EP3640566A1

EP3640566A1 - Multi-split system and oil return control method and device of multi-split system

Info

Publication number: EP3640566A1
Application number: EP18922438.9A
Authority: EP
Inventors: Qihui BU; Yongfeng XU; Boqi LIANG; Hongwei Li; Shilong DONG; Kongxiang WU; Xiaohong Wu; Yu Zhang
Original assignee: Midea Group Co Ltd; GD Midea Heating and Ventilating Equipment Co Ltd
Current assignee: Midea Group Co Ltd; GD Midea Heating and Ventilating Equipment Co Ltd
Priority date: 2018-06-13
Filing date: 2018-12-18
Publication date: 2020-04-22
Also published as: WO2019237698A1; CN108759174A; US20210148615A1; CN108759174B

Abstract

Provided are a multi-split air conditioning system and a method and device for controlling oil return of the multi-split air conditioning system. The method includes: acquiring a minimum refrigerant flux required for carrying a lubricating oil in the system back to an outdoor unit and a current refrigerant flux of the system every a first preset time during operation of the system; detecting and determining that the current refrigerant flux is less than or equal to the minimum refrigerant flux; acquiring a total oil discharge amount of a compressor since last time oil return; detecting and determining that the total oil discharge amount is greater than a maximum safe oil discharge amount; and controlling the system to return oil. Therefore, the present disclosure can ensure that the compressor will not operate in short of oil due to great oil discharge amount and small oil return amount, and can effectively improve the capability and energy efficiency of the multi-split air conditioning system; furthermore, the method is simple and feasible.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority to Chinese patent application No. 201810606528.1, filed on June 13, 2018 , the content of which is incorporated herein by reference in its entirety.

FIELD

The present disclosure relates to the technical field of cooling, in particular to a method for controlling oil return of a multi-split air conditioning system, a device for controlling oil return of a multi-split air conditioning system, and a multi-split air conditioning system.

BACKGROUND

With the constant increase of multi-split air conditioner models (multi-split air conditioner for short), the multi-split air conditioner is increasingly widely used in large buildings and villas owing to the advantages of energy saving, intelligent regulation, accurate temperature control, high automation degree, flexible use, convenient management and the like. Generally, a multi-split air conditioner has a long tube, and numbers of oil storage positions. With the increase of system operation time, more and more lubricating oil will be accumulated in the tube of the system. If the oil is not returned in time, then a compressor will be damaged due to the shortage of the lubricating oil.
With regard to the above-described problem, a conventional method is to perform an oil return operation after the multi-split air conditioner operates for a fixed period of time; the oil return operation is generally an operation switched to a cooling mode, so as to carry the lubricating oil in the tube of the system back to an outdoor unit, and avoid the compressor from operating in short of oil.
However, the operation conditions of the multi-split air conditioner are inconsistent, and the oil discharge amount during operation is also inconsistent. If the oil discharge amount is great and the oil return amount is small under the operation condition, the compressor may also operate in short of oil in a fixed time.

