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

Abstract

The application discloses a series heat pump unit control method, system and readable storage medium, relates to the heat pump field, and is used for reasonably distributing the load of series compressors. The series heat pump unit control method comprises the following steps: judging the series heat pump unit to execute one of the following according to the temperature difference between the outlet water temperature of the series heat pump unit and the set water temperature: unit keeping, unit loading and unit unloading; the series heat pump unit comprises series first-stage compression compressors and second-stage compression compressors, and the pressure stage of the second-stage compression compressors is higher than that of the first-stage compression compressors; if the unit is loaded, the frequency of at least one of the first-stage compression compressors and the second-stage compression compressors is increased; if the unit is unloaded, the frequency of at least one of the first-stage compression compressors and the second-stage compression compressors is reduced; and if the unit is kept, the frequencies of the first-stage compression compressors and the second-stage compression compressors remain unchanged. The above technical scheme realizes reasonable distribution of the rotating speeds of double compressors.

Description

Technical Field

[0001] This invention relates to the field of heat pumps, and more specifically to a control method, system, and readable storage medium for a series heat pump unit. Background Technology

[0002] A heat pump is an energy-saving device that transfers heat energy from a low-grade heat source to a high-grade heat source, driven by electrical or thermal energy. Industrial heat pumps, as an active heat recovery device, can raise the temperature of waste heat in industrial processes to meet the heat demands of the same or adjacent processes. For applications where the temperature difference between the heat source and the demand temperature is significant, multi-stage compression heat pumps are used to greatly increase the temperature rise. This involves using compressors in series to output a higher temperature.

[0003] Centrifugal compressors have a simple structure, few moving parts, and low manufacturing precision requirements, resulting in advantages such as low manufacturing cost and high reliability. Although centrifugal compressors are prone to surge under partial load, they offer unparalleled overall system efficiency and a highly competitive payback period in applications with small operating pressure variations and large heating capacities (such as high temperature rise ultra-high temperature heat pumps).

[0004] The inventors discovered that at least the following problems exist in the existing technology: there is an urgent need in the industry to solve the load distribution problem when two compressors are running in series. Summary of the Invention

[0005] This invention proposes a control method, system, and readable storage medium for a series heat pump unit, which can be used to rationally allocate the load of the series compressor.

[0006] This invention provides a control method for a series heat pump unit, comprising the following steps:

[0007] Based on the temperature difference between the outlet water temperature and the set water temperature of the series heat pump unit, the series heat pump unit is determined to perform one of the following operations: unit hold, unit load, unit unload; the series heat pump unit includes a first-stage compressor and a second-stage compressor connected in series, wherein the pressure stage of the second-stage compressor is higher than that of the first-stage compressor;

[0008] If the unit needs to be loaded, the frequency of at least one of the first and second pressure stage compressors is increased; if the unit needs to be unloaded, the frequency of at least one of the first and second pressure stage compressors is decreased; if it needs to be maintained, the frequencies of both the first and second pressure stage compressors remain unchanged.

[0009] In some embodiments, during unit loading, the series heat pump unit control method includes the following steps:

[0010] Simultaneously increase the frequency of the first pressure stage compressor and the second pressure stage compressor;

[0011] Determine whether the first difference between the set parameters of the second-stage compressor and the set parameters of the first-stage compressor is greater than or equal to the set upper limit threshold X. max ;

[0012] If the first difference is greater than or equal to the set upper limit threshold X max If the first difference is less than the set upper limit threshold X, then the compressor frequency corresponding to the larger of the set parameters of the second pressure stage compressor and the first pressure stage compressor is maintained, and the compressor corresponding to the other set parameter is loaded; max Then return to the step of simultaneously increasing the frequency of the first pressure stage compressor and the second pressure stage compressor.

[0013] In some embodiments, during unit loading, if the first difference is greater than or equal to a set upper limit threshold X max After the steps of maintaining the compressor frequency corresponding to the larger of the set parameters of the second-stage compressor and the first-stage compressor, and loading the compressor corresponding to the other set parameter, the series heat pump unit control method further includes the following steps:

[0014] Determine whether the first difference between the set parameters of the first pressure stage compressor and the second pressure stage compressor is less than the set lower limit threshold X. min ;

[0015] If the first difference is less than the set lower limit threshold X min If the first difference is greater than or equal to the set lower limit threshold X, then return to the step of simultaneously increasing the frequency of the first pressure stage compressor and the second pressure stage compressor; min If so, 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 first pressure stage compressor, and loading the compressor corresponding to the other setting parameter.

[0016] In some embodiments, the series heat pump unit control method includes the following steps during unit unloading:

[0017] Simultaneously unload the frequency of the first pressure stage compressor and the second pressure stage compressor;

[0018] Determine whether the first difference between the set parameters of the first pressure stage compressor and the second pressure stage compressor is greater than or equal to the set upper limit threshold X. max ;

[0019] If the first difference is greater than or equal to the set upper limit threshold X max If the frequency of the compressor corresponding to the smaller set parameter between the first and second pressure stage compressors is maintained, the compressor corresponding to the other set parameter is unloaded; if the first difference is less than the set upper limit threshold X max If so, return to the step of simultaneously unloading the frequency of the first pressure stage compressor and the second pressure stage compressor.

