WO2000068621A1

WO2000068621A1 - Method of controlling refrigerating cycle and refrigerating cycle using the method

Info

Publication number: WO2000068621A1
Application number: PCT/JP2000/001266
Authority: WO
Inventors: Nobuhiko Suzuki
Original assignee: Zexel Valeo Climate Control Corporation
Priority date: 1999-05-11
Filing date: 2000-03-03
Publication date: 2000-11-16
Also published as: JP2000320910A

Abstract

A refrigerating cycle using carbon dioxide as a refrigerant, wherein, when a high-pressure side refrigerant pressure is near a critical point, the degree of undercooling is set at approx. 15°C and, as it lowers from near the critical point, the degree of undercooling is decreased gradually and, when the refrigerating cycle is provided with a variable displacement compressor, a radiator, an expansion device, an evaporator, and an inside heat exchanger, the operating conditions of the compressor including its delivery amount are adjusted so as to set a refrigerant pressure on the flow-in side of the expansion device at a high level when a refrigerant temperature is the same as that in a refrigerating cycle which has not the inside heat exchanger and a refrigerating cycle which has a fixed compressor delivery capacity and to set the refrigerant temperature at a low level when the refrigerant pressure is the same as that in these cycles, whereby the refrigerating cycle using carbon dioxide as a refrigerant can be operated efficiently.

Description

Description Refrigerating cycle control method and refrigerating cycle using the method

The present invention, supercritical refrigerant as a refrigerant, for example, a method of controlling a refrigeration cycle using carbon dioxide (C0 _2), and to a refrigeration cycle using the method. Background art

As refrigeration cycle using C 0 ₂ as the refrigerant, Hei 6 - 5 1 0 1 1 1 JP Ya Hei 9 - like 2 646 2 2 No. is known.

The former detects the refrigerant temperature at the gas cooler outlet in a supercritical vapor compression refrigeration cycle composed of a compressor, a gas cooler, a throttle valve, and a receiver, and determines the maximum COP based on the detected refrigerant temperature. The throttle valve is adjusted so that it can be obtained.

In the latter case, the temperature of the outlet-side refrigerant and the outlet-side refrigerant of the radiator are controlled along the optimal control line? 7 _max shown in Fig. The pressure is controlled. That is, according to the latter publication, a compressor that raises the pressure of a refrigerant to a supercritical region, a radiator that cools the high-pressure refrigerant that has reached the supercritical region, and an expansion that depressurizes the refrigerant after being cooled by the radiator In a refrigeration cycle including a valve and an evaporator that evaporates the refrigerant decompressed by the expansion valve, the relationship between the refrigerant temperature and the refrigerant pressure at the expansion device inlet side is controlled by controlling the expansion valve opening. It is shown that the refrigeration cycle can be operated efficiently if it is set on the "max line" in Fig. 1 or Fig. 5 of the publication. However, the former technology plans to always operate the refrigeration cycle under supercritical conditions, and is not necessarily superior in terms of efficiency. That is, for example, when the outside air temperature is low, it is more advantageous in terms of efficiency to use the high-pressure pressure under the critical pressure (about 7.38 MPa) or less in terms of efficiency. If it decreases, it will have to be a subcritical operation. According to FIG. 2 of the publication, since the characteristic line is shown over the subcritical region, if the refrigerant temperature at the outlet of the gas cooler can be reduced, the operation of the refrigeration cycle under subcritical conditions can be considered. When the operating point below the critical pressure is read from the figure, the refrigerant temperature at the outlet of the gas cooler is almost equal to the saturation temperature, and the operation should be performed with the supercooling degree (SC: subcool) being almost o ° c. Becomes

Further, according to the former technique, the throttle valve is controlled based on the refrigerant temperature at the outlet of the gas cooler. Therefore, regardless of the electronically controlled throttle valve, the mechanical throttle valve using heat transfer is When used for automobiles, the gas cooler and the throttle valve are located at a considerable distance from each other, so the length of the temperature-sensitive cylinder tube becomes very long, and control appropriate for the refrigerant temperature at the gas cooler outlet is performed. It is practically impossible to perform with high accuracy.

