CARBON DIOXIDE REFRIGERATION SYSTEM AND A METHOD OF OPERATING THE REFRIGERATION SYSTEM

Abstract

A refrigeration system including one or more first compressor(s) for compressing a carbon dioxide (CO.sub.2) refrigerant, a main heat rejection system for cooling the CO.sub.2 refrigerant, one or more high pressure expansion device(s) for reducing the pressure of the CO.sub.2 refrigerant, a receiver for storing the CO.sub.2 refrigerant, one or more high pressure expansion device(s), an evaporator and a receiver pressure regulating device. The refrigeration system further includes an auxiliary refrigeration system including an auxiliary compressor arranged to compress at least part of the CO.sub.2 refrigerant and thereafter to direct the compressed CO.sub.2 refrigerant to a heat rejection system.

Claims

1. A refrigeration system for transferring heat, wherein the refrigeration system comprises: a main refrigeration system comprising: one or more first compressors for compressing a carbon dioxide (CO.sub.2) refrigerant; a main heat rejection system for cooling the CO.sub.2 refrigerant discharged from the one or more first compressors; one or more high pressure expansion devices for reducing pressure of the CO.sub.2 refrigerant discharged from the main heat rejection system; a receiver for receiving the CO.sub.2 refrigerant from the one or more high pressure expansion devices; one or more low pressure expansion devices for reducing the pressure of a liquid phase of the CO.sub.2 refrigerant from the receiver; an evaporator for evaporating the liquid phase part of the CO.sub.2 refrigerant from the one or more low pressure expansion devices and thereafter directing the evaporated liquid phase part of the CO.sub.2 refrigerant to a suction side of the one or more first compressors; an auxiliary refrigeration system comprising: an auxiliary compressor arranged to compress at least part of a vapour phase of the CO.sub.2 refrigerant from the receiver to generate compressed CO.sub.2 refrigerant and to direct the compressed CO.sub.2 refrigerant to a heat rejection system, wherein a nominal maximal volume rate of flow of the compressed refrigerant of the auxiliary compressor is less than 10% of the nominal maximal volume rate of flow of compressed refrigerant of the one or more first compressors.

2. The refrigeration system according to claim 1, wherein the one or more first compressors are arranged to compress the CO.sub.2 refrigerant from the vapour phase of the CO.sub.2 refrigerant to a supercritical phase and wherein the main heat rejection system is arranged to cool supercritical CO.sub.2 refrigerant discharged from the one or more first compressors.

3. The refrigeration system according to claim 2, wherein the one or more high pressure expansion devices are arranged to reduce the pressure of the CO.sub.2 refrigerant in the supercritical phase, after being discharged from the main heat rejection system, wherein the one or more high pressure expansion devices are further arranged to decompress the CO.sub.2 refrigerant to a subcritical state.

4. The refrigeration system according to claim 1, wherein the receiver is arranged to receive subcritical CO.sub.2 refrigerant from the one or more high pressure expansion devices.

5. The refrigeration system according to claim 1, wherein the auxiliary compressor is arranged to either compress at least part of the vapour phase of the CO.sub.2 refrigerant from the receiver to a supercritical phase and thereafter to direct the compressed CO.sub.2 refrigerant to an auxiliary heat rejection system or compress at least part of the vapour phase of the CO.sub.2 refrigerant from the suction side of the one or more compressors to a supercritical phase and thereafter to direct the compressed CO.sub.2 refrigerant to the main heat rejection system.

6. The refrigeration system according to claim 1, wherein the auxiliary compressor is arranged to direct the compressed CO.sub.2 refrigerant to the main heat rejection system.

7. The refrigeration system according to claim 1, wherein the main heat rejection system is arranged to allow for cooling of the CO.sub.2 refrigerant discharged from the auxiliary compressor and to allow the CO.sub.2 refrigerant to be led through the main heat rejection system, when the main heat rejection system is off.

8. The refrigeration system according to claim 1, wherein the auxiliary compressor is arranged to direct the compressed CO.sub.2 refrigerant to an auxiliary heat rejection system.

