METHOD FOR CONTROLLING VENTILATION FLOW RATE IN A CONTAINER AND A CONTAINER BEING CONTROLABLE BY THE METHOD

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. § 120 of International Application No. PCT/DK2023/050309 filed Dec. 13, 2023, which claims priority to Denmark Application No. DK PA202270636, filed Dec. 20, 2022 under 35 U.S.C. § 119(a). Each of the above-referenced patent applications is incorporated by reference in its entirety.

SUMMARY

The present invention relates to control of the ventilation flow rate in a cargo storage space in a container, such as an intermodal refrigerated container with an integrated refrigeration unit.

Cooling of cargo spaces in containers for storage of commodities is widely used, in particular during transportation of such commodities, and efforts are made to reduce the energy consumption for keeping the commodities at the required temperature for storage, such as by improving the thermal insulation of the container and improving the energy efficiency of the refrigeration unit.

It is an object of the present invention to reduce the energy consumption of cooled storage of commodities, in particular commodities producing respiration heat, such as fruit, vegetables or flowers.

By the present invention the object is achieved by reducing the energy consumption of the fan or fans for driving a ventilation flow from the cargo storage space, across an evaporator of the refrigeration unit and into the cargo storage space. This is achieved by controlling the operational speed of the fan(s) and thereby the energy consumption of the electrical motor(s) driving the fan(s), which in turn reduces the energy consumption of the refrigeration unit for removing the heat generated by the energy consumption of the electrical motor(s) driving the fan(s) from the container.

The present invention relates to a method for controlling a ventilation flow rate of a cargo storage space in a container, wherein a refrigeration unit is provided for refrigeration of the cargo storage space, the refrigeration unit comprising one or more evaporator fans being arranged to drive a ventilation flow from the cargo storage space, across an evaporator of the refrigeration unit and into the cargo storage space, wherein the ventilation flow rate is controlled by controlling the speed of the evaporator fan(s), the method comprising the steps at a first speed, determining a measure of the pressure difference across the evaporator fan(s) when the evaporator fan(s) are operated at said first speed, selecting an operational speed of the evaporator fan(s) based on the determined measure of the pressure difference and a predetermined limit value for said measure, wherein the operational speed is a first predetermined speed in case the determined measure of the pressure difference is lower than a limit value for said measure of the pressure difference, and a second speed in case the determined measure of the pressure difference is higher than the limit value for the determined measure, wherein the second speed is lower than the first predetermined speed, and operating the evaporator fan(s) at the selected operational speed.

The refrigeration unit typically comprises a compressor, a condenser and an expansion valve.

The measure of the pressure difference across the evaporator fan(s) may e.g. be an actual measured or calculated pressure difference ΔP, a measured value of the a current (I_motor) fed into the electric motor(s) driving the evaporator fan(s), which is proportional to the pressure difference or a calculated value of the cargo airflow resistance (R_air), which R_air is found from measuring the motor current (I_motor) of the motors driving the evaporator fans, which together with the rotational speed of the evaporator fans can be translated to a torque acting on the fan rotors, which again is proportional to the backpressure of the ventilation flow, i.e. the pressure difference across the evaporator fans. The pressure difference across the evaporator fan(s) may be calculated from the measured value of the a current (I_motor) fed into the electric motor(s) driving the evaporator fan(s) together with the rotational speed of the evaporator fans and values that can be found form look-up tables or correlations defined for the particular fans.

The measure of the pressure difference may alternatively be determined from the power applied to the evaporator fan(s), which may e.g. be found from the torque measured by means of a strain gauge applied to a driving axle of the fan and the rotational speed of the fan or from the current and the voltage of the motor driving the evaporator fan(s).

By determining whether the determined measure is above or below a limit value, it is determined if the commodities are satisfactory packed inside the storage space, i.e. that the commodities are packed so that the ventilation flow of cooling air meets sufficient resistance for the air to be distributed to all of the commodities. A non-satisfactory packaging of the commodities will result in shortcuts or chimneys for the cooling airflow with the result that the airflow does not interact sufficiently with the commodities and that the commodities or parts of the commodities may not be refrigerated satisfactorily. The limit value may be predefined for the type of the stored commodity or it may be a more general limit value.

