JP2024173281A - Cooling system, cooling method and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To evaporate a refrigerant in an evaporator at an intended temperature without generating dew condensation even when a temperature of outside air is lowered.

SOLUTION: A cooling system includes: a refrigerant circuit having a compressor that compresses a refrigerant flowing out from an evaporator, a condenser that condenses the refrigerant compressed by the compressor by exchanging heat with cooling fluid, an expansion valve that decompresses the refrigerant condensed by the condenser and the evaporator that evaporates the refrigerant decompressed by the expansion valve for heat exchange; a pump for circulating the cooling fluid via the condenser so as to cool the cooling fluid by using outside air; a detection device that detects a detection value related to an evaporation temperature of the refrigerant; and a control device that regulates an opening of the expansion valve so that a control amount obtained from the detection value detected by the detection device becomes a target value determined relative to the control amount and regulates a discharge amount of the pump so that the control amount becomes the target value when the regulation of the opening of the expansion valve reaches a limit.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2024,JPO&INPIT

Description

This disclosure relates to a cooling system, a cooling method, and a program.

For example, a cooling system (hereinafter referred to as a cooling system) is known that uses a mechanism similar to the compressor cycle disclosed in Patent Document 1 and Patent Document 2. In such a cooling system, the refrigerant is evaporated by an evaporator, and heat is exchanged between the refrigerant and the indoor air. This cools the indoor air. The refrigerant that flows out of the evaporator is then compressed by a compressor to increase its pressure, and the increased pressure is condensed by a condenser. The condensed refrigerant is reduced in pressure by an expansion valve until it is in a state where it can evaporate in the evaporator at the desired temperature. The reduced pressure refrigerant evaporates again in the evaporator, and this process is repeated. This method of cooling by changing the pressure of the refrigerant using a compressor and an expansion valve is called a direct expansion method (hereinafter referred to as a direct expansion type).

Patent No. 6509047 Patent No. 6566705

One type of condenser used in the cooling system described above is a condenser that condenses the refrigerant by water cooling. In this type of condenser, the refrigerant is condensed by heat exchange with the circulating cooling water. The circulating cooling water is cooled by outside air, so when the outside air temperature drops, the temperature of the cooling water drops with the drop in outside air temperature. When the temperature of the cooling water drops, the temperature of the refrigerant in the condenser also drops, resulting in a phenomenon in which the pressure of the refrigerant in the condenser drops.

When the refrigerant pressure drops in the condenser, the degree of pressure reduction by the expansion valve decreases. A smaller pressure reduction means that the range of pressure that can be adjusted by the expansion valve becomes narrower. Therefore, when the pressure reduction degree becomes small, even if the expansion valve is opened to its maximum, it may not be possible to evaporate the refrigerant at the desired temperature in the evaporator.

In this case, if the temperature at which the refrigerant evaporates in the evaporator drops below the dew point temperature, a problem occurs in which condensation occurs in the evaporator. When condensation occurs in the evaporator, for example, the condensation can be scattered by air sent to the evaporator by a fan or the like, causing water splashing. When a cooling system is used, for example, to cool server racks in a data center or to cool semiconductor elements in electronic circuits, it has been necessary to waterproof the system or restrict the installation location of the evaporator in order to prevent short circuits in the server equipment or electronic circuits installed in the server racks due to water splashing.

The present disclosure has been made to solve the above problems, and aims to provide a cooling system, a cooling method, and a program that enable refrigerant to evaporate in an evaporator at a desired temperature without causing condensation, even when the outside air temperature drops.

In order to solve the above problems, the cooling system according to the present disclosure includes a refrigerant circuit having a compressor that compresses the refrigerant flowing out of an evaporator, a condenser that condenses the refrigerant compressed by the compressor by heat exchange with a cooling fluid, an expansion valve that reduces the pressure of the refrigerant condensed by the condenser, and the evaporator that evaporates the refrigerant reduced in pressure by the expansion valve and exchanges heat with it, a pump that circulates the cooling fluid so that it is cooled by outside air while passing through the condenser, a detection device that detects a detection value related to the evaporation temperature of the refrigerant, and a control device that adjusts the opening of the expansion valve so that a control amount obtained from the detection value detected by the detection device becomes a target value set for the control amount, and adjusts the discharge amount of the pump so that the control amount becomes the target value when the adjustment of the opening of the expansion valve reaches its limit.

The cooling method according to the present disclosure includes a step of compressing a refrigerant flowing out of an evaporator by a compressor;
The method includes the steps of: a condenser condensing the refrigerant compressed by the compressor by heat exchange with a cooling fluid; an expansion valve reducing the pressure of the refrigerant condensed by the condenser; an evaporator evaporating the refrigerant reduced in pressure by the expansion valve and exchanging heat with it; a pump circulating the cooling fluid so that it passes through the condenser and is cooled by outside air; a detection device detecting a detection value related to the evaporation temperature of the refrigerant; and a control device adjusting the opening of the expansion valve so that a control quantity obtained from the detection value detected by the detection device becomes a target value set for the control quantity, and, when the adjustment of the opening of the expansion valve reaches its limit, adjusting the discharge volume of the pump so that the control quantity becomes the target value.

The program according to the present disclosure is a program for causing a computer functioning as a control device further provided in a cooling system including a compressor that compresses the refrigerant flowing out of an evaporator, a condenser that condenses the refrigerant compressed by the compressor through heat exchange with a cooling fluid, an expansion valve that reduces the pressure of the refrigerant condensed by the condenser, and the evaporator that evaporates the refrigerant reduced by the expansion valve and exchanges heat with it, a pump that circulates the cooling fluid so that it is cooled by outside air while passing through the condenser, and a detection device that detects a detection value related to the evaporation temperature of the refrigerant, to execute a procedure of adjusting the opening of the expansion valve so that a control amount obtained from the detection value detected by the detection device becomes a target value set for the control amount, and, when the adjustment of the opening of the expansion valve reaches its limit, adjusting the discharge amount of the pump so that the control amount becomes the target value.

According to the cooling system, cooling method, and program disclosed herein, even if the outside air temperature drops, the refrigerant can be evaporated in the evaporator at the desired temperature without causing condensation.

1 is a schematic block diagram illustrating a configuration example of a cooling system according to an embodiment of the present disclosure. FIG. 2 is a diagram illustrating an example of installation of an evaporator according to an embodiment of the present disclosure. 1 is a graph showing changes in saturation temperature of a refrigerant, degree of superheat, and cooling water flow rate per unit time with respect to a cooling water inlet temperature in a typical cooling system. 5 is a flowchart showing a flow of processing by a control device according to an embodiment of the present disclosure. 4 is a graph showing changes in refrigerant saturation temperature, degree of superheat, and coolant flow rate per unit time versus coolant inlet temperature in a cooling system according to an embodiment of the present disclosure. FIG. 13 is a schematic block diagram illustrating a configuration example of a cooling system according to another embodiment of the present disclosure. FIG. 2 is a diagram showing an example of a Mollier diagram for each of a direct expansion operation and a free cooling operation in an embodiment of the present disclosure. 1 is a graph showing the relationship between the cooling capacity, the temperature of the cooling water, the direct expansion operation, and the free cooling operation in an embodiment of the present disclosure. FIG. 1 is a schematic block diagram illustrating a configuration of a computer according to at least one embodiment.

