WO2025252527A1

WO2025252527A1 - A refrigeration system adapted to operate in multiple modes depending on ambient conditions

Info

Publication number: WO2025252527A1
Application number: PCT/EP2025/064543
Authority: WO
Inventors: Olaf Stallmann
Original assignee: Nuovo Pignone Technologie SRL
Current assignee: Nuovo Pignone Technologie SRL
Priority date: 2024-06-05
Filing date: 2025-05-26
Publication date: 2025-12-11
Anticipated expiration: 2026-12-05

Abstract

The refrigeration system comprises compressor section and, downstream of the compressor section, an after-cooler, adapted to condense a flow of compressed refrigerant delivered by the compressor section. The system further comprises an evaporation section, including a heat exchanger arrangement, adapted to absorb heat from a consumer by heat exchange with the refrigerant. The evaporation section comprises a cold refrigerant inlet, fluidly coupled with the after-cooler, and a hot refrigerant outlet, adapted to be fluidly coupled with a suction side of the compressor section. A bypass connection fluidly couples the hot refrigerant outlet of the evaporation section with the after-cooler.

Description

A refrigeration system adapted to operate in multiple modes depending on ambient conditions

DESCRIPTION

TECHNICAL FIELD

[0001] The present disclosure concerns refrigeration systems. Embodiments disclosed herein are particularly adapted for use in solvent-based carbon capture systems, such as those based on mixed salt processes (MSP) and chilled ammonia processes (CAP).

BACKGROUND ART

[0002] Carbon capture is currently investigated as one of the most promising routes toward reduction of carbon dioxide emissions. Several carbon capture systems have been developed, some of them based on the use of an aqueous solvent, which is adapted to contact a flue gas flow in an absorber and absorb carbon dioxide contained in a flue gas, before the flue gas is released in the atmosphere. The CCE-rich absorbent is then regenerated in a regenerator and releases carbon dioxide, which is collected and stored or transported.

[0003] Carbon capture systems are characterized by a high power demand. Part of the power required to operate a carbon capture system is used for refrigeration. Moreover, known carbon capture systems have only limited capability to address cold climatic conditions.

[0004] CA2298373 discloses a large size cooling system including a compressor, a condenser, a pump and evaporator forming a closed circuit for a cooling medium. A bypass line is arranged in parallel to the compressor, to couple the outlet of the evaporator directly with the inlet of the condenser. The bypass line provides a by-pass for free cooling. The system disclosed in CA2298373 can thus operate in two different modes depending on environmental conditions and provides a very limited flexibility in the plant operation.

[0005] An aim of the present disclosure is to provide refrigerant systems which may be beneficial, particularly in combination with a carbon capture system, specifically to reduce the amount of energy required for cooling purposes and to provide enhanced capabilities to address cold ambient conditions.

SUMMARY

[0006] According to embodiments disclosed herein, a refrigeration system is provided, which includes a compressor section comprising a first compressor and a second compressor in sequence. A delivery side of the first compressor is fluidly connectable with a suction side of the second compressor. Downstream of the compressor section the system comprises an after-cooler adapted to receive and condense a flow of compressed refrigerant delivered by the compressor section. An evaporation section comprises a heat exchanger arrangement adapted to absorb heat from a consumer by heat exchange with the refrigerant. The evaporation section comprises a cold refrigerant inlet adapted to be fluidly coupled with the after-cooler, and a hot refrigerant outlet adapted to be fluidly coupled with a suction side of the compressor section. The system further incudes a bypass connection adapted to fluidly couple the hot refrigerant outlet of the evaporation section with the after-cooler by-passing the compress section. The system further comprises a first connection line adapted to fluidly couple the delivery side of the first compressor with the aftercooler bypassing the second compressor.

[0007] As will be described in more detail with reference to embodiments illustrated in the attached drawings, the refrigeration system of the present disclosure can operate in three different modes, depending upon the ambient temperature. In some operation modes, the refrigerant is entirely or partly processed through the compressor section. In another operation mode, the refrigerant is not processed through the compressor section, which is bypassed.

