Patent Description:
It is known to provide air-cargo transport refrigeration units with a simple non-adaptive control system, typically maintaining constant speed operation of refrigeration components when active, such as a compressor or condenser fan. However, using constant, fixed or non-adaptive settings may lead to sub-optimal performance of an air-cargo transport refrigeration unit, such as inefficient or ineffective cooling of cargo contained with the air-cargo transport refrigeration unit.

<CIT> describes a refrigerated storage cabinet. A storing section stores data of a pull down cooling characteristic indicative of a time-varying mode of reduction in a target temperature drop. Upon the start of a pull down control, an internal temperature within the refrigerated storage cabinet is detected at predetermined sampling intervals. An actual temperature drop degree is computed on the basis of the detected internal temperature. The computed value is compared with a target value read from the storing section. When the computed value is less than the target value, a rotational speed of an inverter compressor is increased via an inverter circuit. Conversely, when the computed value is larger than the target value, the rotational speed of the compressor is decreased.

According to a first aspect of the present invention as defined in claim <NUM>, there is disclosed a method of controlling an air-cargo transport refrigeration unit. The air-cargo transport refrigeration unit comprises an internal cargo space, and a refrigeration cycle comprising a compressor, a condenser for rejecting heat to ambient air and provided with a condenser fan, and an evaporator for cooling the internal cargo space. The method comprises performing a control loop including:.

It may be that regulating the speed of the compressor to maintain the rate of temperature decrease comprises evaluating a rate of change of the internal temperature based on temperature signals received from an internal temperature sensor at different times and regulating a speed of the compressor based on the rate of change of the internal temperature.

It may also be that regulating the speed of the compressor to maintain the rate of temperature decrease includes increasing the speed of the compressor based on determining that the rate of temperature decrease is less than the threshold rate.

In addition, it may be that regulating a speed of the condenser fan based on monitoring a discharge pressure further comprises:.

The provision of the transitional range or transitional ranges and the maintenance of a prior setting of the condenser fan speed when the discharge pressure is within the transitional range(s) thereby inhibits the condenser fan speed being adjusted owing to fluctuations in discharge pressure about end points (i.e. bounding thresholds) of a respective low, intermediate or high pressure range. For example, if the discharge pressure was previously in the low-pressure range then the low speed setting may be maintained while the discharge pressures is in the transitional range until the discharge pressure increases to reach the intermediate range. In contrast, if the discharge pressure was previously in the intermediate pressure range then the intermediate speed setting may be maintained while the discharge pressure is in the transitional range until the discharge pressure reaches the low-pressure range.

Further, it may be that the first condenser fan regulation comprises:.

It may be that the method further comprises a third condenser fan regulation, comprising:.

It may also be that the compressor speed regulation is a first compressor speed regulation, and wherein the control loop further comprises a second compressor speed regulation comprising:.

It may be that the refrigeration cycle is one of two refrigeration cycles which share the evaporator for cooling the internal cargo space, each refrigeration cycle comprising a separate compressor and condenser, each condenser being provided with an associated condenser fan, and wherein the method comprises:.

The increase of the speed of the compressor may be applied by incrementally increasing a prior speed setting for each operating compressor, wherein each operating compressor is intended to mean each compressor which is selected to operate based on the cooling demand.

It may be that the control loop further comprises:.

For example, the condenser fan speed may be proportionally reduced to <NUM>% of the earlier setting.

According to a second aspect of the present invention as defined in claim <NUM>, there is disclosed a refrigeration module for an air-cargo transport refrigeration unit, the refrigeration module comprising:.

The refrigeration module may comprise two refrigeration cycles in accordance with the first aspect.

According to a third aspect of the present invention as defined in claim <NUM>, there is provided a non-transitory computer-readable storage medium comprising instructions which, when executed by a processor, cause performance of a method in accordance with any of the examples in accordance with the first aspect.

The controller(s) described herein may comprise a processor. The controller and/or the processor may comprise any suitable circuity to cause performance of the methods described herein and as illustrated in the drawings. The controller or processor may comprise: at least one application specific integrated circuit (ASIC); and/or at least one field programmable gate array (FPGA); and/or single or multi-processor architectures; and/or sequential (Von Neumann)/parallel architectures; and/or at least one programmable logic controllers (PLCs); and/or at least one microprocessor; and/or at least one microcontroller; and/or a central processing unit (CPU), to perform the methods and or stated functions for which the controller or processor is configured.

