Multiple Refrigeration Systems

Abstract

To provide a multiple refrigeration system that enables stable operation by simple controlling, while coupling a plurality of normal refrigerators to configure a multiple refrigeration cycle, and can inexpensively achieve energy saving by minimizing man-hour for development / design.SOLUTION: When a target outlet temperature of a cooled liquid in the whole of a plurality of refrigerators 1, 2, and 3 is set as Tout, and when a measurement value of an inlet temperature of the cooled liquid of a Nth refrigerator counted from a most downstream refrigerator among a plurality of refrigerators in operation is set as Tin(N), a control portion of the Nth refrigerator is configured to calculate a target outlet temperature T(N) of the cooled liquid of the Nth refrigerator by using a calculation formula T(N)=Tin(N)-(Tin(N)-Tout) / N.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a multiple refrigeration system having a plurality of refrigeration cycles through which a refrigerant circulates. [Background technology]

[0002] A method of using a double refrigeration cycle is one way to save energy in refrigerators. As shown in Figure 6, a double refrigeration cycle is a method of dividing a single refrigeration cycle (dotted line cycle) into two refrigeration cycles, one for the medium temperature of the cooled liquid and the other for the cooling liquid, in order to obtain a desired temperature of the cooled liquid. In this way, the outlet temperature of the cooling liquid in the low-temperature cycle drops and the condensation pressure decreases, while the outlet temperature of the cooled liquid in the high-temperature cycle rises and the evaporation pressure increases, resulting in a reduction in the required power and achieving energy savings. Each refrigeration cycle is a compression refrigeration cycle using the so-called reverse Rankine cycle or an absorption refrigeration cycle. In general, the improvement in the compression refrigeration cycle has been remarkable, so the double refrigeration cycle is often used in compression refrigerators.

[0003] The number of divided cycles may be three or more, and theoretically, the more the number of divided cycles, the greater the energy saving effect. In this specification, a divided cycle having two or more cycles is called a "multiple refrigeration cycle."

[0004] Compression-type refrigerators using multiple refrigeration cycles have been commercialized in the past, but the refrigeration cycle is divided into two or more parts, which means that the equivalent of two or more compressors is required. For example, in the case of a two-stage compression cycle, two two-stage compression refrigerators are used, or one compressor with two circuits of a two-stage compression compressor is installed, which inevitably results in high costs, and currently the number of units sold is extremely low. As a result, the man-hours required for development and design are relatively large, and the reality is that they are not widely used.

[0005] By connecting two or more normal refrigerators in series, it is possible to obtain the same effect as a multiple refrigeration cycle, but in this case, it is difficult to control the refrigerators by linking the upstream and downstream units and stabilizing the cooled liquid outlet temperature at the target temperature. For example, if you try to operate two refrigerators as one refrigerator, you can operate the upstream and downstream refrigerators with almost the same refrigeration load by making the opening of the metering valves of the two refrigerators equal, or, in the case of variable speed compression type electric refrigerators, making the rotation speed (operating frequency) of the motor equal. However, this requires major changes such as designing a dedicated refrigerator control device, and the development and design burden is extremely large when verification is included.

[0006] It is also possible to set a target temperature for each individual refrigerator and operate it, but as will be described later, in this case, the refrigeration load must be approximately constant or variable flow control must be used to prevent changes in the temperature of the cooled liquid, otherwise operation will be hindered.

[0007] FIG. 7 is a schematic diagram showing a conventional example of a double refrigeration cycle refrigerator in which two compressors are mounted on one refrigerator. The refrigerator of this example is a double refrigeration cycle having two sets of evaporators 501, 502, condensers 503, 504, compressors 505, 506, expansion valves 507, 508, and temperature sensors 511, 512 inside the refrigerator. The double refrigeration cycle is controlled by one control unit 514. The control unit 514 controls the rotation speed of the compressors 505, 506 and a metering valve (such as a suction vane) to increase or decrease the output of the refrigerator. In general, the control unit 514 monitors the cooled liquid outlet temperature T1 of the entire refrigerator, and if T1 falls below the target temperature, it reduces the output of the refrigerator, and if it exceeds the target temperature, it increases the output of the refrigerator to keep the cooled liquid outlet temperature T1 at the target temperature.

[0008] In this case, the control unit 514 generally operates the two refrigeration cycles by giving the same command value (rotation speed and metering valve opening) to them, rather than individually controlling the rotation speed and metering valve of the compressors 505 and 506. This is because, in a double refrigeration cycle, the most energy-saving operation is achieved when the loads of the two refrigeration cycles are almost equal, but since the two refrigeration cycles are operated under conditions with a temperature difference of several degrees, the effect of individually optimally controlling them is small. In addition, for individual control, it is necessary to obtain the intermediate temperature (T2') of the cooled liquid. However, depending on the structure of the refrigerator, it is often difficult to obtain the intermediate temperature itself, and the cost for doing so is not worth it, so there is little need to perform such control. As a result, as described above, it is most appropriate to operate the two refrigeration cycles by giving the same command value when considering the total cost.

[0009] On the other hand, as mentioned above, the number of refrigerators sold for dual refrigeration cycles is low, making it difficult to recover the costs of development. In particular, the development of control systems requires a lot of verification costs (especially the energy costs during testing), making it difficult to recover these costs, and even if the effects of the dual refrigeration cycle are understood, it can be difficult to develop them.

