A multi-split heat pump air conditioner and a flash tank liquid level control method thereof

Abstract

The application discloses a multi-connected heat pump air conditioner and a flash tank liquid level control method thereof, and is applied to a flash tank. The flash tank comprises a tank body, a liquid inlet pipe, a liquid outlet pipe, a flash vapor-liquid separator, a separation pipe and a spray enthalpy pipe. In a heating process, high-pressure and high-temperature gaseous refrigerant is changed into high-pressure and medium-temperature gas-liquid mixed refrigerant after heat exchange, the high-pressure and medium-temperature gas-liquid mixed refrigerant enters the tank body to be gas-liquid separated through the liquid inlet pipe, liquid refrigerant sinks to the bottom of the tank body under the action of gravity, gaseous refrigerant floats to the upper part of the tank body, the gaseous refrigerant enters the flash vapor-liquid separator through the separation pipe, and the gaseous refrigerant of the flash vapor-liquid separator enters the medium-pressure cavity of a compressor through the spray enthalpy pipe. When the flash tank is full of liquid or liquid refrigerant enters the separation pipe, the liquid refrigerant entering the separation pipe is gasified through a spray enthalpy valve and then enters the flash vapor-liquid separator, and the gaseous refrigerant of the flash vapor-liquid separator enters the compressor through the spray enthalpy pipe. In this way, even if the flash tank is in a full liquid level state, liquid hammer of the compressor will not be caused.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of heat pump air conditioners, in particular to a multi-split heat pump air conditioner and a flash tank liquid level control method thereof. BACKGROUND

[0002] The existing products on the market are mainly normal-temperature multi-split air conditioners. The normal-temperature multi-split air conditioner cannot realize ultra-low temperature heating. The low-temperature multi-split air conditioner mainly adopts a jet enthalpy-increasing system. The scheme is mainly to increase the economizer auxiliary road scheme. There is no flash tank product on the market.

[0003] Compared with the conventional air source heat energy system, the flash tank is gradually being researched as a configuration scheme for improving the low-temperature heating capacity of the heat pump due to its low cost, and has more market potential. However, the control of the flash jet enthalpy-increasing system is more complex. In addition, the multi-split long pipe has more refrigerant supplement and various load changes. The liquid level control of the flash tank has always been a difficulty of this system.

[0004] The prior art with publication number CN218821132U discloses a flash tank, a heat pump system and an air conditioner, which comprises a tank body 178, a liquid inlet pipe and a liquid inlet pipe cup cover. The end of the liquid inlet pipe inserted into the tank body 178 is located in the middle region of the tank body 178, and the end of the liquid inlet pipe inserted into the tank body 178 is provided with a liquid inlet pipe cup cover with holes. The mixed refrigerant entering from the liquid inlet pipe flows downward due to the difference in density between the gaseous and liquid refrigerants, and the gaseous refrigerant flows upward. The refrigerant gas-liquid two-phase separation is achieved by setting the end of the liquid inlet pipe inserted into the tank body 178 in the middle region of the tank body 178. In addition, by setting the cup cover with holes at the end of the liquid inlet pipe inserted into the tank body 178, the liquid refrigerant flows out through the holes of the cup cover to avoid splashing everywhere and reduce the disturbance to the liquid level in the flash tank, thereby solving the technical problem that when the liquid level in the flash tank is high, due to the severe disturbance inside, part of the liquid refrigerant enters the air supplement pipe, causing the compressor to supplement liquid.

[0005] The scheme disclosed in the prior art only prevents the refrigerant from splashing inside the flash tank to cause the liquid refrigerant to enter the air supplement pipe inlet of the flash tank. The technical problem of the compressor supplementing liquid is also based on the fact that the liquid level in the flash tank is always in a safe position. Once the flash tank is filled with liquid refrigerant, the scheme disclosed in the prior art is invalid. SUMMARY

[0006] In order to solve the problem of excessive liquid level in the flash tank caused by excessive refrigerant in the long pipe of the multi-split air conditioner system in the prior art, the purpose of the present application is to provide a multi-split heat pump air conditioner and a flash tank liquid level control method thereof.

