigerant line  4  passes in heat exchange relationship with the lower temperature, lower pressure vapor refrigerant passing through refrigerant line  8 . In addition, a receiver  16  and a filter/drier  18  may be included in the refrigerant circuit as in conventional practice. An economizer circuit (not shown) incorporated into the refrigerant circuit in a conventional manner well known in the art. 
     The compressor  20  may comprise a single-stage or multiple-stage compressor such as, for example, a reciprocating compressor or a scroll compressor, although the particular type of compressor used is not germane to or limiting of the invention. In the exemplary embodiment depicted in  FIG. 2 , the compressor is a reciprocating compressor, such as for example, an 06D model reciprocating compressor manufactured by Carrier Corporation or a variant thereof, having a compressing mechanism, an internal electric compressor motor and an interconnecting drive shaft that are all sealed within a common housing of the compressor  20 . The power source  14  fully powers the internal electric motor of the compressor. In an embodiment, the power source  14  generates sufficient electrical power for fully driving the electrical motor of the compressor  20  and also for providing all other electrical power required by the fans  34 ,  44 ,  54 ,  64  and other parts of the refrigeration system  10 . In an electrically powered embodiment of the transport refrigeration system  10 , the power source  14  comprises a single on-board engine driven synchronous generator configured to selectively produce at least one AC voltage at one or more frequencies. An electrically powered transport refrigeration system suitable for use on truck trailer transport vehicles are shown in U.S. Pat. No. 6,223,546, assigned to the assignee of the present application, the entire disclosure of which is incorporated herein by reference. 
     The refrigeration unit also includes an electronic controller  70  that is configured to operate the refrigeration unit  10  to maintain a predetermined thermal environment within the compartments  112 ,  114  and  116  defined within the cargo box  110  wherein the product is stored during transport. The electronic controller  70  maintains the predetermined environment by selectively powering and controlling the operation of the compressor  20 , the condenser fan(s)  34  associated with the condenser  30 , the evaporator fans  44 ,  54  and  64  associated with the evaporators  40 ,  50  and  60 , respectively, and the suction modulation valve  12 . When cooling of the environment within the compartments  112 ,  114  and  116  of the cargo box  110  is required, the electronic controller  70  activates the compressor  20 , the condenser fan(s)  34  and the evaporator fan  44 ,  54  and  64 , as appropriate. Additionally, the electronic controller  70  adjusts the position of the suction modulation valve  12  to increase or decrease the flow of refrigerant supplied to the compressor  20  as appropriate to control and stabilize the temperatures within the respective compartments  112 ,  114  and  116  within the cargo box at the respective set point temperatures, which correspond to the desired product storage temperatures for the particular products stored within the respective compartments  112 ,  114  and  116  of the cargo box  110 . A more detailed discussion of the operation of the transport refrigeration unit  100  under normal steady-state load is provided in the aforementioned U.S. Pat. No. 6,223,546. 
     In one embodiment, the electronic controller  70  includes a microprocessor and an associated memory. The memory of the controller  70  may be programmed to contain preselected operator or owner desired values for various operating parameters within the system, including, but not limited to, temperature set points for various locations within the refrigerant circuit and/or the compartments  112 ,  114  and  116  of the cargo box  110 , pressure limits, current limits, engine speed limits, and any variety of other desired operating parameters or limits within the system. The programming of the controller is within the ordinary skill in the art. The controller  70  may include a microprocessor board that includes the microprocessor, an associated memory, and an input/output board that contains an analog-to-digital converter which receives temperature inputs and pressure inputs from a plurality of sensors located at various points throughout the refrigerant circuit and the refrigerated cargo box, current inputs, voltage inputs, and humidity levels. The input/output board may also include drive circuits or field effect transistors and relays which receive signals or current from the controller  70  and in turn control various external or peripheral devices associated with the transport refrigeration system. In an embodiment, the controller  70  may comprise the MicroLink™ controller available from Carrier Corporation, the assignee of this application. However, the particular type and design of the controller  70  is within the discretion of one of ordinary skill in the art to select and is not limiting of the invention. 
