Cooling system

Abstract

To provide a cooling system capable of easily controlling the refrigeration capacity even if a thermal load is changed.SOLUTION: A cooling system 10 includes a circulation flow path 12 having a high pressure gas flow path part 12a and a low pressure gas flow path part 12b, a compressor 21 for discharging refrigerant gas, precooler 28, an expander 22 for expanding the refrigerant gas cooled by the precooler 28, a brine cooler 23 using cold of the refrigerant gas flowing out of the expander 22 for cooling a brine, a pressure acquisition part 45 for acquiring the pressure of the refrigerant gas in the high pressure gas flow path part 12a, an inverter for controlling the rotation speed of the compressor 22 according to the thermal load of the brine, and an inverter for controlling the rotation speed of the expander 22 so that a pressure obtained by the pressure acquisition part 45 becomes a set pressure.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cooling system that uses a refrigerant gas. [Background technology]

[0002] Conventionally, refrigerators and chillers have sometimes used what is called an air refrigeration cycle, which does not include a liquefaction process of a refrigerant gas.

[0003] The refrigeration device disclosed in Patent Document 1 includes a working circuit that forms a circulation loop. This working circuit contains a working fluid such as nitrogen. The working circuit forms a cycle that includes a mechanism for compressing the working fluid, a mechanism for cooling the working fluid, a mechanism for expanding the working fluid, and a mechanism for heating the working fluid, all in series. The mechanism for compressing the working fluid includes two compressors in series, each driven by two separate motors. The mechanism for expanding the working fluid includes a turbine connected to the drive shaft of the motor of one of the compressors. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2022-544091 Summary of the Invention [Problem to be solved by the invention]

[0005] In the refrigeration system disclosed in Patent Document 1, the compressor and turbine are driven by a single motor, and the rotation speed of the compressor and turbine are determined uniquely according to the rotation speed of the motor. This can make it difficult to control the refrigeration capacity according to fluctuations in the heat load of the object to be cooled.

[0006] The present invention has been made in view of the above-mentioned problems, and has an object to provide a cooling system that can easily control the refrigeration capacity even when the heat load fluctuates. [Means for solving the problem]

[0007] a first control unit that controls the rotation speed of the compressor in accordance with a heat load of the object to be cooled; and a second control unit that controls the rotation speed of the expander so that the pressure obtained by the pressure obtainment unit becomes a set pressure, regardless of the rotation speed of the compressor.

[0008] The cooling system is equipped with a first control unit that controls the rotation speed of the compressor and a second control unit that controls the rotation speed of the expander, allowing the compressor and expander to be controlled independently. This allows the amount of refrigerant gas discharged from the compressor (the amount circulating through the circulation flow path) to be adjusted as desired, and the pressure of the refrigerant gas in the high-pressure gas flow path can be adjusted to a desired set pressure by the expander, allowing the pressure ratio between the high-pressure side pressure and the low-pressure side pressure in the circulation flow path to be adjusted as desired. As a result, the refrigeration capacity can be easily controlled even when the heat load of the object to be cooled (or the cooling load on the object to be cooled) fluctuates.

[0009] The refrigerant gas may be nitrogen gas. In this embodiment, the GWP (global warming potential) can be set to zero.

[0010] The cooling system may further include another pressure acquisition unit provided in the low-pressure gas flow path unit and configured to acquire a pressure of the refrigerant gas. In this case, the second control unit may execute control to increase the rotation speed of the expander when the pressure acquired by the another pressure acquisition unit becomes equal to or less than a first threshold value or when the pressure acquired by the pressure acquisition unit becomes equal to or greater than a second threshold value.

[0011] In this embodiment, the pressure difference between the high-pressure gas passage section and the low-pressure gas passage section can be reduced by increasing the rotation speed of the expander, which prevents outside air from entering the circulation passage due to an excessive drop in pressure in the low-pressure gas passage section and reduces the burden on the equipment due to an excessive increase in pressure in the high-pressure gas passage section.

[0012] The compressor and the expander may each be an oil-free screw-type rotary machine.

[0013] When a turbomachine is used, condensation on the impeller can cause vibration, but by using screw-type rotary machines for the compressor and expander, this problem can be prevented.

[0014] The expander may include a shaft seal that uses nitrogen gas as a seal gas. In this embodiment, frosting can be prevented more effectively than when air is used as the seal gas.

[0015] The cooling system may further include a gas discharge passage that discharges nitrogen gas from the shaft seal, an on-off valve provided in the gas discharge passage, and a valve control unit that controls the on-off valve to open when the compressor and the expander are operating. In this aspect, it is possible to reliably prevent oil from entering the compression chamber or the expansion chamber from the bearing. [Effects of the Invention]

[0016] As described above, according to the present invention, the refrigeration capacity can be easily controlled even if the heat load fluctuates. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a diagram schematically illustrating a cooling system according to a first embodiment. [Figure 2] 4 is a diagram illustrating a shaft seal of a rotating shaft of a compressor provided in the cooling system. FIG. [Figure 3] 4 is a diagram illustrating a shaft seal of a rotary shaft of an expander provided in the cooling system. FIG. [Figure 4] FIG. 3 is a diagram illustrating the operation of the cooling system. [Figure 5] FIG. 10 is a diagram for explaining a control operation in refrigeration capacity control. [Figure 6] 10A and 10B are diagrams for explaining a control operation for monitoring the pressure of the refrigerant gas in the high-pressure gas flow path portion and the low-pressure gas flow path portion. [Figure 7] 10A and 10B are diagrams for explaining a modified example of a control operation for monitoring the pressure of the refrigerant gas in the high-pressure gas flow path portion and the low-pressure gas flow path portion. [Figure 8] FIG. 6 is a diagram schematically illustrating a cooling system according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0018] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0019] (First embodiment) 1, a cooling system 10 according to this embodiment is configured to circulate a refrigerant gas through a circulation flow path 12 to lower the temperature of the refrigerant gas, and then use the lowered temperature refrigerant gas to cool brine flowing through a brine circulation device 14. That is, in this embodiment, the brine is the object to be cooled by the cooling system 10. The cooled brine is sent to a heat load (not shown) that is the object to be cooled, and supplies cold to the heat load.

