TECHNICAL FIELD

[0001] The present invention relates to refrigeration systems for transport units, heat exchangers for refrigeration systems, transport units comprising refrigeration systems, and marine vessels comprising transport units.

BACKGROUND

[0002] Many types of cargo may be stored in transportable storage units, also referred to as transport units, for transporting cargo on container vessels. Such a storage unit may comprise an atmosphere control system for controlling an atmosphere in the storage unit, such as using a refrigeration system. This may be used to facilitate the storage and transportation of perishable goods, such as fruit, vegetables, or fresh or frozen meat or fish, or other goods, such as medicaments, in the transport unit. Transport units include reefer containers, which may be TEU or 2-TEU containers designed to be shipped on container vessels, and/or refrigerated trucks or trailers.

SUMMARY

[0003] Embodiments of the present invention as described herein aim to improve a cooling capacity achievable by refrigeration systems for transport units, and/or to reduce a size of such refrigeration systems, which may provide more space for cargo to be transported in the transport units. Other advantages provided by embodiments of the invention include reducing a cost of, improving an efficiency of, and/or reducing an environmental impact of such refrigeration systems.

[0004] According to a first aspect of the present invention, there is provided a refrigeration system for a transport unit, the refrigeration system comprising a refrigeration cycle comprising: a compressor; an evaporator; a condenser fl uidically coupled downstream of the compressor and upstream of the evaporator; an expansion valve fluidically coupled downstream of the condenser and upstream of the evaporator; and a suction gas heat exchanger comprising a liquid line side and a suction line side, wherein the liquid line side is fluidically coupled downstream of the condenser and upstream of the expansion valve, and the suction line side is fluidically coupled downstream of the evaporator and upstream of the compressor, whereby the suction gas heat exchanger is configured to transfer heat between refrigerant in the liquid line side and refrigerant in the suction line side.

[0005] The refrigerant flowing from the condenser to the expansion valve, in use, may be at a higher temperature than the refrigerant flowing from the evaporator to the compressor. In this way, the suction gas heat exchanger may be configured to further heat the refrigerant flowing from the evaporator to the compressor, such as to superheat all refrigerant above its saturation temperature. In this way, an amount of liquid entering the compressor may be reduced. This may improve an efficiency and/or a longevity of the compressor.

[0006] The suction gas heat exchanger may similarly cause a reduction in a temperature of the refrigerant flowing from the condenser to the expansion valve, in use. This may reduce a temperature of refrigerant downstream of the expansion valve, such as to increase a level of subcooling of refrigerant below its saturation temperature downstream of the expansion valve. This may further improve an efficiency of the evaporator, such as by supplying refrigerant having a higher liquid-phase content to the evaporator, thereby increasing an amount of heat storable in the refrigerant as latent heat. This may also reduce a pressure drop across the evaporator, and/or increase a pressure of refrigerant received by the compressor, which may increase a performance and/or efficiency of the compressor and/or increase a capacity of the refrigeration system as a whole.

[0007] Optionally, the refrigeration system comprises a non-azeotropic refrigerant in the refrigeration cycle.

[0008] Optionally, the non-azeotropic refrigerant exhibits a temperature glide. Optionally, the non- azeotropic refrigerant comprises a mixture of two or more substances, such as two, three, four or more than four substances. The non-azeotropic refrigerant may comprise a mixture of at least a first substance and a second substance, the first and second substances having different saturation temperatures at a given pressure. That is, the first and second substances may boil at different temperatures. Boiling of the first and second substances at different temperatures and/or at different times in a heat exchanger of the refrigeration system, such as in the cascade heat exchanger, the pre-cooler, the suction gas heat exchanger (where provided), and/or the evaporator (where provided), may induce increased turbulence in the refrigerant flow through the heat exchanger, which may in turn provide improved heat exchange characteristics in the heat exchanger. [0009] The refrigeration system may be configured so that only some of the refrigerant leaving the evaporator is superheated above its saturation temperature, in use. This may be by superheating only one of the first and second substances, or a subset of three or more substances in the non-azeotropic refrigerant, where provided. This may cause at least some, or all, of the refrigerant leaving the evaporator to remain in a liquid phase. This, in turn, may improve an efficiency of the evaporator, such as by providing a liquid coating of refrigerant along a greater length of a conduit of the evaporator, which may allow more heat from an external fluid surrounding the evaporator to be absorbed by the refrigerant as latent heat. An efficiency of the refrigeration system may also be improved by a reduction in a pressure drop across the evaporator, and/or an increase a pressure of refrigerant received by the compressor, due to the reduced superheating of the refrigerant.

[0010] The refrigeration system may be configured to use refrigerants comprising high volumes of CO2, such as up to 40% CO2, up to 50% CO2, up to 60% CO2, or over 60% CO2. The refrigeration system may be configured to use the refrigerant R473A, manufactured by Klea™. This may allow the refrigeration system to achieve cooling temperatures of below -40 C, such as below -50 C, below -60 C and/or below -70 C, while using a refrigerant with a relatively low Global Warming Potential (compared to an equivalent mass of CO2).

[0011] Optionally, the compressor is a multi-stage compressor comprising more than one compression stage.

[0012] The multi-stage compressor may be a two-stage compressor. Optionally, the multi-stage compressor comprises a compressor low stage and a compressor high stage. Optionally, the suction line side of the suction gas heat exchanger is fluidically coupled downstream of the evaporator and upstream of the compressor low stage. Providing a multi-stage compressor may allow high compression ratios to be achieved by the refrigeration system while spreading the demand on the compressor over the two or more stages. The multi-stage compressor may be configured to provide a compression ratio, being a ratio of the pressure received and supplied by the compressor, of up to 1 :5, 1 :8, 1 :11 , or over 1 :11. Providing a compressor having two or more stages may improve a performance and/or longevity of the compressor, such as by reducing a likelihood of the compressor overheating, particularly when providing a higher compression ratio. The multi-stage compressor may be particularly beneficial when using refrigerants requiring relatively high compression ratios, such as R473A, and/or when the refrigerant in the refrigeration system reaches extremely low temperatures, such as below -40 C, below -50 C, below -60 C and/or below -70 C. [0013] Optionally, the refrigeration system is a cascade refrigeration system comprising: the refrigeration cycle, wherein the refrigeration cycle is a first refrigeration cycle, the compressor is a first compressor, and the expansion valve is a first expansion valve; a second refrigeration cycle comprising a second compressor and a second expansion valve; and a cascade heat exchanger comprising a condenser side fluidically coupled downstream of the first compressor and upstream of the first expansion valve, and an evaporator side fluidically coupled downstream of the second expansion valve and upstream of the second compressor; and the condenser is the condenser side of the cascade heat exchanger.

[0014] Optionally, the cascade heat exchanger is configured to transfer heat between refrigerant in the first refrigeration cycle and refrigerant in the second refrigeration cycle. In this way, the cascade refrigeration system may be able to cool the transport unit, or a part thereof, to a lower temperature than with a single-cycle refrigeration system or may be able to provide the same amount of cooling with an improved efficiency.

[0015] Optionally, the refrigeration system is configured to use a different refrigerant in the first refrigeration cycle than in the second refrigeration cycle. In other words, the refrigerant in the first cycle may be different to the refrigerant in the second cycle, in use. The cascade refrigeration system may be configured to use refrigerant in the first refrigeration cycle that has a lower saturation temperature at a given pressure than refrigerant in the second refrigeration cycle. Optionally, the refrigeration system is configured so that, in use, a temperature of refrigerant supplied to the condenser side of the cascade heat exchanger is higher than a temperature of refrigerant supplied to the evaporator side. The first and second refrigeration cycles may be a low and high temperature refrigeration cycles, respectively, relative to each other.

[0016] Optionally, the refrigeration system comprises a pre-cooler, the pre-cooler comprising a first side and a second side, the first side being fluidically coupled downstream of the first compressor and upstream of the condenser side of the cascade heat exchanger. The pre-cooler is configured to transfer heat between refrigerant in the first side and refrigerant in the second side.

[0017] The pre-cooler may be configured, in use, to reduce a temperature of refrigerant that is passed through the first side of the pre-cooler using refrigerant. This may be to reduce the temperature of the refrigerant supplied by the first compressor before it enters the condenser side of the cascade heat exchanger. This may provide a lower temperature difference between refrigerant in the condenser side of the cascade heat exchanger and refrigerant in the evaporator side of the cascade heat exchanger, which may improve an efficiency of evaporation of refrigerant in the evaporator side of the cascade heat exchanger, in use. This may, in turn, may provide a more stable level of superheat of refrigerant above its saturation temperature downstream of the evaporator side, and may make the refrigeration system is easier to regulate. By providing the pre-cooler, an efficiency and/or capacity of the cascade heat exchanger may be improved. Alternatively, or in addition, by providing the pre-cooler, a smaller cascade heat exchanger may be used. This is particularly advantageous for transport units, which may have limited space available for the refrigeration system. This may also reduce a cost of the cascade refrigeration system.