The controller or the processor may comprise or be in communication with one or more memories that store that data described herein, and/or that store machine readable instructions (e.g. software) for performing the processes and functions described herein (e.g. determinations of parameters and execution of control routines).

The memory may be any suitable non-transitory computer readable storage medium, data storage device or devices, and may comprise a hard disk and/or solid state memory (such as flash memory). In some examples, the computer readable instructions may be transferred to the memory via a wireless signal or via a wired signal. The memory may be permanent non-removable memory or may be removable memory (such as a universal serial bus (USB) flash drive). The memory may store a computer program comprising computer readable instructions that, when read by a processor or controller, causes performance of the methods described herein, and/or as illustrated in the Figures. The computer program may be software or firmware or be a combination of software and firmware.

Except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive, any feature described herein may be applied to any aspect and/or combined with any other feature described herein.

<FIG> shows an example air-cargo transport refrigeration unit <NUM> comprising an internal cargo space <NUM> and a first refrigeration cycle <NUM>. The refrigeration cycle <NUM> comprises in flow order a first compressor <NUM>, a first condenser <NUM> for rejecting heat to ambient air, a first expansion valve <NUM> and an evaporator <NUM> for cooling the internal cargo space <NUM>. The first condenser <NUM> is provided with a first condenser fan <NUM>.

In the example of <FIG>, there is both a first refrigeration cycle <NUM> and a second refrigeration cycle <NUM> which share the evaporator <NUM> as a common evaporator for cooling the internal cargo space <NUM>. The second refrigeration cycle <NUM> comprises a second compressor <NUM>, a second condenser <NUM> for rejecting heat to ambient air, a second expansion valve and the evaporator <NUM> for cooling the internal cargo space <NUM>. The second condenser <NUM> is provided with a second condenser fan <NUM>.

Each refrigeration cycle comprises a separate compressor and condenser, but in this example flow through a common evaporator, for example along separate flow paths through the evaporator. It will be appreciated that a refrigeration unit according to the disclosure may alternatively have two refrigeration cycles each with their own respective evaporator or may have only a single refrigeration cycle such as the first refrigeration cycle <NUM>.

<FIG> is a flowchart showing an example method <NUM> of controlling an air-cargo transport refrigeration unit. The air-cargo transport refrigeration unit may be in accordance with the example air-cargo transport refrigeration unit <NUM> described above and shown in <FIG>.

The method <NUM> may be separate to a method of selectively activating and deactivating the or each refrigeration cycle of the air-cargo transport refrigeration unit, for example based on a set-point temperature for the internal cargo space. It may be that the or each refrigeration cycle is selectively activated or deactivated based on a determination that a monitored temperature of the internal cargo space is greater than the set-point temperature for the internal cargo space by a threshold value. For instance, it may be that the or each refrigeration cycle is operated in a temperature pull-down mode for cooling the internal cargo space when it is determined that the monitored temperature is greater than the set point temperature.

It may be that the or each refrigeration cycle is selectively activated or deactivated based on a difference between the monitored temperature of the internal cargo space and the set-point temperature for the internal cargo space. It may be that the or each refrigeration cycle is activated in response to a determination that the difference between the monitored temperature and the set-point temperature is greater than a dead-band threshold value. For example, it may be that the or each refrigeration cycle is operated in a temperature pull-down mode for cooling the internal cargo space when it is determined that the monitored temperature is greater than the set point temperature and the difference between the monitored temperature and the set-point temperature is greater than the dead-band threshold value. As a corollary, it may be that a cooling mode of the or each refrigeration cycle is deactivated when the monitored temperature is below the set-point temperature or when the difference between the monitored temperature and the set-point temperature is less than the dead-band threshold value.

