Trailer refrigeration unit and method of limiting the air temperature supplied thereto

Abstract

The present invention relates to a trailer refrigeration unit and a method for limiting the supply air temperature thereof, and specifically discloses a method for operating a trailer refrigeration unit of a refrigerated trailer system, the method comprising: setting a cargo hold set point temperature through a user interface; pushing an air flow along a flow path from a return air inlet port through an evaporator of the trailer refrigeration unit and to a supply air outlet port of the trailer refrigeration unit; monitoring the return air temperature of the air flow flowing through the return air inlet port; monitoring the supply air temperature of the air flow flowing through the supply air outlet port; heating the air flow flowing through the supply air outlet port when the return air temperature is lower than the cargo hold set point temperature; and stopping heating the air flow flowing to the supply air outlet port when the return air temperature is lower than the cargo hold set point temperature and the supply air temperature reaches a threshold value.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 62 / 705,772, filed on July 15, 2020, the contents of which are incorporated herein in their entirety. Technical Field

[0003] Exemplary embodiments relate to techniques for providing a refrigerated environment in a transportation system. More particularly, the present disclosure relates to the construction of a refrigerated trailer system and methods of operating the same. Background Art

[0004] Trailer refrigeration units are used to cool cargo stored in trailers during transport. The type of cargo stored in a particular trailer can vary significantly and may depend on the interests of the trailer owner. In some cases, despite known best practices for loading cargo, workers may not load the cargo optimally, which can lead to erroneous measurements of trailer conditions and negatively impact the control aspects of the refrigerated trailer system and the environmental conditions therein. Therefore, there remains a need in the art for a method of operating a trailer refrigeration unit that does not negatively impact the cargo even if the cargo has been loaded in a suboptimal manner. Summary of the Invention

[0005] A method for operating a trailer refrigeration unit of a refrigerated trailer system is disclosed, the method comprising: setting a cargo hold set point temperature through a user interface; pushing an air flow along a flow path from a return air inlet port through an evaporator of the trailer refrigeration unit and toward a supply air outlet port of the trailer refrigeration unit; monitoring the return air temperature of the air flow flowing through the return air inlet port; monitoring the supply air temperature of the air flow flowing through the supply air outlet port; heating the air flow flowing through the supply air outlet port when the return air temperature is lower than the cargo hold set point temperature; and stopping heating the air flow flowing to the supply air outlet port when the return air temperature is lower than the cargo hold set point temperature and the supply air temperature reaches a threshold value.

[0006] In addition to or as an alternative to one or more of the aspects disclosed above, reaching the threshold includes the supply air temperature reaching an upper supply air temperature limit, the supply air temperature reaching and / or exceeding the upper supply air temperature limit for a first duration, the supply air temperature reaching a maximum supply air temperature, the supply air temperature reaching and / or exceeding the maximum supply air temperature for a second duration, the supply air temperature reaching a temperature offset from the cargo hold set point temperature, the supply air temperature reaching or exceeding a temperature offset from the cargo hold set point temperature, the time derivative of the supply air temperature reaching a derivative threshold, or a combination including at least one of the foregoing.

[0007] In addition to or as an alternative to one or more of the aspects disclosed above, heating the airflow further comprises activating a heating device configured to be thermally connected to the airflow, activating a hot gas bypass operation mode of the trailer refrigeration unit, which results in heating the airflow passing through the evaporator, or a combination comprising at least one of the foregoing; and wherein stopping the heating of the airflow further comprises de-energizing the heating device, deactivating the hot gas bypass operation, or a combination comprising at least one of the foregoing.

[0008] Additionally or alternatively to one or more of the aspects disclosed above, wherein stopping heating further comprises de-energizing one or more electric heaters disposed in thermal communication with the airflow.

[0009] Additionally or alternatively to one or more of the aspects disclosed above, wherein stopping heating further comprises reducing a mass flow rate of refrigerant through the condenser bypass flow path.

