Control method of chiller and chiller

Abstract

The present invention discloses a control method for a chiller and a chiller. The chiller has at least two refrigeration systems, each of which is independently equipped with a compressor that provides refrigerant circulation power. The evaporators of the different refrigeration systems can operate in series or parallel. The control method includes: detecting the total inlet water temperature T1 and the total outlet water temperature T2 of the chiller and calculating the total inlet and outlet water temperature difference ΔT; determining the threshold range within which the total inlet and outlet water temperature difference ΔT falls and matching a corresponding control strategy; and adjusting the series and parallel state of the evaporators and / or the frequency and / or the on / off state of the compressor according to the control strategy. The present invention uses a reasonable control strategy based on the load state of the chiller to stabilize the water temperature output by the unit, reduce the unit failure rate, and improve the user experience.

Description

Technical Field

[0001] The present invention relates to the field of refrigeration technology, and in particular to a control method for a chiller and a chiller. Background Art

[0002] Chillers have the advantages of easy installation, easy maintenance, water and land saving. They are widely used in urban rail transit, data centers, industrial plants, etc. There is often a "big horse pulling a small cart" problem when selecting the model in the project. As the outdoor ambient temperature changes, the indoor load also changes. The following problems may exist during the operation of the chiller: 1. The compressor starts and stops frequently, causing water temperature fluctuations, resulting in poor user experience; 2. Under low load conditions, the refrigeration system pressure is low, and low pressure protection, low exhaust superheat and other protections are prone to occur, which may cause the unit to fail and shut down; 3. When multiple compressors are turned on in the chiller, the compressor operating status is inconsistent and the frequency difference is large.

[0003] Therefore, how to design a control method to improve the operating stability of chillers is a technical problem that needs to be solved urgently in the industry. Summary of the Invention

[0004] In order to solve the defect of unstable operation of existing chillers, the present invention proposes a control method for a chiller and a chiller. The control method is equipped with a reasonable control strategy according to the load state of the chiller to stabilize the water temperature output by the chiller, reduce the failure rate of the chiller, and improve user experience.

[0005] The technical solution adopted by the present invention is to design a control method for a chiller, wherein the chiller has at least two refrigeration systems, each refrigeration system is separately configured with a compressor that provides refrigerant circulation power, and the evaporators of different refrigeration systems can operate in series or parallel. The control method includes: detecting the total inlet water temperature T1 and the total outlet water temperature T2 of the chiller and calculating the total inlet and outlet water temperature difference ΔT; determining the threshold range of the total inlet and outlet water temperature difference ΔT and matching the corresponding control strategy; adjusting the series and parallel state of the evaporator and / or the frequency of the compressor and / or the on / off state of the compressor according to the control strategy.

[0006] Furthermore, determining the threshold range of the total inlet and outlet water temperature difference ΔT and obtaining the corresponding control strategy includes:

[0007] When the total inlet and outlet water temperature difference ΔT ≥ the first threshold ΔT′, the compressors in the chiller are controlled to adjust their frequencies according to the suction and exhaust pressures;

[0008] and / or when the second threshold ΔT″<the total inlet and outlet water temperature difference ΔT<the first threshold ΔT′, controlling the compressors in the chiller to adjust their frequencies according to the total outlet water temperature T2;

[0009] And / or when the total inlet and outlet water temperature difference ΔT is less than the second threshold ΔT″, the series and parallel states of the enabled evaporators in the chiller are controlled according to the total outlet water temperature T2.

[0010] Furthermore, controlling the frequency of the compressors in the chiller to be turned on according to the suction and exhaust pressures includes:

[0011] Detect the compressor suction pressure P d and exhaust pressure P g ;

[0012] When the suction pressure P d ≤Set suction pressure P sd Or exhaust pressure P g ≥Set exhaust pressure P sg When , the compressor keeps the current frequency unchanged;

[0013] When the suction pressure P d >Set suction pressure P sd And the exhaust pressure P g <Set exhaust pressure P sg When the compressor operates at a frequency f according to the suction pressure P d and exhaust pressure P g Calculated.

[0014] Furthermore, controlling the frequency of the compressors in the chiller to be turned on according to the total outlet water temperature T2 includes:

[0015] Calculate the load change ΔQ based on the total outlet water temperature T2 and the outlet water temperature setting value T0;

[0016] When ΔQ>μ, the compressor performs frequency increase;

[0017] When -μ≤ΔQ≤μ, the compressor maintains the current frequency;

[0018] When ΔQ<-μ, the compressor performs frequency reduction action;

[0019] Among them, μ is the set change.

