Composite oil return device and its control method

Abstract

A composite oil return device is provided. The composite oil return device includes a main oil return circuit and an auxiliary oil return circuit. The main oil return circuit performs a passive oil return step. The main oil return circuit includes: a first oil return pipeline connecting the refrigerant discharge outlet of a flooded evaporator to an oil return heat exchange inlet of the oil return heat exchanger, and a second oil return pipeline connecting an oil return heat exchange outlet of the oil return heat exchanger to a suction inlet of a compressor. The auxiliary oil return circuit includes an oil return ejector, which performs an active oil return step when the compressor operates under partial load conditions. Additionally, a control method of composite oil return device and a chiller system are also provided.

Description

Technical Field

[0001] This disclosure relates to a composite oil return device and its control method, and in particular to a composite oil return device and its control method that can improve the ineffective work wasted by conventional flooded evaporators using high-pressure gas as power for oil return. Prior Technology

[0002] In air conditioning systems, chillers produce 7°C chilled water to supply the load side for process or equipment cooling, or to generate cold air. For the past 20 years, domestically produced water-cooled chillers have primarily used flooded evaporators. Traditional chillers using dry-type direct expansion shell-and-tube or plate heat exchangers have a system where the refrigerant oil used for compressor cooling, lubrication, and oil sealing is discharged along with the high-temperature, high-pressure refrigerant gas and enters the refrigeration cycle. The low-temperature refrigerant carrying the refrigerant oil exchanges heat with the chilled water circulating on the shell side (within the evaporator heat transfer copper tubes), expanding into a low-temperature, low-pressure refrigerant gas, and then returns to the compressor along with the contained refrigerant oil. Therefore, chiller systems using direct expansion evaporators do not require additional oil separators or compressor oil return devices.

[0003] Unlike chillers that use dry direct expansion shell-and-tube or plate heat exchangers, modern chillers primarily employ flooded evaporators with pool boiling to achieve higher refrigeration energy efficiency. In the flooded evaporator, refrigerant containing refrigeration oil exchanges heat with the circulating chilled water on the tube side, boiling and evaporating into gaseous refrigerant. This gaseous refrigerant is drawn into the compressor. However, the refrigeration oil discharged from the compressor along with the high-temperature, high-pressure refrigerant, also evaporates and boils into oil bubbles. This oil cannot return to the compressor with the evaporated refrigerant gas and remains in the flooded evaporator. Over time, this leads to oil loss in the compressor, causing it to malfunction. Therefore, to prevent the compressor from losing oil and thus malfunctioning, chiller systems using flooded evaporators require an additional oil separator and a high-pressure refrigerant gas-powered jet pump oil return device. This allows oil to return from the evaporator to the compressor's oil return system, recovering the refrigerant oil discharged from the compressor and reducing the load on the evaporator's oil return device, enabling the compressor to operate normally and efficiently, and to achieve its refrigeration effect. However, conventional technologies using high-pressure refrigerant gas-powered jet pump oil return devices typically require approximately 2-3% of the system's refrigerant circulation volume to be supplied with high-pressure refrigerant gas. This means that 2-3% of the refrigerant circulation volume is directly bypassed from the compressor's discharge end to its suction end, amounting to wasted power from the compressor. Summary of the Invention

[0004] This disclosure provides a composite oil return device and its control method, which saves about 2-3% of the cooling capacity loss of conventional oil return devices using high-pressure gas-powered jet pumps, thereby improving the cooling capacity and energy efficiency of the chiller system.

[0005] This disclosure provides an embodiment of a composite oil return device, including a main oil return circuit and an auxiliary oil return circuit. The main oil return circuit includes a flooded evaporator, a first oil return pipeline, an oil return heat exchanger, a second oil return pipeline, and a compressor. The oil return heat exchanger includes a shell, a refrigerant heat exchange inlet, a refrigerant heat exchange outlet, a heat exchange pipeline, an oil return heat exchange inlet, and an oil return heat exchange outlet. The shell has a refrigerant heat exchange inlet and a refrigerant heat exchange outlet at opposite ends. One side of the shell has the oil return heat exchange inlet and the oil return heat exchange outlet respectively. The heat exchange pipeline is located inside the shell, with one inlet end connected to the oil return heat exchange inlet and one outlet end connected to the oil return heat exchange outlet. The first oil return pipeline connects a refrigerant outlet of the flooded evaporator to the oil return heat exchange inlet of the oil return heat exchanger, and the second oil return pipeline connects the oil return heat exchange outlet of the oil return heat exchanger to a suction inlet of the compressor. The auxiliary oil return circuit includes an oil return injection pump, an oil return heat exchanger, a third oil return line, a fourth oil return line, a fifth oil return line, and the compressor. The fourth oil return line connects the first inlet of one of the oil return injection pumps to the exhaust outlet of one of the compressors. The third oil return line connects the second oil return line to the second inlet of one of the oil return injection pumps. The fifth oil return line connects the outlet of one of the oil return injection pumps to the suction inlet of the compressor.

[0006] In one embodiment, the aforementioned composite oil return device further includes an oil separator oil return circuit. The oil separator oil return circuit includes an oil separator, a compressor, at least one switching device, an oil level detector, and a sixth oil return line, wherein the oil level detector is located on the compressor, the sixth oil return line connects an oil return inlet of the compressor to an oil return outlet of the oil separator, and at least one switching device is located on the sixth oil return line.

[0007] In one embodiment, the first return oil line and the fourth return oil line are each provided with at least one switching device.

[0008] In one embodiment, the at least one switching device is an electronic expansion valve or a solenoid valve.

[0009] Another embodiment of this disclosure proposes a control method for a composite oil return device, including the following steps: controlling the oil return process from a flooded evaporator to a compressor; when the compressor is operating under partial load conditions, executing a power oil return process with an auxiliary oil return circuit to assist the main oil return circuit in returning oil from the flooded evaporator to the compressor, wherein a third oil return line is used to connect the auxiliary oil return circuit to the main oil return circuit. The oil return process from the flooded evaporator to the compressor includes the following steps: performing a non-powered oil return process using a main oil return circuit, including the following steps: transporting oil-rich refrigerant liquid from a flooded evaporator to an oil return heat exchanger via a first oil return line; the oil-rich refrigerant liquid is transported from an oil return heat exchange inlet of the oil return heat exchanger to a heat exchange line inside the oil return heat exchanger; after heat exchange, a refrigerant oil in the oil-rich refrigerant liquid can be led out to a second oil return line via an oil return heat exchange outlet of the oil return heat exchanger; and the refrigerant oil is introduced into a suction inlet of the compressor via the second oil return line.

