JP7641874B2 - Multiple Refrigeration System - Google Patents

Description

This relates to a multiple refrigeration system that has multiple refrigeration cycles through which a refrigerant circulates.

A method for saving energy in refrigerators is to use a double refrigeration cycle. 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 at the medium temperatures of the cooled liquid and the cooling liquid, respectively, in order to obtain a desired temperature of the cooled liquid. In this way, the coolant outlet temperature drops in the low-temperature cycle, reducing the condensation pressure, while the cooled liquid outlet temperature rises in the high-temperature cycle, increasing the evaporation pressure. As a result, the required power is reduced, and energy savings can be achieved. Each refrigeration cycle is a compression refrigeration cycle that uses the so-called reverse Rankine cycle, or an absorption refrigeration cycle. In general, due to significant improvements in the compression refrigeration cycle, the double refrigeration cycle is often used in compression refrigerators.

The number of divided cycles may be three or more, and theoretically the more divided cycles there are, the greater the energy saving effect. In this specification, two or more divided cycles are referred to as "multiple refrigeration cycles."

Compression-type freezers using multiple refrigeration cycles have been commercialized in the past, but they split the refrigeration cycle into two or more, meaning that the equivalent of two or more compressors is required. For example, a two-stage compression cycle requires the use of two two-stage compression freezers, or one compressor with two circuits for two-stage compression compressors, 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 have not yet become widespread.

It is possible to achieve the same effect as a multiple refrigeration cycle by connecting two or more normal refrigerators in series, 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 roughly the same refrigeration load by equalizing the opening of the metering valves of the two refrigerators, or, in the case of variable-speed compression-type electric refrigerators, by equalizing the rotation speed (operating frequency) of the motor. However, this requires major changes such as a dedicated design for the refrigerator control device, and the development and design burden is extremely large when verification is included.

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

Figure 7 is a schematic diagram showing a conventional example of a double refrigeration cycle refrigerator in which two compressors are installed in one refrigerator. The refrigerator in this example is a double refrigeration cycle with 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 the metering valve (such as a suction vane) to increase or decrease the output of the refrigerator. In general, the control unit 514 monitors the outlet temperature T1 of the cooled liquid 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 outlet temperature T1 of the cooled liquid at the target temperature.

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 approximately 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 of doing so is not worth it, so there is little need to perform such control. As a result, as mentioned above, it is most appropriate to operate the two refrigeration cycles by giving the same command value when considering the total cost.

On the other hand, as mentioned above, the number of refrigerators specifically designed for the double refrigeration cycle sold 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 making it difficult to develop them even if the effects of the double refrigeration cycle are understood.

Figure 8 is a schematic diagram showing a conventional example in which two refrigerators are connected to form a double refrigeration cycle. In this example, the two refrigerators, upstream and downstream, are mechanically connected. The control unit 514 of the downstream refrigerator monitors the cooled liquid outlet temperature T1, and controls the rotation speed and metering valve opening of the compressor 505 so that the cooled liquid outlet temperature T1 is maintained at the target temperature. The control unit 515 of the upstream refrigerator gives the same command value (rotation speed and metering valve opening) to the compressor 506 of the upstream refrigerator as the command value (rotation speed and metering valve opening) given to the compressor 505 by the control unit 514, thereby achieving the same effect as the previous example.

However, in practice, this type of control is not easy. One reason is that if the command value from the downstream control unit 514 is sent directly to the upstream compressor 506, slight differences in temperature conditions, etc. can cause problems such as surging. In addition, the digital communication used to transmit the command values is prone to time lags, which can cause problems when controlling the compressor. Furthermore, compressor control involves complex relationships with pressure conditions and other conditions, which in turn incurs development costs. For these reasons, it is difficult to actually apply this type of control.

One way to avoid these problems is to set separate target values for the outlet temperature of the upstream chiller and the downstream chiller, and control each chiller based on these target values. With this method, each chiller can be controlled in exactly the same way as in conventional single-refrigeration cycles, minimizing chiller risk and development costs. However, with this method, as described below, the refrigeration load is concentrated on the downstream chiller, which leads to uneven deterioration of the equipment and reduces the effectiveness of the dual refrigeration cycle, which should share the refrigeration load evenly between the upstream and downstream chillers in the first place.

Figure 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 Figure 8, so 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 returning from the customer's facility, and generally varies greatly depending on the customer's load.