[0020] In some embodiments, the load change ΔQ is calculated as follows: ΔQ = [T2-(T0+c)] × a + [T2-T2 ’ ]×b; where a and b are correction coefficients, c is the cooling correction value, T2 is the current outlet water temperature, T2 ’ It is the outlet water temperature value at the previous moment.

[0021] Furthermore, the control method of the chiller also includes:

[0022] When the second threshold ΔT″<the total inlet and outlet water temperature difference ΔT<the first threshold ΔT′, the actual frequency of each compressor that has been turned on in the chiller is detected at each set interval Δt, and the average frequency of all the compressors that have been turned on is calculated. When the difference between the actual frequency of any compressor and the average frequency exceeds the threshold deviation, the actual frequency is adjusted to the average frequency.

[0023] Furthermore, controlling the series and parallel states of the enabled evaporators in the chiller according to the total outlet water temperature T2 includes:

[0024] When the total outlet water temperature T2 is less than the set value T0, the enabled evaporators in the chiller run in parallel;

[0025] When the total outlet water temperature T2 ≥ the set value T0, the enabled evaporators in the chiller operate in series.

[0026] Furthermore, after the enabled evaporators in the chiller are operated in parallel, the startup time of the compressors in the chiller is detected, and the refrigeration system with the longest startup time is shut down.

[0027] Furthermore, the control method of the chiller also includes:

[0028] Get the start times of each compressor in the chiller;

[0029] When the number of starts is greater than or equal to the set number, the actual frequency of each compressor that has been turned on is detected and the minimum actual frequency is selected;

[0030] Determine whether the minimum actual frequency is greater than the threshold operating frequency;

[0031] If so, at least one more compressor is turned on, and all the compressors that are turned on are adjusted to the set lower limit frequency.

[0032] The present invention also proposes a chiller, comprising: at least two refrigeration systems, each refrigeration system is separately configured with a compressor providing refrigerant circulation power, the evaporators of different refrigeration systems can operate in series or in parallel, and the controller of the chiller executes the above-mentioned control method.

[0033] Furthermore, the water inlet pipes of all evaporators are connected in parallel to the main water inlet pipe, and the water outlet pipes of all evaporators are connected in parallel to the main water outlet pipe. Among any two evaporators, the evaporator located upstream along the water inlet direction is the upper evaporator, and the evaporator located downstream is the lower evaporator. The water outlet pipe of the upper evaporator is connected to a conversion branch, and the outlet of the conversion branch is connected to the main water inlet pipe.

[0034] Furthermore, the water inlet pipe is installed with a water inlet valve, the water outlet pipe is installed with a water outlet valve, the conversion branch is installed with a main switching valve, and the water outlet pipe of the upper evaporator is also installed with a secondary switching valve connected in series between the water outlet valve and the main water outlet pipe. The inlet of the conversion branch is connected between the water outlet valve and the secondary switching valve.

[0035] Compared with the prior art, the present invention adopts a reasonable control strategy according to the load state of the chiller. When the terminal load is large, it adjusts the high and low pressures of the compressor. When the terminal load is normal, the frequency of the compressor is fine-tuned according to the total outlet water temperature T2. When the terminal load is small, the series and parallel states of the evaporator are roughly adjusted according to the total outlet water temperature T2. By controlling the series and parallel states of the evaporators, the start and stop of the compressor, and the frequency of the compressor, the working state of the chiller is accurately adjusted, the operating stability of the unit is improved, and the failure rate of the unit is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention is described in detail below with reference to the embodiments and accompanying drawings, in which:

[0037] Figure 1 2 is a schematic diagram of the connection of a chiller in one embodiment of the present invention. DETAILED DESCRIPTION

[0038] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this patent and are not intended to limit this patent.

[0039] The control method proposed in the present invention is applicable to chillers, especially chillers with more than two refrigeration systems, each refrigeration system including: a compressor, a condenser, a throttling device and an evaporator, the condenser and the condensing fan are installed in the outdoor unit, the compressor independently provides refrigerant circulation power to the refrigeration system in which it is located, the throttling device can adopt an electronic expansion valve, the evaporator adopts a shell and tube heat exchanger, and the evaporators of different refrigeration systems can operate in series or in parallel.

