Patent Publication Number: US-2018051921-A1

Title: Adaptively controlled defrost cycle time for an aircraft vapor cycle refrigeration system

Description:
BACKGROUND 
     The present disclosure relates generally to vapor cycle refrigeration systems, and in particular to defrost operations associated with vapor cycle refrigeration systems. 
     Vapor cycle refrigeration systems, such as those implemented onboard aircraft, typically include an evaporator that converts a liquid refrigerant to a gas, thereby absorbing heat and providing cooling to an airflow. In some applications, the refrigerant employed by the evaporator operates at a temperature below the freezing point of water. As a result, the evaporator through which the refrigerant flows may cause water vapor in the air being cooled to condense and freeze at it passes through the evaporator. Ice buildup within the evaporator both reduces the cooling efficiency of the evaporator and restricts the flow of air through the evaporator. 
     To prevent the accumulation of ice and/or melt an existing ice buildup, periodic defrost operations of the vapor cycle refrigeration system are often initiated (e.g., on a periodic schedule). Typically, the defrost operations include stopping (or decreasing) the flow of refrigerant through the refrigeration system and allowing unconditioned air to flow through the evaporator to melt the ice. However, initiating the defrost operations too often or for too long can degrade the cooling performance of the system, as conditioned air is not provided by the system during the defrost operations. Similarly, initiating the defrost operations too infrequently or for too little time can also degrade system performance by inadequately removing (or preventing) the accumulation of ice and decreasing the cooling efficiency of the evaporator. 
     SUMMARY 
     In one example, a method includes (a) initiating a cooling cycle of a vapor cycle refrigeration system, (b) sensing a pressure of refrigerant at an inlet of a compressor of the vapor cycle refrigeration system during operation of the cooling cycle, and (c) initiating a defrost cycle of the vapor cycle refrigeration system in response to the sensed pressure being less than a threshold pressure. The method further includes (d) terminating the defrost cycle upon expiration of a defrost cycle time that is based upon a time duration of the cooling cycle, and (e) repeating steps (a)-(d). 
     In another example, a vapor cycle refrigeration system of an aircraft includes an evaporator, a compressor, a pressure sensor, and a controller device that includes at least one processor and computer-readable memory. The evaporator is configured to receive refrigerant and provide cooling to an airflow by evaporating the refrigerant. The compressor is configured to compress the refrigerant and produce flow of the refrigerant through the vapor cycle refrigeration system. The pressure sensor is disposed to sense pressure of the refrigerant at an inlet of the compressor. The controller device is operatively coupled to the pressure sensor and to the compressor. The computer-readable memory of the controller device is encoded with instructions that, when executed by the at least one processor, cause the controller device to (a) initiate a cooling cycle of the vapor cycle refrigeration system by increasing a speed of the compressor, (b) receive an indication of sensed pressure of the refrigerant at the inlet of the compressor from the pressure sensor, and (c) initiate a defrost cycle of the vapor cycle refrigeration system in response to the received indication of the sensed pressure of the refrigerant being less than a threshold pressure. The computer-readable memory is further encoded with instructions that, when executed by the at least one processor, cause the controller device to (d) terminate the defrost cycle upon expiration of a defrost cycle time that is based upon a time duration of the cooling cycle, and (e) repeat steps (a)-(d). 
     In another example, a controller device for a vapor cycle refrigeration system of an aircraft includes at least one processor and computer-readable memory. The computer-readable memory is encoded with instructions that, when executed by the at least one processor, cause the controller device to (a) initiate a cooling cycle of the vapor cycle refrigeration system, (b) receive an indication of a sensed pressure of refrigerant at an inlet of a compressor of the vapor cycle refrigeration system, and (c) initiate a defrost cycle of the vapor cycle refrigeration system in response to the indication of the sensed pressure being less than a threshold pressure. The computer-readable memory is further encoded with instructions that, when executed by the at least one processor, cause the controller device to (d) determine a time duration of the cooling cycle, (e) adjust a defrost cycle time based on the time duration of the cooling cycle to determine an adjusted defrost cycle time, (f) terminate the defrost cycle upon expiration of the adjusted defrost cycle time, and (g) repeat steps (a)-(f). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram illustrating an example aircraft vapor cycle refrigeration system that adaptively controls a defrost cycle time based on a time duration of a cooling cycle. 