[0020] In some embodiments, during the unit unloading process, when the first difference is greater than or equal to a set upper limit threshold X max After maintaining the frequency of the compressor corresponding to the smaller set parameter among the first-stage compressor and the second-stage compressor, and unloading the compressor corresponding to the other set parameter, the series heat pump unit control method further includes the following steps:

[0021] Determine whether the first difference between the set parameters of the unloaded second-stage compressor and the retained first-stage compressor is less than a set lower threshold value X. min ;

[0022] If the first difference is less than the set lower threshold X min Then return to the step of simultaneously unloading the frequency 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 X min If so, return to the step of maintaining the frequency of the compressor corresponding to the smaller of the set parameters of the first pressure stage compressor and the second pressure stage compressor, and unloading the compressor corresponding to the other set parameter.

[0023] In some embodiments, the series heat pump unit adopts isobaric 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.

[0024] In some embodiments, the series heat pump unit employs isobaric ratio control, wherein the set upper limit threshold X max Set an upper limit threshold P for the pressure ratio max .

[0025] In some embodiments, the series heat pump unit employs isobaric ratio control, wherein the set lower limit threshold X min Set a lower threshold P for the pressure ratio min .

[0026] In some embodiments, the series heat pump unit adopts equal current percentage control, then 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.

[0027] In some embodiments, the series heat pump unit employs equal current percentage control, wherein the set upper limit threshold X max Set an upper limit threshold I for the current percentage max .

[0028] In some embodiments, the series heat pump unit employs equal current percentage control, wherein the set lower limit threshold X min Set a lower limit threshold I for the current percentage. min .

[0029] In some embodiments, the temperature difference is 0.2℃ to 0.4℃.

[0030] In some embodiments, the operating parameters of the second pressure stage compressor meet the requirements of the anti-surge line.

[0031] This invention also provides a series heat pump unit control system, comprising:

[0032] Memory; and

[0033] A processor coupled to the memory is configured to execute a series heat pump unit control method as provided in any of the technical solutions of the present invention, based on instructions stored in the memory.

[0034] This invention also provides a computer-readable storage medium, characterized in that it stores a computer program thereon, which, when executed by a processor, implements the series heat pump unit control method provided by any of the technical solutions of this invention.

[0035] The series heat pump unit control method provided by the above technical solution includes a second-stage compressor and a first-stage compressor. By connecting the second-stage compressor and the first-stage compressor in series, multi-stage compression is achieved. 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, equal pressure ratio control is adopted; otherwise, equal current control is adopted, ensuring reasonable speed distribution of the two compressors and reliable operation without surge. Attached Figure Description

[0036] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and, together with their description, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings:

[0037] Figure 1 This is a schematic diagram of the three-stage compression structure of a series heat pump unit provided in an embodiment of the present invention.

[0038] Figure 2This is a schematic diagram of the four-stage compression structure of a series heat pump unit provided in an embodiment of the present invention.

[0039] Figure 3 This is a schematic diagram of the control method for a series heat pump unit provided in an embodiment of the present invention.

[0040] Figure 4 This is a logic diagram illustrating the isobaric ratio control method used in the series heat pump unit control method provided in this embodiment of the invention.

[0041] Figure 5 This is a logic diagram illustrating the equal current control method used in the series heat pump unit control method provided in this embodiment of the invention.

[0042] Figure label:

[0043] 1. First-stage compressor; 2. Second-stage compressor; 3. First heat exchanger; 4. Flash evaporator assembly; 5. Second heat exchanger; 6. First make-up gas branch; 7. Second make-up gas branch; 8. Third make-up gas branch;

[0044] 41. First throttling element; 10. Second throttling element; 11. Third throttling element; 42. Flash generator; 43. Fourth throttling element;

[0045] 101, First impeller; 201, Second impeller. Detailed Implementation

[0046] The following is combined Figures 1-5 The technical solutions provided by this invention will be described in more detail below. The descriptions of exemplary embodiments are merely illustrative and are in no way intended to limit this disclosure or its application or use. This disclosure can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided to make this disclosure thorough and complete, and to fully express the scope of this disclosure to those skilled in the art. It should be noted that, unless otherwise specifically stated, the relative arrangement of components and steps, the composition of materials, numerical expressions, and values ​​set forth in these embodiments should be interpreted as merely exemplary and not as limiting.

[0047] The terms “first,” “second,” and similar words used in this disclosure do not indicate any order, quantity, or importance, but are merely used to distinguish different parts. Words such as “including” or “contains” mean that the element preceding the word covers the element listed after the word, and do not exclude the possibility of covering other elements as well.

[0048] In this disclosure, when a specific device is described as being located between a first device and a second device, an intermediary device may or may not be present between the specific device and the first or second device. When a specific device is described as being connected to other devices, the specific device may be directly connected to the other devices without an intermediary device, or it may be not directly connected to the other devices but have an intermediary device.

[0049] All terms used in this disclosure (including technical or scientific terms) have the same meaning as understood by one of ordinary skill in the art to which this disclosure pertains, unless otherwise specifically defined. It should also be understood that terms defined in a general dictionary, such as a dictionary, should be interpreted as having a meaning consistent with their meaning in the context of the relevant art, and not as having an idealized or highly formalized meaning, unless expressly defined herein.

[0050] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, they should be considered part of the specification.

[0051] The dimensions of the various parts shown in the accompanying drawings are not drawn to actual scale. Common structural elements or elements of the same kind are given the same reference numerals in the various drawings, and repeated descriptions of them are omitted where appropriate.

[0052] This invention provides a control method for a series heat pump unit. Before introducing the control method for the series heat pump unit, the specific implementation of the series heat pump unit will be introduced first.