The cooling system, cooling method, and program according to the present disclosure will be described below with reference to FIG. 1, FIG. 2, and FIG. 4 to FIG. 9. FIG. 1 is a schematic block diagram showing a configuration example of a cooling system 1 according to an embodiment of the present disclosure. FIG. 2 is a diagram showing an installation example of an evaporator 11 according to an embodiment of the present disclosure. FIG. 4 is a flowchart showing a processing flow by a control device 5 according to an embodiment of the present disclosure. FIG. 5 is a graph showing changes in the saturation temperature of the refrigerant, the degree of superheat, and the cooling water flow rate per unit time with respect to the cooling water inlet temperature in the cooling system 1 according to an embodiment of the present disclosure. FIG. 6 is a schematic block diagram showing a configuration example of a cooling system 1a according to another embodiment of the present disclosure. FIG. 7 is a diagram showing an example of a Mollier diagram of each of the direct expansion operation and the free cooling operation in an embodiment of the present disclosure. FIG. 8 is a graph showing the relationship between the cooling capacity, the water temperature of the cooling water, the direct expansion operation, and the free cooling operation in an embodiment of the present disclosure. FIG. 9 is a schematic block diagram showing the configuration of a computer according to at least one embodiment. Note that the same or corresponding configurations in each figure are designated by the same reference numerals and the description will be omitted as appropriate.

(Example of cooling system configuration)
1, the cooling system 1 includes a refrigerant circuit 2, which is a system through which a refrigerant circulates, a cooling water circuit 3, which is a system through which cooling water flows to cool the refrigerant, a control device 5, and a detection device 40. The refrigerant circuit 2 includes an evaporator 11, an accumulator 12, a compressor 13, a condenser 14, a receiver 15, an expansion valve 16, and a pump 17. The cooling water circuit 3 includes a pump 21 and a cooling tower 22. Note that the condenser 14 is also one of the components that make up the cooling water circuit 3, as the condenser 14 cools the refrigerant flowing therethrough by the cooling water flowing therethrough.

The refrigerant circuit 2 further includes pipes 101-112 and pipes 121-124 used during free cooling operation, which will be described later, as well as three-way valves 31-36 used to switch between direct expansion operation (hereinafter referred to as direct expansion operation) and free cooling operation. Each of the three-way valves 31-36 is capable of switching the flow path so that the refrigerant flowing in from one inlet flows out from one of two outlets.

During direct expansion operation, the refrigerant flows through the path indicated by the thick solid line, i.e., the path passing through the evaporator 11, accumulator 12, compressor 13, condenser 14, receiver 15, and expansion valve 16, and the refrigerant circulates through the path in the direction indicated by the thick solid arrow. During free cooling operation, the three-way valves 31 to 36 are switched as follows. By switching the three-way valves 31 and 32, the path passing through the pipe 102, accumulator 12, pipe 103, compressor 13, and pipe 104 is switched to the path indicated by the thick dotted line passing through pipe 121. By switching the three-way valves 33 and 34, the path passing through the pipe 108 is switched to the path indicated by the thick dotted line passing through the pipe 122, pump 17, and pipe 123. By switching the three-way valves 35 and 36, the path passing through the pipe 110, the expansion valve 16, and the pipe 111 is switched to the path indicated by the thick dotted line passing through the pipe 124. That is, during free cooling operation, the path through which the refrigerant flows is a path that bypasses the accumulator 12, the compressor 13, and the expansion valve 16, and passes through the evaporator 11, the condenser 14, the receiver 15, and the pump 17, and the refrigerant circulates in the direction indicated by the thick solid arrow and the thick dotted arrow.

The evaporator 11 is a heat exchanger, into which the refrigerant flows from the pipe 112 and into which the refrigerant flows out to the pipe 101. The evaporator 11 has a planar shape, for example, as shown in FIG. 2, and is installed on the rear door side of the server rack 61 installed inside the data center. A blower 62 is installed on the side of the evaporator 11 opposite to the side facing the rear door side of the server rack 61. Air flows into the inside of the server rack 61 from the opening of the server rack 61 by the blowing of air by the blower 62, and the air that flows into the server rack 61 cools the server devices and the like installed in the server rack 61. The air that has become warm by cooling the server devices and the like passes through the evaporator 11, and is cooled by heat exchange with the refrigerant that evaporates in the evaporator 11. The air cooled by the evaporator 11 flows out into the data center room via the blower 62. This suppresses the rise in temperature inside the data center room due to heat generated by the server devices and the like.

The accumulator 12 receives refrigerant from the pipe 102 and discharges the refrigerant to the pipe 103. The accumulator 12 separates the two-phase gas-liquid refrigerant discharged from the evaporator 11 into gas and liquid, and discharges the separated gaseous refrigerant. The compressor 13 can adjust the pressure applied by, for example, the rotation speed, and draws in the gaseous refrigerant discharged from the accumulator 12 through the pipe 103, compresses the drawn refrigerant to increase its pressure, and discharges the pressurized refrigerant to the pipe 104. The condenser 14 is a heat exchanger, and receives refrigerant from the pipe 105 and discharges the refrigerant to the pipe 106. The condenser 14 condenses the refrigerant that flows in from the pipe 105 by exchanging heat with the cooling water flowing inside.

Receiver 15 receives refrigerant from pipe 106 and outputs refrigerant to pipe 107. Receiver 15 separates the gas that has been condensed into liquid by condenser 14 from the refrigerant that has not been condensed and remains in gaseous state. Receiver 15 removes impurities such as moisture from the separated liquid refrigerant using a filter or the like, stores the excess liquid refrigerant, and outputs the required amount of liquid refrigerant.

Expansion valve 16 is a valve that reduces the pressure of the inflowing refrigerant. Refrigerant flows in from pipe 110 and flows out to pipe 111. Expansion valve 16 is, for example, an electronic expansion valve, and its opening is variable. Changing the opening changes the amount of pressure reduction. Note that the smaller the opening of expansion valve 16, the greater the amount of pressure reduction, and conversely, the greater the opening, the smaller the amount of pressure reduction. Pump 17 can adjust the discharge amount by, for example, the rotation speed, and sucks in the refrigerant flowing out of receiver 15 via pipe 122 and discharges the sucked refrigerant into pipe 123.

The cooling water circuit 3 further includes pipes 131 to 133. In the cooling water circuit 3, the path through which the cooling water flows is shown by the thick dashed line, i.e., the path passes through the cooling tower 22, the condenser 14, and the pump 21, and the cooling water circulates in the direction shown by the thick dashed arrow.

Cooling tower 22 receives cooling water from pipe 131 and discharges it to pipe 132. Cooling tower 22 is equipped with a fan inside, and the amount of outside air taken in from outside can be adjusted by the fan's rotation speed. Cooling tower 22 uses the taken-in outside air to cool the cooling water that flows in from pipe 131. Cooling water flows in to condenser 14 from pipe 132 and flows out cooling water to pipe 133. Pump 21 can adjust the discharge amount by, for example, the rotation speed, and sucks in cooling water that flows out of condenser 14 via pipe 133 and discharges cooling water to pipe 131.

The detection device 40 detects a detection value related to the evaporation temperature, which is the temperature at which the refrigerant evaporates in the evaporator 11, i.e., the saturation temperature of the refrigerant in the evaporator 11. The detection device 40 includes a temperature sensor 41 and a pressure sensor 42. The temperature sensor 41 detects the temperature of the refrigerant in the pipe 101. The pressure sensor 42 detects the pressure of the refrigerant in the pipe 101. Therefore, in this case, the detection value related to the evaporation temperature of the refrigerant detected by the detection device 40 is the temperature and pressure values of the refrigerant in the pipe 101.

The control device 5 controls the refrigerant circuit 2 and the coolant circuit 3. In FIG. 1, as an example, control lines connecting the expansion valve 16, the pump 21, the temperature sensor 41, and the pressure sensor 42 to the control device 5 are shown. In addition, although not shown in FIG. 1, the cooling system 1 has control lines connecting the compressor 13, the pump 17, and each of the three-way valves 31-36 to the control device 5.