SUMMARY

The objects of the present disclosure are to solve at least one of the technical problems in the related art to a certain extent. Therefore, a first object of the present disclosure is to provide a method for controlling oil return of a multi-split air conditioning system. The present disclosure can ensure that the compressor will not operate in short of oil due to great oil discharge amount and small oil return amount, and can effectively improve the capability and energy efficiency of the multi-split air conditioning system; furthermore, the method is simple and feasible.
A second object of the present disclosure is to provide a non-transitory computer readable storage medium.
A third object of the present disclosure is to provide a device for controlling oil return of a multi-split air conditioning system.
A fourth object of the present disclosure is to provide a multi-split air conditioning system.
To achieve the above objects, in a first aspect, an embodiment of the present disclosure provides a method for controlling oil return of a multi-split air conditioning system, the multi-split air conditioning system including an outdoor unit and a plurality of indoor units, wherein the outdoor unit includes a compressor; the method includes: acquiring a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit and a current refrigerant flux of the system every a first preset time during operation of the system; detecting and determining that the current refrigerant flux is less than or equal to the minimum refrigerant flux; acquiring a total oil discharge amount of the compressor since last time oil return; detecting and determining that the total oil discharge amount is greater than a maximum safe oil discharge amount; and controlling the system to return oil.
According to an embodiment of the present disclosure, the method for controlling oil return of a multi-split air conditioning system includes: acquiring a minimum refrigerant flux required for carrying a lubricating oil in the system back to an outdoor unit and a current refrigerant flux of the system every a first preset time during operation of the system, detecting and determining that the current refrigerant flux is less than or equal to the minimum refrigerant flux; acquiring a total oil discharge amount of the compressor since last time oil; detecting and determining that the total oil discharge amount is greater than a maximum safe oil discharge amount; and controlling system to return oil. Therefore, the present disclosure can ensure that the compressor will not operate in short of oil due to great oil discharge amount and small oil return amount, and can effectively improve the capability and energy efficiency of the multi-split air conditioning system; furthermore, the method is simple and feasible.
According to an embodiment of the present disclosure, the method further includes: detecting and determining that a duration of the current refrigerant flux being greater than the minimum refrigerant flux is greater than a second preset time when the current refrigerant flux is greater than the minimum refrigerant flux; zero-clearing the total oil discharge amount, detecting and determining that a duration since last time oil return is greater than a third preset time; and controlling the system to return oil, wherein the third preset time > the second preset time > the first preset time.
According to an embodiment of the present disclosure, the method for controlling oil return of a multi-split air conditioning system further includes: acquiring a current operation mode of the system; controlling the system to return oil in a cooling mode when the current operation mode is determined to be the cooling mode; and controlling the system to return oil in a heating mode when the current operation mode is determined to be the heating mode.
According to an embodiment of the present disclosure, acquiring a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit in the cooling mode includes: acquiring a saturation evaporation temperature of the system, and acquiring a saturation evaporation pressure of the system according to the saturation evaporation temperature; acquiring a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system; and acquiring the minimum refrigerant flux from a preset table according to the saturation evaporation pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
According to an embodiment of the present disclosure, acquiring a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit in the heating mode includes: acquiring an exhaust pressure of the system; acquiring a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system; and acquiring the minimum refrigerant flux from a preset table according to the exhaust pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
According to an embodiment of the present disclosure, the acquiring a current refrigerant flux of the system includes: acquiring an exhaust pressure and an air return pressure of the system, and a current operation frequency of the compressor; acquiring a saturation exhaust temperature of the system according to the exhaust pressure; acquiring a saturation air return temperature of the system according to the air return pressure; and acquiring the current refrigerant flux according to the saturation exhaust temperature, the saturation air return temperature and the current operation frequency.
According to an embodiment of the present disclosure, the current refrigerant flux is acquired by the following formula: $G 2 = C 0 + (C 1 * S) + (C 2 * D) + (C 3 * S^{2}) + (C 4 * S * D) + (C 5 * D^{2}) + (C 6 * S^{3}) + (C 7 * D * S^{2}) + (C 8 * S * D^{2}) + (C 9 * D^{3}),$
wherein G2 is the current refrigerant flux; S is the saturation air return temperature; D is the saturation exhaust temperature; C0 to C9 are calculating coefficients and are acquired according to the current operation frequency.
To achieve the above objects, in a second aspect, an embodiment of the present disclosure provides a non-transitory computer readable storage medium having stored therein a computer program that, when executed by a processor, causes the processor to realize the method for controlling oil return of a multi-split air conditioning system.
The non-transitory computer readable storage medium according to the embodiment of the present disclosure, by means of the method for controlling oil return of a multi-split air conditioning system, can ensure that the compressor will not operate in short of oil due to great oil discharge amount and small oil return amount, and can effectively improve the capability and energy efficiency of the multi-split air conditioning system; furthermore, the method is simple and feasible.
To achieve the above objects, in a third aspect, an embodiment of the present disclosure provides a device for controlling oil return of a multi-split air conditioning system, the multi-split air conditioning system including an outdoor unit and a plurality of indoor units, wherein the outdoor unit includes a compressor; the device includes: a first acquisition module, configured to acquire a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit every a first preset time during operation of the system; a second acquisition module, configured to acquire a current refrigerant flux of the system every the first preset time during operation of the system; and a control module, configured to detect and determine that the current refrigerant flux is less than or equal to the minimum refrigerant flux, acquire a total oil discharge amount of the compressor since last time oil return, detect and determine that the total oil discharge amount is greater than a maximum safe oil discharge amount, and control the system to return oil.
In the device for controlling oil return of a multi-split air conditioning system according to the embodiment of the present disclosure, the first acquisition module acquires the minimum refrigerant flux required for carrying the lubricating oil in the system back to the outdoor unit every the first preset time during operation of the system; the second acquisition module acquires the current refrigerant flux of the system every the first preset time during operation of the system; and the control module detects and determines that the current refrigerant flux is less than or equal to the minimum refrigerant flux, acquires the total oil discharge amount of the compressor since last time oil return, detects and determines that the total oil discharge amount is greater than the maximum safe oil discharge amount, and controls the system to return oil. Therefore, the present disclosure can ensure that the compressor will not operate in short of oil due to great oil discharge amount and small oil return amount, and can effectively improve the capability and energy efficiency of the multi-split air conditioning system; furthermore, the method is simple and feasible.
According to an embodiment of the present disclosure, the control module is further specifically configured to: detect and determine, when the current refrigerant flux is greater than the minimum refrigerant flux, that a duration of the current refrigerant flux being greater than the minimum refrigerant flux is greater than a second preset time; zero-clear the total oil discharge amount, detect and determine that a duration since last time oil return is greater than a third preset time, and control the system to return oil, wherein the third preset time > the second preset time > the first preset time.
According to an embodiment of the present disclosure, the device for controlling oil return of a multi-split air conditioning system further includes: a third acquisition module, configured to acquire a current operation mode of the system; wherein the control module is further configured to control the system to return oil in a cooling mode when the current operation mode is determined to be the cooling mode, and control the system to return oil in a heating mode when the current operation mode is determined to be the heating mode.
According to an embodiment of the present disclosure, when the current operation mode is determined to be the cooling mode, the first acquisition module is specifically configured to: acquire a saturation evaporation temperature of the system, and acquire a saturation evaporation pressure of the system according to the saturation evaporation temperature; acquire a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system; and acquire the minimum refrigerant flux from a preset table according to the saturation evaporation pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
According to an embodiment of the present disclosure, when the current operation mode is determined to be the heating mode, the first acquisition module is specifically configured to: acquire an exhaust pressure of the system; acquire a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system; and acquire the minimum refrigerant flux from a preset table according to the exhaust pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
According to an embodiment of the present disclosure, the second acquisition module is specifically configured to: acquire an exhaust pressure and an air return pressure of the system, and a current operation frequency of the compressor; acquire a saturation exhaust temperature of the system according to the exhaust pressure; acquire a saturation air return temperature of the system according to the air return pressure; and acquire the current refrigerant flux according to the saturation exhaust temperature, the saturation air return temperature and the current operation frequency.
According to an embodiment of the present disclosure, the second acquisition module acquires the current refrigerant flux by the following formula: $G 2 = C 0 + (C 1 * S) + (C 2 * D) + (C 3 * S^{2}) + (C 4 * S * D) + (C 5 * D^{2}) + (C 6 * S^{3}) + (C 7 * D * S^{2}) + (C 8 * S * D^{2}) + (C 9 * D^{3}),$
wherein G2 is the current refrigerant flux; S is the saturation air return temperature; D is the saturation exhaust temperature; C0 to C9 are calculating coefficients and are acquired according to the current operation frequency.
To achieve the above objects, in a fourth aspect, an embodiment of the present disclosure provides multi-split air conditioning system, including the device for controlling oil return of a multi-split air conditioning system.
The multi-split air conditioning system according to the embodiment of the present disclosure, by means of the device for controlling oil return of a multi-split air conditioning system, can ensure that the compressor will not operate in short of oil due to great oil discharge amount and small oil return amount, and can effectively improve the capability and energy efficiency of the multi-split air conditioning system; furthermore, the method is simple and feasible.
A part of additional aspects and advantages of the present disclosure will be provided in the descriptions hereafter, and the other part will become apparent from the descriptions hereafter, or can be understood through the practice of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a flow chart of the method for controlling oil return of a multi-split air conditioning system according to one embodiment of the present disclosure;
Fig. 2 is a flow chart of acquiring the minimum refrigeration flux in the cooling mode according to one embodiment of the present disclosure;
Fig. 3 is a flow chart of acquiring the minimum refrigeration flux in the heating mode according to one embodiment of the present disclosure;
Fig. 4 is a flow chart of the method for controlling oil return of a multi-split air conditioning system in the cooling mode according to one embodiment of the present disclosure;
Fig. 5 is a flow chart of the method for controlling oil return of a multi-split air conditioning system in the heating mode according to one embodiment of the present disclosure; and
Fig. 6 is a block diagram of the device for controlling oil return of a multi-split air conditioning system according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