[0018] Optionally, the pre-cooler is configured to transfer heat between refrigerant in the first refrigeration cycle and refrigerant in the second refrigeration cycle. Optionally, the second side of the pre-cooler is fluidically coupled in the second refrigeration cycle. This may improve an efficiency of the refrigeration system, by making further use of the refrigerant in the first refrigeration cycle.

[0019] Alternatively, the pre-cooler may be configured to transfer heat between refrigerant in the first refrigeration cycle and refrigerant in a third refrigeration cycle of the refrigeration system. Optionally, the second side of the pre-cooler is fluidically coupled in the third refrigeration cycle. Alternatively, the pre-cooler may be configured to transfer heat between refrigerant in the first refrigeration cycle and refrigerant in another refrigeration system.

[0020] Optionally, the refrigeration system comprises: an economiser expansion valve fluidically coupled downstream of the second compressor and upstream of the second side of the precooler. Optionally, the refrigeration system comprises an economiser heat exchanger comprising: a first economiser side fluidically coupled downstream of the second compressor and upstream of the second expansion valve; and a second economiser side fluidically coupled downstream of the second compressor and upstream of the second side of the pre-cooler.

[0021] The economiser expansion valve may be configured to cause a reduction in temperature and pressure of the refrigerant passing therethrough, in use. In this way, the refrigerant in the second side of the pre-cooler, which is downstream of the economiser expansion valve, may be at a lower temperature than refrigerant in the first side of the pre-cooler, which is downstream of the first compressor. [0022] By expanding some of the refrigerant in the second refrigeration cycle using the economiser expansion valve, the refrigeration system may be more efficient than, for example, using an external fluid to reduce a temperature of the refrigerant supplied to the condenser side of the cascade heat exchanger.

[0023] Optionally, the economiser expansion valve is in a parallel fluidic connection with the second expansion valve in the second refrigeration cycle. Optionally, the second side of the precooler is fluidically coupled downstream of the economiser expansion valve and upstream of the second compressor.

[0024] Optionally, the economiser heat exchanger is configured to transfer heat between refrigerant in the first economiser side and refrigerant in the second economiser side. Optionally, the second economiser side is in a parallel fluidic connection with the second expansion valve. Optionally, the second economiser side of the economiser heat exchanger is fluidically coupled downstream of the economiser expansion valve and upstream of the second side of the precooler.

[0025] A temperature of refrigerant in the first economiser side may be at a lower temperature than refrigerant in the second economiser side, in use, such as due to the refrigerant in the second economiser side having been expanded to a lower temperature and pressure across the economiser expansion valve. In this way, the economiser heat exchanger may cause a reduction in temperature of refrigerant supplied to the second expansion valve, such as to increase a level of subcooling of refrigerant supplied to the evaporator side of the cascade heat exchanger below its saturation temperature, in use. This may improve a capacity and/or efficiency of the refrigeration system.

[0026] The refrigerant leaving the second economiser side of the economiser heat exchanger, in use, may be at a lower temperature than refrigerant in the first side of the pre-cooler. The refrigeration system may provide a further use of the relatively lower-temperature refrigerant expanded through the economiser expansion valve to pre-cool refrigerant entering the condenser side of the cascade heat exchanger. By supplying refrigerant from the second economiser side of the economiser heat exchanger to the second side of the pre-cooler, an overall efficiency of the refrigeration system may be improved.

[0027] Optionally, the evaporator comprises a first fluid channel, a second fluid channel, an inlet, and a valve arrangement fluidically coupled between the inlet and the first and second fluid channels. The first and second fluid channels are configured to pass refrigerant from the inlet through the evaporator, so that heat can be exchanged between the refrigerant in the first and second fluid channels and an external fluid that is external to the first and second fluid channels, in use. The valve arrangement is configurable in a first configuration to fluidically couple both of the first and second fluid channels to the inlet, or in a second configuration to fluidically couple one of the first and second fluid channels to the inlet and to fluidically isolate the other of the first and second fluid channels from the inlet.

[0028] Optionally, the valve arrangement comprises a first valve fluidically coupled between the inlet and the first fluid channel, and a second valve fluidically coupled between the inlet and the second fluid channel. Optionally, the valve arrangement comprises an isolation valve, or shut-off valve, such as an electronically-operated or manually-operated isolation valve, such as a ball valve and/or a pipe squeezer. Optionally, the valve arrangement comprises a selector valve configured, when in the second configuration, to supply fluid from the inlet to the one of the first and second fluid channels.

[0029] This may provide improved redundancy in the event of a loss of integrity of one of the first and second fluid channels. For instance, under normal operation, the valve arrangement may permit refrigerant to flow through both of the first and second fluid channels. The valve arrangement may be configured to permit refrigerant to flow through only the one of the first and second fluid channels, in the event of a loss of integrity of the other of the first and second fluid channels, such as due to a crack or leak in the other of the first and second fluid channels.

[0030] This is particularly advantageous for a refrigeration system for a transport unit, which may be more prone to vibrations and/or corrosion during transport of the transport unit than other refrigeration systems. This may also allow the refrigeration system to continue operating in the event of such a loss of integrity, such as until it is next able to be repaired. This is similarly particularly advantageous for a refrigeration system for a transport unit, whereby repair of the refrigeration system may be difficult, or impossible, during transport of the transport unit.

[0031] Optionally, the refrigeration system comprises a controller that is configured to determine a loss of integrity of the first and/or the second fluid channel, and/or to determine which of the first and second fluid channels has lost its integrity. This may be by the controller causing operation of the valve arrangement to fluidically couple one of the first and second fluid channels to the inlet and to isolate the other of the first and second fluid channels from the inlet, such as to configure the valve arrangement in the second configuration, then pressurising the other of the first and second fluid channels, and receiving and utilising output of a gas sensor outside of the evaporator to detect a presence of refrigerant leaking from the one of the first and second fluid channels. This may be repeated for the other of the first and second fluid channels.

[0032] The controller may be configured to perform such a determination periodically, and/or in response to an indication of a suspected leak and/or loss of charge in the refrigeration system. The indication may be any one or more of: an inability of the refrigeration system to maintain a setpoint temperature, such as a setpoint temperature of the external fluid; a superheat of refrigerant leaving the evaporator and/or another heat exchanger of the refrigeration system being below a superheat threshold; detection of an atmospheric pressure in the refrigeration system; a lack of cooling of the frequency convertor; and/or any other suitable indication of a loss of integrity of the first and/or second fluid channel. Optionally, the controller may be configured to cause the valve arrangement to be configured in the second configuration, so that the one of the first and second fluid channels is fluidically coupled to the inlet, in the event of a loss of integrity of the other of the first and second fluid channels. Optionally, the determining of a loss of integrity of, and/or the isolating of one of the first and second fluid channels, such as by configuring the valve arrangement in the second configuration, may be performed manually, such as by an operator or maintenance crew, such as in response to an indication of a leak and/or suspected loss of charge as discussed above.

[0033] Optionally, the evaporator is a fin-and-tube heat exchanger, such as comprising the first and second fluid channels and plural fins thermally coupled to the first and second fluid channels. Optionally, the external fluid is passed through, or across, the evaporator in use, for instance so that the external fluid is in contact with the fins and/or an external surface of the first and second fluid channels, in use. This may provide improved heat exchange properties between refrigerant flowing in the first and/or second fluid channels and the external fluid, such as an atmosphere in the cargo space of the transport unit, in use.

[0034] Optionally, the first and second fluid channels are interlaced throughout the evaporator, such as by being arranged in parallel circuitous paths through the evaporator. This may allow the evaporator to operate at up to 50%, up to 60%, up to 70%, or more than 70% capacity, in the event that one of the first and second fluid channels is isolated from the inlet.