         FIGS. 2A and 2B  are flow diagrams that together illustrate example operations to adaptively control a defrost cycle time based on a time duration of a cooling cycle of a vapor cycle refrigeration system. 
     
    
    
     DETAILED DESCRIPTION 
     As described herein, a duration of a defrost cycle of a vapor cycle refrigeration system is adaptively controlled. A pressure of refrigerant at an inlet of a system compressor is sensed. Defrost operations of the vapor cycle refrigeration system are initiated in response to the sensed pressure being less than a threshold pressure. Accordingly, rather than require multiple sensors to measure, for example, temperature and/or differential pressure of refrigerant or airflow at an evaporator, a system implementing techniques of this disclosure can initiate defrost operations to prevent and/or mitigate ice buildup within the evaporator based on a single pressure sensor at an inlet of the compressor. The use of a single pressure sensor can significantly reduce cost and complexity while increasing reliability of the system. Moreover, a duration of the defrost cycle is iteratively adapted based on a time duration of a preceding cooling cycle and/or a number of consecutive successful and unsuccessful operating cycles. As such, the techniques described herein adaptively control (e.g., increase and decrease) the duration of the defrost cycle to account for varying environmental and system conditions, such as changing temperature and humidity of the operational environment, to better approximate an optimal defrost cycle time duration. 
       FIG. 1  is a schematic block diagram illustrating aircraft vapor cycle refrigeration system  10  including controller  12  that adaptively controls a defrost cycle time based on a time duration of a cooling cycle of vapor cycle refrigeration system  10 . As illustrated in  FIG. 1 , vapor cycle refrigeration system  10  further includes condenser  14 , expansion orifice  16 , flash tank  18 , throttling valve  19 , evaporator  20 , compressor  22 , heat sink inlet temperature sensor T HSI , compressor suction temperature sensor T CS , compressor suction pressure sensor P CS , compressor discharge temperature sensor T CD , compressor discharge pressure sensor P CD , compressor speed sensor N, and compressor motor current sensor I. The arrowed lines extending between condenser  14 , expansion orifice  16 , flash tank  18 , throttling valve  19 , evaporator  20 , and compressor  22  indicate a flow and direction of refrigerant circulated in vapor cycle refrigeration system  12 . Vapor cycle refrigeration system  10  can be part of, e.g., an air conditioning pack of a cooling system of an aircraft. 
     As illustrated in  FIG. 1 , refrigerant is supplied to compressor  22  in vapor form from both flash tank  18  and evaporator  20 . Compressor  22  is driven by a compressor motor (not illustrated), a speed of which is controlled by controller  12  to compress the refrigerant to a higher pressure and supply the compressed refrigerant in vapor form to condenser  14 . Operation of compressor  22  to compress the refrigerant drives a flow of refrigerant through vapor cycle refrigeration system  10 . A greater operational speed of compressor  22  (e.g., a greater rotational speed of a shaft of compressor  22 ) increases the compression generated by compressor  22  and the corresponding flow of refrigerant. Similarly, a lesser operational speed of compressor  22  decreases the compression generated by compressor  22  and the corresponding flow of refrigerant. 
