CN115930473A - multi-stage refrigeration system - Google Patents

Abstract

The invention provides a multistage refrigeration system, which forms a multistage refrigeration cycle by connecting a plurality of common refrigerators, can perform stable operation by simple control, reduces the development and design time to the limit, and can realize energy saving at low cost. Assuming that a target outlet temperature of a liquid to be cooled of the entire plurality of refrigerators (1, 2, 3) is Tout and a measured value of an inlet temperature of the liquid to be cooled of an nth refrigerator counted from a refrigerator located at the most downstream refrigerator among the plurality of refrigerators in operation is Tin (N), a control unit of the nth refrigerator is configured to: the target outlet temperature T (N) of the liquid to be cooled of the nth chiller is calculated by the equation T (N) = Tin (N) - (Tin (N) -Tout)/N.

Description

Multi-stage refrigeration system

Technical Field

The present invention relates to a multistage refrigeration system having a plurality of refrigeration cycles through which a refrigerant circulates.

Background

As a method of achieving energy saving of the refrigerator, there is a method of using a dual refrigeration cycle. As shown in fig. 6, the double refrigeration cycle is a method of dividing a single refrigeration cycle (a cycle of a dotted line) having a general configuration into two refrigeration cycles at respective intermediate temperatures of a liquid to be cooled and a cooling liquid in order to obtain the liquid to be cooled at a desired temperature. In this way, the coolant outlet temperature decreases and the condensing pressure becomes small in the low temperature side cycle, and the coolant outlet temperature increases and the evaporating pressure becomes large in the high temperature side cycle, with the result that the required power becomes small, and energy saving can be achieved. Each refrigeration cycle is a compression refrigeration cycle or an absorption refrigeration cycle using a so-called reverse rankine cycle. In general, a double refrigeration cycle is often used in a compression type refrigerator because of remarkable improvement in the compression type refrigeration cycle.

The number of division cycles may be 3 or more, and theoretically, the greater the number of division cycles, the greater the energy saving effect. In the present specification, two or more split cycles are referred to as "multistage refrigeration cycles".

Although compression-type refrigerators using a multistage refrigeration cycle have been commercialized, it is necessary to divide the refrigeration cycle into two or more compressors, that is, two or more compressors. For example, in the case of the two-stage compression cycle, two-stage compression-type refrigerators are used, or one compressor is provided with two-stage compression-type compressors having two circuits, and therefore, the cost is high in any case, and the number of the compressors to be sold is extremely small. Therefore, the number of steps required for development and design is relatively large, and it is difficult to spread the method in practice.

Although the same effect as that of the multi-stage refrigeration cycle can be obtained by connecting two or more common refrigerators in series, it is difficult to control the refrigerators in this case to stabilize the outlet temperature of the liquid to be cooled and the like at a target temperature by interlocking the upstream refrigerator with the downstream refrigerator. For example, if two refrigerators are intended to be operated as one refrigerator, the upstream and downstream refrigerators can be operated at substantially the same cooling load by equalizing the opening degrees of the variable valve of the two refrigerators or equalizing the rotational speeds (operating frequencies) of the motors in the case of a variable-speed compression-type electric refrigerator. However, large-scale modification such as designing a control device for a refrigerator is required, and the load of development and design is extremely large when verification is included.

Although it is possible to perform operation by applying a target temperature to a single refrigerator, as will be described later, in this case, operation is hindered unless the cooling load is substantially constant or the temperature of the liquid to be cooled is not changed by variable flow rate control.

Fig. 7 is a schematic diagram showing a conventional example of a refrigerator having a dual refrigeration cycle in a case where two compressors are mounted on one refrigerator. The refrigerator of the present example is a dual refrigeration cycle having two sets of evaporators 501 and 502, condensers 503 and 504, compressors 505 and 506, expansion valves 507 and 508, and temperature sensors 511 and 512 in the refrigerator. The control of the dual refrigeration cycle is performed by one controller 514. The controller 514 controls the rotational speed of the compressors 505 and 506 and the volume control valve (suction vane, etc.) to increase or decrease the output of the refrigerator. Generally, the control unit 514 monitors the coolant outlet temperature T1 of the entire refrigerator, and lowers the output of the refrigerator if T1 is lower than the target temperature, and increases the output of the refrigerator if T1 is higher than the target temperature, thereby maintaining the coolant outlet temperature T1 at the target temperature.

In this case, the controller 514 does not separately control the rotation speed and the volume control valve of the compressors 505 and 506 for the two refrigeration cycles, and generally operates the two refrigeration cycles by applying the same command value (rotation speed and volume control valve opening). This is because, in the dual refrigeration cycle, the most energy-saving operation is performed when the loads of the two refrigeration cycles are substantially equal to each other, but since the two refrigeration cycles are operated under a condition of a temperature difference of several degrees, the effect is small even if the optimal control is performed independently. In addition, it is necessary to obtain an intermediate temperature (T2') of the liquid to be cooled for individual control. However, since it is difficult to obtain the intermediate temperature itself due to the structure of the refrigerator, the cost is not reasonable, and the necessity of such control is small. As a result, it is most appropriate to give the same command value to both refrigeration cycles to perform the operation in consideration of the total cost as described above.