If the comparison of the determined measure and the limit value indicates that the packaging is satisfactory, i.e. that the cooling airflow is acceptably distributed, and the determined value is higher than the limit value, the speed of the evaporator fan(s) may be reduced in order to reduce the energy consumption while the cooling of the commodities is sufficient. The second speed of the evaporator fan(s) may be a speed predetermined for the commodity, a generally predetermined speed for the refrigerated container or a variable speed. Provided that the second speed is selected, the method may further comprise the steps of determining a measure of the pressure difference across the evaporator fan(s), and adjusting the operational speed of the evaporator fan(s) in order to reach a predetermined set point value for the determined measure of the pressure difference across the evaporator fan(s).

The predetermined set point value for the determined measure may be predetermined for the type of commodities, for the refrigerated container or a generally predetermined value.

The method may comprise the step of receiving data pertaining to the commodity by means of a control unit for controlling the ventilation flow rate by controlling the speed of the evaporator fan(s), wherein the predetermined set point value for the determined measure of the pressure difference across the evaporator fan(s) is included in or derived from said received data. In particular, the received data may comprise identification of the type of commodity, such as “bananas”, “apples” or “flowers”, and the step of deriving the predetermined set point value for the determined measure of the pressure difference across the evaporator fan(s) and possibly other set point values pertaining to the storage of the commodities, such as oxygen content, CO₂ content, and/or relative humidity of the ventilation air, comprises a look-up in predefined values stored in the control unit or at an external data storage with which the control unit is enabled to communicate, e.g. by means of a wireless communication device of the control unit.

A valid speed range or just a minimum limit for the speed may be defined for the evaporator fan(s).

The first predetermined speed may in a preferred embodiment be the maximum speed of the evaporator fan(s).

It is preferred that each evaporator fan comprises a variable speed electric motor arranged to drive the evaporator fan and an electric power converter to feed power to the variable speed electric motor.

The method may include to issue external warning information in case the determined measure of the pressure difference is lower than the limit value for said measure of the pressure difference, so as to warn that the packaging of the commodities is not satisfactory, which may cause a repackaging of the commodities.

It is preferred that the refrigeration unit is operated to achieve and maintain a set point temperature of the cargo storage space, wherein the cargo storage space contains stored commodities and the set point temperature of the cargo storage space is within the range of −5° C. to 30° C., and wherein the stored commodities produces respiration heat at the set point temperature of the cargo storage space.

This is particularly preferred for commodities that produces respiration heat, such as fruit, vegetables or flowers.

The method is preferably applied to a container being an intermodal refrigerated container.

Furthermore, the method is preferably applied to a container where the refrigeration unit is an integrated refrigeration unit in the container.

The present invention also relates to an intermodal refrigerated container comprising an integrated refrigeration unit for refrigeration of a cargo storage space of the container, the integrated refrigeration unit comprising comprises a compressor, a condenser, an expansion valve, an evaporator and at least one evaporator fan, which is arranged to drive a ventilation flow from the cargo storage space, across the evaporator and into the cargo storage space, and a variable speed electric motor arranged to drive the evaporator fan(s), wherein the intermodal refrigerated container comprises a control unit for controlling the ventilation flow rate by controlling the speed of the evaporator fan, the control unit being adapted to perform the steps according to the method disclosed herein.

The refrigerated container may be an intermodal container, the definition of which is currently generally determined by two ISO standards, ISO 668:2013 and ISO 1496-1:2013.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective cutaway view of a part of an insulated wall of a refrigerated container,

FIG. 2 is a cross-section of a refrigerated container,

FIG. 3 is a flow chart illustrating one process of controlling the ventilation flow rate of the cargo space of the refrigerated container, and

FIG. 4 is a flow chart illustrating another process of controlling the ventilation flow rate of the cargo space of the refrigerated container.