[0008] Further features and embodiments of the refrigeration system of the present disclosure are set forth in the attached claims and described below, with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Reference is now made briefly to the accompanying drawings, in which:

Fig.1 illustrates a schematic of a refrigeration system according to the present disclosure, in a first embodiment;

Figs 2, 3 and 4 illustrate the refrigeration system of Fig.1 in three different operating conditions,

Fig.5 illustrates a schematic of a refrigeration system according to the present disclosure, in a further embodiment; and

Figs 6, 7 and 8 illustrate the refrigeration system of Fig.5 in three different operating conditions.

DETAILED DESCRIPTION

[0010] A first embodiment of a refrigeration system of the present disclosure is shown in Fig.l, and three different operating modes, in different environmental temperature conditions are shown in Figs. 2, 3 and 4.

[0011] The refrigeration system 1 comprises a compressor section 3, which can comprise one or more compressors. Each compressor can be a multi-stage compressor. In the schematic of Fig. l the compressor section 3 comprises two compressors 3.1 and 3.2, which are drivingly coupled to a driver 5, for instance an electric motor, a gas turbine, a steam turbine or the like. A shaft 3.3 drivingly couples the two compressors 3.1 and 3.2. A clutch 3.4 is positioned along the shaft 3.3 and is adapted to decouple the second compressor 3.2 from the driver 5 if the operating conditions of the refrigeration system so require.

[0012] In other embodiments, each compressor 3.1, 3.2 of the compressor section 3 can be driven by a separate driver.

[0013] It shall be understood that the configuration of the compressor section 3 with two compressors 3.1 , 3.2 is just shown by way of example and that other embodiments can include more than two compressors. In other embodiments, a single compressor can be envisaged.

[0014] A suction side of the first compressor 3.1 is shown at 3.1 S and a delivery side of the first compressor 3.1 is shown at 3. ID. Similarly, a suction side of the second compressor 3.2 is shown at 3 ,2S and a delivery side of the first compressor 3.2 is shown at 3.2D. [0015] Upstream of the suction side 3.1 S of the first compressor 3.1 a liquid/gas separator 7 is positioned, which is aimed at separating liquid from a gaseous stream of refrigerant entering the first compressor 3.1.

[0016] An inter-cooler 9 is positioned between the delivery side 3. ID of the first compressor 3.1 and the suction side 3.2S of the second compressor 3.2. A further liquid/gas separator 11 can be positioned between the inter-cooler 9 and the suction side 3.2S of the second compressor 3.2. The liquid/gas separator 11 has an inlet 11.1 fluidly coupled with the refrigerant outlet of the inter-cooler 9, a gas outlet 11.2 fluidly coupled with the suction side 3.2S of the second compressor 3.2 and a liquid outlet 11.3, fluidly coupled with a cold refrigerant inlet 13.1 of an evaporation section 13, which further comprises a hot refrigerant outlet 13.2.

[0017] For the purposes which will be clarified later on, the liquid outlet 11.3 of the liquid/gas separator 11 is fluidly coupled with the cold refrigerant inlet 13.1 of the evaporation section 13 through two different connection lines, labeled 15 and 17, respectively. A control valve 19 is positioned along the connection line 15 and a control valve 21 is positioned along the connection line 17.

[0018] A pump 20 can be positioned along the connection line 15. In case the physical arrangement of liquid/gas separator 11 and liquid/gas separator 25 allows, line 17 could be omitted and only line 15 would be present. In such configuration, the level between the liquid/gas separator 11 and the accumulator vessel 25 would equalize by gravity and the refrigerant flows through valve 27 or through the pump 20 as required by the operating mode, see below. Valve 19 in such case could act as an isolation valve only, without any other control function.

[0019] The delivery side 3.2D of the second compressor 3.2 is fluidly coupled with an after-cooler 23, which is positioned downstream of the compressor section 3, between the latter and an accumulator vessel 25. The accumulator vessel 25 comprises an inlet 25.1, fluidly coupled with the refrigerant outlet of the after-cooler 23, and a liquid outlet 25.3, fluidly coupled with the cold refrigerant inlet 13.1 of the evaporation section 13. A valve 27 is positioned between the liquid outlet 25.2 and the cold refrigerant inlet 13.1 of the evaporation section 13. Gas which might accidentally be present in the accumulator vessel 25, for instance leakages from seals of the compressor, can escape from a gas outlet 25.2.