     Condenser  14  condenses the compressed vapor refrigerant received from compressor  22  to liquid form using cooling liquid and/or gaseous flow supplied through the heat sink inlet. Heat from the compressed refrigerant is transferred from the refrigerant to the cooling liquid and/or gaseous flow supplied to condenser  14  through the heat sink inlet and is carried away from vapor cycle refrigeration system  10  via the heat sink outlet. The condensed, liquid refrigerant is supplied from condenser  14  to expansion orifice  16 . As the liquid refrigerant passes through expansion orifice  16 , a rapid pressure reduction of the liquid refrigerant occurs causing an evaporation of a portion of the refrigerant and resulting in two-phase refrigerant (i.e., liquid phase and vapor phase) that is supplied to flash tank  18  where phase separation occurs through, e.g., gravity separation. Expansion orifice  16  can be a fixed orifice configured to cause the pressure reduction in the refrigerant. In some examples, expansion orifice  16  can be implemented as a valve, the position of which is controlled via, e.g., controller  12  to cause and/or control the pressure reduction of the refrigerant. 
     Vapor-form refrigerant is supplied from flash tank  18  to compressor  22 . Liquid refrigerant, cooled by both the heat transfer in condenser  14  and the rapid pressure reduction in expansion orifice  16 , is supplied to throttling valve  19 . A position of throttling valve  19 , sometimes referred to as an expansion valve, is controlled by a motor (not illustrated) via commands from controller  12  to cause an additional rapid pressure reduction of the liquid refrigerants as it passes through throttling valve  19 , thereby causing an evaporation of a portion of the refrigerant (having a further cooling effect on the refrigerant) and resulting in a two-phase refrigerant (i.e., liquid phase and vapor phase) that is supplied to evaporator  20 . 
     Evaporator  20  cools inlet air as it is passed through evaporator  20  through an evaporation process in which the liquid refrigerant is converted (i.e., evaporated) from the liquid state to a mostly or entirely gaseous state. The evaporation process absorbs heat from the inlet air, thereby cooling the inlet air and providing conditioned air for, e.g., a cabin, galley, or other air conditioning system. The evaporated refrigerant is supplied from evaporator  20  to compressor  22 . As such, vapor cycle refrigeration system  10  provides a closed-loop cycle of refrigerant in which heat is transferred from an inlet air supply to the refrigerant to provide cooled, conditioned air, and rejected from vapor cycle refrigeration system  10  via the heat sink inlet and the heat sink outlet at condenser  14 . 
     As inlet air passes through evaporator  20  and is cooled by the evaporation process of the refrigerant to provide the conditioned air, water vapor present in the inlet air can freeze, thereby causing ice to accumulate within evaporator  20 . The accumulation of ice within evaporator  20  can degrade the cooling efficiency of evaporator  20  by hindering the transfer of heat from the inlet air to the refrigerant. This reduced heat transfer efficiency in evaporator  20  caused by the accumulation of ice can further reduce the refrigerant temperature. Due to the two-phase refrigerant heat transfer that occurs in evaporator  20  as refrigerant is evaporated by the transfer of heat from the inlet air, the pressure of the refrigerant within evaporator  20  is correlated with the refrigerant temperature within evaporator  20 . That is, as the temperature of refrigerant within evaporator  20  decreases, the pressure of the refrigerant within evaporator  20  also decreases. Similarly, as the temperature of refrigerant within evaporator  20  increases, the pressure of the refrigerant within evaporator  20  also increases. Accordingly, controller  12  implementing techniques of this disclosure monitors a pressure of the refrigerant downstream of evaporator  20  via compressor suction pressure sensor P CS . Controller  12  initiates a defrost cycle of vapor cycle refrigeration system  10  based on the measured refrigerant pressure from compressor suction pressure sensor P CS  and adaptively controls (e.g., adjusts) a time duration of the defrost cycle based on a time duration of a preceding cooling cycle, as is further described below. 
     Controller  12  can be any electronic device that is operationally coupled (e.g., electrically and/or communicatively coupled) to components of vapor cycle refrigeration system  10  to control real-time operation of the components of the system and to receive inputs from various sensors positioned throughout vapor cycle refrigeration system  10 . As illustrated in  FIG. 1 , controller  12  is operationally connected to receive inputs from compressor suction temperature sensor T CS , compressor discharge temperature sensor T CD , heat sink inlet temperature sensor T HSI , compressor suction pressure sensor P CS , compressor discharge pressure sensor P CD , compressor speed sensor N, and compressor motor current sensor I. In addition, controller  12  is operationally connected to control components of vapor cycle refrigeration system  10 , such as condenser  14 , throttling valve  19 , evaporator  20 , and compressor  22 . 