[0040] There are many ways to connect evaporators in series or in parallel. This article only illustrates one of them and does not limit the specific pipe connection structure between the evaporators.

[0041] like Figure 1As shown, the water inlet pipes of all evaporators are connected in parallel to the main water inlet pipe 100, and the water outlet pipes of all evaporators are connected in parallel to the main water outlet pipe 200. The evaporator located upstream along the water inlet direction of two adjacent evaporators is the upper evaporator, and the evaporator located downstream is the lower evaporator. The water outlet pipe of the upper evaporator is connected to a conversion branch 17, and the outlet of the conversion branch 17 is connected to the main water inlet pipe. The series and parallel states of the evaporators can be switched by controlling the on-off state of the conversion branch 17.

[0042] In order to flexibly switch the working mode of each evaporator in the chiller, the water inlet pipe of the evaporator is installed with an inlet valve, the water outlet pipe of the evaporator is installed with an outlet valve, the conversion branch 17 is installed with a main switching valve 16, and the water outlet pipe of the upper evaporator between any two evaporators is also installed with an auxiliary switching valve 15. The auxiliary switching valve 15 is connected in series between the outlet valve and the main outlet pipe 200, and the inlet of the conversion branch 17 is connected between the outlet valve and the auxiliary switching valve 15.

[0043] Taking two adjacent refrigeration systems as an example for detailed explanation, the first refrigeration system includes a first compressor 1, a first condenser 2, a first condensing fan 3, a first throttling device 4 and a first evaporator 5; the second refrigeration system includes a second compressor 6, a second condenser 7, a second condensing fan 8, a second throttling device 9 and a second evaporator 10. The first evaporator 5 is an upper evaporator, and the second evaporator 10 is a lower evaporator. The water inlet pipe of the first evaporator 5 is installed with a first water inlet valve 12, and the water outlet pipe is installed with a first water outlet valve 11 and a secondary switching valve 15. The water inlet pipe of the second evaporator 10 is installed with a second water inlet valve 14, and the water outlet pipe is installed with a second water outlet valve 13.

[0044] When the first water inlet valve 12, the second water inlet valve 14, the first water outlet valve 11, the second water outlet valve 13 and the auxiliary switching valve 15 are all opened and the main switching valve 16 is closed, the first evaporator 5 and the second evaporator 10 are connected in parallel, and the chilled water in the main water inlet pipe 100 enters the evaporator from the water inlet pipes of the two evaporators to exchange heat with the refrigerant, and then flows into the main water outlet pipe 200 from the water outlet pipe; when the first water inlet valve 12, the second water inlet valve 14, the first water outlet valve 11, the second water outlet valve 13 and the main switching valve 1 6 are both open and the auxiliary switching valve 15 is closed, the first evaporator 5 and the second evaporator 10 are connected in series, the chilled water in the main water inlet pipe 100 enters the evaporators from the water inlet pipes of the two evaporators to exchange heat with the refrigerant, the water flowing out of the water outlet pipe of the first evaporator 5 flows back to the main water inlet pipe 100 from the conversion branch 17, the chilled water in the main water inlet pipe 100 is mixed with the water flowing out of the first evaporator 5 and then enters the second evaporator 10 to exchange heat with the refrigerant, and then flows into the main water outlet pipe 200 from the water outlet pipe of the second evaporator 10.

[0045] The control method of the present invention is based on the system structure of the above-mentioned chiller, and the control method includes the following steps:

[0046] Detect the total inlet water temperature T1 and the total outlet water temperature T2 of the chiller and calculate the total inlet and outlet water temperature difference ΔT, ΔT = T1-T2. The total inlet and outlet water temperatures ΔT reflect the terminal load state of the chiller. Here, the total inlet water temperature T1 refers to the temperature at the inlet of the total inlet pipe 100, and the total outlet water temperature T2 refers to the temperature at the outlet of the total outlet pipe 200.

[0047] Determine the threshold range of the total inlet and outlet water temperature difference ΔT and match the corresponding control strategy;

[0048] The series and parallel state of the evaporator and / or the frequency of the compressor and / or the on / off state of the compressor are adjusted according to the control strategy.

[0049] The present invention adopts a reasonable control strategy according to the load state of the chiller, so as to stabilize the water temperature output by the chiller, reduce the failure rate of the chiller, and improve the user experience.