The control device 5 takes in the detection value detected by the detection device 40 and output to the control line. Although not shown in FIG. 1, various sensors are installed in the refrigerant circuit 2 and the cooling water circuit 3 in addition to the temperature sensor 41 and the pressure sensor 42. The control device 5 is connected to various sensors not shown via control lines, and also acquires the detection values detected by these various sensors. Based on the acquired detection value, for example, when direct expansion operation is performed in the cooling system 1, the control device 5 performs feedback control corresponding to the direct expansion operation, and outputs various command values calculated by the feedback control to the control objects corresponding to each of the command values, i.e., the expansion valve 16, the compressor 13, and the pump 21. On the other hand, when free cooling operation is performed in the cooling system 1, the control device 5 performs feedback control corresponding to the free cooling operation, and outputs various command values calculated by the feedback control to the control objects corresponding to each of the command values, i.e., the pump 17 and the pump 21. The command value for the expansion valve 16 is the opening degree, and the command values for the compressor 13, pump 17, and pump 21 are the rotation speeds.

(Specific examples of issues)
Here, a specific example of the above problem will be described using the configuration of the cooling system 1 shown in FIG. 1 as an example. In a general cooling system, there is no control line from the control device 5 to the pump 21 of the cooling water circuit 3, and the pump 21 operates autonomously at a constant rotation speed. FIG. 3 is a graph showing changes in various parameters in such a general cooling system, and in FIGS. 3(a) to 3(c), the horizontal axis is the cooling water inlet temperature in the condenser 14, that is, the temperature of the cooling water in the pipe 132 near where the condenser 14 and the pipe 132 are connected. The horizontal axis indicates that the temperature increases toward the right. The vertical axis of FIG. 3(a) is the saturation temperature of the refrigerant, and indicates that the saturation temperature of the refrigerant increases toward the upward direction. The vertical axis of FIG. 3(b) is the degree of superheat, and indicates that the degree of superheat increases toward the upward direction. The vertical axis of FIG. 3(c) is the cooling water flow rate per unit time, and indicates that the cooling water flow rate per unit time increases toward the upward direction.

In the graph of FIG. 3(c), the characteristic indicated by the reference symbol 55 is a characteristic showing the change in the coolant flow rate per unit time (hereinafter referred to as coolant flow rate characteristic 55). As described above, in a typical cooling system, the pump 21 of the coolant circuit 3 operates at a constant rotation speed, so that the coolant flow rate per unit time flowing through the pipes 131-133 of the coolant circuit 3 is constant, regardless of changes in the coolant inlet temperature, as shown in the coolant flow rate characteristic 55.

When the temperature of the outside air drops, the cooling water inlet temperature also drops. When the cooling water inlet temperature drops, the temperature of the refrigerant flowing through the condenser 14 drops, and the pressure of the refrigerant flowing through the condenser 14 drops. When the pressure of the refrigerant drops, the saturation temperature of the refrigerant drops. In the graph of FIG. 3(a), the characteristic indicated by the symbol 51 is a characteristic (hereinafter referred to as saturation temperature characteristic 51) that shows the change in the saturation temperature of the refrigerant at the inlet of the expansion valve 16. Here, the saturation temperature of the refrigerant at the inlet of the expansion valve 16 is the saturation temperature of the refrigerant in the pipe 110 near where the expansion valve 16 and the pipe 110 are connected. The characteristic indicated by the symbol 52 is a characteristic (hereinafter referred to as saturation temperature characteristic 52) that shows the change in the saturation temperature of the refrigerant at the outlet of the expansion valve 16. Here, the saturation temperature of the refrigerant at the outlet of the expansion valve 16 is the saturation temperature of the refrigerant in the pipe 111 near where the expansion valve 16 and the pipe 111 are connected.

Note that the refrigerant pressure in the path from the outlet of the compressor 13 to the inlet of the expansion valve 16 is almost the same, although there are slight differences due to pressure losses in the pipes 104-110, and therefore the saturation temperature characteristic 51 can be regarded as the characteristic of the saturation temperature of the refrigerant discharged by the compressor 13. Similarly, the refrigerant pressure in the path from the outlet of the expansion valve 16 to the inlet of the compressor 13 is almost the same, although there are slight differences due to pressure losses in the pipes 112, 101-103, and therefore the saturation temperature characteristic 52 can be regarded as the characteristic of the saturation temperature of the refrigerant sucked into the compressor 13.

The control device 5 performs feedback control so that the evaporation temperature of the refrigerant in the evaporator 11 is constant, so the saturation temperature of the refrigerant at the outlet of the expansion valve 16 is also constant. Therefore, in the saturation temperature characteristic 52, the saturation temperature of the refrigerant is constant in the portion to the right of the timing indicated by the symbol 50 (hereinafter referred to as timing 50). In contrast, when the refrigerant pressure in the condenser 14 decreases, the refrigerant pressure at the inlet of the expansion valve 16 decreases. Therefore, as shown in the saturation temperature characteristic 51, the saturation temperature of the refrigerant at the inlet of the expansion valve 16 decreases as the cooling water inlet temperature decreases.

The difference between the saturation temperature characteristic 51 and the saturation temperature characteristic 52 at the same cooling water inlet temperature is the difference in the saturation temperature of the refrigerant caused by the pressure reduction by the expansion valve 16, and this difference indirectly indicates the amount of pressure reduction by the expansion valve 16. At the time of timing 50, the value of the saturation temperature characteristic 51 and the value of the saturation temperature characteristic 52 are the same, and at this timing 50, the opening of the expansion valve 16 is at its maximum, the pressure reduction by the expansion valve 16 is not performed, and there is no difference in the pressure of the refrigerant before and after the expansion valve 16. Therefore, when the cooling water inlet temperature is in the left part of the timing 50, the adjustment of the opening of the expansion valve 16 is at its limit at the time of timing 50, so the expansion valve 16 cannot reduce the pressure, and the saturation temperature characteristic 52, like the saturation temperature characteristic 51, shows a change that decreases as the cooling water inlet temperature decreases. Thereafter, when the saturation temperature of the refrigerant at the outlet of the expansion valve 16, shown in the saturation temperature characteristic 52, drops and reaches, for example, the dew point temperature shown by the reference symbol 53, the refrigerant evaporates in the evaporator 11 at the dew point temperature, causing condensation in the evaporator 11.

(Example of cooling system operation)
The control process by the control device 5 of the cooling system 1 and the operation of the cooling system 1 by the control process will be described with reference to Fig. 4. Note that the process shown in Fig. 4 is performed when the refrigerant circuit 2 is performing a direct expansion operation.

The temperature sensor 41 and the pressure sensor 42 each detect the temperature and pressure of the refrigerant in the pipe 101, for example, at regular intervals. The temperature sensor 41 outputs the detected temperature value to the control device 5, and the pressure sensor 42 outputs the detected pressure value to the control device 5. The control device 5 acquires the temperature value output by the temperature sensor 41 and the pressure value output by the pressure sensor 42 (S1).

The control device 5 incorporates a mechanism for calculating a function that indicates the relationship between the pressure of the refrigerant and the saturation temperature of the refrigerant. The control device 5 substitutes the captured pressure value into the function to calculate the saturation temperature of the refrigerant. The control device 5 determines the degree of superheat by subtracting the calculated saturation temperature of the refrigerant from the captured temperature value, i.e., the temperature detected by the temperature sensor 41. The control device 5 determines the degree of superheat by using the calculated degree of superheat as a control amount and performs feedback control so that the control amount becomes a predetermined target value for the degree of superheat. Here, the predetermined target value for the degree of superheat is, for example, the following value. In the graph of FIG. 3(b), the characteristic indicated by the symbol 54 is a characteristic that indicates the change in the degree of superheat (hereinafter referred to as superheat characteristic 54). In the superheat characteristic 54, the part to the right of the timing 50 indicates a constant value, and this constant value becomes, for example, a predetermined target value for the degree of superheat.