Accordingly, the method <NUM> as described herein may not directly control whether or not the refrigeration cycle is operating in response to a demand for cooling but may control operating parameters of the refrigeration cycle for improved performance when it is operating, for example, in a temperature pull-down mode. It should be understood that any reference to the components of the first refrigeration cycle <NUM> or control thereof in the following description may apply equally to equivalent components of the second refrigeration cycle <NUM> and the control thereof, unless indicated otherwise.

The method comprises performing a control loop <NUM>. The control loop <NUM> includes a compressor speed regulation <NUM> and a condenser fan regulation <NUM>. The expression "regulation" is used herein to refer to a regulating action or control action for the respective component, and for example may include any of monitoring, calculation and actuation steps in order to cause a change in the control of the respective component based on monitoring an associated parameter. The compressor speed regulation <NUM> and the condenser fan regulation <NUM> may be executed sequentially or in parallel in the control loop <NUM>. Preferably, the compressor speed regulation <NUM> and the condenser fan regulation <NUM> may be executed sequentially. More preferably, the condenser fan regulation <NUM> may be executed prior to the compressor speed regulation <NUM>.

The compressor speed regulation <NUM> comprises monitoring a temperature in the internal cargo space of the air-cargo transport refrigeration unit (for example using a temperature sensor installed in the internal cargo space). The compressor speed regulation <NUM> further comprises regulating a speed of a compressor of a refrigeration cycle of the air-cargo transport refrigeration unit to maintain a rate of temperature decrease in the internal cargo space above a threshold rate during a temperature pulldown mode. The speed of the or each compressor is linked to a cooling capacity of the or each refrigeration cycle <NUM>. For example, an increase in the speed of the or each compressor <NUM> causes an increase in the cooling capacity of the or each refrigeration cycle <NUM>. Accordingly, the compressor speed regulation <NUM> is able to regulate the cooling capacity of the or each refrigeration cycle <NUM> to ensure that the internal temperature of the cargo space <NUM> decreases at an acceptable rate in the temperature pull-down mode.

The condenser fan regulation <NUM> comprises regulating a speed of a condenser fan of the refrigeration cycle of the air-cargo transport refrigeration unit based on monitoring a discharge pressure of a refrigerant of the refrigeration cycle. The discharge pressure of the refrigerant may be directly monitored using a pressure sensor located between the or each compressor <NUM> and the or each condenser <NUM>. Otherwise, the discharge pressure of the refrigerant may be indirectly monitored by monitoring other thermodynamic properties of refrigerant between the and/or each compressor <NUM> and the or each condenser <NUM>, and/or thermodynamic properties of ambient air.

<FIG> is a flowchart showing an example implementation of the compressor speed regulation <NUM> of the control loop <NUM> with further detail of the step of regulating the speed of the compressor to maintain the rate of temperature decrease (block <NUM>). At block 224a, the method includes evaluating a rate of change of the internal temperature based on temperature signals received from an internal temperature sensor at different times.

As an example, temperature signals may be received in sequence at periodic intervals governed by a refresh rate of the internal temperature sensor. The rate of change of the internal temperature may then be evaluated by dividing a difference between sequential temperature signals by a time period between receipt of sequential temperature signals. As another example, temperature signals may be received in sequence according to a selected sampling rate at respective sample times. The rate of change of internal temperature or the temperature slope may then be evaluated based on a difference between a temperature signal received at a first sampling time, a temperature signal received at a second sampling time and a time elapsed between the first sampling time and the second sampling time.

At block 224b, the method includes regulating a speed of the compressor based on the rate of change of the internal temperature.

Regulating the speed of the compressor may comprise incrementally increasing the speed of the compressor based on determining that the rate of temperature decrease is less than the threshold rate. The incremental increase applied to the speed of the compressor provides a relatively stable control regime for the compressor while in the temperature pull-down mode while ensuring adequate cooling to the internal cargo space.

<FIG> is a flowchart showing an implementation of the second condenser fan regulation <NUM> of the control loop <NUM> according to the invention. The step of regulating a speed of the condenser fan based on monitoring a discharge pressure may comprise a number of sub-steps. In <FIG>, the method step comprises sub-block 232a, in which a determination is made with respect to the monitored discharge pressure. The method then proceeds to select between operation as described below and illustrated with reference to blocks 232b, 232c or 232d and optionally 232e or 232f.

