CONTROL DEVICE, REFRIGERANT CIRCUIT SYSTEM AND CONTROL METHOD

Abstract

 A control device (100) capable of performing an oil return operation corresponding to a load is provided. The control device (100) which controls a refrigerant circuit system (1) controls an enthalpy difference (Δi, Δi') between an inlet side and an outlet side of a condenser (11) and a frequency of a compressor (10A, 10B) so that a difference between a heating capacity of a refrigerant circuit system during an oil return operation in a heating cycle (Q') and that of the refrigerant circuit system during a normal operation (Q) is within a predetermined range. The enthalpy difference (Δi, Δi') during the oil return operation is controlled to be smaller than that during the normal operation by setting a target temperature (T') on an outlet side of a condenser (11) to be higher than a temperature during the normal operation.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] Priority is claimed on Japanese Patent Application No. 2016-197364, filed October 5,2016, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

[0002] The present invention relates to a control device, a refrigerant circuit system and a control method.

Description of Related Art

[0003] Conventionally, in a refrigerant circuit including a multistage compressor, a refrigerant circuit with an injection circuit in which some of a high-pressure refrigerant condensed by a condenser is caused to diverge and the diverged refrigerant is injected into the compressor to improve a cooling capacity and a coefficient of performance (COP) is known. For example, Patent Document 1 discloses a technique for improving the COP by switching between operation and non-operation of an injection operation according to a load level or a ratio between a condensation pressure and an evaporation pressure in a refrigerant circuit with an injection circuit.

[0004] Further, conventionally, an oil return operation which recovers refrigerator oil having flowed into a refrigerant circuit to a compressor is performed. For example, Patent Document 2 discloses a technique in which an excessive increase in a refrigerant pressure can be inhibited by increasing a capacity of the compressor during a heating operation, increasing a circulation amount of a refrigerant and reducing an air volume of a fan on the side of a heat source and thus the oil return operation can be stably performed even during the heating operation.

[Citation List]

[Patent Documents]

[0005] 

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2003-185286

[Patent Document 2] Japanese Patent No. 2760218

[0006] Generally, in the oil return operation, a frequency of the compressor is increased, the circulation amount of the refrigerant is increased, and thus a flow rate of the refrigerant in a pipe is controlled such that it maintained at a constant speed or more necessary for recovering the refrigerator oil. However, when the frequency of the compressor is increased while an outlet side temperature of the condenser is maintained at a predetermined target temperature during the heating operation, there is a problem that the heating capacity may become excessive, or the like.

SUMMARY OF THE INVENTION

[0007] Accordingly, it is an object of the present invention to provide a control device, a refrigerant circuit system, and a control method which are capable of solving the above-described problems.

[0008]  A first aspect of the present invention is a control device which controls a refrigerant circuit system, wherein an enthalpy difference between an inlet side and an outlet side of a condenser provided in the refrigerant circuit system and a refrigerant circulation amount flowing in the refrigerant circuit system are controlled so that a difference between a heating capacity of the refrigerant circuit system during an oil return operation in a heating cycle and that of the refrigerant circuit system during normal operation is within a predetermined range.

[0009] The control device in a second aspect of the present invention may set an enthalpy on the outlet side of the condenser to be higher than that in the normal operation by maintaining the enthalpy on the inlet side of the condenser at a predetermined target value and setting a target temperature on the outlet side of the condenser in the oil return operation to a temperature higher than that in the normal operation.

[0010] In the control device in a third aspect of the present invention, the enthalpy difference of the condenser during the oil return operation may be calculated so that a product of a frequency of a compressor provided in the refrigerant circuit system and the enthalpy difference during the normal operation is the same as that during the oil return operation, and an outlet side temperature of the condenser according to the calculated enthalpy difference may be set as the target temperature during the oil return operation, and the outlet side temperature of the condenser may be controlled such that it becomes the target temperature.

[0011] The control device in a fourth aspect of the present invention may control an opening degree of an expansion valve provided on the outlet side of the condenser on the basis of the target temperature on the outlet side of the condenser.

[0012] A refrigerant circuit system in a fifth aspect of the present invention may include a main flow circuit in which a plurality of compressors configured to compress a refrigerant, a condenser configured to condense the compressed refrigerant, a first expansion valve configured to depressurize the condensed refrigerant, a receiver configured to store some of the refrigerant depressurized in the first expansion valve, a second expansion valve configured to depressurize the refrigerant flowing out from the receiver, and an evaporator configured to evaporate the refrigerant depressurized in the second expansion valve are connected; and an injection circuit which causes divergence of some of the refrigerant flowing out from the receiver and then supplies the diverged refrigerant to a suction side of a predetermined one of the plurality of compressors and includes a third expansion valve configured to depressurize the branched refrigerant, and an intermediate heat exchanger configured to exchange heat between the refrigerant passing through the third expansion valve and the refrigerant passing through the main flow circuit, and the control device may set the target temperature on the outlet side of the condenser during the normal operation to a temperature higher than the temperature of a utilization-side medium flowing into the condenser by a predetermined temperature and may set the target temperature on the outlet side of the condenser during the oil return operation to a temperature higher than the target temperature in the normal operation.