[0035] Optionally, the second refrigeration cycle comprises a second-cycle condenser fluidically connected downstream of the second compressor and upstream of each of the first expansion valve and the economiser expansion valve. The second-cycle condenser is configured to transfer heat between refrigerant in the second refrigeration cycle flowing through the condenser and a further external fluid, such as an ambient atmosphere surrounding the refrigeration system and/or a transport unit in which the refrigeration system is installed. In this way, heat from the atmosphere in the cargo space of the transport unit may be transferred to the refrigerant in the first cycle via the evaporator. This heat may then be transferred to the first refrigeration cycle via the cascade heat exchanger, and then expelled into an external atmosphere via the second-cycle condenser. Providing two refrigeration cycles configured to exchange heat with each other via the cascade heat exchanger may provide further cooling of the atmosphere in the cargo space of the transport unit than might otherwise be obtainable with a single-cycle refrigeration system. Optionally, the second-cycle condenser comprises a first condenser fluid channel, a second condenser fluid channel, a condenser inlet, and a condenser valve arrangement, which are configurable in the same way as, respectively, the first fluid channel, the second fluid channel, the inlet and the valve arrangement, as optionally provided above for the evaporator. It will be appreciated that the above optional features of the valve arrangement are equally applicable to the condenser valve arrangement of the second-cycle condenser.

[0036] Optionally, the first refrigeration cycle comprises a gas cooler fluidically connected downstream of the first compressor and upstream of the first side of the cascade pre-cooler. The gas cooler is configured to transfer heat between refrigerant in the first refrigeration cycle flowing through the gas cooler and the external fluid, which as noted above may be an ambient air surrounding the refrigeration system and/or a transport unit. The gas cooler may be configured to reduce a temperature of refrigerant leaving the compressor, before it enters the pre-cooler and/or the cascade heat exchanger, in use, particularly where the temperature of refrigerant leaving the compressor is higher than the temperature of the external fluid. This may improve an efficiency of the refrigeration in a similar way to the pre-cooler.

[0037] Optionally, the first refrigeration cycle comprises a first-cycle receiver configured to receive refrigerant from the condenser side of the cascade heat exchanger, and to supply the refrigerant to the first expansion valve. Optionally, the second refrigeration cycle comprises a second-cycle receiver configured to receive refrigerant from the second-cycle condenser, and to supply refrigerant to the second expansion valve, and optionally also to the economiser expansion valve.

[0038] Optionally, the first refrigeration cycle comprises a gas injector valve fluidically coupled downstream of the condenser side of the cascade heat exchanger and upstream of the second compressor. Optionally, the gas injector valve is fluidically coupled downstream of the receiver and upstream of the second compressor. Optionally, the gas injector valve is in a parallel fluid connection with the first expansion valve. Optionally, the first compressor comprises a gas injector port. Optionally, the gas injector valve is fluidically coupled downstream of the condenser side of the cascade heat exchanger and upstream of the gas injector port. Optionally, where the first compressor is a two-stage compressor, the gas injector port is fluidically coupled downstream of the first compressor low stage and upstream of the first compressor high stage. Optionally, the gas injector port opens into the first compressor at a location such that a pressure at the gas injector port is between a pressure at an inlet of the first compressor low stage and an outlet of the first compressor high stage. In this way, refrigerant expanded by the gas injector valve may be used to reduce a temperature of a part of the first compressor, such as a motor and/or frequency convertor of the first compressor. This may be to maintain the temperature of the part of the first compressor, such as sensed by a first compressor temperature sensor, below a predetermined temperature threshold.

[0039] Optionally, the second compressor comprises an economiser port. Optionally, the second side of the pre-cooler is fluidically coupled upstream of an economiser port of the second compressor. Optionally, where the second compressor is a two-stage compressor, the economiser port is fluidically coupled downstream of the second compressor low stage and upstream of the second compressor high stage. Optionally, the economiser port opens into the compressor at a location such that a pressure at the economiser port is between a pressure at an inlet of the second compressor low stage and an outlet of the second compressor high stage. In this way, refrigerant expanded by the economiser expansion valve may be used to reduce a temperature of a part of the second compressor, such as a motor and/or frequency convertor of the second compressor. This may be to maintain the temperature of the part of the second compressor, such as sensed by a second compressor temperature sensor, below a predetermined temperature threshold.

[0040] Optionally, any one or more of the cascade heat exchanger, the pre-cooler, the economiser heat exchanger and the suction gas heat exchanger is a plate heat exchanger. Optionally, any one or more of the plate heat exchangers in the refrigeration system are orientated horizontally, or at an angle between horizontal and vertical, in use. For instance, any one or more of the plate heat exchangers may comprise: a first fluid inlet for receiving a first refrigerant; a first fluid outlet for expelling the first refrigerant; and a first plate and a second plate defining a first cavity therebetween, through which the first refrigerant can flow between the first fluid inlet and the first fluid outlet. The first and second plates may define plural first fluid pathways through the cavity. The first and second plates may be planar, and may be orientated orthogonal, or substantially orthogonal, to a horizontal plane, in use. A plate heat exchanger orientated at a shallow angle may comprise: the first fluid inlet fluidically coupled to open into an upper portion of the first cavity at a first end of the first cavity; and the first fluid outlet fluidically coupled to open into an upper portion of the first cavity at a second end of the first cavity, opposite to the first end; wherein, the plate heat exchanger is orientated so that the first fluid outlet is substantially level to, or higher than, the first fluid inlet. In this way, in use, the first refrigerant may enter the first cavity through the first fluid inlet, and any of the first refrigerant in the liquid phase may, due to the action of gravity, better fill the first cavity, or at least a portion of the cavity below the first fluid inlet and the first fluid outlet. Optionally, the first refrigerant is either the refrigerant used in the first cycle or the refrigerant used in the second cycle.

[0041] Optionally, any one or more of the plate heat exchangers in the refrigeration system comprise a third plate located to define a second cavity between the second plate and the third plate. The third and second plates may together define plural second fluid pathways through the second cavity. The first, second and third plates may be arranged in a laminar, or stacked, structure. Optionally, any one or more of the plate heat exchangers comprise a second fluid inlet and a second fluid outlet, each fluidically coupled to open into the second cavity at opposing ends of the cavity. The second fluid outlet may open into the second cavity at first end of the second cavity, such as corresponding to the first end of the first cavity, optionally at a position lower than the first fluid inlet. The second fluid inlet may open into the second cavity at a second, opposite, end of the second cavity, such as corresponding to the second end of the second cavity, optionally at a position lower than the first fluid outlet.

[0042] A second aspect of the present invention provides a heat exchanger for a refrigeration system, the heat exchanger comprising a first fluid channel, a second fluid channel, an inlet, and a valve arrangement fluidically coupled between the inlet and the first and second fluid channels. The first and second fluid channels are configured to pass refrigerant from the inlet through the evaporator, so that heat can be exchanged between the refrigerant in the first and second fluid channels and an external fluid that is external to the first and second fluid channels, in use. The valve arrangement is configurable in a first configuration to fluidically couple both of the first and second fluid channels to the inlet, or in a second configuration to fluidically couple one of the first and second fluid channels to the inlet and to fluidically isolate the other of the first and second fluid channels from the inlet. [0043] Optionally, the heat exchanger is an evaporator or a condenser for the refrigeration system. Optionally, the refrigeration system is a refrigeration system for a transport unit. Optionally, the heat exchanger is a heat exchanger for the refrigeration system of the first aspect. Optionally, the refrigeration system of the first aspect comprises the heat exchanger. The heat exchanger may be the evaporator and/or second-cycle condenser of the refrigeration system of the first aspect.

[0044] Where heat exchanger is an evaporator, the external fluid may be an atmosphere in a cargo space of the transport unit. Alternatively, where the heat exchanger is a condenser of the refrigeration system, the external fluid may be an ambient atmosphere surrounding the refrigeration system and/or the transport unit.

[0045] Any of the optional features and advantages provided for the evaporator and the condenser of the first aspect above equally apply to the heat exchanger of the second aspect.

[0046] A third aspect of the present invention provides an atmosphere control system for a transport unit, the atmosphere control system comprising the refrigeration system of the first aspect and/or the heat exchanger of the second aspect.

[0047] It will be appreciated that the atmosphere control system may comprise any of the optional features of the first and/or the second aspect, and/or may benefit from any of the advantages ascribed to any of the first and/or the second aspect.

[0048] A fourth aspect of the present invention provides a transport unit comprising a cargo space for storing cargo and the refrigeration system of the first aspect, wherein the evaporator is configured to transfer heat between the cargo space and refrigerant in the evaporator.

[0049] Optionally, the transport unit comprises the heat exchanger of the second aspect or the atmosphere control system of the third aspect.

