Series heat pump unit control method and system, and readable storage medium

Abstract

A series heat pump unit control method and system, and a readable storage medium, which relate to the field of heat pumps, and are used to rationally allocate the loads of series compressors. The series heat pump unit control method comprises the following steps: on the basis of a temperature difference between a water output temperature of a series heat pump unit and a set water temperature, determining that the series heat pump unit executes one of the following operations: unit keeping, unit loading, and unit unloading, wherein the series heat pump unit comprises a first pressure-level compressor (1) and a second pressure-level compressor (2), which are connected in series, and the pressure level of the second pressure-level compressor (2) is higher than the pressure level of the first pressure-level compressor (1); if the series heat pump unit executes unit loading, increasing the frequency of at least one of the first pressure-level compressor (1) and the second pressure-level compressor (2); if the series heat pump unit executes unit unloading, reducing the frequency of at least one of the first pressure-level compressor (1) and the second pressure-level compressor (2); and if the series heat pump unit executes unit keeping, keeping both the frequency of the first pressure-level compressor (1) and the frequency of the second pressure-level compressor (2) unchanged.

Description

Series heat pump unit control method, system and readable storage medium

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This disclosure is based on and claims priority to an application with CN application number 202311727335.9 and filing date December 15, 2023. The disclosure content of this CN application is hereby incorporated into this disclosure as a whole. Technical Field

[0003] The present disclosure relates to the field of heat pumps, and in particular to a control method and system for a series heat pump unit, and a readable storage medium. Background Art

[0004] A heat pump is an energy-saving device that, driven by electricity or thermal energy, transfers heat from a low-quality heat source to a high-quality heat source. Industrial heat pumps, as active heat recovery devices, can raise the temperature of waste heat in industrial processes to a higher temperature, allowing it to be used for the heat needs of the same or adjacent processes. For applications where the temperature difference between the heat source and the required temperature is significant, the industry utilizes multi-stage compression heat pumps, which can significantly increase the temperature rise. Multi-stage compression heat pumps utilize compressors connected in series to achieve higher output temperatures.

[0005] Centrifugal compressors have a simple structure, few moving parts, and require less precision to manufacture, resulting in low manufacturing costs and high reliability. Although centrifugal compressors carry the risk of surge under partial load, they offer excellent overall system efficiency in applications with a narrow operating pressure range and high heating capacity (such as high-temperature-rise ultra-high-temperature heat pumps).

[0006] Summary of the Invention

[0007] The inventors have discovered that the relevant technologies have at least the following deficiencies: the industry urgently needs to solve the problem of load distribution when dual compressors are operated in series.

[0008] The present disclosure provides a control method, system and readable storage medium for a series heat pump unit, which are used to reasonably distribute the load of each compressor in the series compressor.

[0009] Some embodiments of the present disclosure provide a control method for a series heat pump unit, comprising the following steps:

[0010] Determining, based on a temperature difference between an outlet water temperature of the series heat pump unit and a set water temperature, whether the series heat pump unit performs one of the following operations: unit maintenance, unit loading, and unit unloading; the series heat pump unit includes a first-stage compressor and a second-stage compressor connected in series, wherein the pressure level of the second-stage compressor is higher than the pressure level of the first-stage compressor;

[0011] If the unit loading operation needs to be performed, the frequency of at least one of the first pressure stage compressor and the second pressure stage compressor is increased; if the unit unloading operation needs to be performed, the frequency of at least one of the first pressure stage compressor and the second pressure stage compressor is reduced; if the unit holding operation needs to be performed, the frequencies of the first pressure stage compressor and the second pressure stage compressor remain unchanged.

[0012] In some embodiments, during the unit loading operation, the series heat pump unit control method performs the following steps:

[0013] Simultaneously increasing the frequency of the first pressure stage compressor and the second pressure stage compressor;

[0014] Determine whether the first difference between the setting parameter of the second pressure stage compressor and the setting parameter of the first pressure stage compressor is greater than or equal to the set upper limit threshold value X max ;

[0015] If the first difference is greater than or equal to the set upper limit threshold X max , then the frequency of the compressor corresponding to the larger of the setting parameters of the second pressure stage compressor and the setting parameters of the first pressure stage compressor is maintained, and the compressor corresponding to the other setting parameter is loaded; if the first difference is less than the set upper limit threshold value X max , then return to the step of simultaneously increasing the frequencies of the first pressure stage compressor and the second pressure stage compressor.

[0016] In some embodiments, during the process of performing the unit loading operation, if the first difference is greater than or equal to the set upper limit threshold value X max , then maintaining the frequency of the compressor corresponding to the larger of the setting parameters of the second-stage compressor and the setting parameters of the first-stage compressor, and loading the compressor corresponding to the other setting parameter, the control method of the series heat pump unit further includes the following steps:

[0017] Determine whether the first difference between the setting parameters of the first pressure stage compressor and the second pressure stage compressor is less than the set lower limit threshold value X min ;

[0018] If the first difference is less than the set lower limit threshold X min , then return to the step of simultaneously increasing the frequencies of the first pressure stage compressor and the second pressure stage compressor; if the first difference is greater than or equal to the set lower limit threshold value X min , then return to the step of maintaining the frequency of the compressor corresponding to the larger of the setting parameters of the second pressure stage compressor and the setting parameters of the first pressure stage compressor, and loading the compressor corresponding to the other setting parameter.

[0019] In some embodiments, during the unloading operation of the unit, the control method of the series heat pump unit performs the following steps:

[0020] frequency of simultaneously unloading the first pressure stage compressor and the second pressure stage compressor;

[0021] Determine whether the first difference between the setting parameters of the first pressure stage compressor and the second pressure stage compressor is greater than or equal to the set upper limit threshold value X max ;

[0022] If the first difference is greater than or equal to the set upper limit threshold X max , then the frequency of the compressor corresponding to the smaller setting parameter of the first pressure stage compressor and the second pressure stage compressor is maintained, and the compressor corresponding to the other setting parameter is unloaded; if the first difference is less than the set upper limit threshold value X max , then return to the step of simultaneously unloading the first pressure stage compressor and the second pressure stage compressor.