The control device 5 determines whether the controlled variable matches the target value (S3). When the control device 5 determines that the controlled variable matches the target value (S3, Yes), the control device 5 waits for the temperature sensor 41 to output a temperature value and the pressure sensor 42 to output a pressure value, and when the temperature sensor 41 outputs a temperature value and the pressure sensor 42 outputs a pressure value, the control device 5 performs the process of S1 again.

On the other hand, when the control device 5 determines that the controlled amount does not match the target value (S3, No), the control device 5 determines whether the controlled amount exceeds the target value (S4). Assume that the control device 5 determines that the controlled amount does not exceed the target value (S4, No). In this case, the controlled amount is less than the target value, and the degree of superheat is reduced, in other words, the refrigerant pressure is high and the saturation temperature of the refrigerant is rising. Therefore, the control device 5 performs feedback control to reduce the opening of the expansion valve 16 and increase the amount of pressure reduction of the expansion valve 16. That is, the control device 5 calculates a command value indicating the opening of the expansion valve 16 that makes the controlled amount approach the target value based on the controlled amount and the target value, and outputs the calculated command value to the expansion valve 16 (S5). When the expansion valve 16 receives a command value from the control device 5, it reduces the opening and increases the amount of pressure reduction according to the command value. This reduces the pressure of the refrigerant in the pipe 101 and lowers the saturation temperature of the refrigerant, increasing the degree of superheat and bringing it closer to the target value.

On the other hand, the control device 5 determines that the control amount exceeds the target value (S4, Yes). In this case, the control amount exceeds the target value, and the degree of superheat is increasing, in other words, the refrigerant pressure is decreasing, and the saturation temperature of the refrigerant is decreasing. The control device 5 determines whether the adjustment of the opening degree of the expansion valve 16 has reached its limit (S6). For example, each time the control device 5 outputs a command value to the expansion valve 16, the control device 5 records information indicating the most recent command value output to the expansion valve 16, i.e., the most recent opening degree of the expansion valve 16, in an internal memory area. The control device 5 determines whether the adjustment of the opening degree of the expansion valve 16 has reached its limit based on whether the most recent opening degree of the expansion valve 16 is less than a predetermined threshold value. That is, if the most recent opening degree of the expansion valve 16 is less than the threshold value, the control device 5 determines that the adjustment of the opening degree of the expansion valve 16 has not reached its limit, and if the most recent opening degree of the expansion valve 16 is not less than the threshold value, the control device 5 determines that the adjustment of the opening degree of the expansion valve 16 has reached its limit. Note that here, the predetermined threshold value is, for example, the maximum opening degree of the expansion valve 16.

The control device 5 determines that the opening of the expansion valve 16 is not at its limit (S6, No). In this case, since there is room to increase the opening of the expansion valve 16, the control device 5 performs feedback control to increase the opening of the expansion valve 16 and reduce the amount of pressure reduction of the expansion valve 16. That is, the control device 5 calculates a command value indicating the opening of the expansion valve 16 that makes the control amount approach the target value based on the control amount and the target value, and outputs the calculated command value to the expansion valve 16 (S7). When the expansion valve 16 receives the command value from the control device 5, it increases the opening and reduces the amount of pressure reduction according to the command value. As a result, the pressure of the refrigerant in the pipe 101 increases and the saturation temperature of the refrigerant increases, so the degree of superheat decreases and approaches the target value.

On the other hand, the control device 5 determines that the opening of the expansion valve 16 has reached its limit (S6, Yes). In this case, the control device 5 is in a state where it cannot further increase the opening of the expansion valve 16. In this state, if the control device 5 does not perform any control, the degree of superheat becomes greater than the target value, as shown in the portion to the left of timing 50 in the superheat characteristic 54 in FIG. 3(b).

To explain in more detail, the degree of superheat is a value obtained by subtracting the saturation temperature of the refrigerant at the outlet of the evaporator 11 from the temperature of the refrigerant at the outlet of the evaporator 11, as described above. The temperature of the refrigerant at the outlet of the evaporator 11 is almost the same as the temperature of the surroundings where the evaporator 11 is installed, so it is almost a constant value. In contrast, the saturation temperature of the refrigerant at the outlet of the evaporator 11 decreases when the pressure of the refrigerant at the outlet of the evaporator 11 decreases. As shown in the saturation temperature characteristic 52 in FIG. 3(a), in the portion to the left of the timing 50, the saturation temperature of the refrigerant at the outlet of the expansion valve 16 decreases with the decrease in the cooling water inlet temperature. As described above, the pressure of the refrigerant in the path from the outlet of the expansion valve 16 to the inlet of the compressor 13 becomes almost the same. Therefore, when the saturation temperature of the refrigerant at the outlet of the expansion valve 16 decreases, the saturation temperature of the refrigerant at the outlet of the evaporator 11 also decreases. Therefore, the saturation temperature of the refrigerant at the outlet of the evaporator 11, which shows a decreasing tendency in the portion to the left of timing 50, is subtracted from the refrigerant temperature at the outlet of the evaporator 11, which is a nearly constant value, so the degree of superheat increases in the portion to the left of timing 50, as shown in the superheat characteristic 54 in FIG. 3(b).

In order to bring this increased degree of superheat closer to the target value, the control device 5 of this embodiment performs the following process. The control device 5 performs feedback control to reduce the discharge rate of the pump 21 of the cooling water circuit 3 while maintaining the opening of the expansion valve 16, thereby reducing the cooling water flow rate per unit time flowing through the cooling water circuit 3. More specifically, based on the control amount and the target value, the control device 5 calculates a command value indicating the rotation speed of the pump 21 that brings the control amount closer to the target value, and outputs the calculated command value to the pump 21 (S8). The pump 21 reduces its rotation speed upon receiving the command value from the control device 5.

After processing S5, S7, and S8, the control device 5 waits for the temperature sensor 41 to output a temperature value and for the pressure sensor 42 to output a pressure value, and when the temperature sensor 41 outputs a temperature value and the pressure sensor 42 outputs a pressure value, it performs processing S1 again.

(Action and Effects)
According to the above-described embodiment, in the cooling system 1, when the control device 5 judges "Yes" in the judgment process of S6 in Fig. 4, in other words, in the cooling system 1, the temperature of the cooling water flowing into the condenser 14 decreases, and the pressure of the refrigerant condensed by the condenser 14 decreases, and even if the opening degree of the expansion valve 16 is set to the maximum opening degree, the superheat degree, which is a controlled variable, may not be maintained at a target value. In this case, the control device 5 changes the control target of the feedback control from the expansion valve 16 to the pump 21 of the cooling water circuit 3 and reduces the rotation speed of the pump 21. As a result, the discharge amount of the pump 21 decreases, so the flow rate of the cooling water per unit time flowing through the cooling water circuit 3 decreases, and the flow rate of the cooling water per unit time flowing through the condenser 14 also decreases. When the flow rate of the cooling water flowing through the condenser 14 decreases, the decrease in the temperature of the refrigerant in the condenser 14 is suppressed, and accordingly, the decrease in the pressure of the refrigerant is also suppressed. As a result, the pressure of the refrigerant before and after the expansion valve 16 increases, and accordingly the saturation temperature of the refrigerant at the outlet of the expansion valve 16 increases, and the degree of superheat decreases. In other words, when the control device 5 judges "Yes" in the judgment process of S6 in Fig. 4, it reduces the flow rate of the cooling water per unit time flowing through the cooling water circuit 3 in the process of S8, thereby intentionally lowering the cooling capacity of the condenser 14.