The embodiments of the present disclosure will be described in detail hereafter, and the examples of the embodiments are shown in the drawings, wherein the same or similar signs from beginning to end denote the same or similar elements or the elements having the same or similar functions. The embodiments described below with reference to the drawings are for illustration only, and are intended to explain the present disclosure, but not to limit the present disclosure.
The method for controlling oil return of a multi-split air conditioning system, the non-transitory computer readable storage medium, the device for controlling oil return of a multi-split air conditioning system, and the multi-split air conditioning system provided according to the embodiments of the present disclosure will be described hereafter with reference to the drawings.
Fig. 1 is a flow chart of the method for controlling oil return of a multi-split air conditioning system according to one embodiment of the present disclosure.
In the embodiment of the present disclosure, the multi-split air conditioning system may include an outdoor unit and a plurality of indoor units, wherein the outdoor unit includes a compressor.
As shown in Fig. 1, the method for controlling oil return of a multi-split air conditioning system according to the embodiment of the present disclosure may include:
S1, acquiring a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit and a current refrigerant flux of the system every a first preset time during operation of the system, wherein the first preset time can be calibrated according to practical situations.
To be specific, the minimum refrigerant flux required for carrying the lubricating oil in the system back to the outdoor unit and the current refrigerant flux of the system can be acquired according to an operation parameter of the system during operation of the system.
According to an embodiment of the present disclosure, as shown in Fig. 2, acquiring a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit in the cooling mode includes:
S201, acquiring a saturation evaporation temperature of the system, and acquiring a saturation evaporation pressure of the system according to the saturation evaporation temperature.
To be specific, during operation of the system in the cooling mode, a temperature of an indoor heat exchanger tube of each indoor unit in an operating state in the plurality of indoor units is respectively acquired, so as to acquire the temperatures of a plurality of indoor heat exchanger tubes; and then the temperatures of the plurality of indoor heat exchanger tubes are averaged, so as to acquire the saturation evaporation temperature of the system. For example, assuming that ten indoor units in the plurality of indoor units are in the operating state, the temperature of the indoor heat exchanger tubes of five indoor units is 7°C, and the temperature of the indoor heat exchanger tubes of the other five indoor units is 8°C, then the acquired saturation evaporation temperature of the system is (5^∗7+8^∗5)/10=7.5. The saturation evaporation pressure of the system is acquired by querying a relationship table between saturation evaporation temperatures and saturation evaporation pressures according to the saturation evaporation temperature.
S202, acquiring a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system.
S203, acquiring the minimum refrigerant flux from a preset table according to the saturation evaporation pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
Specifically, after the system is completely mounted, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube can be pre-stored in the system. During operation of the system in the cooling mode, the saturation evaporation temperature of the system is acquired; then the saturation evaporation pressure of the system is acquired according to the saturation evaporation temperature; and finally the minimum refrigerant flux (equivalent to a minimum gas circulation amount in the refrigeration air tube) is acquired from the preset table according to the saturation evaporation pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.

The preset table can be pre-acquired through a large number of experiments and tests. For example, table 1 shows the minimum refrigeration flux under different tube diameters in the cooling mode when the saturation evaporation pressure is 1200MPa.

Table 1

Cooling mode
Saturation evaporation temperature Te	16.3	Concentration of the lubricating oil	1.0%
Saturation evaporation pressure Pe	1200.0	Density of the lubricating oil	930.0
Density of gaseous refrigerant ρg	50.8	Density of a mixture of the lubricating oil and liquid refrigerant ρl	1098.7
Density of liquid refrigerant p	1100.4

Air tube			Minimum refrigerant flow rate	Minimum refrigerant flux
Outer diameter	Wall thickness	Inner diameter	m/s	kg/h
ϕ15.9	1.0	13.9	1.72	47.64
ϕ19.1	1.0	17.1	1.90	79.97
ϕ22.2	1.0	20.2	2.07	121.28
ϕ25.4	1.0	23.4	2.23	175.17
ϕ28.6	1.2	26.2	2.36	232.37
ϕ31.8	1.2	29.4	2.50	309.95
ϕ34.9	1.3	32.3	2.62	392.13
ϕ38.1	1.4	35.3	2.73	489.63
ϕ41.3	1.5	38.3	2.85	600.38
ϕ44.5	1.7	41.1	2.95	716.20
ϕ54.1	1.9	50.3	3.26	1186.72