[0020] The cooling system 10 includes a circulation flow path 12 for circulating a refrigerant gas. In this embodiment, nitrogen gas is used as the refrigerant gas. However, air may be used as the refrigerant gas instead of nitrogen gas.

[0021] The circulation flow path 12 is arranged with a compressor 21, an expander 22, and a brine cooler 23 in this order. When the compressor 21 and the expander 22 are operated, the refrigerant gas flows through the circulation flow path 12 in the order of the compressor 21, the expander 22, and the brine cooler 23. At this time, the refrigerant gas goes through a compression process and an expansion process, but flows in a gaseous state without undergoing a phase change. In other words, the refrigerant gas cycle in the circulation flow path 12 is not a vapor compression refrigeration cycle that involves phase changes such as condensation and evaporation, but a heat cycle that involves only sensible heat changes such as compression, heat release, expansion, and heat absorption. For this reason, an evaporator and a condenser are not provided.

[0022] The compressor 21 is configured, for example, by an oil-free screw-type rotary machine. That is, the compressor 21 includes a compression section 21a having a pair of screw rotors and a motor 21b (see FIG. 2) that generates a driving force for driving the pair of screw rotors. A compression chamber formed between the screw rotors in the compression section 21a is gradually reduced in size as the screw rotors rotate, thereby compressing the refrigerant gas. In other words, the compressor 21 is configured by a positive displacement rotary machine. Since the screw rotors are of an oil-free type, no lubricating oil is supplied to the screw rotors. Note that the compressor 21 is not limited to a screw-type rotary machine, and may be configured by another type of rotary machine (for example, a centrifugal type) as long as the rotation speed is adjustable.

[0023] The expander 22 is configured, for example, as a screw-type rotary machine having a pair of screw rotors that rotate by the kinetic energy of the refrigerant gas. That is, the expander 22 includes an expansion section 22a having a pair of screw rotors and a generator 22b (see FIG. 3) driven by the pair of screw rotors. The expansion chamber formed between the screw rotors in the expansion section 22a gradually expands as the screw rotors rotate, thereby expanding the refrigerant gas. Because the screw rotors are oil-free, no lubricating oil is supplied to the screw rotors. The expander 22 may also be configured as another type of rotary machine, such as a turbine with an impeller.

[0024] In the circulation flow path 12, in the flow direction of the refrigerant gas, the section from the compressor 21 to the expander 22 is the high-pressure gas flow path section 12a, and the section from the expander 22 to the compressor 21 is the low-pressure gas flow path section 12b. That is, the high-pressure gas flow path section 12a is the portion of the circulation flow path 12 through which the refrigerant gas compressed to high pressure by the compressor 21 flows, and the low-pressure gas flow path section 12b is the portion of the circulation flow path 12 through which the refrigerant gas expanded to low pressure by the expander 22 flows.

[0025] The brine cooler 23 is configured as a heat exchanger that exchanges heat between the brine in the brine circulation device 14 and the refrigerant gas flowing through the circulation flow path 12 to cool the brine. The brine cooler 23 is located downstream of the expander 22 in the circulation flow path 12, and therefore the refrigerant gas that has been cooled by the expander 22 flows into the brine cooler 23. That is, the brine cooler 23 functions as a cooling unit 25 that cools the brine as an object to be cooled using the cold energy of the refrigerant gas that has been expanded by the expander 22 and has its temperature reduced.

[0026] The brine circulation device 14 includes a brine pipe 31 configured to form a closed loop between a thermal load (not shown) that is a cooling target and the brine cooler 23. Brine heated by a thermal load (not shown) is sent to the brine cooler 23, where it is cooled. The brine cooled in the brine cooler 23 is sent through the brine pipe 31 to the thermal load that is the cooling target. The brine is circulated in the brine circulation device 14 by driving a pump 32 arranged in the brine pipe 31.

[0027] An aftercooler 27 that cools the refrigerant gas that has been compressed by the compressor 21 and reached a high temperature is disposed in the high-pressure gas flow path portion 12a of the circulation flow path 12. The aftercooler 27 is configured to cool the refrigerant gas using a cooling fluid such as water. That is, the aftercooler 27 removes excess heat from the refrigerant gas that has been compressed by the compressor 21 and reached a high temperature.

[0028] A precooler 28 is disposed in the circulation flow path 12. The precooler 28 is provided so as to be connected to the high-pressure gas flow path portion 12a and the low-pressure gas flow path portion 12b, and is configured to cause heat exchange between the refrigerant gas flowing through the high-pressure gas flow path portion 12a and the refrigerant gas flowing through the low-pressure gas flow path portion 12b. That is, the precooler 28 is configured to cool the refrigerant gas before it flows into the expander 22 by using the cold energy of the refrigerant gas after the brine cooler 23 supplies cold energy to the brine.