[0053] Series-connected heat pump units can utilize low-grade heat sources to meet the heat supply needs of production processes such as building heating and domestic hot water supply. (See also...) Figure 1 The series-connected heat pump unit includes a first-stage compressor 1, a second-stage compressor 2, a first heat exchanger 3, a flash evaporator assembly 4, and a second heat exchanger 5. The second-stage compressor 2, the first heat exchanger 3, the second heat exchanger 5, and the first-stage compressor 1 form a refrigerant circulation loop. The flash evaporator assembly 4 is used to supply refrigerant to at least one of the first-stage compressor 1 and the second-stage compressor 2.

[0054] The pressure stage 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 center lines of the two first impellers 101 coincide. The communication position (i.e., gas supply position A) between the flash evaporator assembly 4 and the first-stage compressor 1 is located between the two first impellers 101.

[0055] In other embodiments, the first-stage compressor 1 may also include three or more first impellers 101, all of which can be driven by the same drive shaft, and the rotation center lines of all the first impellers 101 coincide. The first-stage compressor 1 is a centrifugal compressor, which has a compact structure, small size, large flow rate, high power, and is conducive to energy saving, thus achieving efficient energy utilization.

[0056] Each first impeller 101 is arranged side by side, and gas can be supplied between every two adjacent first impellers 101 via the flash evaporator assembly 4. The number of gas supply branches of the flash evaporator assembly 4 to the first pressure stage compressor 1 is related to the number of first impellers 101, and the more first impellers 101 there are, the more gas supply branches the flash evaporator assembly 4 can supply to the first pressure stage compressor 1. Specifically, the number of gas supply branches of the flash evaporator assembly 4 to the first pressure stage compressor 1 can be one less than the number of first impellers 101, so that there is a gas supply point 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 gas supply points.

[0057] See also Figure 1 A first gas supply branch 6 is provided between the flash generator assembly 4 and the first pressure stage compressor 1. The connection point between the first gas supply branch 6 and the first pressure stage compressor 1 is located between two of the first impellers 101.

[0058] The first-stage compressor 1 is located upstream of the second-stage compressor 2 and downstream of the second heat exchanger 5. Refrigerant output from the second heat exchanger 5 flows into the first-stage compressor 1 for compression, then flows out of the first-stage compressor 1; it then flows into the second-stage compressor 2 for further compression. The compressed refrigerant from the second-stage compressor 2 enters the first heat exchanger 3 for heat exchange. After passing through the flash evaporator assembly 4, at least a portion of the fluid enters the second heat exchanger 5 for heat exchange. If the flash evaporator assembly 4 needs to supply refrigerant to at least one of the second-stage compressor 2 or the first-stage compressor 1, a portion of the refrigerant will be supplied to the corresponding compressor, and the remaining refrigerant will flow into the second heat exchanger 5. If the flash evaporator assembly 4 does not need to supply refrigerant to at least one of the second-stage compressor 2 or the first-stage compressor 1, all the refrigerant flows directly into the second heat exchanger 5.

[0059] The series heat pump unit provided by the above technical solution adopts three-stage compression and intermediate dual gas replenishment. The gas replenishment method is to replenish gas from the flash evaporator assembly 4. The structure is compact and reasonable and the space utilization rate is high.

[0060] For large-capacity series-connected heat pump units, the heating capacity can reach over 10MW. To meet this capacity, the motor drive power needs to be significantly increased. If a single-compressor drive is used instead of the technical solution of this invention, the motor size will increase and the motor speed will decrease, which is detrimental to improving the efficiency of the series-connected heat pump unit. By adopting the dual-compressor structure provided in this embodiment, the motor power can be distributed across two compressors. This reduces the power consumption of a single compressor, increases the motor speed, and improves the energy efficiency of the series-connected heat pump unit.

[0061] See also Figure 1 The second-stage compressor 2 is located downstream of and connected to the first-stage compressor 1. The second-stage compressor 2 receives and compresses the refrigerant from the first-stage compressor 1. The second-stage compressor 2 includes at least one second impeller 201, and the connection point between the second gas supply branch 7 and the first-stage compressor 1 (i.e., gas supply point B) is located upstream of the uppermost second impeller 201. The second-stage compressor 2 is a centrifugal compressor, which is compact, small in size, has a large flow rate, high power, and is conducive to energy saving, achieving efficient energy utilization.

[0062] The first heat exchanger 3 is installed downstream of the second stage compressor 2 and the two are connected. 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 from the second stage compressor 2 for heat exchange. The first heat exchanger 3 can specifically be a shell-and-tube heat exchanger or other compact and efficient heat exchanger.

[0063] The flash evaporator assembly 4 includes a first throttling element 41 and at least two flash evaporators 42 connected in series. The first throttling element 41 is installed between the two flash evaporators 42 connected in series, and the flash evaporator assembly 4 is formed by a first throttling element 41 in the middle to create two independent flash pressure spaces. The flash evaporators 42 are located downstream of the first heat exchanger 3 and are connected to it. In some embodiments, the flash evaporator assembly 4 includes two independent flash evaporators 42. One flash evaporator 42 is connected to the second pressure stage compressor 2 through a second gas supply branch 7, and the other flash evaporator 42 is connected to the first pressure stage compressor 1 through a first gas supply branch 6. At least one of the first gas supply branch 6 and the second gas supply branch 7 is configured to switch between an on state and an off state. The first gas supply branch 6 and the second gas supply branch 7 are relatively independent, that is, the on or off state of the other gas supply branch is not affected by the on or off state of the first gas supply branch when one gas supply branch is on or off.

