Defrost control method for cascade unit and cascade unit

Abstract

The present invention relates to the technical field of cascade units, and in particular to a defrost control method for cascade units and the cascade units. The defrost control method for cascade units includes a frequency control mode and / or an opening control mode. The frequency control mode includes: detecting the high-temperature stage condensing saturation temperature T1 and the high-temperature stage heat exchanger return water temperature T2, calculating the defrost frequency F, F = A × T1-B × T2, satisfying 1.2 Hz / °C ≤ A ≤ 1.8 Hz / °C, 0.2 Hz / °C ≤ B ≤ 0.8 Hz / °C, and adjusting the low-temperature stage compressor frequency according to the defrost frequency. The opening control mode includes: detecting the low-temperature stage high-pressure saturation temperature T3 and the valve inlet temperature T4 downstream of the low-temperature stage main valve. When T3-T4 ≤ 0°C, the low-temperature stage main valve decreases its opening every first preset time interval t1. When 0°C < T3-T4 ≤ 1°C, the low-temperature stage main valve decreases its opening every second preset time interval t2. When T3-T4 > 1°C, the low-temperature stage main valve opening remains unchanged.

Description

Technical Field

[0001] The present invention relates to the technical field of cascade units, and in particular to a defrosting control method for a cascade unit and the cascade unit. Background Art

[0002] During the defrost process of a conventional cascade chiller, the frequency and opening of the low-temperature stage compressor are set to a fixed value. However, due to the fluctuation of the high-temperature stage return water temperature, the capacity of the low-temperature stage and the high-temperature stage may not match. This can easily lead to high-pressure or low-pressure protection of the high-temperature stage compressor, affecting the stability and reliability of the unit operation. Summary of the Invention

[0003] An object of the present invention is to provide a defrosting control method for a cascade unit, which can improve the stability and reliability of the operation of the cascade unit.

[0004] To achieve this object, the present invention adopts the following technical solutions:

[0005] A defrost control method for a cascade unit is provided, comprising:

[0006] Frequency control mode: Detect the high-temperature stage condensing saturation temperature T1 and the return water temperature T2 of the high-temperature stage heat exchanger, calculate the defrost frequency F, F = A × T1 - B × T2, the first auxiliary coefficient A meets 1.2 Hz / °C ≤ A ≤ 1.8 Hz / °C, and the second auxiliary coefficient B meets 0.2 Hz / °C ≤ B ≤ 0.8 Hz / °C, and adjust the frequency of the low-temperature stage compressor according to the defrost frequency F;

[0007] And / or, opening control mode: detect the low-temperature stage high-pressure saturation temperature T3 and the valve inlet temperature T4 downstream of the low-temperature stage main valve; when T3-T4≤0℃, the low-temperature stage main valve reduces the opening every first preset time t1; when 0℃<T3-T4≤1℃, the low-temperature stage main valve reduces the opening every second preset time t2; when T3-T4>1℃, the opening of the low-temperature stage main valve remains unchanged.

[0008] Optionally, when the defrost frequency F is greater than 90 Hz, the frequency of the low-temperature compressor is adjusted to 90 Hz;

[0009] And / or, when the defrost frequency F is less than 30 Hz, the frequency of the low-temperature compressor is adjusted to 30 Hz.

[0010] Optionally, the frequency control mode and the opening control mode are started synchronously.

[0011] Optionally, when the adjustment target of the opening of the low-temperature main valve is greater than 500P, the opening of the low-temperature main valve is adjusted to 500P;

[0012] And / or, when the adjustment target of the opening of the low-temperature stage main valve is less than 200P, the opening of the low-temperature stage main valve is adjusted to 200P.

[0013] Optionally, when T3-T4≤0℃, the opening degree of the low-temperature stage main valve is C% of the current number of steps per interval of the first preset time t1, and when 0℃<T3-T4≤1℃, the opening degree of the low-temperature stage main valve is D% of the current number of steps per interval of the second preset time t2, 0<D<C≤100.

[0014] Optionally, the first preset time t1 and the second preset time t2 are both 15s;

[0015] and / or, the value of C is 5;

[0016] And / or, the value of D is 3.

[0017] Optionally, before starting the frequency control mode and / or the opening control mode, the following steps are further included:

[0018] Switching mode: Both the low-temperature four-way valve and the high-temperature four-way valve are switched to the defrost connection mode, and then after the third preset time t3, the low-temperature compressor is turned on and the frequency is increased to 45Hz, the high-temperature compressor is turned on, and the low-temperature compressor continues to increase the frequency to 50Hz, maintaining the fourth preset time t4, and then the frequency control mode and / or the opening control mode is turned on.