As a result, in the case of the cooling system 1 of this embodiment, since the degree of superheat does not increase in the portion to the left of the timing 50 shown in FIG. 3(b) and is maintained at the target value, the change in the degree of superheat is shown by the superheat characteristic 54a in FIG. 5(b) instead of the superheat characteristic 54 in FIG. 3(b). Similarly, the change in the saturation temperature of the refrigerant before and after the expansion valve 16 is shown by the saturation temperature characteristics 51a and 52a in FIG. 5(a) instead of the saturation temperature characteristics 51 and 52 in FIG. 3(a) since the saturation temperature of the refrigerant before and after the expansion valve 16 is maintained at a constant value rather than decreasing to the dew point temperature in the portion to the left of the timing 50. Note that the saturation temperature characteristic 51a corresponds to the saturation temperature of the refrigerant at the inlet of the expansion valve 16, and the saturation temperature characteristic 52a corresponds to the saturation temperature of the refrigerant at the outlet of the expansion valve 16. The change in the cooling water flow rate per unit time decreases in the portion to the left of timing 50, so the change is as shown by the cooling water flow rate characteristic 55a in FIG. 5(c) instead of the cooling water flow rate characteristic 55 in FIG. 3(c).

Therefore, by using the cooling system 1 of this embodiment, even if the outside air temperature drops, the refrigerant can be evaporated in the evaporator 11 at the desired temperature without causing condensation. As a result, if the cooling system 1 of this embodiment is used to cool server racks in a data center or semiconductor elements in an electronic circuit, for example, there is no need to waterproof or restrict the installation location of the evaporator 11 to prevent short circuits, etc.

(Another configuration example of the cooling system)
In the above embodiment, the control device 5 uses the degree of superheat as the controlled variable in the feedback control. Incidentally, the reason why the degree of superheat increases in the portion to the left of the timing 50 in the superheat degree characteristic 54 in FIG. 3B is because the saturation temperature of the refrigerant at the outlet of the evaporator 11 decreases as shown in the saturation temperature characteristic 52 in FIG. 3A. Therefore, even if the control device 5 performs feedback control with the saturation temperature of the refrigerant at the outlet of the evaporator 11 as the controlled variable in the feedback control and the target value as a target value previously set for the saturation temperature of the refrigerant at the outlet of the evaporator 11, the same effect as when the controlled variable is the degree of superheat can be obtained. Here, the previously set target value for the saturation temperature of the refrigerant at the outlet of the evaporator 11 is, for example, a constant value shown in the portion to the right of the timing 50 in the saturation temperature characteristic 52 in FIG. 3A.

In this case, the detection device 40 of the cooling system 1 does not need to include a temperature sensor 41, and only needs to include a pressure sensor 42. Therefore, in this case, the detection value related to the evaporation temperature of the refrigerant detected by the detection device 40 is the pressure value of the refrigerant in the pipe 101. In the process of S1 in FIG. 4, the control device 5 acquires the pressure value output by the pressure sensor 42, and in the process of S2, instead of calculating the degree of superheat, substitutes the pressure value into a function to calculate the saturation temperature of the refrigerant and sets it as the controlled variable. In the processes of S3, S4, S5, S7, and S8, the control device 5 performs processing with the saturation temperature of the refrigerant as the controlled variable and the target value as a target value that is predetermined for the saturation temperature of the refrigerant at the outlet of the evaporator 11.

In the above embodiment, the control device 5 uses the degree of superheat as the controlled variable in the feedback control. Generally, in the refrigerant circuit 2, the refrigerant exists in a gas-liquid two-phase state in the range from the downstream of the expansion valve 16 to the middle part of the evaporator 11. Therefore, the temperature of the refrigerant in this range can be regarded as the saturation temperature used to calculate the degree of superheat. Therefore, for example, the cooling system 1a shown in FIG. 6 may be used to calculate the degree of superheat, which is the controlled variable. In the cooling system 1a, the refrigerant circuit 2a has a configuration in which the detection device 40 in the refrigerant circuit 2 of FIG. 1 is replaced with the detection device 40a, and the control device 5 is replaced with the control device 5a. The detection device 40a includes a temperature sensor 41 and a temperature sensor 43. As described above, the temperature sensor 43 may detect the temperature of the refrigerant at any point in the range from the downstream of the expansion valve 16 to the middle part of the evaporator 11, but FIG. 6 shows an example of detecting the temperature of the refrigerant in the pipe 112. The control device 5a has the same configuration as the control device 5, except that the temperature detected by the temperature sensor 43 is used as the saturation temperature of the refrigerant instead of calculating the saturation temperature of the refrigerant from the pressure detected by the pressure sensor 42. That is, the control device 5a acquires the temperature values output by the temperature sensors 41 and 43 in the process of S1 in FIG. 4, and in the process of S2, the control device 5a subtracts the saturation temperature of the refrigerant detected by the temperature sensor 43 from the temperature detected by the temperature sensor 41 to obtain the degree of superheat, and performs the process from S3 onward using this degree of superheat as a control amount.

In the case of the cooling system 1a shown in FIG. 6, even if feedback control is performed with the saturation temperature detected by the temperature sensor 43 as the controlled variable and the target value as the target value previously set for the saturation temperature of the refrigerant in the pipe 112, the same effect as when the controlled variable is the degree of superheat can be obtained. In this case, the detection device 40a of the cooling system 1a does not need to be equipped with the temperature sensor 41, and only the temperature sensor 43 is required. Therefore, in this case, the detection value related to the evaporation temperature of the refrigerant detected by the detection device 40a is the temperature value of the refrigerant in the pipe 112. The control device 5a acquires the temperature value output by the temperature sensor 43 in the process of S1 in FIG. 4, and in the process of S2, the temperature acquired from the temperature sensor 43, i.e., the saturation temperature of the refrigerant, is the controlled variable. The control device 5a performs the processes in S3, S4, S5, S7, and S8 with the saturation temperature of the refrigerant as the controlled variable and the target value as the target value previously set for the saturation temperature of the refrigerant in the pipe 112.

(Free cooling operation)
FIG. 7(a) is a Mollier diagram (also called a p-h (pressure-enthalpy) diagram) when the cooling system 1 performs direct expansion operation, and FIG. 7(b) is a Mollier diagram when the cooling system 1 performs free cooling operation. In FIG. 7(a) and FIG. 7(b), a graph showing temperature is superimposed on the Mollier diagram. In FIG. 7(a) and FIG. 7(b), the horizontal axis is the enthalpy of the refrigerant, and indicates that the enthalpy increases toward the right. The vertical axis on the left side is the pressure of the refrigerant, and indicates that the pressure of the refrigerant increases toward the upward direction. The vertical axis on the right side is the temperature, and indicates that the temperature increases toward the upward direction. The curve indicated by the reference numeral 78 is a saturated liquid line, and the curve indicated by the reference numeral 79 is a saturated gas line. The solid line indicated by the reference numeral 70 indicates the temperature inside the data center where the evaporator 11 is installed, and the dashed line indicated by the reference numeral 71 indicates the temperature of the outside air. In both Figures 7(a) and 7(b), the temperature value on the right axis indicated by the solid line segment indicated by the symbol 70, i.e., the temperature value inside the data center in which the evaporator 11 is installed, is assumed to be the same.

