Multi-connected system and control method of oil return thereof

Abstract

The application discloses a multi-connected system and a control method for oil return of the multi-connected system. The multi-connected system comprises a four-way valve connected with a liquid pipe and a gas pipe; a plurality of indoor units are arranged in parallel and are arranged between the gas pipe and the liquid pipe; an outdoor unit is arranged on the liquid pipe; two ends of a protection pipeline are connected with the gas pipe, and a first gas-liquid separator and a first valve are arranged on the protection pipeline; and the control method comprises the following specific steps: before the multi-connected system sends an oil return start command, the first valve is in a normally closed state; when the multi-connected system sends the oil return start command, the frequency of a compressor is adjusted to a first frequency, and the opening degree of a second valve is adjusted to a first opening degree; during the oil return control process of the multi-connected system, the multi-connected system controls the frequency of the compressor, the opening degree of the second valve and whether the first valve is opened according to an actual gas return superheat degree; and after the multi-connected system sends an oil return end command, the first valve is closed. Thus, the reliability of unit operation and the service life of the unit are greatly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of multi-link heat pump air conditioners, and in particular to a multi-split system and a control method for oil return thereof. BACKGROUND

[0002] A multi-split system is a system in which one outdoor unit is matched with two or more indoor units. The core components of the system include a compressor, a condenser, a throttling component, and an evaporator. The operation of the multi-split system includes four processes, namely, a compression process, a condensation process, a throttling process, and an evaporation process. The compressor compresses low-temperature gaseous refrigerant into high-temperature gaseous refrigerant through work, and the gaseous refrigerant is then transferred to the condenser to exchange heat with outdoor air. The gaseous refrigerant is then phase-changed into liquid refrigerant, which is then flowed into the throttling component to be throttled and cooled, and is then changed into low-temperature and low-pressure liquid refrigerant. The liquid refrigerant is then absorbed heat in the evaporator to be evaporated, and the state of the refrigerant is changed from liquid to gas, and the refrigerant is returned to the compressor to repeat the refrigeration cycle.

[0003] The compressor is the heart of the refrigeration system, and it is extremely important to ensure the safety and reliability of the compressor during operation. Therefore, the compressor must be lubricated with oil to ensure its reliability. However, unlike other air conditioning systems, the pipeline of the multi-split system is much longer than that of a household wall-mounted unit or a unit machine, and the indoor load is diverse. As a result, during operation, some oil will inevitably be carried away from the compressor by the refrigerant. Over time, the amount of oil returned to the compressor will decrease, causing the compressor to idle and damage the compressor. Therefore, each multi-split system must have an oil return process.

[0004] In the oil return control of some manufacturers, the opening degree of the expansion valve during the oil return of the indoor unit is set to a fixed value, and in order to achieve the effect of oil return, the second valve is opened very wide. However, in some low-temperature refrigeration processes, if the customer only opens a small load indoor unit under the premise of adding refrigerant, the evaporation capacity of the indoor unit will not be enough to evaporate so much refrigerant, causing the liquid refrigerant to flow back to the compressor side, resulting in a sudden decrease in the discharge temperature and the return gas superheat of the compressor. In addition, when the compressor frequency is calculated according to the load during normal operation with a small load, the frequency will not be very high, and the superheat will be even more difficult to establish.

[0005] In summary, the sudden increase in the circulation amount of refrigerant during the oil return process, combined with the low frequency of the compressor after the oil return process, will cause the discharge temperature to decrease and the return gas superheat to decrease, and even the risk of compressor liquid return, resulting in liquid hammer. SUMMARY

[0006] In order to solve the problem that the compressor exhaust temperature is lowered, the gas return superheat degree is reduced, and the compressor liquid return risk occurs in the oil return process of the multi-connected system in the prior art, the purpose of the present application is to provide a multi-connected system and an oil return control method thereof, which can prevent the gas return superheat degree from being too low and prevent the compressor liquid return from reducing the reliability of the unit and damaging the compressor during and after the oil return process.