[0020] The evaporation section 13 comprises a heat exchanger arrangement 29, which can include one or more heat exchangers, shown only schematically in Fig.1. The heat exchanger arrangement 29 has a cold side, wherein the refrigerant coming from the cold refrigerant inlet 13.1 flows and absorbs heat from a generic utility, which will be referred to herein as “consumer”. As understood herein a “consumer” is any device, unit, apparatus, process or combination thereof, which requires a refrigeration or cooling duty, i.e. wherefrom heat must be removed. The refrigerant circulating in the heat exchanger arrangement 29 absorbs heat from the consumer and evaporates.

[0021] The hot refrigerant outlet 13.2 of the evaporation section 13 is fluidly coupled through a line 31 with the inlet 7.1 of the liquid/gas separator 7, the gas outlet 7.2 whereof is fluidly coupled with the suction side 3. IS of the first compressor 3.1 Reference number 7.3 designates a liquid outlet of the liquid/gas separator 7. An isolation valve 33 can be positioned along the line 31.

[0022] The refrigerant system 1 further comprises a bypass connection 35, adapted to fluidly couple the hot refrigerant outlet 13.2 of the evaporation section 13 with the discharge side of the first compressor. A control valve 37 can be positioned along the bypass connection 35. The by-pass connection 35 is also coupled with the after-cooler 23 through a connection line 36. The by-pass connection 35 therefore also connects the delivery side 3. ID of the first compressor 3.1 to the after-cooler 23.

[0023] The refrigeration system 1 described so far can operate in three different modes. A first mode of operation is illustrated in Fig.2, in which connections which are interrupted, i.e. wherein no fluid flows, are shown in dashed lines.

[0024] The entire flow of hot refrigerant from the hot refrigerant outlet 13.2 of the evaporation section 13 streams through the liquid/gas separator 7 to remove liquid therefrom, if present, and is then processed in sequence by the first compressor 3.1 and the second compressor 3.2. Compression heat is removed through the inter-cooler 9. The flow of compressed gaseous refrigerant is then cooled and condensed in the aftercooler 23. The resulting flow of condensed refrigerant is passed through the accumulator vessel 25, and fed through valve 27 to the evaporation section 13. The pressure of the refrigerant is reduced through valve 27, which acts as an expansion valve, or Joule-Thomson valve, such that the refrigerant entering the evaporation section 13 has the required temperature suitable to provide the required refrigeration or cooling duty to the consumers through the heat exchanger arrangement 29. For instance, the temperature of the refrigerant entering the evaporation section 13 can range between 2°C and 15°C.

[0025] The cold refrigerant flows through the cold side of the heat exchangers in the heat exchanger arrangement 29, to provide chilling duty to the consumers. The vaporized refrigerant is then directed to the hot refrigerant outlet 13.2 and delivered to the liquid/gas separator 7 again.

[0026] The temperature level of 2°-15°C can be achieved with an inter-cooler 9 and an after-cooler 23 which use air or water as cooling fluid. The final refrigerant temperature at the outlet of the after-cooler 23 can drop below the actual value required to provide the necessary cooling duty to the consumers, since the cooling efficiency depends upon the ambient temperature. This may happen whenever the inter-cooler and after-cooler use a cooling medium, the temperature whereof follows ambient conditions.

[0027] For instance, if the system 1 is installed in a location where the ambient temperature fluctuates considerably during the year, the final temperature of the refrigerant may become too low compared to what is required by the consumers.

[0028] To prevent excessively low temperatures of the refrigerant to occur at the inlet of the evaporation section 13 when the ambient temperature is low, the system 1 can operate in a second mode of operation, shown in Fig.3. Similar to Fig.2, in Fig.3 dashed lines represent portions of the system where no fluid circulation occurs.

