Abstract:
A method of conditioning air in a vehicle load space. The method includes providing a refrigeration circuit including an evaporator, directing refrigerant through the refrigeration circuit, directing load space air across the evaporator, sensing a first condition based on one of a temperature and a pressure of the refrigerant in the refrigeration circuit upstream from the evaporator, determining a second condition based on one of a temperature and a pressure of the refrigerant in the evaporator, determining a difference between the first condition and the second condition, and initiating a defrost process of the evaporator when the difference is greater than a threshold

Description:
RELATED APPLICATIONS  
       [0001]     This application claims the benefit of prior-filed, co-pending U.S. Provisional Patent Application Ser. No. 60/671,716 filed on Apr. 15, 2005, the entire content of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to temperature control systems and, more particularly, to a transport temperature control system and to a method of operating the same.  
       SUMMARY  
       [0003]     Some embodiments of the present invention provide a temperature control system for conditioning air in a load space. The temperature control system can include a compressor, an evaporator coil, a condenser, a refrigeration circuit extending between the compressor, the evaporator coil, and the condenser, and a controller programmed to control operation of the temperature control system and to regulate flow of refrigerant through the refrigeration circuit. The controller can be programmed to operate the temperature control system in a defrost mode based on data received from one or more sensors distributed along the refrigeration circuit.  
         [0004]     In addition, some embodiments of the invention provide a method for controlling operation of a temperature control system having a refrigeration circuit extending between a compressor and an evaporator coil. The method can include the acts of sensing a pressure of refrigerant flowing through the refrigeration circuit, calculating a saturated suction pressure, providing an acceptable range of saturation suction pressure, comparing the saturated suction pressure to the acceptable range of saturation suction pressure, and initiating defrost when the saturation suction pressure is outside the acceptable range of saturation suction pressure.  
         [0005]     In some embodiments, the invention provides a method of conditioning air in a vehicle load space. The method can include the acts of providing a refrigeration circuit including an evaporator, directing refrigerant through the refrigeration circuit, and directing load space air across the evaporator. The method can also include the acts of sensing a first condition based on one of a temperature and a pressure of the refrigerant in the refrigeration circuit upstream from the evaporator, determining a second condition based on one of a temperature and a pressure of the refrigerant in the evaporator, and determining a difference between the first condition and the second condition. The method can include the act of initiating a defrost process of the evaporator when the difference is greater than a threshold.  
         [0006]     The invention can also provide a method of conditioning air in a vehicle load space, the vehicle having an opening communicating between the load space and atmosphere and a door supported on the vehicle adjacent to the opening. The method can include the acts of providing a refrigeration circuit including an evaporator, directing refrigerant through the refrigeration circuit, and directing load space air across the evaporator. The method can also include moving the door relative to the vehicle between an opened position, in which the door is moved away from the opening, and a closed position, in which the door extends across the opening, and sensing a first condition, the first condition being a function of one of a temperature and a pressure of the refrigerant in the refrigeration circuit away from the evaporator. Furthermore, the method can include the acts of measuring one of a temperature and a pressure of the refrigerant in the evaporator, using the one of the temperature and the pressure of the refrigerant in the evaporator to determine a second condition, determining a difference between the first condition and the second condition, and initiating a defrost process of the evaporator when the difference is greater than a threshold and the door is in the opened position.  
         [0007]     In some embodiments, the invention provides a system for conditioning air in a load space of a vehicle. The vehicle can have an opening communicating between the load space and atmosphere and a door movable between an opened position and a closed position. In the opened position, the air is movable through the opening between atmosphere and the load space. In the closed position, the door prevents movement of the air through the opening. The system can include a refrigeration circuit that further includes an evaporator housing refrigerant, first and second sensors, and a controller. The first sensor can be positioned along the refrigeration circuit to sense a first condition that is a function of one of a temperature and a pressure of the refrigerant in the refrigeration circuit away from the evaporator. The second sensor can be positioned along the evaporator to sense one of a temperature and a pressure of the refrigerant in the evaporator. The controller can convert the one of the temperature and the pressure of the refrigerant in the evaporator into a second condition, determine a difference between the first condition and the second condition when the door is in the opened position, and initiate a defrost process of the evaporator when the difference is greater than a threshold  
         [0008]     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a side view of a vehicle having a temperature control system according to an embodiment of the present invention.  