On the other hand, as described above, since the number of refrigerators dedicated to the dual refrigeration cycle is small, it is difficult to recover the cost required for development. In particular, since it is verified that a large amount of cost (particularly, energy cost at the time of test) is spent in the development of the control system, it is difficult to recover the cost, and even if the effect of the double refrigeration cycle is known, it is difficult to develop the control system.

Fig. 8 is a schematic diagram showing a conventional example in which two refrigerators are connected to form a dual refrigeration cycle. In this example, two refrigerators on the upstream side and the downstream side are mechanically coupled. The controller 514 of the downstream-side chiller monitors the coolant outlet temperature T1, and controls the rotation speed of the compressor 505 and the opening degree of the metering valve so that the coolant outlet temperature T1 is maintained at the target temperature. The control unit 515 of the upstream-side chiller can provide the same command value as the command value (rotation speed, opening degree of the metering valve) of the compressor 505 to the control unit 514 to the compressor 506 of the upstream-side chiller, thereby obtaining the same effect as the above example.

In practice, however, such control is not simple. One of the reasons for this is that if the command value of the downstream control unit 514 is applied to the upstream compressor 506 without being changed, a problem such as so-called surge may occur due to a slight difference in temperature conditions or the like. Further, a time lag is likely to occur in digital communication used for transmitting the command value, and this may cause a problem in control of the compressor and the like. In addition, since conditions such as pressure conditions are complicated in controlling the compressor, development cost is also required. For the above reasons, it is difficult to actually apply such control.

In order to avoid such a problem, there is a method of individually determining a target value of the outlet temperature of the upstream-side refrigerator and a target value of the outlet temperature of the downstream-side refrigerator, and controlling the refrigerators based on these target values. According to this method, each of the refrigerators only needs to perform control that is exactly the same as the control of a conventional general single refrigeration cycle, and therefore the risk of the refrigerator is minimized, and the development cost can be minimized. However, in this method, as described below, since the refrigeration load is concentrated on the downstream-side refrigerator, the effect of the dual refrigeration cycle in which the refrigeration load is shared equally by the upstream and downstream refrigerators is reduced due to variations in the degree of degradation of the equipment and the like.

Fig. 9 is a schematic diagram showing a conventional example in which three refrigerators are connected to constitute a triple refrigeration cycle. The three refrigerators 611, 612, and 613 have the same configuration as the refrigerator described with reference to fig. 8, and therefore, the details thereof are omitted. The liquid to be cooled is sent to the equipment of the client at a desired temperature T1 by the refrigerating machine 613, the refrigerating machine 612, and the refrigerating machine 611 in this order. However, the inlet temperature T2 is the temperature of the liquid to be cooled returned from the customer's equipment, and generally varies greatly depending on the load of the customer.

The refrigerators 611, 612, 613 are operated at predetermined temperatures as target values. For example, assuming that the target temperature T1=7 ℃ and the inlet temperature T2=19 ℃ of the liquid to be cooled under the rated load, the intermediate temperature T2' between the refrigerators 613 and 612 under the rated operation is 15 ℃ and the intermediate temperature T2 ″ between the refrigerators 612 and 611 under the rated operation is 11 ℃. Therefore, the temperatures T2', T2 ″, and T1 are set as target temperatures of the refrigerators 613, 612, and 611, respectively. Such operation control can be performed if the refrigeration loads of the refrigerators 611, 612, and 613 are constant or if the refrigerators 611, 612, and 613 are operated so that T2 becomes constant by using so-called control for changing the flow of the liquid to be cooled. However, in many cases, the coolant inlet temperature T2 fluctuates widely.

For example, the cooled liquid inlet temperature T2 is lowered to 12 ℃. At this time, since the cooled liquid inlet temperature T2 is lower than the target temperature T2' =15 ℃, the refrigerator 613 is stopped (light load stop). The intermediate temperature T2' is maintained at 12 ℃. The refrigerator 612 is operated to cool to T2"=11 ℃ which is a target temperature since the inlet temperature thereof is 12 ℃, but is operated at 25% of the normal load. On the other hand, the refrigerator 611 operates at a load of almost 100% because the inlet temperature thereof is 11 ℃. In this way, when the target temperature of a single refrigerator is determined, a large difference occurs in the load concentration in the subsequent refrigerator, the operation time, and the like, and thus efficient operation cannot be performed. Further, the energy saving effect by reducing the number of operating units is naturally also reduced.

Patent document 1: japanese patent laid-open No. 2007-183077

In this way, if a multistage refrigeration cycle is configured by connecting a plurality of refrigerators, a temperature control method therefor becomes a great problem.