[0010] In one embodiment, the above-mentioned operation of the compressor under partial load conditions is: the compressor operates at a partial load of less than or equal to 50%, wherein the partial load is calculated as 1 / 2 of the compressor's full-load rated current.

[0011] In one embodiment, the above-mentioned operation of the compressor under partial load conditions includes the following steps: detecting the refrigerant side pressure of a flooded evaporator and the chilled water outlet temperature of a chilled water side; calculating a saturated refrigerant temperature corresponding to an evaporation pressure based on the refrigerant type; calculating the temperature difference between the saturated refrigerant temperature corresponding to the evaporation pressure and the chilled water outlet temperature to obtain an evaporator progressive temperature difference; and comparing the calculated evaporator progressive temperature difference value with an evaporator progressive temperature difference set value. When the evaporator progressive temperature difference value is greater than or equal to the evaporator progressive temperature difference set value, the power return oil step is executed.

[0012] In one embodiment, the control method of the above-mentioned composite oil return device further includes the following steps: controlling the oil return process from an oil separator to a compressor using an oil separator oil return circuit, including the following steps: detecting the compressor by an oil level detector; when the compressor is in a low oil level state, opening at least one switching device located on a sixth oil return line, so that the refrigerant oil separated in the oil separator is transported to the compressor from an oil return inlet of the compressor through the sixth oil return line; and when the compressor is in a high oil level state, closing at least one switching device located on the sixth oil return line.

[0013] To make this disclosure clearer and easier to understand, specific embodiments are provided below, along with detailed descriptions in conjunction with the accompanying drawings. Simple Explanation of the Diagram

[0014] Figure 1 is a schematic diagram of a chiller system according to this disclosure. Figure 2 is a schematic diagram of the oil return heat exchanger disclosed herein. Figure 3 is a schematic diagram of one side of the return oil heat exchanger in Figure 2. Figure 4 is a schematic diagram of an embodiment of a compound oil return device in a chiller system according to the present disclosure. Figure 5 is a schematic diagram of the pressure and enthalpy of a conventional chiller system using a jet pump oil return device. Figure 6 is a schematic diagram of the pressure and enthalpy of the chiller system of the composite oil return device disclosed herein. Figure 7 is a flowchart of the control method for the composite oil return device disclosed herein. Implementation

[0015] The following description provides detailed examples and accompanying drawings, but these examples are not intended to limit the scope of this disclosure. Furthermore, the drawings are for illustrative purposes only and are not drawn to scale. For ease of understanding, the same elements will be designated with the same symbols in the following description.

[0016] The terms "including," "containing," and "having" used in this disclosure are all open-ended, meaning "including but not limited to."

[0017] In the description of the various embodiments, when the terms "first," "second," "third," "fourth," etc. are used to describe elements, they are only used to distinguish these elements from each other and do not limit the order or importance of these elements.

[0018] In the description of the various embodiments, the term "coupled" or "connected" may refer to two or more elements making direct physical or electrical contact with each other, or making indirect physical or electrical contact with each other. "Coupled" or "connected" may also refer to two or more elements operating or moving with each other.

[0019] In the description of the various embodiments, the term "module" refers to a hardware module, that is, a hardware component that occupies space. In other embodiments, the term "module" may also refer to a hardware module plus a software module, that is, a "module" has software programs in addition to hardware components.

[0020] Figure 1 is a schematic diagram of a chiller system according to this disclosure. Please refer to Figure 1. The chiller system 100 disclosed herein includes at least a compressor 11, an oil separator 21, a condenser 31, an oil return heat exchanger 41, a flooded evaporator 51, and an oil return jet pump. Ejector 61, at least one switching device (as shown in Figure 1, a first switching device V1, a second switching device V2, a third switching device V3, and a fourth switching device V4), a refrigerant circulation pipeline group PA, and an oil return pipeline group RA, wherein the first switching device V1, the second switching device V2, the third switching device V3, and the fourth switching device V4 may be an electronic expansion valve or a solenoid valve, or other elements capable of performing pipeline switching; the copper tubes of the flooded evaporator 51 are completely immersed in the refrigerant and provide an ice water inlet G1 and an ice water outlet G2, and the ice water of the ice water machine forms a circulating ice water from the ice water inlet G1 to the ice water outlet G2.

[0021] The refrigerant circulation piping group PA consists of the first refrigerant piping P1, the second refrigerant piping P2, the third refrigerant piping P3, the fourth refrigerant piping P4, the fifth refrigerant piping P5, and the sixth refrigerant piping P6. The oil return piping group RA consists of the first oil return piping R1, the second oil return piping R2, the third oil return piping R3, the fourth oil return piping R4, the fifth oil return piping R5, and the sixth oil return piping R6. To distinguish between the refrigerant circulation piping group PA and the oil return piping group RA, the first refrigerant piping P1, the second refrigerant piping P2, the third refrigerant piping P3, the fourth refrigerant piping P4, the fifth refrigerant piping P5, and the sixth refrigerant piping P6 are represented by dashed lines, while the first oil return piping R1, the second oil return piping R2, the third oil return piping R3, the fourth oil return piping R4, the fifth oil return piping R5, and the sixth oil return piping R6 are represented by solid lines.

[0022] The chiller system 100 has a main refrigerant circulation. The compressor 11 is equipped with an intake inlet 112, an exhaust outlet 114, and an oil return inlet 116. The oil separator 21 is equipped with an oil separator inlet 212, an oil separator outlet 214, and an oil return outlet 216. The condenser 31 is equipped with a condenser inlet 312 and a condenser outlet 314. The oil return heat exchanger 41 is equipped with a refrigerant heat exchange inlet 412, a refrigerant heat exchange outlet 414, and an oil return heat exchange inlet 416. And an oil heat exchange outlet 418; a flooded evaporator 51 is provided with an evaporator outlet 512, an evaporator inlet 514, and a refrigerant outlet 516; a first switching device V1 is provided with a first switching inlet V12 and a first switching outlet V14, wherein: the first refrigerant line P1 connects the discharge outlet 114 of the compressor 11 and the oil separator inlet 212 of the oil separator 21, and the second refrigerant line P2 connects the oil separator outlet 214 of the oil separator 21 and the condenser 31. Between the condenser inlet 312 and the condenser outlet 314 of the condenser 31, the third refrigerant line P3 connects to the refrigerant heat exchange inlet 412 of the oil return heat exchanger 41. The fourth refrigerant line P4 connects to the refrigerant heat exchange outlet 414 of the oil return heat exchanger 41 and the first switch inlet V12 of the first switch device V1. The fifth refrigerant line P5 connects to the first switch outlet V14 of the first switch device V1 and the evaporator inlet 514 of the flooded evaporator 51. The sixth refrigerant line P6 connects to... Between the evaporator outlet 512 of the flooded evaporator 51 and the suction inlet 112 of the compressor 11, the aforementioned refrigerant circulation pipeline PA, consisting of the first refrigerant pipeline P1, the second refrigerant pipeline P2, the third refrigerant pipeline P3, the fourth refrigerant pipeline P4, the fifth refrigerant pipeline P5, and the sixth refrigerant pipeline P6, connects the compressor 11, the oil separator 21, the condenser 31, the oil return heat exchanger 41, the first switching device V1, and the flooded evaporator 51, thus forming the main refrigerant circulation of the chiller system 100.