[0029] In the high-pressure gas flow path portion 12a, the precooler 28 is located between the aftercooler 27 and the expander 22. That is, the precooler 28 is a heat exchanger that cools the refrigerant gas so that the temperature of the refrigerant gas introduced into the expander 22 falls within a desired range.

[0030] As shown in Fig. 2, a bearing 21d and a shaft seal 21e are provided on the rotating shaft 21c of the compressor 21. The rotating shaft 21c is provided so as to extend on both sides in the axial direction from the screw rotor. The bearing 21d supports the rotating shaft 21c so that it can rotate freely. The shaft seal 21e is provided to prevent oil in the bearing 21d from flowing into the screw rotor, and is located on the rotating shaft 21c between the bearing 21d and the screw rotor.

[0031] A seal gas consisting of the same type of gas (nitrogen gas) as the refrigerant gas is supplied to shaft seal 21e from gas supply source 35. That is, a supply path 36 is connected to gas supply source 35, and the seal gas is supplied to shaft seal 21e through this supply path 36. A gas discharge path 37 is also connected to shaft seal 21e, and the seal gas in shaft seal 21e is discharged to the atmosphere through gas discharge path 37. Compressor 21 may or may not be provided with shaft seal 21e that uses nitrogen gas as the seal gas. When air is used as the refrigerant gas, air may be used as the seal gas.

[0032] An on-off valve 37a, which is an on-off valve, is provided in the gas discharge path 37. The on-off valve 37a is electrically connected to a valve control unit 38 and opens and closes in response to commands from the valve control unit 38. The valve control unit 38 opens the on-off valve 37a while the compressor 21 is operating. Note that while the compressor 21 is stopped, the on-off valve 37a may be closed to stop the discharge of the seal gas, or the on-off valve 37a may be left open to allow the seal gas to continue flowing.

[0033] 2, the drive shaft 21g of the motor 21b and the rotating shaft 21c of the compressor 21 are connected via the reducer 40, but this is not limiting. The rotating shaft 21c of the compressor 21 may be driven directly by the motor 21b without the reducer 40.

[0034] As shown in Fig. 3, the rotating shaft 22c of the expander 22 is also provided with a bearing (not shown) and a shaft seal 22e. The rotating shaft 22c of the expander 22 is provided so as to extend on both sides in the axial direction from the screw rotor. The bearings support the rotating shaft 22c so that it can rotate freely. The shaft seal 22e is provided to prevent oil from the bearings from flowing into the screw rotor, and is located on the rotating shaft 22c between the bearings and the screw rotor.

[0035] A seal gas consisting of the same type of gas (nitrogen gas) as the refrigerant gas is supplied to the shaft seal portion 22e. This seal gas is supplied from a gas supply source 35 that supplies seal gas to the shaft seal portion 21e of the compressor 21, but it may also be supplied from a separate gas supply source (not shown). If the expander 22 becomes extremely cold, for example, at -100°C, there is a concern that the oil in the bearings will freeze (its viscosity will become zero). To prevent this, nitrogen gas at room temperature is passed through the shaft seal portion 22e as the seal gas to prevent the oil in the bearings from freezing. Note that if air is used as the refrigerant gas, room temperature air may also be used as the seal gas.

[0036] A supply path 42 that supplies seal gas to the shaft seal portion 22e is connected to the supply source 35 or a gas supply source not shown. A gas exhaust path 43 is connected to the shaft seal portion 22e, and the seal gas in the shaft seal portion 22e is exhausted to the atmosphere through the gas exhaust path 43. The expander 22 may or may not include the shaft seal portion 22e that uses nitrogen gas as the seal gas.

[0037] An on-off valve 43a consisting of an on-off valve is provided in the gas discharge path 43. The on-off valve 43a is electrically connected to the valve control unit 38 and opens and closes in response to commands from the valve control unit 38. The valve control unit 38 opens the on-off valve 43a while the expander 22 is operating. Note that while the expander 22 is stopped, the on-off valve 43a may be closed to stop the discharge of the seal gas, or the on-off valve 43a may be left open to allow the seal gas to continue flowing.

[0038] 1, the circulation flow path 12 is provided with a pressure acquisition unit (second pressure acquisition unit 45) that acquires the pressure of the refrigerant gas flowing into the expander 22, and another pressure acquisition unit (first pressure acquisition unit 46) that acquires the pressure of the refrigerant gas flowing out from the expander 22. In addition, the brine piping 31 is provided with a temperature acquisition unit 47 that detects the temperature of the brine flowing into the brine cooler 23. Note that the temperature acquisition unit 47 may be arranged to detect the temperature of the brine flowing out from the brine cooler 23.

[0039] A signal (temperature signal) indicating the temperature (acquired temperature) of the brine detected by the temperature acquisition unit 47 is used to control the rotation speed of the compressor 21 (or the motor 21b). That is, the motor 21b of the compressor 21 is connected to an inverter 21f (see FIG. 2) for controlling the rotation speed of the motor 21b, and the temperature signal is used as a control signal for controlling the inverter 21f.