     Controller  12  can include one or more processors and computer-readable memory encoded with instructions that, when executed by the one or more processors, cause controller  12  to operate in accordance with techniques described herein. Examples of the one or more processors include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other equivalent discrete or integrated logic circuitry. Computer-readable memory of controller  12  can be configured to store information with controller  12  during operation. The computer-readable memory can be described, in some examples, as computer-readable storage media. In some examples, a computer-readable storage medium can include a non-transitory medium. The term “non-transitory” can indicate that the storage medium is not embodied in a carrier wave or a propagated signal. In certain examples, a non-transitory storage medium can store data that can, over time, change (e.g., in RAM or cache). Computer-readable memory of controller  12  can include volatile and non-volatile memories. Examples of volatile memories can include random access memories (RAM), dynamic random access memories (DRAM), static random access memories (SRAM), and other forms of volatile memories Examples of non-volatile memories can include magnetic hard discs, optical discs, floppy discs, flash memories, or forms of electrically programmable memories (EPROM) or electrically erasable and programmable (EEPROM) memories. 
     Compressor suction temperature sensor T CS  and compressor suction pressure sensor P CS  are positioned at or near an upstream inlet of compressor  22  to measure a temperature and pressure of refrigerant entering the upstream (or suction) inlet of compressor  22 . Compressor discharge temperature sensor T CD  and compressor discharge pressure sensor P CD  are positioned at or near a downstream outlet of compressor  22  to measure a temperature and pressure of refrigerant exiting the downstream (or discharge) outlet of compressor  22 . Compressor speed sensor N and compressor current sensor I are integral to or positioned adjacent to compressor  22 . Compressor speed sensor N is configured to sense an operational speed of compressor  22 , such as a rotational speed of a shaft of compressor  22 . Compressor current sensor I is configured to sense an amount of electrical current draw of compressor  22  from a power source integral to or remote from vapor cycle refrigeration system  10 . Heat sink inlet temperature sensor T HSI  is positioned at or near the heat sink inlet of condenser  14  to sense a temperature of the cooling liquid and/or gaseous flow through the heat sink inlet. 
     In operation, controller  12  initiates a cooling cycle of vapor cycle refrigeration system  10  by increasing an operational speed of compressor  22  to start the flow of refrigerant through vapor cycle refrigeration system  10 . During the cooling cycle, refrigerant flowing through evaporator  20  absorbs heat from inlet air to provide cooled, conditioned air to a consuming system, such as an aircraft cabin, galley, or other cooling system. Controller  12  monitors the pressure sensed via compressor suction pressure sensor P CS  and initiates a defrost cycle in response to the sensed pressure being less than a threshold pressure, such as a defined minimum pressure stored in computer-readable memory of controller  12  that is less than normal operational refrigerant pressures of vapor cycle refrigeration system  10 . A value of the threshold pressure can based on an operating state (or flight condition) of an aircraft that includes vapor cycle refrigeration system  10 , such as an on-ground operating state, an in-flight operating state, or other operating states. Controller  12  decreases the operational speed of compressor  22  (or halts compressor  22 ) to decrease or stop the flow of refrigerant during the defrost cycle. Accordingly, during the defrost cycle, unconditioned inlet air passes through evaporator  20  and warms internal components of evaporator  20  to melt accumulated ice that could degrade the efficiency of heat transfer from the inlet air to refrigerant within evaporator  20 . In addition, controller  12  adaptively controls the duration of the defrost cycle based on a time duration of a preceding cooling cycle. For instance, controller  12  can increase a duration of the defrost cycle in response to determining that the time duration of a preceding cooling cycle is less than a threshold time duration, such as a desired minimum continuous runtime of vapor cycle refrigeration system  10  in a cooling cycle without entering a defrost cycle (e.g., ten minutes, fifteen minutes, thirty minutes, one hour, or other minimum continuous cooling cycle runtimes). Controller  12  can decrease the duration of the defrost cycle in response to determining that the time duration of the preceding cooling cycle is greater than the threshold time duration. The defrost cycle time can, in certain examples, be based on an operating state (or flight condition) of an aircraft that includes vapor cycle refrigeration system  10 , such as an on-ground operating state, an in-flight operating state, or other operating states. In some examples, controller  12  can compare the duration of the defrost cycle to a maximum defrost cycle time duration to differentiate between an inadequate defrost cycle duration and, e.g., a system fault condition. For instance, if the desired minimum continuous cooling cycle runtime is not achieved when the defrost cycle time is greater than or equal to the maximum defrost cycle time duration, controller  12  can identify a fault condition of vapor cycle refrigeration system  10  due to, e.g., low refrigerant charge, a fault condition of throttling valve  19 , or other fault conditions. Controller  12  can output an indication identifying that the fault condition is present for, e.g., annunciation at a display device or other consuming system. 