[0029] The second compressor 3.2 can be switched off. This may occur e.g. by decoupling the second compressor 3.2 from the driver 5 by opening the clutch 3.4.

[0030] The full hot refrigerant flow from the hot refrigerant outlet 13.2 of the evaporation section 13 flows through the liquid/gas separator 7 to remove liquid therefrom, if present, and is then processed in the first compressor 3.1 only. The delivery side 3. ID of the first compressor 3.1 is fluidly coupled with the inlet of the inter-cooler 9 and with the inlet of the after-cooler 23. The inter-cooler 9 and the after-cooler 23 now operate both in condensation mode, i.e. as condensers, in parallel and condense the compressed refrigerant delivered from the first compressor 3.1, thus maximizing the condensation duty. Condensed refrigerant is collected from the inter-cooler 9 in the liquid/gas separator 11 and from the after-cooler 23 in the accumulator vessel 25. The two flows of condensed refrigerant are expanded in valves 21 and 27 and the low- temperature, low-pressure refrigerant enters the evaporation section 13 at the cold refrigerant inlet 13.1 and is delivered to the cold side of the heat exchangers in the heat exchanger arrangement 29. After evaporation, the hot refrigerant is recovered at the hot refrigerant outlet 13.2 and recycled towards the suction side 3. IS of the first compressor 3.1 through the liquid/gas separator 7.

[0031] In some circumstances, the ambient temperature may become even lower, for instance 5 °C or lower. In such situation, the compressors of the refrigeration system 1 can be switched off completely and the cooling medium circulating in the inter-cooler 9 and after-cooler 23 at ambient temperature can be sufficient to chill the refrigerant at the temperature required to operate the evaporation section 13 such that the required temperature in the cold side of the heat exchangers of the heat-exchanger arrangement 29 is reached.

[0032] This mode of operation is shown in Fig.4, where inoperative sections of the system 1 are shown again in dashed lines.

[0033] The entire refrigerant flow from the hot refrigerant outlet 13.2 of the evaporation section 13 is delivered, through the by-pass connection 35, to the inter-cooler 9 and the after-cooler 23, which operate in parallel as condensers. Condensed refrigerant is collected from the inter-cooler 9 in the liquid/gas separator 11 and from the aftercooler 23 in the accumulator vessel 25. Therefrom, the refrigerant which has been condensed by heat exchange against ambient temperature using water or air as cooling medium, for instance, is expanded in valve 19 and delivered to the cold refrigerant inlet 13.1 of the heat exchanger arrangement 29. After evaporation by heat exchange in the heat exchangers of the heat exchanger arrangement 29, the evaporated refrigerant leaves the evaporation section 13 through the hot refrigerant outlet 13.2 and enters the by-pass connection 35. [0034] Since the compressors 3.1 and 3.2 are both switched off, the system requires some means to promote circulation of the refrigerant. In the embodiment shown in Figs. 1 to 4, the pump 20 is provided for this purpose. The pump 20 pumps low-pressure condensed refrigerant from the valve 19 into the evaporation section 13 through the cold refrigerant inlet 13.1.

[0035] In other embodiments, it may be possible to use gravity to promote circulation of the refrigerant in the system 1. This can happen for instance if the heat exchanger arrangement 29 is positioned at a lower height and the driving force to circulate the refrigerant is provided by liquid static height and the density difference between liquefied and evaporated refrigerant. Vaporizing refrigerant flows upwards and condenses at the inter-cooler 9 and the after-cooler 23 that are arranged at the highest elevation. The liquid refrigerant with a high density is collected in the liquid/gas separator 11 and in the accumulator vessel 25 and the elevation difference between the liquid/gas separator 11 and the accumulator vessel 25 and heat exchanger arrangement 29 is used to transfer the liquid refrigerant.

[0036] The refrigeration system 1 described so far is therefore adapted to operate in one of three different modes, depending upon the ambient temperature. The best mode is selected based on the ambient temperature such as to minimize energy consumption of the refrigerant system 1 and maximizing the efficiency thereof.