[0023] The refrigerant circulation process proceeds sequentially as follows: refrigerant heat absorption (evaporation), compression, condensation, and oil return. The flooded evaporator 51 is a type of evaporator used in the refrigeration or chiller system 100. The refrigerant absorbs heat from the circulating chilled water (chilled water inlet G1, chilled water outlet G2) in the flooded evaporator 51. The liquid refrigerant circulates through the pipes of the flooded evaporator 51 and gradually evaporates into gaseous refrigerant. During this process, the refrigerant gradually transforms from liquid to gas, while simultaneously absorbing heat from the circulating chilled water from the chilled water inlet G1 to the chilled water outlet G2. The flooded evaporator 51 provides more efficient heat exchange, maintains the refrigerant's saturated state, and maximizes heat exchange efficiency. Next, the gaseous refrigerant enters the compressor 11, where it is compressed into a high-temperature, high-pressure gas. The compressor 11 in the chiller system 100 functions by compressing the low-pressure, low-temperature refrigerant vapor into a high-pressure, high-temperature refrigerant gas. The compressor 11 draws in refrigerant vapor from the evaporator, pressurizes it, and delivers it to the condenser 31. The temperature and pressure of the compressed refrigerant gas increase significantly. Since the lubricating oil used in the compressor 11 is mixed with the refrigerant, the oil separator 21 separates the oil from the refrigerant, reducing the amount of oil entering the subsequent condenser 31. Next, the high-temperature, high-pressure gaseous refrigerant enters the condenser 31, releases heat, and cools into liquid refrigerant. The refrigerant, after compression, becomes a high-temperature, high-pressure gas and enters the condenser 31. The condenser 31 cools the refrigerant gas, causing it to lose heat and transform into liquid refrigerant. The condenser 31 typically absorbs heat using water or air, condensing the refrigerant gas into high-pressure liquid refrigerant. Then, the liquid refrigerant passes through the condenser 31 and the oil return heat exchanger 41, and re-enters the flooded evaporator 51, continuing to absorb heat and re-evaporate, forming a closed refrigerant cycle. This refrigerant circulation system is the core working principle of the chiller system 100. Through processes such as compression, condensation, and evaporation, it ensures that the chiller system 100 provides efficient cooling.

[0024] In addition, the chiller system 100 also has a composite oil return device RD consisting of an oil return heat exchanger 41, an oil return pipeline group RA, and an oil return jet pump 61. The oil return jet pump 61 has a first oil return jet pump inlet 612, a second oil return jet pump inlet 614, and an oil return jet pump outlet 616. The first oil return pipeline R1 connects the refrigerant outlet 516 of the flooded evaporator 51 to the oil return heat exchange inlet 416 of the oil return heat exchanger 41. The second oil return pipeline R2 connects the oil return heat exchange outlet 418 of the oil return heat exchanger 41 to the first inlet P61 of the sixth refrigerant pipeline P6. The third oil return pipeline R2 connects the oil return heat exchange outlet 418 of the oil return heat exchanger 41 to the first inlet P61 of the sixth refrigerant pipeline P6. Pipeline R3 connects the second return oil pipeline R2 to the second inlet 614 of the return oil injection pump 61. The fourth return oil pipeline R4 connects the first inlet 612 of the return oil injection pump 61 to the second refrigerant pipeline P2. The third switch device V3 is located in the fourth return oil pipeline R4. The fifth return oil pipeline R5 connects the outlet 616 of the return oil injection pump 61 to the second inlet P62 of the sixth refrigerant pipeline P6. The sixth return oil pipeline R6 connects the return oil inlet 116 of the compressor 11 to the return oil outlet 216 of the oil separator 21. The sixth return oil pipeline R6 is different from the first refrigerant pipeline P1. The above-mentioned oil return heat exchanger 41, oil return injection pump 61, and oil return pipeline group RA consisting of first oil return pipeline R1, second oil return pipeline R2, third oil return pipeline R3, fourth oil return pipeline R4, fifth oil return pipeline R5, and sixth oil return pipeline R6 are connected to compressor 11, oil separator 21, condenser 31, and flooded evaporator 51 to form a composite oil return device RD for chiller system 100.

[0025] Figure 2 is a schematic diagram of the oil return heat exchanger disclosed herein. Figure 3 is a schematic diagram of one side of the oil return heat exchanger in Figure 2. Referring to Figures 1, 2 and 3, the oil return heat exchanger 41 disclosed herein includes a shell 411, a refrigerant heat exchange inlet 412, a refrigerant heat exchange outlet 414, a heat exchange pipeline 415, an oil return heat exchange inlet 416, and an oil return heat exchange outlet 418. The housing 411 has a refrigerant heat exchange inlet 412 and a refrigerant heat exchange outlet 414 at opposite ends along the length direction L. A third refrigerant line P3 is provided between the housing 411 inlet (i.e., refrigerant heat exchange inlet 412) of the oil return heat exchanger 41 and the condenser outlet 314 of the condenser 31. A fourth refrigerant line P4 is provided between the housing 411 outlet (refrigerant heat exchange outlet 414) of the oil return heat exchanger 41 and the first switch inlet V12 of the first switch device V1. Here, the third refrigerant line P3 and the fourth refrigerant line P4 are respectively high-pressure liquid refrigerant connection lines.