[0007] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme: an oil return control method of a multi-connected system, which is used for controlling the oil return process of the multi-connected system in a refrigeration mode; the multi-connected system comprises a four-way valve, four ports of the four-way valve are connected with an oil separator, a liquid pipe, a gas pipe and a second gas-liquid separator respectively; an outlet of the second gas-liquid separator is connected with an inlet of a compressor, an outlet of the compressor is connected with an inlet of the oil separator, an oil outlet of the oil separator is connected with the compressor, a plurality of indoor units are arranged side by side, the plurality of indoor units are arranged between the gas pipe and the liquid pipe, two ports of the indoor unit are connected with the gas pipe and the liquid pipe respectively, an outdoor unit is arranged on the liquid pipe, and the outdoor unit is located between the indoor unit and the four-way valve; the oil return control method further comprises a second valve for controlling the refrigerant flow passing through the indoor unit, a plurality of second valves are arranged in one-to-one correspondence with the plurality of indoor units; two ends of a protection pipeline are connected with the gas pipe, and a first gas-liquid separator and a first valve are installed on the protection pipeline.

[0008] The specific steps of the oil return control method of the multi-connected system are as follows:

[0009] Before the multi-connected system sends an oil return start command, the first valve is in a normally closed state;

[0010] When the multi-connected system sends the oil return start command, the compressor frequency is adjusted to a first frequency, and the opening degree of the second valve is adjusted to a first opening degree;

[0011] During the oil return control process of the multi-connected system, the multi-connected system controls the compressor frequency, controls the opening degree of the second valve, and controls whether the first valve is opened according to the actual gas return superheat degree;

[0012] After the multi-connected system sends an oil return end command, the first valve is closed.

[0013] As a preferred, the lower the outdoor environment temperature is when the multi-connected system sends the oil return start command, the smaller the first opening degree of the second valve is.

[0014] As a preferred, the opening degree of the second valve of the indoor unit in a standby state is smaller than the opening degree of the second valve of the indoor unit in a start state when the multi-connected system sends the oil return start command.

[0015] As a preferred, the actual gas return superheat degree = gas return temperature TS - low pressure saturated temperature Ps-t.

[0016] As preferred, in the oil return control process of the multi-connected system, the system periodically detects the actual return gas superheat degree, and adjusts the compressor frequency, the opening degree of the second valve and whether the first valve is opened according to the actual return gas superheat degree.

[0017] As preferred, in the oil return control process of the multi-connected system, when the actual return gas superheat degree is greater than the target return gas superheat degree, the first valve is closed, the opening degree of the second valve is unchanged, and the compressor frequency is unchanged; when the actual return gas superheat degree is less than the target return gas superheat degree, the compressor frequency is adjusted to the second frequency, the first valve is opened, and the opening degree of the second valve is adjusted.

[0018] As preferred, when the actual return gas superheat degree is less than the target return gas superheat degree, the lower the ambient temperature, the greater the number of steps required for adjusting the second valve.

[0019] As preferred, when the actual return gas superheat degree is less than the target return gas superheat degree, the number of steps required for adjusting the second valve of the indoor unit in the standby state is greater than the number of steps required for adjusting the second valve of the indoor unit in the start-up state.

[0020] As preferred, in the oil return control process of the multi-connected system, the lower the outdoor ambient temperature, the lower the target return gas superheat degree.

[0021] A multi-connected system applying the oil return control method of the multi-connected system.

[0022] The technical scheme of the present application has the following beneficial effects: in the oil return process, the system refrigerant flow can be detected in time, and the system can be protected in time by adjusting the opening degree of the valve and the frequency of the compressor; the above method accurately simulates whether the compressor has a risk of returning liquid through the return gas superheat degree, and thus protects the compressor in time to prevent excessive liquid return from causing liquid hammer, greatly improving the reliability of the unit operation and the service life of the unit. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 A system diagram of the multi-connected system.

[0024] Reference signs: 1, compressor; 2, oil separator; 3, high-pressure switch; 4, four-way valve; 5, second gas-liquid separator; 6, outdoor unit; 7, liquid accumulator; 8, indoor unit; 9, second valve; 10, first gas-liquid separator; 11, first valve; 12, liquid pipe; 13, gas pipe; 14, protection pipe. DETAILED DESCRIPTION

[0025] Embodiments of the present application are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference signs represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by reference to the drawings are exemplary and are intended to explain the present application, and cannot be understood as a limitation of the present application.

