Starting method of air energy heat pump with parallel inverter compressor

Abstract

According to the starting method of the air energy heat pump with the parallel inverter compressors, the air energy heat pump comprises the first inverter compressor and the second inverter compressor which are connected in parallel, and during operation, the starting method comprises the following steps that firstly, power-on operation is conducted, and the second step is executed; step 2, opening a third one-way solenoid valve, and entering step 3; thirdly, the central controller of the air energy heat pump opens the step number of the electronic expansion valve to the maximum value, and the fourth step is executed; 4, the central controller opens a first one-way electromagnetic valve and a second one-way electromagnetic valve, and the step 5 is executed; and 5, the central controller starts the first inverter compressor, and the step 6 is executed. The high-pressure sensor and the low-pressure sensor are used for detecting the high-pressure value and the low-pressure value of the air energy heat pump system correspondingly, the high-low pressure difference delta P is obtained, then smooth, stable and normal starting of the second inverter compressor is ensured by controlling the high-low pressure difference delta P, and liquid impact can be effectively prevented.

Description

Technical Field

[0001] The invention relates to an air energy heat pump, in particular to a starting method of an air energy heat pump with a parallel variable frequency compressor. Background Art

[0002] Under the environmental protection policy of carbon neutrality and energy conservation and emission reduction, the demand for air energy heat pumps in some large commercial and public places is increasing, and the energy efficiency requirements for air energy heat pumps are also increasing. Multiple variable frequency compressors in parallel sharing the same water channel also often appear on construction sites. At present, the startup control scheme or engineering installation of multiple variable frequency compressors in parallel is unreasonable, which is prone to the problem of variable frequency compressor startup failure.

[0003] Some manufacturers have made some improvements to this situation. For example, Chinese patent document No. CN 104048453 A disclosed a compressor system oil equalization control method on September 17, 20219, including the following steps: setting N parallel-connected rotor compressors; starting N1 of the N parallel-connected rotor compressors to run simultaneously; detecting the phase angle of the motor rotor of the N1 compressor; determining whether the phase angles of the motor rotors of the N1 compressors are consistent; if they are consistent, running the normal operation control program to control the operation of the N1 compressors; if they are inconsistent, adjusting the phase angle of the motor rotor of the N1 compressors, and running the normal operation control program to control the operation of the N1 compressors after the phase angles are consistent; N and N1 are integers greater than 1, where N≥N1. When starting up these N parallel-connected compressors, it is still easy to fail to start, so there is room for improvement. Summary of the invention

[0004] The object of the present invention is to provide a method for starting an air energy heat pump with a parallel variable frequency compressor, which can start smoothly, so as to overcome the shortcomings of the prior art.

[0005] A method for starting an air-energy heat pump with a parallel variable-frequency compressor designed for this purpose is characterized in that the air-energy heat pump comprises a first variable-frequency compressor, a second variable-frequency compressor, an outdoor heat exchanger and a four-way valve, the high-pressure outlet of the first variable-frequency compressor is connected to the inlet of an oil separator through a first one-way valve and the high-pressure outlet of the second variable-frequency compressor is connected to the inlet of an oil separator through a second one-way valve, the first outlet of the oil separator is connected to the first interface of the four-way valve, a high-pressure pressure sensor for detecting the high-pressure pressure of the system is arranged between the first outlet of the oil separator and the first interface of the four-way valve, the second interface of the four-way valve is connected to one end of the outdoor heat exchanger, the other end of the outdoor heat exchanger is connected to one end of a water-side heat exchanger through an electronic expansion valve and a first stop valve, the other end of the water-side heat exchanger is connected to the fourth interface of the four-way valve through the second stop valve, the third interface of the four-way valve is connected to the inlet of a gas-liquid separator, and the outlets of the gas-liquid separator are respectively connected to the The low-pressure inlet of the first variable-frequency compressor and the low-pressure inlet of the second variable-frequency compressor are connected, a first one-way solenoid valve for balancing the high and low pressures of the first variable-frequency compressor is connected in series between the high-pressure outlet and the low-pressure inlet of the first variable-frequency compressor, a second one-way solenoid valve for balancing the high and low pressures of the second variable-frequency compressor is connected in series between the high-pressure outlet and the low-pressure inlet of the second variable-frequency compressor, the second outlet of the oil separator is connected to the low-pressure inlet of the first variable-frequency compressor through a first oil return capillary, the second outlet of the oil separator is connected to the low-pressure inlet of the second variable-frequency compressor through a second oil return capillary, the first outlet of the oil separator is connected to the inlet of the gas-liquid separator through a third one-way solenoid valve for balancing the high and low pressures of the system, a low-pressure sensor for detecting the low pressure of the system is arranged between the third interface of the four-way valve and the inlet of the gas-liquid separator, and the output pressure and capacity of the first variable-frequency compressor are different from those of the second variable-frequency compressor;