Disclosure of Invention

Accordingly, the present invention provides a multistage refrigeration system that can realize a multistage refrigeration cycle by connecting a plurality of ordinary refrigerators, can perform stable operation with simple control, and can reduce the number of development and design steps to the limit, thereby realizing energy saving at low cost.

In one aspect, there is provided a multistage refrigeration system including a plurality of refrigerators, each of the plurality of refrigerators including: 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 liquid to be cooled that flows into the evaporator, an outlet temperature sensor that measures an outlet temperature of the liquid to be cooled that flows out from the evaporator, and a circulation flow rate control unit that controls the refrigerant, wherein evaporators of the plurality of refrigerators are connected in series, and the control unit of the nth refrigerator is configured to, when a target outlet temperature of the liquid to be cooled of the entire plurality of refrigerators is Tout and a measured value of an inlet temperature of the liquid to be cooled of the nth refrigerator from a refrigerator located at a most downstream refrigerator in operation is Tin (N):

the target outlet temperature T (N) of the liquid to be cooled of the nth chiller is calculated by the formula T (N) = Tin (N) - (Tin (N) -Tout)/N.

According to the present invention, since the loads of the refrigerators are balanced, the effect of the multistage refrigeration cycle can be maximized. In addition, the operation time of each refrigerator can be balanced. Further, according to the present invention, since there is no transmission/reception of a control signal between refrigerators in the operation of a plurality of refrigerators, there is no delay in the control signal, and stable operation can be performed with simple control. As a result, the number of development and design steps can be reduced to the limit, and a multi-stage refrigeration system that can perform energy-saving operation at low cost can be realized.

In one aspect, the control unit is configured to: the target outlet temperature T (N) is calculated using the equation at a cycle longer than a cycle of an output control operation for eliminating a difference between the current outlet temperature of the liquid to be cooled and the target outlet temperature T (N).

According to the present invention, the control of the cooling capacity based on the difference between the current outlet temperature of the liquid to be cooled and the target outlet temperature T (N) is stabilized, and therefore the operation of each of the refrigerators is stabilized.

In one aspect, the control unit is configured to: gradually raising the target outlet temperature of the nth chiller 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 cooling capacity based on the difference between the current outlet temperature of the liquid to be cooled and the target outlet temperature T (N) is stabilized, and therefore the operation of each of the refrigerators is stabilized.

In one aspect, one of the plurality of control units functions as a master unit, and the other control units function as slave units, wherein the master unit is configured to: when the refrigeration load of each of the plurality of refrigeration machines is lower than a predetermined minimum refrigeration load, the operation of at least one of the plurality of refrigeration machines is stopped.

If the refrigeration load of each refrigerator is lower than the minimum refrigeration load, the operating efficiency of the refrigerator decreases. According to the present invention, when the refrigeration load is lower than the predetermined minimum refrigeration load, the operation of at least one of the refrigerators is stopped, and therefore the refrigeration load of the refrigerator in operation rises and exceeds the minimum refrigeration load. As a result, the operation efficiency of each refrigerator is improved.

In one aspect, the host mechanism is: and a signal indicating that the refrigerator is the nth refrigerator counted from the most downstream refrigerator among the refrigerators in operation is transmitted to the slave unit of the nth refrigerator.

When the slave unit receives a signal indicating that its own refrigerator is the nth refrigerator, the slave unit can calculate the target outlet temperature T (N) of the liquid to be cooled by the above equation, completely independently of the master unit, without transmitting and receiving the signal to and from the master unit.

According to the present invention, since the loads of the refrigerators are balanced, the effect of the multistage refrigeration cycle can be maximized. In addition, the operation time of each refrigerator can be balanced. Further, according to the present invention, since there is no transmission/reception of a control signal between the refrigerators during the operation of the plurality of refrigerators, there is no delay in the control signal, and stable operation can be performed with simple control. As a result, the number of development and design steps can be reduced to the limit, and a multi-stage refrigeration system that can perform energy-saving operation at low cost can be realized.

Drawings

FIG. 1 is a schematic diagram illustrating one embodiment of a multi-stage refrigeration system having three chillers connected in series.

Fig. 2 is a schematic diagram illustrating another embodiment of a multi-stage refrigeration system.

Fig. 3 is a schematic diagram illustrating yet another embodiment of a multi-stage refrigeration system.

Fig. 4 is a schematic diagram illustrating yet another embodiment of a multi-stage refrigeration system.

Fig. 5 is a schematic diagram showing an embodiment of the control unit.

Fig. 6 is a schematic diagram showing a conventional example of a dual refrigeration cycle.

Fig. 7 is a schematic diagram showing a conventional example of a refrigerator having a dual refrigeration cycle in a case where two compressors are mounted on one refrigerator.

Fig. 8 is a schematic diagram showing a conventional example in which two refrigerators are connected to form a dual refrigeration cycle.

Fig. 9 is a schematic diagram showing a conventional example in which three refrigerators are connected to constitute a triple refrigeration cycle.