[0026] In the description of the present application, it is to be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "clockwise", "counterclockwise" and the like indicate the orientation or positional relationship shown in the drawings, which are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation of the present application.

[0027] In addition, the terms "first", "second" are only for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features referred to. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, unless otherwise stated, the meaning of "a plurality of" is two or more, unless otherwise explicitly limited.

[0028] In the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connecting", "fixing" and the like should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0029] In the present application, unless otherwise explicitly specified and limited, the first feature "above" or "below" the second feature can include the first and second features directly contacting, or the first and second features not directly contacting but contacting through another feature between them. Moreover, the first feature "above", "above" and "above" the second feature includes the first feature directly above and obliquely above the second feature, or only indicates that the horizontal height of the first feature is higher than that of the second feature. The first feature "below", "below" and "below" the second feature includes the first feature directly below and obliquely below the second feature, or only indicates that the horizontal height of the first feature is less than that of the second feature. Embodiment

[0030] As Figure 1The multi-connected system comprises a four-way valve 4, four ports of the four-way valve 4 are connected with an oil separator 2, a liquid pipe 12, a gas pipe 13 and a second gas-liquid separator 5 respectively, an outlet of the second gas-liquid separator 5 is connected with an inlet of a compressor 1, an outlet of the compressor 1 is connected with an inlet of the oil separator 2, an oil outlet of the oil separator 2 is connected with the compressor 1, a high-pressure switch 3 is installed between the oil separator and the four-way valve, a plurality of indoor units 8 are arranged in parallel, the plurality of indoor units 8 are arranged between the gas pipe 13 and the liquid pipe 12, two ports of each indoor unit 8 are connected with the gas pipe 13 and the liquid pipe 12 respectively, an outdoor unit 6 is arranged on the liquid pipe 12, and the outdoor unit 6 is located between the indoor unit 8 and the four-way valve 4; the multi-connected system further comprises a second valve 9 for controlling the refrigerant flow through the indoor unit 8, and a plurality of second valves 9 are arranged in one-to-one correspondence with the plurality of indoor units 8.

[0031] Two ends of a protection pipe 14 are connected with the gas pipe 13, and a first gas-liquid separator 10 and a first valve 11 are installed on the protection pipe 14.

[0032] In the embodiment, a liquid accumulator is further installed on the pipeline between the indoor unit 8 and the outdoor unit 6, a gas pipe stop valve is installed on the gas pipe 13, and a liquid pipe stop valve is installed on the liquid pipe 12.

[0033] In the embodiment, the first valve 11 is a ball valve, and the second valve 9 is an electronic expansion valve.

[0034] The oil return control method of the multi-connected system comprises the following steps:

[0035] Before the multi-connected system sends an oil return start command, the first valve 11 is in a normally closed state; when the multi-connected system sends an oil return command, the compressor frequency is adjusted to a first frequency, and the opening degree of the second valve 9 is adjusted to a first opening degree; during the oil return control process of the multi-connected system, the multi-connected system controls the compressor frequency, controls the opening degree of the second valve 9 and controls whether the first valve 11 is opened according to the actual gas return superheat degree; after the multi-connected system sends an oil return end command, the first valve 11 is closed, and the system enters a soft start.

[0036] In this way, during the oil return process, the system refrigerant flow can be detected in time, and the system can be protected in time by adjusting the opening degree of the valve and the frequency of the compressor 1; the above method accurately simulates whether the compressor 1 has a liquid return risk through the gas return superheat degree, and thus protects the compressor 1 in time to prevent excessive liquid return from causing liquid hammer, greatly improving the reliability of the unit operation and the service life of the unit.

[0037] In the embodiment, the first frequency of the compressor 1 is 70 HZ. In this way, since the liquid refrigerant and the lubricating oil have mutual solubility, the high frequency of the compressor 1 can accelerate the flow rate of the refrigerant, and the high-speed refrigerant can take the oil attached to the pipeline back to the compressor 1.

[0038] In the embodiment, the lower the outdoor ambient temperature, the smaller the first opening degree of the second valve 9 when the multi-split system sends the oil return start command. In this way, the lower the ambient temperature, the more the flow of refrigerant returning to the compressor 1 needs to be controlled, thereby preventing excessive system liquid return from causing liquid hammering;

[0039] The first opening degree of the second valve 9 corresponding to the indoor unit 8 in the standby state is smaller than the first opening degree of the second valve 9 corresponding to the indoor unit 8 in the start-up state.