         [0010]      FIG. 2  is a schematic representation of the temperature control system shown in  FIG. 1 .  
         [0011]      FIGS. 3A-3D  are flowcharts illustrating a method of monitoring and controlling operation of the temperature control system shown in  FIG. 1 .  
         [0012]      FIG. 4  shows a first lookup and data table for calculating a refrigerant saturation temperature according to an embodiment of the present invention.  
         [0013]      FIG. 5  shows a second lookup and data table for calculating a refrigerant saturation temperature according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.  
         [0015]     As should also be apparent to one of ordinary skill in the art, the systems shown in the figures are models of actual systems. As noted, many of the modules and logic structures described are capable of being implemented in software executed by a microprocessor or a similar device or of being implemented in hardware using a variety of components including, for example, application specific integrated circuits (“ASICs”). Terms like “controller” may include or refer to hardware and/or software. Furthermore, throughout the specification capitalized terms are used. Such terms are used to conform to common practices and to help correlate the description with the coding examples, equations and/or drawings. However, no specific meaning is implied or should be inferred simply due to the use of capitalization. Thus, the claims should not be limited to the specific examples or terminology or to any specific hardware or software implementation or combination of software or hardware.  
         [0016]      FIG. 1  illustrates a temperature control system  10  according to some embodiments of the present invention. The temperature control system  10  is especially suitable for use in transport applications and can be mounted on a container, truck, trailer, and the like. The illustrated embodiment of  FIG. 1  shows the temperature control system  10  mounted on a trailer  14  having a load space  16 . The trailer  14  is pulled by a tractor  18 , as is understood by those skilled in the art. In other embodiments (not shown), the temperature control system  10  can be mounted on a storage container or another vehicle, such as, for example, a truck, a railcar, a van, etc.  
         [0017]     As used herein, the term “load space” includes any space to be temperature and/or humidity controlled, including transport and stationary applications for the preservation of food, beverages, plants, flowers, and other perishables and maintenance of a desired atmosphere for the shipment of industrial products. Also, as used herein, the term “refrigerant” includes any conventional refrigeration fluid, such as, for example, chlorofluorocarbons (CFCs), hydrocarbons, cryogens (e.g., CO 2 , and N 2 ), etc. In addition, as used herein, the term “refrigerant” refers to fluids commonly used for heating and defrosting purposes.  
         [0018]     The temperature control system  10  controls the temperature of the load space  16  to a desired temperature range adjacent to a predetermined set point temperature. More particularly, the temperature control system  10  maintains the temperature of the load space  16  within a range surrounding the set point temperature (e.g., ±5° C.). As shown in  FIG. 2 , the temperature control system  10  includes a closed refrigerant circuit or flow path  20 , which includes a refrigerant compressor  22  driven by a drive unit  24 . In the illustrated embodiment, the drive unit  24  includes an internal-combustion engine  26  and a stand-by electric motor  28 . The engine  26  and the motor  28 , when both are utilized, are connected to the compressor  22  by a clutch or coupling  30  which disengages the engine  26  while the motor  28  is in operation.  
         [0019]     In some embodiments, such as the illustrated embodiment of  FIG. 2 , the temperature control system  10  can include a dedicated engine  26 . In other embodiments, the vehicle engine can also or alternately supply power to the temperature control system  10  or elements of the temperature control system  10 .  
         [0020]     A discharge valve  34  and a discharge line  36  connect the compressor  22  to a three-way valve  38 . A discharge pressure transducer  40  is located along the discharge line  36 , upstream from the three-way valve  38  to measure the discharge pressure of the compressed refrigerant. The three-way valve  38  includes a first outlet port  42  and a second outlet port  44 .  
         [0021]     When the temperature control system  10  is operated in a COOLING mode, the three-way valve  38  is adjusted to direct refrigerant from the compressor  22  through the first outlet port  42  and along a first circuit or flow path (represented by arrows  48 ). When the temperature control system  10  is operated in HEATING and DEFROST modes, the three-way valve  28  is adjusted to direct refrigerant through the second outlet port  44  and along a second circuit or flow path (represented by arrows  50 ).  