[0010] FIG. 8 is a schematic diagram showing a conventional example in which two refrigerators are connected to form a double refrigeration cycle. In this example, two refrigerators, one upstream and one downstream, are mechanically connected. A control unit 514 of the downstream refrigerator monitors the cooled liquid outlet temperature T1, and controls the rotation speed and amount-adjusting valve opening of compressor 505 so that the cooled liquid outlet temperature T1 is maintained at a target temperature. A control unit 515 of the upstream refrigerator can obtain an effect equivalent to that of the previous example by giving the same command value (rotation speed and amount-adjusting valve opening) to compressor 506 of the upstream refrigerator as the command value (rotation speed and amount-adjusting valve opening) given to compressor 505 by control unit 514.

[0011] However, in practice, such control is not easy. One of the reasons is that if the command value of the downstream control unit 514 is directly sent to the upstream compressor 506, problems such as so-called surging may occur due to slight differences in temperature conditions, etc. Also, the digital communication used to transmit the command value is prone to time lags, which may cause problems in controlling the compressor. Furthermore, compressor control involves complex relationships with conditions such as pressure conditions, which increases development costs. For these reasons, it is difficult to actually apply such control.

[0012] To avoid such problems, there is a method of individually setting the target value of the outlet temperature of the upstream chiller and the target value of the outlet temperature of the downstream chiller, and controlling each chiller based on these target values. With this method, each chiller can be controlled exactly the same as in the conventional single refrigeration cycle, minimizing the risk to the chillers and the development costs. However, with this method, as described below, the refrigeration load is concentrated on the downstream chiller, which causes uneven deterioration of the equipment and reduces the effectiveness of the dual refrigeration cycle, which should share the refrigeration load equally between the upstream and downstream chillers in the first place.

[0013] Fig. 9 is a schematic diagram showing a conventional example in which three refrigerators are connected to form a triple refrigeration cycle. The configuration of each of the three refrigerators 611, 612, and 613 is the same as that of the refrigerator described with reference to Fig. 8, so the details are omitted. The cooled liquid is transferred in the order of refrigerator 613, refrigerator 612, and refrigerator 611, and is cooled to the desired temperature T1 before being sent to the customer's facility. However, the inlet temperature T2 is the temperature of the cooled liquid returned from the customer's facility, and generally varies greatly depending on the customer's load.

[0014] The chillers 611, 612, 613 are operated with a predetermined temperature as a target value. For example, if the target temperature T1 is 7°C and the inlet temperature T2 of the cooled liquid is 19°C at rated load, then the intermediate temperature T2' between the chillers 613, 612 at rated operation is 15°C, and the intermediate temperature T2" ​​between the chillers 612, 611 at rated operation is 11°C. Therefore, the temperatures T2', T2", and T1 are set to the target temperatures of the chillers 613, 612, 611, respectively. If the refrigeration loads of the chillers 611, 612, 613 are constant, or if the chillers 611, 612, 613 are operated so that T2 is constant using so-called cooled liquid flow conversion control, then such operation control may be acceptable. However, in reality, the cooled liquid inlet temperature T2 often fluctuates greatly.

[0015] For example, assume that the cooled liquid inlet temperature T2 drops to 12°C. At this time, since the cooled liquid inlet temperature T2 is lower than the target temperature T2' = 15°C, the refrigerator 613 stops (light load stop). The intermediate temperature T2' remains at 12°C. Since the refrigerator 612 has an inlet temperature of 12°C, it operates to cool down to the target temperature T2'' = 11°C, but it operates at 25% of the normal load. On the other hand, since the inlet temperature of the refrigerator 611 is 11°C, which is the temperature during rated operation, it operates at almost 100% load. In this way, if the target temperatures of the individual refrigerators are set, the load is concentrated on the refrigerators in the later stages, causing large differences in operation time, etc., and making it difficult to operate efficiently. Furthermore, the energy saving effect naturally decreases as the number of operating units decreases. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] JP 2007-183077 A Summary of the Invention [Problem to be solved by the invention]

[0017] When attempting to configure a multiple refrigeration cycle by connecting a plurality of refrigerators in this manner, the method of temperature control becomes a major issue.

[0018] Therefore, the present invention provides a multiple refrigeration system that can configure a multiple refrigeration cycle by connecting multiple normal refrigerators, while enabling stable operation with simple control and achieving energy savings at low cost by minimizing the man-hours required for development and design. [Means for solving the problem]

[0019] In one aspect, a multiple refrigeration system having a plurality of refrigerators, each of the plurality of refrigerators includes an evaporator that evaporates a liquid phase refrigerant to generate a refrigerant gas, a compressor that compresses the refrigerant gas, a condenser that condenses the compressed refrigerant gas to generate a liquid phase refrigerant, an inlet temperature sensor that measures an inlet temperature of a cooled liquid flowing into the evaporator, an outlet temperature sensor that measures an outlet temperature of the cooled liquid flowing out from the evaporator, and a control unit that controls a circulation flow rate of the refrigerant, the evaporators of the plurality of refrigerators being connected in series, and assuming that a target outlet temperature of the cooled liquid in the entirety of the plurality of refrigerators is Tout and a measured value of the inlet temperature of the cooled liquid in the Nth refrigerator counting from the most downstream refrigerator among the plurality of refrigerators in operation is Tin(N), the control unit of the Nth refrigerator Calculation formula T(N)=Tin(N)-(Tin(N)-Tout) / N The multiple refrigeration system is configured to calculate a target outlet temperature T(N) of the cooled liquid of the Nth refrigerator using the above formula:

[0020] According to the present invention, the load of each refrigerator is equalized, maximizing the effect of the multiple refrigeration cycle. Also, the operating time of each refrigerator can be equalized. Furthermore, according to the present invention, since there is no transmission or reception of control signals between the refrigerators while the multiple refrigerators are operating, there is no delay in the control signals, and stable operation can be achieved with simple control. As a result, the man-hours required for development and design can be minimized, and a multiple refrigeration system that can be operated inexpensively and energy-savingly can be realized.