     As such, controller  12  controls a time duration of defrost operations to adapt to changing environmental and system conditions, such as temperature and/or moisture content of the inlet air. The adaptive control can help to ensure that a duration of the defrost cycle is sufficient to prevent accumulation of ice within evaporator  20  that could prevent continuous runtime in a cooling cycle that is less than a prescribed minimum time duration while preventing excessive defrosting that could degrade the overall cooling performance of vapor cycle refrigeration system  10 . 
       FIGS. 2A and 2B  are flow diagrams illustrating example operations to adaptively control a defrost cycle time based on a time duration of a cooling cycle of a vapor cycle refrigeration system. For purposes of clarity and ease of discussion, the example operations of  FIGS. 2A and 2B  are described together within the context of vapor cycle refrigeration system  10  of  FIG. 1 . 
     A cooling cycle of vapor cycle refrigeration system  10  is initiated (Step  24 ). For example, controller  12  can command operation of compressor  22  to start a flow of refrigerant through vapor cycle refrigeration system  10 . Vapor cycle refrigeration system  10  operates in a cooling cycle mode (Step  26 ). During the cooling cycle mode, operation of compressor  22  causes refrigerant to flow through evaporator  20  where heat from inlet air is absorbed by the refrigerant, thereby producing a cooled, conditioned airflow for one or more consuming systems. 
     Pressure of the refrigerant is sensed at an inlet of compressor  22  during operation of the cooling cycle (Step  28 ). For example, compressor suction pressure sensor P CS  senses pressure of refrigerant downstream of evaporator  20  at or near an inlet of compressor  22  and transmits the sensed pressure to controller  12  via one or more electrical and/or communicative connections. Controller  12  determines whether the sensed pressure at the inlet of compressor  22  is less than a threshold pressure (Step  30 ). For example, controller  12  can store a predetermined minimum pressure threshold that is less than normal operational pressures of refrigerant within vapor cycle refrigeration system  10 . The threshold pressure can, in certain examples, be based on an operating state (or flight condition) of an aircraft that includes vapor cycle refrigeration system  10 , such as an on-ground operating state, an in-flight operating state, or other operating states. A pressure of refrigerant sensed by compressor suction pressure sensor P CS  that is less than the threshold pressure can correspond to a temperature of refrigerant within evaporator  20  that is indicative of the accumulation of ice within evaporator  20 . In response to determining that the sensed pressure at the inlet of compressor  22  is not less than the threshold pressure (“NO” branch of Step  30 ), controller  12  continues to cause vapor cycle refrigeration system  10  to operate in the cooling cycle mode (Step  26 ). 