[0050] Specifically, the threshold range of the total inlet and outlet water temperature difference ΔT is determined and the corresponding control strategy is obtained for the following three cases.

[0051] In the first case, when the total inlet and outlet water temperature difference ΔT ≥ the first threshold ΔT′, it indicates that the terminal load of the chiller is large. The compressors that are turned on in the chiller are controlled to adjust the frequency according to the suction and exhaust pressure. While maintaining the current number of compressors that are turned on, the suction and exhaust pressure of the compressors are prevented from being abnormal, thereby reducing the number of compressor starts and stops of the chiller.

[0052] In some embodiments of the present invention, the frequency adjustment method according to the suction and exhaust pressure is as follows: detecting the suction pressure P of the compressor d and exhaust pressure P g , when the suction pressure P d ≤Set suction pressure P sd Or exhaust pressure P g ≥Set exhaust pressure P sg When the suction pressure P d >Set suction pressure P sd And the exhaust pressure P g <Set exhaust pressure P sg When the compressor operates at a frequency f according to the suction pressure P d and exhaust pressure P g Calculation shows that f = f ’ +f ’ (P d ,P g ), f ’ is the current operating frequency of the compressor, f ’ (P d ,P g ) can be expressed as λ1×P d+λ2×P g , λ1 and λ2 are correction coefficients.

[0053] In the second case, when the second threshold ΔT″ is less than the total inlet and outlet water temperature difference ΔT and less than the first threshold ΔT′, it indicates that the terminal load of the chiller is normal. The compressors that are turned on in the chiller are controlled to adjust the frequency according to the total outlet water temperature T2 to ensure that the chiller can stably output a constant water temperature.

[0054] In some embodiments proposed in the present invention, the frequency adjustment method according to the total outlet water temperature T2 is: the load change ΔQ is calculated according to the total outlet water temperature T2 and the outlet water temperature setting value T0. When ΔQ>μ, μ is the set change, and its value can be 0.5, indicating that the terminal load changes greatly, and the compressor performs a frequency increase action to increase the cooling output to meet the load demand; when -μ≤ΔQ≤μ, it indicates that the terminal load is stable and the compressor maintains the current frequency unchanged; when ΔQ<-μ, it indicates that the terminal load changes little, and the compressor performs a frequency reduction action to reduce the cooling output and improve the energy efficiency of the chiller.

[0055] It should be noted that the frequency increase and frequency decrease actions can be performed at a fixed amplitude or by other existing methods. The calculation method of the load change amount can be selected from the scheme that has appeared in the prior art, or the calculation method of the load change amount ΔQ can be designed as: ΔQ = [T2-(T0+c)] × a + [T2-T2 ’ ]×b; where a and b are correction coefficients, and c is the cooling correction value. Generally, it is believed in engineering that a 0.5°C fluctuation in outlet water temperature does not affect customer comfort. [T2-T2 ’ ]×b reflects the rate of change of water temperature, T2 is the outlet water temperature value at the current moment, T2 ’ It is the outlet water temperature value at the previous moment, which can be the previous 60 seconds, that is, T2 ’ It is the outlet water temperature value in the first 60 seconds.

[0056] In some embodiments proposed in the present invention, in the second case, the actual frequency of each compressor that has been turned on in the chiller is detected at each interval set time Δt, and the average frequency of all the compressors that have been turned on is calculated. When the difference between the actual frequency of any compressor and the average frequency exceeds the threshold deviation, the actual frequency is adjusted to the average frequency. The purpose of this design is to prevent the operating frequencies of the compressors in the chiller from being too different, which affects the operating stability of the chiller.

[0057] It should be noted that the threshold deviation is the average frequency multiplied by the set ratio, and the average frequency is calculated by dividing the sum of the actual frequencies of the turned-on compressors by the number of the turned-on compressors.

[0058] In the third case, when the total inlet and outlet water temperature difference ΔT is less than the second threshold ΔT″, the series and parallel states of the enabled evaporators in the chiller are controlled according to the total outlet water temperature T2, so as to meet the water outlet requirements while avoiding frequent starting and stopping of the compressor.