Description of the reference numerals: 1.2, 3. Refrigerator; 11. 21, 31. 12. 22, 32. 13. 23, 33.. A condenser; 14. an inlet temperature sensor; 15. an outlet temperature sensor; 16. 26, 36.. Control section; 17. 27, 37. An expansion valve; a cooled fluid delivery line; a coolant delivery line; a main control circuit; a signal generator; a signal switch.

Detailed Description

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Fig. 1 is a schematic diagram showing an embodiment of a multistage refrigeration system in which three refrigerators are connected in series. As shown in fig. 1, the multistage 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 a 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 of the evaporator 11, and a control unit 16 that controls a circulation flow rate of the refrigerant.

The evaporator 11, the compressor 12, and the condenser 13 are connected by refrigerant pipes depicted by arrows. The refrigerant circulates through the evaporator 11, the compressor 12, and the condenser 13 via refrigerant pipes. 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 a refrigerant pipe 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, and the pressure and temperature of the refrigerant are lowered. The refrigerant having 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 the above-described components of the second refrigerator 2 and the third refrigerator 3 are basically the same as those of the first refrigerator 1, and therefore, redundant descriptions thereof are omitted.

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

The liquid to be cooled (for example, chilled water) first flows into the evaporator 31 of the third refrigerator 3 through the liquid to be cooled transfer line 41. Heat exchange between the liquid to be cooled and the refrigerant liquid is performed in the evaporator 31. As a result of this heat exchange, the liquid to be cooled is cooled, while the refrigerant liquid is heated and evaporated by the liquid to be cooled to become a refrigerant gas. The cooled liquid to be cooled is sent to the evaporator 21 of the second refrigerator 2 through the liquid-to-be-cooled delivery line 41, while the refrigerant gas is sent to the compressor 32. The compressor 32 compresses a 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 coolant sent through the coolant sending line 42, and becomes a refrigerant liquid. In the second refrigerator 2 and the first refrigerator 1, the liquid to be cooled is also cooled in the same manner, and finally flows out from the evaporator 11 of the first refrigerator 1. It goes without saying that the respective elements of the refrigerators 1, 2, and 3 may be designed to be used as a whole to meet the demand. The number of connected refrigerators is not limited to three, and may be two or four or more.

There is an idea of reducing the flow rate of the liquid to be cooled as much as possible in order to reduce the transportation power, and increasing the temperature difference between the inlet and outlet of the liquid to be cooled, thereby realizing a so-called large temperature difference air conditioner that saves energy. The effect is increased as the difference in the temperature of the liquid to be cooled between the inlet and the outlet of the multi-stage refrigeration cycle is increased, and therefore the multi-stage refrigeration cycle is suitable for air conditioning with a large temperature difference. Further, in the large temperature difference air conditioner, since the flow rate of the liquid to be cooled is low, there is a case where special design is necessary without departing from the standard design condition of the refrigerator, but as shown in fig. 1, by connecting a plurality of refrigerators 1, 2, 3 in series, the difference in temperature of the liquid to be cooled in and out of each refrigerator can be suppressed to be small, and the refrigerators 1, 2, 3 can be designed under a condition close to the standard design condition (generally, with a difference of 5 degrees celsius), and therefore, there is an advantage that the range corresponding to the standard specification can be expanded.

For example, when one refrigerator is designed under the condition of an inlet/outlet temperature difference of 12 ℃, the flow rate of the liquid to be cooled is about 42% compared with the design under the condition of an inlet/outlet temperature difference of 5 ℃, and the flow velocity in the tube is extremely reduced in this state. Therefore, a countermeasure such as increasing the number of paths is required. As a result, the structure of the water chamber and the heat transfer pipe 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 the present embodiment, the difference in temperature between the inlet and outlet of the liquid to be cooled per refrigerator becomes 4 ℃, the flow rate of the liquid to be cooled is 1.25 times that of the standard design, and the refrigerator can be used while maintaining the standard design.

Further, since the inlet temperature of the coolant is determined by the cooling tower and the conditions of the outside air and the outlet temperature of the coolant is determined by the flow rate, if the flow rate of the coolant is not changed, the outlet temperature of the coolant is increased, and the required power is increased. As the circulation, it is desirable to maintain the outlet temperature by increasing the flow rate of the coolant, but since the power required by the coolant pump increases and the differential pressure extremely increases, the condensers through which the coolant flows may be connected in parallel instead of in series. Fig. 2 is a schematic diagram showing an embodiment in which the condensers 13, 23, 33 are connected in parallel.

In fig. 1 and 2, the control unit 36 of the third chiller 3 functions as a "master unit". The operation command to the plurality of refrigerators 1, 2, and 3 is given to the control unit 36 of the third refrigerator 3 by a manual input or a signal from a remote place. The control unit 36 of the third refrigerator 3 determines whether or not the three refrigerators 1, 2, and 3 are operating, based on the load state and the like, and gives the control unit 26 of the second refrigerator 2 and the control unit 16 of the first refrigerator 1 operation commands CMD2 and CMD1. There are various methods as to which refrigerator is operated in what combination. For example, in the case where 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 the number is increased and stop the refrigerator with the longest operating time at that time when the number is decreased. This makes it possible to make the operating times of the refrigerators uniform, and to suppress an increase in the number of times of maintenance and the like due to an increase in the operating time of only a specific refrigerator.