     When the refrigeration unit system  10  is in operation, air within the forward compartment  112  of the refrigerated cargo box  110  is circulated by the evaporator fan(s)  44  through the evaporator  40  and back into the interior volume of the refrigerated forward compartment  112  of the cargo box  110 . Additionally, if the solenoid operated flow shut-off valve  58  is open, air within the central compartment  114  of the refrigerated cargo box  110  is circulated by the evaporator fan(s)  54  through the evaporator  50  and back into the interior volume of the refrigerated forward compartment  114  of the cargo box  110 . If the solenoid operated flow shut-off valve  68  is open, air within the aft compartment  116  of the refrigerated cargo box  110  is circulated by the evaporator fan(s)  64  through the evaporator  60  and back into the interior volume of the refrigerated aft compartment  116  of the cargo box  110 . As in conventional practice, the air passing through the evaporators  40 ,  50  and  60  is cooled as it passes in heat exchange relationship with refrigerant circulated through the tubes of the finned heat transfer coils of the respective evaporators  40 ,  50  and  60 . 
     Perishable product is loaded into the compartments  112 ,  114 ,  116  of the refrigerated cargo box  110  of the trailer  100  through the doors  113 ,  115 ,  117 , respectively. When loading product, it is customary to leave the doors open to facilitate the loading operation. With the doors open, relatively warmer moisture bearing ambient air from externally of the cargo box  110  may enter into any of the compartments  112 ,  114  and  116  whose respective door or doors are open. Relatively colder refrigerated air may simultaneously pass out of the interior volume of any compartment whose doors or doors are open. The net effect being to raise the temperature of and add moisture to the air resident within the interior volume of those open compartments, particularly in regions with warm and humid ambient conditions. 
     Consequently, many operators choose to run the transport refrigeration system  10  during the product loading operation to counter the impact of the infiltration of relatively warmer ambient air and to reduce the time required to pull down the temperature of the interior volume of the compartment(s) and the temperature of the freshly added product within the compartment(s) when the loading operation has been completed. As a result of the increased moisture within the interior volume the compartments of the cargo box  110  being loaded, frost will build-up on the finned tube heat exchanger coil of the main evaporator  40  if compartment  112  is being loaded and the finned tube heat exchanger coils of the remote evaporators  50  and  60  if the compartments  114  and  116 , respectively, are being loaded, and will do so more quickly than under normal steady-state operating conditions. As frost builds up on the heat exchanger coils of the respective evaporators, the cooling capacity and the pull down capability of the refrigeration unit  12  decreases and air flow through the affected evaporator decreases. 
     The controller  70  is configured to determine the degree of superheat in the refrigerant passing through refrigerant line  8  that connects the respective refrigerant lines  6   a,    6   b  and  6   c  downstream of the respective outlets of the heat exchanger coils of the evaporators  40 ,  50  and  60  in refrigerant flow communication with the suction inlet of the compressor  20 . The controller  70  compares the determined degrees of superheat, commonly referred to as suction superheat (SSV), to a low suction superheat threshold (LSS). if the determined suction superheat (SSV) is less than the low suction superheat threshold (LSS) during the loading operation, the controller  60  will interrupt operation of the refrigeration unit  12  in the refrigeration mode and initiate an electric defrost cycle to clear the heat exchanger coil of the affected evaporator or evaporators of the evaporators  40 ,  50  and  60  of the frost built-up thereon. 
     In an embodiment, the controller  70  uses the refrigerant suction temperature and the refrigerant suction pressure to calculate the degree of superheat at the suction inlet. A temperature sensor  72  operatively associated with refrigerant line  8  monitors the temperature of the refrigerant upstream of the suction inlet (CST) of the compressor  20  and a pressure sensor  74  operatively associated with the compressor  20  monitors the pressure of the refrigerant at the suction inlet (CSP) of the compressor  20 . In the exemplary embodiment depicted in  FIG. 2 , the temperature sensor  72  comprises a thermocouple, thermistor or other conventional thermostatic device mounted on the refrigerant line  8  at a location downstream with respect to refrigerant flow of the suction modulation valve. In the exemplary embodiment depicted in  FIG. 2 , the pressure sensor  74  comprises a conventional pressure transducer mounted in operative association with the suction inlet side of the compression chamber of the compressor  20  to sense the pressure of the refrigerant vapor at the suction inlet. 