[0006] The operation includes the following steps:

[0007] Step 1: Power on and run, then go to step 2;

[0008] Step 2, open the third one-way solenoid valve and proceed to step 3;

[0009] Step 3: The central controller of the air energy heat pump opens the number of steps of the electronic expansion valve to the maximum value, and then enters step 4;

[0010] Step 4: The central controller opens the first one-way solenoid valve and the second one-way solenoid valve, and then proceeds to step 5;

[0011] Step 5: The central controller starts the first variable frequency compressor and proceeds to step 6;

[0012] Step 6: The central controller increases the current operating frequency F1 of the first variable frequency compressor at a first rate V1, and then proceeds to step 7, wherein the value range of V1 is 1 Hz / 10s to 1 Hz / 1s;

[0013] Step 7, the central controller determines whether F1<P1 is established. If yes, it goes to step 6, otherwise it goes to step 8, P1 is the first frequency operation platform, and the value range of P1 is 25Hz~65Hz;

[0014] Step 8: After the operation period T, enter step 9, where the value range of T is 0.5 minutes to 20 minutes;

[0015] Step nine, the central controller determines whether ΔP≤Pm is established. If yes, it goes to step ten, otherwise it goes to step eight, where ΔP is the high and low pressure difference, ΔP=PH1-PL1, PH1 is the high pressure sensor detection value, PL1 is the low pressure sensor detection value, Pm is the threshold, Pm=K*PL1, K is a constant, and the value range of K is 8 to 12;

[0016] Step 10, the central controller starts the second variable frequency compressor and proceeds to step 11;

[0017] Step 11: The central controller increases the current operating frequency F2 of the second variable frequency compressor at a second rate V2, and enters step 12, wherein the value range of V2 is 1 Hz / 10s to 1 Hz / 1s;

[0018] Step 12, the central controller determines whether F2<P2 is established. If yes, it goes to step 11, otherwise it goes to step 13, P2 is the second frequency operation platform, and the value range of P2 is 25Hz~65Hz;

[0019] Step 13, after the operation cycle T, proceed to step 14;

[0020] Step 14, the central controller determines whether ΔP≤Pm is established. If yes, it proceeds to step 15 and step 23 at the same time, otherwise it proceeds to step 13;

[0021] Step 15: The central controller increases the current operating frequency F1 of the first variable frequency compressor at a third rate V3, and enters step 16, wherein the value range of V3 is 1 Hz / 10s to 1 Hz / 1s;

[0022] Step 16, the central controller determines whether F1<P3 is established. If yes, it proceeds to step 15, otherwise it proceeds to step 17, wherein P3 is the second frequency operation platform, and the value range of P3 is 45Hz~80Hz;

[0023] Step 17: After the operation cycle T, proceed to step 18;

[0024] Step 18, the central controller determines whether ΔP≤Pm is established, if it is yes, it goes to step 19, otherwise it goes to step 17;

[0025] Step 19: The central controller increases the current operating frequency F1 of the first variable frequency compressor at a fifth rate V5, and enters step 20, wherein the value range of V5 is 1 Hz / 10s to 1 Hz / 1s;