9. The refrigeration system according to claim 1, wherein the auxiliary refrigeration system further comprises one or more auxiliary high pressure expansion devices for expanding the CO.sub.2 refrigerant received from the main heat rejection system.

10. The refrigeration system according to claim 1, further comprising one or more control systems for controlling operation of the auxiliary refrigeration system, including the auxiliary compressor, based on one or more inputs.

11. The refrigeration system according to claim 1, wherein the refrigeration system comprises one or more measuring devices for measuring a value representative of a pressure and/or temperature of the refrigeration system and providing an input accordingly to one or more control systems of the refrigeration system.

12. The refrigeration system according to claim 1, wherein the auxiliary refrigeration system comprises a control for controlling a flow of CO.sub.2 refrigerant to the auxiliary compressor.

13. The refrigeration system according to claim 1, wherein the auxiliary refrigeration system comprises a control comprising one or more pulse width modulation (PWM) valves, and wherein a control system of the refrigeration system is arranged to control the operation of the one or more PWM valves based on one or more inputs representing a pressure and/or temperature of the refrigeration system.

14. The refrigeration system according to claim 1, wherein the auxiliary refrigeration system is arranged to be powered by an auxiliary power supply separate from a power supply of the main refrigeration system.

15. A method of operating a refrigeration system in the event a main refrigeration system of the refrigeration system being off, wherein the refrigeration system comprises: a main refrigeration system comprising: one or more first compressors for compressing a carbon dioxide (CO.sub.2) refrigerant; a main heat rejection system for cooling the CO.sub.2 refrigerant discharged from the one or more first compressors; one or more high pressure expansion devices for reducing pressure of the CO.sub.2 refrigerant discharged from the main heat rejection system; a receiver for receiving the CO.sub.2 refrigerant from the one or more high pressure expansion devices; one or more low pressure expansion devices for reducing the pressure of a liquid phase of the CO.sub.2 refrigerant from the receiver; an evaporator for evaporating the liquid phase part of the CO.sub.2 refrigerant from the one or more low pressure expansion devices and thereafter directing the evaporated liquid phase part of the CO.sub.2 refrigerant to a suction side of the one or more first compressors; an auxiliary refrigeration system comprising: an auxiliary compressor arranged to compress at least part of a vapour phase of the CO.sub.2 refrigerant from the receiver to generate compressed CO.sub.2 refrigerant and to direct the compressed CO.sub.2 refrigerant to a heat rejection system, wherein a nominal maximal volume rate of flow of the compressed refrigerant of the auxiliary compressor is less than 10% of the nominal maximal volume rate of flow of compressed refrigerant of the one or more first compressors; wherein the method comprises: detecting a value representing pressure and/or temperature of the refrigeration system by a measuring device and providing an input accordingly to a control system; controlling operation of the auxiliary compressor and if present, a control of the auxiliary refrigeration system, by the control system based on the input.

16. The refrigeration system according to claim 1, wherein the nominal maximal volume rate of flow of compressed refrigerant of the auxiliary compressor is less than 5% of the nominal maximal volume rate of flow of the compressed refrigerant of the one or more first compressors.

17. The refrigeration system according to claim 1, wherein the nominal maximal volume rate of flow of compressed refrigerant of the auxiliary compressor is less than 1% of the nominal maximal volume rate of flow of the compressed refrigerant of the one or more first compressors.

18. The refrigeration system according to claim 1, wherein the nominal maximal volume rate of flow of compressed refrigerant of the auxiliary compressor is less than 0.5% of the nominal maximal volume rate of flow of the compressed refrigerant of the one or more first compressors.

19. The refrigeration system according to claim 10, wherein the one or more inputs represent a pressure and/or temperature of the refrigeration system.

20. The refrigeration system according to claim 8, wherein the auxiliary refrigeration system further comprises one or more auxiliary high pressure expansion devices for expanding the CO.sub.2 refrigerant received from the auxiliary heat rejection system.