[0040] Specifically, if the temperature indicated by the temperature signal (acquired temperature) is a preset temperature (reference temperature), the inverter 21f adjusts the rotation speed of the motor 21b to the preset rotation speed (reference rotation speed). That is, the reference rotation speed is set so as to obtain a circulating flow rate of the refrigerant gas such that the reference temperature is obtained as the temperature in the temperature acquisition unit 47 when the brine is flowing at a predetermined flow rate by the pump 32 in the brine piping 31. Note that, at this time, the condition may be that the acquired temperature matches the reference temperature, or that the acquired temperature is within a predetermined range of the reference temperature.

[0041] When the acquired temperature is higher than the reference temperature (or higher than the reference temperature by a predetermined value or more), the inverter 21f controls the motor 21b to increase the rotation speed of the motor 21b from the reference rotation speed. In this case, since the thermal load of the brine (the cooling load on the brine) is higher, the inverter 21f may increase the rotation speed of the motor 21b by an amount corresponding to the difference between the acquired temperature and the reference temperature.

[0042] On the other hand, when the acquired temperature is lower than the reference temperature (or lower than the reference temperature by a predetermined value or more), the inverter 21f controls the motor 21b to reduce the rotation speed of the motor 21b from the reference rotation speed. In this case, since the thermal load of the brine (the cooling load on the brine) is smaller, the inverter 21f may reduce the rotation speed of the motor 21b by an amount corresponding to the difference between the reference temperature and the acquired temperature. In this way, the inverter 21f functions as a first control unit that controls the rotation speed of the compressor 21 according to the thermal load of the object to be cooled.

[0043] A signal (second pressure signal) indicating the pressure of the refrigerant gas (second acquired pressure P2) detected by the second pressure acquisition unit 45 is used to further adjust the rotation speed of the expander 22 (or the generator 22b), which is obtained according to the kinetic energy of the refrigerant gas flowing into the expansion section 22a of the expander 22. An inverter 22f (see FIG. 3) for controlling the rotation speed of the expander 22 is connected to the generator 22b of the expander 22, and the second pressure signal is used as a control signal for controlling the inverter 22f. That is, the screw rotor of the expander 22 attempts to rotate at a rotation speed according to the kinetic energy of the refrigerant gas flowing into the expansion section 22a, but the screw rotor may be braked by the inverter 22f of the expander 22 controlling the rotation speed of the generator 22b. In this case, the inverter 22f functions as a brake that brakes the rotation speed of the expander 22.

[0044] Specifically, if the pressure indicated by the second pressure signal (second acquired pressure P2) is a preset pressure (set pressure), the inverter 22f of the expander 22 maintains the current control of the generator 22b. That is, the inverter 22f controls the rotation speed of the expander 22 so that the second acquired pressure P2 becomes the set pressure. Note that, at this time, the condition may be that the second acquired pressure P2 matches the set pressure, or that the second acquired pressure P2 falls within a predetermined range of the set pressure.

[0045] When the second acquired pressure P2 is higher than the set pressure (or higher than the set pressure by a predetermined value or more), the inverter 22f of the expander 22 controls the generator 22b to increase the rotation speed of the expander 22 (or the generator 22b). That is, in this case, the pressure of the refrigerant gas in the high-pressure gas passage section 12a is higher than the set pressure, so the inverter 22f increases the rotation speed of the expander 22 (or the generator 22b) by an amount corresponding to the difference between the second acquired pressure P2 and the set pressure. This reduces the pressure ratio before and after the expander 22, allowing the second acquired pressure P2 to approach the set pressure.

[0046] On the other hand, when the second acquired pressure P2 is lower than the set pressure (or is lower than the set pressure by a predetermined value or more), the inverter 22f of the expander 22 controls the generator 22b to reduce the rotation speed of the expander 22 (or the generator 22b). In this case, since the pressure of the refrigerant gas in the high-pressure gas passage section 12a is lower than the set pressure, the inverter 22f reduces the rotation speed of the expander 22 (or the generator 22b) by an amount corresponding to the difference between the second acquired pressure P2 and the set pressure. This increases the pressure ratio before and after the expander 22, allowing the second acquired pressure P2 to approach the set pressure. In this way, the inverter 22f functions as a second control section that controls the rotation speed of the expander 22 so that the second acquired pressure P2 becomes the set pressure. When the inverter 22f controls the rotation speed of the expander 22, the inverter 22f controls the rotation speed without being affected by the rotation speed of the compressor 21.

[0047] A signal (second pressure signal) indicating the pressure of the refrigerant gas (second acquired pressure P2) detected by the second pressure acquisition unit 45 is also used to monitor whether the refrigerant gas in the high-pressure gas flow path unit 12a exceeds a predetermined upper limit value.

[0048] That is, to prevent excessive load on the equipment arranged in the high-pressure gas passage section 12a, such as the compressor 21, a second threshold value is set for the pressure of the refrigerant gas in the high-pressure gas passage section 12a. The second threshold value is a value higher than the set pressure described above. Furthermore, to prevent outside air from entering the circulation passage 12, a first threshold value is set for the pressure of the refrigerant gas in the low-pressure gas passage section 12b. The second threshold value is, for example, 1.5 MPaA, and the first threshold value is, for example, 0.1 MPaA.