Further, in the latter technology, the operating and maintaining a high COP C 0 to ₂ cycles better is it is that the desired arbitrary to 1 o ° about c Celsius to 1 ° degree of supercooling It is, but this degree of subcooling, when interpreted with reference to the diagram of the optimal control line 7? m _a in FIG. 1 of the publication, if you operate a refrigeration cycle in subcritical conditions, regardless of the pressure We plan to keep the degree of supercooling constant within the range of 1 ° C;

In the conventional refrigeration cycle using a nozzle or the like as a refrigerant, it is operated under subcritical conditions that are considerably lower than the critical pressure in the first place. That the enthalpy does not change much Et al., Although the degree of supercooling is Ru der what suffices is controlled at 0 ° C~ 5 ° C and a small range, in the case of a refrigeration cycle using a C 0 ₂ as refrigerant, the first place the operation under supercritical conditions Average When the refrigerant pressure on the high pressure side is near the critical point, the refrigerant enthalpy at the inlet of the expansion valve greatly changes due to a change in the refrigerant temperature. (Because the interval between the isothermal curves becomes larger near the critical point) Also, as the refrigerant pressure at the high pressure side gradually decreases from the pressure at the critical point, the expansion valve inlet side responds to changes in the refrigerant temperature. It should be taken into account that the change in the refrigerant rupee of the refrigerant becomes smaller (when the refrigerant pressure is lower than the critical pressure, the interval between the isothermal curves becomes smaller).

Therefore, when operating the C 0 ₂ cycles subcritical conditions, as in the conventional technology, to control so that the degree of supercooling of about o ° c, such that a constant at a given temperature It is considered that the optimum efficiency cannot always be obtained depending on the control method described above.

In the refrigeration cycle for the co ₂ refrigerant, the ratio of the cooling capacity of the evaporator for the work of the compressor (hereinafter, referred to as COP or the coefficient of performance) and the refrigerant temperature T of the expansion valve inlet side in order to maximize the The fact that there is a certain relationship between the refrigerant pressure P and the refrigerant pressure P is disclosed in the above-mentioned Japanese Patent Application Laid-Open No. Hei 9-246466, but the relationship between the refrigerant temperature and the refrigerant pressure shown here is shown. Is intended to improve cycle efficiency only by controlling the expansion valve, assuming a cycle without an internal heat exchanger that exchanges heat between the refrigerant flowing out of the evaporator and the refrigerant in the supercritical region of the high pressure line. In addition, this applies only to a refrigeration cycle in which the discharge capacity of the compressor is constant.

C 0 in the refrigeration cycle using a supercritical refrigerant ₂ such as, in order to improve one layer of COP, providing the internal heat exchanger, for example KOKOKU 7 -Although it is a known configuration in 186002, etc., when a refrigeration cycle having such an internal heat exchanger is used, the refrigerant is further cooled by the internal heat exchanger and expanded. Since the refrigerant reaches the valve, the refrigerant temperature on the inflow side of the expansion valve that maximizes the COP is further lowered, and the optimum control characteristic indicated by “max” differs from the actually required control characteristic.

According to the study of the present inventor, in a refrigeration cycle having an internal heat exchanger, if a compressor capable of freely adjusting the discharge amount is used, the discharge capacity of the compressor can be reduced over the course of determining the optimum control point. In addition, it has been found that the conventional configuration that seeks to obtain the optimum control point only by controlling the opening degree of the expansion valve causes a shift of the control point which cannot be predicted. That is, in a refrigeration cycle including an internal heat exchanger and a compressor capable of changing the discharge capacity, the refrigeration cycle is balanced so as to satisfy the relationship between the refrigerant temperature and the refrigerant pressure used in the above-described conventional technology. However, good C 0 P cannot be obtained.

Therefore, in the present invention, in a refrigeration cycle of a supercritical fluid, such as C 0 ₂ refrigerant as refrigerant, as main aims to further improve the cycle efficiency. More specifically, it is an object of the present invention to provide a refrigeration cycle that can be operated efficiently by performing supercooling degree control when operating under subcritical conditions, and a method of controlling the refrigeration cycle. In addition, an object of the present invention is to provide a refrigeration cycle that can prevent a decrease in control accuracy even when an expansion valve that does not rely on electric control using a temperature sensing cylinder or the like is used. In particular, it is another object of the present invention to obtain good cycle efficiency in a refrigeration cycle including an internal heat exchanger and a compressor capable of changing a discharge amount. Disclosure of the invention In order to achieve the above object, the present inventors have conducted intensive studies and found that when operating under subcritical conditions in a refrigeration cycle using a carbon dioxide refrigerant, sufficient efficiency can be obtained at the conventional control point. In order to solve this, it was found that an efficient cycle operation was possible by controlling the degree of supercooling, and the present invention was completed.

That is, in the control of the refrigeration cycle using carbon dioxide gas as the refrigerant, when operating in the subcritical region, if the high-pressure side refrigerant pressure is near the critical point, the degree of supercooling is set to about 15 ° C, and the high-pressure side refrigerant is The supercooling degree is gradually reduced as the pressure becomes lower from the vicinity of the critical point.