[0019] Optionally, before switching the mode, the following steps are further included:

[0020] Preparation mode: the low-temperature compressor stops after reducing the frequency to 40Hz, and the high-temperature compressor stops. After the fifth preset time t5, the low-temperature main valve opens to the first defrost initial opening, and the high-temperature main valve opens to the second defrost initial opening, and then enters the switching mode.

[0021] Optionally, the first defrost initial opening degree and the second defrost initial opening degree are both 400P;

[0022] And / or, the third preset time length t3 is 5s;

[0023] And / or, the fourth preset time length t4 is 30s;

[0024] And / or, the fifth preset time length t5 is 60s.

[0025] Another object of the present invention is to provide a cascade unit that can improve the stability and reliability of the operation of the cascade unit.

[0026] To achieve this object, the present invention adopts the following technical solutions:

[0027] A cascade unit is provided, which applies the above-mentioned defrost control method of the cascade unit. The cascade unit includes a low-temperature stage cycle, a high-temperature stage cycle and an intermediate heat exchanger. The low-temperature stage cycle includes the low-temperature stage compressor, the low-temperature stage four-way valve, the low-temperature stage heat exchanger and the low-temperature stage main valve. The high-temperature stage cycle includes the high-temperature stage compressor, the high-temperature stage four-way valve, the high-temperature stage heat exchanger and the high-temperature stage main valve.

[0028] Beneficial effects of the present invention:

[0029] The present invention provides a defrost control method for a cascade unit, including a frequency control mode and / or an opening control mode. The frequency control mode includes: detecting the high-temperature stage condensing saturation temperature T1 and the return water temperature T2 of the high-temperature stage heat exchanger, calculating the defrost frequency F, F = A × T1 - B × T2, with a first auxiliary coefficient A satisfying 1.2 Hz / °C ≤ A ≤ 1.8 Hz / °C and a second auxiliary coefficient B satisfying 0.2 Hz / °C ≤ B ≤ 0.8 Hz / °C, and adjusting the frequency of the low-temperature stage compressor according to the defrost frequency F. The opening control mode includes: detecting the low-temperature stage high-pressure saturation temperature T3 and the valve inlet temperature T4 downstream of the low-temperature stage main valve. When T3-T4 ≤ 0°C, the low-temperature stage main valve decreases its opening every first preset time interval t1. When 0°C < T3-T4 ≤ 1°C, the low-temperature stage main valve decreases its opening every second preset time interval t2. When T3-T4 > 1°C, the low-temperature stage main valve maintains its opening. By adjusting the frequency of the low-temperature stage compressor alone, adjusting the opening of the low-temperature stage main valve alone, or adjusting the frequency of the low-temperature stage compressor and the opening of the low-temperature stage main valve at the same time, the capacities of the low-temperature stage and the high-temperature stage of the cascade unit can be balanced during the defrosting process, preventing low-pressure and high-pressure failures in the high-temperature stage, thereby improving the stability and reliability of the cascade unit's operation.

[0030] The present invention also provides a cascade unit that utilizes the aforementioned defrost control method for a cascade unit. The cascade unit includes a low-temperature cycle, a high-temperature cycle, and an intermediate heat exchanger. The low-temperature cycle includes a low-temperature compressor, a low-temperature four-way valve, a low-temperature heat exchanger, and a low-temperature main valve. The high-temperature cycle includes a high-temperature compressor, a high-temperature four-way valve, a high-temperature heat exchanger, and a high-temperature main valve. Application of the aforementioned defrost control method to this cascade unit can improve the stability and reliability of the unit's operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of a defrost control method for a cascade unit provided by an embodiment of the present invention;

[0032] Figure 2 It is a structural schematic diagram of a cascade unit provided by an embodiment of the present invention.

[0033] In the picture:

[0034] 1. Low-temperature compressor; 2. Low-temperature four-way valve; 3. Low-temperature heat exchanger; 4. Low-temperature main valve; 5. Low-temperature gas-liquid separator; 6. Economizer; 7. Low-temperature auxiliary valve; 8. Low-temperature high-pressure switch; 9. Low-temperature high-pressure sensor; 10. Low-temperature low-pressure switch; 11. Low-temperature suction temperature sensor; 12. Low-temperature needle valve; 13. Valve front temperature sensor; 14. Valve rear temperature sensor; 15. Coil temperature sensor; 16. Low-temperature exhaust temperature sensor; 1 7. High-temperature compressor; 18. High-temperature four-way valve; 19. High-temperature heat exchanger; 20. High-temperature main valve; 21. High-temperature gas-liquid separator; 22. High-temperature high-pressure switch; 23. High-temperature high-pressure sensor; 24. High-temperature low-pressure switch; 25. High-temperature low-pressure sensor; 26. High-temperature suction temperature sensor; 27. High-temperature needle valve; 28. Outlet water temperature sensor; 29. ​​Return water temperature sensor; 30. High-temperature exhaust temperature sensor; 31. Intermediate heat exchanger. DETAILED DESCRIPTION

[0035] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. It should also be noted that, for ease of description, the accompanying drawings only show portions relevant to the present invention, not all of them.