As shown in Fig. 1, in the cooling mode, when the acquired saturation evaporation temperature of the system is 16.3°C, the corresponding saturation evaporation pressure of the system would be 1200MPa; if the concentration of the lubricating oil is 1.0%, the density of the lubricating oil is 930.0kg/m³, and the inner diameter of the refrigeration air tube is 32.3cm, then the current minimum refrigerant flux required for carrying the lubricating oil in the system back to the outdoor unit would be 392.13kg/h.
According to an embodiment of the present disclosure, as shown in Fig. 3, acquiring a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit in the heating mode includes:
S301, acquiring an exhaust pressure of the system.
Specifically, the exhaust pressure of the system can be acquired by acquiring a pressure at an exhaust port of the compressor.
It should be noted that in the heating mode, the exhaust pressure of the system is also a condensation pressure of the system which can be directly acquired via a pressure sensor disposed at the exhaust port of the compressor; alternatively, the exhaust pressure can also be acquired by the following means: first, an exhaust temperature, that is the condensation temperature of the system, is acquired via a temperature sensor disposed at the exhaust port of the compressor, and then a condensation temperature of the system, that is the exhaust pressure, is acquired according to the condensation temperature.
S302, acquiring a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system.
S303, acquiring the minimum refrigerant flux from a preset table according to the exhaust pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
Specifically, after the system is completely mounted, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube can be pre-stored in the system. During operation of the system in the heating mode, the exhaust pressure of the system is acquired; and then the minimum refrigerant flux (equivalent to a minimum gas circulation amount in the refrigeration air tube) is acquired from the preset table according to the exhaust pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.

The preset table can be pre-acquired through a large number of experiments and tests. For example, table 2 shows the minimum refrigeration flux under different tube diameters in the heating mode when the exhaust pressure is 2200MPa.

Table 2

Heating mode
Condensation temperature Tc	38.0	Concentration of the lubricating oil	1.0%
Condensation pressure Pc	2200.0	Density of the lubricating oil	930.0
Density of gaseous refrigerant pg	97.1	Density of a mixture of the lubricating oil and liquid refrigerant ρ1	987.2
Density of liquid refrigerant ρ	987.7

Air tube			Minimum refrigerant flow rate	Minimum refrigerant flux
Outer diameter	Wall thickness	Inner diameter	m/s	kg/hr
ϕ15.9	1.0	13.9	1.18	62.43
ϕ19.1	1.0	17.1	1.31	104.79
ϕ22.2	1.0	20.2	1.42	158.93
ϕ25.4	1.0	23.4	1.53	229.54
ϕ28.6	1.2	26.2	1.62	304.50
ϕ31.8	1.2	29.4	1.71	406.16
ϕ34.9	1.3	32.3	1.79	513.85
ϕ38.1	1.4	35.3	1.88	641.60
ϕ41.3	1.5	38.3	1.95	786.73
ϕ44.5	1.7	41.1	2.02	938.50
ϕ54.1	1.9	50.3	2.24	1555.06

As shown in Fig. 2, in the heating mode, when the acquired exhaust pressure of the system is 2200MPa, if the concentration of the lubricating oil is 1.0%, the density of the lubricating oil is 930.0kg/m³, and the inner diameter of the refrigeration air tube is 32.3cm, then the current minimum refrigerant flux required for carrying the lubricating oil in the system back to the outdoor unit would be 392.13kg/h.
According to an embodiment of the present disclosure, the acquiring a current refrigerant flux of the system includes: acquiring an exhaust pressure and an air return pressure of the system, and a current operation frequency of the compressor; acquiring a saturation exhaust temperature of the system according to the exhaust pressure; acquiring a saturation air return temperature of the system according to the air return pressure; and acquiring the current refrigerant flux according to the saturation exhaust temperature, the saturation air return temperature and the current operation frequency.
According to an embodiment of the present disclosure, the current refrigerant flux can be acquired by the following formula (1): $G 2 = C 0 + (C 1 * S) + (C 2 * D) + (C 3 * S^{2}) + (C 4 * S * D) + (C 5 * D^{2}) + (C 6 * S^{3}) + (C 7 * D * S^{2}) + (C 8 * S * D^{2}) + (C 9 * D^{3})$
wherein G2 is the current refrigerant flux; S is the saturation air return temperature; D is the saturation exhaust temperature; C0 to C9 are calculating coefficients and are acquired according to the current operation frequency.
Specifically, during operation of the system, the saturation air return temperature and the saturation exhaust temperature can be first calculated according to the air return pressure and exhaust pressure of the compressor; and then ten coefficients provided by a compressor manufacturer are utilized to calculate the current refrigerant flux, that is the actual gas circulation amount, according to the operation frequency of the compressor.

For example, table 3 shows ten coefficients provided by a certain compressor manufacturer.

Table 3

Reference number	Capability C	Primary side power P	Primary side current A	Mass flow M
C0	157083.7038	-147.4029839	5.22452E-05	506.1576401
C1	5461.243994	-15.66866598	3.17939E-05	20.23765343
C2	-0.070791694	290.0853125	0.32866752	10.88823094
C3	56.50627815	-0.598613169	0.001438798	0.336176128
C4	-0.189212999	0.855442311	0.000855527	0.215924562
C5	-19.38408202	0.618120874	0.007960909	-0.159218374
C6	0.016318866	-0.000650971	1.89498E-05	-0.00096547
C7	-0.025955152	-0.004744646	-5.70037E-05	-0.000151881
C8	-0.452236172	0.003124316	-1.11086E-05	-0.002222256
C9	0.068265115	-0.00785332	-5.86867E-05	0.000640792

