Adaptive control of multi-compartment transport refrigeration system

A multi-compartment transport refrigeration system (10) includes a first evaporator (40) having an first evaporator inlet coupled to a first evaporator expansion device (140) and a first evaporator outlet coupled to a compressor inlet path, the first evaporator for cooling a first compartment of a container at a first temperature; a second evaporator (609 having a second evaporator inlet coupled to a second evaporator expansion device (160) and a second evaporator outlet coupled to the compressor inlet path, the second evaporator for cooling a second compartment of the container at a second temperature greater than the first temperature; and a controller (550) for controlling the first evaporator expansion device in response to a first superheat setpoint and controlling the second evaporator expansion device in response to a second superheat setpoint, the controller adjusting the second superheat setpoint in response to the second temperature and the first temperature.

BACKGROUND OF THE INVENTION

Embodiments relate generally to transport refrigeration systems, and more particularly to adaptive control of a multi-compartment transport refrigeration system.

The refrigerated container of a truck trailer requires a refrigeration unit for maintaining a desired temperature environment within the interior volume of the container. A wide variety of products, ranging for example, from freshly picked produce to deep frozen seafood, are commonly shipped in refrigerated truck trailers and other refrigerated freight containers. To facilitate shipment of a variety of products under different temperature conditions, some truck trailer containers are compartmentalized into two or more separate compartments, each of which typically having a door that opens directly to the exterior of the trailer. The container may be compartmentalized into a pair of side-by-side axially extending compartments, or into two or more back-to-back compartments, or a combination thereof.

Conventional transport refrigeration units used in connection with compartmentalized refrigerated containers of truck trailers include a refrigerant compressor, a condenser, a main evaporator and one or more remote evaporators connected via appropriate refrigerant lines in a closed refrigerant flow circuit. The refrigeration unit must have sufficient refrigeration capacity to maintain the product stored within the various compartments of the container at the particular desired compartment temperatures over a wide range of outdoor ambient temperatures and load conditions.

In addition to the afore-mentioned main evaporator, one or more remote evaporators, typically one for each additional compartment aft of the forward-most compartment, are provided to refrigerate the air or other gases within each of the separate aft compartments. The remote evaporators may be mounted to the ceiling of the respective compartments or mounted to one of the partition walls of the compartment, as desired. The remote evaporators are generally disposed in the refrigerant circulation circuit in parallel with the main evaporator.

Multiple temperature compartment transport refrigeration systems create significant control and refrigeration system complexity. Existing systems couple the main evaporator and remote evaporators to a common compressor suction plenum. When two or more compartments cool simultaneously in a system with a common suction plenum, the saturated evaporation temperature is shared between all compartments and evaporators. The resulting common evaporating temperature is dictated by coldest temperature compartment. Controls need to be put in place to prevent a perishable compartment from flooding or over feeding the frozen compartment. This is due to the fact that the perishable saturated evaporating temperature is significantly lower than perishable compartment air temperature. When saturation temperatures in the perishable compartment are low, the resulting sensed superheat for the perishable compartment is high. This causes the expansion device (e.g., mechanical or electrical) to open to 100%, which can result in flooding of the evaporator in the frozen compartment.

Existing systems employ pulsed cooling to prevent the perishable compartment capacity demand from flooding and diminishing all available capacity in the frozen or colder compartment. To combat this problem controls are put on a liquid solenoid valve to limit the available capacity to the perishable compartment. Typically a fixed pulse width modulation (PWM) cycle is used to control the liquid flow to the compartment that is trying to cool at the higher air temperature compartment when simultaneous cooling is required. This pulse width modulated approach and a high sensed superheat can cause a very dynamic power disturbance on the engine and introduce engine and control instability.

BRIEF DESCRIPTION OF THE INVENTION

According to one embodiment of the invention a multi-compartment transport refrigeration system includes a compressor having a suction port and a discharge port, the compressor suction port coupled to a compressor inlet path; a heat rejecting heat exchanger downstream of the compressor discharge port; a first evaporator expansion device downstream of the heat rejecting heat exchanger; a first evaporator having an first evaporator inlet coupled to the first evaporator expansion device and a first evaporator outlet coupled to the compressor inlet path, the first evaporator for cooling a first compartment of a container at a first temperature; a second evaporator expansion device downstream of the heat rejecting heat exchanger; a second evaporator having a second evaporator inlet coupled to the second evaporator expansion device and a second evaporator outlet coupled to the compressor inlet path, the second evaporator for cooling a second compartment of the container at a second temperature greater than the first temperature; and a controller for controlling the first evaporator expansion device in response to a first superheat setpoint and controlling the second evaporator expansion device in response to a second superheat setpoint, the controller adjusting the second superheat setpoint in response to the second temperature and the first temperature.