[0010] Additionally or alternatively to one or more of the aspects disclosed above, further comprising setting a heating mode supply air temperature upper limit equal to a cargo hold set point temperature plus an offset temperature.

[0011] Additionally or alternatively to one or more of the aspects disclosed above, wherein the offset temperature is between 0.5°C and 10°C.

[0012] Also disclosed is a refrigerated trailer system, comprising: a fan for pushing an airflow along a flow path from a return air inlet port to a supply air outlet port; a vapor compression system comprising an evaporator, wherein the evaporator is configured to be thermally connected to the airflow; a return air temperature sensor for monitoring the return air temperature; a supply air temperature sensor for monitoring the supply air temperature; and a controller in control communication with the vapor compression system, the return air temperature sensor, and the supply air temperature sensor, wherein the controller is configured to heat the airflow when the return air temperature is lower than a cargo hold set point temperature, and to stop heating the airflow when the return air temperature is lower than the cargo hold set point temperature and the supply air temperature reaches a threshold value.

[0013] In addition to or as an alternative to one or more of the aspects disclosed above, reaching the threshold includes the supply air temperature reaching an upper supply air temperature limit, the supply air temperature reaching and / or exceeding the upper supply air temperature limit for a first duration, the supply air temperature reaching a maximum supply air temperature, the supply air temperature reaching and / or exceeding the maximum supply air temperature for a second duration, the supply air temperature reaching a temperature offset from the cargo hold set point temperature, the supply air temperature reaching or exceeding a temperature offset from the cargo hold set point temperature, the time derivative of the supply air temperature reaching a derivative threshold, or a combination including at least one of the foregoing.

[0014] Additionally or alternatively to one or more of the aspects disclosed above, wherein one or more heaters are provided in thermal communication with the airflow, and the controller is configured to be in controlling communication with the one or more heaters.

[0015] Additionally or alternatively to one or more of the aspects disclosed above, wherein the controller further comprises a control loop configured to control the supply air temperature to a supply air temperature set point value. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The following description should not be considered limiting in any way. Referring to the accompanying drawings, like elements are numbered the same:

[0017] Figure 1 is a cross-sectional diagram of a refrigerated trailer system.

[0018] Figure 2 is a schematic diagram of a trailer refrigeration unit of a refrigerated trailer system.

[0019] Figure 3 is a schematic diagram of a method of operating a refrigerated trailer system.

[0020] Figure 4 is a schematic diagram of the parameters of a trailer refrigeration unit in heating mode without supply air temperature limits.

[0021] Figure 5 is a schematic diagram of the parameters of a trailer refrigeration unit in heating mode with supply air temperature limits.

[0022] Figure 6 is a schematic diagram of the parameters of a trailer refrigeration unit in heating mode with supply air temperature limits. DETAILED DESCRIPTION

[0023] With reference to the accompanying drawings, a detailed description of one or more embodiments of the disclosed apparatus and method is given herein by way of example and not limitation.

[0024] Figure 1FIG2 is a schematic diagram of a cross-sectional view of a refrigerated trailer system 50, including a trailer 10 and a trailer refrigeration unit 20 attached thereto. The trailer refrigeration unit 20 may include a fan 22 for driving air along a flow path 35 from a return air inlet port 40, through an evaporator 23 (e.g., a centralized evaporator or a remote evaporator) of the trailer refrigeration unit 20, and toward a supply air outlet port 30 of the trailer refrigeration unit 20. A return air temperature sensor 41 may be positioned in thermal communication with the airflow passing through the return air inlet port 40 and may be used to monitor the overall air temperature of the airflow returning from the cargo hold 11 of the trailer 10. A supply air temperature sensor 31 may be positioned in thermal communication with the airflow passing through the supply air outlet port 30 and may be used to monitor the overall air temperature of the airflow supplied by the trailer refrigeration unit 20 to the cargo hold 11. Upon exiting the supply air outlet port 30, the airflow may be driven to the top 14 of the cargo hold 11. The flow path of the airflow may diverge as the airflow traverses the length of the cargo hold 11. When it reaches the rear door 16 , the airflow may be directed toward the floor 18 and back through the cargo 13 , where it may be drawn into the return air inlet port 40 (eg, via a pressure gradient caused by the fan 22 ).