         [0022]     The first flow path  48  extends from the compressor  22  through the first outlet port  42  of the three-way valve  38 , a condenser coil  52 , a one-way condenser check valve CV 1 , a receiver  56 , a liquid line  58 , a refrigerant drier  60 , a heat exchanger  62 , an expansion valve  64 , a refrigerant distributor  66 , an evaporator coil  68 , an electronic throttling valve  70 , a suction pressure transducer  72 , a second path  74  through the heat exchanger  62 , an accumulator  76 , a suction line  78 , and back to the compressor  22  through a suction port  80 . The expansion valve  64  is controlled by a thermal bulb  82  and an equalizer line  84 .  
         [0023]     The second flow path  50  can bypass a section of the refrigeration circuit  51 , including the condenser coil  52  and the expansion valve  64 , and can connect the hot gas output of compressor  22  to the refrigerant distributor  66  via a hot gas line  88  and a defrost pan heater  90 . The second flow path  50  continues from the refrigerant distributor  66  through the evaporator coil  68 , the throttling valve  70 , the suction pressure transducer  72 , the second path  74  through the heat exchanger  62 , and the accumulator  76  and back to the compressor  22  via the suction line  78  and the suction port  80 .  
         [0024]     A hot gas bypass valve  92  is disposed to inject hot gas into the hot gas line  88  during operation in the COOLING mode. A bypass or pressurizing line  96  connects the hot gas line  88  to the receiver  56  via check valves  94  to force refrigerant from the receiver  56  into the second flow path  50  during operation in the HEATING and DEFROST modes.  
         [0025]     Line  100  connects the three-way valve  38  to the low-pressure side of the compressor  22  via a normally closed pilot valve  102 . When the valve  102  is closed, the three-way valve  38  is biased (e.g., spring biased) to select the first outlet port  42  of the three-way valve  38 . When the evaporator coil  52  requires defrosting and when heating is required, valve  92  is energized and the low pressure side of the compressor  22  operates the three-way valve  38  to select the second outlet port  44  to begin operation in the HEATING mode and/or DEFROST modes.  
         [0026]     A condenser fan or blower  104  directs ambient air (represented by arrows  106 ) across the condenser coil  52 . Return air (represented by arrows  108 ) heated by contact with the condenser fan  104  is discharged to the atmosphere. An evaporator fan  110  draws load space air (represented by arrows  112 ) through an inlet  114  in a bulkhead or wall  116  and upwardly through conduit  118 . A return air temperature sensor  120  measures the temperature of air entering the inlet  114 . An evaporator coil temperature sensor  136  can be positioned adjacent to or on the evaporator coil  68  for recording the evaporator coil temperature. In other embodiments, the evaporator coil temperature sensor  136  can be positioned in other locations. In still other embodiments, other sensors, such as, for example, the return air temperature sensor  120  and/or the discharge air temperature sensor (described below)  126  can also or alternately be used to calculate the evaporator coil temperature.  
         [0027]     Discharge air (represented by arrow  122 ) is returned to the load space  14  via outlet  124 . Discharge air temperature sensor  126  is positioned adjacent to the outlet  124  and measures the discharge air temperature. During the DEFROST mode, a damper  128  is moved from an opened position (shown in  FIG. 2 ) toward a closed position (not shown) to close the discharge air path to the load space  14 .  
         [0028]     The temperature control system  10  also includes a controller  130  (e.g., a microprocessor). The controller  130  receives data from sensors, including the return air temperature sensor  124  and the discharge air temperature sensor  126 . Additionally, given temperature data and programmed parameters, the controller  130  determines whether cooling, heating, or defrosting is required by comparing the data collected by the sensors with the set point temperature.  
         [0029]      FIGS. 3A-3D  illustrate a method of monitoring and controlling operation of the temperature control system  10 . Particularly,  FIGS. 3A-3D  show a flow chart of an exemplary defrost process  200  that may be carried out by a combination of software, firmware, or hardware.  
         [0030]     Each time the temperature control system  10  is switched on (e.g., booted-up), the controller  130  initiates a startup routine. Among other things, the startup routine determines if the temperature control system  10  is operating correctly and searches for errors in the controller&#39;s programming and mechanical failures in the temperature control system  10 .  