[0050] Optionally, the atmosphere control system and/or the refrigeration system is configured to control an atmosphere in the cargo space. Optionally, the atmosphere control system and/or the refrigeration system comprises an evaporator fluid moving device, such as a fan, that is configured to move the atmosphere in the cargo space across the evaporator, such as to cause a transfer of heat between refrigerant in the evaporator and the atmosphere in the cargo space, such as to cool the atmosphere in the cargo space. [0051] Optionally, the transport unit is a reefer container, such as for transporting cargo on a marine vessel, such as on a container ship. Optionally, the transport unit is a refrigerated truck, or trailer.

[0052] Optionally, the transport unit comprises a parts storage space for storing replacement parts for the refrigeration system of the first aspect, and/or the heat exchanger of the second aspect. Optionally, the parts storage space is located so as to be easily accessible by service personnel, such as outside the cargo space of the transport unit, such as at a front or rear of the transport unit or in proximity to an access port, or hatch, associated with the refrigeration system and/or the atmosphere control system. In this way, the replacement parts may be retrieved without requiring access to the cargo space. This may be particularly advantageous where access to the cargo space is limited, such when cargo is stored in the cargo space, and/or when the atmosphere in the cargo space is being controlled, such as by the refrigeration system and/or the atmosphere control system, such as during transit. Optionally, the transport unit comprises the replacement parts in the parts storage space. Optionally, the replacement parts comprise any one or more of: a temperature sensor, such as for sensing a temperature of refrigerant in the first and/or the second refrigeration cycle, and/or a temperature of the external fluid; a pressure sensor, such as for as for sensing a pressure of refrigerant in the first and/or the second refrigeration cycle, and/or a pressure of the external fluid; a humidity sensor, such as for sensing a humidity, such as a relative humidity, of the external fluid; an expansion valve, such as a replacement first expansion valve, second expansion valve, third expansion valve, and/or gas injection valve; and a pressure relief valve, such as a pressure relief valve for the first compressor, the second compressor, the first-cycle receiver, and/or the second cycle receiver. Optionally, the replacement parts comprise important parts, without which operation of the refrigeration system cannot be properly maintained. The important parts may comprise parts for the first refrigeration cycle, such as one or more temperature sensors for the first refrigeration cycle, a replacement first expansion valve, and/or a pressure relief valve for the first refrigeration cycle. This may allow the refrigeration system to be maintained during transport, such as in the event of a failure of any one of the corresponding parts installed in the refrigeration system. This may facilitate any goods being transported in the transport unit being kept at or below a required temperature. Optionally, the parts storage space comprises, and/or is configured to store, specialised tools for facilitating the maintenance of the refrigeration system. [0053] It will be appreciated that the transport unit may comprise any of the optional features of the first to third aspects, and/or may benefit from any of the advantages ascribed to any of the first to third aspects.

[0054] A fifth aspect of the present invention provides a marine vessel comprising the refrigeration system of the first aspect, the heat exchanger of the second aspect, or the transport unit of the fourth aspect.

[0055] Optionally, the marine vessel comprises the atmosphere control system of the third aspect.

[0056] Optionally, the marine vessel is a container ship.

[0057] It will be appreciated that the marine vessel may comprise any of the optional features of the first to fourth aspects, and/or may benefit from any of the advantages ascribed to any of the first to fourth aspects.

BRIEF DESCRIPTION OF DRAWINGS

[0058] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0059] Figure 1 shows a schematic view of a transport unit comprising a refrigeration system, according to an example;

[0060] Figure 2 shows a schematic view of an example refrigeration system of the transport unit of Figure 1 ;

[0061] Figure 3 shows a schematic view of an example heat exchanger arrangement of the refrigeration system of Figure 2;

[0062] Figure 4 shows a schematic exploded view of an example plate heat exchanger of the refrigeration system of Figure 2;

[0063] Figure 5 shows a schematic view of an example orientation of the plate heat exchanger of Figure 4; and [0064] Figure 6 shows a schematic view of a marine vessel comprising the transport unit of Figure 1 , according to an example.

DETAILED DESCRIPTION

[0065] Figure 1 shows an example storage unit 10, which is specifically a transport unit 10 for transporting cargo 15. More specifically, the transport unit 10 is a reefer container for transporting the cargo 15 on a marine vessel, but may alternatively be a refrigerated truck or trailer.

[0066] The cargo 15 in the illustrated example is fresh or frozen produce. This may include respirating and/or ripenable produce, such as fruit and vegetables, and/or non-respirating fresh or frozen produce, such as meat and/or fish. In other examples, the storage unit 10 may be for transporting any other suitable cargo 15, for example medicaments, such as vaccines. It will be appreciated, however, that the cargo 15 may be any other suitable cargo 15, and may advantageously be cargo 15 that requires, or benefits from, being stored in an atmosphere- controlled space.

[0067] The storage unit 10 comprises a cargo space 12 for storing the cargo 15, and an atmosphere control system 20 for controlling an atmosphere in the cargo space 12. Specifically, the atmosphere control system 20 is configured to supply conditioned gas, such as cooled or heated gas, or gas with a specific composition, into the cargo space 12, such as through one or both of a first port 21a and a second port 21b that each open into the cargo space 12, or via any other suitable fluidic connection between the atmosphere control system 20 and the cargo space 12. In other examples, the atmosphere control system 20, or a part thereof, is located in the cargo space 12.

[0068] The illustrated atmosphere control system 20 comprises a refrigeration system 100 configured to condition the gas to be the supplied to the cargo space 12. For clarity, Figure 1 shows only a part of the refrigeration system 100. The refrigeration system 100 is shown in more detail in Figure 2 and will be described in more detail below. The part of the refrigeration system 100 shown in Figure 1 comprises an evaporator 110, which acts as a heat exchanger to cool gas supplied to the cargo space 12. Components of the refrigeration system 100 are fluidically coupled to one another by conduits, some of which are shown as directional arrows in Figure 1. The conduits are configured to pass refrigerant between the respective components, specifically in the direction of the arrows. [0069] The refrigeration system 100 also comprises an evaporator fluid moving device 111 , which here is a fan 111 , to draw the gas through, or across, the evaporator 110. The evaporator 110 comprises a fin-and-tube heat exchanger for exchanging heat between a refrigerant flowing in the evaporator 110 and the gas passed through the evaporator 110, but may alternatively be of any other suitable construction. The evaporator fluid moving device 111 is specifically configured to draw gas from the cargo space 12, such as through the second port 21b, and to supply gas conditioned by the evaporator 110 to the cargo space 12, such as through the first port 21a. The evaporator fluid moving device 111 may be selectively operable in a forward and a reverse direction, such as to change which of the first and second ports 21a, 21 b the conditioned gas is supplied to and/or received from. In other examples, the evaporator 110 and/or the evaporator fluid moving device 111 may be located in the cargo space 12. A specific example configuration of the evaporator 110 is described in more detail below with reference to Figure 3.

[0070] Turning now to Figure 2, shown is an example of the refrigeration system 100 of Figure 1 . Specifically, the refrigeration system 100 is a cascade refrigeration system 100 comprising a first refrigeration cycle 101 , a second refrigeration cycle 102 and a cascade heat exchanger 150 configured to transfer heat between refrigerant in the first refrigeration cycle 101 and refrigerant in the second refrigeration cycle 102. The cascade heat exchanger 150 will be described in more detail below. Herein, refrigerant comprised in the first and second refrigeration cycles may be referred to respectively as a “first-cycle refrigerant” and a “second-cycle refrigerant”. Moreover, reference herein to a specific component of the refrigeration system 100 being “downstream” of one component and “upstream” of another component is in the context of a single refrigeration cycle centred on either the upstream or downstream component referenced. For example, if a component B is disclosed as being downstream of a component A and upstream of a component C, then refrigerant flowing from component A, in use, would flow through component B before component C, and would not flow through component C before component B.

[0071] The evaporator 110 is fluidically coupled in the first refrigeration cycle 101. At a high level, the first refrigeration cycle 101 also comprises a first compressor 120 and a first expansion valve 140 that is fluidically coupled downstream of the first compressor 120 and upstream of the evaporator 110. The second refrigeration cycle 102 comprises a second compressor 220, a second expansion valve 240, and a second-cycle condenser 210 that is fluidically coupled downstream of the second compressor 220 and upstream of the second expansion valve 240. [0072] The cascade heat exchanger 150 comprises a condenser side 151 fluidically coupled downstream of the first compressor 120 and upstream of the first expansion valve 140, and an evaporator side 152 fluidically coupled downstream of the second expansion valve 240 and upstream of the second compressor 220. In this way, the cascade heat exchanger 150 can function as a condenser of the first refrigeration cycle 101 and an evaporator of the second refrigeration cycle 102.