[0026] Step 20, the central controller determines whether F1<Pn is established. If yes, it proceeds to step 19, otherwise it proceeds to step 21, where Pn is the target frequency operating platform, and the value range of Pn is 45Hz~110Hz;

[0027] Step 21: After the operation cycle T, proceed to step 22;

[0028] Step 22, the central controller determines whether ΔP≤Pm is established, if yes, proceeds to step 31, otherwise proceeds to step 21;

[0029] Step 23: The central controller increases the current operating frequency F2 of the second variable frequency compressor at a fourth rate V4, and enters step 12, wherein the value range of V4 is 1 Hz / 10s to 1 Hz / 1s;

[0030] Step 24, the central controller determines whether F2<P4 is established. If yes, it proceeds to step 11, otherwise it proceeds to step 13, P4 is the second frequency operation platform, and the value range of P4 is 25Hz~80Hz;

[0031] Step 25: After the operation cycle T, proceed to step 26;

[0032] Step 26, the central controller determines whether ΔP≤Pm is established, if it is yes, it goes to step 27, otherwise it goes to step 25;

[0033] Step 27: The central controller increases the current operating frequency F2 of the second variable frequency compressor at a fourth rate V6, and enters step 18, wherein the value range of V6 is 1 Hz / 10s to 1 Hz / 1s;

[0034] Step 28, the central controller determines whether F2<Pn is established, if it is yes, it goes to step 27, otherwise it goes to step 29;

[0035] Step 29: After the operation cycle T, proceed to step 30;

[0036] Step 30, the central controller determines whether ΔP≤Pm is established. If yes, it proceeds to step 31, otherwise it proceeds to step 29;

[0037] Step 31, parallel startup is completed.

[0038] The present invention detects the high pressure value and the low pressure value of the air energy heat pump system respectively through a high pressure sensor and a low pressure sensor, and obtains the high and low pressure difference ΔP, and then ensures the smooth and stable normal startup of the second variable frequency compressor by controlling the high and low pressure difference ΔP, and can effectively prevent liquid hammer. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a structural schematic diagram of an embodiment of the present invention.

[0040] Figure 2 This is one of the control flow charts of the present invention.

[0041] Figure 3 This is the second control flow chart of the present invention.

[0042] In the figure: 1 is the first stop valve, 2 is the second stop valve, 3 is the electronic expansion valve, 4 is the outdoor heat exchanger, 5 is the four-way valve, 6 is the high-pressure pressure sensor, 7 is the first one-way valve, 8 is the second one-way valve, 9 is the second variable frequency compressor, 10 is the first variable frequency compressor, 11 is the gas-liquid separator, 12 is the low-pressure pressure sensor, 13 is the third one-way solenoid valve, 14 is the second one-way solenoid valve, 15 is the second oil return capillary, 16 is the first oil return capillary, 17 is the oil separator, 18 is the first one-way solenoid valve, D is the first interface of the four-way valve, C is the second interface of the four-way valve, S is the third interface of the four-way valve, and E is the fourth interface of the four-way valve. DETAILED DESCRIPTION