[0013] A sixth aspect of the present invention is a refrigerant circuit system including a main flow circuit in which a plurality of compressors configured to compress a refrigerant, a condenser configured to condense the compressed refrigerant, a first expansion valve configured to depressurize the condensed refrigerant, a receiver configured to store some of the refrigerant depressurized in the first expansion valve, a second expansion valve configured to depressurize the refrigerant flowing out from the receiver, and an evaporator configured to evaporate the refrigerant depressurized in the second expansion valve are connected; an injection circuit which branches some of the refrigerant flowing out from the receiver and then supplies the branched refrigerant to a suction side of a predetermined one of the plurality of compressors and includes a third expansion valve configured to depressurize the branched refrigerant, and an intermediate heat exchanger configured to exchange heat between the refrigerant passing through the third expansion valve and the refrigerant passing through the main flow circuit; and a control device according to any one of the above-described devices.

[0014] A seventh aspect of the present invention is control method, wherein a control device which controls a refrigerant circuit system controls an enthalpy difference between an inlet side and an outlet side of a condenser provided in a refrigerant circuit system and a refrigerant circulation amount flowing in the refrigerant circuit system so that a difference between the heating capacity of the refrigerant circuit system during an oil return operation in a heating cycle and the heating capacity of the refrigerant circuit system in a normal operation is within a predetermined range.

[0015] According to the present invention, the oil return operation can be performed while a heating capacity corresponding to a load is maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] 

FIG. 1 is a view showing an example of a refrigerant circuit system in one embodiment of the present invention.

FIG. 2 is a first P-h diagram of the refrigerant circuit system in one embodiment of the present invention.

FIG. 3 is a second P-h diagram of the refrigerant circuit system in one embodiment of the present invention.

FIG. 4 is a first diagram showing a control method during an oil return operation in one embodiment of the present invention.

FIG. 5 is a second diagram showing the control method during the oil return operation in one embodiment of the present invention.

FIG. 6 is a diagram showing an effect of an oil return operation in one embodiment of the present invention.

FIG. 7 is a flowchart of a control device in one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

<Embodiment>

[0017] Hereinafter, a refrigerant circuit system according to one embodiment of the present invention will be described with reference to FIGS. 1 to 7.

[0018] FIG. 1 is a view showing an example of a refrigerant circuit system in one embodiment of the present invention.

[0019] A refrigerant circuit system 1 is used in a water heater or the like and constitutes a refrigerant circuit which is operated in a heating cycle (warming cycle). The refrigerant circuit system 1 increases the amount of supply water (utilization-side medium) supplied from an outside to a predetermined set target temperature (for example, 80°C) and then supplies the hot water to a user. In the embodiment, the set target temperature (hot water temperature) of the feed water supplied to the user is constant. The set target temperature of the feed water is referred to as a utilization-side outlet temperature. Also, the temperature of the feed water supplied from the outside is referred to as a utilization-side inlet temperature.

[0020] As shown in FIG. 1, the refrigerant circuit system 1 includes a main flow circuit including a high stage-side compressor 10A, a low stage-side compressor 10B, a utilization-side heat exchanger (condenser) 11, a first expansion valve 12, a receiver 13, a second expansion valve 14, a heat source-side heat exchanger (evaporator) 15, an accumulator 16 and a main flow pipe 17 which connects the above-described elements, an injection circuit including an injection pipe 20, a third expansion valve 21 and an intermediate heat exchanger 22, and a control device 100.

[0021] Further, a specific constitution of the refrigerant circuit system 1 shown in FIG. 1 schematically shows a basic constitution of the refrigerant circuit system 1, and may further include other elements.

[0022] The high stage-side compressor 10A and the low stage-side compressor 10B compress a refrigerant and discharge a high pressure refrigerant. The high stage-side compressor 10A and the low stage-side compressor 10B are connected in series. A suction side of the low stage-side compressor 10B is connected to the accumulator 16. Further, a discharge side of the low stage-side compressor 10B is connected to a suction side of the high stage-side compressor 10A. The low stage-side compressor 10B suctions and compresses a low pressure refrigerant supplied from the accumulator 16 and discharges an intermediate pressure refrigerant to the high stage-side compressor 10A side. Further, the injection pipe 20 is connected to the suction side of the high stage-side compressor 10A, and the intermediate pressure refrigerant is supplied from the injection pipe 20 as described later.

[0023] Frequencies of the high stage-side compressor 10A and the low stage-side compressor 10B are controlled through the control device 100 by an inverter circuit. In the embodiment, the frequency of the high stage-side compressor 10A is controlled by the control device 100 so that a discharge pressure saturation temperature of the high stage-side compressor 10A becomes a predetermined temperature (for example, 82°C) corresponding to a preset utilization-side outlet temperature (for example, 80°C). As described above, in the embodiment, a target high pressure is determined according to the utilization-side outlet temperature, and a value thereof is controlled to be constant, regardless of a change in the utilization-side inlet temperature. The high temperature and high pressure refrigerant discharged from the high stage-side compressor 10A is supplied to the utilization-side heat exchanger 11.