Here, the degree of supercooling when the high-pressure side refrigerant pressure is near the critical point is a value obtained as a result of simulation, and an error actually occurs between the calculation result and the actual value. Here, about 15 ° C is expected to be in the range of 10 ° C to 20 ° C. In order to gradually reduce the degree of supercooling as the pressure of the high-pressure refrigerant decreases from the vicinity of the critical point, the degree of supercooling is reduced by about 1 at the high-pressure refrigerant pressure (7.38 MPa) near the critical point. It is conceivable that the temperature is set to 5 ° C and the refrigerant pressure is gradually reduced by, for example, a linear change so that the refrigerant pressure is 3.5 MPa and the degree of supercooling is about 1 ° C.

Therefore, with this configuration, the refrigerant enthalpy at the expansion valve inlet near the critical point greatly changes due to a change in the refrigerant temperature.Therefore, it is possible to increase the refrigeration effect by increasing the degree of subcooling. When the high-pressure side refrigerant pressure gradually decreases from the critical pressure, the change in refrigerant temperature and the refrigerant temperature change gradually decreases, so the required refrigeration effect even if the degree of subcooling is gradually reduced. Can be secured. For this reason, when a carbon dioxide gas cycle normally operated under supercritical conditions is operated under subcritical conditions, the size is controlled by controlling the degree of supercooling as described above. It is possible to further improve the vehicle efficiency.

Also, in order to improve the cycle efficiency of a refrigeration cycle equipped with an internal heat exchanger and a compressor that allows the discharge amount to be changed, compression that enables the change in the refrigerant discharge amount that raises the pressure of the refrigerant to the supercritical range A radiator that cools the refrigerant that has reached the supercritical region; an expansion device that depressurizes the refrigerant after being cooled by the radiator; an evaporator that evaporates the refrigerant depressurized by the expansion device; In controlling a refrigeration cycle including an internal heat exchanger that exchanges heat between the refrigerant flowing out of the evaporator and the refrigerant in the supercritical region, operating conditions including a discharge amount of the compressor are adjusted. The refrigerant temperature on the inflow side of the expansion device and the refrigerant pressure on the inflow side of the expansion device are controlled by a refrigerating cycle having no internal heat exchanger, and a refrigerating cycle having a fixed discharge capacity of the compressor. Compared with the cycle, the refrigerant If the temperature is the same, the refrigerant pressure may be set higher, and if the refrigerant pressure is the same, the refrigerant temperature may be set lower.

More specifically, when the refrigerant temperature on the inflow side of the expansion device is T [° C] and the refrigerant pressure on the inflow side of the expansion device is P [MPa], T and P are expressed by T ≤2.41P + 4.86, T≥2.52P-7.41

Here, the internal heat exchanger only needs to have a refrigerant that exchanges heat with the refrigerant flowing out of the evaporator on a high-pressure line from the compressor to the expansion device. You may make it heat-exchange with a refrigerant | coolant. As the refrigerant, using a carbon dioxide (C 0 ₂₎ refrigerant, the compressor and capable of changing the refrigerant discharge amount, the duty ratio control the energization of the variable displacement compressor and an electromagnetic click latch having a variable displacement mechanism It is preferable to use a compressor that can be controlled, and an electric motor-driven compressor that can control the number of revolutions.

Therefore, the high-temperature and high-pressure refrigerant, which is pressurized by the compressor and becomes supercritical, is cooled by the radiator and flows out of the evaporator due to internal heat exchange The refrigerant is further cooled by the refrigerant, and then decompressed by the expansion device to become low-temperature and low-pressure wet steam, which is vaporized by the evaporator and sent to the compressor after being heated by the refrigerant in the high-pressure line by internal heat exchange. It is boosted again. By adjusting the operating conditions including the discharge amount of the compressor and setting the refrigerant temperature and refrigerant pressure on the inflow side of the expansion device within the above-described range, an internal heat exchanger is provided, and the discharge capacity can be freely varied. Good cycle efficiency can be obtained even in a refrigeration cycle equipped with a compressor.

In contrast to the control methods described above, the refrigeration cycle that implements the former control method includes a compressor that uses carbon dioxide gas as a refrigerant and pressurizes the refrigerant, a radiator that cools the pressurized refrigerant, Pressure adjusting means for reducing the pressure of the refrigerant cooled by the heater, an evaporator for evaporating the refrigerant reduced in pressure by the pressure adjusting means, and detecting means for detecting the refrigerant pressure and the refrigerant temperature upstream of the pressure adjusting means. When the refrigerant pressure is near the critical point based on the detection result of the detection means, the pressure reduction amount of the pressure adjustment means is adjusted so that the degree of supercooling on the inflow side of the pressure adjustment means is about 15 ° C. And control means for controlling the pressure reduction amount of the pressure adjustment means so as to gradually reduce the degree of supercooling on the inflow side of the pressure adjustment means as the refrigerant pressure decreases from near the critical point. Thinking It is.