[0043] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0044] See also Figure 1A method for starting an air energy heat pump with a parallel variable frequency compressor, characterized in that the air energy heat pump comprises a first variable frequency compressor 10, a second variable frequency compressor 9, an outdoor heat exchanger 4 and a four-way valve 5, the high pressure outlet of the first variable frequency compressor 10 is connected to the inlet of the oil separator 17 through a first check valve 7 and the high pressure outlet of the second variable frequency compressor 9 is connected to the inlet of the oil separator 17 through a second check valve 8, the first outlet of the oil separator 17 is connected to the first interface D of the four-way valve 5, and a high pressure pressure sensor 6 for detecting the high pressure of the system is provided The first outlet of the oil separator 17 is disposed between the first interface D of the four-way valve 5, the second interface C of the four-way valve 5 is connected to one end of the outdoor heat exchanger 4, the other end of the outdoor heat exchanger 4 is connected to one end of the water side heat exchanger through the electronic expansion valve 3 and the first stop valve 1, the other end of the water side heat exchanger is connected to the fourth interface E of the four-way valve 5 through the second stop valve 2, the third interface S of the four-way valve 5 is connected to the inlet of the gas-liquid separator 11, and the outlet of the gas-liquid separator 11 is respectively connected to the first frequency conversion The low-pressure inlet of the compressor 10 and the low-pressure inlet of the second variable-frequency compressor 9 are connected, a first one-way solenoid valve 18 for balancing the high and low pressures of the first variable-frequency compressor 10 is connected in series between the high-pressure outlet and the low-pressure inlet of the first variable-frequency compressor 10, a second one-way solenoid valve 14 for balancing the high and low pressures of the second variable-frequency compressor 9 is connected in series between the high-pressure outlet and the low-pressure inlet of the second variable-frequency compressor 9, the second outlet of the oil separator 17 is connected to the low-pressure inlet of the first variable-frequency compressor 10 through the first oil return capillary 16, the second outlet of the oil separator 17 is connected to the low-pressure inlet of the second variable-frequency compressor 9 through the second oil return capillary 15, the first outlet of the oil separator 17 is connected to the inlet of the gas-liquid separator 11 through the third one-way solenoid valve 13 for balancing the high and low pressures of the system, a low-pressure pressure sensor 12 for detecting the low pressure of the system is arranged between the third interface S of the four-way valve 5 and the inlet of the gas-liquid separator 11, and the output pressure and capacity of the first variable-frequency compressor 10 are different from those of the second variable-frequency compressor 9.

[0045] See also Figure 2-Figure 3 , the operation includes the following steps:

[0046] Step 1: Power on and run, then go to step 2;

[0047] Step 2, open the third one-way solenoid valve 13 and proceed to step 3;

[0048] Step 3: The central controller of the air energy heat pump opens the number of steps of the electronic expansion valve 3 to the maximum value, and enters step 4;

[0049] Step 4: The central controller opens the first one-way solenoid valve 18 and the second one-way solenoid valve 14, and then proceeds to step 5;

[0050] Step 5: The central controller starts the first variable frequency compressor 10 and proceeds to step 6;

[0051] Step 6: The central controller increases the current operating frequency F1 of the first variable frequency compressor 10 at a first rate V1, and then proceeds to step 7, wherein the value range of V1 is 1 Hz / 10s to 1 Hz / 1s;

[0052] Step 7, the central controller determines whether F1<P1 is established. If yes, it goes to step 6, otherwise it goes to step 8, P1 is the first frequency operation platform, and the value range of P1 is 25Hz~65Hz;

[0053] Step 8: After the operation period T, enter step 9, where the value range of T is 0.5 minutes to 20 minutes;

[0054] Step nine, the central controller determines whether ΔP≤Pm is established. If yes, it goes to step ten, otherwise it goes to step eight, where ΔP is the high and low pressure difference, ΔP=PH1-PL1, PH1 is the high pressure sensor detection value, PL1 is the low pressure sensor detection value, Pm is the threshold, Pm=K*PL1, K is a constant, and the value range of K is 8 to 12;

[0055] Step 10, the central controller starts the second variable frequency compressor 9 and proceeds to step 11;

[0056] Step 11: The central controller increases the current operating frequency F2 of the second variable frequency compressor 9 at a second rate V2, and enters step 12, wherein the value range of V2 is 1 Hz / 10s to 1 Hz / 1s;

[0057] Step 12, the central controller determines whether F2<P2 is established. If yes, it goes to step 11, otherwise it goes to step 13, P2 is the second frequency operation platform, and the value range of P2 is 25Hz~65Hz;

[0058] Step 13, after the operation cycle T, proceed to step 14;

[0059] Step 14, the central controller determines whether ΔP≤Pm is established. If yes, it proceeds to step 15 and step 23 at the same time, otherwise it proceeds to step 13;

[0060] Step 15: The central controller increases the current operating frequency F1 of the first variable frequency compressor 10 at a third rate V3, and enters step 16, wherein the value range of V3 is 1 Hz / 10s to 1 Hz / 1s;