DETAILED DESCRIPTION

The insulated wall 1 of a refrigerated container 2 may typically comprise the layers shown in FIG. 1, where on the outside of the container 2 an outer layer 3 of corrugated steel sheet provides the external surface. On the inside, an inner layer 4 is provided made from e.g. aluminium sheets or glass fibre reinforced polymer sheets. Optionally, a plywood layer 5 may be provided under the inner layer 4. Between the outer layer 3 and the inner layer 4, an insulating material 6 such as insulating foam of polyurethane and vertical U-shaped steel beams 7 connecting the inner 4 and outer layers 3 of the container.

In the cross-section of a refrigerated container 2 shown in FIG. 2, the box temperature T_box at the cargo storage space 8 inside the container 2 inside the container and the ambient temperature T_ambient are indicated. The commodities to be refrigerated are to be stored in the cargo storage space 8. The atmosphere in the storage space 8 is cooled by means of the evaporator 12 delivering the refrigeration effect Q_Ref by evaporation of the liquid refrigerant received from the compressor 13. The evaporator fans 9 drive a flow of air from the storage space 8 of the container 2 and past the evaporator 12 in order to cool the air, which is returned to the storage space 8. The amount of water condensing at the evaporator 12 is determined by the condensation sensor 11.

Air exchange between the surroundings of the container 2 and the storage space 8 inside the container for the purpose of controlling the content of the atmosphere inside the container in the storage space 8, in particular the CO₂ contents, is controlled by means of the fresh air ventilators 10.

A controller 14 is arranged to control the operation of the various parts of the equipment in the refrigerated container 2.

An example of how the methods disclosed herein is provided below with reference to the flow chart in FIG. 3.

The initial step 15 is to store the commodities in the cargo storage space 8 and enter the set point temperature for the cargo storage space 8 into the controller 14. The set point temperature may be entered manually directly into the controller 14, or the set point temperature may be transferred electronically together with other data pertaining to the storage of the commodities, such as oxygen content, CO₂ content, and/or relative humidity of the ventilation air, as well as limit value and/or set point value of a measure (I_motor, ΔP, R_air) of the pressure difference (ΔP) across the evaporator fan(s) 9. Alternatively, the type of commodities can be entered into the controller 14, e.g. by scanning of an identifier of the commodities by a scanner connected to the controller 14 or a wireless data transmission to the controller 14.

The compressor 13 and evaporator 12 are in the first operational step 16 operated in a pull-down mode for rapid lowering of the temperature T_box inside the cargo storage space 8, also known as the box temperature. The evaporator fans 9 are in this step 16 also operated at maximum speed.

When the box temperature T_box inside the cargo storage space 8 has reached the set point temperature, the set point temperature is maintained in step 17 for a period of time until an equilibrium has been reached, which is understood as a substantially stable refrigeration effect Q_Ref of the evaporator 12, which may be calculated from the mass flow of refrigerant multiplied by the difference between the specific enthalpy of the refrigerant before it reaches the evaporator and the specific enthalpy of the refrigerant after leaving the evaporator. The equilibrium state can e.g. be defined as the refrigeration effect Q_Ref of the evaporator 12 varying less than 15% during one hour of operation, such as 10% during one hour of operation.

In the next step 18, the speed of the evaporator fans 9 is reduced to 75% of the maximum speed to obtain a second value of airflow resistance in order to calculate a reliable value for the cargo airflow resistance R_air, measured in pressure drop per flow rate (ΔP/m³/s). The airflow resistance R_air is found from measuring the motor current I_motor of the motors driving the evaporator fans 9, which together with the rotational speed of the evaporator fans 9 can be translated to a torque acting on the fan rotors, which again is proportional to the backpressure of the ventilation flow. The values can be found form look-up tables or correlations defined for the particular fan 9.

With two measured points at two different speed of the evaporator fans 9 obtained in step 17 and 18, a reliable value of the airflow resistance R_air is determined. The airflow resistance R_air is indicative of how well the commodities have been packed in the cargo storage space 8, which is important for the heat transfer between the commodities and the cooling airflow driven by the evaporator fans 9. In addition, there is a minimum differential pressure difference ΔP or back pressure of the evaporator fans 9 for ensuring that the cooling airflow reaches all of the cargo storage space 8 and thereby all of the stored commodities. Such minimum differential pressure ΔP depends on the type of stored commodity and the packaging of the commodities in the storage space 8. A non-satisfactory packaging of the commodities will result in shortcuts or chimneys for the cooling airflow with the result that the airflow does not interact sufficiently with the commodities and that the commodities or parts of the commodities may not be refrigerated satisfactorily.