The refrigeration cycle that realizes the latter control method includes a compressor that boosts the refrigerant to a supercritical region, a radiator that cools the refrigerant that has reached the supercritical region, and a refrigerant that is cooled by the radiator. An expansion device for reducing the pressure of the refrigerant, an evaporator for evaporating the refrigerant decompressed by the expansion device, and an internal heat exchanger for exchanging heat between the refrigerant flowing out of the evaporator and the refrigerant in the supercritical region, The refrigerant discharge amount of the compressor can be changed, and operating conditions including the discharge amount of the compressor are adjusted to adjust the refrigerant temperature on the inflow side of the expansion device and the refrigerant pressure on the inflow side of the expansion device. A refrigeration cycle without the internal heat exchanger; and And, compared to a refrigeration cycle in which the discharge capacity of the compressor is fixed, the refrigerant pressure is set higher if the refrigerant temperature is the same, and the refrigerant temperature is set if the refrigerant pressure is the same. A configuration in which the setting is low can be considered. Here, assuming that the refrigerant temperature at the inflow side of the expansion device is T [° C] and the refrigerant pressure at the inflow side of the expansion device is P [MPa], T and P are defined as T≤2. 4 IP + 4.86, T≥2.52 Ρ-7.41 It should be set within the range enclosed by 1. Brief description of drawings

FIG. 1 is a diagram showing a configuration example of a refrigeration cycle according to the present invention. FIG. 2 is a Mollier chart of a carbon dioxide gas refrigerant. FIG. 3 is a diagram showing a simulation result of a target refrigerant pressure and a refrigerant temperature on the expansion valve inflow side of the refrigeration cycle according to the present invention. FIG. 4 is a flowchart showing an example of a control operation by the control unit of FIG. FIG. 5 is a flowchart illustrating a calculation process of the refrigerant temperature and the refrigerant pressure on the inlet side of the expansion valve for obtaining the maximum C 0 °. FIG. 6 is a characteristic diagram showing the relationship between the refrigerant pressure on the inlet side of the expansion valve and the refrigerant temperature. In FIG. 6, the mark `` x '' represents a plot of the simulation result of the refrigerant pressure and the refrigerant temperature at the expansion valve inlet at the maximum COP in the cycle using the existing components. The symbol “〇” indicates a plot of a simulation result of the refrigerant pressure and the refrigerant temperature at the inlet of the expansion valve, which is a maximum of C 0 P, in a cycle in which the component efficiency is improved. In addition, C in the figure represents a line of T = 24.1P + 4.86, and D represents a line of Τ = 2.52Ρ−7.41. Fig. 7 is a characteristic diagram showing the relationship between the expansion valve opening (or compressor discharge amount), C Ο Ρ, and cooling capacity Q. The broken line is a conventional refrigeration cycle using a fixed displacement compressor. , The solid line is the compression that can change the discharge capacity The present refrigeration cycle using the machine is shown below. FIG. 8 shows a Mollier diagram of a conventional refrigeration cycle using a fixed displacement compressor and a refrigeration cycle using a compressor capable of changing a discharge capacity. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In FIG. 1, a refrigeration cycle 1 includes a compressor 2 for compressing a refrigerant, a radiator 3 for cooling the refrigerant, and an internal heat exchanger 4 for exchanging heat between a refrigerant on a high pressure line and a refrigerant on a low pressure line. It comprises an expansion valve 5 for reducing the pressure of the refrigerant, an evaporator 6 for evaporating and evaporating the refrigerant, and an accumulator 7 for gas-liquid separation of the refrigerant flowing out of the evaporator 6. In this cycle, the discharge side of the compressor 2 is connected via the radiator 3 to the high pressure passage 4 a of the internal heat exchanger 4, the outlet side of the high pressure passage 4 a is connected to the expansion valve 5, and the compressor 2 A high-pressure side line 8 is formed by a path from the pressure to the high-pressure side of the expansion valve 5. The low pressure side of the expansion valve 5 is connected to an evaporator 6, and the outlet side of the evaporator 6 is connected to a low pressure passage 4 b of the internal heat exchanger 4 via an accumulator 7. The outflow side of the low pressure passage 4 is connected to the suction side of the compressor 2, and a path from the outflow side of the expansion valve 5 to the suction side of the compressor 2 forms a low pressure side line 9.