[0007] In order to achieve the above-mentioned purpose, the application adopts the following technical solutions: a flash tank liquid level control method, characterized in that: applied to a flash tank, the flash tank comprises a tank body, a liquid inlet pipe, a liquid outlet pipe, a flash vapor-liquid separator, a separation pipe and a flash pipe, one end of the liquid inlet pipe and one end of the liquid outlet pipe are inserted into the tank body, two ends of the separation pipe are connected with the tank body and the inlet of the separator respectively, a flash valve is installed on the separation pipe, and two ends of the flash pipe are connected with the outlet of the flash vapor-liquid separator and the medium-pressure cavity of the compressor respectively; wherein, the state of the refrigerant entering the flash vapor-liquid separator can be controlled by controlling the opening degree of the flash valve; in the heating process, the refrigerant is compressed by the compressor to become high-pressure high-temperature gaseous refrigerant, the high-pressure high-temperature gaseous refrigerant is changed into high-pressure medium-temperature gaseous-liquid mixture or pure liquid refrigerant after heat exchange, the high-pressure medium-temperature gaseous-liquid mixture or pure liquid refrigerant enters the tank body through the liquid inlet pipe to carry out preliminary gas-liquid separation, the liquid refrigerant sinks to the bottom of the tank body under the action of gravity, the gaseous refrigerant floats to the upper part of the tank body, the gaseous refrigerant enters the flash vapor-liquid separator through the separation pipe, and the gaseous refrigerant of the flash vapor-liquid separator enters the medium-pressure cavity of the compressor through the flash pipe; when the flash tank is full of liquid or the liquid refrigerant enters the separation pipe, the liquid refrigerant entering the separation pipe is vaporized after the flash valve and then enters the flash vapor-liquid separator, and the gaseous refrigerant of the flash vapor-liquid separator enters the medium-pressure cavity of the compressor through the flash pipe.

[0008] As a preferred, the inlet of the flash valve and the outlet of the liquid outlet pipe are both provided with a temperature sensor for detecting temperature; the liquid level height of the liquid refrigerant in the flash tank is determined as follows: when the difference between the inlet temperature Tpi of the flash valve and the outlet temperature Tso of the liquid outlet pipe is greater than a, it indicates that the tank body is not filled with liquid refrigerant; at this time, the inlet temperature Tpi of the flash valve is greater than the outlet temperature Tso of the liquid outlet pipe; a is a constant; when the inlet temperature Tpi of the flash valve is equal to the outlet temperature Tso of the liquid outlet pipe, it indicates that the tank body is filled with liquid refrigerant.

[0009] As a preferred, after the tank body is filled with liquid refrigerant, the liquid refrigerant in the tank body is reduced in the following manner: the opening degree of the flash valve is reduced until the difference between the inlet temperature Tpi of the flash valve and the outlet temperature Tso of the liquid outlet pipe is greater than b, at this time, it indicates that the liquid level in the flash tank is at a safe liquid level position, wherein, b is greater than a, and a and b are both constants.

[0010] As a preferred, the opening condition of the flash valve is: detecting the compressor frequency, the compressor discharge temperature Tda and the system high-pressure corresponding saturated temperature Pd-t; when Tda-Pd-t>c and Tda>d, and the compressor frequency is greater than or equal to e, the flash valve is opened, the initial opening degree of the flash valve is set to 60 pls, and the opening degree of the flash valve is periodically adjusted; when Tda-Pd-t≤c or the discharge is less than or equal to d or the compressor frequency is less than e, the flash valve is not allowed to be opened.

[0011] As preferred, the EEV adjustment process is as follows: the EEV opening degree change value is △EEV=KIErrn+Kp(Errn-Errn-1), wherein, Errn=T

current EEV superheat

target EEV superheat

current exhaust gas superheat

target exhaust gas superheat

current EEV superheat

[0012] As preferred, the current EEV opening degree is Pcurrent opening degree, and the target EEV opening degree is Pheating target opening degree, Pheating target opening degree=Pcurrent opening degree+△EEV.