[0025] Manufacturer's recommendations for loading cargo 13 in the cargo hold 11 may suggest leaving air gaps between the cargo and the front wall 12 (e.g., from about 6 centimeters (cm) to about 12 cm), between the cargo and the side walls (e.g., at least about 2 cm to about 5 cm), and between the cargo and the rear door (e.g., about 10 cm). Additional recommendations include minimizing or eliminating the use of packaging material (e.g., plastic wrap) because it can impede airflow to the cargo (e.g., particularly around pallets of boxed goods), and avoiding mixed cargo (e.g., a combination of fish, fruit, vegetables, meat, etc.), which when loaded can have varying thermal masses and average temperatures and can result in uneven temperature distribution (e.g., hot and / or cold spots) throughout the cargo hold 11.

[0026] Thus, when properly loaded, cargo 13 may be arranged in cargo hold 11 with open space for airflow along front wall 12, side walls, roof 14, rear door 16, and floor 18 of cargo hold 11. The additional spacing between items of cargo 13 may allow for improved airflow to the items (e.g., individual pallets, boxes, containers, or the cargo therein).

[0027] Figure 2is a schematic diagram of a trailer refrigeration unit 20. The trailer refrigeration unit 20 may include a controller 54 configured to operate to heat or cool the supply airflow flowing along the flow path 35. For example, the trailer refrigeration unit 20 may include a vapor compression system comprising an evaporator 23, an expansion device 24, a condenser 25, and a compressor 26. The trailer refrigeration unit 20 may cool the supply airflow flowing along the flow path 35 (e.g., into the return air inlet port 40) by passing the supply airflow across the evaporator 23, which is disposed in thermal communication with the supply airflow and can absorb heat from the airflow. The trailer refrigeration unit 20 may also heat the supply airflow by passing the supply airflow across one or more optional heaters 27 (e.g., electric heaters, hydrocarbon-fueled heaters, etc.), which are disposed in thermal communication with the airflow flowing along the flow path 35. For example, the trailer refrigeration unit 20 may include a first heater and a second heater, each disposed in thermal communication with the airflow flowing through the supply air outlet port 30. The first heater may have a higher heat output than the second heater. The trailer refrigeration unit 20 can be configured to partially or fully heat a single heater of one or more heaters 27, or any combination thereof. For example, the trailer refrigeration unit 20 can include a first heater and a second heater, and can be configured to heat the first heater, the second heater, or both heaters, thereby allowing three non-zero heat output settings for heating the supply airflow. Thus, the number of heat output settings for heating the supply airflow can be a function of the number of discrete heaters in the refrigerated trailer. Furthermore, the heat output of a single heater or a group of heaters can be controlled (e.g., via power supplied to the heaters, fuel and / or air flow to the heaters, etc.), which can further refine the heater output based on the conditions of the refrigerated trailer system 50. In one example, the trailer refrigeration unit 20 can include a first heater having a heat output capacity of approximately 5250 watts (W) and a second heater having a heat output capacity of approximately 3450 W, with the heaters positioned in thermal communication with the airflow flowing along the flow path 35. In another example, the trailer refrigeration unit 20 may include a first heater and a second heater having equal heat output capabilities of approximately 3960 W, which are disposed in thermal communication with the airflow flowing along the flow path 35. Furthermore, when electric heaters are used, the one or more heaters 27 may be configured for use with any suitable power source, including alternating current (AC) or direct current (DC) power, such as 580 volts AC (VAC) at 65 hertz (Hz) or 480 VAC at 60 Hz, etc.