The refrigeration cycle 1, C_〇 ₂ have been used as the refrigerant, the refrigerant compressed by the compressor 2 enters into the radiator 3 as a supercritical refrigerant of high temperature and high pressure is cooled by heat radiation here. Thereafter, the internal heat exchanger 4 exchanges heat with the low-temperature refrigerant in the low-pressure line 9 to be further cooled and sent to the expansion valve 5 without being liquefied. Then, the pressure is reduced in the expansion valve 5 to become low-temperature and low-pressure wet steam, heat exchange with the air passing therethrough in the evaporator 6 to become gaseous, and then the high-temperature side of the high-pressure side line 8 in the internal heat exchanger 4. It is heated by exchanging heat with the refrigerant and returned to the compressor 2. here, The expansion valve 5 controls the signal from the refrigerant temperature sensor 10 for detecting the refrigerant temperature on the expansion valve inlet side and the signal from the pressure sensor 12 for detecting the refrigerant pressure on the expansion valve inlet side. The control unit 11 is input to the control unit 11 to control the opening according to a program given in advance.

Considering the case where the above-described refrigeration cycle 1 is used under subcritical conditions where the high pressure is equal to or lower than the critical pressure (7.38 MPa), in this case, as shown in FIG. The opening of the expansion valve 5 is controlled so that the degree of supercooling indicated by the control line for obtaining the optimum efficiency is obtained.

That is, when the refrigerant pressure at the inlet side of the expansion valve 5 is near the critical pressure, the degree of supercooling at the inlet side of the expansion valve 5 is set to 10 to 20 ° C., preferably about 15; The supercooling degree is gradually reduced as the high pressure side refrigerant pressure becomes lower than the vicinity of the critical pressure.In this configuration example, the refrigerant pressure at the inlet side of the expansion valve 5 is about 3.5 MPa ( The supercooling degree is linearly changed to about 1 ° C when the temperature of the refrigerant flowing into the evaporator 6 becomes about 0 ° C).

Such control-ray is, C 0 ₂ a refrigeration cycle using on to operate in a subcritical region, the present invention as the range for obtaining obtains the maximum or a state close thereto with good cycle efficiency COP Were found based on the following findings and simulations.

If the refrigeration cycle using C 0 _2, although it is common to operate at supercritical region, if it is sought cane operate in a subcritical region Te is critical圧近near odor, as the expansion valve inlet of the above-described Since the ruby refrigerant changes greatly with changes in the refrigerant temperature, the refrigeration effect can be greatly changed even if the subcooling degree is slightly changed, and the size of the subcooling degree must be reduced in order to obtain optimum efficiency. It makes sense to control properly. In contrast, the refrigerant pressure on the high pressure side When the force gradually decreases from the critical pressure, the rate of change of the refrigerant at the inlet of the expansion valve with respect to the change of the refrigerant temperature decreases, so that even if the degree of subcooling is slightly changed, the refrigeration effect is significantly changed. I can no longer do it. For this reason, in a region where the refrigerant pressure on the high-pressure side is low, controlling the degree of supercooling does not have much significance, and it is sufficient to set the degree of supercooling to the same level as before. Therefore, in the refrigeration cycle using carbon dioxide as the refrigerant, the degree of supercooling is increased when the high-pressure side refrigerant pressure is near the critical point, and the degree of subcooling is reduced as the high-pressure side refrigerant pressure decreases from near the critical point. It was discovered that it would be better to gradually reduce it.

By the way, in order to determine how much the supercooling degree should be increased when the high-pressure side refrigerant pressure is near the critical point, the present inventor conducted intensive research based on the above-mentioned findings, and found that the critical point They found that the degree of supercooling in the vicinity should be about 15 ° C.

The reason why the degree of supercooling in the vicinity of the critical point was set to about 15 ° C is that, as shown in FIG. 3, when the above-described refrigeration cycle was operated in the subcritical region under predetermined conditions, The target refrigerant pressure and the refrigerant temperature on the inflow side of the expansion valve at which high efficiency is obtained are plotted by simulation, and these are approximated by a known method such as the least-squares method. Are the results of comparing. According to this approximation line, the degree of supercooling gradually decreases as the high-pressure side refrigerant pressure decreases from near the critical pressure, and when the high-pressure side refrigerant pressure reaches approximately 3.5 MPa, the degree of supercooling decreases. It has been confirmed that the temperature will be about 1 ° C.

However, there is naturally a difference between the calculation result by the simulation and the optimum degree of supercooling required in the actual refrigeration cycle, and there is a variation in the actual refrigeration cycle. Assuming that the degree of supercooling obtained by the refrigeration is 5 ° C, the simulation results when the refrigeration cycle is operated under different conditions in the subcritical region and under various conditions in the supercritical region It has been found by the present inventor that the variation of the control is almost completely covered (control line 5, a). Therefore, setting the degree of supercooling near the critical point to about 15 ° C means that the degree of supercooling in this part is in the range of 10 ° C to 20 ° C. From the state in which the degree of supercooling near the critical point is set, it has been found that cycle control in which the degree of supercooling is gradually reduced as the high-pressure refrigerant pressure decreases becomes desirable. In FIG. 4, an example of cycle control in the subcritical region by the control unit 11 is shown as a flowchart. Hereinafter, the control of the supercooling degree of the cooling cycle based on this will be described. G11 is a signal from the refrigerant temperature sensor 10 for detecting the refrigerant temperature at the inlet side of the expansion valve.