[0024] The utilization-side heat exchanger 11 serves as a condenser. The high pressure refrigerant supplied to the utilization-side heat exchanger 11 exchanges heat with the feed water used by the user, radiates heat and is condensed and liquefied. Meanwhile, the feed water supplied to the utilization-side heat exchanger 11 absorbs heat from the high pressure refrigerant, is raised to the predetermined set target temperature (utilization-side outlet temperature) and is then provided to the user. Further, in the drawing, a dotted arrow indicates a flow direction of feed water, and a solid arrow indicates the flow direction (heating cycle) of the refrigerant.

[0025] The first expansion valve 12 is a flow control valve which depressurizes the refrigerant. The high pressure refrigerant after the heat exchange with the utilization-side heat exchanger 11 is decompressed and expanded by the first expansion valve 12 and is supplied to the receiver 13. An opening degree of the first expansion valve 12 is controlled by the control device 100. The control device 100 controls the opening degree of the first expansion valve 12 so that the outlet side temperature of the utilization-side heat exchanger 11 becomes a predetermined value. For example, during a normal operation, the control device 100 controls the outlet side temperature of the utilization-side heat exchanger 11 such that it is higher than the utilization-side inlet temperature by a predetermined value (for example, 2°C). For example, when the utilization-side inlet temperature is 5°C, the set target temperature of the outlet side temperature of the utilization-side heat exchanger 11 is 7°C. Further, during an oil return operation, the control device 100 controls the opening degree of the first expansion valve 12 so that the outlet side temperature of the utilization-side heat exchanger 11 has a temperature, at which the heating capacity of the refrigerant circuit system 1 is equal to that during the normal operation, as a target temperature.

[0026] The receiver 13 is a pressure container for temporarily storing some of the supplied refrigerant. In the receiver 13, a two-phase refrigerant in which a gas and a liquid are mixed is stored. The first expansion valve 12 is provided on an upstream side of the receiver 13, and a branch to the injection pipe 20, the intermediate heat exchanger 22 and the second expansion valve 14 are respectively provided on a downstream side thereof. Some of the refrigerant flowing out from the receiver 13 is caused to diverge to the injection pipe 20, and the remaining refrigerant flows through the main flow circuit.

[0027] The second expansion valve 14 is a flow control valve which depressurizes the refrigerant. The liquid refrigerant flowing in the main flow circuit is cooled in the intermediate heat exchanger 22 by heat exchange with some of the refrigerant flowing in (caused to diverge to) the injection pipe 20, is decompressed and expanded in the second expansion valve 14 and becomes a low pressure refrigerant.

[0028] The heat source-side heat exchanger 15 serves as an evaporator. The heat source-side heat exchanger 15 evaporates the low pressure refrigerant introduced from the second expansion valve 14 by heat absorption from a heat source such as external air. The refrigerant passed through the heat source-side heat exchanger 15 is supplied to the accumulator 16. The refrigerant is separated into gas and liquid by the accumulator 16, and only the gas refrigerant is suctioned into the low stage-side compressor 10B. The low stage-side compressor 10B compresses the refrigerant and discharges the compressed refrigerant to the high stage-side compressor 10A side.

[0029] Meanwhile, the refrigerant caused to diverge on the downstream side of the receiver 13 is supplied to the suction side of the high stage-side compressor 10A via the injection pipe 20. The third expansion valve 21 and the intermediate heat exchanger 22 are provided at the injection pipe 20.

[0030] The third expansion valve 21 is a flow control valve which depressurizes part of the diverged refrigerant.

[0031] The intermediate heat exchanger 22 exchanges heat between the refrigerant passing through the third expansion valve 21 and the refrigerant passing through the main flow pipe 17. The refrigerant depressurized by the third expansion valve 21 is heated by the heat exchange in the intermediate heat exchanger 22, is returned to the high stage-side compressor 10A and is then recompressed. Using the injection circuit, the COP of a refrigeration cycle can be improved as is well known.

[0032] The control device 100 is a computer device such as a microcomputer. For example, as described above, the control device 100 controls devices constituting the refrigerant circuit such as the high stage-side compressor 10A and the first expansion valve 12. Particularly, in the embodiment, the control device 100 controls the outlet side temperature of the utilization-side heat exchanger 11 so that, regarding an enthalpy difference between an inlet and an outlet of the utilization-side heat exchanger 11, the ratio of the enthalpy difference during the oil return operation to the enthalpy difference during the normal operation is a reciprocal of the increase rate of the refrigerant circulation amount due to the increase in the frequencies of the high stage-side compressor 10A and the low stage-side compressor 10B at the time of switching from the normal operation to the oil return operation. Specifically, the control device 100 controls the opening degree of the first expansion valve 12 so that the outlet side temperature of the utilization-side heat exchanger 11 becomes the target temperature.