[0040] In the embodiment, the actual return gas superheat degree = return gas temperature Ts - low pressure saturation temperature Ps-t.

[0041] In the embodiment, during the oil return control process of the multi-split system, the multi-split system periodically detects the actual return gas superheat degree and adjusts the compressor frequency, the opening degree of the second valve 9, and whether the first valve 11 is opened according to the actual return gas superheat degree.

[0042] Specifically, during the oil return control process of the multi-split system, when the actual return gas superheat degree > target return gas superheat degree, the second valve 9 is closed, and the compressor frequency remains unchanged.

[0043] When the actual return gas superheat degree < target return gas superheat degree, the multi-split system considers that too much refrigerant is returning to the compressor 1, and needs to close the valve and increase the frequency to establish the return gas superheat degree, so the compressor frequency is increased to the second frequency, the first valve 11 is opened, and the second valve 9 is closed.

[0044] Further preferably, the target return gas superheat degree is related to the outdoor ambient temperature; specifically, the lower the outdoor ambient temperature, the lower the target return gas superheat degree,

[0045] Further, during the oil return control process, the multi-split system detects the actual return gas superheat degree at an interval of 3s; specifically, the multi-split system detects the ambient temperature, return gas temperature Ts, and low pressure saturation temperature Ps-t every 3s.

[0046] Further preferably, during the oil return control process of the multi-split system, when the actual return gas superheat degree < target return gas superheat degree, the lower the ambient temperature, the greater the opening degree of the second valve 9 adjusted each time.

[0047] Because the indoor unit 8 in the start-up state has an evaporation amount, in order to ensure the reliability of the oil return of the multi-split system, the second valve 9 of the indoor unit 8 in the standby state needs to be adjusted to a larger opening than the second valve 9 of the indoor unit 8 in the start-up state. In this way, the opening of the second valve 9 is reduced to reduce the flow of refrigerant returning to the compressor 1, and because the indoor unit 8 in the start-up state has an evaporation amount, in order to ensure the reliability of the oil return of the multi-split system, the number of steps by which the opening of the second valve 9 of the indoor unit 8 in the start-up state is reduced is smaller than the number of steps by which the opening of the second valve 9 of the indoor unit 8 in the standby state is reduced each time.

[0048] In the actual superheat detection gap, the second valve 9 opening is dynamically adjusted.

[0049] Further preferably, the second frequency of the compressor 1 is 75 HZ; in the oil return control process of the multi-split system, when the actual superheat is less than the target superheat, the frequency of the compressor is adjusted to 75 HZ.

[0050] In this embodiment, during the adjustment of the second valve 9, if the actual superheat reaches the target superheat and lasts for 1 minute and 30 seconds, the opening of the second valve 9 is not adjusted, the second valve 9 maintains the current opening, and the first valve 11 is closed until the oil return process of the multi-split system is completed. In this way, it is considered that the superheat of the multi-split system has been established at this time, the opening of the second valve 9 does not need to be reduced, the first gas-liquid separator 10 does not need to participate in system protection, and the ball valve can be closed.

[0051] In this embodiment, after the oil return is completed, the multi-split system enters soft start, at this time, the second valve 9 of the indoor unit 8 in the standby state is closed, the first valve 11 is closed, the opening of the second valve 9 corresponding to the indoor unit 8 in the start-up state is adjusted according to the ambient temperature and the load, and is opened to the soft start opening, and the frequency of the compressor is also set according to the original soft start control.

[0052] Specifically, in a low-temperature environment, the indoor unit 8 with a small number of matches cannot have a large opening when starting up, because the compressor 1 does little work at the start, a small opening of the second valve 9 is needed to raise the exhaust gas; the evaporation area of the indoor unit 8 with a relatively large number of matches will be larger, so the opening of the second valve 9 can be slightly larger.

[0053] Further preferably, in the soft start process of the multi-split system, the superheat fault point is judged by detecting the return gas temperature Ts and the low-pressure saturation temperature Ps-t, and the lower the ambient temperature, the lower the superheat fault point.