[0028] Alternatively, the trailer refrigeration unit 20 can be configured to heat the supply airflow (e.g., to increase the total enthalpy or reduce the cooling potential of the supply airflow) without or in addition to the use of one or more heaters 27. For example, hot refrigerant can alternatively be transferred from the high-pressure side of the vapor compression cycle (e.g., upstream of the expansion device 24) directly to the low-pressure side (e.g., the inlet of the compressor 26) without passing through the condenser 25. This can be achieved by opening the optional hot gas bypass valve 28 to allow the refrigerant to bypass the condenser 25 along the optional flow path 29. This action can increase the suction temperature of the cycle and result in an increased refrigerant temperature throughout the cycle, thereby increasing the temperature of the refrigerant flowing through the evaporator 23 and increasing the temperature capacity of the supply airflow flowing through the evaporator 23. The same effect can be achieved by utilizing a three-way valve in addition to, or in lieu of, the optional hot gas bypass 28 and / or expansion device 24. The use of the hot bypass valve 28 can be combined with the use of one or more heaters 27 to heat the supply airflow flowing to the supply air outlet port.

[0029] The refrigerated trailer system 50 can be operated to cool the cargo 13 to a cargo hold setpoint temperature, which can be set by an operator of the refrigerated trailer system 50. For example, the operator of the refrigerated trailer system 50 can use the control interface 55 of the refrigerated trailer system 50 to set the cargo hold setpoint temperature. The cargo hold setpoint temperature can be set to any suitable temperature. For example, the cargo hold setpoint temperature can be set to any value from approximately -12°C to approximately 20°C (inclusive), or from approximately -5°C to approximately 18°C ​​(inclusive), or from approximately -2°C to approximately 18°C ​​(inclusive), or from approximately -1°C to approximately 18°C ​​(inclusive), or from approximately 0°C to approximately 18°C ​​(inclusive), or from approximately 0°C to approximately 4°C (inclusive), or from approximately 0°C to approximately 3°C (inclusive). In particular, the cargo hold set point temperature may be set to about -1°C, or about 0°C, or about 1°C, or about 2°C, or about 3°C, or about 4°C, or about 5°C, or about 6°C, or about 7°C, or about 8°C, or about 9°C, or about 10°C, or about 11°C, or about 12°C, or about 13°C, or about 14°C, or about 15°C, or about 16°C, or about 17°C, or about 18°C. The cargo hold set point temperature may also be set to a decimal value within the above ranges, for example, to 2.5°C, 3.5°C, 4.5°C, etc.

[0030] The control interface 55 may include any suitable interface for an operator to set a cargo hold setpoint temperature. For example, the control interface 55 may include a mobile phone software application, a control panel configured to communicate with the controller of the refrigerated trailer system 50 (e.g., a control panel located in the cab 8 of the refrigerated trailer system 50, a control panel located on the housing 21 of the trailer refrigeration unit 20, a control panel located on an interior wall of the trailer 10 of the trailer refrigeration unit 20, etc.). The trailer refrigeration unit 20 may be operated to maintain the return air temperature sensor 41 at a set cargo hold setpoint temperature. For example, an operator may select 3°C ​​as the cargo hold setpoint temperature (e.g., on a control application on their mobile phone). In response, the trailer refrigeration unit 20 may automatically increase or decrease the supply air temperature measured by the supply air sensor 31 in an effort to bring the return air temperature (e.g., measured by the return air inlet sensor 41) into alignment with the cargo hold setpoint temperature. The trailer or cargo temperature sensor 15 may be configured to further influence the supply air flow temperature control.

[0031] In some cases, best practices for cargo loading can be ignored or disregarded. This can result in cargo 13 being positioned too close to the front wall 12, side walls, ceiling 14, rear door 16, and / or floor 18 of the cargo hold 11, or too close to one or more trailers and / or cargo temperature sensors 15 located in the cargo hold 11. When cargo is positioned in a manner that reduces the space for airflow to reach, around, and / or through the cargo 13 (e.g., against a side wall, ceiling, front wall 12, or rear door 16, or on the floor 18 without a pallet underneath, or when packed too tightly together within the cargo hold 11), when cargo is covered by packaging material, or when various types of cargo are mixed, it can lead to hot and / or cold spots in the cargo hold 11 due to inadequate airflow distribution around the cargo 13. Furthermore, when cargo is positioned too close to trailers and / or cargo temperature sensors 15, it can erroneously affect aspects of the trailer refrigeration unit 20 control, such as supply air temperature control.