[0021] In one embodiment, the control unit is configured to calculate the target outlet temperature T(N) using the calculation formula at a period longer than the period of an output control calculation for eliminating the difference between the current outlet temperature of the cooled liquid and the target outlet temperature T(N). According to the present invention, the control of the refrigeration capacity based on the difference between the current outlet temperature of the cooled liquid and the target outlet temperature T(N) is stable, so that the operation of each refrigerator is stable.

[0022] In one aspect, the control unit is configured to gradually increase the target outlet temperature of the Nth refrigerator from a temperature lower than the calculated target outlet temperature T(N) to the calculated target outlet temperature T(N). According to the present invention, the control of the refrigeration capacity based on the difference between the current outlet temperature of the cooled liquid and the target outlet temperature T(N) is stable, so that the operation of each refrigerator is stable.

[0023] In one embodiment, one of the multiple control units functions as a parent unit and the other control units function as child units, and the parent unit is configured to stop operation of at least one of the multiple refrigerators when the refrigeration load of each of the multiple refrigerators falls below a predetermined minimum refrigeration load. When the refrigeration load of each chiller falls below the minimum refrigeration load, the operation efficiency of the chiller falls. According to the present invention, when the refrigeration load falls below a predetermined minimum refrigeration load, the operation of at least one chiller is stopped, so that the refrigeration load of the operating chiller rises and exceeds the minimum refrigeration load. As a result, the operation efficiency of each chiller is improved.

[0024] In one aspect, the parent unit is configured to transmit a signal to the child unit of the Nth refrigerator indicating that the child unit is the Nth refrigerator counted from the most downstream refrigerator among the refrigerators in operation. Once the slave unit receives a signal indicating that its own refrigerator is the Nth refrigerator, it can calculate the target outlet temperature T(N) of the cooled liquid using the above formula completely independent of the master unit without transmitting or receiving any signal to or from the master unit. Effect of the Invention

[0025] According to the present invention, the load of each refrigerator is equalized, maximizing the effect of the multiple refrigeration cycle. Also, the operating time of each refrigerator can be equalized. Furthermore, according to the present invention, since there is no transmission or reception of control signals between the refrigerators while the multiple refrigerators are operating, there is no delay in the control signals, and stable operation can be achieved with simple control. As a result, the man-hours required for development and design can be minimized, and a multiple refrigeration system that can be operated inexpensively and energy-savingly can be realized. [Brief description of the drawings]

[0026] [Figure 1] FIG. 1 is a schematic diagram showing an embodiment of a multiple refrigeration system in which three refrigerators are connected in series. [Diagram 2] FIG. 13 is a schematic diagram showing another embodiment of a multiple refrigeration system. [Diagram 3] FIG. 13 is a schematic diagram showing yet another embodiment of a multiple refrigeration system. [Figure 4] FIG. 13 is a schematic diagram showing yet another embodiment of a multiple refrigeration system. [Diagram 5] FIG. 2 is a schematic diagram illustrating an embodiment of a control unit. [Figure 6] FIG. 1 is a schematic diagram showing a conventional example of a double refrigeration cycle. [Figure 7] FIG. 1 is a schematic diagram showing a conventional example of a double refrigeration cycle refrigerator in which two compressors are mounted in one refrigerator. [Figure 8] FIG. 1 is a schematic diagram showing a conventional example in which two refrigerators are connected to form a double refrigeration cycle. [Figure 9] FIG. 1 is a schematic diagram showing a conventional example in which three refrigerators are connected to form a triple refrigeration cycle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic diagram showing one embodiment of a multiple refrigeration system in which three refrigerators are connected in series. As shown in Fig. 1, the multiple refrigeration system includes three refrigerators 1, 2, and 3. The first refrigerator 1 includes an evaporator 11 that evaporates a liquid-phase refrigerant to generate a refrigerant gas, a compressor 12 that compresses the refrigerant gas, a condenser 13 that condenses the compressed refrigerant gas to generate a liquid-phase refrigerant, an inlet temperature sensor 14 that measures the inlet temperature of the cooled liquid flowing into the evaporator 11, an outlet temperature sensor 15 that measures the outlet temperature of the cooled liquid flowing out from the evaporator 11, and a control unit 16 that controls the circulation flow rate of the refrigerant.

[0028] The evaporator 11, the compressor 12, and the condenser 13 are connected by refrigerant piping depicted by arrows. The refrigerant circulates through the evaporator 11, the compressor 12, and the condenser 13 through the refrigerant piping. The first refrigerator 1 further includes an expansion valve 17 as an expansion mechanism located between the evaporator 11 and the condenser 13. The expansion valve 17 is attached to the refrigerant piping extending between the evaporator 11 and the condenser 13. The refrigerant flowing from the condenser 13 to the evaporator 11 passes through the expansion valve 17, thereby decreasing the pressure and temperature of the refrigerant. The refrigerant that has passed through the expansion valve 17 flows into the evaporator 11.