[0059] In some embodiments proposed by the present invention, the series and parallel states of the enabled evaporators in the chiller are controlled according to the total outlet water temperature T2 as follows: when the total outlet water temperature T2 ≥ the set value T0, it means that the total outlet water temperature T2 needs to be lowered, and the enabled evaporators in the chiller are operated in series, and the chilled water in the total water inlet pipe 100 is continuously cooled through at least two or more evaporators, effectively reducing the total outlet water temperature T2 so that it meets the terminal usage requirements; when the total outlet water temperature T2 < the set value T0, it means that the total outlet water temperature T2 is lower than the terminal usage requirements, the cooling capacity output by the chiller is redundant, and the outlet water temperature is higher than the set value T0. The enabled evaporators in the chiller are operated in parallel, and the chilled water in the total water inlet pipe 100 is cooled through at most one evaporator, reducing cooling capacity waste.

[0060] In some embodiments proposed by the present invention, in the third case, after the enabled evaporators in the chiller are operated in parallel, the startup time of the compressors that have been turned on in the chiller is detected, and the refrigeration system with the longest startup time is shut down, that is, the compressor, the water inlet valve of the evaporator, and the water outlet valve of the evaporator of the refrigeration system are shut down. The refrigeration system does not participate in the cooling of the chiller at all, thereby reducing the number of refrigeration systems that are turned on in the chiller and reducing the energy consumption of the chiller.

[0061] It should be understood that the control method of the present invention is to adjust the working state of the chiller after it is turned on. The start-up conditions of the chiller can adopt the conventional schemes that have appeared in the prior art, and the present invention does not require this. In addition, for the start-up of the compressor, the control method of the chiller also includes: obtaining the start-up times of each compressor that has been turned on in the chiller; when the start-up times are ≥ the set times, it means that the compressor is frequently started and the number of compressors currently turned on in the chiller is insufficient. At this time, the actual frequency of each compressor that has been turned on is detected and the minimum actual frequency is screened out to determine whether the minimum actual frequency is greater than the threshold operating frequency. If so, it means that the compressor that has been turned on is already overloaded, and it is necessary to increase the start-up of at least one compressor to balance the load, improve the operating stability of the chiller, and prevent compressor failure. After the new compressor is turned on, all the compressors that have been turned on are adjusted to the set lower limit frequency, so that the compressor frequencies of the chiller are roughly the same, and the operating stability of the chiller is higher.

[0062] In some embodiments, the set number of times is a variable constant, and the value range of the set number of times is usually 3 to 10 times. The threshold operating frequency is m times the set lower limit frequency, m>1, for example, m is equal to 2. In actual application, it can be designed according to specific needs.

[0063] The present invention adopts a reasonable control strategy according to the load state of the chiller. When the terminal load is large, the high and low pressures of the compressor are adjusted. When the terminal load is normal, the frequency of the compressor is fine-tuned according to the total outlet water temperature T2. When the terminal load is small, the series and parallel state of the evaporator is roughly adjusted according to the total outlet water temperature T2. By controlling the series and parallel state of the evaporator, the start and stop of the compressor, and the frequency of the compressor, the working state of the chiller is accurately adjusted, the operating stability of the unit is improved, and the failure rate of the unit is reduced. In actual application, the control method of the chiller can select only one of the load states and the corresponding control strategy, or select two of the load states and the corresponding control strategy. Those skilled in the art can make a choice according to specific needs.

[0064] It should be noted that the various coefficients appearing in the above formulas can be obtained through statistical experimental data, and their values ​​vary greatly depending on the model of the unit, application scenario, etc., and the present invention does not impose specific restrictions on this.

[0065] In addition, the terms used above are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof. The use of words such as "first" and "second" to define parts is only for the convenience of distinguishing the corresponding parts. If not otherwise stated, the above words do not have special meanings and are therefore not to be construed as limiting the scope of protection of this application. Unless otherwise specified, the relative arrangement of the parts and steps, numerical expressions and numerical values ​​set forth in these embodiments do not limit the scope of this application.

[0066] Techniques, methods, and apparatus known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, such techniques, methods, and apparatus should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not limiting. Therefore, other examples of the exemplary embodiments may have different values. It should be noted that similar reference numerals and letters represent similar items in the following figures, and therefore, once an item is defined in one figure, it need not be further discussed in subsequent figures.