[0013] As preferred, the initial EEV opening degree is f, and the EEV adjustment interval is g seconds, both f and g are constants.

[0014] A multi-connected heat pump air conditioner applies the above-mentioned flash tank liquid level control method.

[0015] The beneficial effects of the technical scheme of the present application are as follows: in the open state of the EEV, the gaseous refrigerant entering the EEV of the compressor is ensured, even in the full-liquid level state of the flash tank, the structure of the present application will not cause liquid strike of the compressor; compared with the EEV multi-connected system, the cost of the flash tank system is lower; compared with the normal-temperature multi-connected system, the flash tank system can improve the low-temperature heating capacity and realize super-low-temperature heating. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 It is a principle diagram of a multi-connected heat pump air conditioner system;

[0017] Figure 2 It is a structure of a flash tank Figure 1 ;

[0018] Figure 3 It is a structure of a flash tank Figure 2 ;

[0019] Figure 4 It is a structure of a flash tank Figure 3 .

[0020] Reference signs: 11, compressor; 12, oil-liquid separator; 13, four-way valve; 14, air pipe stop valve; 15, air conditioner indoor unit; 16, liquid pipe stop valve; 17, flash tank; 18, main gas-liquid separator; 19, condenser; 20, main valve;

[0021] 171, liquid inlet pipe; 172, liquid outlet pipe; 173, flash gas-liquid separator; 174, separation pipe; 175, EEV; 176, EEV pipe; 177, connecting bracket; 178, tank body. EMBODIMENT

[0022] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0023] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0024] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more, unless explicitly defined otherwise.

[0025] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0026] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature. Example

[0027] A control method for a flash evaporator, applied to, for exampleFigures 2-4 The flash tank 17 includes a tank body 178, a liquid inlet pipe 171, a liquid outlet pipe 172, a flash vapor-liquid separator 173, a separation pipe 174, and a flash vapor injection pipe 176. One end of the liquid inlet pipe 171 and one end of the liquid outlet pipe 172 are inserted into the tank body 178. The separation pipe 174 is connected to the tank body 178 and the inlet of the separator, respectively. The flash vapor injection valve 175 is installed on the separation pipe 174. The flash vapor injection pipe 176 is connected to the outlet of the flash vapor-liquid separator 173 and the medium-pressure cavity of the compressor, respectively. The state of the refrigerant entering the flash vapor-liquid separator 173 can be controlled by controlling the opening degree of the flash vapor injection valve 175.

[0028] During the heating process, the refrigerant is compressed by the compressor to become high-pressure and high-temperature gaseous refrigerant. After heat exchange, the high-pressure and high-temperature gaseous refrigerant becomes high-pressure and medium-temperature gaseous-liquid mixture or pure liquid refrigerant. The high-pressure and medium-temperature gaseous-liquid mixture or pure liquid refrigerant enters the tank body 178 through the liquid inlet pipe 171 for preliminary gas-liquid separation. The liquid refrigerant sinks to the bottom of the tank body 178 under the action of gravity, and the gaseous refrigerant floats to the upper part of the tank body 178. The gaseous refrigerant enters the flash vapor-liquid separator 173 through the separation pipe 174. The gaseous refrigerant in the flash vapor-liquid separator 173 enters the medium-pressure cavity of the compressor through the flash vapor injection pipe 176. When the flash tank 17 is full of liquid or liquid refrigerant enters the separation pipe 174, the liquid refrigerant entering the separation pipe 174 is vaporized by the flash vapor injection valve 175 and enters the flash vapor-liquid separator 173. The gaseous refrigerant in the flash vapor-liquid separator 173 enters the medium-pressure cavity of the compressor through the flash vapor injection pipe 176.