[0029] Similarly, the second refrigerator 2 includes an evaporator 21, a compressor 22, a condenser 23, an expansion valve 27, a control unit 26, an inlet temperature sensor 24, and an outlet temperature sensor 25, and the third refrigerator 3 also includes an evaporator 31, a compressor 32, a condenser 33, an expansion valve 37, a control unit 36, an inlet temperature sensor 34, and an outlet temperature sensor 35. The functions of these components of the second refrigerator 2 and the third refrigerator 3 are basically the same as those of the first refrigerator 1, so duplicated explanations thereof will be omitted.

[0030] The multiple evaporators 11, 12, 13 of the multiple refrigerators 1, 2, 3 are connected in series by a cooled liquid transfer line 41. That is, the cooled liquid outlet of the evaporator 31 of the third refrigerator 3 is connected to the cooled liquid inlet of the evaporator 21 of the second refrigerator 2, and the cooled liquid outlet of the evaporator 21 of the second refrigerator 2 is connected to the cooled liquid inlet of the evaporator 11 of the first refrigerator 1. The multiple condensers 13, 23, 33 of the multiple refrigerators 1, 2, 3 are connected in series by a coolant transfer line 42. That is, the coolant outlet of the condenser 13 of the first refrigerator 1 is connected to the coolant inlet of the condenser 23 of the second refrigerator 2, and the coolant outlet of the condenser 23 of the second refrigerator 2 is connected to the coolant inlet of the condenser 33 of the third refrigerator 3.

[0031] The liquid to be cooled (e.g., cold water) first flows into the evaporator 31 of the third refrigerator 3 through the cooled liquid transfer line 41. Heat exchange between the cooled liquid and the refrigerant liquid takes place in the evaporator 31. As a result of this heat exchange, the cooled liquid is cooled, while the refrigerant liquid is heated by the cooled liquid and evaporates to become a refrigerant gas. The cooled liquid to be cooled is sent to the evaporator 21 of the second refrigerator 2 through the cooled liquid transfer line 41, while the refrigerant gas is sent to the compressor 32. The compressor 32 compresses the refrigerant gas and sends the compressed refrigerant gas to the condenser 33. In the condenser 33, the refrigerant gas is cooled and condensed by the cooling liquid transferred through the cooling liquid transfer line 42, to become a refrigerant liquid. In the second refrigerator 2 and the first refrigerator 1, the liquid to be cooled is cooled in the same manner, and finally flows out of the evaporator 11 of the first refrigerator 1. It goes without saying that the specifications of the chillers 1, 2, and 3 should be designed so as to meet the overall needs of the consumer. The number of connected chillers is not limited to three, but may be two, or may be four or more.

[0032] There is a concept of so-called large temperature difference air conditioning, which aims to save energy by reducing the flow rate of the cooled liquid as much as possible and increasing the temperature difference between the inlet and outlet of the cooled liquid in order to reduce the power required for transport. The larger the temperature difference between the inlet and outlet of the cooled liquid, the greater the effect of the multiple refrigeration cycle, so the multiple refrigeration cycle is suitable for large temperature difference air conditioning. In addition, in large temperature difference air conditioning, the flow rate of the cooled liquid is low, so there are cases where a special design is unavoidable that deviates from the standard design conditions of the refrigerator. However, by connecting multiple refrigerators 1, 2, and 3 in series as shown in Figure 1, the temperature difference between the inlet and outlet of the cooled liquid per refrigerator can be kept relatively small, and refrigerators 1, 2, and 3 can be designed under conditions close to the standard design conditions (generally 5°C difference), which has the advantage of widening the range that can be handled by the standard specifications.

[0033] For example, when designing one refrigerator under the condition of 12°C inlet / outlet temperature difference, the flow rate of the cooled liquid is about 42% of that under the condition of 5°C inlet / outlet temperature difference, and the flow rate in the tube is extremely reduced if left as is. Therefore, it is necessary to take measures such as increasing the number of passes. As a result, the structure of the water chamber and heat transfer tube of the refrigerator may become complicated, and the heat transfer efficiency may deteriorate. In contrast, when three refrigerators 1, 2, and 3 are connected in series as in this embodiment, the inlet / outlet temperature difference of the cooled liquid per unit is 4°C, and the cooled liquid flow rate is 1.25 times that of the standard design, which is not significantly different, and the refrigerator can be used with the standard design.

[0034] The inlet temperature of the coolant is determined by the cooling tower and outside air conditions, and the outlet temperature of the coolant is determined by the flow rate, so if the flow rate of the coolant does not change, the outlet temperature of the coolant will rise, and the required power will increase. As a cycle, it is ideal to maintain the outlet temperature by increasing the flow rate of the coolant, but this may increase the required power of the coolant pump or cause the differential pressure to increase excessively, so the condensers through which the coolant flows may be connected in parallel rather than in series. Figure 2 is a schematic diagram showing an embodiment in which condensers 13, 23, and 33 are connected in parallel.