[0064] See also Figure 1 The second heat exchanger 5 is located downstream of the flash evaporator assembly 4; the second heat exchanger 5 is connected to the first stage compressor 1. Specifically, the second heat exchanger 5 can be a shell-and-tube heat exchanger or other compact heat exchanger with high heat exchange efficiency.

[0065] See 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 flash evaporator assembly 4. The second throttling element 10 throttles the refrigerant output from the first heat exchanger 3, and the throttled refrigerant enters the flash evaporator assembly 4. When the opening of the second throttling element 10 is adjusted to its maximum, the second throttling element 10 no longer throttles, but instead serves to open the refrigerant branch.

[0066] See also Figure 1 The series heat pump unit also includes a third throttling element 11, which is installed between the flash evaporator assembly 4 and the second heat exchanger 5. The third throttling element 11 throttles the refrigerant output from the flash evaporator assembly 4, and the throttled refrigerant enters the second heat exchanger 5. When the opening of the third throttling element 11 is adjusted to its maximum, the third throttling element 11 no longer throttles, but instead serves to open the refrigerant branch.

[0067] See Figure 1 During the refrigeration cycle, the refrigerant flows along 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 suction port (B port) of the second stage compressor 2, and the flash gas throttled by the first throttling element 41 is supplied to the discharge port (A port) of the first stage compressor 1.

[0068] See Figure 2 , Figure 2 This illustrates four levels of compression. Unlike the previous embodiment, in... Figure 2 In the illustrated embodiment, the second-stage compressor 2 employs two second impellers 201. The second-stage compressor 2 has more options for its gas supply location; the supply point can be located before the upstream second impeller 201 or between the two second impellers 201. The flash evaporator assembly 4 includes a first throttling element 41 and three flash evaporators 42 connected in series. The entire series-connected heat pump unit is a four-stage compression system with three intermediate gas supply lines. In addition to the second gas supply branch 7 and the first gas supply branch 6 described above, the series-connected heat pump unit also has a third gas supply branch 8, all of which receive gas from the flash evaporator assembly 4.

[0069] In four-stage compression, the flash evaporator assembly 4 has two internal baffles, forming three independent flash pressure spaces. The refrigeration cycle is as follows: second heat exchanger 5, first stage compressor 1, second stage compressor 2, first heat exchanger 3, second throttling element 10, flash evaporator assembly 4, first throttling element 41, fourth throttling element 43, and third throttling element 11. Each throttling element can be a fixed orifice plate, an electric butterfly valve, etc. This scheme achieves multi-stage gas replenishment using only one flash evaporator assembly, saving space and offering high cost-effectiveness.

[0070] The flash gas after the second throttling element 10 is supplied to the exhaust port of the third-stage compressor, that is, to the exhaust port (C port) of the first-stage impeller of the second-stage compressor 2. The flash gas after the fourth throttling element 43 is supplied to the exhaust port (A port) of the first-stage compressor of the first-stage compressor 1. The flash gas after the first throttling element 41 is supplied to the intake port (B port) of the second-stage compressor 2.

[0071] In the above embodiments, after the series-connected heat pump unit is designed and manufactured, the pressure ratios of the second-stage compressor 2 and the first-stage compressor 1 are 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-connected heat pump unit uses an equal pressure ratio control method to control the load of the two 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-connected heat pump unit uses an equal current percentage control method to control the load of the two compressors.

[0072] This example uses a three-stage compression system. The first-stage compressor 1 uses two-stage compression, and the second-stage compressor 2 uses single-stage compression. The series-connected heat pump unit is described using an isobaric ratio control method. 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 limit threshold X max Set an upper limit threshold of P for the pressure ratio. max Set a lower threshold X min Set a lower threshold value of P for the pressure ratio. min The first difference is the absolute value of the difference between the square of the pressure ratio of the second-stage compressor 2 and the pressure ratio of the first-stage compressor 1.

[0073] See Figure 3 and Figure 4 The following describes the specific implementation method of the control method for series heat pump units.

[0074] The control method for series heat pump units includes the following steps:

[0075] Step S100: Based on the temperature difference between the outlet water temperature and the set water temperature of the series heat pump unit, determine whether the series heat pump unit will perform one of the following operations: unit hold, unit load, or unit unload. The series heat pump unit includes a first-stage compressor 1 and a second-stage compressor 2 connected in series, wherein the pressure stage of the second-stage compressor 2 is higher than that of the first-stage compressor 1.

[0076] See Figure 1 In some embodiments, the temperature difference is 0.2℃ to 0.4℃. This article uses 0.2℃ as an example.

[0077] The control target for a 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, the appropriate operation for the series heat pump unit can be determined. △T = Set temperature - Actual outlet water temperature. △T > 0 indicates that the set temperature is higher than the actual outlet water temperature; △T < 0 indicates that the set temperature is lower than the actual outlet water temperature.

[0078] When -0.2℃≤△T≤0.2℃, it indicates that the operating parameters of the series heat pump unit meet the set requirements. At this time, both the first stage compressor 1 and the second stage compressor 2 maintain their current frequency.

[0079] If △T>+0.2℃, it means that the set temperature is higher than the actual outlet water temperature and the difference between the actual outlet water 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.

[0080] If ΔT < -0.2℃, it indicates that the actual outlet water temperature is higher than the set temperature and the difference between the actual outlet water temperature and the set water temperature is relatively large. The unit will then unload, and the compressor frequency will decrease. Execution capacity refers to the compressor's operating frequency. If the compressor maintains its current execution capacity, it means the compressor maintains its current operating frequency. If loaded, the compressor's operating frequency will increase. If unloaded, the compressor's operating frequency will decrease.