[0037] A further embodiment of a refrigeration system according to the present disclosure is shown in Fig.5 and three different modes of operation thereof are pictorially represented in Figs 6, 7 and 8.

[0038] Referring first to Fig.5, the refrigerant system 101 comprises a compressor section 103, which can comprise one or more compressors. Each compressor can be a multi-stage compressor. In the schematic of Fig.5 the compressor section 103 comprises two compressors 103.1 and 103.2, which are drivingly coupled to a driver 105, for instance an electric motor, a gas turbine, a steam turbine or the like. A shaft 103.3 drivingly couples the two compressors 103.1 and 103.2. A clutch 103.4 is positioned along the shaft 103.3 and is adapted to decouple the second compressor 103.2 from the driver 105 if the operating conditions of the refrigeration system so require. [0039] In other embodiments, each compressor 103.1, 103.2 of the compressor section 103 can be driven by a separate driver.

[0040] It shall be understood that the configuration of the compressor section 103 with two compressors 103.1, 103.2 is just shown by way of example and that other embodiments can include more than two compressors. In other embodiments, a single compressor can be envisaged.

[0041] A suction side of the first compressor 103.1 is shown at 103. IS and a delivery side of the first compressor 103.1 is shown at 103. ID. Similarly, a suction side of the second compressor 103.2 is shown at 103.2S and a delivery side of the first compressor

103.2 is shown at 103.2D.

[0042] Upstream of the suction side 103.1 S of the first compressor 103.1 a liquid/gas separator 107 is positioned, which is aimed at separating liquid form a gaseous stream of refrigerant entering the first compressor 103.1.

[0043] A further liquid/gas separator 111 can be positioned between the delivery side 103. ID of the first compressor 103 and the suction side 103.2S of the second compressor 103.2. In this embodiment no inter-cooler is provided. In other embodiments, not shown, an inter-cooler can be positioned between the first compressor 103.1 and the second compressor 103.2, similarly to inter-cooler 9 in Fig.l.

[0044] In this embodiment, the liquid/gas separator 111 has an inlet 111.1 fluidly coupled with the delivery side 103. ID of the first compressor 103.1, and a gas outlet

111.2 fluidly coupled with the suction side 103.2S of the second compressor 103.2.

[0045] The delivery side 103.2D of the second compressor 103.2 is fluidly coupled with an after-cooler 123, which is positioned downstream of the compressor section 103, between the latter and an accumulator vessel 125. The accumulator vessel 125 comprises an inlet 125.1, fluidly coupled with the refrigerant outlet of the after-cooler 123, and a liquid outlet 125.3, fluidly coupled with a first cold refrigerant inlet 113.1 of an evaporation section 113, which further features a hot refrigerant outlet 113.2. A gas outlet 125.2 can be provided to remove gaseous species accidentally accumulating in the vessel, for instance due to compressor seal leakages. In this embodiment, the hot refrigerant outlet 113.2 actually features a first hot refrigerant outlet 113.21 and a second hot refrigerant outlet 113.22. A pressure adjusting valve or pressure control valve 114 can be positioned at the second hot refrigerant outlet 113.22, downstream of a heat exchanger arrangement to be described.

[0046] In a different embodiment, not shown, the liquid/gas separator 111 can be fluidly coupled to the hot refrigerant outlet 113.2 and not to the delivery side 103. ID of the first compressor 103.1. In this case, the delivery side 103. ID of the first compressor 103.1 can be fluidly coupled directly to the suction side 103.2S of the second compressor 103.2. In yet further embodiments, the liquid/gas separator 111 can be omitted altogether.

[0047] In the embodiment illustrated in Fig.5, the evaporation section 113 is a two- temperature level evaporation section, which includes a first heat exchanger arrangement 129.1 and a second heat exchanger arrangement 129.2. The cold refrigerant inlet 113.1 splits into a first inlet line 130.1 which delivers cold refrigerant to the first heat exchanger arrangement 129.1 and a second inlet line 130.2 which delivers cold refrigerant to the second heat exchanger arrangement 129.1. A first valve 127.1, which can operate as an expansion valve or a Joule-Thompson valve, is positioned along the first inlet line 129.1 and a second valve 127.2, which can operate as an expansion valve or a Joule-Thompson valve, is positioned along the second inlet line 129.2.