         [0031]     In some embodiments, the controller  130  prompts the operator to enter load parameters. For example, the controller  130  can prompt the operator to enter the set point temperature (e.g., 0° C.), a low temperature limit (e.g., 5° C.), and a high temperature limit (e.g., 5° C.). In other embodiments, the controller  130  prompts the operator to enter the type of load (e.g., lettuce, bananas, flowers, ice cream, milk, etc.) and the anticipated travel time (e.g., one hour, two hours, etc.). In these embodiments, the controller  130  recalls previously programmed set point temperature, low temperature limit, and high temperature limit values for the selected load type.  
         [0032]     During startup, the controller  130  initiates temperature control operations. More particularly, the controller  130  receives temperature and/or pressure data from sensors, such as, for example, temperature sensors  120 ,  126 ,  136  and the discharge pressure transducer  40 . If the temperature data supplied to the controller  130  is above the high temperature limit, the controller  130  can be programmed to initiate operation in a high speed or HS COOLING mode or a low speed LS COOLING mode.  
         [0033]     During operation in the HS COOLING mode and the LS COOLING mode, the controller  130  is programmed to activate the compressor  22 , the condenser fan  104 , the evaporator fan  110 , the return air temperature sensor  120 , and the discharge air temperature sensor  126  and to direct refrigerant along the first flow path  48  to provide relatively low temperature refrigerant to the evaporator coil  68 .  
         [0034]     If the temperature data supplied to the controller  130  is below the low temperature limit, the controller  130  can be programmed to initiate operation in a HEATING mode. During operation in the HEATING mode, refrigerant is directed along the second flow path  50  to provide heat to the load space  16  as explained above.  
         [0035]     During operation in the HS COOLING mode and/or the LS COOLING mode, frost and/or ice can accumulate on the evaporator coil  68 . In applications in which relatively warm loads or loads that are not pre-cooled are loaded into the load space  16  and in applications in which a load space door is left open to the atmosphere for extended periods of time, relatively large quantities of frost and ice can accumulate on the evaporator coil  68  relatively rapidly. In these applications, the ice and frost can act as an insulator, reducing and/or preventing heat transfer between load space air  112  and the refrigerant flowing through the evaporator coil  68 .  
         [0036]     In some embodiments of the present invention, the controller  130  can be programmed to periodically operate in a DEFROST mode to remove and/or reduce the formation of frost and ice. In these embodiments, the temperature control system  10  can be operated in the DEFROST mode periodically (e.g., for ten minutes every hour) and/or when the controller  130  calculates that the ice and frost is reducing the heat transfer between the load space air  112  and the refrigerant flowing through the evaporator coil  68 .  
         [0037]     In some embodiments, the controller  130  can include a DEFROST CONTROL ALGORITHM or a defrost process  200  for initiating operation in the DEFROST mode. As noted, the algorithm determines if the system  10  is equipped with an electronic throttle valve (“ETV”) at step  204  as shown in  FIG. 3A . If the defrost process  200  determines at step  204  that the system  10  is not equipped with an electronic throttle valve (“No” at step  204 ), the process  200  is disabled or shut down at step  208  and an alarm may be activated to alert an operator. However, if the defrost process  200  determines at step  204  that the system  10  is equipped with an electronic throttle valve (“Yes” at step  204 ), the process  200  proceeds to determine if the process  200  has been selected or enabled at step  212 . If the process  200  has not been selected or enabled as determined (“No” at step  212 ), the process  200  is disabled or shut down at step  208  and an alarm may be activated to alert an operator.  
         [0038]     If the process  200  has been selected or enabled (“Yes” at step  212 ), the process  200  proceeds to clear and reset a number of parameters and timers (e.g., a frozen coil defrost timer) at step  216  such that these parameters and timers have respective predetermined initial values. Thereafter, the process  200  enters a start-up delay at step  220  to allow time for the system  10  to stabilize. In some embodiments, the start-up delay is about 5 minutes.  
         [0039]     At step  224 , the process  200  determines if the system  10  is running at a high speed (“HS”) cooling mode so as to determine if the system  10  or the compressor  22  is running at a predetermined capacity. If the system  10  is not in the high-speed cooling mode (“No” at step  224 ), the process  200  returns to step  216  to clear and reset the parameters and timers. Otherwise, if the process determines that the system  10  is running at the high-speed cooling mode at step  224  (“Yes” at step  224 ), the process  200  proceeds to determine if the system  10  is transitioning between different operating modes at step  228 .  