[0035] In FIG. 1 and FIG. 2, the control unit 36 ​​of the third refrigerator 3 functions as a "parent unit." Operation commands for the multiple refrigerators 1, 2, and 3 are given to the control unit 36 ​​of the third refrigerator 3 by manual input or remote signal. The control unit 36 ​​of the third refrigerator 3 judges whether to operate each of the three refrigerators 1, 2, and 3 based on the load state, etc., and gives operation commands CMD2 and CMD1 to the control unit 26 of the second refrigerator 2 and the control unit 16 of the first refrigerator 1. There are various methods for determining which refrigerators to operate and in what combination. For example, when refrigerators of the same type are connected in series, the simplest method is to start the refrigerator with the shortest operating time at that time when increasing the number, and to stop the refrigerator with the longest operating time at that time when decreasing the number. In this way, the operating times of the refrigerators can be made uniform, and it is possible to prevent the operating time of only a specific refrigerator from being long, which increases the frequency of maintenance, etc.

[0036] Each of the control units 16, 26, 36 includes a storage device in which a program is stored, and a calculation device that executes calculations according to instructions included in the program. Generally, control devices for embedding in other devices, such as microcomputers and sequencers, are used as the control units 16, 26, 36, but the specific configuration of the control units 16, 26, 36 is not limited to these examples.

[0037] In this embodiment, in order to level out the load on the operating chillers, the control unit of the N-th chiller, counting from the most downstream chiller among the multiple operating chillers, is configured to calculate a target outlet temperature T(N) of the cooled liquid in the N-th chiller using the following calculation formula T(N): Calculation formula T(N)=Tin(N)-(Tin(N)-Tout) / N Here, Tout is the target outlet temperature of the cooled liquid in all of the multiple refrigerators 1, 2, and 3, and Tin(N) is the measured inlet temperature of the cooled liquid in the Nth refrigerator. The above calculation formula is stored in the memory device of each of the control units 16, 26, and 36.

[0038] For example, when all three chillers 1, 2, and 3 are in operation, the third chiller 3 is the third chiller from the most downstream, so N is 3. The control unit 36 ​​of the third chiller 3 calculates the target outlet temperature T(3) as follows. T(3)=Tin(3)-(Tin(3)-Tout) / 3 Tin(3) is measured by an inlet temperature sensor 34 that measures the inlet temperature of the liquid to be cooled flowing into the evaporator 31 of the third refrigerator 3. The target outlet temperature Tout is a preset temperature. In the same manner, the control unit 26 of the second refrigerator 2 calculates the target outlet temperature T(2) as follows. T(2)=Tin(2)-(Tin(2)-Tout) / 2 Tin( 2 ) is measured by an inlet temperature sensor 24 that measures the inlet temperature of the liquid to be cooled flowing into the evaporator 21 of the second refrigerator 2 .

[0039] Applying the aforementioned conditions to the above formula, Tin(3) = 12°C, Tout = 7°C, so T(3) ≒ 10.3°C, T(2) ≒ 8.6°C, and the load on each of the chillers 1, 2, and 3 is approximately 42%, so the load is leveled out. In addition, because three chillers 1, 2, and 3 are operating, the refrigeration cycle is divided into three, maximizing energy saving effects.

[0040] It is desirable to perform the calculation using the above calculation formula at a time interval sufficiently longer than the response time to temperature in the temperature control of the refrigerators 1, 2, 3, and update the target outlet temperature T(N) of each refrigerator 1, 2, 3. Specifically, each of the control units 16, 26, 36 is configured to calculate the target outlet temperature T(N) using the above calculation formula at a period longer than the period of the output control calculation for eliminating the difference between the current outlet temperature of the cooled liquid and the target outlet temperature T(N). The current outlet temperature of the cooled liquid is measured by the outlet temperature sensor 15, 25, 35. For example, since the output control calculation for eliminating the difference between the current outlet temperature of the cooled liquid and the target outlet temperature T(N) is performed at a period of about 1 second, it is desirable to perform the calculation of the target outlet temperature T(N) using the above calculation formula at a period sufficiently longer than the period of the output control calculation, for example, about 1 minute. According to such an operation, the control of the refrigeration capacity based on the difference between the current outlet temperature of the cooled liquid and the target outlet temperature T(N) is stable, and therefore the operation of each refrigerator is stable.

[0041] One of the multiple control units 16, 26, 36 functions as a parent unit, and the other control units function as child units. In this embodiment, the control unit 36 ​​of the third refrigerator 3 functions as a parent unit. The parent unit is configured to stop the operation of at least one of the multiple refrigerators 1, 2, 3 when the refrigeration load of each of the multiple refrigerators 1, 2, 3 falls below a predetermined minimum refrigeration load. Normally, when the refrigeration load of each refrigerator falls below the minimum refrigeration load, the operation efficiency of the refrigerator decreases. According to this embodiment, when the refrigeration load falls below the predetermined minimum refrigeration load, the operation of at least one refrigerator is stopped, so that the refrigeration load of the refrigerator in operation increases and exceeds the minimum refrigeration load. As a result, the operation efficiency of each refrigerator is improved.

[0042] When the number of operating units is increased or decreased due to a change in the refrigeration load, the target outlet temperature T(N) may change suddenly. Therefore, each of the control units 16, 26, 36 is configured to gradually increase the target outlet temperature of the Nth refrigerator from a temperature lower than the calculated target outlet temperature T(N) to the calculated target outlet temperature T(N). More specifically, each of the control units 16, 26, 36 increases the target outlet temperature of the Nth refrigerator at a predetermined increase rate (for example, an increase rate of 1° C. per minute) from a temperature lower than the calculated target outlet temperature T(N) to the calculated target outlet temperature T(N).