Description

DRAWING

[0101] Aspects of the present disclosure will be described in the following with reference to the figures in which:

[0102] FIG. 1 shows a schematic diagram of a refrigeration system according to an embodiment of the invention,

[0103] FIG. 2 shows a schematic diagram of a refrigeration system according to an embodiment of the invention,

DETAILED DESCRIPTION OF THE INVENTION

[0104] Preferred embodiments of the present invention will be described hereinafter with reference to the accompanying drawing. In a first and second embodiment, a refrigeration system 1 using CO.sub.2 as refrigerant 2 is shown in FIG. 1 and FIG. 2. The refrigeration system 1 comprises a first compressor 3, a main heat rejection system 4, a low-pressure expansion device 7, a receiver 6, a high-pressure expansion device 8 and an evaporator 5 sequentially connected to each other. The refrigeration system 1 is arranged as a refrigeration cycle 20, which when viewed along a counter-clockwise direction in FIGS. 1-2, allows for the refrigerant 2 to travel along a first line 21 from the receiver 6 to the low pressure expansion device 7, along a second line 22 from the low pressure expansion device 7 to the evaporator 5, along a third line 23 from the evaporator 5 to the first compressor 3, along a fourth line 24 from the first compressor 3 to the main heat rejection system 4, along a fifth line 25 from the main heat rejection system 4 to the high pressure expansion device 8 and along a sixth line 26 from the high pressure expansion device 8 to the receiver 6. This does not exclude, that there may be additional elements, such as devices and/or components, which may be arranged between the shown elements of the refrigeration system 1 of FIGS. 1-2, e.g. valves, such as cut off valves and check valves may be arranged at one or more locations on the cycle 20.

[0105] Additionally, the refrigeration system 1 may further comprise a seventh line 27, an eight line 28 and a receiver pressure regulating device 12 for receiver pressure control, wherein the seventh line 27 is arranged from the receiver to the receiver pressure regulating device 12 and the eight line 28 is arranged form the receiver pressure regulating device 12 to the third line 23 or to the first compressor 3. Additionally, valves, such as GBC and/or NRV valves may also be fitted in the seventh and/or eight line.

[0106] FIG. 1 further shows the refrigeration system 1 comprising an auxiliary refrigeration system comprising an auxiliary compressor 10 arranged in fluid communication with the receiver 6 at the suction side and in fluid communication with the main heat rejection system 4 at the pressure side. The auxiliary refrigeration system may have a ninth line 29 extending from the seventh line 27 to an auxiliary compressor 10 and a tenth line 30 extending from the auxiliary compressor 10 to the fourth line 24. In one or more embodiments, the auxiliary compressor 10 is connected to the high pressure side of the main refrigeration cycle of the refrigeration system 1 such that at least the low pressure expansion device 7, the evaporator 5 and the first compressor 3 and in some embodiments, also the receiver pressure regulating device 12 is bypassed. Alternatively, the auxiliary compressor 10 may be connected directly between the suction side of the first compressor 4, such as by a line extending from the 8th (28) or the 3rd (23) line to the auxiliary compressor 10.

[0107] In one or more embodiments, and as shown in FIG. 1, the refrigeration system 1 may additionally comprise an auxiliary high-pressure expansion device 9 connected in parallel with the high-pressure expansion device 8 of the main refrigeration system, which may be arranged for pressure control. The auxiliary high-pressure expansion device 9 may be connected to the fifth line 25 on the inlet side by an eleventh line 31 and to the sixth line 26 on the outlet side by a twelfth line 32.

[0108] The refrigeration system 1 may preferably be arranged so that the auxiliary refrigeration system is powered independently from the main refrigeration system, such that the auxiliary refrigeration system may turned on, either manually or automatically by a control system, even when there is no power being feed to the main refrigeration system. In the event of power failure or cut of power to the main refrigeration system, e.g. due to maintenance etc. it is likely that the refrigerant 2 present in the main refrigeration system, especially in the receiver 6, due to the relatively large volume, will experience an increase in temperature and thereby also in pressure. The pressure increase produced during the main refrigeration system off-period may be directed through the auxiliary compressor 10 to the main refrigeration cycle, i.e. through the main heat rejection system 4 and back towards the receiver 6. Even through the main heat rejection system 4 is not powered; it is preferably of a cooling capacity to allow sufficient cooling of the refrigerant 2. After the main heat rejection system 4, the refrigerant 2 is preferably led through the auxiliary high-pressure expansion device 9, where after the refrigerant 2 is returned and collected in the receiver 6. In one or more embodiments, parts of the main refrigeration system, such as the fourth 24, fifth 25 and/or the sixth lines 26 or a combination thereof may be passively arranged to lead the refrigerant 2 through the refrigeration system 1, even when the main refrigeration system is not powered.