[0081] In step S200, if unit loading is required, the frequency of the first-stage compressor 1 and the second-stage compressor 2 is increased. If unit unloading is required, the frequency of the first-stage compressor 1 and the second-stage compressor 2 is decreased. If maintenance is required, the frequencies of both the first-stage compressor 1 and the second-stage compressor 2 remain unchanged.

[0082] In step S200 above, when the capacity is loaded according to the water temperature requirement, the frequencies of two compressors are loaded simultaneously, and the square of the pressure ratio of the second-stage compressor is determined. Pressure ratio P of the first stage compressor 1 d The difference, the absolute value of which is the first difference under the isobaric ratio mode. When hour, and P d The compressors with larger values ​​are kept operational, while those with smaller values ​​are loaded.

[0083] See Figure 3 and Figure 4 Specifically, during the unit loading process, i.e. Figure 4 The content corresponding to the left branch, the control method of the series heat pump unit specifically includes the following steps: simultaneously increasing the set parameters of the second-stage compressor 2 and the frequency of the first-stage compressor 1. Then, 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 P. max .

[0084] If the first difference Greater than or equal to the set upper limit threshold P max If the set parameters of the second-stage compressor 2 and the first-stage compressor 1 are greater, the compressor frequency corresponding to the compressor with the larger set parameter will be maintained, and the compressor corresponding to the other set parameter will be loaded. If the first difference is less than the set upper limit threshold P max If so, the operation of loading both the first-stage compressor 1 and the second-stage compressor 2 is performed, so that the frequency of both the first-stage compressor 1 and the second-stage compressor 2 is increased.

[0085] Specifically, if the first difference Greater than or equal to the set upper limit threshold P max ,and Greater than P d If P..., then the second-stage compressor 2 maintains its operating capability, and the first-stage compressor 1 is loaded independently. d Greater than Then, the operating capability of the first-stage compressor 1 is maintained, and the second-stage compressor 2 is loaded separately. If the first difference... Less than the set upper limit threshold P max If so, return to the step of simultaneously loading the first stage compressor 1 and the second stage compressor 2.

[0086] During the above-mentioned unit loading and adjustment process, the first difference It changes in real time. During unit loading, when entering a special control phase where one compressor is loaded while the other remains in place, the compressor requiring loading will be loaded to... Then, restore the normal control mode of synchronous loading, that is, when Then it will revert to the mode where both the first-stage compressor 1 and the second-stage compressor 2 are loaded.

[0087] See Figure 3and Figure 4 As mentioned above, during the unit loading process, when entering a special control phase where one compressor is loaded and the other is held, 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 X. min Specifically, if the first difference is less than the set lower threshold P min If so, then return to execute the operation of loading both the first-stage compressor 1 and the second-stage compressor 2. If the first difference Greater than or equal to the set lower threshold P min If so, 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. In more detail, if... and Then for The corresponding second-stage compressor 2 maintains its operating capability for P. d The corresponding first-stage compressor 1 is loaded until... Restore loading to both the first-stage compressor 1 and the second-stage compressor 2. If and Then for The corresponding second-stage compressor 2 is loaded, for P d The corresponding first-stage compressor 1 remains in place until... Restore loading to both the first-stage compressor 1 and the second-stage compressor 2.

[0088] See Figure 3 and Figure 4 In some embodiments, if the series-connected heat pump unit is in a standby state, then the corresponding Figure 4 For the content corresponding to the intermediate branch, the first stage compressor 1 and the second stage compressor 2 will both maintain their current frequencies without adjustment.

[0089] See Figure 3 and Figure 4 Specifically refers to Figure 4 The content corresponding to the right branch, during the unit unloading process, the series heat pump unit control method specifically includes the following steps: simultaneously unloading the frequencies of the first-stage compressor 1 and the second-stage compressor 2; then determining 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 P. max .

[0090] If the first difference is greater than or equal to the set upper limit threshold P maxIf the first pressure stage compressor 1 and the second compressor 2 remain in the smaller state, the larger one will be unloaded 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-stage compressor 1 and the second-stage compressor 2. Specifically, if the first difference... Greater than or equal to the set upper limit threshold P max ,and Greater than P d Then, the second-stage compressor 2 will be unloaded separately, while maintaining the operating capacity of the first-stage compressor 1. If P d Greater than Then, only the first-stage compressor 1 is unloaded, while the operating capacity of the second-stage compressor 2 remains unchanged. If the first difference... Less than the set upper limit threshold P max If so, return to the step of simultaneously unloading the first stage compressor 1 and the second stage compressor 2.

[0091] During the aforementioned unit unloading adjustment process, when entering the state of maintaining the smaller of the first-stage compressor 1 and the second compressor 2, and unloading the larger one separately, the first difference... It changes in real time. When If the first stage compressor 1 and the second stage compressor 2 are both unloaded, then return to the step of keeping the smaller of the first stage compressor 1 and the second stage compressor 2 and unloading the larger one alone.

[0092] Specifically, if and Then individually The corresponding second-stage compressor 2 is unloaded, and P is... d The corresponding first-stage compressor 1 maintains its current operating capacity until... Revert to the operation where both the first-stage compressor 1 and the second-stage compressor 2 are unloaded. If and Then for The corresponding second-stage compressor 2 maintains its current operating capacity, and P is operated independently. d The corresponding first-stage compressor 1 is unloaded until... Revert to the step of unloading both the first-stage compressor 1 and the second-stage compressor 2.