[0043] For example, when three refrigerators 1, 2, and 3 are operating, if the inlet temperature Tin(3) of the cooled liquid of the third refrigerator 3 is 17°C and the target outlet temperature Tout is 7°C, the inlet temperature of the cooled liquid of the second refrigerator 2 is 13.6°C. According to the above calculation formula, the target outlet temperature T(2) of the second refrigerator 2 is calculated to be 10.3°C. If the third refrigerator 3 is stopped due to a decrease in the refrigeration load, the inlet temperature of the cooled liquid of the second refrigerator 2 rises to 17°C. As a result, the target outlet temperature T(2) of the second refrigerator 2 is calculated to be 12°C, and the target temperature rises by 1.7°C. If this target outlet temperature T(2) is immediately applied to the control unit 26 of the second refrigerator 2 as a target value for the output control calculation, there is a concern that the control may become unstable.

[0044] Therefore, the control unit 26 of the second refrigerator 2 increases the target outlet temperature of the second refrigerator 2 at a predetermined increase rate from a temperature lower than the calculated target outlet temperature T(2) to the calculated target outlet temperature T(2). For example, the control unit 26 of the second refrigerator 2 first increases the target outlet temperature from 10.3°C to 11.3°C, and one minute later increases it from 11.3°C to 12°C. Such a stepwise increase can prevent the control operation of the control unit 26 of the second refrigerator 2 from becoming unstable. The actual increase rate and calculation cycle should be adjusted according to the characteristics of the refrigerator.

[0045] As is clear from the above calculation formula, the control unit of each chiller can calculate the target outlet temperature T(N) from the above calculation formula by only acquiring the number N-1 of chillers operating downstream of itself and the common target outlet temperature Tout. Therefore, there is no need for analog communication to transmit the inlet temperature Tin(3) of the most upstream cooled liquid, or to separately calculate the refrigeration load factor. In other words, a major advantage of this embodiment is that it can be implemented with exactly the same control logic as an auxiliary function of each chiller.

[0046] To calculate the target outlet temperature T(N), each chiller needs to obtain information on the number N-1 of chillers in operation downstream of itself. One of the multiple control units 16, 26, 36 functions as a parent unit, and the other control units function as child units. The parent unit is configured to send a signal to the child unit of the Nth chiller indicating that it is the Nth chiller counting from the most downstream chiller among the chillers in operation. Once the child unit receives the signal indicating that it is the Nth chiller, it can calculate the target outlet temperature T(N) of the cooled liquid using the above calculation formula completely independently of the parent unit without sending or receiving a signal with the parent unit.

[0047] In this embodiment, the control unit 36 ​​of the third refrigerator 3, which is the most upstream, is the "parent unit", and the parent unit shares the operation command given to the child units (the control unit 26 of the second refrigerator 2 and the control unit 16 of the first refrigerator 1), so that the child units can obtain information on the number of operating units N-1 downstream of themselves. That is, when the control unit 36 ​​(parent unit) of the third refrigerator 3 commands the child units to operate, if it wants to operate the second refrigerator 2, it activates the operation command CMD2, and if it wants to operate the first refrigerator 1, it activates the operation command CMD1. Note that activating an operation command means, for example, outputting a specified voltage such as +5V for a voltage-applied signal (or source signal), and connecting the signal line to ground potential (0V) for a voltage-free signal (or sink signal). However, the format of the operation command is not particularly limited as long as it can transmit an operation command and a stop command by an agreement (protocol).

[0048] The control unit 26 of the second refrigerator 2, which is located between the third refrigerator 3 and the first refrigerator 1, receives the status (active or inactive) of the operation command CMD2 from the control unit 36 ​​(parent unit) of the third refrigerator 3, and at the same time receives the status (active or inactive) of the operation command CMD1 from the control unit 36 ​​(parent unit) of the third refrigerator 3. Therefore, when the operation command CMD2 is active, the control unit 26 of the second refrigerator 2 can obtain information as to whether the first refrigerator 1 is operating downstream, depending on whether the operation command CMD1 is active or inactive.

[0049] The control unit 26 of the second refrigerator 2 calculates the target outlet temperature T(2) according to the above-mentioned calculation formula when the operation command CMD1 is active, and determines the target outlet temperature T(1)=Tout when the operation command CMD1 is inactive. The control unit 26 of the second refrigerator 2 may receive information on whether the first refrigerator 1 is in operation or not as a signal directly from the control unit 16 of the first refrigerator 1. The control unit 36 ​​of the third refrigerator 3, which is the parent unit, naturally has information on the number of refrigerators in operation, since it is the one that determines whether to stop operation. When the first refrigerator 1 is in operation, the target outlet temperature of the cooled liquid of the first refrigerator 1 is Tout regardless of the conditions.

[0050] By doing so, each chiller can calculate its own target outlet temperature at which it should be operated using only simple signal lines while using a control unit having a common program, and it becomes possible to connect and operate multiple chillers in series with simple effort.

[0051] According to this embodiment, the load of each refrigerator is equalized, maximizing the effect of the multiple refrigeration cycle. In addition, the operating time of each refrigerator can also be equalized. Furthermore, according to this embodiment, there is no transmission and reception of control signals between the refrigerators while the multiple refrigerators are operating, so there is no delay in the control signals, and stable operation is possible with simple control. As a result, the number of development and design man-hours can be minimized, and a multiple refrigeration system that can be operated inexpensively and energy-savingly can be realized.