[0036] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood broadly. For example, they may refer to fixed or detachable connections; mechanical or electrical connections; direct or indirect connections through an intermediary; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention.

[0037] In the present invention, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may include the first and second features being in contact not directly but through another feature between them. Furthermore, a first feature being "above," "above," and "above" a second feature may include the first feature being directly above or obliquely above the second feature, or may simply mean that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may include the first feature being directly below or obliquely below the second feature, or may simply mean that the first feature is lower in level than the second feature.

[0038] During the defrost process in a conventional cascade unit, the frequency and opening of the low-temperature stage compressor are set to fixed values. However, variations in the high-temperature stage return water temperature can lead to a mismatch between the low-temperature and high-temperature stage capabilities, which can easily cause high-pressure or low-pressure protection in the high-temperature stage, impacting the unit's operational stability and reliability. To balance the low-temperature and high-temperature stage capabilities and improve the operational stability and reliability of the cascade unit, this embodiment provides a defrost control method for a cascade unit.

[0039] like Figure 1 As shown, the defrost control method of the cascade unit includes a frequency control mode and / or an opening control mode. When the frequency control mode is turned on, the frequency of the low-temperature stage compressor 1 can be adjusted, and when the opening control mode is turned on, the opening of the low-temperature stage main valve 4 can be adjusted.

[0040] Among them, the frequency control mode includes: detecting the high-temperature stage condensation saturation temperature T1 and the return water temperature T2 of the high-temperature stage heat exchanger 19, calculating the defrost frequency F, F = A×T1-B×T2, the first auxiliary coefficient A satisfies 1.2Hz / ℃≤A≤1.8Hz / ℃, the second auxiliary coefficient B satisfies 0.2Hz / ℃≤B≤0.8Hz / ℃, and adjusting the frequency of the low-temperature stage compressor 1 according to the defrost frequency F.

[0041] Optionally, when the defrost frequency F is greater than 90 Hz, the frequency of the low-temperature stage compressor 1 is adjusted to 90 Hz, that is, in the frequency control mode, the frequency of the low-temperature stage compressor 1 is not higher than 90 Hz. Optionally, when the defrost frequency F is less than 30 Hz, the frequency of the low-temperature stage compressor 1 is adjusted to 30 Hz, that is, in the frequency control mode, the frequency of the low-temperature stage compressor 1 is not lower than 30 Hz.

[0042] Optionally, the frequency of the low-temperature compressor 1 is adjusted according to the rounded-off result of the defrost frequency F. Rounding the calculated result can facilitate adjustment and reduce the accuracy requirement for the frequency of the low-temperature compressor 1.

[0043] The opening control mode includes detecting the low-temperature stage high-pressure saturation temperature T3 and the downstream valve inlet temperature T4 of the low-temperature stage main valve 4. When T3-T4 ≤ 0°C, the low-temperature stage main valve 4 decreases its opening at intervals of a first preset time duration t1. When 0°C < T3-T4 ≤ 1°C, the low-temperature stage main valve 4 decreases its opening at intervals of a second preset time duration t2. When T3-T4 > 1°C, the opening of the low-temperature stage main valve 4 remains unchanged. Specifically, when the difference between the low-temperature stage high-pressure saturation temperature T3 and the downstream valve inlet temperature T4 is less than or equal to 1°C, the opening of the low-temperature stage main valve 4 needs to be decreased. When the difference between the low-temperature stage high-pressure saturation temperature T3 and the downstream valve inlet temperature T4 is greater than 1°C, it can be considered that the capacities of the low-temperature and high-temperature stages are balanced, and the opening does not need to be adjusted.

[0044] Optionally, in this embodiment, when T3-T4≤0°C, the opening of the low-temperature stage main valve 4 is closed by C% of the current number of steps per first preset time interval t1; when 0°C<T3-T4≤1°C, the opening of the low-temperature stage main valve 4 is closed by D% of the current number of steps per second preset time interval t2, and 0<D<C≤100. That is, when the difference between the low-temperature stage high-pressure saturation temperature and the valve inlet temperature is less than or equal to 0°C, the opening is closed more gradually; when the difference between the low-temperature stage high-pressure saturation temperature and the valve inlet temperature is greater than 0°C but still less than or equal to 1°C, the opening is closed less gradually.