(Step 50), and a signal from the pressure sensor 12 are input (Step 52). The control unit 11 stores a map data of the control line or an arithmetic expression in advance, and based on the refrigerant temperature and the refrigerant pressure input in Steps 51 and 52, the control unit 11 detects the temperature at the expansion valve inlet side. A value such that the degree of supercooling falls on the control line is calculated (step 54), and the opening of the expansion valve 5 is electrically controlled based on the calculation result (step 56).

Therefore, according to the configuration described above, in the subcritical region, near the critical point where the change in the degree of subcooling has a great effect on the efficiency of the refrigeration cycle. In the low pressure region where the change in the degree of supercooling gradually becomes less effective in the efficiency of the refrigeration cycle, the degree of subcooling decreases as the high-pressure pressure decreases, and the subcooling decreases to the conventional level. By gradually approaching it, it becomes possible to make the carbon dioxide cycle more suitable in terms of efficiency when operated in the subcritical region.

In the configuration example described above, the optimal control line is obtained by the electric expansion valve. However, the expansion valve 5 detects the refrigerant temperature and pressure upstream of the expansion valve even if the expansion valve 5 is not of the type electrically controlled by the control unit 11 as described above. A type in which the expansion valve 5 is operated by a non-electrical method using a temperature-sensitive member and a pressure-sensitive member may be used. According to such a configuration, in addition to the above-described effects, the refrigerant temperature and pressure are detected in the vicinity of the expansion valve. Therefore, even when a refrigeration cycle is to be mounted on the vehicle body, the cable can be connected to the cable. This eliminates the need for routing, and can prevent a decrease in control accuracy.

By the way, in the refrigerating cycle 1 shown in FIG. 1, when using a variable displacement type compressor in which the amount of discharged refrigerant is variable as the compressor 2, the refrigerant temperature T [° C.] on the inlet side of the expansion valve 5 And the refrigerant pressure P [MPa] during normal operation, the area indicated by the sand in Fig. 6, that is, T≤2.41P + 4.886 (C line) and Τ≥2 5 2 Ρ-7.4 1 (D line) It is desirable to set so as to be within the range enclosed by. This region is a range for obtaining a good cycle efficiency by obtaining a state of COP at or near the maximum, and has been found by the following simulations and findings.

First, under various operating conditions, the refrigerant pressure and the refrigerant temperature on the expansion valve inlet side where the maximum COP is obtained are found by simulation. This method will be described with reference to the flowchart shown in FIG. 5. First, in step 60, the operating conditions of the refrigeration cycle 1 are input to the simulator 1. The operating conditions are, for the compressor 2, the rotation speed or discharge amount, efficiency (volume efficiency, mechanical efficiency, adiabatic compression efficiency), etc., and for the radiator 3 or the evaporator 6, the heat exchange efficiency, The volume, the temperature and humidity of the air passing therethrough, the wind speed, etc., and the internal heat exchanger 4 is the heat exchange efficiency.

Then, in the next step 62, a control point at which the refrigeration cycle 1 balances under the above operating conditions is calculated. The calculation of this balanced control point is as follows: (i) The refrigerant pressure initial value of the high-pressure side line 8 is set to, for example, 14 MPa, and the compressor suction refrigerant temperature is temporarily determined to be, for example, the evaporation temperature + 15 ° C. After that, (ϋ) Since the capacity of each component of the refrigeration cycle 1 is determined in advance, the provisionally determined value is recalculated using this as a constraint. (Iii) If there is a difference between the tentatively determined value and the recalculated value that is equal to or greater than a predetermined range, the recalculated value is used as a new tentative value, and the calculation of (ii) is further performed. This is repeated until the difference falls within the predetermined range.

According to the conventional method of calculating the optimum high pressure while keeping the refrigerant temperature at the inlet of the expansion valve 5 or the refrigerant temperature at the outlet of the radiator 3 constant, such a balance calculation is required in practice. However, if the same cooling capacity is to be obtained by lowering the high pressure, the amount of circulating refrigerant increases, and as a result, the refrigerant temperature at the expansion valve inlet or the radiator outlet increases, which is different from the actual cycle characteristics. The purpose is to obtain characteristics that match the actual cycle as much as possible. Thus, as described above, the high pressure and the refrigerant temperature at the compressor inlet that balance the refrigeration cycle 1 are obtained, and then, in step 64, the coefficient of performance (COP) at that time is calculated. After the COP at the time of the balance is obtained, the high pressure, the refrigerant temperature at the compressor inlet, the COP, the refrigerant temperature at the radiator outlet, and the like change due to the change in the discharge amount of the compressor 2. At 66, the discharge amount of the compressor 2 is changed as a parameter, and the refrigerant pressure P and the refrigerant temperature T at the inlet of the expansion valve at which the COP is maximum are found.