     The defrost control mode of the controller  70  during the period in which the transport refrigeration system  10  is running while product is being loaded into the cargo box  110  is presented schematically in block diagram of  FIG. 3 . At block  202 , the temperature sensor  72  detects the compressor suction temperature (CST) and transmits a signal indicative of the CST to the input/output module of the controller  70 . At block  204 , the pressure sensor  74  detects the compressor suction pressure (CSP) and transmits a signal indicative of the CSP to the input/output board of the controller  70 . The analog input signals received from the sensors  72  and  74  are converted to digital signals by means of an analog-to-digital converter associated with the input/outlet module. If the sensors  72  and  74  are of a type that generate and transmit digital signals, rather than analog signals, this conversion is not needed. 
     The controller  70  uses the sensed CST and CSP to determine the degrees of superheat in the refrigerant at the suction inlet (CSS) at the current operating conditions of the transport refrigerant unit  12 . In an embodiment, at block  206 , the controller  70  determines the degree of superheat through reference to a look-up table of suction saturated temperature (SST) versus saturated pressure (equal to CSP) for the refrigerant with which the refrigerant unit  12  is charged and then, at block  208 , subtracts that suction saturated temperature (SST) from the sensed compressor suction temperature (CST) to determine the degrees of superheat (CSS) at the suction inlet of the compressor. 
     Having determined the actual degrees of superheat of the refrigerant at the suction inlet (CSS) at the compressor suction inlet, at block  210 , the controller  70  compares the CSS to a threshold valve representative of the permissible lower limit for the degrees of superheat of the refrigerant at the suction inlet (LSS) to the compressor  20  and indicative of unacceptable frost build-up on one or more of the respective heat exchanger coils of the main evaporator  40  and the remote evaporators  50  and  60 . 
     If the actual degrees of superheat of the refrigerant at the suction inlet (CSS) is equal to or less than the permissible lower limit for the degrees of superheat of the refrigerant at the suction inlet (LSS), the controller  70 , at block  212 , interrupts the operation of the refrigeration unit  12  in the refrigeration mode and activates the electric defrost elements  80  and deactivates fan or fans  44 ,  54  and  64 , as appropriate, to initiate an electric defrost of the frosted heat exchanger coils of the respective evaporators  40 ,  50  and  60 . After a preprogrammed time delay following initiation of the electric defrost cycle, the extent of which is within the ordinary skill in the art to select for the particular evaporator heat exchanger coils in use, the controller  70  will deactivate the electric defrost elements  80  to terminate the electric defrost cycle and reactivate fan or fans  44 ,  54  and  64 , as appropriate, to return the transport refrigeration unit  12  to operation in the refrigeration mode, at block  214 . If, however, the actual degree of superheat of the refrigerant at the suction inlet (CSS) is greater than the permissible lower limit for the degrees of superheat of the refrigerant at the suction inlet (LSS), the controller  70  continues operation of the refrigeration unit  12  in the refrigeration mode. 
     In an embodiment, the step of initiating the defrost mode may comprise initiating a defrost of the main evaporator if product is being loaded into the first compartment and/or initiating a defrost of the remote evaporator if product is being loaded, into the second compartment. Referring now to  FIG. 4 , the controller  70  will carry out step  212  by first determining which of the doors  113 ,  115  and  117 , associated respectively with compartments  112 ,  114  and  116  and then initiating a defrost cycle of the main evaporator  40  if and only if, the door  113  to the forward compartment  112  is open, initiating, a defrost of the remote evaporator  50  if, and only if, the door  115  to the central compartment  114  is open, and initiating a defrost of the remote evaporator  60  if, and only if, the door  117  to the aft compartment  116  is open. To facilitate the determination of which of the compartment doors is open during loading of product, a door sensor (not shown) may be installed, as in conventional practice, in operative association with each of the compartment doors to sense whether its associated door is open or closed and transmit a signal to the controller  70  indicating when that the door is open. 
     While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.