As can be seen from the positions of the solid line segment indicated by the symbol 70 and the dashed line segment indicated by the symbol 71 in FIG. 7(a), FIG. 7(a) shows a case where the temperature of the outside air is higher than the temperature inside the data center. In this case, direct expansion operation is performed. When direct expansion operation is performed, as shown by the arrow 81 in FIG. 7(a), the gaseous refrigerant is compressed by the compressor 13 and becomes high pressure. As shown by the arrow 82, the high-pressure refrigerant is condensed into liquid by heat exchange with the cooling water in the condenser 14. As shown by the arrow 83, the high-pressure liquid refrigerant is depressurized by the expansion valve 16 to lower the saturation temperature of the refrigerant, and as shown by the arrow 84, it evaporates into gas in the evaporator 11. Therefore, in direct expansion operation, the refrigerant changes in the cycle shown by the arrows 81 to 84.

As can be seen from the positions of the solid line segment indicated by the symbol 70 and the dashed line segment indicated by the symbol 71 in FIG. 7(b), FIG. 7(b) shows a case where the outside air temperature is lower than the temperature inside the data center. In this case, free cooling operation is performed. When free cooling operation is performed, the temperature of the cooling water flowing through the cooling water circuit 3 is cooled by the outside air, and is therefore lower than the temperature inside the data center in which the evaporator 11 is installed. Therefore, the gaseous refrigerant flowing out from the evaporator 11 condenses and becomes liquid simply by being cooled by the condenser 14, without being compressed by the compressor 13. Furthermore, it evaporates at the desired temperature in the evaporator 11, even without being depressurized by the expansion valve 16. Therefore, in the case of free cooling operation, if the pressure loss caused by the pipes 101, 121, 105-107, 122, 123, 109, 124, and 112 on the path through which the refrigerant flows is compensated for and the pump 17 applies enough pressure to evaporate the refrigerant at the desired temperature in the evaporator 11, it becomes possible to suppress the rise in temperature inside the data center with less power consumption than in the case of direct expansion operation.

In the case of free cooling operation, the refrigerant is pressurized by pump 17 as shown by arrow 85, and then evaporates into gas in evaporator 11 as shown by arrow 86. The gaseous refrigerant is condensed into liquid through heat exchange with cooling water in condenser 14 as shown by arrow 87. Therefore, in free cooling operation, the refrigerant changes in the cycle shown by arrows 85 to 87.

In the cooling system 1, the direct expansion operation and the free cooling operation are switched by the control device 5. The cooling system 1 is provided with a cooling water temperature sensor (not shown) for detecting the temperature of the cooling water, for example, near the connection between the condenser 14 and the pipe 132, and the control device 5 acquires the temperature of the cooling water detected by the cooling water temperature sensor, for example, at regular intervals. The control device 5 determines whether to switch from the direct expansion operation to the free cooling operation based on the temperature of the cooling water detected by the cooling water temperature sensor and the cooling capacity required for the cooling system 1. When the control device 5 determines to switch to the free cooling operation, it outputs a switching command value to the three-way valves 31 to 36 to switch the path of the refrigerant circuit 2 to the path when the above-mentioned free cooling operation is performed. Then, the control device 5 performs feedback control of the free cooling operation instead of the feedback control of the direct expansion operation.

In the case of free cooling operation, the feedback control performed by the control device 5 may be, as in the case of direct expansion operation, a control amount in which the degree of superheat calculated based on the temperature acquired from the temperature sensor 41 and the pressure acquired from the pressure sensor 42 is used as a control amount, and the control amount may be a control in which the degree of superheat is maintained at a predetermined target value, or a control in which the saturation temperature of the refrigerant calculated based on the pressure acquired from the pressure sensor 42 is maintained at a predetermined target value for the saturation temperature of the refrigerant. In the case of free cooling operation, the control device 5 may or may not perform a process equivalent to the process shown in S8 of FIG. 4, that is, a feedback control that adjusts the rotation speed of the pump 21 so that the control amount becomes the target value. When feedback control for adjusting the rotation speed of the pump 21 is not performed, for example, the control device 5 may output a command value to the pump 21 of the cooling water circuit 3 to maintain the rotation speed of the pump 21 during direct expansion operation before switching to free cooling operation, or when processing S8 in FIG. 4 is performed during direct expansion operation, output a command value to return the rotation speed of the pump 21 to the rotation speed of the pump 21 before processing S8 was performed, or output a command value to set the rotation speed of the pump 21 to a predetermined rotation speed.

As described above, the switch from direct expansion operation to free cooling operation is determined based on the cooling water temperature and the cooling capacity required for the cooling system 1. FIG. 8 is an example of a graph showing the relationship between the cooling water temperature, cooling capacity, direct expansion operation, and free cooling operation. In FIG. 8, the horizontal axis is the cooling capacity, and indicates that the cooling capacity increases toward the right. The vertical axis is the cooling water temperature, and indicates that the cooling water temperature increases toward the top. The value of the cooling capacity indicated by the reference symbol 210 is a value indicating the minimum cooling capacity required for the cooling system 1, and the value of the cooling capacity indicated by the reference symbol 211 is a value indicating the maximum cooling capacity required for the cooling system 1.

In this case, the line indicated by the symbol 200 is the boundary between the region where direct expansion operation is necessary and the region where free cooling operation is preferable (hereinafter referred to as boundary 200). The region above boundary 200 is the region where direct expansion operation is necessary, and the region below boundary 200 is the region where free cooling operation is preferable. If direct expansion operation is possible in the region where free cooling operation is preferable, direct expansion operation may be performed. The feedback control of the pump 21 of the cooling water circuit 3 as the control target shown in the process of S8 in FIG. 4 described above can also be said to be a process that extends the region of direct expansion operation beyond boundary 200 and increases the region where direct expansion operation is possible in the region where free cooling operation is preferable.

Since the relationship between the region where direct expansion operation is necessary and the region where free cooling operation is preferable is as shown in FIG. 8, for example, the cooling capacity required of the cooling system 1 is constant, but the cooling water temperature may not remain within a constant range but may change across the boundary line 200. In this case, if the control device 5 determines whether to switch between direct expansion operation and free cooling operation based on the boundary line 200, an unstable state will result in frequent switching between direct expansion operation and free cooling operation.

Therefore, for example, the control device 5 detects the temperature of the cooling water corresponding to the cooling capacity required for the cooling system 1 at that time based on the boundary line 200, and performs direct expansion operation until the condition that the maximum value of the cooling water temperature within a predetermined time is equal to or lower than the detected temperature is satisfied. As described above, in the cooling system 1 of this embodiment, even if the opening degree of the expansion valve 16 is at the maximum opening degree, direct expansion operation can be performed as long as the rotation speed of the pump 21 can be reduced, so that direct expansion operation can be performed to the extent that the above condition is satisfied. Then, when the condition is satisfied, the control device 5 switches from direct expansion operation to free cooling operation. In this way, it is possible to reduce an unstable state in which direct expansion operation and free cooling operation are frequently switched between.

In addition, when the control device 5 switches from direct expansion operation to free cooling operation based on the boundary line 200, the pump 17 may not operate due to cavitation, pulsation, and backflow due to head differences in the refrigerant circuit. Even in such cases, the cooling system 1 of this embodiment is capable of performing direct expansion operation beyond the boundary line 200, so if the control device 5 detects that the pump 17 is not operating after switching to free cooling operation, it can return to direct expansion operation and continue operation. Then, while operating in direct expansion operation, the operator of the cooling system 1 can investigate the cause of the pump 17 not operating.

The cooling system 1a shown in FIG. 6 may be operated in the free cooling mode as described above. In this case, the feedback control performed by the control device 5a may be, as in the case of the direct expansion operation, a control amount calculated by the control device 5a based on the temperature acquired from the temperature sensor 41 and the saturation temperature acquired from the temperature sensor 43, and the control amount may be maintained at a predetermined target value for the superheat, or the saturation temperature of the refrigerant acquired from the temperature sensor 43 may be maintained at a predetermined target value for the saturation temperature of the refrigerant.