[0048] Vaporized refrigerant from the first heat exchanger arrangement 129.1 flows through the first hot refrigerant outlet 113.21, while vaporized refrigerant from the second heat exchanger arrangement 129.2 flows through the second hot refrigerant outlet 113.22

[0049] The liquid outlet 125.3 of the accumulator vessel 125 is fluidly coupled with the cold refrigerant inlet 113.1 through alternatively a first connection line 132 and a second connection line 134, along which a pump 120 can be positioned. A non-retum valve or check valve 138 can be positioned along the first connection line 132.

[0050] The hot refrigerant outlet 113.2 of the evaporation section 113 is fluidly coupled through a line 131 with the inlet 107.1 of the liquid/gas separator 107, the gas outlet 107.2 whereof is fluidly coupled with the suction side 103. IS of the first compressor 103.1 Reference number 107.3 designates a liquid outlet of the liquid/gas separator 107. An isolation valve 133 can be positioned along the line 131.

[0051] The refrigerant system 1 further comprises a bypass connection 135, adapted to fluidly couple the hot refrigerant outlet 113.2 of the evaporation section 113 with the after-cooler 123. A pressure control valve 137 can be positioned along the bypass connection 135, downstream of the first heat exchanger 129.1. The by-pass connection 135 is coupled with the after-cooler 123 though a connection line 136. The by-pass connection 135 also connects the delivery side 103. ID of the first compressor 103.1 to the after-cooler 123.

[0052] The pressure control valves 137 and 114 are adapted to control the pressure downstream of the first heat exchanger arrangement 129.1 and of the second heat exchanger arrangement 129.2 when the first compressor 103.1 is inoperative, as will be described with reference to the mode of operations illustrated in Fig.8, for instance.

[0053] The refrigeration system 101 described so far can operate in three different modes. A first mode of operation is illustrated in Fig.6, in which connections which are interrupted, i.e. wherein no fluid flows, are shown in dashed lines.

[0054] The flow of hot refrigerant from the first hot refrigerant outlet 113.21 of the evaporation section 113 streams through the liquid/gas separator 107 to remove liquid therefrom, if present, and is then processed in sequence by the first compressor 103.1 and the second compressor 103.2. The flow of compressed gaseous refrigerant is passed through the after-cooler 123 and through the accumulator vessel 125, to remove any gaseous fraction possibly present in the stream and fed through the first connection line 132 toward the cold refrigerant inlet 113.1 of the evaporation section 113 and split into the first inlet line 130.1 and second inlet line 130.2, where the refrigerant is expanded in valves 127.1 and 127.2.

[0055] The flow of hot refrigerant from the second hot refrigerant outlet 113.22 of the evaporation section 113 streams directly in the liquid/gas separator 111, with the refrigerant flow coming from the delivery side 103. ID of the first compressor 103.1 and is therefore compressed only through the second compressor 103.2. [0056] The pressure of the refrigerant is reduced through valves 127.1 and 127.2, which act as expansion valves, or Joule-Thomson valves, such that the refrigerant entering the heat exchanger arrangements 129.1 and 129.2 has the required temperature suitable to provide the required refrigeration or cooling duty to the consumers through the heat exchanger arrangements 129.1 and 129.2. The pressure and temperature levels of the two separate refrigerant streams evaporating in the two exchanger arrangements 129.1 and 129.2 can be different from one another.

[0057] In some embodiments, the first heat exchanger arrangement 129.1 may require refrigerant at a lower temperature, for instance around 2-7°C, while the second heat exchanger arrangement 129.2 may require refrigerant at a higher temperature, for instance around 12-17°C. These temperature levels are provided by way of example only and shall not be construed as limiting the scope of the present disclosure. Such temperature levels are for instance useful in a chilled ammonia process.