[0052] FIG. 3 is a schematic diagram showing yet another embodiment of the multiple refrigeration system. In this embodiment, five refrigerators 1 to 5 are connected. The detailed configuration of each refrigerator is the same as that of the embodiment described with reference to FIG. 1, so detailed illustration thereof is omitted. In this embodiment, the control units 16 to 56 of the refrigerators 1 to 5 are connected by one communication line (CMDx). This is a so-called bus connection, and all the control units 16 to 56 receive signals sent to each control unit, and each refrigerator operates according to the given command. In this case, as in the previous embodiment, the parent unit issues an operation command to the child unit. Alternatively, an operation instruction device (not shown) that executes the number control may be separately provided, and the command may be issued from this operation instruction device. In this case, the operation instruction device issues an operation command to the refrigerators 1 to 5. At this time, it goes without saying that the number of refrigerators to be operated and which refrigerator to operate can be determined by referring to the above-mentioned operation time, etc.

[0053] Here, an example of operation control of the multiple refrigeration system shown in Fig. 3 will be described. For example, when an operation command is sent to the control unit 36 ​​of the third refrigerator 3 via the communication line CMDx, the control units 16-56 of the first refrigerator 1-fifth refrigerator 5 also receive the operation command for the third refrigerator 3. Therefore, the control units 46, 56 of the fifth refrigerator 5 and fourth refrigerator 4 can recognize that one more refrigerator is operating downstream of them. Similarly, by performing the same process for a stop command, each control unit can always recognize how many refrigerators are operating downstream of the refrigerator it is responsible for.

[0054] In this embodiment, if each control unit knows the installation order of the chillers to which operation commands were given, starting from the most downstream, and its own installation order, it can know whether the number of operating units is increased or decreased by simply judging whether the two are larger or smaller, and appropriate operation is also possible using a common program in each control unit. Therefore, the control unit of each chiller can set its own target outlet temperature in the same way as in the above embodiment.

[0055] The recognition of the number of operating units by each control unit may be performed when the control unit receives an operation command or at regular intervals to check whether other chillers are operating, or each control unit may periodically notify the control unit of the upstream chiller that it is operating. Also, when an operation command is given to a control unit of a certain chiller from an operation instruction device (not shown), the number of chillers operating downstream of that chiller may be notified. In any case, such communication means allows the control unit of each chiller to recognize the number of chillers operating downstream of itself. Note that, if there is a faulty chiller, for example, the operation instruction device does not give an operation command to that chiller. By not giving an operation command to the faulty chiller, that chiller is not included in the number of operating units, so that the entire chiller system can be operated even if some chillers are faulty.

[0056] The frequency at which the number of operating units is checked can be much shorter than the frequency at which the temperature of the cooled liquid, which changes from moment to moment, is measured. In other words, unlike the compressor control described in the prior art, there is no problem with using digital communication. Therefore, there is no need to frequently exchange temperature information (analog values) as in this embodiment, which reduces the load on the communication lines, and this is also an advantage of this embodiment.

[0057] Theoretically, the more chillers connected, the greater the energy saving effect, but in reality, connecting too many chillers in series makes the temperature difference between the inlet and outlet of each chiller extremely small, which often makes it difficult to design the evaporator. For this reason, the actual number of units connected is usually 2 to 3, and although it depends on the customer's temperature conditions (such as a large temperature difference between the inlet and outlet of the cooled liquid), in practice it is considered sufficient to connect up to about 5 units in series. For this reason, it is acceptable to set an upper limit on the number of units that can be connected in the specifications.

[0058] Figure 4 is a schematic diagram showing yet another embodiment of the multiple refrigeration system. The detailed configuration of each refrigerator is the same as that of the embodiment described with reference to Figure 1, so detailed illustration is omitted. This embodiment is a multiple refrigeration system in which a minimum of two refrigerators 1 and 2 are connected. There is no parent-child relationship between the control units 16 and 26 of the two refrigerators 1 and 2, and the two refrigerators 1 and 2 operate according to direct or remote instructions, respectively.

[0059] In this embodiment, the control unit 16 of the first refrigerator 1 sends an operation signal ACK1 indicating that the first refrigerator 1 is operating to the control unit 26 of the second refrigerator 2. As a result, the control unit 26 of the second refrigerator 2 recognizes that the first refrigerator 1 is operating, and switches the target outlet temperature of the cooled liquid from Tout to T(2) based on this operation signal. In this case, by utilizing the function of the control unit 26 to switch the target outlet temperature of the cooled liquid, such a function can be implemented simply by adding a circuit to a conventional control unit.

[0060] FIG. 5 is a schematic diagram showing one embodiment of the structure of the control unit 26 of the second refrigerator 2 shown in FIG. 4. The control unit 26 has a main control circuit 61, a signal generator 62, and a signal switch 63. The signal switch 63 is connected to the main control circuit 61 and the signal generator 62. The temperature Tin(2) of the cooled liquid flowing into the second refrigerator 2 is measured by the inlet temperature sensor 24 (see FIG. 1). The measured value of the cooled liquid temperature Tin(2) is input to the main control circuit 61 and is further output from the main control circuit 61. The measured value of the temperature Tin(2) is output from the main control circuit 61 in the form of an analog signal consisting of an electric signal. For example, the analog signal is an electric signal of 4 to 20 mA corresponding to 0 to 50°C. That is, a temperature value in the range of 0 to 50°C corresponds one-to-one to an electric signal in the range of 4 to 20 mA.