[0054] When the actual gas return superheat is less than the gas return superheat fault point at the current ambient temperature, and continues for 1 min 20 s, the multi-split system enters shutdown protection immediately after the soft start ends, and displays a fault code; if it does not continue for 1 min 20 s, the multi-split system continues to follow the original control to continue the soft start process until the multi-split system runs smoothly.

[0055] This embodiment takes a 1P indoor unit and two 3P indoor units as an example, and the specific steps are as follows:

[0056] 1) The system runs in a 18℃ environment, and the 1P indoor unit is turned on for refrigeration.

[0057] 2) The system runs until the oil return is started, at which time the compressor frequency is increased to 70HZ

[0058] 3) The 1P indoor unit's expansion valve is opened to 180 steps, and the other two 3P indoor units' expansion valves are opened to 170 steps, at which time the return gas temperature Ts and the low-pressure saturation temperature Ps-t are detected, and the following situations will be encountered:

[0059] ① When Ts is 10℃ and Ps-t is 3℃, at this time the actual gas return superheat is 7℃> the target gas return superheat is 4℃, then the 1P indoor unit continues to maintain 180 steps, the two standby indoor units maintain 170 steps, and the gas return superheat is always 4℃, then the valve opening degree continues to maintain unchanged, the frequency maintains 70HZ, the ball valve is not opened, until the oil return ends, enters the soft start, at this time the two standby indoor units' valves are closed, the 1P indoor unit's valve is opened to 130 steps, and the gas return superheat is calculated to be 3℃ at the same time, then the system continues to control the soft start, and enters the normal control operation;

[0060] ②When Ts is 5℃ and Ps-t is 1.5℃, the actual superheat of the return gas is 3.5℃, which is less than the target superheat of 4℃. At this time, the compressor frequency is increased to 75HZ, the ball valve is opened, the indoor units in operation and standby are controlled, the expansion valve of the 1P indoor unit is closed by 3 steps per second, and the expansion valves of the other two 3P indoor units are closed by 5 steps per second. After 10 seconds, the expansion valve of the 1P indoor unit is closed to 150 steps, and the expansion valves of the two standby indoor units are closed to 120 steps. At this time, the system detects that the return gas temperature Ts is 7℃ and the low-pressure saturated temperature Ps-t is 3℃, so the superheat of the return gas is 4℃, and the superheat of the return gas is always equal to 4℃ after 1 minute and 35 seconds. At this time, the expansion valve of the 1P indoor unit is maintained at 150 steps, the expansion valves of the other two standby indoor units are maintained at 120 steps, the compressor frequency is continuously maintained at 75HZ, and the ball valve is closed until the oil return is completed. The system enters soft start, the expansion valves of the standby indoor units are closed, and the expansion valve of the 1P indoor unit is opened from 150 steps to 130 steps. At this time, the actual superheat of the return gas detected is 3℃, so the system continues to control the soft start and enters the normal control operation.

[0061] ③When Ts is 10℃ and Ps-t is 8℃, the superheat of the exhaust gas is 2℃, which is less than 4℃. At this time, the system frequency is increased to 75HZ, the indoor units in operation and standby are controlled according to the control, the ball valve is opened, the opening of the expansion valve of the 1P indoor unit is closed by 3 steps per second, the opening of the expansion valves of the other two 3P indoor units is closed by 5 steps per second, the opening of the expansion valve of the indoor unit in operation is always closed to 140 steps, and the opening of the expansion valve of the standby indoor unit is always closed to 120 steps. At this time, the expansion valve is no longer closed, the compressor frequency is maintained at 75HZ, and the expansion valve is maintained at this opening until the oil return is completed. At this time, the soft start is entered, the standby valve is closed, the 1P indoor unit in operation is opened to 130 steps, and the ball valve is closed. At this time, the actual superheat of the return gas calculated is -0.5℃ and lasts for 1 minute and 30 seconds. After the system is unblocked, it will immediately report a low return gas superheat fault, and enter the shutdown protection state.

[0062] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0063] Although the embodiments of the present application have been shown and described above, it is understood that the above-described embodiments are exemplary and are not to be construed as limiting the present application, and that changes, modifications, substitutions and variations can be made by those skilled in the art without departing from the spirit and scope of the present application.