[0058] The temperature levels required in the two heat exchanger arrangements 129.1 and 129.2 can be achieved with an after-cooler 123 which use air or water as cooling fluid. The final refrigerant temperature at the outlet of the after-cooler 123 can drop below the actual value needed to provide the required cooling duty to the consumers, since the cooling efficiency depends upon the ambient temperature. This may happen whenever the after-cooler 123 uses a cooling medium, the temperature whereof follows ambient conditions.

[0059] To prevent excessively low temperatures of the refrigerant to occur at the inlet of the evaporation section 113 when the ambient temperature is low, the system 101 can operate in a second mode of operation, shown in Fig.7. Similar to Fig.5, in Fig.7 dashed lines represent portions of the system where no fluid circulation occurs.

[0060] The second compressor 103.2 can be switched off. This may occur e.g. by decoupling the second compressor 103.2 from the driver 5 by opening the clutch 103.4.

[0061] The hot refrigerant from the first hot refrigerant outlet 113.21 of the evaporation section 13 flows through the liquid/gas separator 107 to remove liquid therefrom, if present, and is then processed in the first compressor 103.1 only. The delivery side 103. ID of the first compressor 103.1 is fluidly coupled through the connection line 136 to the after-cooler 123. The hot refrigerant flow from the second hot refrigerant outlet 113.22 is delivered directly through the connection line 135 and 136 to the after-cooler 123 and delivered through the accumulator vessel 125 and then to the cold refrigerant inlet 113.1 of the evaporation section 113. The condensed refrigerant is pumped into the cold refrigerant inlet 113.1 by pump 120.

[0062] Thus, in this operation mode, similarly to the operation mode of system 1 show in Fig.3, only one compressor is active, which saves energy.

[0063] In some circumstances, the ambient temperature may become even lower, for instance 5°C or lower. In such situation, the compressors of the refrigeration system 101 can be switched off completely and the cooling medium circulating in after-cooler 123 at ambient temperature can be sufficient to chill the refrigerant at the temperature required to operate the evaporation section 113 such that the required temperature in the cold side of the heat exchangers of both heat-exchanger arrangements 129.1 and 129.2 is reached.

[0064] This mode of operation is shown in Fig.8, where inoperative sections of the system 1 are shown again in dashed lines.

[0065] The entire refrigerant flow from both hot refrigerant outlets 113.21, 113.22 of the evaporation section 113 is delivered, through the by-pass connection 135, to the after-cooler 1, which operates as condenser. Condensed refrigerant is collected from the after-cooler 123 in the accumulator vessel 125. Therefrom, the refrigerant which has been condensed by heat exchange against ambient temperature using water or air as cooling medium, for instance, is expanded in valves 127.1 and 127.2 and delivered through the heat exchanger arrangements 129.1 and 129.2. After evaporation by heat exchange in the heat exchangers of the heat exchanger arrangements 129.1 and 129.2, the two flows of evaporated refrigerant leave the evaporation section 113 through the hot refrigerant outlets 113.21 and 113.22 and enter the by-pass connection 135.

[0066] Since the compressors 103.1 and 103.2 are both switched off, the system requires some means to promote circulation of the refrigerant. In the embodiment shown in Figs. 5 to 8, the pump 120 is provided for this purpose. The pump 120 pumps low- pressure condensed refrigerant into the evaporation section 113 through the cold refrigerant inlet 113.1.

[0067] As mentioned in connection with Figs. 1 to 4, also in case of the system 101 in other embodiments, it may be possible to use gravity to promote circulation of the refrigerant in the system 101. This can happen for instance if the heat exchanger ar- rangements 129.1, 129.2 are positioned at a lower height and the driving force to circulate the refrigerant is provided by heat absorbed by the refrigerant through the heat exchanger arrangements 129.1, 129.2. Vaporizing refrigerant spontaneously flows upwards and circulates towards the after-cooler 123.

[0068] Similar to the refrigeration system 1, also the refrigeration system 101 de- scribed so far is therefore adapted to operate in one of three different modes, depending upon the ambient temperature. The best mode is selected based on the ambient temperature such as to minimize energy consumption of the refrigerant system 1 and maximizing the efficiency thereof.