[0093] The series heat pump unit control method provided by the above technical solution adopts the isobaric control method, which makes the speed distribution reasonable, reduces the compressor shaft wear phenomenon, avoids the compressor power being too high or too low, and makes the series heat pump unit operate reliably. It avoids the phenomenon that when the speed of the first stage compressor 1 is too high and the speed of the second stage compressor 2 is too low, the refrigerant discharged by the first stage compressor 1 cannot be consumed, resulting in excessive power of the whole unit but low capacity and energy efficiency.

[0094] See Figure 5 The following section introduces the content regarding the use of equal current control.

[0095] See Figure 5 In some embodiments, if the design parameters of the series heat pump unit are such that the pressure ratios of the first-stage compressor 1 and the second-stage compressor 2 are different, then the series heat pump unit employs equal current percentage control. In this mode, the first difference is the absolute value of the difference between the current percentage of the second-stage compressor 2 and the current percentage of the first-stage compressor 1. An upper limit threshold X is set. max Set an upper limit threshold I for the current percentage max Set a lower threshold X. min Set a lower limit threshold I for the current percentage. min The current percentage is the ratio of the compressor's current actual operating current to its rated current at full load. The current percentage for the first-stage compressor (Id) is denoted as Id, and for the higher-stage compressors as Ig. The actual current corresponding to full-load operation of the unit is set as the compressor's rated current, and the maximum difference in current percentage is I. max The minimum difference in current percentage is I. min .

[0096] The control method for series heat pump units includes the following steps:

[0097] First, the temperature difference between the outlet water temperature and the set water temperature of the series heat pump unit determines which operation the unit should perform: unit hold, unit loading, or unit unloading. The series heat pump unit consists of a first-stage compressor 1 and a second-stage compressor 2 connected in series, with the second-stage compressor 2 having a higher pressure stage than the first-stage compressor 1.

[0098] The control target for a series heat pump unit is the heat pump outlet water temperature. The difference between the actual outlet water temperature and the set temperature is calculated as ΔT = set temperature - actual outlet water temperature. The following example uses 0.2℃. The control logic is the same as described above: when -0.2℃ ≤ ΔT ≤ 0.2℃, both the first-stage compressor 1 and the second-stage compressor 2 maintain their current frequencies. If ΔT > +0.2℃, a unit loading operation is performed. If ΔT < -0.2℃, a unit unloading operation is performed.

[0099] Secondly, if the unit needs to be loaded, the frequency of at least one of the first-stage compressor 1 and the second-stage compressor 2 is increased. If the unit needs to be unloaded, the frequency of at least one of the first-stage compressor 1 and the second-stage compressor 2 is decreased. If it needs to be maintained, the frequencies of both the first-stage compressor 1 and the second-stage compressor 2 remain unchanged.

[0100] Based on the water temperature requirement, when the unit is loaded, both compressor frequencies are applied simultaneously, and then the current percentage of the second-stage compressor (I) is determined. g Current percentage I of the first stage compressor 1 d Difference. When |I g -I d |≥I max At that time, I g and I d The compressors with larger values ​​are kept operational, while the compressors with smaller values ​​are loaded.

[0101] See Figure 5 Specifically, during the unit loading process, i.e. Figure 5 The content corresponding to the left branch in the middle, the series heat pump unit control method also 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 .

[0102] If the first difference |I g -I d |Greater than or equal to the set upper limit threshold I max If the set parameters of the second-stage compressor 2 and the first-stage compressor 1 are greater, the compressor frequency corresponding to the compressor with the larger set parameter will be maintained, and the compressor corresponding to the other set parameter will be loaded. If the first difference is less than the set upper limit threshold I... max If so, return to the step where both the first pressure stage compressor 1 and the second pressure stage compressor 2 are loaded.

[0103] Specifically, in the equal current control mode, during unit load control, if the first difference |I g -I d |Greater than or equal to the set upper limit threshold I max And I g Greater than I d If so, the second-stage compressor 2 maintains its operating capability, and the first-stage compressor 1 is loaded independently. If I d Greater than I g If the first pressure stage compressor 1 maintains its operating capability, then the second pressure stage compressor 2 is loaded independently. If the first difference |I g -I d| Less than the set upper limit threshold I max If so, return to the step of simultaneously loading the first pressure stage compressor 1 and the second pressure stage compressor 2.

[0104] See Figure 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 first-stage compressor 1, and loading the compressor corresponding to the other set parameter, the series heat pump unit control method further includes the following step: determining whether the first difference between the set parameters of the second-stage compressor 2 and the first-stage compressor 1 is less than a set lower limit threshold I. min .

[0105] If the first difference |I g -I d | Less than the set lower threshold I min If so, 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 threshold I. min If so, return to the step of maintaining the frequency of the compressor corresponding to the larger of the set parameters of the first pressure stage compressor 1 and the second pressure stage compressor 2, and loading the compressor corresponding to the other set parameter.

[0106] Specifically, if |I g -I d |≥I min , and I g >I d Then for I g The corresponding second-stage compressor 2 maintains its operating capability, separately for I. d The corresponding first-stage compressor 1 is loaded until |I g -I d |<I min Return to the steps where both the first-stage compressor 1 and the second-stage compressor 2 are loaded. If |I g -I d |≥I min , and I g <I d Then, for I alone g The corresponding second-stage compressor 2 is loaded, for I d The corresponding first-stage compressor 1 maintains its current operating capacity until |I g -Id|<I min Then return to the step of loading both the first stage compressor 1 and the second stage compressor 2.