[0049] When the second acquired pressure P2 indicated by the second pressure signal becomes equal to or greater than the second threshold, the inverter 22f of the expander 22 increases the rotation speed of the expander 22. Normally, the expander 22 is controlled so that the second acquired pressure P2 becomes equal to the set pressure, and therefore the second acquired pressure P2 does not become equal to or greater than the second threshold. However, if the thermal load of the cooling target to which the brine is supplied suddenly increases for some reason, the temperature of the brine acquired by the temperature acquisition unit 47 (i.e., the thermal load of the brine) may suddenly increase. In such a case, the inverter 22f of the expander 22 may suddenly increase the rotation speed of the compressor 21. In such a case, the inverter 22f of the expander 22 is controlled based on the second pressure signal to prevent excessive load from being placed on the compressor 21, etc.

[0050] A signal (first pressure signal) indicating the pressure of the refrigerant gas (first acquired pressure P1) detected by the first pressure acquisition unit 46 is used to monitor whether the refrigerant gas in the low-pressure gas flow path unit 12b falls below a predetermined lower limit value.

[0051] When the first acquired pressure P1 indicated by the first pressure signal falls below a first threshold, which is a lower limit, the inverter 22f of the expander 22 is configured to increase the rotation speed of the expander 22. Normally, the expander 22 is controlled so that the second acquired pressure P2 becomes the set pressure, and the first acquired pressure P1 does not fall below the first threshold. However, if the thermal load of the cooling target to which the brine is supplied suddenly increases for some reason, the inverter 22f may be controlled to suddenly increase the rotation speed of the compressor 21. In such a case, the first acquired pressure P1 may fall below the first threshold. Therefore, the inverter 22f of the expander 22 is controlled based on the first pressure signal to increase the rotation speed of the expander 22, and the pressure of the refrigerant gas in the low-pressure gas flow path portion 12b of the circulation flow path 12 is maintained above the lower limit.

[0052] Here, the operation of the cooling system 10 according to this embodiment will be described with reference to FIG.

[0053] When starting up the cooling system 10, first, the circulation flow path 12 is filled with refrigerant gas (dry nitrogen gas) (step ST11). That is, in the cooling system 10, when a start button (not shown) is pressed, the refrigerant gas begins to be replenished into the circulation flow path 12, and it is determined whether the pressure in the system has increased to a specified range. As a result, the refrigerant gas is replenished so that the pressure of the refrigerant gas in the circulation flow path 12 is within the specified range. That is, even if the cooling system 10 has been operated before, refrigerant gas may leak from the circulation flow path 12, so the refrigerant gas is replenished so that the pressure in the system falls within the specified range. Note that the pressure of the nitrogen gas when replenishment is equal to or higher than the suction pressure of the compressor 21. This allows the refrigerant gas to be replenished stably into the circulation flow path 12.

[0054] When it is detected that the pressure in the system has increased to a specified range, a controller (not shown) starts the compressor 21 and the expander 22 (step ST12). Then, the rotation speeds of the compressor 21 and the expander 22 are gradually increased, and when the compressor 21 reaches a specified rotation speed, refrigeration capacity control is performed to adjust the amount of cold energy supplied to the brine in the brine cooler 23 (step ST13).

[0055] In the refrigeration capacity control (step ST13), as shown in FIG. 5, control is performed so that the refrigeration capacity exerted by the cooling system 10 falls within a predetermined range. Specifically, first, it is determined whether the refrigeration capacity is too high (step ST21). To determine whether the refrigeration capacity is too high, a temperature signal indicating the brine temperature (acquired temperature) acquired by the temperature acquisition unit 47 is used. If the acquired temperature matches a preset temperature (reference temperature) or falls within a predetermined range relative to the reference temperature (NO in step ST21), it is determined that the refrigeration capacity is not too high, and the process proceeds to step ST22, where it is determined whether the refrigeration capacity is too low. If the acquired temperature matches a preset temperature (reference temperature) or falls within a predetermined range relative to the reference temperature, the determination in step ST22 is also NO, and the process proceeds to step ST23. In step ST23, it is confirmed whether a stop condition for the cooling system 10 is satisfied. If the stop condition is not satisfied, the process returns to step ST21. Therefore, when the refrigeration capacity is within a predetermined range, the compressor 21 and the expander 22 continue to operate at the current rotation speed.

[0056] On the other hand, when the stop condition is satisfied, for example, when a command to stop operation is received, the process proceeds to step ST24, where stop control is performed. In the stop control, the rotation speeds of the compressor 21 and the expander 22 are gradually reduced, and the compressor 21 and the expander 22 are eventually stopped.

[0057] If it is determined in step ST21 that the refrigeration capacity is too high, i.e., if it is determined that the temperature acquired by the temperature acquisition unit 47 is lower than the reference temperature by a predetermined value or more, it is determined whether the rotation speed of the compressor 21 is equal to or lower than a lower limit (step ST25). That is, it is determined whether the compressor 21 can be stably operated at a rotation speed lowered from the current rotation speed. If the rotation speed of the compressor 21 is not equal to or lower than the lower limit (if the determination in step ST25 is NO), the rotation speed of the compressor 21 is lowered (step ST26). This reduces the flow rate of the refrigerant gas circulating through the circulation flow path 12, thereby reducing the amount of cold supplied to the brine in the brine cooler 23. That is, the rotation speed of the compressor 21 is controlled according to the thermal load of the object to be cooled. At this time, the brine circulates between the brine cooler 23 and the thermal load of the object to be cooled, to which the brine is supplied. Therefore, when the amount of cold energy supplied to the brine by the brine cooler 23 is reduced, the temperature acquired by the temperature acquisition unit 47, which is arranged upstream of the brine cooler 23 in the brine piping 31, also increases.