[0069] Exemplary embodiments have been disclosed above and illustrated in the ac- companying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

1. A refrigeration system (1; 101) comprising: a compressor section comprising a first compressor (3.1; 103.1) and a second compressor (3.2; 103.2) in sequence; wherein a delivery side (3. ID; 103. ID) of the first compressor (3.1; 103.1) is fluidly connectable with a suction side (3.2S; 103.2S) of the second compressor (3.2; 103.2); and wherein the first compressor (3.1; 103.1) and the second compressor (3.2; 103.2) are adapted to operate independently of each other; downstream of the compressor section (3; 103), an after-cooler (23; 123) adapted to receive and condense a flow of compressed refrigerant delivered by the compressor section (3; 103); an evaporation section (13; 113), comprising a heat exchanger arrangement (29; 129.1, 129.2), adapted to absorb heat from a consumer by heat exchange with the refrigerant; wherein the evaporation section comprises a cold refrigerant inlet (13.1; 113.1) adapted to be fluidly coupled with the after-cooler (23; 123), and a hot refrigerant outlet (13.2; 113.2) adapted to be fluidly coupled with a suction side of the compressor section (3; 103); a bypass connection (35; 135) adapted to fluidly couple the hot refrigerant outlet (13.2; 113.2) of the evaporation section (13; 113) with the after-cooler (23; 123) bypassing the compressor section (3; 103); and a first connection line (35, 36; 135; 136) adapted to fluidly couple the delivery side (3. ID; 103. ID) of the first compressor with the aftercooler (23; 123) bypassing the second compressor (3.2; 103.2).

2. The refrigeration system of claim 1, wherein the bypass connection (35; 135) is adapted to fluidly couple the hot refrigerant outlet (13.2) of the evaporation section (13) with the first connection line (35, 36; 135, 136).

3. The refrigeration system (101) of claim 1 or 2, wherein the heat exchanger arrangement (129.1, 129.2) comprises a first heat exchanger arrangement (129.1) which exchanges heat at a first temperature level, and a second heat exchanger arrangement (129.2) which exchanges heat at a second temperature level, different from the first temperature level.

4. The refrigeration system (101) of claim 3, further comprising a first expansion valve (127.1) upstream of the first heat exchanger arrangement (129.1) and a second expansion valve (127.2) upstream of the second heat exchanger arrangement (129.2).

5. The refrigeration system (1; 101) of any one of the preceding claims, further comprising a pump (20; 120) adapted to circulate condensed refrigerant from the after-cooler (23; 123) into the evaporation section (29; 129).

6. The refrigeration system (1) of any one of the preceding claims, further comprising an inter-cooler (9) arranged between the delivery side (3. ID) of the first compressor (3.1) and the suction side (3.2S) of the second compressor (3.2).

7. The refrigeration system (1) of claim 6, further comprising a second connection line (15, 17) between a refrigerant outlet of the inter-cooler (9) and the cold refrigerant inlet (13.1) of the evaporation section (13), said second connection line (15, 17) by-passing the after-cooler (23).

8. The refrigeration system (1) of claim 7, further comprising an expansion valve (19; 21) along the second connection line (15; 17), between the intercooler (9) and the cold refrigerant inlet (13.1) of the evaporation section (13).

9. The refrigeration system (1) of any one of claims 6 to 8, further comprising a pump (20) adapted to circulate condensed refrigerant from the inter-cooler (9) into the evaporation section (13).

10. The refrigeration system (1) of any one of claims 6 to 9, further comprising a liquid/gas separator (11) between the intercooler (9) and the suction side (3.2S) of the second compressor (3.2).

11. The refrigeration system of claim 3 or 4, further comprising a first pressure control valve (137) downstream of the first heat exchanger arrangement (129.1) and a pressure control valve (114) downstream of the second heat exchanger arrangement (129.2); and wherein the evaporated refrigerant from the first heat exchanger arrangement (129.1) is mixed with the evaporated refrigerant from the second heat exchanger arrangement (129.2).