[0032] Applicants have discovered that even when the trailer is lightly or unloaded, the disclosed method is able to maintain the environmental conditions of the cargo hold 11 while not exceeding the maximum supply air temperature that could result in cargo loss, taking into account at least the above-mentioned best loading practices.

[0033] like Figure 3As shown, a first aspect 200 of the disclosed method may include setting a cargo hold setpoint temperature. Setting the cargo hold setpoint temperature may be performed manually by an operator of the refrigerated trailer system 50, automatically by the controller 54 in response to one or more control parameters of the control system, or by a combination of manual and automatic control. The cargo hold setpoint temperature may be set remotely by the operator using a remote control interface 55 (e.g., a mobile phone, a central station, etc.). For example, the cargo hold setpoint temperature may be automatically set by a schedule manually input by the operator, or may be set by the type of cargo (e.g., when the operator inputs the type of cargo and the controller uses a corresponding default cargo hold setpoint temperature).

[0034] The second aspect 210 of the disclosed method may include monitoring the return air temperature of the airflow flowing through the return air inlet port 40. The return air temperature may be monitored by a return air temperature sensor 41. The return air temperature sensor 41 may be positioned in any suitable location to detect the overall air temperature of the return airflow. For example, the return air temperature sensor 41 may be located at the return air inlet port 40, in a duct extending between the return air inlet port 40 and the fan 23, or the like.

[0035] The third aspect 220 of the disclosed method can include monitoring the supply air temperature of the airflow flowing through the supply air outlet port 30. The supply air temperature can be monitored by a supply air temperature sensor 31. The supply air temperature sensor 31 can be positioned in any suitable location to detect the overall air temperature of the supply airflow. For example, the supply air temperature sensor 31 can be located at the supply air outlet port 30, in a duct extending between the fan 23 and the supply air inlet port 40, or the like.

[0036] A fourth aspect 230 of the disclosed method may include heating the airflow to the supply air outlet port 30 when the return air temperature is below the cargo hold set point temperature. When the return air temperature is below the cargo hold set point temperature, the trailer refrigeration unit may operate in a heating mode. The heating mode may include stopping the cooling air flowing along the flow path 35 (e.g., stopping the compressor 25 of the vapor compression system), supplying heat to the air flowing along the flow path 35, or a combination thereof, so as to increase the temperature of the supply air flowing out of the supply air outlet port 30 and raise the temperature of the cargo 13 to the cargo hold set point temperature. The heating may include heat supplied by one or more heaters 27, which are arranged to be in thermal communication with the airflow flowing along the flow path 35 to and / or through the supply outlet port 30. The one or more heaters 27 may include an electric heater. The one or more heaters 27 may include a hydrocarbon fuel-fired heater or its equivalent, which heats the air through a combustion process.

[0037] A fifth aspect 230 of the disclosed method may include ceasing heating of the airflow to the supply air outlet port 30 when the return air temperature is below the cargo hold setpoint temperature and the supply air temperature exceeds a threshold. Ceasing heating may include deactivating one or more heaters 27 for timed cooling. The duration of the timed cooling may be set by an operator of the trailer refrigeration system 50 (e.g., using the control interface 55) or may be preset by the system manufacturer. The threshold may include a supply air temperature limit (e.g., as measured by the supply air temperature sensor 31) or a surrogate thereof, a return air inlet port temperature limit (e.g., as measured by the return air temperature sensor 41) or a surrogate thereof, an offset temperature from the cargo hold setpoint temperature (e.g., a maximum offset from the cargo hold setpoint temperature measured by the return air temperature sensor 41), a maximum trailer and / or cargo temperature (e.g., as measured by the trailer and / or cargo temperature sensor 15), a rate of change of one or more of the temperatures, etc. For example, the controller 54 may be configured to deactivate or reduce the heat output of one or more of the one or more heaters 27 when the supply air temperature reaches the supply air temperature limit. Furthermore, when hot gas bypass operation is included in the vapor compression system, the controller 54 may be configured to close the hot gas bypass flow path 29 (e.g., by closing the hot gas bypass valve 28 or a functionally equivalent three-way valve) when the supply air temperature reaches a supply air temperature limit.