     In response to determining that the sensed pressure at the inlet of compressor  22  is less than the threshold pressure (“YES” branch of Step  30 ), controller  12  determines whether the duration of the cooling cycle of vapor cycle refrigeration system  10  is greater than a threshold time duration (Step  32 ). For example, controller  12  can store a threshold time duration corresponding to a desired minimum continuous runtime of vapor cycle refrigeration system  10  in a cooling cycle without entering a defrost cycle (e.g., fifteen minutes or other threshold durations of time). The threshold time duration can, in some examples, be based on an operating state (or flight condition) of an aircraft that includes vapor cycle refrigeration system  10 , such as an on-ground operating state, an in-flight operating state, or other operating states. Controller  12  can measure a duration of the preceding cooling cycle, such as by determining a difference between a time when the immediately-preceding cooling cycle was initiated (e.g., at Step  24 ) and a time when the sensed pressure at the inlet of compressor  22  was determined to be below the threshold pressure (e.g., at the “YES” branch of Step  30 ). 
     In response to determining that the duration of the cooling cycle of vapor cycle refrigeration system  10  is not greater than the threshold time duration (“NO” branch of Step  32 ), controller  12  determines whether the time duration of the defrost cycle is greater than (or equal to) a maximum defrost cycle time duration stored at, e.g., computer-readable memory of controller  12  (Step  34 ). In response to determining that the time duration of the defrost cycle is greater than (or equal to) the maximum defrost cycle time duration (“YES” branch of Step  34 ), controller  12  exits operation of the cooling and defrost cycles of vapor cycle refrigeration system  10  (e.g., shuts down vapor cycle refrigeration system  10  by decreasing an operational speed of or stopping operation of compressor  22 ) and outputs an indication of a fault condition of vapor cycle refrigeration system  10  (Step  36 ). That is, a defrost cycle time duration that is greater than the maximum defrost cycle time duration can indicate a fault condition of vapor cycle refrigeration system due to, e.g., low refrigerant charge, a fault condition of throttling valve  19 , or other fault conditions. As such, controller  12  can identify and output an indication of the presence of a fault condition of vapor cycle refrigeration system  10  based at least in part on the time duration of a preceding cooling cycle. 
     In response to determining that the defrost cycle time duration is not greater than (or equal to) the maximum defrost cycle time duration (“NO” branch of Step  34 ), controller  12  increases the time duration of the defrost cycle (Step  38 ). For example, controller  12  can store (e.g., in computer-readable memory) an initial baseline defrost cycle time duration, such as three minutes, five minutes, or other time durations. Controller  12  can increase the defrost cycle time duration from the baseline duration, such as by an amount of thirty seconds, one minute, or other increased time durations to arrive at an adjusted defrost cycle time duration. Controller  12  can increase the defrost cycle time duration from the baseline duration during, e.g., a first iteration of the operations of  FIGS. 2A and 2B , and can increase the defrost cycle time duration from the adjusted defrost cycle time duration during subsequent iterations of the operations of  FIGS. 2A and 2B . In some examples, controller  12  can increase the defrost cycle time by an amount that is proportional to a difference between the threshold time duration and the time duration of the preceding cooling cycle. 
     In response to determining that the duration of the cooling cycle of vapor cycle refrigeration system  10  is greater than the threshold time duration (“YES” branch of Step  32 ), controller  12  determines whether the defrost cycle time duration is less than (or equal to) a minimum defrost cycle time duration stored at, e.g., computer-readable memory of controller  12  (Step  40 ). In response to determining that the defrost cycle time duration is less than (or equal to) the minimum defrost cycle time duration (“YES” branch of step  40 ), controller  12  continues to operate the defrost cycle using the minimum defrost cycle time duration by, e.g., setting the defrost cycle time duration as the minimum defrost cycle time duration (Step  42 ). 
     In response to determining that the defrost cycle time duration is not less than (or equal to) the minimum defrost cycle time duration (“NO” branch of Step  40 ), controller  12  decreases the time duration of the defrost cycle (Step  44 ) to arrive at an adjusted defrost cycle time. Controller  12  can decrease the defrost cycle time from a baseline defrost cycle time duration during, e.g., a first iteration of the operations of  FIGS. 2A and 2B . Controller  12  can decrease the defrost cycle time from the adjusted defrost cycle time during, e.g., subsequent iterations of the operations of  FIGS. 2A and 2B . In some examples, controller  12  can decrease the defrost cycle time by an amount that is proportional to a difference between the threshold time duration and the time duration of the preceding cooling cycle. 