It should be noted that the calculating coefficients C0 to C9 in the formula (1) are respectively the values corresponding to capability C in table 3.
In practical use, different compressor operation frequencies correspond to different ten coefficients; a compressor manufacturer generally will provide ten coefficients under a plurality of operation frequencies, for example the ten coefficients under the operation frequencies of 30Hz, 60Hz, 75Hz, 90Hz and the like. In this case, first, the ten coefficients under the current operation frequency can be acquired through an interpolation algorithm according to the ten coefficients under different operation frequencies; and then the current refrigerant flux of the system is acquired through calculation by the formula (1) according to the ten coefficients under the current operation frequency. For example, assuming that the current operation frequency of the compressor is 70Hz, first, the ten coefficients under the operation frequency 70Hz can be acquired through the interpolation algorithm according to the ten coefficients under the operation frequencies 60Hz and 75Hz, wherein the values corresponding to the capability C in the ten coefficients are respectively the calculating coefficients C0 to C9 under the operation frequency; and then the current refrigerant flux of the system is acquired through calculation by the formula (1) according to the calculating coefficients C0 to C9, the acquired saturation air return temperature and the saturation exhaust temperature.
Certainly, the following calculation method is also acceptable: first, the refrigerant flux of the system under the operation frequency 60Hz is acquired through calculation by the formula (1) according to the ten coefficients under the operation frequency 60Hz, the acquired saturation air return temperature and the saturation exhaust temperature; and the refrigerant flux of the system under the operation frequency 75Hz is acquired through calculation by the formula (1) according to the ten coefficients under the operation frequency 75Hz, the acquired saturation air return temperature and the saturation exhaust temperature; then, the refrigerant flux of the system under the operation frequency 70Hz, that is the current refrigerant flux of the system, is calculated through the interpolation algorithm. It can be understood that compared with the former method, the calculation amount of the latter method is greatly reduced. Therefore, the latter method is preferably adopted.
It should be noted that the acquisition methods of the current refrigeration flux under the cooling mode and the heating mode are the same.
S2, it is detected and determined that the current refrigerant flux is less than or equal to the minimum refrigerant flux.
S3, a total oil discharge amount of the compressor since last time oil return is acquired.
Specifically, each time the compressor operates, oil will be discharged; and the oil discharge rate (equivalent to an oil drainage rate) of the compressor is known. Therefore, the total oil discharge amount of the compressor can be acquired through calculation according to the oil discharge rate of the compressor.

For example, the rate discharge rate of the compressor can be provided by a compressor manufacturer, as shown in table 4.

Table 4

Minimum oil amount	350ml
Ex-factory oil amount	500ml
Safe oil drainage amount for calculation	470cm³
Compressor exhaust volume	36cm³/rev
Oil dilution limit	0.4
Minimum rotating speed	30rps
Maximum rotating speed	90rps
30rps oil drainage rate (%)	0.41%
60rps oil drainage rate (%)	0.73%
90rps oil drainage rate (%)	1.56%
120rps oil drainage rate (%)	2.00%

Table 4 provides different oil drainage rates under different rotating speeds; rotating speeds are proportional to operation frequencies; therefore, oil drainage rates under different rotating speeds can be acquired according to the proportional relationship therebetween; then the oil drainage rate under the current operation frequency can be acquired through the interpolation algorithm; and finally the total oil discharge amount of the compressor can be acquired according to the oil drainage rate under the current operation frequency, the current refrigerant flux, an operation time of the compressor, and the density of the lubricating oil. For example, the total oil discharge amount of the compressor = the oil drainage rate under the current operation frequency ^∗ the current refrigerant flux ^∗ the operation time of the compressor / the density of the lubricating oil.
It should be noted that for a system provided with an oil separator, the total oil discharge amount can also be acquired according to a separation efficiency of the oil separator. For example, the total oil discharge amount of the compressor = the oil drainage rate under the current operation frequency ^∗ the current refrigerant flux ^∗ (1- the separation efficiency of the oil separator) ^∗ the operation time of the compressor / the density of the lubricating oil, wherein the separation efficiency of the oil separator is generally 90%.
S4, it is detected and determined that the total oil discharge amount is greater than a maximum safe oil discharge amount.
The maximum safe oil discharge amount can be acquired from data provided by the compressor manufacturer. As shown in table 4, the maximum safe oil discharge amount of the compressor is 470ml.
S5, the system is controlled to return oil.
To be specific, the minimum refrigerant flux required for carrying the lubricating oil in the system back to the outdoor unit and the current refrigerant flux of the system are acquired in real time during operation of the system; and whether the current refrigerant flux is less than or equal to the minimum refrigerant flux is determined. If so, then the current refrigerant flux is too low, and the lubricating oil cannot normally return to the outdoor unit; in this case, the summarized oil discharge amount, that is the total oil discharge amount of the compressor, is calculated, and whether the total oil discharge amount is greater than a maximum safe oil discharge amount; if so, then the current oil discharge amount is too great, which would be easy to cause the compressor to operate in short of oil; therefore, the system is controlled to return oil, so as to ensure that the compressor would not be damaged due to operation in short of oil. Therefore, the present disclosure can ensure that the compressor will not operate in short of oil due to great oil discharge amount and small oil return amount, and can effectively improve the capability and energy efficiency of the multi-split air conditioning system; furthermore, the method is simple and feasible.
Further, according to an embodiment of the present disclosure, the method further includes: detecting and determining whether a duration of the current refrigerant flux being greater than the minimum refrigerant flux is greater than a second preset time when the current refrigerant flux is greater than the minimum refrigerant flux; zero-clearing the total oil discharge amount, detecting and determining that a duration since last time oil return is greater than a third preset time; and controlling the system to return oil, wherein the third preset time > second preset time > the first preset time; the first preset time, the second preset time and the third preset time can be calibrated according to practical situations.
To be specific, if the current refrigerant flux is greater than the minimum refrigerant flux and the duration is greater than the second preset time, then the current refrigerant flux is sufficient; in this case, the lubricating oil can normally return to the outdoor unit without specifically controlling oil return; and when the operation time of the system since last time oil return is greater than the third preset time, the system is controlled to return oil. Therefore, under the situation that the refrigerant flux of the system is sufficient, special oil return processes should be reduced as few as possible, so as to ensure indoor comfort.
According to an embodiment of the present disclosure, the method for controlling oil return of a multi-split air conditioning system further includes: acquiring a current operation mode of the system; controlling the system to return oil in a cooling mode when the current operation mode is determined to be the cooling mode; and controlling the system to return oil in a heating mode when the current operation mode is determined to be the heating mode.
To be specific, if the system is currently in the cooling mode, then the system returns oil in the cooling mode directly; and if the system is currently in the heating mode, then the system returns oil in the heating mode directly, which can be realized by utilizing the prior art, thus effectively avoiding the influence on indoor comfort during oil return. For example, in the heating mode, when oil return is required, if the system is switched to the cooling mode, then the indoor temperature would reduce, and the indoor comfort would be influenced.
Further, in order to enable a person skilled in the art to more clearly understand the present disclosure, the specific steps of the method for controlling oil return of a multi-split air conditioning system in the cooling mode and in the heating mode will be respectively described in detail hereafter. Fig. 4 is a flow chart of the method for controlling oil return of a multi-split air conditioning system in the cooling mode according to one embodiment of the present disclosure. As shown in Fig. 4, the method for controlling oil return of a multi-split air conditioning system may include:

S401, the multi-split air conditioning system operates in a cooling mode;
S402, acquiring a temperature of an indoor heat exchanger tube of the indoor unit;
S403, calculating the minimum refrigerant flux G1;
S404, acquiring an air return pressure Pe and an air return temperature Te;
S405, calculating the current refrigerant flux G2;
S406, determining whether the current refrigerant flux G2 is greater than the minimum refrigerant flux G1; if so, then executing the step S407; and if not, then executing the step S411;
S407, determining whether a duration of the current refrigerant flux G2 being greater than the minimum refrigerant flux G1 is greater than t1; if so, then executing the step S408; and if not, then executing the step S402;
S408, zero-clearing the total oil discharge amount;
S409, determining whether a duration since last time oil return is greater than t2; if so, then executing the step S410; and if not, then executing the step S402;
S410, returning oil in the cooling mode;
S411, acquiring a total oil discharge amount of the compressor since last time oil return;
S412, determining whether the total oil discharge amount is greater than a maximum safe oil discharge amount; if so, then executing the step S410; and if not, then executing the step S402.

Fig. 5 is a flow chart of the method for controlling oil return of a multi-split air conditioning system in the heating mode according to one embodiment of the present disclosure. As shown in Fig. 5, the method for controlling oil return of a multi-split air conditioning system may include:

S501, the multi-split air conditioning system operates in the heating mode;
S502, acquiring an exhaust pressure Pc;
S503, calculating the minimum refrigerant flux G1;
S504, acquiring an air return pressure Pe and an air return temperature Te;
S505, calculating the current refrigerant flux G2;
S506, determining whether the current refrigerant flux G2 is greater than the minimum refrigerant flux G1; if so, then executing the step S507; and if not, then executing the step S511;
S507, determining whether a duration of the current refrigerant flux G2 being greater than the minimum refrigerant flux G1 is greater than t1; if so, then executing the step S508; and if not, then executing the step S502;
S508, zero-clearing the total oil discharge amount;
S509, determining whether a duration since last time oil return is greater than t2; if so, then executing the step S510; and if not, then executing the step S502;
S510, returning oil in the heating mode;
S511, acquiring a total oil discharge amount of the compressor since last time oil return;
S512, determining whether the total oil discharge amount is greater than a maximum safe oil discharge amount; if so, then executing the step S510; and if not, then executing the step S502.