[0033] Further, a detection unit such as a temperature sensor and a pressure sensor is installed in the refrigerant circuit system 1. For example, a temperature sensor 31 is provided at the inlet of the utilization-side heat exchanger 11 through which the feed water (utilization-side medium) passes. The temperature sensor 31 measures the utilization-side inlet temperature. Also, a temperature sensor 33 is provided on the outlet side of the utilization-side heat exchanger 11. The temperature sensor 33 measures a temperature of the refrigerant on the outlet side of the utilization-side heat exchanger 11. The temperature sensor 31 and the temperature sensor 33 output information on the measured temperatures to the control device 100. Also, a pressure sensor 32 is provided on the inlet side of the utilization-side heat exchanger 11. The pressure sensor 32 measures a pressure of the refrigerant on the inlet side of the utilization-side heat exchanger 11. The pressure sensor 32 outputs information on the measured pressure to the control device 100. Next, a change in the heating capacity during each of the normal operation and the oil return operation will be described using the refrigeration cycle in the refrigerant circuit system 1 shown in FIG. 1.

[0034] FIG. 2 is a first P-h diagram of the refrigerant circuit system in one embodiment of the present invention. FIG. 2 is a diagram of the relationship between the pressure and the enthalpy which indicates the refrigeration cycle when the refrigerant circuit system 1 is operated. In the P-h diagram of FIG. 2, each symbol indicates the following state. That is, A1 indicates the state of the refrigerant discharged from the high stage-side compressor 10A, A2 indicates the state of the refrigerant on the outlet side of the utilization-side heat exchanger 11, A3 indicates the state of the refrigerant on the outlet side of the first expansion valve 12, A4 indicates the state of the refrigerant on the inlet side of the second expansion valve 14, A5 indicates the state of the refrigerant on the outlet side of the second expansion valve 14, A6 indicates the state of the refrigerant on the outlet side of the heat source-side heat exchanger 15, A7 indicates the state of the refrigerant discharged from the low stage-side compressor 10 B, A8 indicates the state of the refrigerant on the outlet side of the third expansion valve 21, and A9 indicates the state of the refrigerant on the suction side of the high stage-side compressor 10A.

[0035] The high temperature and high pressure refrigerant (state A1) discharged from the high stage-side compressor 10A radiates heat in the utilization-side heat exchanger 11, is condensed and liquefied and becomes a high pressure liquid refrigerant (state A2). In the embodiment, by making the outlet side temperature of the utilization-side heat exchanger 11 close to the utilization-side outlet temperature, the enthalpy difference is increased, the heating capacity is increased, and thus the COP is increased. The high pressure liquid refrigerant becomes a refrigerant (state A3) which is depressurized and expanded by the first expansion valve 12 and then flows into the receiver 13. In the refrigerant flowing out from the receiver 13, the refrigerant flowing through the main flow circuit is cooled in the intermediate heat exchanger 22 (state A4) and reaches the second expansion valve 14. Additionally, the refrigerant is further depressurized by the second expansion valve 14 (state A5), flows into the heat source-side heat exchanger 15 to be evaporated and thus becomes a low pressure gas refrigerant (state A6). The low pressure gas refrigerant is pressurized to a predetermined intermediate pressure by the low stage-side compressor 10B (state A7) and is supplied to the suction side of the high stage-side compressor 10A. Meanwhile, the refrigerant caused to diverge into the injection circuit is depressurized to a predetermined intermediate pressure by the third expansion valve 21 (state A8), absorbs heat through the intermediate heat exchanger 22 and is supplied to the suction side of the high stage-side compressor 10A through the injection pipe 20. Here, an intermediate pressure refrigerant in which the refrigerant supplied through the injection pipe 20 and the refrigerant compressed by the low stage-side compressor 10B are mixed and the temperature thereof is reduced (state A9), is supplied to the high stage-side compressor 10A. The high stage-side compressor 10A compresses the intermediate pressure refrigerant and discharges a high temperature and high pressure refrigerant (state A1). After that, the same cycle is repeated.

[0036] In FIG. 2, the enthalpy difference of the refrigerant passed through the utilization-side heat exchanger 11 in the inlet (state A1) and the outlet (state A2) is indicated by Δi in the drawing. The heating capacity Q of the refrigeration cycle is expressed by the product of the enthalpy difference Δi and the refrigerant circulation amount Gr.

[0037] Here, assuming that the refrigerant circulation amount Gr is the refrigerant circulation amount in the normal operation, the heating capacity Q in the normal operation can be calculated by the above-described Equation (1).

[0038] Next, the heating capacity Qα when the oil control operation is performed by a conventional control method will be described. Generally, during the oil return operation, the frequency of the compressor is increased so that the flow rate of the refrigerant becomes a predetermined speed or more (for example, flooding speed, zero penetration speed, or the like). When the frequency of the compressor is increased, the refrigerant circulation amount Gr' is also increased. Therefore, when the oil return operation is performed at the same operation point as in the refrigeration cycle shown in FIG. 2, the heating capacity Qα in that case can be calculated by the following Equation (2).