At block 232b, the method comprises setting the speed of the condenser fan at a high condenser fan speed in response to a determination that the discharge pressure is in a high-pressure range. At block 232c, the method comprises setting the speed of the condenser fan at an intermediate condenser fan speed in response to a determination that the discharge pressure is in an intermediate pressure range between the high-pressure range and a low-pressure range. At block 232d, the method comprises setting the speed of the condenser fan at a low condenser fan speed in response to a determination that the discharge pressure is in the low-pressure range.

It may be that the high-pressure range, the intermediate-pressure range, and the low-pressure range do not overlap. The high-pressure range, the intermediate-pressure range and the low-pressure range may be contiguous. The high-pressure range, the intermediate-pressure range, and the low-pressure range may be delimited by a high operating pressure threshold (the lower limit of the high pressure range) and a low operating threshold (the upper limit of the low pressure range). Discharge pressures between the low operating threshold and the high operating threshold may correspond to particularly efficient operation of the compressor and/or the refrigeration cycle.

For example, if the discharge pressure is above the high operating pressure threshold, an amount of work done on the refrigerant by the compressor may be excessively high and lead to inefficient operation of the system. Such a situation may arise if there is insufficient heat rejection at the condenser, resulting in heat accumulation and a consequent rise in the condensing (saturation) temperature, and a commensurate rise in the pressure at the condenser, to which the discharge pressure from the compressor is directly related.

Setting the condenser fan speed at the high condenser fan speed increases the effectiveness of convective heat transfer from the condenser to ambient air. This results in a reduction in both the saturation (condensing) temperature and the pressure at the condenser, and a corresponding reduction of the discharge pressure. Accordingly, the work done on the refrigerant by the compressor reduces, while a work done on ambient air by the condenser fan increases by a lesser amount, improving the overall efficiency of the refrigeration cycle.

Further, there may be an operating envelope for the refrigeration cycle, which may be a function of performance characteristics of the components (in particular the compressor, condenser and evaporator) and the pressure-temperature relationship for the selected refrigerant. The operating envelope may correspond to an operating capability of the refrigeration cycle, and/or a particularly efficient set of operating conditions for the refrigeration cycle. The operating envelope may be delimited by minimum and maximum condensing temperatures, and minimum and maximum evaporating temperatures - these being the saturation temperature of the refrigerant at the condenser and evaporator respectively.

As discussed above, the condensing (saturation) temperature and discharge pressure may vary depending on whether there is sufficient heat rejection at the condenser. As ambient temperature reduces, heat rejection at the condenser may be enhanced which may result in a reduction in the condensing temperature below the operating envelope for the refrigeration cycle, with a commensurate reduction in the discharge pressure.

Reducing the condenser fan speed decreases the effectiveness of convective heat transfer from the condenser to ambient air and may therefore lead to an increase in the condensing (saturation) temperature and the discharge pressure as discussed above to maintain operation of the refrigeration cycle within the operating envelope. Conversely, insufficient heat rejection may result in the condensing (saturation) temperature rising to exceed the maximum defined by the operating envelope, with a commensurate rise in the discharge pressure. Accordingly, the condenser fan speed may be increased to increase convective heat transfer and maintain the condensing (saturation) temperature within the operating envelope for the refrigeration cycle.

Accordingly, by controlling the condenser fan speed based on discharge pressure as described may result in efficient operation of the refrigeration cycle, and maintenance of the refrigeration cycle within a predetermined operating envelope.

The high pressure range, the intermediate pressure range and the low pressure range may be non-contiguous, and there may be transitional ranges between them. Optionally, the method of regulating a speed of the condenser fan based on monitoring a discharge pressure may further comprise blocks 232e and/or 232f. At block 232e, the method comprises maintaining a prior setting of the condenser fan speed responsive to determining that the discharge pressure is in a transitional range between the low-pressure range and the intermediate pressure range. At block 232f, the method comprises maintaining a prior setting of the condenser fan speed responsive to determining that the discharge pressure is in a transitional range between the intermediate pressure range and the high-pressure range.