In the method for controlling oil return of a multi-split air conditioning system according to the embodiment of the present disclosure, the minimum refrigerant flux required for carrying the lubricating oil in the system back to the outdoor unit and the current refrigerant flux of the system are acquired every the first preset time during operation of the system; whether the current refrigerant flux is less than or equal to the minimum refrigerant flux is detected and determined; the total oil discharge amount of the compressor since last time oil return is acquired; whether the total oil discharge amount is greater than the maximum safe oil discharge amount is detected and determined; the system is controlled to return oil. Therefore, the present disclosure can ensure that the compressor will not operate in short of oil due to great oil discharge amount and small oil return amount, and can effectively improve the capability and energy efficiency of the multi-split air conditioning system; furthermore, the method is simple and feasible.
In addition, an embodiment of the present disclosure further provides a non-transitory computer readable storage medium having stored therein a computer program that, when executed by a processor, causes the processor to realize the method for controlling oil return of a multi-split air conditioning system.
The non-transitory computer readable storage medium provided according to the embodiment of the present disclosure, by means of the method for controlling oil return of a multi-split air conditioning system, can ensure that the compressor will not operate in short of oil due to great oil discharge amount and small oil return amount, and can effectively improve the capability and energy efficiency of the multi-split air conditioning system; furthermore, the method is simple and feasible.
Fig. 6 is a block diagram of the device for controlling oil return of a multi-split air conditioning system according to one embodiment of the present disclosure.
In the embodiment of the present disclosure, the multi-split air conditioning system includes an outdoor unit and a plurality of indoor units, wherein the outdoor unit includes a compressor.
As shown in Fig. 6, the device for controlling oil return of a multi-split air conditioning system according to the embodiment of the present disclosure may include: a first acquisition module 100, a second acquisition module 200 and a control module 300.
The first acquisition module 100 is configured to acquire a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit every a first preset time during operation of the system; the second acquisition module 200 is configured to acquire a current refrigerant flux of the system every the first preset time during operation of the system; and the control module 300 is configured to detect and determine that the current refrigerant flux is less than or equal to the minimum refrigerant flux, acquire a total oil discharge amount of the compressor since last time oil return, detect and determine that the total oil discharge amount is greater than a maximum safe oil discharge amount, and control the system to return oil.
According to an embodiment of the present disclosure, the control module 300 is further specifically configured to: detect and determine, when the current refrigerant flux is greater than the minimum refrigerant flux, that a duration of the current refrigerant flux being greater than the minimum refrigerant flux is greater than a second preset time; zero-clear the total oil discharge amount, detect and determine that a duration since last time oil return is greater than a third preset time, and control the system to return oil, wherein the third preset time > the second preset time > the first preset time.
According to an embodiment of the present disclosure, the device for controlling oil return of a multi-split air conditioning system further includes: a third acquisition module (not shown in the figure), wherein the third acquisition module is configured to acquire a current operation mode of the system; the control module 300 is further configured to control the system to return oil in a cooling mode when the current operation mode is determined to be the cooling mode, and control the system to return oil in a heating mode when the current operation mode is determined to be the heating mode.
According to an embodiment of the present disclosure, when the current operation mode is determined to be the cooling mode, the first acquisition module 100 is specifically configured to: acquire a saturation evaporation temperature of the system, and acquire a saturation evaporation pressure of the system according to the saturation evaporation temperature; acquire a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system; and acquire the minimum refrigerant flux from a preset table according to the saturation evaporation pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
According to an embodiment of the present disclosure, when the current operation mode is determined to be the heating mode, the first acquisition module 100 is specifically configured to: acquire an exhaust pressure of the system; acquire a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system; and acquire the minimum refrigerant flux from a preset table according to the exhaust pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
According to an embodiment of the present disclosure, the second acquisition module 200 is specifically configured to: acquire an exhaust pressure and an air return pressure of the system, and a current operation frequency of the compressor; acquire a saturation exhaust temperature of the system according to the exhaust pressure; acquire a saturation air return temperature of the system according to the air return pressure; and acquire the current refrigerant flux according to the saturation exhaust temperature, the saturation air return temperature and the current operation frequency.
According to an embodiment of the present disclosure, the second acquisition module 200 acquires the current refrigerant flux by the following formula: $G 2 = C 0 + (C 1 * S) + (C 2 * D) + (C 3 * S^{2}) + (C 4 * S * D) + (C 5 * D^{2}) + (C 6 * S^{3}) + (C 7 * D * S^{2}) + (C 8 * S * D^{2}) + (C 9 * D^{3}),$
wherein G2 is the current refrigerant flux; S is the saturation air return temperature; D is the saturation exhaust temperature; C0 to C9 are calculating coefficients and are acquired according to the current operation frequency.
In the device for controlling oil return of a multi-split air conditioning system according to the embodiment of the present disclosure, the first acquisition module acquires the minimum refrigerant flux required for carrying the lubricating oil in the system back to the outdoor unit every the first preset time during operation of the system; the second acquisition module acquires the current refrigerant flux of the system every the first preset time during operation of the system; and the control module detects and determines that the current refrigerant flux is less than or equal to the minimum refrigerant flux, acquires the total oil discharge amount of the compressor since last time oil return, detects and determines that the total oil discharge amount is greater than the maximum safe oil discharge amount, and
controls the system to return oil. Therefore, the present disclosure can ensure that the compressor will not operate in short of oil due to great oil discharge amount and small oil return amount, and can effectively improve the capability and energy efficiency of the multi-split air conditioning system; furthermore, the method is simple and feasible.
In addition, an embodiment of the present disclosure further provides a multi-split air conditioning system, the multi-split air conditioning system including the device for controlling oil return of a multi-split air conditioning system.
The multi-split air conditioning system provided according to the embodiment of the present disclosure, by means of the device for controlling oil return of a multi-split air conditioning system, can ensure that the compressor will not operate in short of oil due to great oil discharge amount and small oil return amount, and can effectively improve the capability and energy efficiency of the multi-split air conditioning system; furthermore, the method is simple and feasible.
In addition, In the descriptions of the present disclosure, the azimuth or position relationships indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "anticlockwise", "axial direction", "radial direction", "circumferential" and the like are on the basis of the azimuth and position relationships as shown in the drawings, and are only intended to facilitate and simplify the description of the present disclosure, but not intended to indicate or imply that the designated devices or elements must have a specific azimuth or are constructed and operated in a specific azimuth. Therefore, the terms cannot be considered to limit the present disclosure.
In addition, the terms "first" and "second" are used for the purpose of description only, but cannot be considered to indicate or imply relative importance or implicitly indicate the number of the indicated technical features. Therefore, a feature defined by "first" or "second" may explicitly or implicitly include at least one of the features. In the description of the present disclosure, unless otherwise stated, "a plurality of' means at least two, for example, two, three and the like.
In the present disclosure, unless otherwise specified and stated, a first feature being "on" or "under" a second feature means that the first feature and the second feature can be in direct contact, or in indirect contact via an intermediate medium. Furthermore, the first feature being "on", "above" and "over" the second feature means that the first feature can be right above or obliquely above the second feature, or only denotes that the horizontal height of the first feature is greater than that of the second feature. The first feature being "under", "below" and "underneath" the second feature means that the first feature can be right below or obliquely below the second feature, or only denotes that the horizontal height of the first feature is less than that of the second feature.
In the description of the specification, the reference terms "one embodiment", "some embodiments", "example", "a specific example" or "some examples" and the like mean that the specific characteristic, structure, material or feature described in combination with the embodiment or the example are contained in at least one embodiment or example of the present disclosure. In the specification, the schematic recitation of the above-described terms does not necessarily refer to the same embodiment or example. Furthermore, the described specific characteristic, structure, material or feature can be combined in an appropriate manner in any one or more embodiments or examples. In addition, under the situation of having no conflict, a person skilled in the art can combine or incorporate different embodiments or examples described in the specification and the features of the different embodiments or examples.
Although the embodiments of the present disclosure have been shown and described hereabove, it can be understood that the above-described embodiments are only for illumination, but not intended to limit the present disclosure. And a person skilled in the art can make various changes, modifications, substitutions and variations to the above-described embodiments in the scope of the present disclosure.

Claims

A method for controlling oil return of a multi-split air conditioning system, the multi-split air conditioning system comprising an outdoor unit and a plurality of indoor units, wherein the outdoor unit comprises a compressor; the method comprises:
acquiring a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit and a current refrigerant flux of the system every a first preset time during operation of the system;

detecting and determining that the current refrigerant flux is less than or equal to the minimum refrigerant flux;

acquiring a total oil discharge amount of the compressor since last time oil return;

detecting and determining that the total oil discharge amount is greater than a maximum safe oil discharge amount; and

controlling the system to return oil.
The method for controlling oil return of a multi-split air conditioning system according to claim 1, further comprising:
detecting and determining that a duration of the current refrigerant flux being greater than the minimum refrigerant flux is greater than a second preset time when the current refrigerant flux is greater than the minimum refrigerant flux;

zero-clearing the total oil discharge amount, detecting and determining that a duration since last time oil return is greater than a third preset time; and