[0061] Step 16, the central controller determines whether F1<P3 is established. If yes, it proceeds to step 15, otherwise it proceeds to step 17, wherein P3 is the second frequency operation platform, and the value range of P3 is 45Hz~80Hz;

[0062] Step 17: After the operation cycle T, proceed to step 18;

[0063] Step 18, the central controller determines whether ΔP≤Pm is established, if it is yes, it goes to step 19, otherwise it goes to step 17;

[0064] Step 19: The central controller increases the current operating frequency F1 of the first variable frequency compressor 10 at a fifth rate V5, and enters step 20, wherein the value range of V5 is 1 Hz / 10s to 1 Hz / 1s;

[0065] Step 20, the central controller determines whether F1<Pn is established. If yes, it proceeds to step 19, otherwise it proceeds to step 21, where Pn is the target frequency operating platform, and the value range of Pn is 45Hz~110Hz;

[0066] Step 21: After the operation cycle T, proceed to step 22;

[0067] Step 22, the central controller determines whether ΔP≤Pm is established, if yes, proceeds to step 31, otherwise proceeds to step 21;

[0068] Step 23: The central controller increases the current operating frequency F2 of the second variable frequency compressor 9 at a fourth rate V4, and enters step 12, wherein the value range of V4 is 1 Hz / 10s to 1 Hz / 1s;

[0069] Step 24, the central controller determines whether F2<P4 is established. If yes, it proceeds to step 11, otherwise it proceeds to step 13, P4 is the second frequency operation platform, and the value range of P4 is 25Hz~80Hz;

[0070] Step 25: After the operation cycle T, proceed to step 26;

[0071] Step 26, the central controller determines whether ΔP≤Pm is established, if it is yes, it goes to step 27, otherwise it goes to step 25;

[0072] Step 27: The central controller increases the current operating frequency F2 of the second variable frequency compressor 9 at a fourth rate V6, and enters step 18, wherein the value range of V6 is 1 Hz / 10s to 1 Hz / 1s;

[0073] Step 28, the central controller determines whether F2<Pn is established, if it is yes, it goes to step 27, otherwise it goes to step 29;

[0074] Step 29: After the operation cycle T, proceed to step 30;

[0075] Step 30, the central controller determines whether ΔP≤Pm is established. If yes, it proceeds to step 31, otherwise it proceeds to step 29;

[0076] Step 31, parallel startup is completed.

[0077] Application Examples

[0078] The air-source heat pump unit with a heating capacity of 90kW is configured with a first variable-frequency compressor with a displacement of 80cm3 / rev and a second variable-frequency compressor with a displacement of 90cm3 / rev. The constant K value is 8, which is pre-set by the manufacturer when leaving the factory, and the heating target value is set to the maximum value of 55°C.

[0079] After receiving the heating start command, the central controller of the air energy heat pump unit issues a command to open the third one-way solenoid valve 13 located between the total exhaust side and the total return air side, and directly open the opening of the electronic expansion valve 3 to the maximum step number 480B.

[0080] The central controller controls the first one-way solenoid valve 18 and the second one-way solenoid valve 14 located between the exhaust port and the return port of the first variable frequency compressor 10 and the second variable frequency compressor 9 to open at the same time. After 30 seconds, the central controller controls the first variable frequency compressor to start and increase to 45Hz at a rate of 1s / 1Hz and maintain it. The central controller obtains the detected high pressure PH1 value of 21bar and the low pressure PL1 value of 7bar.

[0081] High and low pressure difference ΔP=PH1-PL1=14bar.

[0082] At this time, the threshold value Pm=K*PL1=8*7bar=56bar, so ΔP≤Pm holds.

[0083] Next, the central controller controls the second variable frequency compressor to start and increase to 45Hz at a rate of 1s / 1Hz and maintain. The central controller obtains the detected high pressure PH1 value of 24bar and the low pressure PL1 value of 5.8bar, with a high and low pressure difference ΔP=24bar-5.8bar=18.2bar.

[0084] At this time, the threshold value Pm=K*PL1=8*5.8bar=46.4bar, so ΔP≤Pm holds.