[0058] At this time, the rotation speed of the compressor 21 may be reduced by a predetermined rotation speed, or the reduced rotation speed may be adjusted according to the difference between the acquired temperature and the reference temperature. After the rotation speed of the compressor 21 is reduced, the process returns to step ST21.

[0059] If the temperature of the cooling fluid for cooling the refrigerant gas fluctuates in the aftercooler 27, the temperature of the refrigerant gas that has passed through the aftercooler 27 may fluctuate. In this case, the temperature of the brine in the brine circulation device 14 fluctuates, and the temperature acquired by the temperature acquisition unit 47 changes. Therefore, if the rotation speed of the compressor 21 is controlled based on the temperature acquired by the temperature acquisition unit 47, it is possible to deal with the temperature fluctuation of the cooling fluid.

[0060] If the rotation speed of the compressor 21 is equal to or lower than the lower limit (if the determination in step ST25 is YES), instead of decreasing the rotation speed of the compressor 21, the rotation speed of the expander 22 is increased (step ST27). That is, the inverter 22f of the expander 22 normally controls the rotation speed of the expander 22 so that the pressure indicated by the second pressure signal from the second pressure acquisition unit 45 (second acquired pressure P2) becomes the set pressure. However, if the rotation speed of the compressor 21 is equal to or lower than the lower limit, the set pressure is changed (in this case, reduced) by a controller (not shown). Because the inverter 22f controls the second acquired pressure P2 to become the set pressure, the reduction in the set pressure increases the rotation speed of the expander 22.

[0061] As a result, the flow rate of the refrigerant gas flowing from the high-pressure gas passage section 12a to the low-pressure gas passage section 12b in the circulation passage 12 increases, thereby decreasing the ratio between the pressure of the refrigerant gas in the high-pressure gas passage section 12a and the pressure of the refrigerant gas in the low-pressure gas passage section 12b (i.e., the expansion ratio). This increases the temperature of the refrigerant gas in the low-pressure gas passage section 12b, thereby reducing the amount of cold supplied to the brine in the brine cooler 23. At this time, the rotation speed of the expander 22 may be increased by a predetermined number of rotations, or the increased rotation speed may be adjusted according to the difference between the acquired temperature and the reference temperature. In this way, the set pressure is changed in a predetermined case. In this case, the inverter 22f is configured to control the rotation speed of the expander 22 so that the second acquired pressure P2 becomes equal to the changed set pressure. Therefore, the refrigeration capacity can be adjusted simply by changing the set pressure. The extent to which the set pressure should be changed can be determined based on the results of a preliminary test or the like.

[0062] When the rotation speed of the expander 22 is increased in step ST27, the process returns to step ST21. Note that the set pressure after the change is maintained.

[0063] If it is determined in step ST22 that the refrigeration capacity is too low, i.e., if it is determined that the temperature acquired by the temperature acquisition unit 47 is higher than the reference temperature by a predetermined temperature or more, it is determined whether the rotation speed of the compressor 21 is equal to or higher than the upper limit (step ST28). That is, it is determined whether the operation of the compressor 21 can be continued at a rotation speed higher than the current rotation speed. If the rotation speed of the compressor 21 is not equal to or higher than the upper limit (if the determination in step ST28 is NO), control is performed to increase the rotation speed of the compressor 21 (step ST29). That is, the rotation speed of the compressor 21 is controlled according to the thermal load of the object to be cooled. This increases the flow rate of the refrigerant gas circulating through the circulation flow path 12, thereby increasing the amount of cold supplied to the brine in the brine cooler 23. Note that at this time, the rotation speed of the compressor 21 may be increased by a predetermined rotation speed, or the increased rotation speed may be adjusted according to the difference between the acquired temperature and the reference temperature. Once the rotation speed of the compressor 21 is increased, the process returns to step ST21.

[0064] On the other hand, if the rotation speed of the compressor 21 is equal to or higher than the upper limit (if the determination in step ST28 is YES), instead of increasing the rotation speed of the compressor 21, control is performed to decrease the rotation speed of the expander 22 (step ST30). That is, the inverter 22f of the expander 22 normally controls the rotation speed of the expander 22 so that the pressure indicated by the second pressure signal from the second pressure acquisition unit 45 (second acquired pressure P2) becomes the set pressure. However, if the rotation speed of the compressor 21 is equal to or higher than the upper limit, the set pressure is changed (in this case, increased) by a controller (not shown). Because the inverter 22f controls the second acquired pressure P2 to become the set pressure, increasing the set pressure decreases the rotation speed of the expander 22.

[0065] This reduces the flow rate of refrigerant gas flowing from the high-pressure gas passage section 12a to the low-pressure gas passage section 12b in the circulation passage 12, thereby increasing the ratio between the pressure of the refrigerant gas in the high-pressure gas passage section 12a and the pressure of the refrigerant gas in the low-pressure gas passage section 12b (i.e., the expansion ratio).This reduces the temperature of the refrigerant gas in the low-pressure gas passage section 12b, thereby increasing the amount of cold supplied to the brine in the brine cooler 23.At this time, the rotation speed of the expander 22 may be reduced by a predetermined rotation speed, or the reduced rotation speed may be adjusted according to the difference between the acquired temperature and the reference temperature.After the rotation speed of the expander 22 is reduced, the process returns to step ST21.

[0066] Next, with reference to FIG. 6, monitoring of the pressure of the refrigerant gas in the high-pressure gas passage portion 12a and the low-pressure gas passage portion 12b will be described.