[0061] The measured value (electrical signal) of temperature Tin(2) output from the main control circuit 61 is input again to the main control circuit 61 via the signal switcher 63. The main control circuit 61 has a converter (not shown) that converts the inputted electrical signal into a temperature value, and this converter is pre-adjusted to convert an electrical signal of 4 to 20 mA into a temperature value of 3.5 to 28.5°C. An electrical signal in the range of 4 to 20 mA corresponds one-to-one to a temperature value in the range of 3.5 to 28.5°C. This temperature value range of 3.5 to 28.5°C is a range obtained by converting in advance the range of 0 to 50°C using the following calculation formula: T(2)=Tin(2)-(Tin(2)-Tout) / 2 Here, Tout is 7°C.

[0062] A relay can be used for the signal switch 63. When the operation signal ACK1 is active, the measurement value (electrical signal) of the temperature Tin(2) output from the main control circuit 61 is input to the main control circuit 61 via the signal switch 63. When the operation signal ACK1 is inactive, the signal generated by the signal generator 62 is input to the main control circuit 61 via the signal switch 63. The signal of the signal generator 62 is an analog signal (electrical signal) corresponding to Tout. Therefore, when the first refrigerator 1 is not operating, the second refrigerator 2 operates with Tout as the target outlet temperature. According to this embodiment, the second refrigerator 2 is operated at the target outlet temperature T(2) when the first refrigerator 1 is operating, and is operated at the target outlet temperature Tout when the first refrigerator 1 is stopped.

[0063] These signal functions are often provided as standard functions in normal refrigerators, and in this case, it is sufficient to add only the signal switch 63 and the signal generator 62 to the existing refrigerator, so the functions of this embodiment can be implemented in the refrigerator very simply. Furthermore, if the control unit has a function (fail-safe function) that automatically sets the target outlet temperature of the cooled liquid to Tout when Tin(2) is not input to the main control circuit 61, the signal generator 62 is not necessary.

[0064] By doing as described above, it is possible to configure a multiple refrigeration system that aims to save energy by connecting multiple refrigeration units in series and appropriately allocating load using a standardized (unified) program with minimal communication (if the number of units is small, only simple contact signals), even if the number of operating units changes.

[0065] The above-described embodiments have been described for the purpose of enabling a person having ordinary skill in the art to practice the present invention. Various modifications of the above-described embodiments are naturally possible for a person skilled in the art, and the technical idea of ​​the present invention can be applied to other embodiments. Therefore, the present invention is not limited to the described embodiments, but is to be interpreted in the broadest scope according to the technical idea defined by the claims. [Explanation of symbols]

[0066] 1,2,3 Refrigeration unit 11,21,31 Evaporator 12,22,32 Compressor 13,23,33 Condenser 14,24,34 Inlet temperature sensor 15,25,35 Outlet temperature sensor 16,26,36 Control section 17, 27, 37 Expansion valve 41 Cooled liquid transfer line 42 Coolant transfer line 61 Main control circuit 62 Signal Generator 63 Signal Switch

Claims

1. A multiple refrigeration system having a plurality of refrigerators, Each of the plurality of refrigerators includes an evaporator that evaporates a liquid phase refrigerant to generate a refrigerant gas, a compressor that compresses the refrigerant gas, a condenser that condenses the compressed refrigerant gas to generate a liquid phase refrigerant, an inlet temperature sensor that measures an inlet temperature of a cooled liquid flowing into the evaporator, an outlet temperature sensor that measures an outlet temperature of the cooled liquid flowing out from the evaporator, and a control unit that controls a circulation flow rate of the refrigerant, The evaporators of the plurality of refrigerators are connected in series, Let Tout be a target outlet temperature of the cooled liquid in all of the plurality of refrigerators, and Tin(N) be a measured inlet temperature of the cooled liquid in the N-th refrigerator counting from the most downstream refrigerator among the plurality of refrigerators in operation. The control unit of the N-th refrigerator Calculation formula T(N)=Tin(N)-(Tin(N)-Tout) / N The target outlet temperature T(N) of the cooled liquid of the Nth refrigerator is calculated using the above formula: A multiple refrigeration system, wherein the control unit is configured to calculate the target outlet temperature T(N) using the calculation formula at a period longer than the period of an output control calculation for eliminating the difference between the current outlet temperature of the cooled liquid and the target outlet temperature T(N).

2. 2. The multiple refrigeration system of claim 1, wherein the control unit is configured to gradually increase the target outlet temperature of the Nth refrigerator from a temperature lower than the calculated target outlet temperature T(N) to the calculated target outlet temperature T(N).

3. one of the plurality of control units functions as a parent unit and the other control units function as child units, and the parent unit is configured to stop operation of at least one of the plurality of refrigerators when a refrigeration load of each of the plurality of refrigerators falls below a predetermined minimum refrigeration load; 3. The multiple refrigeration system according to claim 1 or 2, wherein the parent unit is configured to transmit a signal to the child unit of the Nth refrigerator indicating that the child unit is the Nth refrigerator counting from the most downstream refrigerator among the refrigerators in operation.

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