[0026] The shell side of the casing 411 is provided with an oil return heat exchange inlet 416 and an oil return heat exchange outlet 418. A heat exchange pipeline 415 is located inside the casing 411 and is a shell-ring type heat exchanger, used as an oil return or subcooler. In this way, refrigerant enters the casing 411 from the oil return heat exchange inlet 416, undergoes heat exchange through the heat exchange pipeline 415, and the refrigerant after heat exchange flows out from the refrigerant heat exchange outlet 414. Furthermore, the inlet end B1 of the heat exchange pipeline 415 is connected to the oil return heat exchange inlet 416, and the outlet end B2 of the heat exchange pipeline 415 is connected to the oil return heat exchange outlet 418, forming an oil return circulation path.

[0027] Figure 4 is a schematic diagram of an embodiment of the composite oil return device in the chiller system disclosed herein. Referring to Figures 4 and 2, the composite oil return device RD disclosed herein is configured with a main oil return circuit RDA, an auxiliary oil return circuit RDB, and an oil separator oil return circuit RDC via an oil return heat exchanger 41, an oil return pipeline assembly RA, and an oil return jet pump 61. The main oil return circuit RDA performs a non-powered oil return step for a long time, while the auxiliary oil return circuit RDB assists the main oil return circuit RDA under specific conditions so that oil can be returned from the flooded evaporator 51 to the compressor 11. The specific conditions include insufficient oil bubble lift when the compressor 11 is operating under partial load.

[0028] Please refer to Figures 2 to 4. The main oil return circuit RDA includes a flooded evaporator 51, a first oil return line R1, an oil return heat exchanger 41, a second oil return line R2, and a compressor 11.

[0029] A first oil return line R1 is provided between the oil return heat exchanger 41 and the flooded evaporator 51, so that the refrigerant outlet 516 of the flooded evaporator 51 serves as an oil return guide outlet. In one embodiment, the first oil return line R1 connects the refrigerant outlet 516 of the flooded evaporator 51 and the oil return heat exchange inlet 416 of the oil return heat exchanger 41, and the first oil return line R1 may be equipped with a second switching device V2 as a switch to open or close the refrigerant outlet 516. The second switching device V2 is, for example, a solenoid valve.

[0030] At least one return oil line is connected between the return oil heat exchange outlet 418 of the return oil heat exchanger 41 and the suction inlet 112 of the compressor 11. Taking Figure 4 as an example, the second return oil line R2 connects the return oil heat exchange outlet 418 of the return oil heat exchanger 41 and the first inlet P61 of the sixth refrigerant line P6. The first inlet P61 of the sixth refrigerant line P6 is connected to the suction inlet 112 of the compressor 11, so that the return oil heat exchanger 41 can be connected to the compressor 11 through the second return oil line R2.

[0031] In this way, the oil-rich refrigerant liquid in the flooded evaporator 51 is transported from the refrigerant outlet 516 to the heat exchange pipeline 415 inside the oil return heat exchanger 41 via the first oil return pipeline R1 and the oil return heat exchanger 41's oil return heat exchanger 41. At this time, the high-pressure, high-temperature liquid refrigerant in the condenser 31 is transferred to the interior of the shell inlet (i.e., refrigerant heat exchange inlet 412) of the oil return heat exchanger 41 via the third refrigerant pipeline P3, so that the high-pressure, high-temperature liquid refrigerant can exchange heat with the oil-rich refrigerant liquid located in the heat exchange pipeline 415. The low-pressure oil-rich refrigerant liquid in the heat exchange pipeline 415 evaporates into gas due to the heat obtained from the high-pressure, high-temperature refrigerant liquid. The 5% of the refrigeration oil in the oil-rich refrigerant liquid generates lift due to the boiling oil bubbles caused by the evaporation of the refrigerant. The refrigeration oil generating lift is led out through the oil return heat exchange outlet 418 of the oil return heat exchanger 41 to the second oil return pipeline R2, and enters the suction inlet 112 of the compressor 11 through the second oil return pipeline R2, achieving the effect of unpowered oil return (eliminating the need for high-pressure gas). This process allows the oil bubbles from the evaporation of the oil-rich refrigerant liquid in the flooded evaporator 51 to exert lift on the compressor 11, and also generates subcooling in the high-pressure, high-temperature liquid state, thereby increasing the cooling effect of the flooded evaporator 51 and improving the cooling energy efficiency of the compressor 11. In one embodiment, the second oil return line R2 is connected to the compressor 11 through the first inlet P61 of the sixth refrigerant line P6.

[0032] Please refer to Figures 2 to 4. The auxiliary return oil circuit RDB includes a return oil injection pump 61, a return oil heat exchanger 41, a third return oil line R3, a fourth return oil line R4, a fifth return oil line R5, and a compressor 11.

[0033] A third return oil line R3 is provided between the second inlet 614 of the return oil injection pump 61 and the second return oil line R2. The third return oil line R3 connects the second return oil line R2 and the second inlet 614 of the return oil injection pump 61. The second return oil line R2 serves as the connecting line of the main return oil circuit RDA. The auxiliary return oil circuit RDB is connected to the main return oil circuit RDA through the third return oil line R3.

[0034] The connecting pipes (first refrigerant pipe P1 and second refrigerant pipe P2) between the first inlet 612 of the return oil injection pump 61 and the exhaust outlet 114 of the compressor 11 are connected to at least one return oil pipe. Taking Figure 4 as an example, the fourth return oil pipe R4 is connected between the first inlet 612 of the return oil injection pump 61 and the second refrigerant pipe P2, so that the fourth return oil pipe R4 is connected between the first inlet 612 of the return oil injection pump 61 and the exhaust outlet 114 of the compressor 11. The third switching device V3 is provided in the fourth return oil pipe R4. The third switching device V3 is, for example, a solenoid valve.

[0035] At least one return oil line is connected between the oil return pump outlet 616 of the return oil injection pump 61 and the connecting line (sixth refrigerant line P6) of the suction inlet 112 of the compressor 11. Taking Figure 4 as an example, the fifth return oil line R5 connects the oil return pump outlet 616 of the return oil injection pump 61 and the second inlet P62 of the sixth refrigerant line P6, so that the return oil injection pump 61 can be connected to the compressor 11 through the fifth return oil line R5.