[0085] Next, the central controller controls the first variable frequency compressor to increase to 62 Hz and maintain at a rate of 1s / 1 Hz. The central controller obtains the detected high pressure PH1 value of 27 bar and the low pressure PL1 value of 5.3 bar.

[0086] High and low pressure difference ΔP=PH1-PL1=27bar-5.3bar=21.7bar.

[0087] At this time, the threshold value Pm=K*PL1=8*5.3ba=42.4bar, so ΔP≤Pm holds.

[0088] The central controller controls the second variable frequency compressor to increase to 62Hz and maintain at a rate of 1s / 1Hz. The central controller obtains the detected high pressure PH1 value of 28.3bar and the low pressure PL1 value of 4.9bar.

[0089] Pressure difference ΔP=PH1-PL1=28.3bar-4.9bar=23.4bar.

[0090] At this time, the threshold value Pm=K*PL1=8*4.9bar=39.2bar, so ΔP≤Pm holds.

[0091] At this point, the dual-frequency compressor is started successfully.

[0092] In the description of the present invention, it should be understood that the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features.

[0093] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for starting an air energy heat pump with a parallel variable frequency compressor, characterized in that The air energy heat pump comprises a first variable frequency compressor (10), a second variable frequency compressor (9), an outdoor heat exchanger (4) and a four-way valve (5); the high-pressure outlet of the first variable frequency compressor (10) is connected to the inlet of an oil separator (17) through a first one-way valve (7) and the high-pressure outlet of the second variable frequency compressor (9) is connected to the inlet of an oil separator (17) through a second one-way valve (8); the first outlet of the oil separator (17) is connected to the first interface (D) of the four-way valve (5); a high-pressure pressure sensor (6) for detecting the high-pressure pressure of the system is arranged between the first outlet of the oil separator (17) and the four-way valve (5); The first interface (D) of the four-way valve (5) is connected to the second interface (C) of the four-way valve (5) and one end of the outdoor heat exchanger (4); the other end of the outdoor heat exchanger (4) is connected to one end of the water-side heat exchanger through the electronic expansion valve (3) and the first stop valve (1); the other end of the water-side heat exchanger is connected to the fourth interface (E) of the four-way valve (5) through the second stop valve (2); the third interface (S) of the four-way valve (5) is connected to the inlet of the gas-liquid separator (11); the outlet of the gas-liquid separator (11) is connected to the outlet of the first variable frequency compressor (10) and the outlet of the gas-liquid separator (11). The first one-way solenoid valve (18) for balancing the high and low pressures of the first variable frequency compressor (10) is connected in series between the high pressure outlet and the low pressure inlet of the first variable frequency compressor (10), and the second one-way solenoid valve (14) for balancing the high and low pressures of the second variable frequency compressor (9) is connected in series between the high pressure outlet and the low pressure inlet of the second variable frequency compressor (9). The second outlet of the oil separator (17) is connected to the low pressure inlet of the first variable frequency compressor (10) through a first oil return capillary tube (16). The second outlet of the oil separator (17) is connected to the low-pressure inlet of the second variable-frequency compressor (9) through a second oil return capillary (15); the first outlet of the oil separator (17) is connected to the inlet of the gas-liquid separator (11) through a third one-way solenoid valve (13) for balancing the high and low pressures of the system; a low-pressure sensor (12) for detecting the low pressure of the system is arranged between the third interface (S) of the four-way valve (5) and the inlet of the gas-liquid separator (11); the output pressure and capacity of the first variable-frequency compressor (10) are different from those of the second variable-frequency compressor (9); The operation includes the following steps: Step 1: Power on and run, then go to step 2; Step 2, open the third one-way solenoid valve (13), and proceed to step 3; Step 3: The central controller of the air energy heat pump opens the number of steps of the electronic expansion valve (3) to the maximum value, and then proceeds to step 4; Step 4: the