[0029] In this way, when the flash vapor injection valve 175 is open, it is ensured that the gaseous refrigerant enters the injection port of the compressor. Even if the flash tank 17 is full of liquid, the structure of the flash tank 17 + the flash vapor-liquid separator 173 of the flash tank 17 will not cause liquid hammer of the compressor. Compared with the economized flash vapor injection multi-contact system, the cost of the flash tank 17 system is lower. Compared with the normal-temperature multi-contact system, the flash tank 17 system can improve the low-temperature heating capacity and realize super-low-temperature heating.

[0030] Further preferably, the inlet of the separation pipe 174 is located at the top of the tank body 178, the outlet of the liquid inlet pipe 171 is located at the bottom of the tank body 178, the inlet of the liquid outlet pipe 172 is located at the bottom of the tank body 178, the flash vapor-liquid separator 173 is fixedly installed on the tank body 178, and the separation pipe 174 is connected to the bottom of the flash vapor-liquid separator 173.

[0031] Thus, since the liquid inlet pipe 171 extends into the bottom of the tank 178, the refrigerant can be effectively prevented from splashing into the tank 178. The liquid outlet pipe 172 also extracts the liquid refrigerant from the bottom of the tank 178, so that the air conditioning system can have sufficient refrigerant for operation. The inlet of the separation pipe 174 is arranged at the top of the tank 178, so that the tank 178 can contain as much liquid refrigerant as possible.

[0032] Further preferably, the tank 178 is further provided with two connecting supports 177 arranged above and below the tank 178 for fixing the tank 178. The connecting support 177 comprises a mounting seat, and a connecting hole is formed in the mounting seat. The connecting support 177 further comprises a hook fixed to the tank 178, and the hook is hooked in the connecting hole of the mounting seat.

[0033] A multi-split heat pump air conditioner comprises the flash tank 17 and the liquid level control method of the flash tank 17. Figure 1 As shown in the figure, the multi-split heat pump air conditioner further comprises a compressor 11, an oil separator 12, a four-way valve 13, an air conditioner indoor unit 15, the flash tank 17, a condenser 19, and a main gas-liquid separator 18 arranged in a main circulation pipeline. A gas pipe stop valve 14 is arranged in the pipeline between the four-way valve 13 and the air conditioner indoor unit 15. A liquid pipe stop valve 16 is arranged in the pipeline between the air conditioner indoor unit 15 and the flash tank 17. A main valve 20 is arranged between the flash tank 17 and the condenser 19. The flash tank 17 is connected to the medium-pressure cavity of the compressor 11 through a flash gas-liquid separator 173.

[0034] The main circulation pipeline comprises a high-pressure pipeline, a medium-pressure pipeline, and a low-pressure pipeline. Figure 1 As shown in the figure, the dashed line represents the high-pressure pipeline, the high-pressure pipeline is a gas pipe, the double-dot chain line represents the medium-pressure pipeline, the medium-pressure pipeline is a liquid pipe, and the solid line represents the low-pressure pipeline.

[0035] The liquid level control method and operation principle of the flash tank 17 of the multi-split flash system are as follows:

[0036] When the air conditioner is started for heating, the compressor 11 is started after the air conditioner outdoor unit main control board receives a start command. The refrigerant is compressed into high-pressure and high-temperature gaseous refrigerant after passing through the compressor 11. The refrigerant passes through the four-way valve 13, enters the gas pipe, and flows to the air conditioner indoor unit 15 after passing through the gas pipe stop valve 14. The refrigerant is condensed into high-pressure and medium-temperature gas-liquid mixture or pure liquid refrigerant after passing through the air conditioner indoor unit 15, and then enters the liquid pipe and returns to the flash tank 17 after passing through the liquid pipe stop valve 16.

[0037] The condensed gas-liquid mixed refrigerant is preliminarily separated in the flash tank 17, and the liquid refrigerant sinks to the bottom of the flash tank 17 under the action of gravity.