[0039] Here, since the enthalpy difference Δi is equivalent to that during the normal operation, and the refrigerant circulation amount Gr' is increased due to the increase in the frequencies of the high stage-side compressor 10A and the low stage-side compressor 10B, the heating capacity Qα during the oil return operation is a value (Qα> Q) which is larger than the heating capacity Q in the normal operation. Accordingly, a problem such as excessive heating capacity with respect to the load and an increase in the high pressure due to the increase in the frequencies of the high stage-side compressor 10A and the low stage-side compressor 10B occurs. Next, a method of controlling the oil return operation in the embodiment which avoids such a problem will be described.

[0040] FIG. 3 is a second P-h diagram of the refrigerant circuit system in one embodiment of the present invention. In the P-h diagram of FIG. 3, a state indicated by each symbol is the same as that described in FIG. 2. That is, B1 indicates the state of the refrigerant discharged from the high stage-side compressor 10A, B2 indicates the state of the refrigerant on the outlet side of the utilization-side heat exchanger 11, B3 indicates the state of the refrigerant on the outlet side of the first expansion valve 12, B4 indicates the state of the refrigerant on the inlet side of the second expansion valve 14, B5 indicates the state of the refrigerant on the outlet side of the second expansion valve 14, B6 indicates the state of the refrigerant on the outlet side of the heat source-side heat exchanger 15, B7 indicates the state of the refrigerant discharged from the low stage-side compressor 10 B, B8 indicates the state of the refrigerant on the outlet side of the third expansion valve 21, and B9 indicates the state of the refrigerant on the suction side of the high stage-side compressor 10A. Further, since a corresponding relation between the flow of the refrigerant in the refrigerant circuit and the states B1 to B9 is the same as that described in FIG. 2, a description thereof will be omitted.

[0041] Next, a control method of the oil return operation according to the embodiment will be described with reference to FIG. 3. Also in the oil return operation of the embodiment, the pressure and the temperature on a discharge side of the high stage-side compressor 10A are controlled to the same pressure and temperature as those in the normal operation (state B1). Therefore, the enthalpy equal to that during normal operation is maintained on the inlet side of the utilization-side heat exchanger 11. However, the target temperature on the outlet side of the utilization-side heat exchanger 11 is set to be higher than that during the normal operation (state B2). Therefore, the enthalpy difference Δi' between the inlet side and the outlet side of the utilization-side heat exchanger 11 during the oil return operation is smaller than the enthalpy difference Δi during the normal operation (Δi' <Δi). Further, since the pressure difference between the upstream side (state B3) and the downstream side (state B8) of the third expansion valve 21 provided in the injection circuit is large, the refrigerant circulation amount flowing into the injection circuit is increased compared to that during the normal operation, and the refrigerant circulation amount in the main circuit is reduced accordingly.

[0042] Here, the heating capacity Q' in the oil return operation of the embodiment can be calculated by the following Equation (3).

[0043] As described above, the enthalpy difference Δi' is smaller than that during the normal operation (Δi), and the refrigerant circulation amount Gr' is increased due to the increase in the frequencies of the high stage-side compressor 10A and the low stage-side compressor 10B. Therefore, the heating capacity Q' during the oil return operation has a value smaller than the above-described Qα, and an increase amount from the heating capacity Q during the normal operation becomes small. Accordingly, excessive heating capacity during the oil return operation can be inhibited. In addition, since the supercooling area of the utilization-side heat exchanger 11 is reduced, an increase in the high pressure can be inhibited.

[0044] Next, a control method for maintaining the heating capacity Q' during the oil return operation in the embodiment at a level equivalent to the heating capacity Q during the normal operation will be described.

[0045] FIG. 4 is a first diagram showing a control method during the oil return operation in one embodiment of the present invention.

[0046] In the graph shown in FIG. 4, a vertical axis indicates the enthalpy difference, and a horizontal axis indicates the refrigerant circulation amount. The graph shown in FIG. 4 shows a relationship between the refrigerant circulation amount flowing through the refrigerant circuit system 1 realized by the control device 100 of the embodiment and the enthalpy difference between the inlet side and the outlet side in the utilization-side heat exchanger 11. The graph shown in FIG. 4 shows that the product of the refrigerant circulation amount and the enthalpy difference is always a constant value (the refrigerant circulation amount and the enthalpy difference are inversely proportional to each other). That is, the ratio of the enthalpy difference during the normal operation to the enthalpy difference during the oil return operation is a reciprocal of the ratio of the refrigerant circulation amount during the normal operation to the refrigerant circulation amount during the oil return operation. For example, when the refrigerant circulation amount during the oil return operation is 1.2 times that in the normal operation, the enthalpy difference during the oil return operation is 1 / 1.2 times that during the normal operation time.