According to another embodiment of the invention, a method of operating a multi-compartment transport refrigeration system includes operating a first evaporator and first evaporator expansion device to cool a first compartment of a container at a first temperature, a first evaporator outlet coupled to a compressor inlet path; operating a second evaporator and second evaporator expansion device to cool a second compartment of a container at a second temperature greater than the first temperature, a second evaporator outlet coupled to the compressor inlet path; controlling the first evaporator expansion device in response to a first superheat setpoint and controlling the second evaporator expansion device in response to a second superheat setpoint, the controller adjusting the second superheat setpoint in response to the second temperature and the first temperature.

DETAILED DESCRIPTION OF THE INVENTION

Referring now toFIG. 1, there is shown a truck trailer100having a refrigerated container110subdivided, i.e., compartmentalized, by internal partition walls104,106into a forward cargo compartment112, a central cargo compartment114and an aft cargo compartment116. The cargo compartments112,114and116have access doors113,115and117, respectively, which open directly to the exterior of the truck trailer to facilitate loading of product into the respective cargo compartments112,114and116. The container100is equipped with a transport refrigeration system10for regulating and maintaining within each of the respective cargo compartments112,114and116a desired storage temperature range selected for the product being shipped therein. Although embodiments will be described herein with reference to the three compartment, refrigerated container, illustrated inFIG. 1, it is to be understood that embodiments may also be used in connection with truck trailers having compartmentalized containers with the cargo compartments arranged otherwise, and also in connection with other refrigerated transport vessels, including for example refrigerated container of a truck, or a refrigerated freight container of compartmentalized design for transporting perishable product by ship, rail and/or road transport.

Transport refrigeration system10includes a main evaporator40and remote evaporators50and60. Each of the evaporators40,50and60may comprise a conventional finned tube coil heat exchanger. Each compartment includes a return air temperature sensor41,51and61(FIG. 2), to measure the return air temperature from compartments112,114and116, respectively. The transport refrigeration system10is mounted as in conventional practice to an exterior wall of the truck trailer100, for example the front wall102thereof, with the compressor20and the heat rejecting heat exchanger117(FIG. 2) disposed externally of the refrigerated container110in a housing16.

FIG. 2is a schematic representation of the multiple evaporator transport refrigeration unit10in an exemplary embodiment. In the depicted embodiment, compressor20is a scroll compressor, however other compressors such as reciprocating or screw compressors are possible without limiting the scope of the disclosure. Compressor20includes a motor114which may be an integrated electric drive motor driven by a synchronous generator21operating at low speed (for example, 45 Hz) or high speed (for example, 65 Hz). Generator21may be driven by a diesel engine23of a vehicle that tows truck trailer100. Alternatively, generator21may be driven by a stand-alone engine23. In an exemplary embodiment, engine23a diesel engine, such as a four cylinder, 2200 cc displacement diesel engine which operates at a high speed (about 1950 RPM) or at low speed (about 1350 RPM).

High temperature, high pressure refrigerant vapor exits a discharge port of the compressor20then moves to a heat rejecting heat exchanger117(e.g., condenser or gas cooler), which includes a plurality of condenser coil fins and tubes144, which receive air, typically blown by a heat rejecting heat exchanger fan (not shown). By removing latent heat through this step, the refrigerant condenses to a high pressure/high temperature liquid and flows to the receiver120that provides storage for excess liquid refrigerant during low temperature operation. From the receiver120, the refrigerant flows to a subcooler121, which increases the refrigerant subcooling. Subcooler121may be positioned adjacent heat rejecting heat exchanger117, and cooled by air flow from the heat rejecting heat exchanger fan. A filter-drier124keeps the refrigerant clean and dry, and outlets refrigerant to a first refrigerant flow path71of an economizer heat exchanger148, which increases the refrigerant subcooling. Economizer heat exchanger148may be a plate-type heat exchanger, providing refrigerant to refrigerant heat exchange between a first refrigerant flow path71and second refrigerant flow path72.

From the first refrigerant flow path71, refrigerant flows from the economizer heat exchanger148to a plurality of evaporator expansion devices140,150and160, connected in parallel with the first refrigerant flow path71. Evaporator expansion devices140,150and160are associated with evaporators40,50and60, respectively, to control ingress of refrigerant to the respective evaporators40,50and60. The evaporator expansion devices140,150and160may be electronic evaporator expansion devices controlled by a controller550. Controller550is shown as distributed for ease of illustration. It is understood that controller550may be a single device that controls the evaporator expansion devices140,150and160. Evaporator expansion device140is controlled by controller550in response to signals from a first evaporator outlet temperature sensor141and first evaporator outlet pressure sensor142. Evaporator expansion device150is controlled by controller550in response to signals from a second evaporator outlet temperature sensor151and second evaporator outlet pressure sensor152. Evaporator expansion device160is controlled by controller550in response to signals from a third evaporator outlet temperature sensor161and third evaporator outlet pressure sensor162. Evaporator fans (not shown) draw or push air over the evaporators40,50and60to condition the air in compartments112,114, and116, respectively.

Refrigerant vapor from evaporators40,50and60is coupled to a common compressor inlet path200coupled to a compressor suction port through a compressor suction modulation valve201and compressor suction service valve202.