[0038] The refrigerant flowing out of the flash tank 17 continues to circulate along the main path of the system, passes through the main valve 20, the evaporator, and the main gas-liquid separator 18, and then returns to the compressor 11; another part of the refrigerant passes through the separation pipe 174, enters the flash gas-liquid separator 173 through the flash injection valve 175, and then enters the medium-pressure chamber of the compressor 11 through the flash injection pipe 176. At this time, the pressure difference between the flash tank 17 and the flash injection port is smaller than the pressure difference between the flash tank 17 and the main valve 20, so the gaseous refrigerant in the flash tank 17 is pushed into the flash injection port of the compressor 11.

[0039] In the above process, when the gaseous exhaust speed in the flash tank 17 is greater than the generated speed, the flash tank 17 will gradually be filled with liquid refrigerant. At this time, there is no space for the liquid to expand in the pipe of the flash tank 17, and the temperature of the liquid refrigerant in the flash tank 17 will not decrease. Therefore, in this embodiment, the judgment method of the liquid level height of the liquid refrigerant in the flash tank 17 is:

[0040] A temperature sensor is installed at the inlet of the flash injection valve 175 and the outlet of the liquid outlet pipe 172;

[0041] Because the liquid refrigerant at the bottom of the flash tank 17 will be partially gasified due to the pressure loss of the flash tank 17 itself, the gasification absorbs heat, causing the temperature of the liquid refrigerant at the bottom of the flash tank 17 to decrease, and the temperature of the refrigerant at the top of the flash tank 17 to increase due to the heat absorption of gasification; therefore, when the difference between the inlet temperature Tpi of the flash injection valve and the outlet temperature Tso of the liquid outlet pipe is greater than a, it indicates that the liquid refrigerant has not filled the tank 178; at this time, the inlet temperature Tpi of the flash injection valve is greater than the outlet temperature Tso of the liquid outlet pipe; a is a constant; in the test process, the temperature difference between Tpi and Tso is about 2°C.

[0042] When the flash tank 17 is gradually filled with liquid refrigerant, the liquid level of the liquid refrigerant continuously approaches the inlet of the separation pipe 174; therefore, when the inlet temperature Tpi of the flash injection valve is equal to the outlet temperature Tso of the liquid outlet pipe, it indicates that the liquid refrigerant has filled the tank 178.

[0043] When the liquid refrigerant in the tank 178 is full, in order to prevent the liquid refrigerant from being injected into the medium-pressure chamber of the compressor 11 and causing liquid hammer, in this embodiment, the method for reducing the liquid level of the liquid refrigerant in the tank 178 is to reduce the opening of the flash injection valve 175 until the difference between the inlet temperature Tpi of the flash injection valve and the outlet temperature Tso of the liquid outlet pipe is greater than b, at which time it indicates that the liquid level in the flash tank 17 is at a safe liquid level position, wherein b is greater than a, and a and b are constants. In this embodiment, b is 3°C.

[0044] In this embodiment, the opening condition of the flash injection valve 175 is to detect the frequency of the compressor 11, the discharge temperature Tda of the compressor 11, and the corresponding saturated temperature Pd-t of the high pressure of the system;

[0045] When Tda-Pd-t>c and Tda>d, and the compressor 11 frequency≥e, the enthalpy injection valve 175 is opened, and the initial opening degree of the enthalpy injection valve 175 is set to 60 pls, and the opening degree of the enthalpy injection valve 175 is periodically adjusted; when Tda-Pd-t≤c or the exhaust temperature≤d or the compressor 11 frequency

[0046] After the opening condition of the enthalpy injection valve 175 is met, the enthalpy injection valve 175 is opened at the preset initial opening degree, and in the embodiment, the initial opening degree of the enthalpy injection valve 175 is f, and the adjustment interval of the enthalpy injection valve 175 is g seconds, and f and g are both constants.