     Accordingly, controller  12  can adjust the defrost cycle time by increasing (in Step  38 ) or decreasing (in Step  44 ) the defrost cycle time based on the time duration of the cooling cycle. Controller  12  initiates the defrost cycle of vapor cycle refrigeration system  10  (Step  46 ). Controller  12  terminates the defrost cycle (Step  48 ) upon expiration of the determined defrost cycle time (i.e., the adjusted defrost cycle time determined in one of Step  38  and Step  44 ). Controller  12  can initiate the defrost cycle by decreasing an operational speed of compressor  22  (or halting compressor  22 ) to slow or stop a flow of refrigerant through vapor cycle refrigeration system  10 , thereby enabling unconditioned inlet air to flow through evaporator  20  to warm internal components of evaporator  20  and mitigate the accumulation of ice within evaporator  20 . Controller  12  can terminate the defrost cycle by, e.g., increasing the operational speed of compressor  22  to increase or start the flow of refrigerant through vapor cycle refrigeration system  10  and initiate a subsequent cooling cycle (Step  24 ). 
     Accordingly, vapor cycle refrigeration system  10  implementing techniques of this disclosure can adaptively control the duration of a defrost cycle, thereby accounting for varying environmental and system conditions and increasing overall cooling efficiency of the system. Moreover, the techniques can enable such adaptive control based on a single pressure sensor P CS  disposed downstream of evaporator  20  at an inlet of compressor  22 , thereby decreasing the cost, weight, and complexity of implementation of the adaptive control techniques. 
     Discussion of Possible Embodiments 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A method includes (a) initiating a cooling cycle of a vapor cycle refrigeration system, (b) sensing a pressure of refrigerant at an inlet of a compressor of the vapor cycle refrigeration system during operation of the cooling cycle, and (c) initiating a defrost cycle of the vapor cycle refrigeration system in response to the sensed pressure being less than a threshold pressure. The method further includes (d) terminating the defrost cycle upon expiration of a defrost cycle time that is based upon a time duration of the cooling cycle, and (e) repeating steps (a)-(d). 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations, operations and/or additional components: 
     The method can further include adjusting the defrost cycle time based on the time duration of the cooling cycle. 
     Adjusting the defrost cycle time based on the time duration of the cooling cycle can include increasing the defrost cycle time in response to determining that the time duration of the cooling cycle is less than a threshold time duration. 
     Adjusting the defrost cycle time based on the time duration of the cooling cycle can include decreasing the defrost cycle time in response to determining that the time duration of the cooling cycle is greater than a threshold time duration. 
     Adjusting the defrost cycle time based on the time duration of the cooling cycle can include adjusting the time duration of the cooling cycle by an amount that is proportional to a difference between a threshold time duration and the time duration of the cooling cycle. 
     Adjusting the defrost cycle time can include adjusting the defrost cycle time from a baseline defrost cycle time based on a difference between the time duration of the cooling cycle and a threshold time duration. 
     Initiating the defrost cycle can include reducing an operational speed of the compressor of the vapor cycle refrigeration system, and wherein initiating the cooling cycle can include increasing the operational speed of the compressor. 