[0067] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A control method for a chiller, wherein the chiller has at least two refrigeration systems, each of which is independently equipped with a compressor for providing refrigerant circulation power, and the evaporators of different refrigeration systems can operate in series or in parallel, characterized in that: The control method includes: Detecting the total inlet water temperature T1 and the total outlet water temperature T2 of the chiller and calculating the total inlet and outlet water temperature difference ΔT; When the total inlet and outlet water temperature difference ΔT is greater than or equal to a first threshold ΔT´, controlling the compressor in the chiller to adjust the frequency according to the suction and exhaust pressure; When the second threshold ΔT´´<the total inlet and outlet water temperature difference ΔT<the first threshold ΔT´, controlling the compressors in the chiller to adjust their frequencies according to the total outlet water temperature T2; When the total inlet and outlet water temperature difference ΔT is less than the second threshold ΔT´´, if the total outlet water temperature T2 is less than the set value T0, the enabled evaporators in the chiller are operated in parallel; if the total outlet water temperature T2 is greater than or equal to the set value T0, the enabled evaporators in the chiller are operated in series.

2. The control method according to claim 1, characterized in that: Controlling the frequency of the compressor in the chiller to be turned on according to the suction and exhaust pressures includes: Detect the suction pressure P of the compressor d and exhaust pressure P g ; When the suction pressure P d ≤Set suction pressure P sd Or exhaust pressure P g ≥Set exhaust pressure P sg When , the compressor maintains the current frequency unchanged; When the suction pressure P d >Set suction pressure P sd And the exhaust pressure P g <Set exhaust pressure P sg When the operating frequency f of the compressor is adjusted according to the suction pressure P d and the exhaust pressure P g Calculated.

3. The control method according to claim 1, wherein: Controlling the compressor in the chiller to adjust the frequency according to the total outlet water temperature T2 includes: Calculate the load change ΔQ according to the total outlet water temperature T2 and the outlet water temperature setting value T0; When ΔQ>μ, the compressor performs frequency increase; When -μ≤ΔQ≤μ, the compressor maintains the current frequency unchanged; When ΔQ<-μ, the compressor performs frequency reduction action; Among them, μ is the set change.

4. The control method according to claim 3, characterized in that: The calculation method of the load change ΔQ is: ΔQ=[T2-(T0+c)]×a+[T2-T2 ’ ]×b; where a and b are correction coefficients, c is the cooling correction value, T2 is the current outlet water temperature, T2 ’ It is the outlet water temperature value at the previous moment.

5. The control method according to claim 1, characterized in that: The control method further includes: When the second threshold ΔT´´<the total inlet and outlet water temperature difference ΔT<the first threshold ΔT´, the actual frequency of each compressor that has been turned on in the chiller is detected at each set interval Δt, and the average frequency of all the compressors that have been turned on is calculated. When the actual frequency of any compressor differs from the average frequency by more than the threshold deviation, the actual frequency is adjusted to the average frequency.

6. The control method according to claim 1, characterized in that: After the enabled evaporators in the chiller are operated in parallel, the startup time of the compressors in the chiller is detected, and the refrigeration system with the longest startup time is shut down.

7. The control method according to claim 1, characterized in that: The control method further includes: Obtaining the number of startups of each turned-on compressor in the chiller; When the number of starts is greater than or equal to the set number, the actual frequencies of the compressors that have been turned on are detected and the minimum actual frequency is selected; Determining whether the minimum actual frequency is greater than a threshold operating frequency; If so, at least one more compressor is turned on, and all the compressors that are turned on are adjusted to the set lower limit frequency.

8. Chiller, including: At least two refrigeration systems, each of which is individually configured with a compressor that provides refrigerant circulation power, and the evaporators of different refrigeration systems can operate in series or in parallel; it is characterized in that the controller of the chiller executes the control method described in any one of claims 1 to 7.

9. The chiller according to claim 8, characterized in that: The water inlet pipes of all the evaporators are connected in parallel to the main water inlet pipe, and the water outlet pipes of all the evaporators are connected in parallel to the main water outlet pipe. The evaporator upstream along the water inlet direction of any two evaporators is the upper evaporator, and the evaporator downstream is the lower evaporator. The water outlet pipe of the upper evaporator is connected to a conversion branch, and the outlet of the conversion branch is connected to the main water inlet pipe.

10. The chiller according to claim 9, characterized in that: The water inlet pipe is equipped with a water inlet valve, the water outlet pipe is equipped with a water outlet valve, the conversion branch is equipped with a main switching valve, and the water outlet pipe of the upper evaporator is also equipped with a secondary switching valve connected in series between the water outlet valve and the main water outlet pipe. The inlet of the conversion branch is connected between the water outlet valve and the secondary switching valve.

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