[0107] See Figure 5 If the series heat pump unit maintains its current operating parameters, i.e. Figure 5For the content corresponding to the intermediate branch, the first stage compressor 1 and the second stage compressor 2 will both maintain their current frequencies without adjustment.

[0108] See Figure 5 The content corresponding to the right branch, during the unit unloading process, the series heat pump unit control method specifically includes the following steps: simultaneously unloading the frequency of the first stage compressor 1 and the second stage compressor 2; determining 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 .

[0109] If the first difference |I g -I d |Greater than or equal to the set upper limit threshold I max If the set parameters of the first-stage compressor 1 and the second-stage compressor 2 are smaller, the compressor frequency will be maintained, while the other compressor will be unloaded separately; if the first difference |I g -I d | 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 |I g -I d |Greater than or equal to the set upper limit threshold I max And I g Greater than I d Then, for I alone g The corresponding second-stage compressor 2 is unloaded, maintaining I. d The corresponding first-stage compressor 1's operating capacity. If I d Greater than I g Then, only the first-stage compressor 1 is unloaded, while the execution capability of the second-stage compressor 2 remains unchanged. If the first difference |I g -I d | Less than the set upper limit threshold I max If so, return to the step of simultaneously unloading the first stage compressor 1 and the second stage compressor 2.

[0110] See also Figure 5 In the equal current mode, during the unit unloading process, after maintaining the frequency of the compressor corresponding to the smaller set parameter of the first-stage compressor 1 and the second-stage compressor 2, and unloading the other compressor separately, the series heat pump unit control method further includes the following steps: determining the first difference |I| between the set parameter of the unloaded second-stage compressor 2 and the maintained first-stage compressor 1. g -I d Is it less than the set lower threshold? min If the first difference is less than the set lower threshold Imin Then return to the step of unloading both the first-stage compressor 1 and the second-stage compressor 2; if the first difference |I g -I d |Greater than or equal to the set lower threshold I min Then, the process returns to the step of maintaining the frequency of the compressor corresponding to the compressor with the smaller set parameter of the first-stage compressor 1 and the second-stage compressor 2, while unloading the other compressor separately. Specifically, if |I g -I d |≥I min , and I g >I d Then for I g The corresponding second-stage compressor 2 is unloaded, and I... d The corresponding first-stage compressor 1 maintains its current operating capacity until |I g -I d |<I min The process is restored to unloading of both the first-stage compressor 1 and the second-stage compressor 2. If |I g -I d |≥I min , and I g <I d Then for I g The corresponding second-stage compressor 2 maintains its current operating capacity for I. d The corresponding first-stage compressor 1 is unloaded until |I g -I d |<I min The system is restored to unloading both the first-stage compressor 1 and the second-stage compressor 2.

[0111] The series heat pump unit control method provided by the above technical solution adopts an equal current percentage control method, which makes the speed distribution reasonable and avoids the compressor power being too high or too low, so as to ensure the reliable operation of the series heat pump unit.

[0112] The series heat pump unit provided by the above technical solution, due to its dual-compressor equal pressure ratio and equal current percentage control method, allows both compressors to load and unload simultaneously. Only an anti-surge line needs to be set for the second-stage compressor 2 to ensure that it does not surge. With the second-stage compressor 2 not surging, the entire unit does not surge. Returning to the load control mode, under equal pressure ratio or equal current percentage control, when the unit's operating capacity decreases and the unloading frequency decreases, during the synchronous or individual unloading process of the two compressors, when the high-pressure stage unloads to below the minimum frequency +2Hz, the frequencies of both compressors remain constant. At this point, the unit has already unloaded to its minimum load limit and cannot unload further. If the actual load is even smaller, and the unit's minimum load is higher than the actual load, the unit's water temperature will be higher than the set temperature until the unit reaches its normal standby temperature. This completes the entire load and anti-surge control process. The anti-surge control method is simple and highly reliable.

[0113] This invention also provides a series heat pump unit control system, including a memory and a processor coupled to the memory. The processor is configured to execute the series heat pump unit control method provided by any of the technical solutions of this invention based on instructions stored in the memory.

[0114] Other embodiments of the present invention also provide a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the series heat pump unit control method provided by any of the technical solutions of the present invention.

[0115] The processors described herein may include general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs) or other programmable logic devices, 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 alternatives, it 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, multiple microprocessors, one or more microprocessors cooperating with a DSP core, or any other such configuration.

[0116] Storage media 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 disc storage, 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 computer-readable media. For example, if software is transmitted from a website, server, or other remote source using coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then such coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of media. As used herein, disk and disc include compact discs (CDs), laser discs, optical discs, digital multi-purpose discs (DVDs), floppy disks, and Blu-ray discs, where disks typically reproduce data magnetically, and discs reproduce data optically using lasers. Combinations of the above should also be included within the scope of computer-readable media.

[0117] Those skilled in the art will understand that the method embodiments of this disclosure can be provided as a method, system, or computer program product. Therefore, this disclosure can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this disclosure can take the form of a computer program product embodied on one or more computer-usable non-transitory storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0118] This disclosure is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It should be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0119] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0120] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0121] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting the scope of protection of this invention. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0122] In the description of this invention, each technical feature may be combined with other technical features where feasible.