     A vapor cycle refrigeration system of an aircraft includes an evaporator, a compressor, a pressure sensor, and a controller device that includes at least one processor and computer-readable memory. The evaporator is configured to receive refrigerant and provide cooling to an airflow by evaporating the refrigerant. The compressor is configured to compress the refrigerant and produce flow of the refrigerant through the vapor cycle refrigeration system. The pressure sensor is disposed to sense pressure of the refrigerant at an inlet of the compressor. The controller device is operatively coupled to the pressure sensor and to the compressor. The computer-readable memory of the controller device is encoded with instructions that, when executed by the at least one processor, cause the controller device to (a) initiate a cooling cycle of the vapor cycle refrigeration system by increasing a speed of the compressor, (b) receive an indication of sensed pressure of the refrigerant at the inlet of the compressor from the pressure sensor, and (c) initiate a defrost cycle of the vapor cycle refrigeration system in response to the received indication of the sensed pressure of the refrigerant being less than a threshold pressure. The computer-readable memory is further encoded with instructions that, when executed by the at least one processor, cause the controller device to (d) terminate the defrost cycle upon expiration of a defrost cycle time that is based upon a time duration of the cooling cycle, and (e) repeat steps (a)-(d). 
     The vapor cycle refrigeration system of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations, operations and/or additional components: 
     The computer-readable memory of the controller device can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to adjust the defrost cycle time based on the time duration of the cooling cycle. 
     The computer-readable memory of the controller device can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to adjust the defrost cycle time based on the time duration of the cooling cycle by increasing the defrost cycle time in response to determining that the time duration of the cooling cycle is less than a threshold time duration. 
     The computer-readable memory of the controller device can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to adjust the defrost cycle time based on the time duration of the cooling cycle by decreasing the defrost cycle time in response to determining that the time duration of the cooling cycle is greater than a threshold time duration. 
     The computer-readable memory of the controller device can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to adjust the defrost cycle time by an amount that is proportional to a difference between a threshold time duration and the time duration of the cooling cycle. 
     The computer-readable memory of the controller device can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to adjust the defrost cycle time from a baseline defrost cycle time based on a difference between the time duration of the cooling cycle and a threshold time duration. 
     The computer-readable memory of the controller device can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to initiate the defrost cycle by reducing an operational speed of the compressor, and wherein the computer-readable memory of the controller device can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to initiate the cooling cycle by increasing the operational speed of the compressor. 
     A controller device for a vapor cycle refrigeration system of an aircraft includes at least one processor and computer-readable memory. The computer-readable memory is encoded with instructions that, when executed by the at least one processor, cause the controller device to (a) initiate a cooling cycle of the vapor cycle refrigeration system, (b) receive an indication of a sensed pressure of refrigerant at an inlet of a compressor of the vapor cycle refrigeration system, and (c) initiate a defrost cycle of the vapor cycle refrigeration system in response to the indication of the sensed pressure being less than a threshold pressure. The computer-readable memory is further encoded with instructions that, when executed by the at least one processor, cause the controller device to (d) determine a time duration of the cooling cycle, (e) adjust a defrost cycle time based on the time duration of the cooling cycle to determine an adjusted defrost cycle time, (f) terminate the defrost cycle upon expiration of the adjusted defrost cycle time, and (g) repeat steps (a)-(f). 
     The controller device of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations, operations and/or additional components: 
     The computer-readable memory can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to: determine that the time duration of the cooling cycle is less than a threshold time duration; and increase the defrost cycle time by increasing the defrost cycle time in response to determining that the time duration of the cooling cycle is less than the threshold time duration. 
     The computer-readable memory can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to: determine that the time duration of the cooling cycle is greater than a threshold time duration; and decrease the defrost cycle time by increasing the defrost cycle time in response to determining that the time duration of the cooling cycle is greater than the threshold time duration. 
     The computer-readable memory can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to: determine a difference between the time duration of the cooling cycle and a threshold time duration; and adjust the defrost cycle time by an amount that is proportional to the determined difference between the time duration of the cooling cycle and the threshold time duration. 
     The computer-readable memory can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to: determine a difference between the time duration of the cooling cycle and a threshold time duration; and adjust the defrost cycle time from a baseline defrost cycle time based on a the determined difference between the time duration of the cooling cycle and the threshold time duration. 
     The computer-readable memory can be further encoded with instructions that, when executed by the at least one processor, cause the controller device to: initiate the defrost cycle by reducing an operational speed of the compressor of the vapor cycle refrigeration system; and initiate the cooling cycle by increasing the operational speed of the compressor. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.