The control units 16, 26, and 36 each include a storage device that stores a program and an arithmetic device that executes an arithmetic operation in accordance with a command included in the program. As these control units 16, 26, and 36, control devices for incorporating devices such as a microcomputer and a sequencer are generally used in many cases, but the specific configurations of the control units 16, 26, and 36 are not limited to these examples.

In the present embodiment, in order to balance the load on the operating refrigerators, the controller of the nth refrigerator from the most downstream refrigerator among the operating refrigerators is configured to: the target outlet temperature T (N) of the liquid to be cooled of the nth chiller is calculated by the following equation T (N).

The formula T (N) = Tin (N) - (Tin (N) -Tout)/N

Here, tout is a target outlet temperature of the liquid to be cooled of the entire plurality of refrigerators 1, 2, and 3, and Tin (N) is a measured value of the inlet temperature of the liquid to be cooled of the nth refrigerator. The above equations are stored in the storage devices of the control units 16, 26, and 36, respectively.

For example, when all of the three refrigerators 1, 2, and 3 are in operation, the third refrigerator 3 is the third refrigerator from the most downstream side, and therefore 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.

Similarly, 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.

If the above conditions are applied to the above equation, since Tin (3) =12 ℃ and Tout =7 ℃, T (3) ≈ 10.3 ℃ and T (2) ≈ 8.6 ℃ and the loads of the refrigerators 1, 2, and 3 are balanced by about 42%. Since 3 refrigerators 1, 2, and 3 are operated, the refrigerating cycle is divided into 3 units, and the energy saving effect is maximized.

The calculation using the above equation is preferably performed at time intervals sufficiently longer than the response time to the temperature in the temperature control of the refrigerators 1, 2, 3, and updates the target outlet temperatures T (N) of the refrigerators 1, 2, 3. Specifically, each of the control units 16, 26, and 36 is configured to calculate the target outlet temperature T (N) using the above equation at a cycle that is longer than the cycle of the output control calculation for eliminating the difference between the current outlet temperature of the liquid to be cooled and the target outlet temperature T (N). The current outlet temperature of the liquid to be cooled is measured by the outlet temperature sensors 15, 25, 35. For example, since the output control operation for eliminating the difference between the current outlet temperature of the liquid to be cooled and the target outlet temperature T (N) is performed at a cycle of about 1 second, the calculation of the target outlet temperature T (N) using the above equation is preferably performed at a cycle sufficiently longer than the cycle of the output control operation, for example, at a cycle of about 1 minute. According to such an operation, the control of the cooling capacity based on the difference between the current outlet temperature of the liquid to be cooled and the target outlet temperature T (N) is stabilized, and therefore the operation of each of the refrigerators is stabilized.

One of the control units 16, 26, and 36 functions as a master unit, and the other control units function as slave units. In the present embodiment, the control unit 36 of the third refrigerator 3 functions as a master. The main mechanism is configured to stop operation of at least one of the plurality of refrigeration machines 1, 2, and 3 when the refrigeration loads of the plurality of refrigeration machines 1, 2, and 3 are lower than a predetermined minimum refrigeration load. In general, when the refrigeration load of each refrigerator is lower than the minimum refrigeration load, the operating efficiency of the refrigerator decreases. According to the present embodiment, when the refrigeration load is lower than the predetermined minimum refrigeration load, the operation of at least one of the refrigerators is stopped, and therefore the refrigeration load of the refrigerator in operation rises and exceeds the minimum refrigeration load. As a result, the operation efficiency of each refrigerator is improved.

When the number of operating units increases or decreases due to a change in the cooling load, the target outlet temperature T (N) may change rapidly. Therefore, the control units 16, 26, and 36 are configured to gradually increase the target outlet temperature of the nth chiller from a temperature lower than the calculated target outlet temperature T (N) to the calculated target outlet temperature T (N). More specifically, the control units 16, 26, and 36 increase the target outlet temperature of the nth chiller from a temperature lower than the calculated target outlet temperature T (N) to the calculated target outlet temperature T (N) at a predetermined rate of increase (for example, a rate of increase of 1 ℃ per minute).

For example, when 3 refrigerators 1, 2, and 3 are operated, if the inlet temperature Tin (3) of the liquid to be cooled in the third refrigerator 3 is 17 ℃ and the target outlet temperature Tout is 7 ℃, the inlet temperature of the liquid to be cooled in the second refrigerator 2 is 13.6 ℃. According to the above equation, the target outlet temperature T (2) of the second refrigerator 2 is calculated to be 10.3 ℃. When the third refrigerator 3 is stopped due to a decrease in the refrigeration load, the inlet temperature of the liquid to be cooled in the second refrigerator 2 rises to 17 ℃. As a result, the target outlet temperature T (2) of the second refrigerator 2 was calculated to be 12 ℃, and the target temperature increased by 1.7 ℃. If this target outlet temperature T (2) is immediately applied to the control unit 26 of the second chiller 2 as a target value of the output control calculation, there is a concern that the control will be unstable.