[0023] In some embodiments, during the process of performing the unit unloading operation, when the first difference is greater than or equal to the set upper limit threshold value X max , then maintaining the frequency of the compressor corresponding to the smaller set parameter of the first pressure stage compressor and the second pressure stage compressor, and unloading the compressor corresponding to the other set parameter, the control method of the series heat pump unit further includes the following steps:

[0024] Determine whether the first difference between the setting parameters of the second pressure stage compressor after unloading and the first pressure stage compressor maintained is less than the set lower limit threshold value X min ;

[0025] If the first difference is less than the set lower limit threshold X min , then return to the step of simultaneously unloading the first pressure stage compressor and the second pressure stage compressor; if the first difference is greater than or equal to the set lower limit threshold value X min , then return to the step of maintaining the frequency of the compressor corresponding to the smaller set parameter of the first pressure stage compressor and the second pressure stage compressor, and unloading the compressor corresponding to the other set parameter.

[0026] In some embodiments, the series heat pump unit adopts constant pressure ratio control, and the first difference is the absolute value of the difference between the square of the pressure ratio of the second pressure stage compressor and the pressure ratio of the first pressure stage compressor.

[0027] In some embodiments, the series heat pump unit adopts constant pressure ratio control, and the upper limit threshold value X is set maxSet an upper threshold P for the pressure ratio max .

[0028] In some embodiments, the series heat pump unit adopts constant pressure ratio control, and the lower limit threshold value X is set min Set the lower limit threshold P for the pressure ratio min .

[0029] In some embodiments, the series heat pump unit adopts equal current percentage control, and the first difference is the absolute value of the difference between the current percentage of the second pressure stage compressor and the current percentage of the first pressure stage compressor.

[0030] In some embodiments, the series heat pump unit adopts equal current percentage control, and the upper limit threshold value X is set max Set the upper threshold I for the current percentage max .

[0031] In some embodiments, the series heat pump unit adopts equal current percentage control, and the lower limit threshold value X is set min Set the lower threshold I for the current percentage min .

[0032] In some embodiments, the temperature difference is 0.2°C to 0.4°C.

[0033] In some embodiments, the operating parameters of the second pressure stage compressor meet the requirements of the anti-surge line.

[0034] The present disclosure also provides a control system for a series heat pump unit, including:

[0035] Memory; and

[0036] A processor coupled to the memory, the processor being configured to execute a control method for a series heat pump unit as provided by any technical solution of the present disclosure based on instructions stored in the memory.

[0037] An embodiment of the present disclosure further provides a computer-readable storage medium having a computer program stored thereon. When the program is executed by a processor, the control method for a series heat pump unit provided by any technical solution of the present disclosure is implemented.

[0038] The above technical solution provides a control method for a series heat pump unit, which includes a second-stage compressor and a first-stage compressor. Multi-stage compression is achieved by connecting the second-stage compressor and the first-stage compressor in series. During load distribution, the equal pressure ratio control and equal current control methods are determined according to whether the design pressure ratios of the two compressor impellers are consistent. If the design pressure ratios of the two compressor impellers are consistent, the equal pressure ratio control is adopted, otherwise the equal current control method is adopted, thereby ensuring the reasonable distribution of the speeds of the dual compressors and reliable anti-surge operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG1 is a schematic structural diagram of a three-stage compression series heat pump unit provided in some embodiments of the present disclosure.

[0040] FIG2 is a schematic structural diagram of a four-stage compression series heat pump unit provided in some embodiments of the present disclosure.

[0041] FIG3 is a flow chart of a control method for a series heat pump unit provided in some embodiments of the present disclosure.

[0042] FIG4 is a logic diagram of a control method for a series heat pump unit using an equal pressure ratio control method according to some embodiments of the present disclosure.

[0043] FIG5 is a logic diagram of a series heat pump unit control method using an equal current control method provided in some embodiments of the present disclosure.

[0044] Reference numerals:

[0045] 1. First-stage compressor; 2. Second-stage compressor; 3. First heat exchanger; 4. Flasher assembly; 5. Second heat exchanger; 6. First air supply branch; 7. Second air supply branch; 8. Third air supply branch;

[0046] 41. First throttling element; 10. Second throttling element; 11. Third throttling element; 42. Flasher; 43. Fourth throttling element;

[0047] 101, first impeller; 201, second impeller. DETAILED DESCRIPTION

[0048] The technical solutions provided by the present disclosure are described in more detail below with reference to Figures 1 to 5. The description of the exemplary embodiments is merely illustrative and is in no way intended to limit the present disclosure and its application or use. The present disclosure can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided to make the present disclosure thorough and complete and to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that unless otherwise specifically stated, the relative arrangement of the components and steps, the composition of the materials, the numerical expressions and the numerical values ​​described in these embodiments should be interpreted as being merely exemplary and not limiting.

[0049] The terms "first," "second," and similar terms used in this disclosure do not denote any order, quantity, or importance, but are simply used to distinguish different parts. The terms "include," "comprise," and similar terms mean that the elements preceding the term include the elements listed after the term, and do not exclude the possibility of also including other elements.

[0050] In the present disclosure, when a specific device is described as being located between a first device and a second device, an intervening device may or may not be present between the specific device and the first device or the second device. When a specific device is described as being connected to another device, the specific device may be directly connected to the other device without an intervening device, or may be not directly connected to the other device but with an intervening device.

[0051] All terms (including technical or scientific terms) used in this disclosure have the same meaning as those understood by one of ordinary skill in the art to which this disclosure belongs, unless otherwise specifically defined. It should also be understood that terms defined in, for example, general dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an idealized or highly formal sense, unless explicitly defined herein.

[0052] Technologies, methods, and equipment known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, the technologies, methods, and equipment should be considered part of the specification.

[0053] The dimensions of the various parts shown in the drawings are not drawn in accordance with actual proportions. In the drawings, common structural elements or structural elements of the same type are given the same reference numerals, and their repeated descriptions are appropriately omitted.

[0054] Some embodiments of the present disclosure provide a control method for a series heat pump unit. Before introducing the control method for a series heat pump unit, a specific implementation method of the series heat pump unit is first introduced.