Claims

1. A method for controlling the return oil flow in a multi-split air conditioning system, characterized in that: A method for controlling an oil return process of a multi-connected system in a refrigeration mode The multi-connected system comprises a four-way valve (4), four ports of the four-way valve (4) are connected with an oil separator (2), a liquid pipe (12), a gas pipe (13) and a second gas-liquid separator (5) respectively, an outlet of the second gas-liquid separator (5) is connected with an inlet of a compressor (1), an outlet of the compressor (1) is connected with an inlet of the oil separator (2), an oil outlet of the oil separator (2) is connected with the compressor (1), a plurality of indoor units (8) are arranged in parallel, the plurality of indoor units (8) are arranged between the gas pipe (13) and the liquid pipe (12), two ports of the indoor unit (8) are connected with the gas pipe (13) and the liquid pipe (12) respectively, an outdoor unit (6) is arranged on the liquid pipe (12), and the outdoor unit (6) is located between the indoor unit (8) and the four-way valve (4); the multi-connected system further comprises a second valve (9) for controlling a refrigerant flow through the indoor unit (8), and a plurality of second valves (9) are arranged in one-to-one correspondence with the plurality of indoor units (8); Two ends of a protection pipeline (14) are connected with the gas pipe (13), and a first gas-liquid separator (10) and a first valve (11) are arranged on the protection pipeline (14); The specific steps of the multi-connected system oil return control method are as follows: Before the multi-connected system sends an oil return start command, the first valve (11) is in a normally closed state; When the multi-connected system sends the oil return start command, the frequency of the compressor (1) is adjusted to a first frequency, and the opening degree of the second valve (9) is adjusted to a first opening degree; During the oil return control process of the multi-connected system, the multi-connected system controls the frequency of the compressor (1), the opening degree of the second valve (9) and whether the first valve (11) is opened according to an actual gas return superheat degree; After the multi-connected system sends an oil return end command, the first valve (11) is closed.

2. The oil return control method of a multi-split system according to claim 1, characterized by: When the multi-connected system sends the oil return start command, the lower the outdoor environment temperature, the smaller the first opening degree of the second valve (9).

3. The oil return control method of a multi-split system according to claim 1, characterized by: When the multi-connected system sends the oil return start command, the opening degree of the second valve (9) of the indoor unit (8) in a standby state is smaller than that of the second valve (9) of the indoor unit (8) in a running state.

4. The oil return control method of a multi-split system according to claim 1, characterized by: The actual gas return superheat degree = gas return temperature TS - low pressure saturated temperature Ps-t.

5. The oil return control method of a multi-split system according to claim 1, characterized by: During the oil return control process of the multi-connected system, the system periodically detects the actual gas return superheat degree, and adjusts the frequency of the compressor (1), the opening degree of the second valve (9) and whether the first valve (11) is opened according to the actual gas return superheat degree.

6. The oil return control method of a multi-split system according to claim 5, characterized by: During the oil return control process of the multi-connected system, when the actual gas return superheat degree > target gas return superheat degree, the first valve (11) is closed, the opening degree of the second valve (9) is unchanged, and the frequency of the compressor (1) is unchanged; When the actual gas return superheat degree < target gas return superheat degree, the frequency of the compressor (1) is adjusted to a second frequency, the first valve (11) is opened, and the opening degree of the second valve (9) is adjusted.

7. The oil return control method of a multi-split system according to claim 6, characterized by: When the actual gas return superheat degree < target gas return superheat degree, the lower the environment temperature, the greater the number of steps required for the adjustment of the second valve (9).

8. The oil return control method of a multi-split system according to claim 6, characterized by: When the actual gas return superheat is less than the target gas return superheat, the number of steps required to adjust the second valve (9) of the indoor unit (8) in the standby state is greater than the number of steps required to adjust the second valve (9) of the indoor unit (8) in the start-up state.

9. The oil return control method of a multi-split system according to claim 6, characterized by: In the oil return control process of the multi-connected system, the lower the outdoor environment temperature, the lower the target gas return superheat.

10. A multi-split system, characterized in that: The application relates to a kind of oil return control methods of multi-connected system according to any one of the above claims 1-9.

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