[0047] As described above, the control device 100 controls the enthalpy difference during the oil return operation so that the product of the refrigerant circulation amount and the enthalpy difference during the normal operation and the product of the refrigerant circulation amount and the enthalpy difference during the oil return operation coincide with each other. In order to recover the refrigerator oil, it is necessary to set the flow rate of the refrigerant to a certain speed or more, and thus in the embodiment, the control device 100 also performs control to increase the frequencies of the high stage-side compressor 10A and the low stage-side compressor 10B during the oil return operation, as in the conventional method. Since the refrigerant circulation amount is increased substantially in proportion to the increase in the frequency of the compressor, the refrigerant circulation amount is increased similarly to that in the conventional oil returning operation by the frequency control of the high stage-side compressor 10A and the low stage-side compressor 10B. Accordingly, the control device 100 performs the control to reduce the enthalpy difference in order to maintain the relationship shown in the graph of FIG. 4. In the embodiment, since an enthalpy on the inlet side of the utilization-side heat exchanger 11 is constant, the enthalpy difference between the inlet side and the outlet side of the utilization-side heat exchanger 11 is reduced by increasing the enthalpy on the outlet side thereof. Specifically, the control device 100 raises the target temperature on the outlet side of the utilization-side heat exchanger 11 and controls the opening degree of the first expansion valve 12 to aim for the target temperature.

[0048] FIG. 5 is a second diagram showing the control method during the oil return operation in one embodiment of the present invention.

[0049] In the graph shown in FIG. 5, the vertical axis indicates the refrigerant circulation amount, and the horizontal axis indicates the temperature on the outlet side of the condenser. The graph shown in FIG. 5 shows a relationship between an outlet side temperature of the utilization-side heat exchanger 11 and the refrigerant circulation amount for realizing the product relationship between the enthalpy difference and the refrigerant circulation amount described in FIG. 4. Further, since the refrigerant circulation amount discharged from the high stage-side compressor 10A is increased in proportion to the increase in frequency of the high stage-side compressor 10A or the like, the relationship between the frequency of the high stage-side compressor 10A or the like and the outlet side temperature of the utilization-side heat exchanger 11 can be shown by the same graph as that of FIG. 5.

[0050] The control device 100 stores a table for conversion between the refrigerant circulation amount and the outlet side temperature of the utilization-side heat exchanger 11 shown in FIG. 5 or a conversion table between the frequency of the high stage-side compressor 10A or the like and the outlet side temperature of the utilization-side heat exchanger 11 in a built-in memory part (not shown). The conversion tables are prepared in advance by calculation, experiment or the like according to a function or the like showing the relationship between the enthalpy in the utilization-side heat exchanger 11, the temperature of the refrigerant and the pressure of the refrigerant and are recorded in the memory part. Additionally, when the oil return operation starts, the control device 100 calculates the refrigerant circulation amount Gr' in the oil return operation by a known method and calculates an outlet side temperature T' of the utilization-side heat exchanger 11 corresponding to the refrigerant circulation amount Gr' with reference to the conversion table shown in FIG. 5. The calculated outlet side temperature T' is the target temperature on the outlet side of the utilization-side heat exchanger 11 in the oil return operation. The control device 100 controls the opening degree of the first expansion valve 12 so that the outlet side temperature of the utilization-side heat exchanger 11 becomes T'. Then, the product (heating capacity Q') of the refrigerant circulation amount Gr' and the enthalpy difference Δi' at that time has the same value as that of the product (heating capacity Q) of the refrigerant circulation amount Gr and the enthalpy difference Δi during the normal operation. By performing control as described above, the oil return operation can be performed while the heating capacity during the normal operation is maintained.

[0051] FIG. 6 is a diagram showing an effect of the oil return operation in one embodiment of the present invention.

[0052] A lower diagram of FIG. 6 shows the refrigerant circulation amount in the normal operation and the oil return operation. The refrigerant circulation amount during the normal operation is indicated by a square mark, the refrigerant circulation amount during the oil return operation according to the conventional control method is indicated by a triangle mark, and the refrigerant circulation amount during the oil return operation according to the control method of the embodiment is indicated by a circle mark. Further, in the drawing described below, it is also assumed that an operation state of the refrigerant circuit system 1 corresponding to the square mark, the triangle mark and the circle mark is the same. As shown in the lower diagram of FIG. 6, to circulate the refrigerant at the flow rate necessary for recovering the refrigerating machine oil both in the conventional oil returning operation and the oil returning operation in the present embodiment, the refrigerant circulation amount during the oil return operation is larger than that during normal operation.

[0053] A middle diagram of FIG. 6 shows the enthalpy difference between normal operation and the oil return operation. As shown in the middle diagram of FIG. 6, the enthalpy difference during the normal operation is equal to the enthalpy difference during the oil return operation according to the conventional control method, but the enthalpy difference is reduced in the oil return operation according to the control method of the embodiment.

[0054] An upper diagram of FIG. 6 shows the heating capability in the normal operation and the oil return operation. As shown in the upper diagram of FIG. 6, the heating capacity in the normal operation is equal to that in the oil return operation according to the control method of the embodiment without any change. Meanwhile, in the oil return operation according to the conventional control method, the heating capacity is increased as compared with this.

[0055] Next, a processing flow in the oil return operation will be described with reference to FIG. 7.

[0056] FIG. 7 is a flowchart of a control device in one embodiment of the present invention.