[0055] A series heat pump unit can utilize low-grade heat sources to meet the heat supply needs of production processes such as building heating and domestic hot water. Referring to Figure 1, the series heat pump unit includes a first-stage compressor 1, a second-stage compressor 2, a first heat exchanger 3, a flasher assembly 4, and a second heat exchanger 5. The second-stage compressor 2, first heat exchanger 3, second heat exchanger 5, and first-stage compressor 1 form a refrigerant circulation loop. The flasher assembly 4 is used to replenish air to at least one of the first-stage compressor 1 and the second-stage compressor 2.

[0056] The pressure level of the second-stage compressor 2 is higher than that of the first-stage compressor 1, and the pressure ratio of the second-stage compressor 2 is selected as needed. The pressure ratio of the first-stage compressor 1 is also selected as needed. In some embodiments, the first-stage compressor 1 includes at least two coaxially mounted first impellers 101. The two first impellers 101 are mounted on the same drive shaft, and the rotation centerlines of the two first impellers 101 coincide. The connection position between the flash assembly 4 and the first-stage compressor 1 (i.e., the air supply position A) is located between the two first impellers 101.

[0057] In other embodiments, the first-stage compressor 1 may include three or more first impellers 101. All first impellers 101 may be driven by the same drive shaft, and the rotational centerlines of all first impellers 101 may coincide. The first-stage compressor 1 is a centrifugal compressor, which has a compact structure, small size, high flow rate, high power, and is energy-saving, thereby achieving efficient energy utilization.

[0058] The first impellers 101 are arranged side by side, and air can be supplied between every two adjacent first impellers 101 through the flasher assembly 4. The number of branches of the flasher assembly 4 supplying air to the first-stage compressor 1 is related to the number of first impellers 101, and the more first impellers 101 there are, the more branches of the flasher assembly 4 supplying air to the first-stage compressor 1. Specifically, the number of branches of the flasher assembly 4 supplying air to the first-stage compressor 1 can be one less than the number of first impellers 101, so that there is an air supply position between every two adjacent first impellers 101. The above technical solution improves the energy efficiency of the series heat pump unit by reasonably setting the air supply position.

[0059] 1 , a first air supply branch 6 is provided between the flasher assembly 4 and the first pressure stage compressor 1 . The first air supply branch 6 is connected to the first pressure stage compressor 1 between two first impellers 101 .

[0060] The first-stage compressor 1 is located upstream of the second-stage compressor 2 and downstream of the second heat exchanger 5. The refrigerant output by the second heat exchanger 5 flows into the first-stage compressor 1 for compression, then flows out of the first-stage compressor 1; then flows into the second-stage compressor 2 for further compression. The refrigerant compressed by the second-stage compressor 2 enters the first heat exchanger 3 for heat exchange. Then, after passing through the flasher assembly 4, at least part of the fluid enters the second heat exchanger 5 for heat exchange. If the flasher assembly 4 needs to replenish air to at least one of the second-stage compressor 2 and the first-stage compressor 1, some refrigerant will be added to the corresponding compressor, and the remaining refrigerant will flow into the second heat exchanger 5. If the flasher assembly 4 does not need to replenish air to at least one of the second-stage compressor 2 and the first-stage compressor 1, all the refrigerant will flow directly into the second heat exchanger 5.

[0061] The series heat pump unit provided by the above technical solution adopts three-stage compression and double air supply in the middle, and the air supply method is to supply air from the flash unit assembly 4. It has a compact and reasonable structure and high space utilization.

[0062] For large-capacity series heat pump units, the heating capacity can reach more than 10MW. If this heating capacity is to be met, the motor drive power needs to be greatly increased. If the technical solution of the present invention is not adopted, and a single compressor drive method is adopted, the motor volume will be increased and the motor speed will be reduced, which is not conducive to improving the efficiency of the series heat pump unit. By adopting the dual compressor structure provided in the embodiment of the present invention, the motor power can be shared among the two compressors, which can reduce the power loss of a single electrode, increase the motor speed, and help improve the energy efficiency of the series heat pump unit.

[0063] Continuing to refer to Figure 1, the second-stage compressor 2 is located downstream of the first-stage compressor 1 and the two are connected. The second-stage compressor 2 receives the refrigerant transmitted by the first-stage compressor 1 and compresses it. The second-stage compressor 2 includes at least one second impeller 201, and the connection position between the second air supply branch 7 and the first-stage compressor 1 (i.e., the air supply position B) is located upstream of the most upstream second impeller 201. The second-stage compressor 2 adopts a centrifugal compressor with a compact structure, small size, large flow rate, high power, and is conducive to energy saving, which can achieve efficient use of energy.

[0064] The first heat exchanger 3 is installed downstream of the second-stage compressor 2 and communicates with the second-stage compressor 2. Specifically, the first heat exchanger 3 is, for example, a condenser. The condenser is located downstream of the second-stage compressor 2 to utilize the high-temperature refrigerant output by the second-stage compressor 2 for heat exchange. Specifically, the first heat exchanger 3 can be a compact, highly efficient heat exchanger, such as a shell-and-tube heat exchanger.

[0065] The flasher assembly 4 includes a first throttling element 41 and at least two flashers 42 connected in series. A first throttling element 41 is installed between the two flashers 42 connected in series. A first throttling element 41 is used in the middle of the flasher assembly 4 to form two independent flash pressure spaces. The flasher 42 is located downstream of the first heat exchanger 3 and the two are connected. In some embodiments, the flasher assembly 4 includes two independent flashers 42 as an example. One of the flashers 42 is connected to the second pressure stage compressor 2 through the second air supply branch 7, and the other flasher 42 is connected to the first pressure stage compressor 1 through the first air supply branch 6. At least one of the first air supply branch 6 and the second air supply branch 7 is constructed to switch between an on state and an off state. The first air supply branch 6 and the second air supply branch 7 are relatively independent, that is, the on or off state of one of the air supply branches does not affect the on-off state of the other air supply branch.

[0066] 1 , the second heat exchanger 5 is located downstream of the flash unit 4 and is in communication with the first-stage compressor 1. The second heat exchanger 5 can be a shell-and-tube heat exchanger or other compact and efficient heat exchanger.

[0067] Referring to Figure 1 , in some embodiments, the series heat pump unit further includes a second throttling element 10, which is installed between the first heat exchanger 3 and the flasher assembly 4. The second throttling element 10 throttles the refrigerant output from the first heat exchanger 3, and the throttled refrigerant enters the flasher assembly 4. When the second throttling element 10 is adjusted to its maximum opening, it no longer throttles the refrigerant and instead serves to open a refrigerant branch.