Therefore, the control unit 26 of the second chiller 2 increases the target outlet temperature of the second chiller 2 from a temperature lower than the calculated target outlet temperature T (2) to the calculated target outlet temperature T (2) at a predetermined rate of increase. For example, the controller 26 of the second refrigerator 2 first increases the target outlet temperature from 10.3 ℃ to 11.3 ℃ and 1 minute thereafter increases the target outlet temperature from 11.3 ℃ to 12 ℃. Such a stepwise increase can suppress the control operation of the control unit 26 of the second chiller 2 from becoming unstable. The actual increase rate and calculation cycle can be adjusted according to the characteristics of the refrigerator.

As can be seen from the above equation, if the control unit of each refrigerating machine acquires only the number N-1 of the refrigerating machines operating downstream of the control unit and the common target outlet temperature Tout, the target outlet temperature T (N) can be calculated from the above equation. Therefore, analog communication for transmitting the inlet temperature Tin (3) of the cooling target fluid at the most upstream side and the calculation of the cooling duty are not required. That is, it is a great advantage of the present embodiment that the individual refrigeration machines can be equipped with exactly the same control logic as their associated functions.

In order to calculate the target outlet temperature T (N), each of the refrigerators needs to acquire information on the number N-1 of operating units downstream of the refrigerator. One of the control units 16, 26, and 36 functions as a master unit, and the other control units function as slave units. The master unit is configured to transmit a signal indicating the number of nth refrigerators from the most downstream refrigerator among the operating refrigerators to the slave unit of the nth refrigerator. When the slave unit receives a signal indicating that its own refrigerator is the nth refrigerator, the slave unit can calculate the target outlet temperature T (N) of the liquid to be cooled using the above equation, completely independently of the master unit, without transmitting and receiving the signal to and from the master unit.

In the present embodiment, the control unit 36 of the third refrigerator 3 at the most upstream is set as the "master unit", and the slave units can acquire information on the number N-1 of operating units downstream of the slave units by sharing the operation command given from the master unit to the slave units (the control unit 26 of the second refrigerator 2 and the control unit 16 of the first refrigerator 1). That is, when the control unit 36 (master unit) of the third refrigerator 3 issues an operation command to the slave unit, the operation command CMD2 is validated when the second refrigerator 2 is to be operated, and the operation command CMD1 is validated when the first refrigerator 1 is to be operated. The term "enable" of the operation command means, for example, that a predetermined voltage such as +5V is output when a voltage signal (or a source signal) is present, and that a signal line is connected to a ground potential (0V) when a voltage signal (or a synchronization signal) is absent. However, the form is not particularly limited as long as the operation command can be transmitted by determining (agreeing) the instruction to operate and the instruction to stop the operation.

The control unit 26 of the second refrigerator 2 located between the third refrigerator 3 and the first refrigerator 1 receives the state (valid or invalid) of the operation command CMD2 from the control unit 36 (master) of the third refrigerator 3, and receives the state (valid or invalid) of the operation command CMD1 from the control unit 36 (master) of the third refrigerator 3. Therefore, when the operation command CMD2 is valid, the control unit 26 of the second chiller 2 can obtain information on whether the first chiller 1 is operating downstream, based on whether the operation command CMD1 is valid or invalid.

The control unit 26 of the second chiller 2 calculates the target outlet temperature T (2) according to the above equation when the operation command CMD1 is valid, and determines the target outlet temperature T (1) = Tout when the operation command CMD1 is invalid. The control unit 26 of the second chiller 2 may receive information as to whether or not the first chiller 1 is operating as a direct signal from the control unit 16 of the first chiller 1. The control unit 36 of the third chiller 3, which is the master, naturally has information on the number of operating units because it determines that the operation is stopped by itself. When the first refrigerator 1 is in operation, the target outlet temperature of the liquid to be cooled of the first refrigerator 1 is Tout under any conditions.

As described above, each of the refrigerators can calculate the target outlet temperature to be operated by using the control unit having the common program and by using only a simple signal line, and a plurality of refrigerators can be connected in series and operated with a simple time and effort.

According to the present embodiment, since the loads of the refrigerators are balanced, the effect of the multistage refrigeration cycle can be maximized. In addition, the operation time of each refrigerator can be balanced. Further, according to the present embodiment, since there is no transmission/reception of a control signal between the refrigerators in the operation of the plurality of refrigerators, there is no delay of the control signal, and stable operation can be performed with simple control. As a result, the number of development and design steps can be reduced to the limit, and a multi-stage refrigeration system that can perform energy-saving operation at low cost can be realized.