[0073] An example mode of operation of the refrigeration system 100 may be as follows. A high- pressure, high-temperature gaseous (or vaporous) first-cycle refrigerant provided by the first compressor 120 is condensed in the condenser side 151 of the cascade heat exchanger 150. The condensed (or at least partly condensed) first-cycle refrigerant from the cascade heat exchanger 150 is then passed to the first expansion valve 140. The first expansion valve 140 is configured to reduce a pressure and temperature of the first-cycle refrigerant supplied to the first evaporator 110, which in turn can be used to cool the gas to be supplied to the cargo space 12 of the transport unit 10. The first-cycle refrigerant evaporated in the first evaporator 110 is then returned to the first compressor 120.

[0074] Similarly, the second-cycle condenser 210 is configured to receive a high-pressure, high- temperature second-cycle refrigerant from the second compressor 220. The second-cycle condenser 210 is configured to condense the second-cycle refrigerant by exchanging heat between the second-cycle refrigerant and an external fluid surrounding the second-cycle condenser 210. In the illustrated example, at least a part of the second-cycle condenser 210 is in fluidic communication with an ambient atmosphere surrounding the transport unit 10, and the external fluid is the ambient atmosphere. The refrigeration system 100 also comprises a condenser fluid moving device 211 that is operable to cause the external fluid to move through, or across, the second-cycle condenser 210, in use. A specific example configuration of the second-cycle condenser 210 is described in more detail below with reference to Figure 3.

[0075] The condensed (or at least partly condensed) second-cycle refrigerant from the second- cycle condenser 210 is then passed to the second expansion valve 240. The second expansion valve 240 is configured to reduce a pressure and temperature of the second-cycle refrigerant supplied to the evaporator side 152 of the cascade heat exchanger 150. In use, the second-cycle refrigerant supplied to the evaporator side 152 of the cascade heat exchanger 150 is at a lower temperature than the first-cycle refrigerant supplied to the condenser side 151 of the cascade heat exchanger 151. In this way, heat exchange between the first-cycle refrigerant and second- cycle refrigerant in the cascade heat exchanger 150 causes the second-cycle refrigerant to evaporate and the first-cycle refrigerant to condense in the cascade heat exchanger 150. The second-cycle refrigerant evaporated in the evaporator side of the cascade heat exchanger 150 is then returned to the second compressor 220.

[0076] In this way, broadly speaking, heat from the cargo space 12 of the transport unit 10 is transferred to the first -cycle refrigerant in the first refrigeration cycle 101 via the evaporator 110. This heat is then transferred to the second-cycle refrigerant in the second refrigeration cycle 102 via the cascade heat exchanger 150, and then expelled into the ambient atmosphere surrounding the transport unit 10 via the second-cycle condenser 210.

[0077] In the present example, the refrigeration system 100 comprises a different refrigerant in the first refrigeration cycle 101 than in the second refrigeration cycle 102. In other words, the first- cycle refrigerant is different to the second-cycle refrigerant, in use. Specifically, the first-cycle refrigerant has a lower saturation temperature at a given pressure than the second-cycle refrigerant. This allows the first refrigeration cycle 101 to operate at a lower temperature than the second refrigeration cycle 102, and so provide greater cooling to the gas from the cargo space 12. For this reason, the first refrigeration cycle 101 may be referred to herein as a low temperature cycle of the refrigeration system, and the second refrigeration cycle 102 may be referred to as a high temperature cycle of the refrigeration system. Providing two refrigeration cycles 101 , 102 configured to exchange heat with each other via the cascade heat exchanger 150 may provide further cooling of the cargo space 12 of the transport unit 10 than might otherwise be obtainable with a single-cycle refrigeration system.

[0078] Further components of the refrigeration system 100 are now described in more detail.

[0079] Firstly, as shown in Figure 2, the refrigeration system 100 comprises an economiser heat exchanger 260 comprising a first economiser side 261 and a second economiser side 262. The first economiser side 261 is fluidically coupled downstream of the second compressor 220 and upstream of the second expansion valve 240. The refrigeration system 100 also comprises a third expansion valve 230 fluidically coupled downstream of the second compressor 220 and the second-cycle condenser 211 , and upstream of the second economiser side 262. More specifically, the third expansion valve 230 is fluidically coupled in a parallel fluidic connection with the second expansion valve 240. In other words, the refrigeration system 100 comprises two parallel paths leading from a junction upstream of the second and third expansion valves 240, 230. The second expansion valve 240 is fluidically coupled in one of the parallel paths, and the third expansion valve 230 is fluidically coupled in the other of the parallel paths. [0080] In this way, some of the second-cycle refrigerant flowing in a liquid line between the second-cycle condenser 210 and the second expansion valve 240 may be tapped off and expanded through the third expansion valve 230, such as to reduce a temperature and/or pressure of the refrigerant. This may then be passed through the second economiser side 262 to cool the second-cycle refrigerant in the first economiser side 261 , such as to cause, or increase a level of subcooling of the second cycle refrigerant in the first economiser side below its saturation temperature, before it is passed to the second expansion valve 240. This may improve an overall efficiency and/or heat exchange capacity of the refrigeration system 100.

[0081] The second compressor 220 here comprises a second intermediate port (not shown), which may open into the compressor 220 such that a pressure at the second intermediate port is between a pressure at an inlet of the second compressor 220 and a pressure at an outlet of the second compressor 220. In this way, the second-cycle refrigerant may be expanded across the third expansion valve 230 to a pressure that is higher than the pressure downstream of the second expansion valve 240, such as to the intermediate pressure at the second intermediate port. This may provide a reduction in temperature of the second-cycle refrigerant through the third expansion valve that is sufficient to cause a subcooling of refrigerant in the first economiser side 261 of the economiser heat exchanger 260, without significantly impacting a power demand of the second compressor 220. The second compressor 220 itself will be described in more detail below.

[0082] The refrigeration system 100 shown in Figure 2 also comprises a pre-cooler 180. The precooler 180 comprises a first side 181 and a second side 182. The first side 181 is fluidically coupled in the first refrigeration cycle 101 , specifically downstream of the first compressor 120 and upstream of the condenser side 151 of the cascade heat exchanger 150. The second side 182 is fluidically coupled in the second refrigeration cycle 102, specifically downstream of the second side of the economiser heat exchanger 261 and upstream of the second compressor 220. However, in other examples, the economiser heat exchanger 260 is not present, and the second side 181 of the pre cooler 180 is fluidically coupled downstream of the second expansion valve 240, or is fluidically coupled to any other suitable refrigeration cycle, such as a third cycle of the refrigeration system 100, or a refrigeration cycle of another refrigeration system.

[0083] In any event, in use, a temperature of refrigerant in the second side 182 of the pre-cooler 180 is lower than a temperature of refrigerant in the first side 181 of the pre-cooler 180. In this way, the pre-cooler 180 is configured to transfer heat between the first-cycle refrigerant and the second-cycle refrigerant, in use. This is specifically to pre-cool the first-cycle refrigerant received from the first compressor 120 before it is passed to the condenser side 151 of the cascade heat exchanger 150. This may also cause further evaporation of the second cycle refrigerant in the second side 182 of the pre-cooler 180, such as to ensure more, or all, of the second-cycle refrigerant is in a gaseous phase before it enters the second compressor 220. This may improve a longevity and/or efficiency of the second compressor 220.

[0084] By pre-cooling the first-cycle refrigerant before it enters the cascade heat exchanger 150, a temperature difference between refrigerant in the condenser side 151 and refrigerant in the evaporator side 152 may be reduced. This may be to improve an efficiency of evaporation of the second-cycle refrigerant in the evaporator side 152 of the cascade heat exchanger 150, in use. More specifically, this may reduce, or limit, an occurrence of the so-called “Leidenfrost effect”, in which evaporation of the second-cycle refrigerant is so vigorous that refrigerant close to a heat exchange surface in the evaporator side 152 forms an insulating vapour layer separating some, or all, of the liquid-phase refrigerant from the heat exchange surface. Reducing a temperature difference between the first-cycle refrigerant and the second cycle refrigerant may result in less vigorous, and therefore more efficient, evaporation of the second-cycle refrigerant in the evaporator side 152 of the cascade heat exchanger 150. The present invention achieves this more efficiently by using the pre-cooler 180, which in turn makes further use of the second-cycle refrigerant expanded by the third expansion valve 230 and passed through the economiser heat exchanger 260.