[0068] Continuing with Figure 1 , the series heat pump unit also includes a third throttling element 11, which is installed between the flasher assembly 4 and the second heat exchanger 5. This throttling element 11 throttles the refrigerant output from the flasher assembly 4, allowing the throttled refrigerant to enter the second heat exchanger 5. When the third throttling element 11 is fully opened, it no longer throttles the refrigerant and instead serves to open a branch refrigerant circuit.

[0069] Referring to Figure 1 , during the refrigeration cycle, the refrigerant flows through the following path: second heat exchanger 5, first compressor 1, second compressor 2, first heat exchanger 3, second throttling element 10, flash gas assembly 4, first throttling element 41, and third throttling element 11. The flash gas throttled by the second throttling element 10 is supplied to the intake port (port B) of the second-stage compressor 2, while the flash gas throttled by the first throttling element 41 is supplied to the exhaust port (port A) of the first-stage compressor 1.

[0070] Refer to Figure 2, which illustrates four-stage compression. Different from some of the above-mentioned embodiments, in some embodiments illustrated in Figure 2, the second-stage compressor 2 uses two second impellers 201. There are more options for the air supply position of the second-stage compressor 2. The air supply position can be located before the most upstream second impeller 201, or between the two second impellers 201. The flasher assembly 4 includes a first throttling element 41 and three flashers 42 connected in series. The entire series heat pump unit is a four-stage compression unit, which uses three intermediate air supplies. In addition to the second air supply branch 7 and the first air supply branch 6 described above, the series heat pump unit is also provided with a third air supply branch 8, both of which supply air from the flasher assembly 4.

[0071] During four-stage compression, two partitions are installed within the flash ignition assembly 4, creating three independent flash pressure chambers. The refrigeration cycle proceeds as follows: second heat exchanger 5, first-stage compressor 1, second-stage compressor 2, first heat exchanger 3, second throttling element 10, flash ignition assembly 4, first throttling element 41, fourth throttling element 43, and third throttling element 11. Each throttling element can be a fixed orifice plate, electric butterfly valve, or other suitable design. This solution utilizes a single flash ignition assembly to achieve multi-stage air supply, saving space and achieving high cost-effectiveness.

[0072] The flash gas after the second throttling element 10 is supplied to the exhaust port of the third-stage compression, that is, the exhaust port (port C) of the first-stage impeller of the second-stage compressor 2. The flash gas throttled by the fourth throttling element 43 is supplied to the exhaust port (port A) of the first-stage compression of the first-stage compressor 1. The flash gas after the first throttling element 41 is supplied to the intake port (port B) of the second-stage compressor 2.

[0073] In each of the aforementioned embodiments, after the series heat pump unit is designed and manufactured, the pressure ratio between the second-stage compressor 2 and the first-stage compressor 1 is determined. If the design parameters specify that the pressure ratios of the second-stage compressor 2 and the first-stage compressor 1 are the same, the series heat pump unit uses equal pressure ratio control to control the loads of both compressors. If the design parameters specify that the pressure ratios of the second-stage compressor 2 and the first-stage compressor 1 are different, the series heat pump unit uses equal current percentage control to control the loads of both compressors.

[0074] Here, a three-stage compression is used as an example. The first-stage compressor 1 adopts two-stage compression, and the second-stage compressor 2 adopts single-stage compression. The series heat pump unit is described by taking the constant pressure ratio control method as an example. The pressure ratio of the first-stage compressor 1 is P d , the pressure ratio of the high compression stage compressor is P g , set the upper threshold value X max Set the upper threshold value of the pressure ratio to P max , set the lower threshold X min Set the lower limit threshold for the pressure ratio to P min The first difference is the absolute value of the difference between the square of the pressure ratio of the second pressure stage compressor 2 and the pressure ratio of the first pressure stage compressor 1 .

[0075] 3 and 4 , the specific implementation of the control method for the series heat pump units is described below.

[0076] The control method of the series heat pump unit includes the following steps:

[0077] Step S100 determines, based on the temperature difference between the outlet water temperature of the series heat pump unit and the set water temperature, whether the series heat pump unit performs one of the following operations: unit maintenance, unit loading, or unit unloading. The series heat pump unit includes a first-stage compressor 1 and a second-stage compressor 2 connected in series, where the pressure level of the second-stage compressor 2 is higher than the pressure level of the first-stage compressor 1.

[0078] 1 , in some embodiments, the temperature difference is 0.2° C. to 0.4° C. Here, 0.2° C. is used as an example.

[0079] The control target for a series heat pump unit is the heat pump outlet temperature. By calculating the difference between the actual outlet temperature and the setpoint temperature, the appropriate action for the series heat pump unit can be determined. ΔT = setpoint temperature - actual outlet temperature. A ΔT greater than 0 indicates that the setpoint temperature is higher than the actual outlet temperature; a ΔT less than 0 indicates that the setpoint temperature is lower than the actual outlet temperature.

[0080] When -0.2°C ≤ ΔT ≤ 0.2°C, it indicates that the operating parameters of the series heat pump unit meet the set requirements. At this time, the first-stage compressor 1 and the second-stage compressor 2 both maintain the current frequency.

[0081] If △T>+0.2℃, it means that the set temperature is higher than the actual water outlet temperature and the difference between the actual water outlet temperature and the set water temperature is relatively large. The execution capacity of the series heat pump unit needs to be increased and the frequency of the compressor needs to be increased, that is, loading operation needs to be performed.

[0082] If ΔT is less than -0.2°C, the actual outlet water temperature is higher than the set temperature, and the difference between the two is significant. The unit is unloaded, and the compressor frequency is unloaded. Capacity refers to the compressor's operating frequency. If the compressor maintains its current capacity, it maintains its current operating frequency. If it is loaded, the compressor frequency is increased. If it is unloaded, the compressor frequency is decreased.

[0083] In step S200, if the unit needs to be loaded, the frequencies of the first-stage compressor 1 and the second-stage compressor 2 are increased. If the unit needs to be unloaded, the frequencies of the first-stage compressor 1 and the second-stage compressor 2 are decreased. If the frequencies need to be maintained, the frequencies of the first-stage compressor 1 and the second-stage compressor 2 remain unchanged.