         [0040]     If it is determined at step  228  that the system  10  is transitioning between different operating modes (“Yes” at step  228 ), the process  200  returns to step  216  to clear and reset the parameters and timers. However, if the system  10  is not transitioning between different operating modes at step  228  (“No” at step  228 ), the process  200  checks to determine if there is a leak in the refrigerant circuit  20 , if a refrigerant pressure is unacceptable, or if the refrigerant charge is at an unacceptable level at step  232 . If there is a leak in the refrigerant circuit  20 , if the refrigerant pressure is unacceptable, or if the refrigerant charge is at an unacceptable level (“Yes” at step  232 ), the process  200  is disabled or shut down at step  208  and an alarm may be activated to alert an operator. Otherwise, if there is no leak in the refrigerant circuit  20 , the refrigerant pressure is acceptable, and the refrigerant charge is at an acceptable level (“No” at step  232 ), the process  200  proceeds to check for any sensor errors, detailed hereinafter.  
         [0041]     In the embodiment shown, the process  200  determines if the temperature sensors are functioning properly in the system  10 . For example, the process  200  checks to determine if the return air temperature (“RA TEMP”) sensor  120  is functioning properly at step  236 . In some embodiments, if the return air temperature sensor  120  is not functioning properly, an RA TEMP sensor alarm is generated. If an RA TEMP sensor alarm has been generated, or is active as determined at step  236  (“Yes” at step  236 ), the process  200  continues to check if other temperature sensors are functioning properly in the system  10 . For example, the process  200  checks to determine if the discharge air temperature (“DA TEMP”) sensor  126  is functioning properly at step  240 . If the DA TEMP sensor  126  is not functioning properly (“Yes” at step  240 ), the process  200  is disabled or shut down at step  208  and an alarm may be activated to alert an operator.  
         [0042]     If the RA TEMP sensor alarm is not active (“No” at step  236 ), the process  200  proceeds to compare the RA TEMP with a predetermined temperature at step  244  to determine if the load includes fresh products or frozen products. If the comparison between the RA TEMP and the predetermined temperature indicates frozen products (“No” at step  244 ), the process  200  returns to step  216  to clear and reset the parameters and timers. Particularly, the controller  130  is programmed to determine whether the RA TEMP corresponds to a “fresh” load or a “frozen” load. In applications in which the return air temperature is less than or equal to a FRFZ value (e.g., about 15° F./−2° C., or about 24° F./−4° C.), the controller  130  is programmed to exit the DEFROST CONTROL ALGORITHM or the process  200 . In applications in which the RA TEMP is greater than the FRFZ value, the controller  130  is programmed to continue operation in the DEFROST CONTROL ALGORITHM or the process  200 .  
         [0043]     However, if the comparison between the RA TEMP and the predetermined temperature indicates fresh products (“Yes” at step  244 ), the process  200  queries a counter or a timer to confirm that the process  200  has not been activated for a least a predetermined time threshold at step  248 . In some embodiments, the time threshold is about  30  minutes. If the time elapsed is less than the time threshold (“No” at step  248 ), the process  200  returns to step  216  to clear and reset the defrost timer. Otherwise, if the time elapsed is at least equal to the time threshold (“Yes” at step  248 ), the process  200  continues as follows. Particularly, if the controller  130  determines that the temperature control system  10  has been operated in the DEFROST mode within the minimum allowable time or the time threshold, the controller  130  can be programmed to exit the DEFROST CONTROL ALGORITHM or the process  208  to prevent the temperature control system  10  from repeatedly or continually operating in the DEFROST mode.  
         [0044]     Referring back to step  240 , if the DA TEMP sensor  126  is functioning properly (“No” at step  240 ), the process  200  continues to determine from the DA TEMP if the load includes fresh products at step  252 . Particularly, if the DA TEMP indicates that the load includes frozen products (“No” at step  252 ), the process  200  returns to step  216  to clear and reset the parameters and timers. However, if the DA TEMP indicates that the load includes fresh products (“Yes” at step  252 ), the process  200  enters step  248  as described earlier.  