[0085] A further reduction in the temperature of the first cycle refrigerant upstream of the cascade heat exchanger 150 is achieved by the refrigeration system comprising a gas cooler 190 fluidically coupled in the first refrigeration cycle 101 downstream of the first compressor 120 and upstream of the first side 181 of the pre-cooler 180. The gas cooler 190 is physically located near to the second-cycle condenser 210, and is configured to transfer heat between the first-cycle refrigerant and an external fluid, which may be the external fluid that is passed through the second-cycle condenser 210, which as noted above may be an ambient air surrounding the refrigeration system 100 and/or the transport unit 10. The refrigeration system 100 shown also comprises a gas cooler fluid moving device 191 configured to move the external fluid through, or across, the gas cooler 190, such as across fins and/or pipes of the gas cooler 190. In other examples, the condenser fluid moving device 211 is configured to pass the external fluid through, or across, both the second-cycle condenser 210 and the gas cooler 190. In some examples, the second-cycle condenser 210 and the gas cooler 190 are a part of the same heat exchanger, which may reduce a physical footprint of the condenser 210 and gas cooler 190. The gas cooler is able to provide further pre-cooling of the first-cycle refrigerant, particularly where the temperature of the first-cycle refrigerant leaving the first compressor 120 is higher than the temperature of the external fluid. This may improve an efficiency of the refrigeration system 100 in a similar way to the pre-cooler 180.

[0086] Turning now to the first refrigeration cycle 101 , fluidically coupled downstream of the cascade heat exchanger 150 and upstream of the first compressor 120 is a suction gas heat exchanger 160. The suction gas heat exchanger comprises a liquid line side 161 and a suction line side 162. The liquid line side 161 is fluidically coupled downstream of the condenser side 151 of the cascade heat exchanger 150 and upstream of the first expansion valve 140, and the suction line side 162 is fluidically coupled downstream of the evaporator 110 and upstream of the first compressor 120. The suction gas heat exchanger 160 is configured to transfer heat between refrigerant in the liquid line side and refrigerant in the suction line side, in use. In other words, the suction gas heat exchanger 160 is configured to transfer heat between first-cycle refrigerant flowing in a first-cycle liquid line fluidically coupling the condenser side 151 of the cascade heat exchanger 150 to the first expansion valve 140, and first-cycle refrigerant flowing in a first-cycle suction line fluidically coupling the evaporator 110 to the compressor 120.

[0087] In use, first-cycle refrigerant in the first-cycle liquid line, and thus in the liquid line side 161 , is at a higher temperature than first-cycle refrigerant in the first-cycle suction line, and thus the suction line side 162, due to a reduction in temperature and pressure of the first-cycle refrigerant across the first expansion valve 140. In this way, the suction gas heat exchanger 160 is configured to use the higher-temperature refrigerant flowing in the first-cycle liquid line to further heat refrigerant flowing in the first-cycle suction line from the evaporator 110 to the first compressor 120, such as to superheat all of the first-cycle refrigerant above its saturation temperature at a pressure in the suction line. This can reduce an amount of liquid entering the first compressor 120, and thereby improve an efficiency and/or longevity of the first compressor 120. This may also provide additional subcooling of the first-cycle refrigerant in the liquid line below its saturation temperature at a pressure in the liquid line. This can lead to a higher proportion of liquid-phase refrigerant in the first-cycle refrigerant downstream of the first expansion valve 140, and/or may reduce a pressure in the suction line, thereby improving an efficiency of the refrigeration system 100. [0088] It will be appreciated that, in this way, the suction line heat exchanger 160 can facilitate a higher proportion of liquid-phase refrigerant being provided to and/or leaving the evaporator 110. In some such examples, only some, or none, of the refrigerant leaving the evaporator 110 is superheated above its saturation temperature at a pressure in the suction line, in use. In other such examples, the first-cycle refrigerant provided to the evaporator 110 is entirely in a liquid phase. Providing more liquid-phase refrigerant in the evaporator 110, and/or through a greater proportion of the evaporator 110, may be referred to as operating the refrigeration system 100 with a “wet”, or “flooded” evaporator 110. This may allow more latent heat transfer to occur between the refrigerant and the gas to be supplied to and/or received from the cargo space 12, which may improve a cooling capacity and/or efficiency of the refrigeration system 100.

[0089] In the illustrated example, the first refrigeration cycle 101 comprises a first-cycle refrigerant comprising a mixture, or blend, of refrigerants. Specifically, the first-cycle refrigerant is a non- azeotropic refrigerant (or “zeotropic” refrigerant), comprising a mixture of at least a first substance and a second substance, wherein the first and second substances have different saturation temperatures at a given pressure, such as the refrigerant R473A, manufactured by Klea™. In other words, the non-azeotropic first-cycle refrigerant exhibits a temperature glide. The second refrigeration cycle 102, in contrast, comprises a second-cycle refrigerant having a single refrigerant compound, such as R134a. In other examples, the second-cycle refrigerant comprises a mixture, or blend, of refrigerants, such as an azeotropic refrigerant comprising a mixture of substances having the same saturation temperatures at a given pressure, such as R513a. In other examples, either of the first-cycle and second-cycle refrigerants is any other suitable refrigerant. For example, either of the first-cycle and second-cycle refrigerants may be a “pure” refrigerant having a single refrigerant compound or may be an azeotropic or non-azeotropic refrigerant.

[0090] In general, the refrigeration system 100, and particularly the first refrigeration cycle 101 , is, in various examples, configured to use refrigerants comprising high volumes of CO2, such as up to 40% CO2, up to 50% CO2, up to 60% CO2, or over 60% CO2. This can allow the refrigeration system to achieve cooling temperatures of below -40 C, such as below -50 C, below -60 C and/or below -70 C, while using a refrigerant with a relatively low Global Warming Potential (compared to an equivalent mass of CO2). That is, the refrigeration system 100 is, in some examples, capable of cooling a temperature of the gas supplied to the cargo space 12 to below - 40 C, such as below -50 C, below -60 C and/or below -70 C. [0091] An example heat exchanger arrangement 300 is now described with reference to Figure 3. In the illustrated example, the heat exchanger arrangement 300 is the evaporator 110. It will be appreciated that, in other examples, the heat exchanger arrangement 300 is the condenser 210.

[0092] The heat exchanger arrangement 300 is specifically a fin-and-tube heat exchanger 300, comprising a first and second fluid channels 311 , 312 and plural fins 310 thermally coupled to the first and second fluid channels 311 , 312. The first and second fluid channels 311 , 312 are here interlaced throughout the heat exchanger arrangement 300, specifically by being arranged in parallel circuitous paths through the fins 310. It will be appreciated that other configurations of the first and second fluid paths 311 , 312 are possible.

[0093] In use, an external fluid is passed through, or across, the heat exchanger arrangement 300, so that the external fluid is in contact with the fins 310 and/or an external surface of the first and second fluid channels 311 , 312. It will be appreciated that, where the heat exchanger arrangement 300 is the evaporator 110, the external fluid is the gas to be supplied to the cargo space 12. Where the heat exchanger arrangement 300 is the second-cycle condenser 210, the external fluid is the ambient atmosphere.

[0094] The heat exchanger arrangement 300 also comprises an inlet 330, and a valve arrangement 320 fluidically coupled between the inlet 330 and the first and second fluid channels 311 , 312. In this way, the first and second fluid channels 311 , 312 are configured to pass refrigerant from the inlet 330 through the heat exchanger arrangement 300, specifically to an outlet 350. The valve arrangement 320 is configurable in a first configuration to fluidically couple both of the first and second fluid channels 311 , 312 to the inlet 330, and in a second configuration to fluidically couple one of the first and second fluid channels 311 , 312 to the inlet 330 and to fluidically isolate the other of the first and second fluid 311 , 312 channels from the inlet 330.