[0084] In the above step S200, when the capacity loading is executed according to the water temperature requirement, the two compressor frequencies are loaded at the same time, and the square of the pressure ratio of the second stage compressor 2 is determined. The pressure ratio P of the first stage compressor 1 d The absolute value of the difference is the first difference in the equal pressure ratio mode. hour, and P d The compressor corresponding to the larger value is kept at its capacity, while the compressor corresponding to the smaller value is loaded.

[0085] Referring to Figures 3 and 4, specifically, during the unit loading process, i.e., the content corresponding to the left branch in Figure 4, the control method for the series heat pump unit specifically includes the following steps: simultaneously increasing the setting parameters of the second pressure stage compressor 2 and the frequency of the first pressure stage compressor 1. Then, it is determined whether the first difference between the setting parameters of the second pressure stage compressor 2 and the setting parameters of the first pressure stage compressor 1 is greater than or equal to the set upper limit threshold value Pmax .

[0086] If the first difference Greater than or equal to the set upper threshold P max , then the frequency of the compressor corresponding to the larger of the setting parameters of the second pressure stage compressor 2 and the setting parameters of the first pressure stage compressor 1 is maintained, and the compressor corresponding to the other setting parameter is loaded. max , then the operation of loading both the first-stage compressor 1 and the second-stage compressor 2 is returned to be executed, so that the frequencies of both the first-stage compressor 1 and the second-stage compressor 2 are increased.

[0087] Specifically, if the first difference Greater than or equal to the set upper threshold P max ,and Greater than P d , then maintain the performance capacity of the second pressure stage compressor 2 and load the first pressure stage compressor 1 alone. d Greater than The performance capacity of the first pressure stage compressor 1 is maintained, and the second pressure stage compressor 2 is loaded separately. Less than the set upper threshold P max , then return to the step of loading the first pressure stage compressor 1 and the second pressure stage compressor 2 at the same time.

[0088] During the above unit loading adjustment process, the first difference It changes in real time. During the unit loading process, when a special control is entered where one compressor is loaded and the other compressor is kept, when the compressor that needs to be loaded is loaded to Then, the normal control mode of synchronous loading is restored, that is, when Then the mode is restored to loading both the first-stage compressor 1 and the second-stage compressor 2 .

[0089] 3 and 4, as described above, during the unit loading process, when entering the special control where one compressor is loaded and the other compressor is maintained, it is necessary to determine whether the first difference between the set parameters of the second-stage compressor 2 and the loaded first-stage compressor 1 is less than the set lower limit threshold value X min Specifically, if the first difference is less than the set lower limit threshold P min , then return to the operation of loading both the first pressure stage compressor 1 and the second pressure stage compressor 2. If the first difference Greater than or equal to the set lower limit threshold P min, then the operation returns to maintaining the frequency of the compressor corresponding to the larger of the set parameters of the first-stage compressor 1 and the second-stage compressor 2, and loading the compressor corresponding to the other set parameter. and Then The corresponding second-stage compressor 2 maintains its performance capacity, d The corresponding first pressure stage compressor 1 is loaded until Restore to load both the first stage compressor 1 and the second stage compressor 2. and Then The corresponding second stage compressor 2 is loaded, d The corresponding first pressure stage compressor 1 is maintained until Restore to loading both the first pressure stage compressor 1 and the second pressure stage compressor 2.

[0090] 3 and 4 , in some embodiments, if the series heat pump unit is in the unit holding state, corresponding to the content of the middle branch in FIG4 , the first-stage compressor 1 and the second-stage compressor 2 both maintain the current frequency without adjustment.

[0091] Referring to Figures 3 and 4, specifically referring to the content corresponding to the right branch of Figure 4, during the unloading process of the unit, the control method of the series heat pump unit specifically includes the following steps: simultaneously unloading the frequencies of the first pressure stage compressor 1 and the second pressure stage compressor 2; then determining whether the first difference between the setting parameters of the first pressure stage compressor 1 and the second pressure stage compressor 2 is greater than or equal to the set upper limit threshold value P max .

[0092] If the first difference is greater than or equal to the set upper limit threshold P max , then keep the state corresponding to the smaller one of the first pressure stage compressor 1 and the second pressure stage compressor 2, and unload the larger one separately; if the first difference is less than the set upper limit threshold P max , then return to the step of unloading both the first pressure stage compressor 1 and the second pressure stage compressor 2. Specifically, if the first difference Greater than or equal to the set upper threshold P max ,and Greater than P d , then unload the second stage compressor 2 alone to maintain the performance capacity of the first stage compressor 1. d Greater than Then the first pressure stage compressor 1 is unloaded separately, and the execution capacity of the second pressure stage compressor 2 is kept unchanged. Less than the set upper threshold P max, then return to the step of simultaneously unloading the first pressure stage compressor 1 and the second pressure stage compressor 2.

[0093] In the above-mentioned unit unloading adjustment process, when the state of keeping the smaller of the first pressure stage compressor 1 and the second pressure stage compressor 2 corresponding to each other is entered and the larger one is unloaded separately, the first difference It changes in real time. Then return to the step of unloading both the first-stage compressor 1 and the second-stage compressor 2; otherwise, return to the step of keeping the smaller of the first-stage compressor 1 and the second compressor 2 and unloading the larger one alone.

[0094] Specifically, if and Then separately The corresponding second pressure stage compressor 2 is unloaded, and P d The corresponding first pressure stage compressor 1 maintains the current execution capacity until Restore to the operation of unloading both the first pressure stage compressor 1 and the second pressure stage compressor 2. and Then The corresponding second-stage compressor 2 maintains the current execution capacity, and the P d The corresponding first pressure stage compressor 1 is unloaded until The process returns to the step of unloading both the first-stage compressor 1 and the second-stage compressor 2 .

[0095] The control method for the series heat pump unit provided by the above technical solution adopts an equal pressure ratio control method, which makes the speed distribution reasonable, reduces the compressor shaft grinding phenomenon, avoids the compressor power being too high or too low, and makes the series heat pump unit operate reliably; it avoids the situation where the speed of the first-stage compressor 1 is too high and the speed of the second-stage compressor 2 is too low and the refrigerant discharged from the first-stage compressor 1 cannot be consumed, resulting in the phenomenon that the power of the whole machine is too large but the capacity and energy efficiency are low.