[0057] As a premise, the control device 100 acquires information on the temperature measured by the temperature sensor 31 and the temperature sensor 33 and information on the pressure measured by the pressure sensor 32 at predetermined time intervals and stores the information on the acquired temperatures and pressures in the built-in memory part (not shown). Further, the conversion table for the refrigerant circulation amount and the outlet side temperature of the condenser (or the conversion table for the frequency of the high stage-side compressor 10A or the like and the temperature on the outlet side of the condenser) are recorded in the memory part (not shown) of the control device 100. Furthermore, the control device 100 controls each of the frequencies so that the pressure (value measured by the pressure sensor 32) on the discharge side of the high stage side compressor 10A becomes a pressure (for example, a pressure at which a temperature higher by 2°C than the utilization-side outlet temperature is set as the saturation temperature) according to the utilization-side outlet temperature and the high stage-side compressor 10A and the low stage-side compressor 10B have a uniform pressure ratio. Further, the control device 100 controls the opening degree of the first expansion valve 12 so that the temperature on the outlet side of the utilization-side heat exchanger 11 (measured value by the temperature sensor 33) becomes a target temperature which is higher than the utilization-side inlet temperature (measured value by the temperature sensor 31) by a predetermined temperature (for example, 2°C). Also, for example, it is assumed that the control device 100 starts the oil return operation on the basis of predetermined conditions, for example, a cumulative execution time of the normal operation after the previous oil return operation becoming a threshold value or more, being satisfied.

[0058] First, the control device 100 increases the frequencies of the two compressors (the high stage-side compressor 10A and the low stage-side compressor 10B) (Step S11). For example, the frequency at which the flow rate of the refrigerant flowing through the pipe becomes a flow rate necessary for recovering the refrigerator oil is predetermined for each compressor and is recorded in the memory part, and the control device 100 increases the frequencies of the high stage-side compressor 10A and the low stage-side compressor 10B according to this information. Then, the control device 100 calculates the outlet side temperature of the condenser (Step S12). The outlet side temperature of the condenser is the target temperature on the outlet side of the utilization-side heat exchanger 11 which is calculated by making the product of the enthalpy difference Δi and the refrigerant circulation amount Gr (or the frequency of the high stage-side compressor 10 A or the like may be used) during normal operation equivalent to the product of the enthalpy difference Δi' and the refrigerant circulation amount Gr' during the oil return operation. The control device 100 calculates the outlet side temperature of the condenser with reference to the conversion table shown in FIG. 5 by using the frequency of each compressor or the refrigerant circulation amount calculated in Step S11.

[0059] Next, the control device 100 controls the opening degree of the first expansion valve 12 so that the outlet side temperature of the utilization-side heat exchanger 11 becomes the outlet side temperature of the condenser which is calculated in Step S12 (Step S13). For example, a data table or the like in which the opening degree of the valve is set according to a difference between the temperature measured by the temperature sensor 33 and the target temperature may be recorded in the memory part for each target temperature, and thus the control device 100 may adjust the opening degree of the first expansion valve 12 on the basis of the data table. Accordingly, the enthalpy difference between the inlet side and the outlet side of the utilization-side heat exchanger 11 can be a target enthalpy difference, and the heating capacity during the oil return operation can be equal to that in the normal operation.

[0060] Further, the method of calculating the outlet side temperature of the condenser in Step S12 is not limited to the calculation method according to the conversion table exemplified in FIG. 5. For example, even if the product of the refrigerant circulation amount and the enthalpy difference during the normal operation does not coincide with the product of the refrigerant circulation amount and the enthalpy difference during the oil return operation, control may be performed when an arbitrary temperature at which a difference between them is within a predetermined range is set as the outlet side temperature of the condenser. Therefore, excessive capacity can be alleviated in the oil return operation.

[0061] According to the embodiment, the control device 100 controls the enthalpy difference between the inlet side and the outlet side of the utilization-side heat exchanger 11 and the frequencies of the high stage-side compressor 10A and the low stage-side compressor 10B so that the difference between the heating capacity in the oil return operation of the heating cycle and the heating capacity in the normal operation is within a predetermined range or the heating capacity in the oil return operation and the heating capacity in the normal operation are equal to each other. More specifically, the control device 100 keeps the enthalpy on the inlet side of the utilization-side heat exchanger 11 at a predetermined target value and sets the enthalpy to be high during the oil return operation by setting the target temperature on the outlet side of the utilization-side heat exchanger 11 to a high temperature side. Therefore, the enthalpy difference becomes smaller than that in the normal operation, and thus the increase in the refrigerant circulation amount due to the increase in the frequency of the high stage-side compressor 10A or the like is offset. Further, preferably, the enthalpy difference in the oil return operation in which the product of the frequency of the high stage-side compressor 10A and the like and the enthalpy difference in the normal operation is the same as that in the oil return operation may be calculated, and the temperature on the outlet side of the utilization-side heat exchanger 11 according to the calculated enthalpy difference may be set as the target temperature. Then, even during the oil return operation, the heating capacity corresponding to the load can be maintained as in the normal operation. Accordingly, conventional problems such as excessive capacity which tends to occur during the oil return operation in the heating cycle, temporary stopping of the compressor due to an operation of high pressure protection control in response to an increase to a high pressure, consequential temperature fluctuation, and deterioration of feeling can be prevented.