The refrigerators 611, 612, and 613 are operated with a predetermined temperature as the 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 the rated load, the intermediate temperature T2' between the refrigerators 613 and 612 at the rated operation is 15°C, and the intermediate temperature T2" between the refrigerators 612 and 611 at the rated operation is 11°C. Therefore, the temperatures T2', T2", and T1 are set to the target temperatures of the refrigerators 613, 612, and 611, respectively. If the refrigeration load of the refrigerators 611, 612, and 613 is constant, or if the refrigerators 611, 612, and 613 are operated so that T2 is constant using so-called cooled liquid flow control, such operation control may be used. However, in reality, the cooled liquid inlet temperature T2 often fluctuates greatly.

For example, suppose 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 inlet temperature of refrigerator 612 is 12°C, it operates to cool 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 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 individual refrigerators are set, the load is concentrated on the refrigerators in the later stages, resulting in large differences in operating time, etc., and making efficient operation impossible. In addition, the energy saving effect is naturally reduced by reducing the number of operating units.

JP 2007-183077 A

When trying to create a multiple refrigeration cycle by connecting multiple refrigerators in this way, the method of temperature control becomes a major issue.

The present invention provides a multiple refrigeration system that can achieve energy savings at low cost by linking multiple normal refrigerators to form a multiple refrigeration cycle, while enabling stable operation with simple control and minimizing the man-hours required for development and design.

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 a target outlet temperature of the cooled liquid in the entire 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:

According to the present invention, the load on each refrigerator is leveled, maximizing the effect of the multiple refrigeration cycle. The operating time of each refrigerator can also be leveled. Furthermore, according to the present invention, there is no transmission or reception of control signals between the refrigerators while the multiple refrigerators are in operation, so there is no delay in the control signals, and stable operation is possible with simple control. As a result, the amount of development and design work can be minimized, and a multiple refrigeration system that can be operated inexpensively and energy-savingly can be realized.

In one aspect, 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.

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.

In one aspect, 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.

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 counting 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 calculation formula completely independent of the master unit without transmitting or receiving any signal to or from the master unit.

According to the present invention, the load on each refrigerator is leveled, maximizing the effect of the multiple refrigeration cycle. The operating time of each refrigerator can also be leveled. Furthermore, according to the present invention, there is no transmission or reception of control signals between the refrigerators while the multiple refrigerators are in operation, so there is no delay in the control signals, and stable operation is possible with simple control. As a result, the amount of development and design work can be minimized, and a multiple refrigeration system that can be operated inexpensively and energy-savingly can be realized.

FIG. 1 is a schematic diagram showing an embodiment of a multiple refrigeration system in which three refrigerators are connected in series. FIG. 13 is a schematic diagram showing another embodiment of a multiple refrigeration system. FIG. 13 is a schematic diagram showing yet another embodiment of a multiple refrigeration system. FIG. 13 is a schematic diagram showing yet another embodiment of a multiple refrigeration system. FIG. 2 is a schematic diagram illustrating an embodiment of a control unit. FIG. 1 is a schematic diagram showing a conventional example of a double refrigeration cycle. 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. FIG. 1 is a schematic diagram showing a conventional example in which two refrigerators are connected to form a double refrigeration cycle. FIG. 1 is a schematic diagram showing a conventional example in which three refrigerators are connected to form a triple refrigeration cycle.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
Fig. 1 is a schematic diagram showing an 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 an inlet temperature of the liquid to be cooled flowing into the evaporator 11, an outlet temperature sensor 15 that measures an outlet temperature of the liquid to be cooled flowing out from the evaporator 11, and a control unit 16 that controls the circulation flow rate of the refrigerant.

The evaporator 11, compressor 12, and condenser 13 are connected by refrigerant piping depicted by arrows. The refrigerant circulates through the evaporator 11, compressor 12, and 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.

The second refrigerator 2 similarly 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 duplicate explanations will be omitted.

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 cooling liquid transfer line 42. That is, the cooling liquid outlet of the condenser 13 of the first refrigerator 1 is connected to the cooling liquid inlet of the condenser 23 of the second refrigerator 2, and the cooling liquid outlet of the condenser 23 of the second refrigerator 2 is connected to the cooling liquid inlet of the condenser 33 of the third refrigerator 3.