[0038] For example, once the supply air temperature exceeds the supply air temperature limit for a specified duration (e.g., 30 seconds), the heater 27 can be deactivated (e.g., so as to no longer supply heat). The supply air temperature limit can be set as a function of the cargo hold set point temperature. For example, the supply air temperature limit can be set to a value from about 0°C to about 20°C, or from about 2.5°C to about 10.0°C above the cargo hold set point temperature. For example, the supply air temperature limit can be set to a value from about 2.5°C, or about 3.0°C, or about 3.5°C, or about 4.0°C, or about 4.5°C, or about 5.0°C, or about 5.5°C, or about 6.0°C, or about 6.5°C, or about 7.0°C, or about 7.5°C, or about 8.0°C, or about 8.5°C, or about 9.0°C, or about 9.5°C, or about 10.0°C above the cargo hold set point temperature. The supply air temperature limit may also be set to a decimal value within the above range, for example, to 2.5° C., 3.5° C., 4.5° C., etc. In one embodiment, when in heating mode (e.g., when the return air temperature is below the cargo hold set point temperature) and the supply air temperature sensor 31 indicates that the supply air temperature exceeds the cargo hold set point temperature by more than 3° C. for 30 seconds, one or more heaters 27 may be deactivated (e.g., power is removed, fuel flow is stopped, etc.).

[0039] Once deactivated, the one or more heaters 27 may remain deactivated for the duration of the off time to allow the temperature distribution of the cargo to even out and become more uniform before reactivating the one or more heaters 27 .

[0040] As an alternative or in addition to on / off heater control, the heater's heat output can be controlled based on the supply air temperature sensor 31. For example, one or more heaters 27 can be driven by a control loop that includes a proportional, integral, and / or derivative controller that modulates heater output power based on the measured supply air temperature. The proportional, integral, and / or derivative controller can use the difference between the supply air temperature and a supply air temperature setpoint, the integral of the difference over time, and / or the time derivative of the difference to influence the heat output of the one or more heaters 27. In this way, the controller 54 can reduce or eliminate the cyclical rising / falling temperature variations associated with on / off heater control methods. Additionally, a pulse width modulation controller can be used to modulate the heat output of one or more heaters 27 in response to control parameters of the refrigerated trailer system 50 (e.g., supply air temperature, return air temperature, cargo temperature, etc.). The pulse width modulation controller can modulate the length of time that discretely activated ones of the one or more heaters 27 are activated. For example, a pulse width modulation controller may respond to a decrease in supply air temperature by extending the time the heater is activated relative to the time it is deactivated within a given time interval, which may add more heat to the supply air flow and correspondingly increase the supply air temperature.

[0041] Figure 4 is a schematic diagram of the temperature of the refrigerated trailer system 50 during heating mode without any supply air temperature limits. These situations may be caused by the cargo 13 being improperly loaded into the cargo hold 11. For example, when best practices for loading the cargo 13 are not followed as previously described. During heating mode, the return air temperature 120 is below the cargo hold set point temperature 110. As a result, one or more heaters 27 may be activated, causing the supply air temperature 100 to increase sharply, and the return air temperature 120 to increase accordingly. In the absence of supply air temperature limits, the supply air temperature 100 may increase above the maximum supply air temperature 130 even when the return air temperature 120 remains below the cargo hold set point temperature 110. This may unnecessarily expose the cargo 13 to air temperatures above the maximum supply air temperature 130 (e.g., set by the operator in the controller 54).