Accordingly, when the discharge pressure is in either of the transitional ranges, the condenser fan regulation <NUM> is prevented from adjusting an earlier setting corresponding to the discharge pressure being in an adjacent pressure range. The technical advantages of this example of the method <NUM> are explained with reference to <FIG>.

<FIG> is a graph showing an example of how a condenser fan speed varies in accordance with an example implementation of the method <NUM> as the discharge pressure of a refrigerant of the refrigeration cycle <NUM> varies with time. The low-pressure range is denoted by reference numeral <NUM>, the intermediate-pressure range is denoted by reference numeral <NUM> and the high-pressure range is denoted by reference numeral <NUM>. In addition, a transitional range between the low-pressure range <NUM> and the intermediate-pressure range <NUM> is denoted by <NUM>, while a transitional range between the intermediate-pressure range <NUM> and the high-pressure range <NUM> is denoted by <NUM>.

In the example shown in <FIG>, the low-pressure range is defined as the region beneath the low pressure operating threshold as represented by the short-dashed line <NUM>. The intermediate pressure range is defined as the region between a low-pressure hysteresis threshold represented by the long-dashed line <NUM> and a high-pressure hysteresis threshold represented by the long-dashed line <NUM>. The high-pressure range is defined as the region above the high pressure operating threshold as represented by the short-dashed line <NUM>. The transitional range <NUM> between the low-pressure range <NUM> and the intermediate-pressure range <NUM> is defined as the region between the low pressure operating threshold <NUM> and the low-pressure hysteresis threshold <NUM>, while the transitional range <NUM> between the intermediate-pressure range <NUM> and the high-pressure range <NUM> is defined as the region between the high-pressure hysteresis threshold <NUM> and the high pressure operating threshold <NUM>.

The provision of the transitional range or transitional ranges and the maintenance of a prior setting of the condenser fan speed when the discharge pressure is within the transitional range(s) inhibits the condenser fan speed being adjusted owing to relatively minor fluctuations in discharge pressure about end points (i.e. bounding thresholds) of a respective low, intermediate or high pressure range. For example, if the discharge pressure was previously in the low-pressure range then the low speed setting may be maintained while the discharge pressures is in the transitional range until the discharge pressure increases to reach the intermediate range (as shown by the close-dashed line <NUM> representing low-speed operation extending through the lower transitional range <NUM>). In contrast, if the discharge pressure was previously in the intermediate pressure range then the intermediate speed setting may be maintained while the discharge pressure is in the transitional range until the discharge pressure reaches the low-pressure range (as shown by the dash-dot line <NUM> representing intermediate speed operation extending through the lower transitional range <NUM>).

It follows that the method <NUM> is able to reduce a number of condenser fan speed changes required under a variety of operating conditions. A high number of condenser fan speed changes is associated with reduced stability of the refrigeration cycle <NUM> and increased wear on drive components of the condenser fan <NUM>. The method <NUM> may therefore increase stability of the refrigeration cycle <NUM> and reduces wear on drive components of the condenser fan <NUM>.

In accordance with the invention and as shown by <FIG>, the condenser fan regulation <NUM> is a second condenser fan regulation <NUM> which is preceded in order of execution in the control loop <NUM> by a first condenser fan regulation <NUM>. Accordingly, the control loop comprises both the first condenser fan regulation <NUM> and the second condenser fan regulation <NUM>, with the first condenser fan regulation <NUM> being performed prior to the second condenser fan regulation <NUM> within the control loop <NUM>. Any action or result of the first condenser regulation <NUM> may be overwritten by the subsequent second condenser regulation <NUM>, such that an action or a result of the second condenser regulation <NUM> is applied in preference to an action or a result of the first condenser regulation <NUM>. As discussed above, the second condenser fan speed regulation <NUM> may determine to maintain at a prior setting (e.g. as set by the first condenser fan regulation or in an earlier iteration of the control loop), such that a prior setting is not overwritten.

The first condenser fan regulation <NUM> comprises regulating a speed of the condenser fan based on monitoring a temperature of the ambient air. The temperature of the ambient air may be directly monitored using an ambient temperature sensor for monitoring the temperature of the ambient air.