[0067] A first threshold value is set for the pressure of the refrigerant gas in the low-pressure gas passage section 12b, and a second threshold value is set for the pressure of the refrigerant gas in the high-pressure gas passage section 12a. During operation of the compressor 21 and the expander 22, pressure monitoring is performed using the pressures P1 and P2 acquired by the first pressure acquisition section 46 and the second pressure acquisition section 45.

[0068] That is, when the compressor 21 and the expander 22 are operating, it is determined whether or not the first acquired pressure P1 obtained by the first pressure acquisition unit 46 is greater than a first threshold value (step ST41). If the first acquired pressure P1 is greater than the first threshold value, the process proceeds to step ST42, where it is determined whether or not the second acquired pressure P2 obtained by the second pressure acquisition unit 45 is less than a second threshold value. If the second acquired pressure P2 is less than the second threshold value, the process returns to step ST41. Therefore, if the first acquired pressure P1 is greater than the first threshold value and the second acquired pressure P2 is less than the second threshold value, the rotation speed of the expander 22 is not adjusted.

[0069] On the other hand, when the first acquired pressure P1 becomes equal to or less than the first threshold value (NO in step ST41), the inverter 21f of the expander 22 increases the rotation speed of the expander 22 (step ST43). That is, when the pressure of the refrigerant gas in the low-pressure gas passage portion 12b becomes equal to or less than the lower limit value, the rotation speed of the expander 22 is increased to prevent outside air from entering the circulation passage 12. In this case, the rotation speed of the expander 22 may be increased by a predetermined rotation speed.

[0070] Furthermore, when the second acquired pressure P2 becomes equal to or greater than the second threshold value (NO in step ST42), the inverter 21f of the expander 22 increases the rotation speed of the expander 22 (step ST43). That is, when the pressure of the refrigerant gas in the high-pressure gas passage portion 12a becomes equal to or greater than the upper limit value, the rotation speed of the expander 22 is increased so as to prevent excessive load on devices such as the compressor 21. In this case, the rotation speed of the expander 22 may be increased by a predetermined rotation speed.

[0071] 7, instead of increasing the rotation speed of the expander 22, the rotation speed of the compressor 21 may be decreased. That is, when the first acquired pressure P1 becomes equal to or less than the first threshold value (NO in step ST41), or when the second acquired pressure P2 becomes equal to or more than the second threshold value (NO in step ST42), the inverter 21f of the compressor 21 may decrease the rotation speed of the compressor 21.

[0072] Furthermore, when the first acquired pressure P1 becomes equal to or lower than the first threshold value (NO in step ST41), or when the second acquired pressure P2 becomes equal to or higher than the second threshold value (NO in step ST42), the inverter 21f of the expander 22 may increase the rotation speed of the expander 22, and the inverter 21f of the compressor 21 may decrease the rotation speed of the compressor 21.

[0073] As described above, in this embodiment, the inverter 21f controls the rotation speed of the compressor 21 and the inverter 22f controls the rotation speed of the expander 22, allowing the compressor 21 and the expander 22 to be independently controlled. This allows the amount of refrigerant gas discharged from the compressor 21 (the amount of refrigerant gas circulating through the circulation flow path 12) to be adjusted as desired, and the expander 22 adjusts the pressure of the refrigerant gas in the high-pressure gas flow path portion 12a to a desired set pressure. That is, the rotation speed of the expander 22 can be adjusted so that the pressure of the refrigerant gas at the inlet side of the expander 22 becomes the desired set pressure, regardless of the rotation speed of the compressor 21. Furthermore, because the circulation flow path 12 is a closed loop, adjusting the pressure of the refrigerant gas at the inlet side of the expander 22 to the desired set pressure also adjusts the pressure ratio between the high-pressure side pressure and the low-pressure side pressure in the circulation flow path 12 accordingly. This adjusts the refrigeration capacity, allowing the temperature of the refrigerant gas flowing out of the expander 22 to be set as desired. As a result, the refrigeration capacity can be easily controlled even if the heat load of the object to be cooled (or the cooling load on the object to be cooled) fluctuates.

[0074] Furthermore, in this embodiment, nitrogen gas is used as the refrigerant gas, so that the GWP (global warming potential) can be set to zero.

[0075] Furthermore, in this embodiment, the inverter 22f of the expander 22 increases the rotation speed of the expander 22 when the pressure acquired by the first pressure acquisition unit 46 (first acquired pressure P1) becomes equal to or lower than the first threshold value, or when the pressure acquired by the second pressure acquisition unit 45 (second acquired pressure P2) becomes equal to or higher than the second threshold value. This reduces the pressure difference between the high-pressure gas passage section 12a and the low-pressure gas passage section 12b. As a result, it is possible to prevent outside air from entering the circulation passage 12 due to an excessive drop in pressure in the low-pressure gas passage section 12b, and it is also possible to suppress strain on the equipment due to an excessive increase in pressure in the high-pressure gas passage section 12a.

[0076] In this embodiment, the compressor 21 and the expander 22 are each an oil-free screw-type rotary machine. When a turbomachine is used, condensation on the impeller can cause vibration. However, by using a screw-type rotary machine as the compressor 21 and the expander 22, such a problem can be prevented.

[0077] In this embodiment, the expander 22 includes a shaft seal 22e that uses nitrogen gas as a seal gas, which can prevent frost formation more effectively than when air is used as a seal gas.