central controller opens the first one-way solenoid valve (18) and the second one-way solenoid valve (14), and then proceeds to step 5; Step 5: the central controller starts the first variable frequency compressor (10) and proceeds to step 6; Step 6: The central controller increases the current operating frequency F1 of the first variable frequency compressor (10) at a first rate V1, and then proceeds to step 7, wherein the value range of V1 is 1 Hz / 10s to 1 Hz / 1s; Step 7, the central controller determines whether F1<P1 is established. If yes, it goes to step 6, otherwise it goes to step 8, P1 is the first frequency operation platform, and the value range of P1 is 25Hz~65Hz; Step 8: After the operation period T, enter step 9, where the value range of T is 0.5 minutes to 20 minutes; Step nine, the central controller determines whether ΔP≤Pm is established. If yes, it goes to step ten, otherwise it goes to step eight, where ΔP is the high and low pressure difference, ΔP=PH1-PL1, PH1 is the high pressure sensor detection value, PL1 is the low pressure sensor detection value, Pm is the threshold, Pm=K*PL1, K is a constant, and the value range of K is 8 to 12; Step 10, the central controller starts the second variable frequency compressor (9), and then proceeds to step 11; Step 11: the central controller increases the current operating frequency F2 of the second variable frequency compressor (9) at a second rate V2, and enters step 12, wherein the value range of V2 is 1 Hz / 10s to 1 Hz / 1s; Step 12, the central controller determines whether F2<P2 is established. If yes, it goes to step 11, otherwise it goes to step 13, P2 is the second frequency operation platform, and the value range of P2 is 25Hz~65Hz; Step 13, after the operation cycle T, proceed to step 14; Step 14, the central controller determines whether ΔP≤Pm is established. If yes, it proceeds to step 15 and step 23 at the same time, otherwise it proceeds to step 13; Step 15: the central controller increases the current operating frequency F1 of the first variable frequency compressor (10) at a third rate V3, and proceeds to step 16, wherein the value range of V3 is 1 Hz / 10s to 1 Hz / 1s; Step 16, the central controller determines whether F1<P3 is established. If yes, it proceeds to step 15, otherwise it proceeds to step 17, wherein P3 is the second frequency operation platform, and the value range of P3 is 45Hz~80Hz; Step 17: After the operation cycle T, proceed to step 18; Step 18, the central controller determines whether ΔP≤Pm is established, if it is yes, it goes to step 19, otherwise it goes to step 17; Step 19: the central controller increases the current operating frequency F1 of the first variable frequency compressor (10) at a fifth rate V5, and enters step 20, wherein the value range of V5 is 1 Hz / 10s to 1 Hz / 1s; Step 20, the central controller determines whether F1<Pn is established. If yes, it proceeds to step 19, otherwise it proceeds to step 21, where Pn is the target frequency operating platform, and the value range of Pn is 45Hz~110Hz; Step 21: After the operation cycle T, proceed to step 22; Step 22, the central controller determines whether ΔP≤Pm is established, if yes, proceeds to step 31, otherwise proceeds to step 21; Step 23: the central controller increases the current operating frequency F2 of the second variable frequency compressor (9) at a fourth rate V4, and enters step 12, wherein the value range of V4 is 1 Hz / 10s to 1 Hz / 1s; Step 24, the central controller determines whether F2 < P4 is established. If yes, it goes to step 11, otherwise it goes to step 13, P4 is the second frequency operation platform, and the value range of P4 is 25Hz~80Hz; Step 25: After the operation cycle T, proceed to step 26; Step 26, the central controller determines whether ΔP≤Pm is established, if it is yes, it goes to step 27, otherwise it goes to step 25; Step 27: the central controller increases the current operating frequency F2 of the second variable frequency compressor (9) at a fourth rate V6, and enters step 18, wherein the value range of V6 is 1 Hz / 10s to 1 Hz / 1s; Step 28, the central controller determines whether F2<Pn is established, if it is yes, it goes to step 27, otherwise it goes to step 29; Step 29: After the operation cycle T, proceed to step 30; Step 30, the central controller determines whether ΔP≤Pm is established. If yes, it proceeds to step 31, otherwise it proceeds to step 29; Step 31, parallel startup is completed.

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