(Other configuration examples)
Although the embodiments of the present disclosure have been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and design changes and the like that do not depart from the gist of the present disclosure are also included.

For example, in the above embodiment, water flows through the coolant circuit 3, but it may be a cooling fluid other than water, i.e., a cooling liquid such as antifreeze or a refrigerant.

In the above embodiment, the temperature sensor 41 and the pressure sensor 42 are installed so as to detect the state of the refrigerant inside the pipe 101, but the positions at which the temperature sensor 41 and the pressure sensor 42 are installed are not limited to the pipe 101, and they may be installed at any position between the outlet of the evaporator 11 and the inlet of the compressor 13. For example, the temperature sensor 41 may be installed in the pipe 103, and the pressure sensor 42 may be installed in the pipe 101, and so on, and each may be installed at a different position.

The above-mentioned degree of superheat is a subtraction value obtained by subtracting the saturation temperature of the refrigerant calculated from the pressure of the refrigerant in the pipe 101 detected by the pressure sensor 42 from the temperature of the refrigerant in the pipe 101 detected by the temperature sensor 41, and indicates the degree of superheat at the outlet of the evaporator 11. Since the compressor 13 is generally installed in the same room as the evaporator 11, the temperature of the refrigerant flowing through the pipes 101, 102, and 103 is almost the same. Also, as described above, the pressure of the refrigerant in the path from the outlet of the expansion valve 16 to the inlet of the compressor 13 is also almost the same. Therefore, even if the temperature sensor 41 and the pressure sensor 42 are installed at any position from the outlet of the evaporator 11 to the inlet of the compressor 13, almost the same degree of superheat can be obtained.

In the above embodiment, the evaporator 11 is shown as being installed between the server rack 61 and the blower 62 as shown in FIG. 2, but the evaporator 11 may be, for example, an evaporator 11 of a size large enough to be installed in a semiconductor element on an electronic circuit board and used to cool the semiconductor element.

In the above embodiment, the fan of the cooling tower 22 may be included as a control target of the feedback control of the control device 5. In this case, the control device 5 calculates the rotation speed of the fan of the cooling tower 22 that brings the temperature of the cooling water at the inlet of the condenser 14 to a predetermined target value, and outputs the calculated rotation speed as a command value to the fan of the cooling tower 22. In the above embodiment, the cooling equipment that cools the cooling water is not limited to the cooling tower 22, and a cooling equipment that cools the cooling water other than the cooling tower 22 may be provided, or a storage tank or the like that stores the cooling water may be inserted between the pipes 131 and 132, and the cooling water may come into contact with the outside air in the storage tank or the like, so that the cooling water is naturally cooled by the outside air.

In the above embodiment, the refrigerant circuit 2 is configured to be able to switch between direct expansion operation and free cooling operation, but it may be configured to perform only direct expansion operation. That is, the refrigerant circuit 2 may not include the three-way valves 31-36, the pipes 121-124, and the pump 17, and each of the three-way valves 31-36 may directly connect the two pipes connected to each of them, for example, in the case of the three-way valve 31, the pipes 101 and 102 may be directly connected. The refrigerant circuit 2 may also be configured to not include both or either of the accumulator 12 and the receiver 15.

In the above embodiment, in the process of S6 in FIG. 4, it is determined whether the adjustment of the opening degree of the expansion valve 16 has reached its limit based on a predetermined threshold value for the opening degree of the expansion valve 16. In contrast, for example, two pressure sensors are provided to detect the refrigerant pressure at the inlet and outlet of the expansion valve 16, and the control device 5 may determine that the adjustment of the opening degree of the expansion valve 16 has reached its limit when it determines that the pressure values detected by the two pressure sensors match, or when it determines that the difference between the pressure values detected by the two pressure sensors is less than or equal to a predetermined value.

In the above embodiment, the threshold value in the process of S6 in FIG. 4 is, for example, the maximum opening of the expansion valve 16. In contrast, the threshold value may be set to an opening slightly smaller than the maximum opening of the expansion valve 16. For example, it is assumed that the control device 5 judges "No" in the process of S6 and performs the process of S7, and as a result, even if the opening of the expansion valve 16 is set to the maximum opening by the process of S7, the control amount will not be the target value. In order to avoid such a state, it is necessary to provide a margin for the adjustment amount of the opening of the expansion valve 16 in the process of S7, and therefore the threshold value may be set to an opening slightly smaller than the maximum opening of the expansion valve 16. In this case, even if the opening of the expansion valve 16 is not the maximum opening in the process of S6, the control device 5 will judge that the adjustment of the opening of the expansion valve 16 has reached its limit if it is no longer less than the threshold. In the process of S6, the control device 5 performs processing to determine, for example, whether the opening degree of the expansion valve 16 is less than a threshold value. However, depending on the threshold value set, it may be possible to determine whether the opening degree of the expansion valve 16 is equal to or less than the threshold value.

In the above embodiment, the control device 5 applies the rotation speed of the pump 21 as a command value for the pump 21, and adjusts the discharge volume of the pump 21 according to the rotation speed. In contrast, depending on the type of pump 21, a parameter other than the rotation speed may be used as a command value to adjust the discharge volume of the pump 21.

(Computer Configuration)
FIG. 9 is a schematic block diagram showing the configuration of a computer according to at least one of the above-described embodiment and other embodiments shown in the above-described other configuration examples.
The computer 90 comprises a processor 91 , a main memory 92 , a storage 93 , and an interface 94 .
The above-mentioned control device 5 is implemented in a computer 90. The operation of the above-mentioned control device 5 is stored in a storage 93 in the form of a program. The processor 91 reads the program from the storage 93, expands it in the main memory 92, and executes each process such as the process shown in FIG. 4 according to the program. The processor 91 also secures, for example, a storage area in the main memory 92 corresponding to the storage area inside the above-mentioned control device 5 according to the program. The above-mentioned control lines are connected to the interface 94, and, for example, the interface 94 receives detection values output by the detection device 40 and the like, and transmits command values to the compressor 13, the expansion valve 16, and the pumps 17 and 21 that are to be controlled.

The program may be for realizing some of the functions to be performed by the computer 90. For example, the program may be for realizing the functions by combining with other programs already stored in the storage, or by combining with other programs implemented in other devices. In other embodiments, the computer may include a custom LSI (Large Scale Integrated Circuit) such as a PLD (Programmable Logic Device) in addition to or instead of the above configuration. Examples of PLDs include PAL (Programmable Array Logic), GAL (Generic Array Logic), CPLD (Complex Programmable Logic Device), FPGA (Field Programmable Gate Array), etc. In this case, some or all of the functions realized by the processor may be realized by the integrated circuit.

Examples of the storage 93 include a hard disk drive (HDD), a solid state drive (SSD), a magnetic disk, a magneto-optical disk, a compact disc read only memory (CD-ROM), a digital versatile disc read only memory (DVD-ROM), and a semiconductor memory. The storage 93 may be an internal medium directly connected to the bus of the computer 90, or an external medium connected to the computer 90 via an interface 94 or a communication line. In addition, when the program is distributed to the computer 90 via a communication line, the computer 90 that receives the program may expand the program into the main memory 92 and execute each process such as the process shown in FIG. 4 above. In the above embodiment and at least one of the other embodiments, the storage 93 is a non-transitory tangible storage medium.

<Additional Notes>
The cooling systems 1 and 1a described in the above embodiments can be understood, for example, as follows.