[0096] Referring to FIG5 , the following describes the content of adopting the equal current control method.

[0097] Referring to FIG5 , in some embodiments, if the design parameters of the series heat pump unit are that the pressure ratios of the first pressure stage compressor 1 and the second pressure stage compressor 2 are different, the series heat pump unit adopts equal current percentage control. In this mode, the first difference is the absolute value of the difference between the current percentage of the second pressure stage compressor 2 and the current percentage of the first pressure stage compressor 1. Set the upper threshold value X max Set the upper threshold I for the current percentage max . Set the lower threshold value X min Set the lower threshold I for the current percentagemin . The current percentage is the ratio of the current actual operating current of the compressor to the full-load rated current. The current percentage of the first-stage compressor 1 is I d , and the current percentage of the high-stage compressor is I g . Set the actual current value corresponding to the full-load operation of the unit as the rated current of the compressor. The maximum difference in current percentage is I max , and the minimum difference in current percentage is I min .

[0098] The control method of the series heat pump unit includes the following steps:

[0099] First, determine which operation the series heat pump unit performs according to the temperature difference between the outlet water temperature and the set water temperature of the series heat pump unit: unit hold, unit loading or unit unloading. The series heat pump unit includes a series-connected first-stage compressor 1 and a second-stage compressor 2, and the compression stage of the second-stage compressor 2 is higher than that of the first-stage compressor 1.

[0100] The control target of the series heat pump unit is the heat pump outlet water temperature. By calculating the difference between the actual outlet water temperature and the set temperature of the series heat pump unit, △T = set temperature - actual outlet water temperature. Hereinafter, 0.2 °C is still taken as an example. The control logic is the same as the temperature content described above: when -0.2 °C ≤ △T ≤ 0.2 °C, at this time, both the first-stage compressor 1 and the second-stage compressor 2 maintain their current frequencies. If △T > +0.2 °C, perform the unit loading operation. If △T < -0.2 °C, perform the unit unloading operation.

[0101] Secondly, if the unit needs to be loaded, increase the frequency of at least one of the first-stage compressor 1 and the second-stage compressor 2. If the unit needs to be unloaded, reduce the frequency of at least one of the first-stage compressor 1 and the second-stage compressor 2. If it needs to be held, the frequencies of the first-stage compressor 1 and the second-stage compressor 2 remain unchanged.

[0102] According to the water temperature requirement, when performing the unit loading, load the frequencies of both compressors at the same time, and then determine the current percentage I g of the second-stage compressor 2 d and the current percentage I g of the first-stage compressor 1 d difference. When 丨I max -I g 丨 ≥ I d , the compressor corresponding to the larger value in I

[0103] Referring to FIG. 5, specifically, during the unit loading process, that is, the content corresponding to the left branch in FIG. 5, the control method of the series heat pump unit further includes the following steps: determining whether the first difference between the set parameters of the second-stage compressor 2 and the set parameters of the first-stage compressor 1 is greater than or equal to the set upper limit threshold I max .

[0104] If the first difference │I g -I d │ is greater than or equal to the set upper limit threshold I max , then maintain the frequency of the compressor corresponding to the larger of the set parameters of the second-stage compressor 2 and the set parameters of the first-stage compressor 1, and load the compressor corresponding to the other set parameter. If the first difference is less than the set upper limit threshold I max , then return to execute the step of loading both the first-stage compressor 1 and the second-stage compressor 2

[0105] Specifically, in the equal-current control mode, during the unit loading control, if the first difference │I g -I d │ is greater than or equal to the set upper limit threshold I max , and I g is greater than I d , then maintain the execution ability of the second-stage compressor 2 and load the first-stage compressor 1 alone. If I d is greater than I g , then maintain the execution ability of the first-stage compressor 1 and load the second-stage compressor 2 alone. If the first difference │I g -I d │ is less than the set upper limit threshold I max , then return to execute the step of loading both the first-stage compressor 1 and the second-stage compressor 2

[0106] Referring to FIG. 5, during the unit loading process, after entering the step of maintaining the frequency of the compressor corresponding to the larger of the set parameters of the second-stage compressor 2 and the set parameters of the first-stage compressor 1, and loading the compressor corresponding to the other set parameter, the control method of the series heat pump unit further includes the following steps: determining whether the first difference between the set parameters of the second-stage compressor 2 and the first-stage compressor 1 is less than the set lower limit threshold I min .

[0107] If the first difference │I g -I d │ is less than the set lower limit threshold I min , then return to the step of loading both the first-stage compressor 1 and the second-stage compressor 2. If the first difference is greater than or equal to the set lower limit threshold I min, then return the step of executing and maintaining the frequency of the compressor corresponding to the larger of the set parameters in the first-stage compressor 1 and the second-stage compressor 2, and loading the compressor corresponding to the other set parameter.

[0108] Specifically, if 丨I g -I d 丨≥I min , and I g >I d , then maintain the execution ability of the second-stage compressor 2 corresponding to I g , and separately load the first-stage compressor 1 corresponding to I d until 丨I g -I d 丨<I min , and return to the step of loading both the first-stage compressor 1 and the second-stage compressor 2. If 丨I g -I d 丨≥I min , and I g <I d , then separately load the second-stage compressor 2 corresponding to I g , and maintain the current execution ability of the first-stage compressor 1 corresponding to I d until 丨I g -I d 丨<I min , and return to the step of loading both the first-stage compressor 1 and the second-stage compressor 2.

[0109] Referring to Fig. 5, if the series heat pump unit maintains the current operating parameters, that is, the content corresponding to the middle branch in Fig. 5, then both the first-stage compressor 1 and the second-stage compressor 2 maintain the current frequency and do not need to be adjusted.

[0110] Referring to the content corresponding to the right branch in Fig. 5, during the unit unloading process, the control method of the series heat pump unit specifically includes the following steps: simultaneously unload the frequencies of the first-stage compressor 1 and the second-stage compressor 2; determine whether the first difference between the set parameters of the first-stage compressor 1 and the second-stage compressor 2 is greater than or equal to the set upper limit threshold I max .