<FIG> is a flowchart showing an implementation of the first condenser fan regulation <NUM> of the control loop <NUM> according to the invention. The step of regulating a speed of
the condenser fan based on monitoring a temperature of the ambient air may comprise a number of sub-steps. In the example shown in <FIG> at block 242a a determination is made with respect to the monitored ambient temperature. The method then selectively proceeds to blocks 242b, 242c or 242d.

At block 242b, the method comprises setting the speed of the condenser fan at the high condenser fan speed in response to a determination that the ambient temperature is in a high-temperature range. At block 242c, the method comprises setting the speed of the condenser fan at an intermediate condenser fan speed in response to a determination that the ambient temperature is in an intermediate-temperature range between the high-temperature range and a low-temperature range. At block 242d, the method comprises setting the speed of the condenser fan at a low condenser fan speed in response to a determination that the ambient temperature is in the low-temperature range.

It may be that the high-temperature range, the intermediate-temperature range, and the low-temperature range do not overlap, and it may be that they are contiguous. The high-temperature range, the intermediate-temperature range, and the low-temperature range may be delimited by a high ambient temperature parameter and a low ambient temperature parameter. As described above, heat rejection at the condenser is a function of the temperature difference between the condensing (saturation) temperature and the temperature of the ambient air and a flow rate of air passed the condenser, with a rate of heat transfer increasing with increasing temperature difference, and increasing with increasing fan speed. Consequently, setting the condenser fan speed based on ambient temperature permits the resultant rate of heat transfer to be managed. As described above, this can prevent excess heat accumulation and increase of the discharge pressure (which leads to excessive compressor work) and can also maintain the refrigeration cycle within its operating envelope.

Returning once again to the example shown by <FIG>, it may be that the control loop <NUM> further comprises a third condenser fan regulation <NUM> which is preceded in order of execution in the control loop <NUM> by the first condenser fan regulation <NUM> and the second condenser fan regulation <NUM>. The third condenser fan regulation <NUM> comprises evaluating a fault condition of an ambient temperature sensor for monitoring the temperature of the ambient air. The third condenser fan regulation <NUM> comprises setting the speed of the condenser fan at the high condenser fan speed when the evaluation is indicative of a fault associated with the ambient temperature sensor.

The third condenser fan regulation <NUM> ensures that adequate heat rejection from the or each condenser <NUM> to ambient air (and therefore adequate cooling to the internal cargo space) is maintained in the event of a fault in an ambient temperature sensor, despite a possible detrimental effect to the overall efficiency of the or each refrigeration cycle <NUM> (e.g. from running the condenser speed higher than optimal). This improves a reliability of the air-cargo transport refrigeration unit <NUM>.

In addition, the third condenser fan regulation <NUM> ensures that the temperature of refrigerant in the condenser does not become very high in the event of an ambient temperature sensor fault and a high ambient air temperature, which in turn prevents a temperature at the or each condenser <NUM> from becoming very high and a commensurate rise in condensing (saturation) temperature and discharge pressure.

It may be that the compressor speed regulation <NUM> is a first compressor speed regulation <NUM>, and the control loop <NUM> comprises a second compressor speed regulation <NUM> which is preceded in order of execution in the control loop <NUM> by the first compressor speed regulation <NUM>. Accordingly, any action or result of the first compressor speed regulation <NUM> may be overwritten by the subsequent second compressor speed regulation <NUM>, such that an action or a result of the second compressor speed regulation <NUM> is applied in preference to an action or a result of the first compressor speed regulation <NUM>.

The second compressor speed regulation <NUM> comprises an evaluation of whether the air-cargo transport refrigeration unit <NUM> is in an extreme operating condition. The evaluation results in a determination that the air-cargo transportation unit <NUM> is in an extreme operating when the compressor speed is set to the high compressor speed and the temperature of the internal cargo space is above a threshold setting. When the extreme operating condition is determined, the second compressor speed regulation <NUM> comprises reducing the speed of the compressor. When the air-cargo transport refrigeration unit <NUM> is in the extreme operating condition, it may be that the compressor <NUM> is being operated to generate a large pressure ratio and to operate at a high speed. Accordingly, a total load on the compressor <NUM> may be very high when the air-cargo transport refrigeration unt <NUM> is in the extreme condition. A reduction in compressor speed when the air-cargo transport refrigeration unit <NUM> is in the extreme condition reduces the total load on the compressor, which in turn may cause the compressor to operate more efficiently and/or prevents components of the compressor from being subject to excessive wear.