Fig. 3 is a schematic diagram illustrating yet another embodiment of a multi-stage refrigeration system. In the present embodiment, 5 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, and thus, detailed illustration thereof is omitted. In the present embodiment, the control units 16 to 56 of the refrigerators 1 to 5 are connected by 1 communication line (CMDx). This is a so-called bus connection, and all of the control units 16 to 56 receive signals transmitted to the respective control units, and the respective refrigerators operate in accordance with the given commands. In this case, the host computer issues an operation command to the slave computer, as in the previous embodiment. Alternatively, an operation instruction device (not shown) for executing the number control may be separately provided, and the operation instruction device may give an instruction. In this case, the operation instruction device gives an operation instruction to the refrigerators 1 to 5. In this case, the number of operating refrigerators and which refrigerator to operate may be determined by referring to the above-described operation time.

Here, an example of the operation control of the multistage refrigeration system shown in fig. 3 will be described. For example, when the operation command is transmitted to the control unit 36 of the third chiller 3 through the communication line CMDx, the control units 16 to 56 of the first to 5 th chillers 1 to 5 receive the operation command to the third chiller 3 in the same manner. Therefore, the control units 46 and 56 of the 5 th refrigerator 5 and the 4 th refrigerator 4 can recognize that 1 refrigerator operated downstream of the control units has been added. Similarly, by performing the same processing on the stop command, each control unit can always recognize that several refrigerators are operating downstream of its own refrigerator.

In this embodiment, if the control units know the installation order from the most downstream side of the chiller to which the operation command is given and the own installation order, the increase or decrease in the number of operating units can be known only by simple size determination of the both, and appropriate operation can still be performed by using a common program in the control units. Therefore, the control unit of each refrigerator can determine its own target outlet temperature in the same manner as in the above embodiment.

The identification of the number of operating refrigerators by each control unit may be an operation of confirming whether or not another refrigerator is operating when the control unit receives an operation command or at regular intervals, or may be an operation of periodically notifying the upstream refrigerator control unit from each control unit that the refrigerator is operating. Further, when an operation instruction device (not shown) gives an operation instruction to a control unit of a certain refrigerator, the number of refrigerators operating downstream of the certain refrigerator may be notified. In short, the control unit of each of the refrigerators can recognize the number of refrigerators operating downstream of the control unit by the communication means. In addition, when there is a malfunctioning refrigerator, for example, the operation instruction device does not give an operation instruction to the refrigerator. Since the number of operating refrigerators is not counted by not giving an operation command to the malfunctioning refrigerator, the entire refrigeration system can be operated even if some of the refrigerators are malfunctioning.

In addition, the cycle of checking the number of the operation units may be very small compared to the measurement cycle of the temperature of the liquid to be cooled which changes from time to time. That is, unlike the control of the compressor described in the conventional art, there is no problem even if digital communication is used. Therefore, it is not necessary to exchange temperature information (analog value) frequently as in the present embodiment, and the load on the communication line is reduced, which is also an advantage of the present embodiment.

Further, although the energy saving effect is theoretically high as the number of connected refrigerators increases, in practice, if too many refrigerators are connected in series, the difference in the inlet and outlet temperatures of 1 refrigerator becomes extremely small, and thus it is often difficult to design the evaporator. Therefore, the number of actually connected units is usually 2 to 3, and it is considered that about 5 units are connected in series in practice, although it depends on the temperature condition of the client (such as a large difference in temperature between the cooling target fluid and the cooling target fluid). Therefore, an upper limit of the specification may be set for the number of connected stations.

Fig. 4 is a schematic diagram illustrating yet another embodiment of a multi-stage refrigeration system. The detailed configuration of each refrigerator is the same as that of the embodiment described with reference to fig. 1, and thus, detailed illustration thereof is omitted. The present embodiment is a multistage refrigeration system in which two refrigerators 1 and 2 having the smallest size are connected. The controllers 16 and 26 of the two refrigerators 1 and 2 do not have a master-slave relationship, and the two refrigerators 1 and 2 are operated in accordance with a direct instruction or a remote instruction.

In the present embodiment, the control unit 16 of the first chiller 1 transmits an operation signal ACK1 indicating that the first chiller 1 is in operation to the control unit 26 of the second chiller 2. Thereby, the control portion 26 of the second refrigerator 2 recognizes that the first refrigerator 1 is in operation, and switches the target outlet temperature of the liquid to be cooled from Tout to T (2) based on the operation signal. In this case, if the function of switching the target outlet temperature of the liquid to be cooled, which is provided in the control unit 26, is used, such a function can be implemented only by adding a circuit to the conventional control unit.

Fig. 5 is a schematic diagram showing an embodiment of the structure of the control unit 26 of the second refrigerator 2 shown in fig. 4. The control unit 26 includes a main control circuit 61, a signal generator 62, and a signal switch 63. The signal switcher 63 is connected to the main control circuit 61 and the signal generator 62. The temperature Tin (2) of the liquid to be cooled flowing into the second refrigerator 2 is measured by the inlet temperature sensor 24 (see fig. 1). The measured value of the temperature Tin (2) of the liquid to be cooled 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 as an analog signal composed of an electric signal. For example, the analog signal is an electric signal of 4 to 20mA corresponding to 0 to 50 ℃. That is, the temperature values within the range of 0 to 50 ℃ correspond to the electric signals within the range of 4 to 20mA one by one.