         [0045]     After the process  200  has determined at step  248  that the time elapsed is at least equal to the time threshold (“Yes” at step  248 ), the process  200  determines if the suction pressure (“SP”) transducer or sensor is functioning properly at step  256 . If the SP sensor  72  is not functioning properly, a SP alarm is activated. If the SP alarm is activated (“Yes” at step  256 ), the process  200  is disabled or shut down at step  208 . Otherwise, if the SP sensor  72  is functioning properly (“No” at step  256 ), or if the SP alarm is not active, the process  200  determines if the coil sensor  136  is functioning properly at step  260 .  
         [0046]     If the coil sensor  136  is functioning properly, a coil sensor alarm is deactivated. Otherwise, if the coil sensor  136  does not function properly, the coil sensor alarm is activated. If the process  200  determines that the coil sensor alarm is active at step  260  (“Yes” at step  260 ), the process  200  is disabled at step  208 . Otherwise, if the coil sensor alarm is deactivated as determined at step  260  (“No” at step  260 ), the process  200  proceeds to determine if the electronic throttle valve (“ETV”)  70  is functioning properly at step  264 . If the electronic throttle valve  70  is not functioning properly (“Yes” at step  264 ), an ETV alarm is activated. If the ETV alarm is activated (“Yes” at step  264 ), the process  200  is disabled or shut down at step  208 . However, if the electronic throttle valve  70  is functioning properly (“No” at step  264 ), the process  200  continues as follows.  
         [0047]     As shown in  FIG. 3B , after the process  200  determines that all sensors and valves function properly, the process  200  compares a temperature (T COIL ) measured at the coil sensor  136  with a defrost initiation temperature (T DEF ) at step  304 . In some embodiments, the defrost initiation temperature T DEF  is about 45° F. or about 7° C. If the process  200  determines that T COIL  is greater than T DEF  (“No” at step  304 ), the process  200  returns to step  216  of FIG.  3 A. Otherwise, if the coil temperature T COIL  is less than or equal to the defrost initiation temperature T DEF  (“Yes” at step  304 ), the process  200  measures a position of the ETV  70  against a predetermined value at step  308 . In some embodiments, the predetermined position is a fully opened position. If the ETV position is less than fully opened as determined at step  308  (“Yes” at step  308 ), the process  200  returns to step  216  of  FIG. 3A . If the ETV position is fully opened as determined at step  308  (“No” at step  308 ), the process  200  compares the suction pressure (“SP”) measured at the SP sensor  72  with a predetermined pressure value P 1  (e.g., 100 PSIG) at step  312 .  
         [0048]     If the suction pressure SP is greater than the predetermined pressure value P 1  (“Yes” at step  312 ), the process  200  enters step  316  to set a saturation suction temperature T SAT  to a predetermined temperature value. In some embodiments, T SAT  is set to be about 50° F. or 10° C. However, other predetermined temperature values can also be used at step  316 . After the process  200  has set a saturation suction temperature T SAT  at step  316 , the process  200  returns to step  216  of  FIG. 3A . If the suction temperature SP is not greater than the predetermined pressure value P 1  as determined at step  312  (“Yes” at step  312 ), the process  200  continues as follows.  
         [0049]     If the suction pressure SP is less than the predetermined pressure value P 1  (“No” at step  312 ), the process  200  determines if the suction pressure SP falls within a range as shown in  FIG. 3C . If suction pressure SP is greater than or equal to a high end predetermined pressure value P 2  (“Yes” at step  320 ), the process  200  converts the suction pressure SP into a saturation suction temperature T SAT  using a conversion process at step  324 .  
         [0050]     In the illustrated embodiment, the saturation pressure SP is converted into a saturation suction temperature T SAT  using a first curve-fit conversion formula at step  324 . Exemplary curve-fit formulas include, but are not limited to, non-parametric fitting using splines and interpolants, linear parametric fitting models, such as, straight line approximation, and non-linear parametric fitting models, such as, polynomials derived by curve-fitting techniques, such as, the least square method, weighted least square method, autoregressive moving average, interpolation, extrapolation, differentiation, and integration of fits. In the embodiment shown, the first curve-fit formula is a second order polynomial of the form a 1 x 2 +b 1 x+c 1 . In some embodiments, the coefficients a 1 , b 1 , and c 1  are −0.0045, 1.3076, and −39.891, respectively. In other embodiments, the process  200  can also convert the suction pressure SP into a saturation suction temperature T SAT  with other pressure-to-temperature formulas or conversion methods at step  324 .  