In an example of the air-cargo transport refrigeration unit <NUM> which comprises both the first refrigeration cycle <NUM> and the second refrigeration cycle <NUM>, the method <NUM> may further comprise a step of selecting which of the first compressor <NUM> and the second compressor <NUM> to operate based on a cooling demand of the air-cargo transport refrigeration unit <NUM>. The cooling demand of the air-cargo transport may be based on, for example, a difference between the temperature of the ambient air and the temperature of the internal cargo space, a difference between the temperature of the internal cargo space and a temperature set-point, an indication of a type of cargo stored in the internal cargo space <NUM> and/or an indication of an amount of cargo stored in the internal cargo space <NUM>.

The method <NUM> may comprise selecting to operate only a single one of the first compressor <NUM> and the second compressor <NUM> in response to a determination that the cooling demand is below a cooling demand threshold. Alternatively, the method <NUM> may comprise selecting to operate both of the first compressor <NUM> and the second compressor <NUM> in response to a determination that the cooling demand is above a cooling demand threshold. It may be advantageous to operate only a single one of the compressors when the cooling demand is relatively low, since operating both compressors when the cooling demand is low may cause each compressor to operate in a region corresponding to a relatively low efficiency on a respective operating map. On the other hand, operating only a single one of the compressors may cause the selected compressor to operate in a region corresponding to a relatively high efficiency on its operating map. Consequently, the overall efficiency of the air-cargo transport refrigeration unit <NUM> may be improved. Further, by only operating one compressor when possible, the total operating time of each compressor may be reduced for a given operational time of the air-cargo refrigeration unit.

Optionally, when operating only a single one of the first compressor <NUM> and the second compressor <NUM>, the method may include selecting which compressor to operate based on an operating time parameter associated with each compressor. For example, the method <NUM> may comprise selecting to operate the compressor for which the associated operating time parameter indicates it has logged less operating time since installation and/or since a service interval. This reduces unequal use and wear, and reduces the likelihood that the first compressor <NUM> or the second compressor <NUM> will log excessive operating time since installation and/or between service intervals. It may therefore reduce the likelihood of premature failure due to wear of the first compressor <NUM> and/or the second compressor <NUM>.

It may be that the control loop <NUM> further comprises an evaluation of an in-flight criterion, represented by block <NUM>. The in-flight criterion corresponds to whether or not the air-cargo transport refrigeration unit <NUM> is in an in-flight condition (that is, the air-cargo transport refrigeration unit is being transported by means of an airborne aircraft).

The evaluation of the in-flight criterion comprises determining whether an acceleration of the air-cargo transport refrigeration unit <NUM> corresponds to the in-flight condition. The determination is based on a signal received from an accelerometer <NUM> of the air-cargo transport refrigeration unit <NUM>. For instance, the determination may be based on a signal received from the accelerometer <NUM> corresponding to a take-off acceleration parameter or a landing acceleration parameter. The determination may be further based on a history of signals received from the accelerometer <NUM> of the air-cargo transport refrigeration unit <NUM>. For example, the determination may be based on comparing the history of signals received from the accelerometer <NUM> with a profile of acceleration consistent with a take-off profile.

Additionally or alternatively, the evaluation of the in-flight criterion may comprise determining whether an altitude of the air-cargo transport refrigeration unit <NUM> corresponds to the in-flight condition, based on a signal received from an altimeter <NUM> of the air-cargo transportation unit <NUM>. For instance, the determination may be based on a signal received from the altimeter <NUM> corresponding to an airborne altitude parameter. The determination may be further based on a history of signals received from the altimeter <NUM> of the air-cargo transport refrigeration unit <NUM>. For example, the determination may be based on comparing the history of signals received from the altimeter <NUM> to an altitude profile consistent with a phase of flight, such as take-off or climb.