[0047] In the embodiment, the specific control mode of the enthalpy injection valve 175 is as follows: the opening degree change value of the enthalpy injection valve is △EEV=KIErrn+Kp(Errn-Errn-1),

[0048] Wherein, Errn=T

current enthalpy injection valve superheat

target enthalpy injection valve superheat

current exhaust superheat

target exhaust superheat

current enthalpy injection valve superheat

[0049] In the embodiment, the current opening degree of the enthalpy injection valve 175 is Pcurrent opening degree, the target opening degree of the enthalpy injection valve is Pheating target opening degree, and Pheating target opening degree=Pcurrent opening degree+△EEV.

[0050] The specific example is as follows:

[0051] In the current operating state, the exhaust temperature Tda of the compressor 11 is 55℃, and the system high-pressure corresponding saturation temperature Pd-t is 36℃, and the difference between the two is greater than 15℃, so the enthalpy injection valve 175 is opened; in the embodiment, the initial opening degree f of the enthalpy injection valve 175 is 60 pls, the adjustment interval g of the enthalpy injection valve 175 is 2 seconds, K1=2 / 3, and KP=1 / 3.

[0052] After the enthalpy injection valve 175 is opened, the outlet temperature Tso of the liquid outlet pipe of the flash tank 17 is 36℃, and the inlet temperature Tpi of the enthalpy injection valve 175 is 38℃, after 2s, the outlet temperature Tso of the liquid outlet pipe is 36℃, and the inlet temperature Tpi of the enthalpy injection valve 175 is 38℃, at this time, the calculated target opening degree Pheating target opening degree of the enthalpy injection valve 175 is Pcurrent opening degree+△EEV=60 pls+*1 / 3)=61 pls, and the enthalpy injection valve 175 is opened by 1 pls after rounding down.

[0053] When the continuous operation reaches the flash tank 17 outlet temperature Tso = 38℃, the spray enthalpy valve 175 inlet temperature Tpi = 38℃, at this time according to experience the flash tank 17 liquid level is full, need to close small valve step, at this time the calculated target spray enthalpy valve 175 opening P heating target opening = P current opening + △EEV = 61 pls + * 1 / 3 = 60, rounded down to 60 pls, then the spray enthalpy valve 175 needs to close 1 pls.

[0054] The minimum opening of the spray enthalpy valve 175 is defined as 40 pls, when the opening of the spray enthalpy valve 175 is at the minimum opening 40 pls and continuously operates for 10 min, if the calculated △EEV is negative, since the opening is small at this time, the liquid in the gas-liquid separator in the flash tank 17 is not enough to fill, so there is no problem of liquid hammering of the compressor 11.

[0055] 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 mean 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.

[0056] Although the embodiments of the present application have been shown and described above, it can be understood that the above-described embodiments are exemplary and cannot be understood as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above-described embodiments within the scope of the present application without departing from the principles and purposes of the present application.

Claims

1. A method for controlling the liquid level in a flash tank, characterized in that: This invention relates to a flash evaporator, which includes a tank body (178), an inlet pipe (171), an outlet pipe (172), a flash vapor-liquid separator (173), a separation pipe (174), and an enthalpy injection pipe (176). One end of the inlet pipe (171) and one end of the outlet pipe (172) are inserted into the tank body (178). The two ends of the separation pipe (174) are respectively connected to the tank body (178) and the inlet of the separator. An enthalpy injection valve (175) is installed on the separation pipe (174). The two ends of the enthalpy injection pipe (176) are respectively connected to the outlet of the flash vapor-liquid separator (173) and the intermediate pressure chamber of the compressor. The state of the refrigerant entering the flash vapor-liquid separator (173) can be controlled by controlling the opening degree of the enthalpy injection valve (175). During the heating process, the refrigerant is compressed by the compressor and becomes a high-pressure, high-temperature gaseous refrigerant. After heat exchange, the high-pressure, high-temperature gaseous refrigerant becomes a high-pressure, medium-temperature gas-liquid mixture or pure liquid refrigerant. The high-pressure, medium-temperature gas-liquid mixture or pure liquid refrigerant enters the tank (178) through the liquid inlet pipe (171) for preliminary gas-liquid separation. The liquid refrigerant sinks to the bottom of the tank (178) under the action of gravity, while the gaseous refrigerant floats to the top of the tank (178). The gaseous refrigerant enters the flash vapor-liquid separator (173) through the separation pipe (174). The gaseous refrigerant from the flash vapor-liquid separator (173) enters the medium-pressure chamber of the compressor through the enthalpy injection pipe (176). When the flash tank is full of liquid or liquid refrigerant enters the separator (174), the liquid refrigerant entering the separator (174) is vaporized through the injection enthalpy valve (175) and enters the flash vapor-liquid separator (173). The gaseous refrigerant in the flash vapor-liquid separator (173) enters the medium-pressure chamber of the compressor through the injection enthalpy pipe (176).