[0036] In this configuration, the auxiliary oil return circuit RDB assists the main oil return circuit RDA when the oil bubble lift effect is insufficient during partial load operation of the compressor 11. It activates the third switch V3 on the fourth oil return line R4 in the auxiliary oil return circuit RDB. At this time, the slightly reduced high-pressure refrigerant gas serves as the power source for the oil return injection pump 61, driving it to draw incompletely evaporated oil-rich refrigerant liquid from the oil return heat exchanger 41 through the oil return heat exchange outlet 418, the second oil return line R2, and the third oil return line R3, thus... High-pressure refrigerant air evaporates the oil-rich refrigerant liquid, which then returns to the compressor 11 via the second inlet P62 of the fifth oil return line R5 and the suction inlet 112 of the compressor 11. Since the compressor 11 operates under partial load, the loss of cooling capacity and the impact on cooling energy efficiency are relatively small. This oil return assistance to the compressor 11 is achieved by using high-pressure gas to drive the oil return jet pump 61, which draws the refrigerant oil from the oil return outlet (i.e., the oil return heat exchange outlet 418) on the tube side of the oil return heat exchanger 41 back to the compressor 11. Therefore, the disclosed composite oil return device RD can selectively activate the oil return jet pump 61 to assist in oil return via the fifth oil return line R5, depending on the oil return status. This reduces the loss of cooling capacity and ineffective power in conventional systems that rely entirely on high-pressure gas.

[0037] Please refer to Figure 4. The oil separator return circuit RDC includes a compressor 11, an oil separator 21, and a sixth oil return line R6. The compressor 11 has an oil level detector 118 and at least one oil return inlet (such as oil return inlet 116). The sixth oil return line R6 connects the oil return inlet 116 of the compressor 11 to the oil return outlet 216 of the oil separator 21. A fourth switch device V4 is installed on the sixth oil return line R6. The oil level detector 118 of the compressor 11 is used to control the fourth switch device V4 on the sixth oil return line R6, so that the refrigerant oil in the oil separator 21 returns to the compressor 11. This prevents the refrigerant oil that is not intercepted by the oil separator 21 from accumulating in the flooded evaporator 51 with the discharge of the compressor 11. This reduces or prevents the oil concentration in the flooded evaporator 51 from increasing, prevents the evaporation temperature from deteriorating, and prevents the oil bubbles during refrigerant boiling from gradually increasing and causing the compressor 11 to be liquid compressed, thus preventing the compressor 11 from losing oil.

[0038] Figure 5 is a schematic diagram of the pressure and enthalpy of a conventional chiller system using a jet pump oil return device. The changes in pressure and enthalpy caused by the compression, condensation, and evaporation processes are shown at state points 10, 20, 30, and 40. The conventional oil return device using a jet pump oil return device is powered by high-pressure refrigerant gas. It uses the high-pressure refrigerant gas at the outlet of the oil separator to draw a mixture of refrigerant and oil from the opening of the flooded evaporator with a higher oil concentration. Then, it passes through the condenser and exchanges heat with the high-temperature liquid refrigerant, which increases the subcooling of the high-pressure liquid refrigerant and causes the refrigerant containing high oil concentration to evaporate. Then, it can be selectively passed through the oil separator again to exchange heat with the high-temperature refrigeration oil. After the refrigerant has fully evaporated, it returns to the compressor through the compressor suction pipe.

[0039] The ejector pump uses high-pressure gas as its power source. Specifically, from state point 20 in the pressure and enthalpy diagram (Figure 5), 2-3% of the high-pressure gas bypasses and enters the ejector pump. This draws the oil-rich refrigerant liquid from the flooded evaporator into the ejector pump. The two fluids mix and are discharged from the ejector pump outlet, then return to the compressor via the compressor suction pipe, thus performing oil return to the compressor. Generally, after the refrigerant discharged from the compressor passes through an oil separator to separate the refrigerant oil, approximately 0.1% (wt%) of refrigerant oil from the refrigerant gas mass flow rate will still enter the condenser. This means that 0.1% of the refrigerant mass flow rate of the compressor loses refrigerant oil into the system, accumulating in the flooded evaporator. Based on empirical sampling, the oil concentration in the flooded evaporator is approximately 5%. Since the compressor discharges 0.1% refrigerant oil, the mass of refrigerant oil drawn from the flooded evaporator must also be 0.1% of the refrigerant gas mass flow rate discharged from the compressor. This ensures that the compressor's refrigerant oil is balanced and there is no risk of oil loss.

[0040] Since the refrigerant oil concentration in a flooded evaporator is 5%, a 2% refrigerant mass flow rate (5% oil-rich refrigerant) needs to be drawn from the flooded evaporator to recover the 0.1% refrigerant oil lost by the compressor. The jet pump's gas-liquid ratio is 1:1, meaning one part high-pressure gas plus one part oil-rich refrigerant, equivalent to a 4% total refrigerant mass flow rate, is needed to balance the oil discharged and returned by the compressor. In Figure 5, the red dashed line RL from state point 20 represents the high-pressure gas being mixed with the oil-rich refrigerant at state point 4f from the evaporator, and then mixed with the saturated state point 1g to reach state point 10 before entering the compressor. The 2% hot gas discharged from the compressor and the 2% oil-rich refrigerant from the flooded evaporator bypass the compressor without any refrigeration process, meaning the compressor loses up to 4% of its refrigeration capacity by using the jet pump to return oil.

[0041] Conventional technologies utilize jet pump oil return devices powered by high-pressure refrigerant gas. Typically, this requires approximately 2-3% of the system's refrigerant circulation volume to be supplied with high-pressure refrigerant gas. This means that 2-3% of the refrigerant circulation volume is directly bypassed from the compressor's discharge end to its suction end, which is equivalent to wasting the compressor's power.