When the evaluation is indicative of the air-cargo transport refrigeration unit being in the in-flight condition, the method proceeds to block <NUM>, in which in-flight regulations are performed. The in-flight regulations may comprise an in-flight condenser fan regulation, and an in-flight compressor speed regulation.

In the in-flight compressor speed regulation, the speed of the or each compressor <NUM> is set to a predetermined constant speed when it is determined that the discharge pressure is in the high-pressure range.

In the in-flight condenser fan regulation, the condenser fan speed is proportionally reduced from an earlier setting as set in any condenser fan regulation of the control loop. For example, the condenser fan speed may be proportionally reduced to <NUM>% of an earlier setting of the condenser fan speed, or to <NUM>% of the speed that would have been set when the in-flight condition is not determined.

As described elsewhere herein, a reduction in condenser fan speed may cause an increase in discharge pressure. The predetermined constant speed of the compressor may be selected to limit and/or offset an increase in the discharge pressure.

This ensures that an airflow generated by the or each condenser fan <NUM> within a cargo hold in which the air-cargo transport refrigeration unit may be disposed does not become excessively large. If the airflow generated by the or each condenser fan <NUM> within the cargo hold were to become excessively large, it may be that an air flow within a cargo hold of an aircraft is disrupted. For example, such a disruption may interfere with operation of safety alarms (e.g. temperature sensitive alarms related to fire detection) associated with the cargo-hold. Accordingly, the steps represented by blocks <NUM> and <NUM> provide that the method <NUM> and control loop <NUM> are more suitable for air-cargo transportation applications.

<FIG> shows a refrigeration module <NUM> for an air-cargo transportation unit <NUM>. The refrigeration module <NUM> comprises a controller <NUM>, a first refrigeration cycle <NUM> and a second refrigeration cycle <NUM>. The first refrigeration cycle <NUM> and the second refrigeration cycle <NUM> may be in accordance with the cycles described with respect to <FIG> above, with like reference numerals indicating common components.

As described with reference to <FIG>, each refrigeration cycle comprises a separate compressor and condenser but in this example flow through a common evaporator, for example along separate flow paths through the evaporator. It will be appreciated that a refrigeration unit according to the disclosure may alternatively have two refrigeration cycles each with their own respective evaporator or may have only a single refrigeration cycle such as the first refrigeration cycle <NUM>.

Claim 1:
A method (<NUM>) of controlling an air-cargo transport refrigeration unit (<NUM>) comprising: an internal cargo space (<NUM>), and a refrigeration cycle (<NUM>) comprising a compressor (<NUM>), a condenser (<NUM>) for rejecting heat to ambient air and provided with a condenser fan (<NUM>), and an evaporator (<NUM>) for cooling the internal cargo space;
the method (<NUM>) comprising performing a control loop (<NUM>) including:
a compressor speed regulation (<NUM>) comprising:
monitoring (<NUM>) a temperature in the internal cargo space;
regulating (<NUM>) a speed of the compressor to maintain a rate of temperature decrease in the internal cargo space above a threshold rate during a temperature pulldown mode;
the method characterised in that the control loop further includes:
a first condenser fan regulation (<NUM>) comprising: regulating a speed of the condenser fan based on monitoring a temperature of the ambient air; and
a second condenser fan regulation (<NUM>) comprising:
regulating a speed of the condenser fan based on monitoring a discharge pressure of a refrigerant of the refrigeration cycle;
wherein regulating the speed of the condenser fan based on monitoring a discharge pressure comprises:
setting (232b) the speed of the condenser fan at a high condenser fan speed in response to a determination (232a) that the discharge pressure is in a high-pressure range (<NUM>);
setting (232d) the speed of the condenser fan at a low condenser fan speed in response to a determination (232a) that the discharge pressure is in a low-pressure range (<NUM>); and
setting (232c) the speed of the condenser fan at an intermediate condenser fan speed in response to a determination (232a) that the discharge pressure is in an intermediate-pressure range (<NUM>) between the low-pressure range and the high-pressure range, and
wherein within the control loop, the first condenser fan regulation is performed prior to the second condenser fan regulation, such that a result of the second condenser fan regulation is applied in preference to a result of the first condenser fan regulation.