The results of plotting the refrigerant pressure and the refrigerant temperature at the inlet of the expansion valve, which are the maximum C 0 P for each of the above calculations under various conditions, are `` X '' and `` 〇 '' in Fig. 6. . Also, the maximum COP obtained by each simulation does not change significantly even if the pressure or the expansion valve opening slightly changes, so the cooling on the expansion valve inlet side under each condition where the maximum COP can be obtained. If the distribution range of the medium temperature T [° C] and the refrigerant pressure P [MPa] is defined as described above, it is possible to obtain an operating state of at most C0P or close to this, which is desirable for the present refrigeration cycle. It will be.

In other words, in the conventional refrigeration cycle without the internal heat exchanger 4 and with a fixed displacement compressor with a fixed discharge capacity, the optimal control line is shown by broken line A in FIG. In addition, when the compressor 2 is of a fixed displacement type having the internal heat exchanger 4, the optimal control line is indicated by a broken line B in FIG. On the other hand, when a good control line is found using the compressor 2 having the internal heat exchanger 4 and having a variable capacity as described above, a refrigeration cycle having an A or B control line can be obtained. In comparison, it is possible to set the refrigerant temperature T and the refrigerant pressure P on the inflow side of the expansion valve to be higher if the refrigerant temperature is the same, and to lower the refrigerant temperature if the refrigerant pressure is the same. It will be useful.

This is because, as shown in FIGS. 7 and 8, the present refrigerating cycle 1 having the compressor 2 capable of arbitrarily changing the discharge capacity is compared with the conventional cycle having the conventional fixed displacement compressor. In the refrigeration cycle 1, with the discharge capacity smaller than that of the conventional cycle, or with the expansion valve 5 narrowed, the COP that achieves the same cooling capacity can be improved except at high load. In this case, the refrigerant flow rate of the refrigeration cycle can be reduced, and as a result, the refrigerant temperature at the radiator 3 outlet and the expansion valve 5 inlet can be lowered, and the refrigerant temperature can be further lowered with respect to the high-pressure side line 8 As a means for setting the refrigerant temperature T and the refrigerant pressure P on the inflow side of the expansion valve 5 to a sandy range as shown in FIG. 6, the discharge capacity of the compressor 2 is adjusted. And external control In the case of the expansion valve 5 whose opening can be controlled by a control signal, by adjusting the degree of engagement so that the refrigerant temperature and the refrigerant pressure on the inflow side of the expansion valve 5 become the target values in the region, Equalization type expansion valve If it is, the amount of the charged gas to be equalized with the refrigerant pressure is adjusted, or if the expansion valve uses a bimetal, the characteristic is such that the refrigerant temperature and the refrigerant pressure on the inflow side are adjusted within the above range. It is preferable to use a metal material having

Industrial applicability

As described above, according to the present invention, when a refrigeration cycle using carbon dioxide gas as a refrigerant is operated under subcritical conditions, when the high-pressure side refrigerant pressure is near the critical point, the degree of supercooling is reduced by about 15 °. C, since the supercooling degree is controlled so as to gradually decrease as the high-pressure side refrigerant pressure decreases from near the critical point, good cycle efficiency can be obtained.

Further, as a refrigeration cycle for realizing this, a compressor for increasing the pressure of the refrigerant, a radiator for cooling the refrigerant, a pressure adjusting means for reducing the pressure of the refrigerant after being cooled by the radiator, and a pressure reducing means for reducing the pressure of the refrigerant. The evaporator for evaporating the heated refrigerant, and detecting means for detecting the refrigerant pressure and the refrigerant temperature on the upstream side of the pressure adjusting means, the refrigerant pressure on the high pressure side based on the detection result of the detecting means. If it is determined that the supercooling degree is near the critical point, the pressure reduction amount of the pressure adjusting means is controlled so that the supercooling degree is approximately 15 ° C, and the supercooling degree decreases as the refrigerant pressure on the high pressure side decreases from the vicinity of the critical point. If the pressure reducing amount of the pressure adjusting means is controlled so as to gradually decrease the pressure, the good cycle efficiency can be achieved by controlling the pressure adjusting means and adjusting the degree of supercooling. In addition, since the temperature and the like of the refrigerant are detected by the detecting means on the upstream side of the pressure adjusting means, a decrease in control accuracy is prevented even when an expansion valve that does not rely on electric control using a temperature-sensitive cylinder or the like is used. Can be.