[0111] If the first difference 丨I g -I d 丨 is greater than or equal to the set upper limit threshold I max , then maintain the frequency of the compressor corresponding to the smaller of the set parameters of the first-stage compressor 1 and the second-stage compressor 2, and separately unload the other compressor; if the first difference 丨I g -I d 丨 is less than the set upper limit threshold I max, then return to the step of unloading both the first-stage compressor 1 and the second-stage compressor 2. Specifically, if the first difference 丨I g -I d 丨 is greater than or equal to the set upper threshold I max , and I g is greater than I d , then unload only the second-stage compressor 2 corresponding to I g , and maintain the execution ability of the first-stage compressor 1 corresponding to I d . If I d is greater than I g , then unload only the first-stage compressor 1 and keep the execution ability of the second-stage compressor 2 unchanged. If the first difference 丨I g - I d 丨 is less than the set upper threshold I max , then return to the step of unloading both the first-stage compressor 1 and the second-stage compressor 2 simultaneously.

[0112] Continue to refer to FIG. 5. In the equal-current mode, during the unit unloading process, after the step of maintaining the frequency of the compressor corresponding to the smaller of the set parameters of the first-stage compressor 1 and the second-stage compressor 2 and separately unloading the other compressor, the control method of the series heat pump unit further includes the following steps: Determine whether the first difference 丨I g -I d 丨 of the set parameters of the unloaded second-stage compressor 2 and the maintained first-stage compressor 1 is less than the set lower threshold I min ; if the first difference is less than the set lower threshold I min , then return to the step of unloading both the first-stage compressor 1 and the second-stage compressor 2; if the first difference 丨I g -I d 丨 is greater than or equal to the set lower threshold I min , then return to the step of maintaining the frequency of the compressor corresponding to the smaller of the set parameters of the first-stage compressor 1 and the second-stage compressor 2 and separately unloading the other compressor. Specifically, if 丨I g -I d 丨≥I min , and I g >I d , then unload the second-stage compressor 2 corresponding to I g , maintain the current execution ability of the first-stage compressor 1 corresponding to I d until 丨I g -I d 丨<I min , and resume unloading both the first-stage compressor 1 and the second-stage compressor 2. If 丨I g -Id |≥I min and I g <I d then for I g the corresponding second-stage compressor 2 maintains its current execution capacity, and unloads the first-stage compressor 1 corresponding to I d until |I g -I d |<I min and it resumes to unload both the first-stage compressor 1 and the second-stage compressor 2.

[0113] The control method for the series heat pump unit provided by the above technical solution adopts an equal current percentage control method, enabling reasonable distribution of speeds, avoiding too high or too low compressor power, and enabling the reliable operation of the series heat pump unit. [[ID= 21]]

[0114] For the series heat pump unit provided by the above technical solution, due to adopting a control method of equal pressure ratio and equal current percentage for the two compressors, the two compressors are loaded and unloaded simultaneously. Only an anti-surge line needs to be set for the second-stage compressor 2 to ensure that the second-stage compressor 2 does not surge. In the case where the second-stage compressor 2 does not surge, the entire unit does not surge. At this time, it returns to the load control mode. In the equal pressure ratio or equal current percentage control mode, when the execution capacity of the unit decreases the unloading frequency, during the process of synchronous unloading or individual unloading of the two compressors, when the high-pressure stage unloads to below the minimum frequency + 2Hz, the frequencies of the two compressors are maintained. At this time, the unit has unloaded to the limit minimum load and cannot be unloaded further. If the actual load is smaller and the minimum load of the unit is higher than the actual load, the water temperature of the unit will be higher than the set temperature until the unit runs to the standby temperature and stands by normally, thus completing the entire load and anti-surge control. The anti-surge control method is simple and highly reliable.

[0115] Some embodiments of the present disclosure also provide a control system for a series heat pump unit, including a memory and a processor coupled to the memory. The processor is configured to execute the control method for the series heat pump unit provided by any technical solution of the present disclosure based on the instructions stored in the memory.

[0116] Some other embodiments of the present disclosure also provide a computer-readable storage medium, on which a computer program is stored. When the program is executed by a processor, it implements the control method for the series heat pump unit provided by any technical solution of the present disclosure.

[0117] The processors described herein may include a general-purpose processor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in an alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. The processor may also be implemented as a combination of computing devices, such as a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

[0118] A storage medium can be any available medium that can be accessed by a computer. By way of example and not limitation, such computer-readable media may include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Any connection is also properly referred to as a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwaves, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwaves are included in the definition of medium. As used herein, disks and discs include compact discs (CDs), laser discs, optical discs, digital versatile discs (DVDs), floppy disks, and Blu-ray discs, where disks often reproduce data magnetically, while discs reproduce data optically with lasers. The above combinations should also be included within the scope of computer-readable media.

[0119] Those skilled in the art will appreciate that the method embodiments of the present disclosure may be provided as methods, systems, or computer program products. Therefore, the present disclosure may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Furthermore, the present disclosure may take the form of a computer program product implemented on one or more computer-usable non-transient storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0120] The present disclosure is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present disclosure. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device produce a device for implementing the functions specified in one or more processes in the flowchart and / or one or more boxes in the block diagram.

[0121] These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce a product including an instruction device that implements the functions specified in one or more processes in the flowchart and / or one or more boxes in the block diagram.

[0122] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, so that the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in one or more processes in the flowchart and / or one or more boxes in the block diagram.

[0123] In the description of the present disclosure, it should be understood that the terms "center", "longitudinal", "lateral", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present disclosure and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the scope of protection of the present disclosure.

[0124] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present disclosure and not to limit it. Although the present disclosure has been described in detail with reference to the preferred embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present disclosure can still be modified or some technical features can be replaced by equivalents without departing from the spirit of the technical solution of the present disclosure, which should be included in the scope of the technical solution for protection requested by the present disclosure.

Claims

1. A control method for a series heat pump unit, The following steps are involved: According to the temperature difference between the outlet water temperature of the series heat pump unit and the set water temperature, it is judged that the series heat pump unit performs one of the following operations: unit maintenance, unit loading, and unit unloading; the series heat pump unit comprises a first pressure stage compressor (1) and a second pressure stage compressor (2) connected in series, and the pressure stage of the second pressure stage compressor (2) is higher than the pressure stage of the first pressure stage compressor (1); If it is necessary to perform a unit loading operation, the frequency of at least one of the first pressure-stage compressor (1) and the second pressure-stage compressor (2) is increased; if it is necessary to perform a unit unloading operation, the frequency of at least one of the first pressure-stage compressor (1) and the second pressure-stage compressor (2) is reduced; if it is necessary to perform a unit maintaining operation, the frequencies of the first pressure-stage compressor (1) and the second pressure-stage compressor (2) remain unchanged.