controlling the system to return oil, wherein the third preset time > the second preset time > the first preset time.
The method for controlling oil return of a multi-split air conditioning system according to claim 1 or 2, further comprising:
acquiring a current operation mode of the system;

controlling the system to return oil in a cooling mode when the current operation mode is determined to be the cooling mode; and

controlling the system to return oil in a heating mode when the current operation mode is determined to be the heating mode.
The method for controlling oil return of a multi-split air conditioning system according to claim 3, wherein acquiring a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit in the cooling mode comprises:
acquiring a saturation evaporation temperature of the system, and acquiring a saturation evaporation pressure of the system according to the saturation evaporation temperature;

acquiring a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system; and

acquiring the minimum refrigerant flux from a preset table according to the saturation evaporation pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
The method for controlling oil return of a multi-split air conditioning system according to claim 3, wherein acquiring a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit in the heating mode comprises:
acquiring an exhaust pressure of the system;

acquiring a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system; and

acquiring the minimum refrigerant flux from a preset table according to the exhaust pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
The method for controlling oil return of a multi-split air conditioning system according to claim 1 or 2, wherein the acquiring the current refrigerant flux of the system comprises:
acquiring an exhaust pressure and an air return pressure of the system, and a current operation frequency of the compressor;

acquiring a saturation exhaust temperature of the system according to the exhaust pressure;

acquiring a saturation air return temperature of the system according to the air return pressure; and

acquiring the current refrigerant flux according to the saturation exhaust temperature, the saturation air return temperature and the current operation frequency.
The method for controlling oil return of a multi-split air conditioning system according to claim 6, wherein the current refrigerant flux is acquired by the following formula: $G 2 = C 0 + (C 1 * S) + (C 2 * D) + (C 3 * S^{2}) + (C 4 * S * D) + (C 5 * D^{2}) + (C 6 * S^{3}) + (C 7 * D * S^{2}) + (C 8 * S * D^{2}) + (C 9 * D^{3}),$
wherein G2 is the current refrigerant flux; S is the saturation air return temperature; D is the saturation exhaust temperature; C0 to C9 are calculating coefficients and are acquired according to the current operation frequency.
A non-transitory computer readable storage medium having stored therein a computer program that, when executed by a processor, causes the processor to realize the method for controlling oil return of a multi-split air conditioning system according to any one of claims 1 to 7.
A device for controlling oil return of a multi-split air conditioning system, the multi-split air conditioning system comprising an outdoor unit and a plurality of indoor units, wherein the outdoor unit comprises a compressor; the device comprises:
a first acquisition module, configured to acquire a minimum refrigerant flux required for carrying a lubricating oil in the system back to the outdoor unit every a first preset time during operation of the system;

a second acquisition module, configured to acquire a current refrigerant flux of the system every the first preset time during operation of the system; and

a control module, configured to detect and determine that the current refrigerant flux is less than or equal to the minimum refrigerant flux, acquire a total oil discharge amount of the compressor since last time oil return, detect and determine that the total oil discharge amount is greater than a maximum safe oil discharge amount, and control the system to return oil.
The device for controlling oil return of a multi-split air conditioning system according to claim 9, wherein the control module is further specifically configured to: detect and determine, when the current refrigerant flux is greater than the minimum refrigerant flux, that a duration of the current refrigerant flux being greater than the minimum refrigerant flux is greater than a second preset time; zero-clear the total oil discharge amount, detect and determine that a duration since last time oil return is greater than a third preset time, and control the system to return oil, wherein the third preset time > the second preset time > the first preset time.
The device for controlling oil return of a multi-split air conditioning system according to claim 9 or 10, wherein the device further comprises:
a third acquisition module, configured to acquire a current operation mode of the system,

wherein the control module is further configured to control the system to return oil in a cooling mode when the current operation mode is determined to be the cooling mode, and control the system to return oil in a heating mode when the current operation mode is determined to be the heating mode.
The device for controlling oil return of a multi-split air conditioning system according to claim 11, wherein when the current operation mode is determined to be the cooling mode, the first acquisition module is specifically configured to
acquire a saturation evaporation temperature of the system, and acquire a saturation evaporation pressure of the system according to the saturation evaporation temperature;
acquire a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system; and
acquire the minimum refrigerant flux from a preset table according to the saturation evaporation pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
The device for controlling oil return of a multi-split air conditioning system according to claim 11, wherein when the current operation mode is determined to be the heating mode, the first acquisition module is specifically configured to
acquire an exhaust pressure of the system;
acquire a concentration of the lubricating oil, a density of the lubricating oil, and an inner diameter of a refrigerant air tube in the system; and
acquire the minimum refrigerant flux from a preset table according to the exhaust pressure, the concentration of the lubricating oil, the density of the lubricating oil, and the inner diameter of the refrigerant air tube.
The device for controlling oil return of a multi-split air conditioning system according to claim 9 or 10, wherein the second acquisition module is specifically configured to
acquire an exhaust pressure and an air return pressure of the system, and a current operation frequency of the compressor;
acquire a saturation exhaust temperature of the system according to the exhaust pressure;
acquire a saturation air return temperature of the system according to the air return pressure; and
acquire the current refrigerant flux according to the saturation exhaust temperature, the saturation air return temperature and the current operation frequency.
The device for controlling oil return of a multi-split air conditioning system according to claim 14, wherein the second acquisition module is configured to acquire the current refrigerant flux by the following formula: $G 2 = C 0 + (C 1 * S) + (C 2 * D) + (C 3 * S^{2}) + (C 4 * S * D) + (C 5 * D^{2}) + (C 6 * S^{3}) + (C 7 * D * S^{2}) + (C 8 * S * D^{2}) + (C 9 * D^{3}),$
wherein G2 is the current refrigerant flux; S is the saturation air return temperature; D is the saturation exhaust temperature; C0 to C9 are calculating coefficients and are acquired according to the current operation frequency.
A multi-split air conditioning system, comprising the device for controlling oil return of a multi-split air conditioning system according to any one of claims 9 to 15.