[0042] Figure 6 is a schematic diagram of the pressure and enthalpy of the chiller system of the compound oil return device disclosed herein. Please refer to Figure 6 and Figures 2 to 4. When the compressor 11 is operating under high load conditions, the oil-rich refrigerant liquid in the flooded evaporator 51 is guided into the pipe-side circuit (i.e., heat exchange pipe 415) within the oil return heat exchanger 41 to exchange heat with the high-pressure, high-temperature refrigerant liquid from the condenser 31. The low-pressure oil-rich refrigerant liquid in the pipe side (i.e., heat exchange pipe 415) gains heat from the high-pressure, high-temperature refrigerant liquid, causing the refrigerant to evaporate into gas. The 5% of the refrigeration oil in the refrigerant liquid generates lift due to the boiling oil bubbles caused by the refrigerant evaporation, and is drawn out from the pipe-side outlet (i.e., the oil return heat exchange outlet 418 of the oil return heat exchanger 41) and enters the suction inlet 112 of the compressor 11, achieving the effect of non-powered oil return (eliminating the need for high-pressure gas). The high-pressure, high-temperature refrigerant liquid from the outlet of condenser 31 enters through the inlet on the shell side of the return oil heat exchanger 41 (i.e., refrigerant heat exchange inlet 412), and exchanges with the low-pressure, oil-rich refrigerant liquid evaporating on the pipe side (i.e., heat exchange pipe 415). This causes a change from state point 30 in Figure 6 to state point 301, resulting in subcooling. After passing through the expansion valve, the refrigerant expands to state point 401. Compared to the original state point 30 without a return oil heat exchanger, the refrigeration effect within the flooded evaporator 51 is significantly improved. With the increase in enthalpy difference (effect), compared to the enthalpy difference he1 in Figure 5, which is ∆he=h10-h40, the enthalpy difference he2 disclosed in this invention is ∆he=h10-h401. That is, ∆he=h10-h40 becomes ∆he=h10-h401, and the enthalpy difference increases to h40-h401. Under the same compressor 11 suction mass flow rate, this disclosure, compared to conventional technology, shows an increased refrigeration effect due to the increased enthalpy difference, thus improving the cooling capacity. The high load condition mentioned above refers to situations where the compressor 11 needs to handle a greater workload or demand than usual during operation, such as increased flow rate demand or the compressor needing to operate at higher pressures. The high load condition can be interpreted according to the actual situation.

[0043] Figure 7 is a flowchart of the control method of the composite oil return device disclosed herein. Please refer to Figure 7 and Figures 2 to 4. The control method S100 of the composite oil return device disclosed herein includes the following steps S110 to S120.

[0044] First, step S110 is performed to control the oil return process from a flooded evaporator 51 to a compressor 11, which includes steps S112 to S114. In step S112, a non-powered oil return process is executed using a main oil return circuit RDA. The non-powered main oil return circuit RDA is open for an extended period. Step S112 includes the following steps: Oil-rich refrigerant liquid from the flooded evaporator 51 is transported to an oil return heat exchanger 41 via a first oil return line R1. The first oil return line R1 connects the refrigerant outlet 516 of the flooded evaporator 51 to the oil return heat exchange inlet 416 of the oil return heat exchanger 411, so that the refrigerant outlet 516 of the flooded evaporator 51 serves as an oil return guide. The oil-rich refrigerant liquid is transported from the refrigerant outlet 516 to the return oil heat exchange inlet 416 of the return oil heat exchanger 41 via the first return oil pipeline R1. The oil-rich refrigerant liquid is then transported from the return oil heat exchange inlet 416 of the return oil heat exchanger 41 to the heat exchange pipeline 415 inside the return oil heat exchanger 41. In this way, the oil-rich refrigerant liquid in the flooded evaporator 51 is transported from the refrigerant outlet 516 to the heat exchange pipeline 415 inside the return oil heat exchanger 41 via the first return oil pipeline R1 and the return oil heat exchange inlet 416 of the return oil heat exchanger 41. At this time, the high-pressure, high-temperature liquid refrigerant of the condenser 31 is transferred to the interior of the shell inlet (i.e., refrigerant heat exchange inlet 412) of the return oil heat exchanger 41 via the third refrigerant pipeline P3, so that the high-pressure, high-temperature liquid refrigerant can exchange heat with the oil-rich refrigerant liquid located in the heat exchange pipeline 415; after the heat exchange, a piece of refrigeration oil in the oil-rich refrigerant liquid can be led out to a second return oil pipe through a return oil heat exchange outlet 418 of the return oil heat exchanger 41. In the oil-rich refrigerant liquid in heat exchange pipe 415, the low-pressure refrigerant liquid gains heat from the high-pressure, high-temperature refrigerant liquid, causing the refrigerant to evaporate into gas. The 5% of refrigerant oil in the oil-rich refrigerant liquid generates lift due to the boiling oil bubbles caused by the refrigerant evaporation. This lift-generating refrigerant oil is led out through the oil return heat exchange outlet 418 of the oil return heat exchanger 41 to the second oil return pipe R2; and through the second oil return pipe R2, the refrigerant oil enters the suction inlet 112 of the compressor 11. Entering the suction inlet 112 of the compressor 11 via the second oil return pipe R2 achieves a non-powered oil return effect (eliminating the need for high-pressure gas). This process allows the lift effect of the oil bubbles from the evaporation of the oil-rich refrigerant liquid in the flooded evaporator 51 to return oil to the compressor 11, and also creates subcooling in the high-pressure, high-temperature liquid state, thereby increasing the cooling effect of the flooded evaporator 51 and improving the cooling energy efficiency of the compressor 11. In one embodiment, the second oil return line R2 is connected to the compressor 11 through the first inlet P61 of the sixth refrigerant line P6.

[0045] Therefore, the composite oil return device RD disclosed herein can be applied to the chiller system 100 to draw oil-rich refrigerant liquid from the flooded evaporator 51 for oil return to the compressor 11 without power, saving about 2-3% of the cooling capacity loss of the traditional jet pump oil return device technology that uses high-pressure gas as power, thereby improving the cooling capacity and energy efficiency of the chiller system 100.

[0046] In the above step S112 process, if the compressor 11 is operating under partial load conditions, step S114 is executed. When the compressor 11 is operating under partial load conditions, an auxiliary oil return circuit RDB performs a power oil return step to assist the main oil return circuit RDA in returning oil from the flooded evaporator 51 to the compressor 11. The auxiliary oil return circuit RDB is connected to the main oil return circuit RDA through a third oil return line R3.

[0047] In step S114 above, the compressor 11 operates under partial load conditions as follows: the compressor 11 operates under partial load conditions of less than or equal to 50%, wherein the partial load is calculated as 1 / 2 of the rated current of the compressor 11 at full load.

[0048] In step S114 above, when the compressor 11 operates under partial load conditions, the following steps are included: detecting the refrigerant side pressure of one side of the flooded evaporator 51 and the chilled water outlet temperature of one side of the chilled water, wherein the refrigerant side pressure can be measured by a pressure gauge installed at the evaporator outlet 512, and the chilled water outlet temperature can be measured by a temperature gauge installed at the chilled water outlet G2. Next, based on the refrigerant type, a saturated refrigerant temperature corresponding to an evaporation pressure is calculated; the temperature difference between the saturated refrigerant temperature corresponding to the evaporation pressure and the chilled water outlet temperature is calculated to obtain an evaporator progressive temperature difference; and the calculated evaporator progressive temperature difference value is compared with an evaporator progressive temperature difference set value. When the evaporator progressive temperature difference value is greater than or equal to the evaporator progressive temperature difference set value, the power return oil step is executed.