[0095] More specifically, the valve arrangement 320 comprises a first valve 321 fluidically coupled between the inlet 330 and the first fluid channel 311 , and a second valve 322 fluidically coupled between the inlet 330 and the second fluid channel 312. The first and second valves 321 , 322 are electronically-operated valves, such as electronically-operated isolation valves or shut-off valves. Alternatively, the first and second valves 321 , 322 are manually-operated isolation valves, such as pipe squeezers and/or ball valves. [0096] In this way, as shown by the dashed line in Figure 3, the second valve 322 may be closed to prevent refrigerant from flowing through the second fluid channel 312 from the inlet 330, while the first valve 321 may remain open to permit refrigerant to flow from the inlet 330 through the first fluid channel 311. Alternatively, the first valve 321 may be closed and the second valve 322 may be open to permit refrigerant to flow from the inlet 330 through the second fluid channel 312, but not through the first fluid channel 311. Shutting both of the first and second valves 321 , 322 would prevent refrigerant from flowing through either of the first and second fluid channels 311 , 312 from the inlet 330. In the event of, for example, a rupture or other source of refrigerant leakage from one of the first and second fluid channels 311 , 312, the affected fluid channel may be isolated by closing the respective valve 321 , 322, thereby permitting the refrigeration system 100 to continue operating using only the other of the first and second fluid channels 311 , 312. The interlaced nature of the first and second fluid channels 311 312 can allow the heat exchanger arrangement 300 to operate at up to 50%, up to 60%, up to 70%, or more than 70% capacity, in the event that one of the first and second fluid channels 311 , 312 is isolated from the inlet 330.

[0097] In the illustrated example, the valve arrangement 320 also comprises first and second outlet valves 341 , 342 configured to selectively fluidically couple the respective first and second fluid channels 311 , 312 to the outlet 350. The first and second outlet valves 341 , 342 are any suitable valves as provided for the first and second valves 321 , 321 described above. The first and second outlet valves 341 , 342 are configured to be closed, to prevent refrigerant flowing therethrough, at the same time as the respective first and second valves 321 , 322. This may inhibit a back-flow of refrigerant from the outlet 350 through a respective isolated first and/or second fluid channel 311 , 312, and/or prevent external fluid from entering the refrigeration system 100 through, for example, a rupture in the respective first and/or second fluid channel 311 , 312. In other examples, there is no such first and/or second outlet valve 341 , 342. In other examples, the first and/or the second outlet valve 341 , 342 comprises a non-return valve, which may permit a flow of refrigerant from the respective first and/or second fluid channel 311 , 321 to the outlet 350, but prevent a flow of refrigerant from the outlet 350 to the respective first and/or second fluid channel 311 , 321. In other examples not shown here, the valve arrangement 320 instead comprises a single, or more than one, selector valve that is configured, when in the second configuration, to selectively fluidically couple the inlet 330 and/or the outlet 350 to the one of the first and second fluid channels 311 , 312.

[0098] In the illustrated example, the refrigeration system 100 comprises a controller 360 that is configured to determine a loss of integrity of the first and/or the second fluid channel 311 , 312, and/or to determine which of the first and second fluid channels 311 , 312 has lost its integrity. This is by the controller 360 causing operation of the valve arrangement 320 to fluidically couple one of the first and second fluid channels 311 , 312 to the inlet and to isolate the other of the first and second fluid channels from the inlet, specifically to configure the valve arrangement in the second configuration. The controller 360 then causes pressurisation the other of the first and second fluid channels 311 , 312, and receives and utilises output of a gas sensor 370 located outside of the first and second fluid channels 311 , 312 to detect a presence of refrigerant leaking from the one of the first and second fluid channels 311 , 312. This may be repeated for the other of the first and second fluid channels 311 , 312.

[0099] The controller 360 is configured to perform this determination periodically, but may alternatively perform the determination in response to an indication of a leak and/or a suspected loss of charge in the refrigeration system 100. The indication may, for example, be an inability of the refrigeration system to maintain a setpoint temperature; a superheat of refrigerant leaving the one or more heat exchangers of the refrigeration system 100 being below a superheat threshold; detection of an atmospheric pressure in the refrigeration system 100; and/or any other suitable indication of a loss of integrity of the first and/or second fluid channel 311 , 312. In some examples, the controller 360 is configured to cause the valve arrangement to be configured in the second configuration, so that one of the first and second fluid channels 311 , 312 is fluidically coupled to the inlet 330 (and/or the outlet 350), in the event of a loss of integrity of the other of the first and second fluid channels 311 , 312.

[0100] In other examples, the determining of a loss of integrity of, and/or the isolating of one of the first and second fluid channels 311 , 312, such as by configuring the valve arrangement in the second configuration, is performed manually, such as by an operator or maintenance crew, such as in response to an indication of a leak and/or suspected loss of charge as discussed above.

[0101] Turning now to Figure 4, shown is a schematic exploded view of an example plate heat exchanger 400 according to an example. Any one or more of the cascade heat exchanger 150, the economiser heat exchanger 260, the pre-cooler 180, and the suction line heat exchanger 160 may be, or may comprise, such a plate heat exchanger 400. The plate heat exchanger 400 comprises a plurality of plates 410a to 41 Oe arranged parallel to one another in a laminar, or stacked, arrangement. The plate heat exchanger 400 shown in Figure 4 specifically comprises first to fifth plates 410a to 41 Oe configured so as to define a respective cavity 420a to 420d between each adjacent pair of the plates 410a to 41 Oe. In this way, a first cavity 420a is defined between the first and second plates 410a, 410b, a second cavity 420b is defined between the second and third plates 410b, 410c, and so on. First to fourth cavities 420a to 420d are shown in Figure 4.

[0102] The plate heat exchanger 400 also comprises a first fluid inlet 430a for receiving a first fluid, a first fluid outlet 430b for expelling the first fluid, a second fluid inlet 440a for receiving a second fluid, and a second fluid outlet 440b for expelling the second fluid. The first fluid may be a refrigerant, such as the first-cycle or second-cycle refrigerant, and the second fluid may be a further refrigerant. The second fluid may be the first fluid but at a different point in the refrigeration system 100, such as where the plate heat exchanger 400 is the suction gas heat exchanger 160 or the economiser heat exchanger 260, or it may be a different fluid, such as where the plate heat exchanger 400 is the pre-cooler 180 or the cascade heat exchanger 150.

[0103] The plate heat exchanger 400 is configured so that the first fluid is able to pass from the first fluid inlet 430a to the first fluid outlet 430b via the second and fourth cavities 420b, 420d, as shown with a solid line in Figure 4. The second fluid is able to pass from the second fluid inlet 440a to the second fluid outlet 440b via the first and second cavities 420a, 420c, as shown with a dashed line in Figure 4. In this way, each internal plate 410b to 410d (i.e. excluding the first and fifth plates 410a, 41 Oe, which are endplates) defines a heat exchange interface for exchanging heat between the first fluid and the second fluid. It will be appreciated that the first and third cavities 420a, 420c may define a first “side” of the plate heat exchanger 400, while the second and third cavities 420b, 420d define a second “side”. In some examples, the plates 410a to 410e are shaped so as to define plural fluid flow pathways within each cavity.

[0104] The first fluid inlet 430a opens into the second and fourth cavities 420b, 420d at respective first ends 401 of the second and fourth cavities 420b, 420d, which here correspond to respective first ends 401 of the plates 410a to 41 Oe. The first fluid outlet 430b opens into the second and fourth cavities 420b, 420d at respective second, opposite ends 402 of the second and fourth cavities 420b, 420d, which here correspond to respective second ends 402 of the plates 410a, 41 Oe, opposite to the respective first ends 401 of the plates 410a to 41 Oe, specifically in a longitudinal dimension of the plates 410a to 41 Oe.

[0105] Figure 5 shows an example orientation of the plate heat exchanger 400 in the refrigeration system 100. Specifically, the plate heat exchanger 400 is shown orientated at an angle between horizontal and vertical. In other examples, the plate heat exchanger 400 is orientated horizontally. [0106] It will be appreciated that the first to fifth plates 410a to 41 Oe are planar. In the orientation shown in Figure 5, the plates 410a to 41 Oe are orientated orthogonally, or substantially orthogonally, to a horizontal plane, which is the x-y plane using the coordinate system shown in Figure 5 (the y-direction being into the page). In other words, the plates 410a to 410e are aligned with the x-z plane in Figure 5. Figure 5 specifically shows a schematic cross-sectional diagram of the plate heat exchanger 400 through the second cavity 420b, to show the third plate 410c.

[0107] In the orientation shown, the first fluid inlet 430a is fluidically coupled to open into respective upper portions 450 of the second and fourth cavities 420b, 420d at the respective first ends 401 of the second and third cavities, and the first fluid outlet is fluidically coupled to open into respective upper portions 450 of the second and fourth cavities 420b, 420d at the respective second ends 402 of the second and fourth cavities 420b, 420d. Here, an “upper portion” is any part of a cavity 420a to 420d on an upper side of a centreline (not shown) that extends along a centre of the cavity 420a to 420d, or an associated plate 410a to 41 Od, from the first end 401 to the second end 402.