2. The control method of a series heat pump unit according to claim 1, wherein during the process of performing the unit loading operation, the control method of the series heat pump unit performs the following steps: Simultaneously increasing the frequency of the first pressure stage compressor (1) and the second pressure stage compressor (2); Determine whether a first difference between a setting parameter of the second pressure stage compressor (2) and a setting parameter of the first pressure stage compressor (1) is greater than or equal to a setting upper limit threshold value X max ; If the first difference is greater than or equal to the set upper limit threshold value X max , the frequency of the compressor corresponding to the larger one of the setting parameters of the second pressure stage compressor (2) and the setting parameters of the first pressure stage compressor (1) is maintained, and the compressor corresponding to the other setting parameter is loaded; if the first difference is less than the set upper limit threshold value X max , then return to the step of simultaneously increasing the frequencies of the first pressure stage compressor (1) and the second pressure stage compressor (2).

3. The control method of a series heat pump unit according to claim 2, wherein in the process of performing the unit loading operation, if the first difference is greater than or equal to the set upper limit threshold value X max , then maintaining the frequency of the compressor corresponding to the larger one of the setting parameters of the second pressure stage compressor (2) and the setting parameters of the first pressure stage compressor (1), and performing a loading operation on the compressor corresponding to the other setting parameter, the control method for the series heat pump unit further comprises the following steps: Determine whether a first difference between the setting parameters of the first pressure stage compressor (1) and the second pressure stage compressor (2) is less than a set lower limit threshold value X min ; If the first difference is less than the set lower limit threshold X min , then return to execute the said while increasing the first pressure The frequency of the first compressor (1) and the second compressor (2) is increased; if the first difference is greater than or equal to the set lower limit threshold value X min , then the step of returning to execute the step of maintaining the frequency of the compressor corresponding to the larger of the setting parameters of the second pressure stage compressor (2) and the setting parameters of the first pressure stage compressor (1), and loading the compressor corresponding to the other setting parameter.

4. The control method of a series heat pump unit according to claim 1, wherein during the process of performing the unit unloading operation, the control method of the series heat pump unit performs the following steps: frequency of simultaneously unloading the first pressure stage compressor (1) and the second pressure stage compressor (2); Determine whether a first difference between the setting parameters of the first pressure stage compressor (1) and the second pressure stage compressor (2) is greater than or equal to a setting upper limit threshold value X max ; If the first difference is greater than or equal to the set upper limit threshold value X max , the frequency of the compressor corresponding to the smaller setting parameter of the first pressure stage compressor (1) and the second pressure stage compressor (2) is maintained, and the compressor corresponding to the other setting parameter is unloaded; If the first difference is less than the set upper limit threshold X max , then the step of simultaneously unloading the first pressure stage compressor (1) and the second pressure stage compressor (2) is returned to execute.

5. The control method of a series heat pump unit according to claim 4, wherein in the process of performing the unit unloading operation, when the first difference is greater than or equal to the set upper limit threshold value X max , then the frequency of the compressor corresponding to the smaller set parameter of the first pressure stage compressor (1) and the second pressure stage compressor (2) is maintained, and after the step of unloading the compressor corresponding to the other set parameter, the control method of the series heat pump unit further comprises the following steps: Determine whether a first difference between the setting parameters of the second pressure stage compressor (2) after unloading and the first pressure stage compressor (1) maintained is less than a set lower limit threshold value X min ; If the first difference is less than the set lower limit threshold X min , then return to the step of simultaneously unloading the first pressure stage compressor (1) and the second pressure stage compressor (2); if the first difference is greater than or equal to the set lower limit threshold value X min , then the process returns to the step of maintaining the frequency of the compressor corresponding to the smaller set parameter of the first pressure stage compressor (1) and the second pressure stage compressor (2), and unloading the compressor corresponding to the other set parameter.

6. The control method for a series heat pump unit according to any one of claims 1 to 5, wherein the series heat pump unit adopts equal pressure ratio control, and the first difference is the absolute value of the difference between the square of the pressure ratio of the second pressure stage compressor (2) and the pressure ratio of the first pressure stage compressor (1).

7. The control method of a series heat pump unit according to claim 2, wherein the series heat pump unit adopts equal pressure ratio control, and the upper limit threshold value X is set max Set an upper threshold P for the pressure ratio max .

8. The control method of a series heat pump unit according to claim 3, wherein the series heat pump unit adopts equal pressure ratio control, and the lower limit threshold value X is set min Set the lower threshold P for the pressure ratio min .

9. The control method for a series heat pump unit according to any one of claims 1 to 5, wherein the series heat pump unit adopts equal current percentage control, and the first difference is the absolute value of the difference between the current percentage of the second pressure stage compressor (2) and the current percentage of the first pressure stage compressor (1).

10. The control method of a series heat pump unit according to claim 2 or 3, wherein the series heat pump unit adopts equal current percentage control, and the upper limit threshold value X is set max Set the upper threshold I for the current percentage max .

11. The control method of a series heat pump unit according to claim 3, wherein the series heat pump unit adopts equal current percentage control, and the lower limit threshold value X is set min Set the lower threshold I for the current percentage min .

12. The control method for a series heat pump unit according to any one of claims 1 to 11, wherein the temperature difference is 0.2°C to 0.4°C.

13. The control method for a series heat pump unit according to any one of claims 1 to 11, wherein the operating parameters of the second pressure stage compressor (2) meet the requirements of the anti-surge line.

14. A control system for a series heat pump unit, comprising: Memory; and A processor coupled to the memory, wherein the processor is configured to execute the control method of the series heat pump unit according to any one of claims 1 to 13 based on instructions stored in the memory.

15. A computer-readable storage medium, wherein a computer program is stored thereon, and when the program is executed by a processor, the control method of a series heat pump unit according to any one of claims 1 to 13 is implemented.

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