[0049] Therefore, when the compressor 11 is operating under partial load conditions, the temperature difference between the high-pressure refrigerant liquid in the condenser 31 and the low-pressure rich oil refrigerant liquid drawn by the self-filling evaporator 51 is reduced. As a result, the low-pressure rich oil refrigerant liquid may not be able to evaporate completely, resulting in insufficient lift from the oil bubbles. At this time, the slightly reduced high-pressure gas is used as the power for the return oil injection pump 61 to draw the incompletely evaporated rich oil refrigerant liquid to evaporate and return the oil to the compressor 11 through the suction pipe (taking Figure 4 as an example, the path of the sixth refrigerant line P6 connected to the suction inlet 112 of the compressor 11). Since it is operating under partial load conditions, the loss of cooling capacity and the impact on cooling energy efficiency are also relatively small.

[0050] The control method S100 of the disclosed compound oil return device further includes the following step S120: controlling the oil return process from an oil separator 21 to a compressor 11 by an oil separator return circuit RDC, including the following step: detecting the compressor 11 by an oil level detector 118. When the compressor 11 is in a low oil level state, at least one switching device (such as the fourth switching device V4) located on a sixth oil return line R6 is turned on, so that the refrigerant oil separated in the oil separator 21 is transported to the compressor 11 through the sixth oil return line R6 from the oil return inlet 116 of the compressor 11. This prevents the refrigerant oil that is not intercepted by the oil separator 21 from accumulating in the flooded evaporator 51 with the discharge of the compressor 11, thereby reducing or preventing the oil concentration in the flooded evaporator 51 from increasing, preventing the evaporation temperature from deteriorating, and preventing the oil bubbles from gradually increasing during refrigerant boiling, which would lead to liquid compression of the compressor 11, thus preventing the compressor 11 from losing oil. When the compressor 11 is in a high oil level state, at least one switching device (such as the fourth switching device V4) located on the sixth oil return line R6 is turned off, so as to stop the oil separator 21 from returning oil to the compressor 11 through the sixth oil return line R6.

[0051] Furthermore, in one embodiment, this disclosure addresses the oil return control for compressor 11 when it experiences hydraulic compression oil loss. The control steps are as follows: Detecting the refrigerant discharge pressure (discharge pressure) and discharge temperature of compressor 11; then, calculating the saturated refrigerant temperature corresponding to the discharge pressure based on the refrigerant type; next, calculating the discharge superheat of compressor 11, which is equal to the discharge temperature minus the saturated refrigerant temperature corresponding to the discharge pressure; if the discharge superheat of compressor 11 is less than the discharge superheat set value, it is determined that compressor 11 is experiencing hydraulic compression oil loss. The process of eliminating the hydraulic compression oil loss condition of compressor 11 is then initiated. The compressor 11 controls the oil return from liquid compression loss, exceeding the chilled water return or outlet temperature tracking control; it controls the opening of the electronic expansion valve (as shown in Figure 1, the first switch device V1) to ensure that the refrigerant saturation temperature corresponding to the evaporation pressure of the flooded evaporator 51 is 2±1℃; the compressor 11 is loaded and operated so that its chilled water outlet temperature is not lower than 5.0±1℃, and the low-pressure switch of the compressor 11 is not activated; then, the oil return injection pump 61 of the compound oil return device RD and the auxiliary oil return circuit RDB are activated to recover the refrigerant oil in the flooded evaporator 51 and return it to the compressor. The compressor 11 maintains the above conditions and operates for 30 minutes as one cycle; after one cycle of operation to eliminate the abnormality of oil loss from liquid compression of the compressor 11, the steps of detecting the refrigerant discharge pressure (discharge pressure) and discharge temperature of the compressor 11 are repeated, and this cycle of operation and judgment is repeated until the oil loss condition of the compressor 11 is resolved.

[0052] In summary, this disclosure guides the oil-rich refrigerant liquid from the flooded evaporator into the pipe-side circuit of the oil return heat exchanger during compressor operation. There, it exchanges heat with the high-pressure, high-temperature refrigerant liquid from the condenser. The low-pressure, oil-rich refrigerant liquid on the pipe side gains heat from the high-pressure, high-temperature refrigerant liquid, causing the refrigerant to evaporate into gas. The 5% refrigerant oil in the refrigerant liquid generates lift due to the boiling oil bubbles caused by the refrigerant evaporation, and is drawn from the pipe-side outlet into the compressor's suction inlet, achieving a non-powered oil return effect (eliminating the need for high-pressure gas). At the same compressor suction mass flow rate, this disclosure, compared to conventional technology, increases the enthalpy difference, thus increasing the refrigeration effect of the chiller system and consequently improving its cooling capacity.

[0053] Furthermore, this disclosure states that when the compressor operates under partial load conditions, the temperature difference between the high-pressure refrigerant liquid in the condenser and the low-pressure rich oil refrigerant liquid drawn from the self-filling evaporator decreases. As a result, the low-pressure rich oil refrigerant liquid may not evaporate completely, leading to insufficient lift from the oil bubbles. In this case, the slightly reduced high-pressure gas is used as the power source for the return oil jet pump to draw the incompletely evaporated rich oil refrigerant liquid to evaporate and return it to the compressor via the suction pipe. Moreover, because it operates under partial load conditions, the loss of cooling capacity and the impact on cooling energy efficiency of the chiller system are relatively small.

[0054] Furthermore, the composite oil return device disclosed herein can selectively activate the oil return injection pump to assist in oil return via the oil return pipeline assembly, depending on the oil return status. This can reduce the loss of cooling capacity and ineffective power in conventional systems that rely entirely on high-pressure gas for power.

[0055] In addition, this disclosure discloses the setting of an oil separator return circuit, so that the refrigeration oil in the oil separator returns to the compressor, preventing the refrigeration oil that is not intercepted by the oil separator from gradually accumulating in the flooded evaporator with the compressor discharge, thereby reducing or preventing the oil concentration in the flooded evaporator from increasing, preventing the evaporation temperature from deteriorating, and preventing the oil bubbles from gradually increasing during refrigerant boiling, which would lead to compressor liquid compression, thus preventing the compressor from losing oil.

[0056] Although this disclosure has been disclosed above with reference to embodiments, it is not intended to limit this disclosure. Anyone skilled in the art may make some modifications and refinements without departing from the spirit and scope of this disclosure. Therefore, the scope of protection of this disclosure shall be determined by the appended claims.