[0078] In addition, in this embodiment, the valve control unit 38 opens the on-off valves 37a, 43a when the compressor 21 and the expander 22 are operating, so that oil can be reliably prevented from entering the compression chamber or the expansion chamber from the bearing 21d.

[0079] (Second embodiment) 8 shows the second embodiment. Note that the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0080] In the first embodiment, a brine cooler 23 is provided in the circulation flow path 12, and the brine cooler 23 functions as a cooling unit 25 that cools brine as an object to be cooled. In contrast, in the second embodiment, the brine cooler 23 that performs heat exchange between the refrigerant gas and the brine is not provided. Instead, a cooler 50 that functions as the cooling unit 25 is provided in the circulation flow path 12. The cooler 50 is thermally connected to an object to be cooled, and is configured to cool the object to be cooled by the cold energy of the refrigerant gas passing through the cooler 50. The object to be cooled may be, for example, a pipe that sends cooled air to a heat load that is an object to be cooled (not shown), or the heat load itself.

[0081] Although the description of other configurations, actions, and effects will be omitted, the description of the first embodiment can be applied to the second embodiment.

[0082] (Other embodiments) The disclosed embodiments should be considered to be illustrative and not restrictive in all respects. The present invention is not limited to the above-described embodiments, and various modifications and improvements are possible without departing from the spirit and scope of the present invention. For example, in the above-described embodiments, the temperature acquisition unit 47 is arranged to detect the temperature of the brine flowing into the brine cooler 23, and the inverter 21f of the compressor 21 controls the discharge rate of the compressor 21 so that the brine temperature at the inlet side of the brine cooler 23 becomes the reference temperature. Alternatively, the temperature acquisition unit 47 may be arranged to detect the temperature of the brine flowing out from the brine cooler 23, and the inverter 21f of the compressor 21 may control the discharge rate of the compressor 21 so that the brine temperature at the outlet side of the brine cooler 23 becomes the reference temperature. In other words, because the brine circulates between the thermal load of the brine supply destination and the brine cooler 23, the refrigeration capacity can be controlled based on the thermal load of the brine using either the temperature change at the inlet side or the temperature change at the outlet side of the brine cooler 23. In addition, the actual discharge volume of the compressor 21 may be measured by various means, and the inverter 21f of the compressor 21 may control the motor 21b so that the discharge volume becomes a preset value according to the thermal load of the object to be cooled.

[0083] In the above embodiment, in step ST26 of the refrigeration capacity control shown in Fig. 5, control is performed to reduce the rotation speed of the compressor 21. However, this is not limited to this, and for example, control may be performed to reduce the rotation speed of the compressor 21 and increase the rotation speed of the expander 22.

[0084] In the above embodiment, in step ST29 of the refrigeration capacity control shown in Fig. 5, control is performed to increase the rotation speed of the compressor 21. However, this is not limited to this, and for example, control may be performed to increase the rotation speed of the compressor 21 and decrease the rotation speed of the expander 22. [Explanation of symbols]

[0085] 10: Cooling system 12: Circulation flow path 12a: High-pressure gas flow path 12b: Low-pressure gas flow path 21: Compressor 21f: Inverter 22: Expander 22e: Shaft seal part 22f: Inverter 23: Brine Cooler 25: Cooling section 28: Precooler 38: Valve control section 43: Gas exhaust channel 43a: On-off valve 45: Second pressure acquisition unit 46: First pressure acquisition unit 50: Cooler

Claims

1. A cooling system that cools an object to be cooled using a refrigerant gas, a circulation flow path having a high-pressure gas flow path portion and a low-pressure gas flow path portion for circulating a refrigerant gas; a compressor that discharges refrigerant gas toward the high-pressure gas flow path; a precooler for exchanging heat between the refrigerant gas in the high-pressure gas flow path and the refrigerant gas in the low-pressure gas flow path; an expander that expands the refrigerant gas cooled in the precooler and causes the expanded refrigerant gas to flow into the low-pressure gas flow path; a cooling section in the low-pressure gas flow path section that cools an object to be cooled using cold energy of the refrigerant gas flowing out from the expander; a pressure acquisition unit provided in the high-pressure gas flow path unit and configured to acquire a pressure of the refrigerant gas; a first control unit that controls a rotation speed of the compressor in accordance with a thermal load of the object to be cooled; a second control unit that controls the rotation speed of the expander so that the pressure obtained by the pressure obtaining unit becomes a set pressure, regardless of the rotation speed of the compressor; A cooling system comprising:

2. The cooling system of claim 1 , wherein the refrigerant gas is nitrogen gas.

3. Further, another pressure acquisition unit is provided in the low-pressure gas flow path unit and acquires the pressure of the refrigerant gas. The cooling system described in claim 1 or 2, wherein the second control unit executes control to increase the rotation speed of the expander when the pressure acquired by the other pressure acquisition unit becomes equal to or lower than a first threshold value, or when the pressure acquired by the pressure acquisition unit becomes equal to or higher than a second threshold value.

4. The refrigeration system of claim 1 , wherein the compressor and the expander are each oil-free rotary screw machines.

5. The cooling system according to claim 4 , wherein the expander includes a shaft seal portion that uses nitrogen gas as a seal gas.

6. a gas discharge path for discharging nitrogen gas from the shaft seal portion; an on-off valve provided in the gas discharge path; a valve control unit that controls the on-off valve to be open when the compressor and the expander are in operation; The cooling system of claim 5 further comprising:

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