The measured value (electric signal) of the temperature Tin (2) output from the main control circuit 61 is input to the main control circuit 61 again via the signal switch 63. The main control circuit 61 includes a converter (not shown) for converting an input electric signal into a temperature value, and the converter is adjusted in advance so as to convert an electric signal of 4 to 20mA into a temperature value of 3.5 to 28.5 ℃. The electric signals within the range of 4-20 mA correspond to the temperature values within the range of 3.5-28.5 ℃ one by one. The range of the temperature value from 3.5 to 28.5 ℃ is a range obtained by previously converting the temperature value to a range from 0 to 50 ℃ by using the following equation.

T(2)＝Tin(2)－(Tin(2)－Tout)/2

Tout here is 7 ℃.

The signal switcher 63 can use a relay (relay). When the operation signal ACK1 is valid, the measured value (electric 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 invalid, a signal from 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 (electric signal) corresponding to Tout. Therefore, when the first refrigerator 1 is not operated, the second refrigerator 2 is operated with Tout as the target outlet temperature. According to the present embodiment, the second refrigerator 2 is operated at the target outlet temperature T (2) when the first refrigerator 1 is operated, and is operated at the target outlet temperature Tout when the first refrigerator 1 is stopped.

These signal functions are often provided in a normal refrigerator as standard functions, and in this case, only the signal switcher 63 and the signal generator 62 need to be added to an existing refrigerator, so that the functions of the present embodiment can be extremely easily mounted on the refrigerator. Further, when Tin (2) is not input to the main control circuit 61, the signal generator 62 is not necessary if the control unit has a function (fail-safe function) of automatically setting the target outlet temperature of the liquid to be cooled to Tout.

As described above, even when a plurality of refrigerators are connected in series and the number of operating refrigerators varies, appropriate load distribution can be performed with minimum communication (only simple contact signals if the number of refrigerators is small) using a standardized (unified) program, and a multistage refrigeration system in which energy saving is achieved can be configured.

The above-described embodiments are 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 embodiments can be naturally implemented by those 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 embodiments described above, and can be interpreted within the maximum scope based on the technical idea defined by the claims.

Claims (7)

1. A multi-stage refrigeration system having a plurality of refrigerators,

the plurality of refrigerators each include: 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 liquid to be cooled that flows into the evaporator, an outlet temperature sensor that measures an outlet temperature of the liquid to be cooled that flows out of the evaporator, and a circulation flow rate control unit that controls the refrigerant,

the evaporators of the plurality of refrigerators are connected in series,

assuming that a target outlet temperature of the liquid to be cooled of the entire plurality of refrigerators is Tout and a measured value of an inlet temperature of the liquid to be cooled of an nth refrigerator counted from a refrigerator located at a most downstream refrigerator among the plurality of refrigerators in operation is Tin (N), the controller of the nth refrigerator is configured to:

the target outlet temperature T (N) of the liquid to be cooled of the nth chiller is calculated by the formula T (N) = Tin (N) - (Tin (N) -Tout)/N.

2. The multi-stage refrigeration system of claim 1,

the control unit is configured to: the target outlet temperature T (N) is calculated using the equation at a cycle longer than a cycle of an output control operation for eliminating a difference between the current outlet temperature of the liquid to be cooled and the target outlet temperature T (N).

3. The multi-stage refrigeration system of claim 1 or 2,

the control unit is configured to: gradually raising the target outlet temperature of the nth chiller from a temperature lower than the calculated target outlet temperature T (N) to the calculated target outlet temperature T (N).

4. The multi-stage refrigeration system of claim 1 or 2,

one of the plurality of control units functions as a master unit, and the other control units function as slave units, wherein the master unit is configured to: when the refrigeration load of each of the plurality of refrigeration machines is lower than a predetermined minimum refrigeration load, the operation of at least one of the plurality of refrigeration machines is stopped.

5. The multi-stage refrigeration system of claim 3,

one of the plurality of control units functions as a master unit, and the other control units function as slave units, wherein the master unit is configured to: when the refrigeration load of each of the plurality of refrigeration machines is lower than a predetermined minimum refrigeration load, the operation of at least one of the plurality of refrigeration machines is stopped.

6. The multi-stage refrigeration system of claim 4,

the main mechanism is as follows: and transmitting a signal indicating that the refrigerator is the nth refrigerator counted from the most downstream refrigerator among the operating refrigerators to a slave unit of the nth refrigerator.

7. The multi-stage refrigeration system of claim 5,

the main mechanism is as follows: and transmitting a signal indicating that the refrigerator is the nth refrigerator counted from the most downstream refrigerator among the operating refrigerators to a slave unit of the nth refrigerator.

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