[0057] 1g: Saturation point 4f,10,20,30,301,40,401: State points 100: Chiller Water System 11: Compressor 112: Inhalation Inlet 114: Exhaust outlet 116: Return oil inlet 118: Oil Level Detector 21: Oil separator 212: Oil separator inlet 214: Oil separator outlet 216: Return oil outlet 31: Condenser 312: Condenser inlet 314: Condenser outlet 41: Return oil heat exchanger 411: Casing 412: Refrigerant heat exchange inlet 414: Refrigerant heat exchange outlet 415: Heat exchange piping 416: Return oil heat exchange inlet 418: Return oil heat exchange outlet 51: Flooded Evaporator 512: Evaporator outlet 514: Evaporator Inlet 516: Refrigerant outlet 61: Return oil injection pump 612: First inlet of return oil injection pump 614: Second inlet of return oil injection pump 616: Return oil injection pump outlet B1: Entry point B2: Export end G1: Ice Water Inlet G2: Ice Water Outlet he1, he2: enthalpy difference L: Length direction PA: Refrigerant circulation piping assembly P1: First refrigerant piping P2: Second refrigerant pipeline P3: Third refrigerant piping P4: Fourth refrigerant piping P5: Fifth refrigerant pipeline P6: Sixth refrigerant pipeline P61: First Entrance P62: Second Entrance RA: Return oil pipeline assembly R1: First return oil line R2: Second return oil line R3: Third return oil line R4: Fourth return oil line R5: Fifth oil pipeline R6: Sixth oil pipeline RD: Composite oil return device RDA: Main return oil circuit RDB: Auxiliary oil return circuit RDC: Oil separator return circuit RL: Red dashed line S100: Control Method of Composite Oil Return Device S110, S112, S114, S120: Steps V1: First switching device V12: First switch input V14: First switch output V2: Second switching device V3: Third switching device V4: Fourth Switching Device

Claims

1. A composite oil return device, comprising: A main oil return circuit includes a flooded evaporator, a first oil return pipeline, an oil return heat exchanger, a second oil return pipeline, and a compressor. The oil return heat exchanger includes a housing, a refrigerant heat exchange inlet, a refrigerant heat exchange outlet, a heat exchange pipeline, an oil return heat exchange inlet, and an oil return heat exchange outlet. The housing has the refrigerant heat exchange inlet and the refrigerant heat exchange outlet at opposite ends. The oil return heat exchange inlet and the oil return heat exchange outlet are respectively provided on one side of the housing. The heat exchange pipeline is located inside the housing. One inlet end of the heat exchange pipeline is connected to the oil return heat exchange inlet, and one outlet end of the heat exchange pipeline is connected to the oil return heat exchange outlet. The first oil return line connects a refrigerant outlet of the flooded evaporator to the oil return heat exchange inlet of the oil return heat exchanger; the second oil return line connects the oil return heat exchange outlet of the oil return heat exchanger to the suction inlet of the compressor; and an auxiliary oil return circuit includes an oil return injection pump, the oil return heat exchanger, a third oil return line, a fourth oil return line, a fifth oil return line, and the compressor, wherein the fourth oil return line connects a first inlet of one of the oil return injection pumps to a discharge outlet of the compressor; the third oil return line connects the second oil return line to a second inlet of one of the oil return injection pumps; and the fifth oil return line connects the outlet of one of the oil return injection pumps to the suction inlet of the compressor.

2. The composite oil return device as described in claim 1 further comprises: An oil separator return circuit includes an oil separator, the compressor, at least one switching device, an oil level detector, and a sixth return oil line, wherein the oil level detector is located on the compressor, the sixth return oil line connects an oil return inlet of the compressor to an oil return outlet of the oil separator, and the at least one switching device is located on the sixth return oil line.

3. The composite oil return device as claimed in claim 2, wherein the at least one switching device is an electronic expansion valve or a solenoid valve.

4. The composite oil return device as claimed in claim 1, wherein the first oil return line and the fourth oil return line are each provided with at least one switching device.

5. The composite oil return device as claimed in claim 4, wherein the at least one switching device is an electronic expansion valve or a solenoid valve.

6. A control method for a composite oil return device, comprising the following steps: controlling the oil return process from a flooded evaporator to a compressor, comprising the following steps: executing a non-powered oil return process using a main oil return circuit, comprising the following steps: transporting an oil-rich refrigerant liquid from a flooded evaporator to an oil return heat exchanger via a first oil return pipeline; the oil-rich refrigerant liquid being transported from an oil return heat exchange inlet of the oil return heat exchanger to a heat exchange pipeline inside the oil return heat exchanger; After heat exchange, the refrigerant oil in the oil-rich refrigerant liquid can be led out to a second return oil line through a return oil heat exchange outlet of the return oil heat exchanger; and the refrigerant oil can be introduced into a suction inlet of the compressor through the second return oil line; and when the compressor is operating under partial load conditions, a power return oil step is performed by an auxiliary return oil circuit to assist the main return oil circuit in returning oil from the flooded evaporator to the compressor, wherein the auxiliary return oil circuit is connected to the main return oil circuit through a third return oil line.

7. The control method of the compound oil return device as described in claim 6, wherein the compressor operates under the partial load condition as follows: the compressor operates under the partial load condition at less than or equal to 50%, wherein the partial load is calculated as 1 / 2 of the compressor's full-load rated current.

8. A control method for the composite oil return device as described in claim 6, wherein when the compressor operates under partial load conditions, the method includes the following steps: detecting a refrigerant-side pressure and a chilled water outlet temperature on the chilled water side of the flooded evaporator; calculating a saturated refrigerant temperature corresponding to an evaporation pressure based on the refrigerant type; calculating a temperature difference between the saturated refrigerant temperature and the chilled water outlet temperature corresponding to the evaporation pressure to obtain an evaporator progressive temperature difference; and comparing the calculated evaporator progressive temperature difference value with an evaporator progressive temperature difference set value, wherein when the evaporator progressive temperature difference value is greater than or equal to the evaporator progressive temperature difference set value, the power oil return step is executed.

9. The control method for the composite oil return device as described in claim 6 further includes the following steps: controlling the oil return process from an oil separator to the compressor via an oil separator oil return circuit, including the following steps: detecting the compressor by an oil level detector; when the compressor is in a low oil level state, opening at least one switching device located on a sixth oil return line, so that a refrigerant oil separated in the oil separator is transported to the compressor from an oil return inlet of the compressor via the sixth oil return line; and when the compressor is in a high oil level state, closing the at least one switching device located on the sixth oil return line.

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