[0123] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. However, these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A control method for a series heat pump unit, characterized in that, Includes the following steps: The series heat pump unit is determined to perform one of the following operations based on the temperature difference between the outlet water temperature and the set water temperature: unit hold, unit load, unit unload; the series heat pump unit includes a first-stage compressor (1) and a second-stage compressor (2) connected in series, wherein the pressure stage of the second-stage compressor (2) is higher than that of the first-stage compressor (1); If the unit needs to be loaded, the frequency of at least one of the first pressure stage compressor (1) and the second pressure stage compressor (2) is increased; if the unit needs to be unloaded, the frequency of at least one of the first pressure stage compressor (1) and the second pressure stage compressor (2) is decreased; if it needs to be maintained, the frequencies of the first pressure stage compressor (1) and the second pressure stage compressor (2) remain unchanged. The series heat pump unit control method during unit loading includes the following steps: At the same time, increase the frequency of the first pressure stage compressor (1) and the second pressure stage compressor (2); Determine whether the first difference between the set parameters of the second pressure stage compressor (2) and the set parameters of the first pressure stage compressor (1) is greater than or equal to the set upper limit threshold. ; If the first difference is greater than or equal to the set upper limit threshold If the frequency of the compressor corresponding to the larger of the set parameters of the second pressure stage compressor (2) and the first pressure stage compressor (1) is maintained, the compressor corresponding to the other set parameter is loaded; if the first difference is less than the set upper limit threshold Then return to the step of simultaneously increasing the frequency of the first pressure stage compressor (1) and the second pressure stage compressor (2).

2. The control method for a series heat pump unit according to claim 1, characterized in that, During unit loading, if the first difference is greater than or equal to a set upper limit threshold... After maintaining the frequency of the compressor corresponding to the larger of the set parameters of the second-stage compressor (2) and the first-stage compressor (1), and loading the compressor corresponding to the other set parameter, the series heat pump unit control method further includes the following steps: Determine whether the first difference between the set parameters of the first pressure stage compressor (1) and the second pressure stage compressor (2) is less than the set lower threshold. ; If the first difference is less than the set lower threshold If the first difference is greater than or equal to the set lower threshold, then return to the step of simultaneously increasing the frequency of the first pressure stage compressor (1) and the second pressure stage compressor (2); 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 (2) and the first pressure stage compressor (1), and loading the compressor corresponding to the other setting parameter.

3. The control method for a series heat pump unit according to claim 1, characterized in that, During the unloading process of the unit, the control method for the series heat pump unit includes the following steps: Simultaneously unload the frequency of the first pressure stage compressor (1) and the second pressure stage compressor (2); Determine whether the first difference between the set 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. ; If the first difference is greater than or equal to the set upper limit threshold If the frequency of the compressor corresponding to the smaller set parameter between the first pressure stage compressor (1) and the second pressure stage compressor (2) is maintained, the compressor corresponding to the other set parameter is unloaded; if the first difference is less than the set upper limit threshold If so, return to the step of simultaneously unloading the frequency of the first pressure stage compressor (1) and the second pressure stage compressor (2).

4. The control method for a series heat pump unit according to claim 3, characterized in that, During the unit unloading process, if the first difference is greater than or equal to the set upper limit threshold... After the step of maintaining the frequency of the compressor corresponding to the smaller set parameter among the first pressure stage compressor (1) and the second pressure stage compressor (2), and unloading the compressor corresponding to the other set parameter, the series heat pump unit control method further includes the following steps: Determine whether the first difference between the set parameters of the unloaded second pressure stage compressor (2) and the retained first pressure stage compressor (1) is less than the set lower threshold. ; If the first difference is less than the set lower threshold If the first difference is greater than or equal to the set lower threshold, then return to the step of simultaneously unloading the first pressure stage compressor (1) and the second pressure stage compressor (2). If the frequency of the compressor corresponding to the smaller of the set parameters of the first pressure stage compressor (1) and the second pressure stage compressor (2) is maintained, the compressor corresponding to the other set parameter is unloaded.

5. The control method for a series heat pump unit according to any one of claims 1 to 4, characterized in that, The series heat pump unit adopts isobaric 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).

6. The control method for a series heat pump unit according to claim 1, characterized in that, The series heat pump unit adopts isobaric ratio control, and the set upper limit threshold is... Set an upper limit threshold for the pressure ratio. .

7. The control method for a series heat pump unit according to claim 2, characterized in that, The series heat pump unit adopts isobaric ratio control, and the set lower threshold value... Set a lower threshold for the pressure ratio .

8. The control method for a series heat pump unit according to any one of claims 1 to 4, characterized in that, The series heat pump unit adopts equal current percentage control, then 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).

9. The control method for a series heat pump unit according to claim 1, characterized in that, The series heat pump unit adopts equal current percentage control, and the set upper limit threshold Set an upper limit threshold for the current percentage. .

10. The control method for a series heat pump unit according to claim 2, characterized in that, The series heat pump unit adopts equal current percentage control, and the set lower threshold value... Set a lower threshold for the current percentage. .

11. The control method for a series heat pump unit according to any one of claims 1 to 4, characterized in that, The temperature difference is 0.2℃~0.4℃.

12. The control method for a series heat pump unit according to any one of claims 1 to 4, characterized in that, The operating parameters of the second pressure stage compressor (2) meet the requirements of the anti-surge line.

13. A control system for a series heat pump unit, characterized in that, include: Memory; and A processor coupled to the memory, the processor being configured to execute the series heat pump unit control method as described in any one of claims 1 to 12 based on instructions stored in the memory.

14. A computer-readable storage medium, characterized in that, It stores a computer program that, when executed by a processor, implements the series heat pump unit control method as described in any one of claims 1 to 12.

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