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 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, becoming refrigerant liquid. In the second refrigerator 2 and the first refrigerator 1, the cooled liquid 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 refrigerators 1, 2, and 3 should be designed to meet the overall needs of the consumer. Also, the number of refrigerators connected together is not limited to three, but may be two, or may be four or more.

The idea behind so-called large temperature difference air conditioning is to reduce the flow rate of the cooled liquid as much as possible to reduce the power required for transport, and to increase the temperature difference between the cooled liquid and the inlet and outlet, thereby saving energy. The larger the temperature difference between the cooled liquid and the outlet, 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 in some cases 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 cooled liquid and the inlet and outlet 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 a difference of 5°C), which has the advantage of broadening the range that can be handled by the standard specifications.

For example, when designing a single refrigerator with a 12°C inlet/outlet temperature difference, the flow rate of the cooled liquid is about 42% of that when compared to a design with a 5°C inlet/outlet temperature difference, and the flow rate inside the tube drops drastically. This requires measures such as increasing the number of passes. As a result, the structure of the refrigerator's water chamber and heat transfer tubes 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 a significant change, and the refrigerator can be used with the standard design.

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. Therefore, if the flow rate of the coolant does not change, the outlet temperature of the coolant will rise, and the power required will increase. Ideally, the cycle would be to maintain the outlet temperature by increasing the flow rate of the coolant, but this could increase the power required for the coolant pump or cause the pressure difference to increase dramatically. Therefore, the condensers through which the coolant flows are sometimes connected in parallel rather than in series. Figure 2 is a schematic diagram showing one embodiment in which condensers 13, 23, and 33 are connected in parallel.

1 and 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 a 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 extended, which increases the number of maintenance operations.

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 the instructions included in the program. Generally, control devices for embedding in 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.

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.

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 cooled liquid 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 cooled liquid flowing into the evaporator 21 of the second refrigerator 2 .

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 refrigerators 1, 2, and 3 is approximately 42%, so the load is leveled out. Also, because three refrigerators 1, 2, and 3 are operating, the refrigeration cycle is divided into three, maximizing energy savings.

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, and 3, and to update the target outlet temperature T(N) of each refrigerator 1, 2, and 3. Specifically, each of the control units 16, 26, and 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 sensors 15, 25, and 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 calculate 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. This operation stabilizes 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), resulting in stable operation of each refrigerator.

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 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 operating 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 operating refrigerator increases and exceeds the minimum refrigeration load. As a result, the operating efficiency of each refrigerator is improved.

When the number of operating units is increased or decreased due to fluctuations in the refrigeration load, the target outlet temperature T(N) may change suddenly. Therefore, each of the control units 16, 26, and 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, and 36 increases the target outlet temperature of the Nth refrigerator at a predetermined rate of increase (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).

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 will be 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 will rise 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.

Therefore, the control unit 26 of the second refrigerator 2 increases the target outlet temperature of the second refrigerator 2 at a predetermined rate of increase 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. This gradual increase can prevent the control operation of the control unit 26 of the second refrigerator 2 from becoming unstable. The actual rate of increase and calculation period should be adjusted according to the characteristics of the refrigerator.

As is clear from the above calculation formula, the control unit of each chiller needs only to obtain the number N-1 of chillers operating downstream of itself and the common target outlet temperature Tout, and can calculate the target outlet temperature T(N) from the above calculation formula. 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 additional function of each chiller.

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 a 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.

In this embodiment, the control unit 36 of the third refrigerator 3, which is the most upstream, is the "parent unit", and by sharing the operation command given from the parent unit to the child units (the control unit 26 of the second refrigerator 2 and the control unit 16 of the first refrigerator 1), the child units can obtain information on the number of operating units N-1 downstream from 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 operation and stop commands by agreement (protocol).

The control unit 26 of the second refrigerator 2, which is 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 on whether the first refrigerator 1 is operating downstream, depending on whether the operation command CMD1 is active or inactive.

When the operation command CMD1 is active, the control unit 26 of the second refrigerator 2 calculates the target outlet temperature T(2) according to the above-mentioned calculation formula, and when the operation command CMD1 is inactive, it determines the target outlet temperature T(1) = Tout. 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 parent refrigerator 3 naturally has information on the number of refrigerators in operation, since it is the one that decides 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.

By doing this, each refrigerator can calculate the target outlet temperature at which it should operate using only simple signal lines while using a control unit with a common program, making it possible to easily connect and operate multiple refrigerators in series.