[0062] In addition, it is possible to substitute the elements in the above-described embodiment with well-known elements within a scope not deviating from the gist of the present invention. Further, the technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

EXPLANATION OF REFERENCES

[0063] 

1 Refrigerant circuit system

10A High stage-side compressor

10B Low stage-side compressor

11 Utilization-side heat exchanger

12 First expansion valve

13 Receiver

14 Second expansion valve

15 Heat source-side heat exchanger

16 Accumulator

17 Main flow pipe

20 Injection pipe

21 Third expansion valve

22 Intermediate heat exchanger

31,33 Temperature sensor

32 Pressure sensor

100 Control device

Claims

1. A control device (100) which controls a refrigerant circuit system (1), wherein an enthalpy difference (Δi, Δi') between an inlet side and an outlet side of a condenser (11) provided in the refrigerant circuit system (1) and a refrigerant circulation amount (Gr, Gr') flowing in the refrigerant circuit system (1) are controlled so that a difference between a heating capacity of the refrigerant circuit system during an oil return operation in a heating cycle (Q') and that of the refrigerant circuit system during normal operation (Q) is within a predetermined range.

2. The control device (100) according to claim 1, wherein an enthalpy on the outlet side of the condenser (11) is set higher than that in the normal operation by maintaining the enthalpy on the inlet side of the condenser (11) at a predetermined target value and setting a target temperature on the outlet side of the condenser (11) in the oil return operation to a temperature higher than that in the normal operation.

3. The control device (100) according to claim 1 or 2, wherein the enthalpy difference (Δi') of the condenser (11) during the oil return operation is calculated so that a product of a frequency of a compressor (10A, 10B) provided in the refrigerant circuit system (11) and the enthalpy difference (Δi) during the normal operation is the same as that during the oil return operation, and an outlet side temperature (T') of the condenser (11) according to the calculated enthalpy difference is set as the target temperature during the oil return operation, and the outlet side temperature of the condenser (11) is controlled to be the target temperature.

4. The control device (100) according to claim 3, wherein an opening degree of an expansion valve (12) provided on the outlet side of the condenser (11) is controlled on the basis of the target temperature on the outlet side of the condenser (11).

5. The control device (100) according to any one of claims 1 to 4, wherein the refrigerant circuit system (1) includes a main flow circuit in which a plurality of compressors (10A, 10B) configured to compress a refrigerant, a condenser (11) configured to condense the compressed refrigerant, a first expansion valve (12) configured to depressurize the condensed refrigerant, a receiver (13) configured to store some of the refrigerant depressurized in the first expansion valve (12), a second expansion valve (14) configured to depressurize the refrigerant flowing out from the receiver (13), and an evaporator (15) configured to evaporate the refrigerant depressurized in the second expansion valve (14) are connected; and an injection circuit which branches some of the refrigerant flowing out from the receiver (14) and then supplies the branched refrigerant to a suction side of a predetermined one of the plurality of compressors (10A, 10B) and includes a third expansion valve (21) configured to depressurize the branched refrigerant, and an intermediate heat exchanger (22) configured to exchange heat between the refrigerant passing through the third expansion valve (21) and the refrigerant passing through the main flow circuit, and

during the normal operation, the target temperature on the outlet side of the condenser (11) is set to a temperature higher than the temperature of a utilization-side medium flowing into the condenser by a predetermined temperature, and

during the oil return operation, the target temperature on the outlet side of the condenser (11) is set to a temperature higher than the target temperature in the normal operation.

6. A refrigerant circuit system (1) comprising:

a main flow circuit in which a plurality of compressors (10A, 10B) configured to compress a refrigerant, a condenser (11) configured to condense the compressed refrigerant, a first expansion valve (12) configured to depressurize the condensed refrigerant, a receiver (13) configured to store some of the refrigerant depressurized in the first expansion valve (12), a second expansion valve (14) configured to depressurize the refrigerant flowing out from the receiver, and an evaporator (15) configured to evaporate the refrigerant depressurized in the second expansion valve (14) are connected;

an injection circuit which branches some of the refrigerant flowing out from the receiver and then supplies the branched refrigerant to a suction side of a predetermined one of the plurality of compressors (10A, 10B) and includes a third expansion valve (21) configured to depressurize the branched refrigerant, and an intermediate heat exchanger (22) configured to exchange heat between the refrigerant passing through the third expansion valve (21) and the refrigerant passing through the main flow circuit; and

a control device (100) according to any one of claims 1 to 5.

7. A control method, wherein a control device (100) which controls a refrigerant circuit system (1) controls an enthalpy difference (Δi, Δi') between an inlet side and an outlet side of a condenser (11) provided in a refrigerant circuit system (1) and a refrigerant circulation amount (Gr, Gr') flowing in the refrigerant circuit system (1) so that a difference between the heating capacity of the refrigerant circuit system during an oil return operation in a heating cycle (Q') and the heating capacity of the refrigerant circuit system in a normal operation (Q) is within a predetermined range.

Drawing