[0042] Figures 5 and 6 is a graphical representation of the temperature of the trailer system 50 being cooled during heating mode with supply air temperature limits. Figure 5In a refrigerated trailer system 50, one or more electric heaters 27 may be used to heat the supply air. During a heating mode, the return air temperature 120 may be lower than the cargo hold setpoint temperature 110. Consequently, the one or more electric heaters 27 may be activated until the supply air temperature 100 reaches an upper supply air temperature limit 131. Upon reaching the upper supply air temperature limit 131, or after reaching and / or exceeding the upper supply air temperature limit 131 for a set duration, the heaters 27 may then be deactivated, which may result in a corresponding decrease in the supply air temperature 100 until the one or more heaters are reactivated.

[0043] The heater 27 can be reactivated based on a wait time, such as a minimum time that one or more heaters 27 remain deactivated (e.g., an off time). For example, when one or more heaters are deactivated, they can remain deactivated for a minimum time period of less than or equal to 30 minutes, 15 minutes, 10 minutes, 7 minutes, 5 minutes, 3 minutes, 1 minute, etc. The controller 54 can adjust, truncate, reset, or otherwise override this minimum time period based on another parameter of the refrigerated trailer system 50 (e.g., a measured or calculated control parameter, etc.). For example, if the return air temperature 120 drops to the minimum return air temperature 140, the supply air temperature 100 drops to the lower supply air temperature limit 141 (e.g., preset by the operator, set based on the cargo hold setpoint temperature 110, etc.), an alarm condition is issued by the controller 54, etc., the controller 54 can reset or otherwise override the deactivation time. Once reactivated, the heater 27 can remain activated until it reaches, or reaches and / or exceeds, the upper supply air temperature limit 131 for a predetermined duration as previously described. This cycling of turning on and then off the heat transferred to the supply air flow by the one or more heaters 27 results in an increase and subsequent decrease in the supply air temperature 100 .

[0044] exist Figure 6 In the embodiment of the present invention, the refrigerated trailer system 50 may include one or more adjustable output heaters 27 for heating the supply air. During heating mode, the return air temperature 120 may be lower than the cargo compartment set point temperature 110. As a result, the heat output of the one or more adjustable output heaters 27 may be adjusted until the supply air temperature 100 reaches the supply air set point temperature 101. The supply air set point temperature may be preset by the operator, may be a function of the cargo compartment set point temperature 110, etc. Once the return air temperature reaches the cargo compartment set point temperature 110, the trailer refrigeration unit may switch from heating mode to cooling mode.

[0045] The term "about" is intended to include the degree of error associated with the measurement of the particular quantity based on the equipment available at the time the application was filed.

[0046] The terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the present disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It should also be understood that when used in this specification, the terms "include" and / or "comprise" specify the presence of stated features, integers, steps, operations, elements, and / or parts, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, parts, and / or combinations thereof.

[0047] Although the present disclosure has been described with reference to one or more exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for its elements without departing from the scope of the present disclosure. Furthermore, many modifications may be made to adapt specific circumstances or materials to the teachings of the present disclosure without departing from the substantive scope of the present disclosure. Therefore, it is intended that the present disclosure be not limited to the specific embodiments disclosed as the best mode contemplated for carrying out the present disclosure, but that the present disclosure encompass all embodiments falling within the scope of the claims.

Claims

1. A method of operating a trailer refrigeration unit of a refrigerated trailer system, comprising: Setting cargo hold set point temperature via the user interface; Propelling an airflow along a flow path from a return air inlet port, through an evaporator of the trailer refrigeration unit, and toward a supply air outlet port of the trailer refrigeration unit; monitoring a return air temperature of the airflow passing through the return air inlet port; monitoring a supply air temperature of the air flow passing through the supply air outlet port; heating the airflow through the supply air outlet port when the return air temperature is below the cargo hold set point temperature; and ceasing heating of the airflow to the supply air outlet port when the return air temperature is below the cargo hold set point temperature and the supply air temperature reaches a threshold; wherein heating the airflow further comprises activating one or more electric heaters disposed in thermal communication with the airflow, activating a hot gas bypass mode of operation of the trailer refrigeration unit, or activating one or more electric heaters disposed in thermal communication with the airflow and activating a hot gas bypass mode of operation, to cause heating of the airflow passing through the evaporator; and Wherein, stopping the heating of the airflow further comprises de-energizing the one or more electric heaters for a duration, deactivating the hot gas bypass operation, or both.