(1) The cooling system 1 according to the first aspect includes a refrigerant circuit 2 having a compressor 13 for compressing the refrigerant flowing out of an evaporator 11, a condenser 14 for condensing the refrigerant compressed by the compressor through heat exchange between the refrigerant and a cooling fluid, an expansion valve 16 for reducing the pressure of the refrigerant condensed by the condenser, and the evaporator for evaporating the refrigerant reduced by the expansion valve and exchanging heat therewith, a pump 21 for circulating the cooling fluid so that it is cooled by outside air while passing through the condenser, a detection device 40, 40a for detecting a detection value related to the evaporation temperature of the refrigerant, and a control device 5, 5a for adjusting the opening of the expansion valve so that a control amount obtained from the detection value detected by the detection device becomes a target value set for the control amount, and adjusting the discharge amount of the pump so that the control amount becomes the target value when the adjustment of the opening of the expansion valve becomes a limit. According to this aspect and each of the following aspects, even if the temperature of the outside air drops, the refrigerant can be evaporated in the evaporator at a desired temperature without causing condensation.

(2) The cooling system 1, 1a according to the second aspect is the cooling system 1, 1a according to (1), in which a threshold indicating the limit of adjustment of the expansion valve is set in advance for the opening degree of the expansion valve, and the control device 5, 5a determines whether the adjustment of the opening degree of the expansion valve has reached its limit based on the opening degree of the expansion valve and the threshold.

(3) The cooling system 1, 1a according to the third aspect is the cooling system 1, 1a according to (2), in which the threshold value is the maximum opening degree of the expansion valve.

(4) The cooling system 1 according to the fourth aspect is the cooling system 1 according to any one of (1) to (3), in which the detection device 40 detects the temperature and pressure of the refrigerant present between the evaporator and the compressor as detection values related to the evaporation temperature of the refrigerant, and the control device 5 calculates a saturation temperature from the pressure of the refrigerant, calculates the degree of superheat by subtracting the calculated saturation temperature from the temperature of the refrigerant, and sets the calculated degree of superheat as a control amount.

(5) The cooling system 1 according to the fifth aspect is the cooling system 1 according to any one of (1) to (3), in which the detection device 40 detects the pressure of the refrigerant present between the evaporator and the compressor as a detection value related to the evaporation temperature of the refrigerant, and the control device 5 calculates a saturation temperature from the pressure of the refrigerant and sets the calculated saturation temperature as a control quantity.

(6) The cooling system 1a according to the sixth aspect is the cooling system 1a according to any one of (1) to (3), in which the detection device 40a detects a first temperature of the refrigerant present between the evaporator and the compressor and a second temperature of the refrigerant present between the downstream of the expansion valve and the middle part of the evaporator as detection values related to the evaporation temperature of the refrigerant, and the control device 5a calculates the degree of superheat by subtracting the second temperature from the first temperature, and sets the calculated degree of superheat as a control amount.

(7) The cooling system 1a according to the seventh aspect is the cooling system 1a described in any one of (1) to (3), in which the detection device 40a detects the saturation temperature of the refrigerant present between the downstream of the expansion valve and the middle part of the evaporator as a detection value related to the evaporation temperature of the refrigerant, and the control device 5a uses the saturation temperature as a control variable.

Reference Signs List 1 Cooling system 2 Refrigerant circuit 3 Cooling water circuit 5 Control device 11 Evaporator 12 Accumulator 13 Compressor 14 Condenser 15 Receiver 16 Expansion valve 17, 21 Pump 22 Cooling tower 31-36 Three-way valve 40 Detection device 41 Temperature sensor 42 Pressure sensors 101-112, 121-124, 131-133 Pipes

Claims (9)

a compressor that compresses the refrigerant flowing out from the evaporator;
a condenser that condenses the refrigerant compressed by the compressor by heat exchange between the refrigerant and a cooling fluid;
an expansion valve for reducing the pressure of the refrigerant condensed by the condenser;
a refrigerant circuit including the evaporator for evaporating the refrigerant decompressed by the expansion valve and exchanging heat therewith;
a pump that circulates the cooling fluid through the condenser so that the cooling fluid is cooled by outside air;
A detection device for detecting a detection value related to an evaporation temperature of the refrigerant;
a control device that adjusts an opening degree of the expansion valve so that a control amount obtained from the detection value detected by the detection device becomes a target value set for the control amount, and adjusts a discharge amount of the pump when the adjustment of the opening degree of the expansion valve reaches a limit so that the control amount becomes the target value;
A cooling system comprising: A threshold value indicating a limit of adjustment of the expansion valve is determined in advance for the opening degree of the expansion valve,
The control device includes:
determining whether or not the adjustment of the opening degree of the expansion valve has reached its limit based on the opening degree of the expansion valve and the threshold value;
The cooling system of claim 1 . The threshold value is a maximum opening degree of the expansion valve.
The cooling system of claim 2 . The detection device includes:
Detecting a temperature and a pressure of the refrigerant present between the evaporator and the compressor as a detection value related to an evaporation temperature of the refrigerant;
The control device includes:
Calculating a saturation temperature from the pressure of the refrigerant;
Calculating a degree of superheat by subtracting the calculated saturation temperature from the temperature of the refrigerant, and setting the calculated degree of superheat as a control variable.
The cooling system of claim 1 . The detection device includes:
detecting a pressure of the refrigerant present between the evaporator and the compressor as a detection value related to an evaporation temperature of the refrigerant;
The control device includes:
A saturation temperature is calculated from the pressure of the refrigerant, and the calculated saturation temperature is set as a control variable.
The cooling system of claim 1 . The detection device includes:
detecting a first temperature of the refrigerant present between the evaporator and the compressor and a second temperature of the refrigerant present between a downstream of the expansion valve and an intermediate portion of the evaporator as detection values related to an evaporation temperature of the refrigerant;
The control device includes:
calculating a degree of superheat by subtracting the second temperature from the first temperature, and setting the calculated degree of superheat as a controlled variable;
The cooling system of claim 1 . The detection device includes:
detecting a saturation temperature of the refrigerant present between the downstream of the expansion valve and a middle portion of the evaporator as a detection value related to an evaporation temperature of the refrigerant;
The control device includes:
The saturation temperature is a control variable.
The cooling system of claim 1 . A compressor compresses the refrigerant flowing from the evaporator;
A step in which a condenser condenses the refrigerant compressed by the compressor by heat exchange between the refrigerant and a cooling fluid;
an expansion valve decompressing the refrigerant condensed by the condenser;
A step in which the evaporator evaporates the refrigerant decompressed by the expansion valve and exchanges heat;
a pump circulating the cooling fluid through the condenser so that the cooling fluid is cooled by outside air;
A detection device detects a detection value related to an evaporation temperature of the refrigerant;
a control device adjusting an opening degree of the expansion valve so that a control amount obtained from the detection value detected by the detection device becomes a target value set for the control amount, and when the adjustment of the opening degree of the expansion valve reaches a limit, adjusting a discharge amount of the pump so that the control amount becomes the target value;
A cooling method comprising: a compressor that compresses the refrigerant flowing out from the evaporator;
a condenser that condenses the refrigerant compressed by the compressor by heat exchange between the refrigerant and a cooling fluid;
an expansion valve for reducing the pressure of the refrigerant condensed by the condenser;
a refrigerant circuit including the evaporator for evaporating the refrigerant decompressed by the expansion valve and exchanging heat therewith;
a pump that circulates the cooling fluid through the condenser so that the cooling fluid is cooled by outside air;
a detection device for detecting a detection value related to the evaporation temperature of the refrigerant; and a computer functioning as a control device further provided in the cooling system,
a step of adjusting an opening degree of the expansion valve so that a control amount obtained from the detection value detected by the detection device becomes a target value set for the control amount, and, when the adjustment of the opening degree of the expansion valve reaches a limit, adjusting a discharge amount of the pump so that the control amount becomes the target value;
A program for executing.

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