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

Figure 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 Figure 1, so detailed illustration 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 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 command device (not shown) that executes the number control may be separately provided, and the command may be issued from this operation command device. In this case, the operation command 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 refrigerators to operate can be determined by referring to the operation time, etc., described above.

Here, an example of operation control of the multiple refrigeration system shown in Figure 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 to 56 of the first refrigerator 1 to the fifth refrigerator 5 each also receive the operation command for the third refrigerator 3. Therefore, the control units 46 and 56 of the fifth refrigerator 5 and the fourth refrigerator 4 can recognize that one more refrigerator is now operating downstream of them. Similarly, by performing the same process for a stop command, each control unit can constantly recognize how many refrigerators are operating downstream of the refrigerator it is responsible for.

In this embodiment, if each control unit knows the installation order of the chillers from the most downstream to which the operation command was given and its own installation order, it can know the increase or decrease in the number of operating units by simply judging whether the two are larger or smaller, and appropriate operation can be achieved 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.

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 refrigerators are operating, or each control unit may periodically notify the control unit of the upstream refrigerator that it is operating. Also, when an operation command is given to a refrigerator control unit from an operation command device (not shown), the number of refrigerators operating downstream of that refrigerator may be notified. In any case, such communication means allows the control unit of each refrigerator to recognize the number of refrigerators operating downstream of itself. Note that if a refrigerator is broken, for example, the operation command device will not give an operation command to that refrigerator. By not giving an operation command to the broken refrigerator, the refrigerator will not be included in the number of operating units, so the entire refrigeration system can operate even if some refrigerators are broken.

The frequency with which the number of operating units is checked can be much shorter than the frequency with which the temperature of the cooled liquid, which changes from moment to moment, is measured. In other words, unlike compressor control as 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.

Theoretically, the more refrigeration units connected, the greater the energy saving effect; however, in practice, connecting too many refrigeration units in series makes the temperature difference between the inlet and outlet of each unit extremely small, which often makes it difficult to design the evaporator. For this reason, the actual number of units connected is usually two to three, depending on the customer's temperature conditions (such as a large temperature difference between the inlet and outlet of the cooled liquid), but in practice, it is considered sufficient to be able to connect up to about five units in series. For this reason, it is acceptable to set a specification upper limit on the number of units that can be connected.

Figure 4 is a schematic diagram showing yet another embodiment of a 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 that connects a minimum of two refrigerators 1 and 2. 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.

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.

Figure 5 is a schematic diagram showing one embodiment of the structure of the control unit 26 of the second refrigerator 2 shown in Figure 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 Figure 1). The measured value of the temperature Tin (2) of the cooled liquid 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 electrical signal. For example, the analog signal is an electrical signal of 4 to 20 mA corresponding to 0 to 50 ° C. That is, temperature values in the range of 0 to 50 ° C. correspond one-to-one to electrical signals in the range of 4 to 20 mA.

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 a signal switcher 63. The main control circuit 61 has a converter (not shown) that converts the input 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.

A relay can be used for the signal switch 63. When the operation signal ACK1 is active, the measured 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 operates at the target outlet temperature T (2) when the first refrigerator 1 is operating, and operates at the target outlet temperature Tout when the first refrigerator 1 is stopped.

These signal functions are often provided as standard functions in ordinary refrigerators, and in this case, only the signal switch 63 and the signal generator 62 need to be added to the existing refrigerator, making it extremely easy to implement the functions of this embodiment in the refrigerator. 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.

By doing the above, multiple refrigeration units can be connected in series, and even if the number of operating units changes, a standardized (unified) program can be used to appropriately distribute the load with minimal communication (only simple contact signals if the number of units is small), creating an energy-saving multiple refrigeration system.

The above-described embodiments have been described for the purpose of enabling a person having ordinary knowledge in the technical field to which the present invention pertains to practice the present invention. Various modifications of the above-described embodiments would naturally be possible for a person skilled in the art, and the technical concept of the present invention may also 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 in accordance with the technical concept defined by the scope of the claims.

Reference Signs List 1, 2, 3 Refrigerating machine 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 unit 17, 27, 37 Expansion valve 41 Cooled liquid transfer line 42 Cooling liquid transfer line 61 Main control circuit 62 Signal generator 63 Signal switch

Claims (3)

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 N-th 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. 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). 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 .

Images