2. The method for controlling the liquid level in a flash tank according to claim 1, characterized in that: Temperature sensors are installed at both the inlet of the enthalpy valve (175) and the outlet of the liquid outlet pipe (172). The method for determining the liquid refrigerant level in the flash tank is as follows: When the difference between the inlet temperature Tpi of the enthalpy injection valve and the outlet temperature Tso of the liquid outlet pipe is greater than a, it indicates that the liquid refrigerant has not filled the tank (178); at this time, the inlet temperature Tpi of the enthalpy injection valve is greater than the outlet temperature Tso of the liquid outlet pipe; a is a constant. When the inlet temperature Tpi of the enthalpy injection valve is equal to the outlet temperature Tso of the liquid outlet pipe, it indicates that the liquid refrigerant has filled the tank (178).

3. The method for controlling the liquid level in a flash tank according to claim 2, characterized in that: After the tank (178) is full of liquid refrigerant, the liquid refrigerant level in the tank (178) is reduced as follows: Reduce the opening of the enthalpy injection valve (175) until the difference between the inlet temperature Tpi of the enthalpy injection valve and the outlet temperature Tso of the liquid outlet pipe is greater than b. At this time, it indicates that the liquid level in the flash tank is at a safe level, where b is greater than a, and a and b are both constants.

4. The method for controlling the liquid level in a flash tank according to claim 2, characterized in that: The conditions for opening the injection enthalpy valve (175) are: detecting the compressor frequency, compressor discharge temperature Tda and the system high pressure corresponding saturation temperature Pd-t; When Tda-Pd-t>c and Tda>d, and the compressor frequency≥e, the injection enthalpy valve (175) is opened. The initial opening of the injection enthalpy valve (175) is set to 60pls, and the opening of the injection enthalpy valve (175) is periodically adjusted. When Tda-Pd-t≤c or exhaust ≤d or compressor frequency <e, the injection enthalpy valve (175) is not allowed to open.

5. The method for controlling the liquid level in a flash tank according to claim 2, characterized in that: The adjustment process of the injection enthalpy valve (175) is as follows: The change in the opening of the injection enthalpy valve is ΔEEV = KIErrn + Kp(Errn - Errn-1). Where Errn = T[current superheat of injection enthalpy valve] - T[target superheat of injection enthalpy valve]; Errn-1 = T[current exhaust superheat] - T[target exhaust superheat], T[current superheat of injection enthalpy valve] = Tso - Tpi, and KI and Kp are both coefficients.

6. The method for controlling the liquid level in a flash tank according to claim 5, characterized in that: The current opening degree of the enthalpy injection valve (175) is P current opening degree, and the target opening degree of the enthalpy injection valve is P heating target opening degree, P heating target opening degree = P current opening degree + ΔEEV.

7. The method for controlling the liquid level in a flash tank according to claim 5, characterized in that: The initial opening of the enthalpy injection valve (175) is f, and the adjustment interval of the enthalpy injection valve (175) is g seconds, where f and g are constants.

8. A multi-split heat pump air conditioner, characterized in that: The method for controlling the liquid level in a flash tank includes any one of the claims 1-7 above.

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