[0109] FIG. 2 shows a second embodiment of the invention. The refrigeration system 1 of FIG. 2 is similar to the refrigeration system 1 of FIG. 1 but differs in that the auxiliary refrigeration system 10 further comprises an auxiliary heat rejection system 11. In this embodiment, the auxiliary system 10 also bypasses the main heat rejection system 4 of the main refrigeration system. As shown in FIG. 2 the auxiliary compressor is connected to the receiver 6 by a ninth line 29 extending from the seventh line 27 to the auxiliary compressor 10, so that the receiver pressure regulating device 12 is bypassed. Alternatively, in one or more embodiments, the auxiliary compressor 10 may be connected by a line extending from the auxiliary compressor 10 to the suction side of the first compressor 4, such as to the eight line 28 or the third line 23. The receiver pressure regulating device 12 may be arranged to allow flow of refrigerant 2 during operation of the auxiliary refrigeration system.

[0110] As shown in FIG. 2, the auxiliary compressor 10 may be connected by a line 30 to the auxiliary heat rejection system 11 and by a thirteenth line to the inlet side of a high pressure expansion device 8, 9. The auxiliary refrigeration cycle is further enabled by the fifth line 25, sixth line 26, eleventh line 31, and/or twelfth line 32, in order to connect a high pressure expansion device 8, 9 and thereafter to the receiver 6.

[0111] In one or more embodiments, the auxiliary refrigeration system 10 may be connected to the main refrigeration system at the fifth line 25 as shown in FIG. 2, or it may be bypassing the fifth line 25 and connected directly to a high pressure expansion device 8, 9.

[0112] In one or more embodiments, the auxiliary refrigeration system may utilize the sixth line 26 to connect to the receiver 6, as shown in FIGS. 1 and 2, or it may alternatively be connected by a separate line e.g. a line from the auxiliary high pressure expansion device 9 to the receiver 6. In one or more embodiments, the auxiliary refrigeration system is only connected to the main refrigeration system at the receiver 6.

[0113] The receiver 6 is preferably arranged to provide for two phases of the refrigerant 2 to be present in the receiver 6 and for allowing the vapour part to be recompressed either through the main refrigeration system or through the auxiliary refrigeration system 10 e.g. when the main refrigeration system is not powered. Simultaneously, the liquid part may be retained in the receiver 6 when the main refrigeration system is not powered on or it may be led through the main refrigeration system, when the main refrigeration system is on.

[0114] In one or more embodiments, the refrigeration system 1 may further be equipped with an oil extraction device for the first compressor 3 and arranged between the first compressor 3 and the main heat rejection system 4, e.g. such as immediately before an inlet of the main heat rejection system 4. The refrigeration system 1 may additionally or alternatively be equipped with an auxiliary oil extraction device for the auxiliary compressor 10 arranged between the heat rejection device 4, 12 and the auxiliary compressor 10.

REFERENCE LIST

[0115] 1 refrigeration system [0116] 2 refrigerant, e.g. CO.sub.2 refrigerant [0117] 3 first compressor [0118] 4 main heat rejection system [0119] 5 evaporator [0120] 6 receiver [0121] 7 low pressure expansion device [0122] 8 high pressure expansion device [0123] 9 auxiliary high pressure expansion device [0124] 10 auxiliary compressor [0125] 11 auxiliary heat rejection system [0126] 12 receiver pressure regulating device [0127] 20 refrigeration cycle [0128] 21 first line [0129] 22 second line [0130] 23 third line [0131] 24 fourth line [0132] 25 fifth line [0133] 26 sixth line [0134] 27 seventh line [0135] 28 eight line [0136] 29 ninth line [0137] 30 tenth line [0138] 31 eleventh line [0139] 32 twelfth line [0140] 33 thirteenth line