[0108] Thereby, the plate heat exchanger 400 being orientated horizontally, or at an angle between horizontal and vertical, comprises that plate heat exchanger 400 being orientated so that the first fluid outlet 430b is substantially level to, or higher than, the first fluid inlet 430a. In this way, in use, the first fluid may enter the upper portions 450 of the second and fourth cavities 420b, 420d through the first fluid inlet 430a, and any of the first fluid that is in a liquid phase may, due to the action of gravity, better fill the second and fourth cavities 420a, 420d below the first fluid inlet 430a and the first fluid outlet 430b. That is, the first fluid may be better distributed through the second and fourth cavities 420a, 420d, as shown by the solid black lines in Figure 5. This is particularly advantageous where the first fluid is a refrigerant to be evaporated, such as a refrigerant in the evaporator side 152 of the cascade heat exchanger 150. In such a case, a liquidphase portion of the refrigerant vapour in the plate heat exchanger

[0109] By contrast, the second fluid can pass through the plate heat exchanger as shown by the dashed lines in Figure 5. That is, the second fluid inlet 440a is higher than the second fluid outlet 440b. In this case, where the second fluid comprises a refrigerant to be condensed, a gas-phase portion of the refrigerant may spread throughout the first and third cavities 420a, 420c, while a liquid-phase portion of the refrigerant may move towards a lower portion 450 of the respective first and third cavities 420a, 420c due to the action of gravity. This may lead to a more natural, and easier, extraction of liquid-phase refrigerant from the plate heat exchanger 400 as it pools towards the second-fluid outlet 440b. [0110] Returning now to Figure 2, the refrigeration system 100 is described in further detail.

[0111] The first refrigeration cycle 101 comprises a first-cycle receiver 170 configured to receive refrigerant from the condenser side 151 of the cascade heat exchanger 150, and to supply the refrigerant to the first expansion valve 140. Similarly, the second refrigeration cycle 102 comprises a second-cycle receiver 270 configured to receive refrigerant from the second-cycle condenser 210, and to supply the refrigerant to the second expansion valve 240 and to the third expansion valve 230. The first-cycle receiver 170 and second-cycle receiver 270 may be configured to store, and supply, refrigerant in a liquid phase, and may act as a buffer for accommodating changes in pressure in the refrigeration system 100. The first-cycle and second-cycle receivers 170, 270 may comprise respective pressure relief valves (not shown) for relieving overpressures in the refrigeration system 100.

[0112] The first compressor 120 in the illustrated example is a two-stage compressor comprising a first compressor low stage and a first compressor high stage. Specifically, the first compressor 120 is a two-stage piston compressor, but may be any other suitable compressor. The first compressor low stage is configured to pressurise the first-cycle refrigerant from a first suction line pressure at an inlet of the first compressor 120 to a first intermediate pressure, while the first compressor high stage is configured to pressurise the first-cycle refrigerant from the first intermediate pressure to a first discharge pressure at an outlet of the first compressor 120. In a similar way, the second compressor 220 in the illustrated example is a two-stage compressor comprising a second compressor low stage and a second compressor high stage. The second compressor low stage is configured to pressurise the second-cycle refrigerant from a second suction line pressure at an inlet of the second compressor 220 to a second intermediate pressure, while the second compressor high stage is configured to pressurise the second-cycle refrigerant from the second intermediate pressure to a second discharge pressure at an outlet of the second compressor 220.

[0113] It will be appreciated that the first and/or second compressor 120, 220 may instead comprise two single-stage compressors fluidically connected in series. In other examples, the first and/or second compressor is a single-stage compressor, such as a single-stage piston compressor, or any other suitable compressor, such as a single rotary-screw compressor. In any case, the first and second compressors 120, 220 comprise respective first and second injector ports 121 , 221 configured to open into a location between the inlets and outlets of the respective first and second compressors 120, 220. In this way, a pressure at the first injector port 121 is the first intermediate pressure, and a pressure at the second injector port 221 is the second intermediate pressure.

[0114] The first refrigeration cycle 101 comprises a gas injector valve 130 fluidically coupled downstream of the first-cycle receiver 170 and upstream of the first compressor 120, in a parallel fluid connection with the first expansion valve 140. Specifically, the gas injector valve 130, which is an expansion valve, is fluidically coupled to the first injector port 121 , so as to supply refrigerant expanded through the gas injector valve 130 to the first compressor 120 via the first injector port 121. In this way, refrigerant expanded by the gas injector valve 130 may be used to reduce a temperature of a part of the first compressor 120, such as a motor and/or frequency convertor of the first compressor 120. This may be to maintain the temperature of the part of the first compressor 120, such as sensed by a first compressor temperature sensor (not shown), below a predetermined temperature threshold.

[0115] In a similar way, in the illustrated example, the second side 182 of the pre-cooler is fluidically coupled to the second injector port 221 of the second compressor 220. In this way, second-cycle refrigerant expanded by the third expansion valve 230 can be used to reduce a temperature of a part of the second compressor 220, such as a motor and/or frequency convertor of the second compressor 220. This may be to maintain the temperature of the part of the second compressor 220, such as sensed by a second compressor temperature sensor (not shown), below a predetermined temperature threshold.

[0116] The refrigeration system 100 also comprises a sensor system 500 comprising a plurality of sensors for sensing various characteristics of the refrigeration system 500. For example, the sensor system 500 comprises supply and return gas temperature sensors 510a, 510b configured to sense a temperature of the gas respectively supplied to, and returned from, the cargo space 12 in use. The supply and return gas temperature sensors 510a, 510b are also shown in Figure 1. In some examples, the sensor system 500 comprises the gas sensor 370. In other examples, the gas sensor 370 is omitted. In other examples, the refrigeration system comprises an ambient temperature sensor 520 for sensing a temperature of the external fluid that is passed through the second-cycle condenser 210 and/or the gas cooler 190, in use. In further examples, though not shown in the Figures, for clarity, the sensor system 500 comprises any suitable number of pressure and/or temperature sensors upstream and/or downstream of any one or more of the expansion valves 130, 140, 230, 240, compressors 120, 220, and/or heat exchangers 110, 150, 180, 190, 210, 260 in the refrigeration system 100. The sensor system may obtain data that can be used to control the refrigeration system 100, and/or to monitor an integrity of the refrigeration system 100, or parts thereof.

[0117] Returning now to Figure 1 , the transport unit 10 of the illustrated example comprises a parts storage space 600 for storing replacement parts for the refrigeration system 100 of the first aspect, and/or the heat exchanger arrangement 400. The parts storage space is located outside the cargo space 12 of the transport unit 10, so as to be easily accessible by service personnel. Specifically, the parts storage space 600 is located in proximity to an access port associated with the refrigeration system 100 and/or the atmosphere control system 20. In other examples, the parts storage space 600 is located in any other suitable location in the transport unit 10 outside of the cargo space 122. In this way, the replacement parts may be retrieved without requiring access to the cargo space 12. In some examples, the replacement parts comprise a temperature sensor for sensing a temperature of the external fluid, such as replacement supply and/or return temperature sensors 510a, 510b, or a replacement ambient temperature sensor 520. In other examples, the replacement parts comprise any one or more sensors of the sensor system 500, such as: temperature sensors for sensing a temperature of refrigerant in the first and/or second refrigeration cycles 101 , 102; pressure sensors for sensing a pressure of refrigerant in the first and/or second refrigeration cycles 101 , 102, and/or a pressure of the external fluid; a humidity sensor, such as for sensing a humidity, such as a relative humidity, of the external fluid; an expansion valve, such as a replacement first expansion valve 140, second expansion valve 240, third expansion valve 230, and/or gas injection valve 130; and a pressure relief valve, such as a pressure relief valve for the first compressor 120, the second compressor 220, the first-cycle receiver 170, and/or the second cycle receiver 270.

[0118] In various examples, the replacement parts comprise important parts, without which operation of the refrigeration system 100 cannot be properly maintained. The important parts here comprise parts for the first refrigeration cycle 101 , such as one or more temperature sensors for the first refrigeration cycle 101 , a replacement first expansion valve 140, and/or a pressure relief valve for the first refrigeration cycle 101 , although the important parts may comprise other parts other examples. The parts storage space 600 also comprises, and/or is configured to store, specialised tools for facilitating maintenance of the refrigeration system 100, such as replacement of the respective parts.

[0119] Finally, Figure 6 shows an example marine vessel 1 , which here is a container ship 1 , comprising the transport unit 10 described above. [0120] Example embodiments of the present invention have been discussed, with particular reference to the examples illustrated. However, it will be appreciated that variations and modifications may be made without departing from the scope of the invention as defined by the appended claims.