         [0051]     If the suction pressure SP is less than the high end predetermined pressure value P 2  (“No” at step  320 ), the process  200  compares the suction pressure SP with a low end predetermined pressure value P 3  (e.g., about −10 PSIG) at step  328 . If the suction pressure SP is not less than the predetermined pressure value P 3 , (“No” at step  328 , the process  200  converts the suction pressure SP into a saturation suction temperature T SAT  using a second curve-fit formula as the conversion process at step  332 . However, if the suction pressure SP is less than P 3  as determined at step  328  (“Yes” at step  328 ), the process  200  sets the saturation suction temperature T SAT  to a predetermined temperature value (e.g., about −90° F. or about −68° C.) step  336 . In the illustrated embodiment, the second curve-fit formula is a second order polynomial of the form a 2 x 2 +b 2 x+c 2 . In some embodiments, the coefficients a 2 , b 2 , and c 2  are −0.0718, 2.8678, and −51.895, respectively. Furthermore, the process  200  can also convert the suction pressure into the saturation suction temperature with other pressure-to-temperature formulas or conversion methods at step  332 .  
         [0052]     After the process  200  has determined the saturation suction temperature T SAT  (at steps  324 ,  332 , or  336 ), the process  200  proceeds to compare the saturation suction temperature T SAT  with the coil temperature T COIL  at step  340 . If saturation suction temperature T SAT  is greater than or equal to the coil temperature T COIL  (“No” at step  340 ), the process  200  clears the defrost timer at step  344 . However, if the saturation suction temperature T SAT  is less the coil temperature T COIL  (“Yes” at step  340 ), the process  200  determines an absolute temperature difference T DIFF  between the coil temperature T COIL  and the saturation suction temperature T SAT  at step  348 . In the illustrated embodiment, the absolute temperature difference T DIFF  between the coil temperature T COIL  and the saturation suction temperature T SAT  is also referred to as an evaporator internal temperature difference (“EITD”). As such, the value of the absolute temperature difference T DIFF  indicates a temperature discrepancy between the coil temperature T COIL  (which is a sensed or measured temperature) and the saturation suction temperature T SAT  (which is a calculated or computed temperature, and approximates what the temperature at the coil  68  should be).  
         [0053]     After the temperature difference T DIFF  has been determined at step  348 , the process  200  compares the temperature difference T DIFF  with a predetermined defrost limit T LIMIT  at step  352 . In some embodiments, the predetermined defrost limit T LIMIT  is stored in the memory and has a default value of about 100° F. or 40° C. for a cold start, and has a range from about 126° F. or 70° C. to about 18° F. or 110° C. If the temperature difference T DIFF  is not greater than the predetermined defrost limit T LIMIT  (“No” at step  352 ), the process  200  clears the defrost timer at step  344 . In such a case, the process  200  has determined that the coil  68  has not been frozen and is not frosted. Otherwise, if the temperature difference T DIFF  is greater than the defrost temperature limit T LIMIT  (“Yes” at step  352 ), the process  200  has determined that the coil  68  is frozen and/or frosted. As such, the process  200  proceeds to determine if the defrost timer is active at step  356  to prepare the system  10  to defrost the coil  68 . In some embodiments, the defrost timer has a limit of about 60 seconds.  
         [0054]     If the defrost timer is active (“Yes” at step  356 ), the coil  68  is being defrosted, and the process  200  proceeds to increment the defrost timer at step  360 . Thereafter, the process  200  proceeds to determine if the defrost timer has expired at step  364 . If the defrost timer has not expired (“No” at step  364 ), the process  200  returns to step  224  of  FIG. 3A . Otherwise, if the defrost timer has expired (“Yes” at step  364 ), the process  200  initiates or continues to defrost the coil  68  at step  368 . After the defrost timer has been cleared at step  344 , or if the defrost timer is inactive (“No” at step  356 ) and after the process  200  has proceeded to start or activate the defrost timer at step  372 , the process  200  returns to step  224  of  FIG. 3A .  
         [0055]     After the process  200  has initiated or continued to defrost the coil  68  at step  368 , the process  200  determines if the defrosting of the coil  68  has been terminated at step  376 . If the defrosting of the coil  68  has not been terminated (“No” at step  376 ), the process  200  proceeds to store a position of the electronic throttle valve  70  at step  380  and repeats step  368 . However, if the defrosting of the coil  68  has been terminated (“Yes” at step  376 ), the process  200  proceeds to determine the position of the electronic throttle valve  70  to determine if the position of the electronic throttle valve  70  is more than fully opened at step  384 .  