The calculated airflow resistance R_air is in step 20 compared to a minimum value R_Min set to determine whether such shortcuts or chimneys in the packaging of the commodities in the cargo storage space are of a magnitude that prevents a lowering of the speed of the evaporator fans 9, in which case step 22 is selected, in which the evaporator fans 9 are operated at maximum speed while the box temperature T_box inside the cargo storage space 8 is maintained at the set point temperature by controlling the operation of the compressor 13.

In case the calculated airflow resistance R_air in step 20 is found to exceed the minimum value R_Min it is determined in step 21 whether a set point speed of the evaporator fans 9, lower than the maximum speed of the evaporator fans 9, has been predetermined for the commodity, in which case the operation moves on to step 23, where the evaporator fans 9 are operated at the predetermined set point speed while the box temperature T_box inside the cargo storage space 8 is maintained at the set point temperature by controlling the operation of the compressor 13. If a set point evaporator fans 9 speed has not been predetermined for the commodity, a cargo thermal conductivity k is calculated in step 24 from data obtained during the operational steps 17 and 18.

The cargo thermal conductivity k is found by for both operational steps 17, 18 determining the actual flow rate F as a function of the speed of the evaporator fans 9 and the determined back pressure ΔP of the evaporator fans 9 and by determining the cargo heat reject P_Cargo, i.e. the actual cooling effect by the cooling airflow driven by the evaporator fans 9. The cargo heat reject P_Cargo may be found from the energy balance equation:

Q Ref = P i ⁢ n ⁢ g ⁢ r ⁢ e ⁢ s + P f ⁢ a ⁢ n ⁢ s + P C ⁢ a ⁢ r ⁢ g ⁢ o

The refrigeration effect Q_Ref of the evaporator 12 may be calculated from the mass flow of refrigerant multiplied by the difference between the specific enthalpy of the refrigerant before it reaches the evaporator and the specific enthalpy of the refrigerant after leaving the evaporator. The effect can be found from using the suction pressure of the compressor, the discharge pressure, the compressor displacement, the volumetric efficiency, and the compressor speed.

P_ingres is the heat ingress from the ambient into the cargo storage space, which can be found from the difference between the ambient temperature T_ambient and the box temperature T_box at the cargo storage space 8 inside the container 2, multiplied by the insulation parameter U of the refrigerated container 2. The value of U for a standard intermodal container is from the manufacturing of the new container known to have a value of 43 W/K±1 but the value may decrease over time due to wear, damages and degradation of the insulating material 6.

The electric effect P_fans consumed by the evaporator fans 9 is determined from the power consumption of the motor driving the evaporator fans9.

Thus, the cargo heat reject P_Cargo may be calculated as:

P Cargo = Q Ref - P ingres - P fans

A second example of how the methods disclosed herein is provided below with reference to the flow chart in FIG. 4.

The initial steps 15, 16 and 17 are identical to the process illustrated in FIG. 3 apart from the entry of a motor current limit value I_Min for that particular commodity is entered into the controller 14.

When the equilibrium is reached in step 17, the current I_motor of the motors driving the evaporator fans 9 is determined and compared to the motor current limit value I_Min in step 28. In case the current I_motor is lower than the limit value I_Min, which indicates that the packaging of the commodities is non-satisfactory, the process continues to step 22, in which the evaporator fans 9 are operated at maximum speed. In case the current I_motor is higher than the limit value I_Min, which indicates that the packaging of the commodities is satisfactory, the process proceeds to step 29, in which the speed of the evaporator fans is gradually (or stepwise) reduced while maintaining box temperature at the set point temperature by means of controlling the cooling effect of the evaporator 12 until an optimised balance is reached, where after the box temperature is maintained by adjusting the cooling effect and/or the evaporator speed in step 30.

Similar processes as disclosed in FIG. 4 could be performed with a limit for the measured or calculated pressure difference ΔP or a calculated value of the cargo airflow resistance R_air.