In a refrigeration cycle having an internal heat exchanger and a compressor capable of adjusting the discharge capacity, the refrigerant temperature T [° C] and the refrigerant pressure P [MPa at the inflow side of the expansion device are set. ] Compared to a refrigeration cycle without an internal heat exchanger and a refrigeration cycle with a fixed compressor discharge capacity. If the refrigerant temperature is the same, the refrigerant pressure is set to be high, and if the refrigerant pressure is the same, the refrigerant temperature is set to be low. Preferably, T ≦ 2.41Ρ + 4.886 and Τ Since Τ and Ρ are set in the range of ≥2.52Ρ-7.41, good cycle efficiency can be obtained under various operating conditions. In addition, when the heat load such as the outside air temperature is low, the high pressure side does not become supercritical and may become a gas-liquid two-phase as in the conventional cycle. It has been confirmed that if it is, moderate supercooling can be obtained at the expansion valve inlet, and good cycle efficiency can be obtained.

Claims

The scope of the claims

1. In a control method used in a refrigeration cycle using carbon dioxide gas as a refrigerant, when operating in a subcritical region, if the high-pressure side refrigerant pressure is near a critical point, the supercooling degree is set to about 15 ° C, A method for controlling a refrigeration cycle, wherein the degree of supercooling is gradually reduced as the high-pressure side refrigerant pressure decreases from near the critical point.

2. A compressor that can increase the pressure of the refrigerant to the supercritical region and change the discharge rate, a radiator that cools the refrigerant that has reached the supercritical region, and decompresses the refrigerant after being cooled by the radiator. A refrigeration system comprising: an expansion device that expands; an evaporator that evaporates the refrigerant depressurized by the expansion device; and an internal heat exchanger that exchanges heat between the refrigerant flowing out of the evaporator and the refrigerant in the supercritical area. In the control method used for the cycle,

The operating conditions including the discharge amount of the compressor are adjusted to control the refrigerant temperature and the refrigerant pressure on the inflow side of the expansion device, a refrigeration cycle without the internal heat exchanger, and the discharge of the compressor. Compared to a refrigeration cycle having a fixed capacity, the refrigerant pressure is set higher if the refrigerant temperature is the same, and the refrigerant temperature is set lower if the refrigerant pressure is the same. And a method for controlling a refrigeration cycle.

3. When the refrigerant temperature at the inflow side of the expansion device is T [° C] and the refrigerant pressure at the inflow side of the expansion device is P [MPa], T and P are:

T≤ 2.4 1 P + 4.8 6

T≥ 2.5 2 Ρ-7.4 1

3. The method for controlling a refrigeration cycle according to claim 2, wherein the range is set so as to satisfy both of the relationships.

4. Use carbon dioxide as refrigerant

A compressor that pressurizes the refrigerant,

A radiator for cooling the pressurized refrigerant,

A pressure adjusting means for reducing the pressure of the refrigerant cooled by the radiator; an evaporator for evaporating the refrigerant reduced in pressure by the pressure adjusting means;

Detecting means for detecting the refrigerant pressure and the refrigerant temperature upstream of the pressure adjusting means;

When the refrigerant pressure is near the critical point based on the detection result of the detection means, the pressure reduction amount of the pressure adjustment means is adjusted so that the degree of supercooling on the inflow side of the pressure adjustment means is about 15 ° C. Control means for controlling the pressure reduction amount of the pressure adjusting means so as to gradually reduce the degree of supercooling on the inflow side of the pressure adjusting means as the refrigerant pressure decreases from near the critical point. Characterized refrigeration cycle.

5. A compressor that boosts the refrigerant to the supercritical region, a radiator that cools the refrigerant that has reached the supercritical region, an expansion device that depressurizes the refrigerant after being cooled by the radiator, and a decompression device that decompresses the refrigerant. A refrigerating cycle, comprising: an evaporator for evaporating the separated refrigerant; and an internal heat exchanger for exchanging heat between the refrigerant flowing out of the evaporator and the refrigerant in the supercritical region.

The discharge amount of the compressor can be changed,

The operating conditions including the discharge amount of the compressor are adjusted to control the refrigerant temperature and the refrigerant pressure on the inflow side of the expansion device, a refrigeration cycle without the internal heat exchanger, and the discharge of the compressor. Compared to a refrigeration cycle having a fixed capacity, the refrigerant pressure is set higher if the refrigerant temperature is the same, and the refrigerant temperature is set lower if the refrigerant pressure is the same. And a refrigeration cycle characterized by the following.

6. The refrigerant temperature at the inflow side of the expansion device is T [° C], and the temperature of the expansion device is Assuming that the refrigerant pressure on the inlet side is P [MPa], T and P are T≤2.41 P + 4.86

T≥2.52 P-7.4 1

6. The refrigeration cycle according to claim 5, wherein the refrigeration cycle is set in a range that satisfies both of the relationships.