Refrigeration system10further includes a second refrigerant flow path72through the economizer heat exchanger148. The second refrigerant flow path72is connected between the first refrigerant flow path71and an intermediate inlet port167of the compressor20. The intermediate inlet port167is located at an intermediate location along a compression path between compressor suction port and compressor discharge port. An economizer expansion device77is positioned in the second refrigerant flow path72, upstream of the economizer heat exchanger148. The economizer expansion device77may be an electronic economizer expansion device controlled by controller550. When the economizer is active, controller550controls economizer expansion device77to allow refrigerant to pass through the second refrigerant flow path72, through economizer heat exchanger148and to the intermediate inlet port167. The economizer expansion device77serves to expand and cool the refrigerant, which proceeds into the economizer counter-flow heat exchanger148, thereby sub-cooling the liquid refrigerant in the first refrigerant flow path71proceeding to evaporator expansion devices140,150and160.

As described in further detail herein, many of the points in the refrigerant vapor compression system10are monitored and controlled by a controller550. Controller550may include a microprocessor and its associated memory. The memory of controller can contain operator or owner preselected, desired values for various operating parameters within the system10including, but not limited to, temperature set points for various locations within the system10or the container, pressure limits, current limits, engine speed limits, and any variety of other desired operating parameters or limits with the system10. In an embodiment, controller550includes a microprocessor board that contains microprocessor and memory, an input/output (I/O) board, which contains an analog to digital converter which receives temperature inputs and pressure inputs from various points in the system, AC current inputs, DC current inputs, voltage inputs and humidity level inputs. In addition, I/O board includes drive circuits or field effect transistors (“FETs”) and relays which receive signals or current from the controller550and in turn control various external or peripheral devices in the system10, such as economizer expansion valve77, for example.

In operation, controller550controls evaporator expansion devices140,150and160based on sensed superheat at the outlet of each of the respective evaporators40,50and60. Controller550determines the sensed superheat for each evaporator40,50and60based on the respective evaporator outlet temperature sensor and evaporator outlet pressure sensor. The sensed superheat is then compared to a superheat setpoint for each evaporator40,50and60to control the evaporator expansion device associated with each evaporator40,50and60. As noted above, as evaporator40,50and60are coupled to common suction plenum200, when simultaneous cooling is required at two compartments at two differing compartment temperatures, the saturation temperature is the same across evaporators40,50and60. This causes the superheat in the higher temperature compartment to be high, causing a large discrepancy between the sensed superheat and the superheat setpoint for that compartment.

An example of the superheat error is provided for illustration. Compartment40contains frozen food (e.g., cooled to OF) and compartment60contains perishable produce (e.g., cooled to 38 F). The common suction plenum200may cause the superheat in compartment60to be excessively high, causing controller550(e.g., executing a PID control process) to excessively open expansion device160. This causes excess refrigerant to flow in evaporator60, which can migrate to evaporator40along the common suction plenum, thereby flooding evaporator40. The excess flow of refrigerant in evaporator40may lead to closing of evaporator expansion devices140.

Embodiments discussed herein address this superheat error, by adjusting the superheat setpoint of the evaporator in the warmer compartment to be a function of the return air temperature of the warmer compartment and return air temperature of the cooler compartment. The return air temperature for each compartment is measured by return air temperature sensors41,51, and61. In an exemplary embodiment, the superheat setpoint for the warmer compartment is adjusted as:
SHadj=SHorg+(RATwarm−RATcold)

where SHadj is the adjusted superheat setpoint for the warmer compartment, SHorg is the original superheat setpoint for the warmer compartment, RATwarm is the return air temperature for the warmer compartment and RATcold is the return air temperature for the colder compartment.

Adjusting the superheat setpoint for the warmer compartment prevents the superheat error (e.g., difference between the sensed superheat and the superheat setpoint) from becoming too large, resulting in overfeeding the frozen compartment.

FIG. 3is a flowchart of a method for controlling the multi-compartment refrigeration system in an exemplary embodiment. The process begins at300where two compartments of the system demand cooling at different temperatures. At302, controller550determines a temperature difference between the return air temperature of the warmer compartment, RATwarm, and the return air temperature of the cooler compartment, RATcold. At304, the original superheat setpoint, SHorg, for the warmer compartment is added to the temperature difference between the return air temperature of the warmer compartment, RATwarm, and the return air temperature of the cooler compartment, RATcold, to define an adjusted superheat setpoint, SHadj, for the warmer compartment. At306, expansion device for the warmer compartment is controlled using the adjusted superheat setpoint, SHadj.

Embodiments discussed herein lead to better system stability and less chance of engine disturbances (stalls). The total system efficiency will improve because less saturation flooding of the frozen evaporator will occur from excessive flow of the perishable compartment. Additionally, expansion device life expectancy will improve due to reduced pulsing. Another benefit is reduced transient flooding and slugging to the compressor.