2. The method according to claim 1, characterized in that Reaching the threshold includes at least one of the following or any combination thereof: the supply air temperature reaches an upper supply air temperature limit, the supply air temperature reaches or exceeds the upper supply air temperature limit for a first duration, the supply air temperature reaches a maximum supply air temperature, the supply air temperature reaches or exceeds the maximum supply air temperature for a second duration, the supply air temperature reaches a temperature offset from the cargo hold set point temperature, the supply air temperature reaches or exceeds the temperature offset from the cargo hold set point temperature, and the time derivative of the supply air temperature reaches a derivative threshold.

3. The method according to any one of claims 1 to 2, characterized in that Ceasing heating further includes de-energizing one or more electric heaters disposed in thermal communication with the airflow.

4. The method according to any one of claims 1 to 2, characterized in that Deactivating heating also includes reducing the mass flow of refrigerant through the condenser bypass flow path.

5. The method according to claim 1, wherein Also included is setting a heating mode supply air temperature upper limit equal to the cargo hold set point temperature plus an offset temperature.

6. The method according to claim 5, characterized in that The offset temperature is between 0.5°C and 10°C.

7. A refrigerated trailer system comprising: a fan for forcing an air flow along a flow path from a return air inlet port to a supply air outlet port, a vapor compression system comprising an evaporator, wherein the evaporator is disposed in thermal communication with the air flow, Return air temperature sensor, which is used to monitor the return air temperature, Supply air temperature sensor, which is used to monitor the supply air temperature, a controller in control communication with the vapor compression system, the return air temperature sensor, and the supply air temperature sensor, wherein the controller is configured to heat the airflow when the return air temperature is below a cargo hold set point temperature and to stop heating the airflow when the return air temperature is below the cargo hold set point temperature and the supply air temperature reaches a threshold; wherein heating the airflow further comprises activating one or more electric heaters disposed in thermal communication with the airflow, activating a hot gas bypass mode of operation of the trailer refrigeration unit, or activating one or more electric heaters disposed in thermal communication with the airflow and activating a hot gas bypass mode of operation, to cause heating of the airflow passing through the evaporator; and Wherein, stopping the heating of the airflow further comprises de-energizing the one or more electric heaters for a duration, deactivating the hot gas bypass operation, or both.

8. The refrigerated trailer system according to claim 7, wherein: Reaching the threshold includes at least one of the following or any combination thereof: the supply air temperature reaches an upper supply air temperature limit, the supply air temperature reaches or exceeds the upper supply air temperature limit for a first duration, the supply air temperature reaches a maximum supply air temperature, the supply air temperature reaches or exceeds the maximum supply air temperature for a second duration, the supply air temperature reaches a temperature offset from the cargo hold set point temperature, the supply air temperature reaches or exceeds the temperature offset from the cargo hold set point temperature, and the time derivative of the supply air temperature reaches a derivative threshold.

9. The refrigerated trailer system according to any one of claims 7 to 8, characterized in that: One or more heaters are disposed in thermal communication with the airflow, and the controller is configured to be in control communication with the one or more heaters.

10. The refrigerated trailer system according to any one of claims 7 to 8, characterized in that: The controller also includes a control loop configured to control the supply air temperature to a supply air temperature set point value.

11. The refrigerated trailer system according to any one of claims 7 to 8, characterized in that: One or more heaters are disposed in thermal communication with the airflow, and the controller is configured to be in control communication with the one or more heaters, and wherein the controller further comprises a control loop configured to control the supply air temperature to a supply air temperature setpoint value.

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