         [0056]     Particularly, once the controller  130  operates the temperature control system  10  in the DEFROST mode, the controller  130  resumes operation in the HS COOLING mode, the LS COOLING mode, or the HEATING mode. If the position of electronic throttle valve  70  is more than fully opened (“Yes” at step  384 ), the process  200  resets the refrigerant charge flag at step  388 , and returns to step  224  of  FIG. 3A . If, however, the position of the electronic throttle valve  70  is less than fully opened (“No” at step  384 ), the process  200  sets the refrigerant charge at step  392  to indicate the level of the refrigerant charge is acceptable, and repeats step  224  of  FIG. 3A . In such cases, there is no refrigerant leak in the system  10 .  
         [0057]     As shown in  FIG. 3D , after the process  200  has been disabled at step  208  of  FIG. 3A , the process  200  determines if the system  10  is equipped with an electronic throttle valve at step  404 . If the system  10  is not equipped with an electronic throttle valve (“No” at step  404 ), the process  200  returns to step  212  of  FIG. 3A . However, if the system  10  is equipped with an ETV (“Yes” at step  404 ), the process  200  determines if the process  200  has been disabled via a memory or the controller  130  at step  408 . If the process  200  determines at step  408  that the algorithm or the defrost process has been disabled via memory or the processor (“Yes” at step  408 ), the process  200  returns to step  212  of  FIG. 3A .  
         [0058]     If the algorithm has not been disabled via the memory or the processor as determined at step  408  (“No” at step  408 ), the process  200  determines if the coil sensor alarm has been set active at step  412 . If the coil sensor alarm has been set active as determined at step  412  (“Yes” at step  412 ), the process  200  returns to step  212  of  FIG. 3A . However, if the coil sensor alarm has not been set active as determined at step  412  (“No” at step  412 ), the process  200  determines if the suction pressure alarm has been set active at step  416 . If the SP alarm has been set active (“Yes” at step  416 ), the process  200  returns to step  212  of  FIG. 3A . Otherwise, if the suction pressure alarm has not been set active as determined at step  416  (“No” at step  416 ), the process  200  determines if the RA TEMP sensor alarm has been set active at step  420 . If the RA TEMP alarm is active as determined at step  420  (“Yes” at step  420 ), the process  200  determines if the DA TEMP sensor alarm has been set active at step  424 . If the RA TEMP sensor alarm has not been set active or if the DA TEMP sensor alarm has not been set active, the process  200  enables the defrost process at step  428  and returns to step  212  of  FIG. 3A . However, if the DA TEMP sensor alarm is active, the process  200  returns to step  212  of  FIG. 3A .  
         [0059]     Other features, actions, steps, and procedures can occur or be directed to occur during operation of the temperature control system  10  and during operation of the DEFROST CONTROL ALGORITHM, which are not described in detail above but are illustrated  FIGS. 3A-3D .  
         [0060]      FIG. 4  shows a first lookup and data table or plot  400  for calculating a refrigerant saturation suction temperature from the measured pressure. The measured or sensed suction pressure values at the suction pressure sensor  72  are measured along an x-axis  404 , and the calculated saturation suction temperature values are measured along a y-axis  408 . Particularly, the measured saturation suction temperature values are shown as curve  412 , and the calculated temperature values determined from the first curve-fitting formula are shown as curve  416 . Furthermore, the pressure values measured range from about −10 PSIG to about 16 PSIG.  
         [0061]     Similarly,  FIG. 5  shows a second lookup and data table or plot  500  for calculating a refrigerant saturation suction temperature from the measured pressure. The measured or sensed suction pressure values at the suction pressure sensor  72  are measured along a second x-axis  504 , and the calculated saturation suction temperature values are measured along a second y-axis  508 . Particularly, the measured saturation suction temperature values are shown as curve  512 , and the calculated temperature values determined from the second curve-fit formula are shown as curve  516 . Furthermore, the pressure values measured range from about 16 PSIG to about 100 PSIG.  
         [0062]     Various features and advantages of the invention are set forth in the following claims.