[0045] Optionally, the first preset time duration t1 and the second preset time duration t2 are both 15 seconds, i.e., the frequency of adjusting the opening is the same in both cases. Optionally, the value of C is 5, i.e., the current opening is reduced by 5% each time the adjustment is made. Optionally, the value of D is 3, i.e., the current opening is reduced by 3% each time the adjustment is made.

[0046] Of course, in other embodiments, the values ​​of C and D can be made consistent, but t1 is less than t2. That is, when the difference between the low-temperature stage high-pressure saturation temperature and the inlet temperature is less than or equal to 0°C, the frequency of small opening and closing is faster. When the difference between the low-temperature stage high-pressure saturation temperature and the inlet temperature is greater than 0°C but still less than or equal to 1°C, the frequency of small opening and closing is slower. Compared with the two adjustment methods, the adjustment scheme of this embodiment has higher adjustment accuracy.

[0047] Optionally, when the adjustment target of the opening of the low-temperature stage main valve 4 is greater than 500P, the opening of the low-temperature stage main valve 4 is adjusted to 500P, that is, in the opening control mode, the opening of the low-temperature stage main valve 4 is not higher than 500P. Optionally, when the adjustment target of the opening of the low-temperature stage main valve 4 is less than 200P, the opening of the low-temperature stage main valve 4 is adjusted to 200P, that is, in the opening control mode, the opening of the low-temperature stage main valve 4 is not lower than 200P.

[0048] Optionally, the adjustment target of the opening of the low-temperature stage main valve 4 also needs to be rounded to facilitate adjustment and reduce the accuracy requirement for the opening of the low-temperature stage main valve 4 .

[0049] In other embodiments, the frequency control mode or the opening control mode may be enabled separately. In this embodiment, the frequency control mode and the opening control mode are enabled simultaneously to achieve the best regulation effect. Of course, by adjusting the frequency of the low-temperature stage compressor 1 separately, adjusting the opening of the low-temperature stage main valve 4 separately, or adjusting the frequency of the low-temperature stage compressor 1 and the opening of the low-temperature stage main valve 4 simultaneously, the capacity of the low-temperature stage and the high-temperature stage of the cascade unit can be balanced during the defrosting process, preventing low-pressure and high-pressure failures in the high-temperature stage, thereby improving the stability and reliability of the cascade unit operation.

[0050] Optionally, before the frequency control mode and / or the opening control mode are turned on, a preparatory mode and a switching mode are also included. The preparatory mode is for preparing the cascade unit to switch from the heating state to the defrosting state, and the switching mode is for officially switching to the defrosting state.

[0051] In standby mode, the low-temperature compressor 1 is reduced in frequency to 40 Hz and then shut down. The high-temperature compressor 17 is also shut down. After a fifth preset time t5, the low-temperature main valve 4 opens to the first initial defrost opening, and the high-temperature main valve 20 opens to the second initial defrost opening. The system then enters the switching mode. Optionally, both the first and second initial defrost openings are 400°. Optionally, the fifth preset time t5 is 60 seconds.

[0052] Switching mode: Both the low-temperature four-way valve 2 and the high-temperature four-way valve 18 are switched to defrost communication mode, and the cascade unit enters the defrost state. Then, after a third preset duration t3, the low-temperature compressor 1 is turned on and the frequency is increased to 45 Hz. The high-temperature compressor 17 is turned on and the low-temperature compressor 1 continues to increase the frequency to 50 Hz for a fourth preset duration t4. The frequency control mode and / or the opening control mode are then activated. Optionally, the third preset duration t3 is 5 seconds. Optionally, the fourth preset duration t4 is 30 seconds.

[0053] like Figure 2 As shown, this embodiment also provides a cascade unit that utilizes the aforementioned defrost control method for a cascade unit. The cascade unit includes a low-temperature stage cycle, a high-temperature stage cycle, and an intermediate heat exchanger 31. The first path of the intermediate heat exchanger 31 passes through the low-temperature stage cycle, and the second path passes through the high-temperature stage cycle. Optionally, the intermediate heat exchanger 31 is an evaporative condenser.

[0054] in, Figure 2 The solid arrows in the figure indicate the flow direction of the refrigerant in the cascade unit in the heating state, and the dotted arrows in the figure indicate the flow direction of the refrigerant in the cascade unit in the defrosting state.

[0055] The low-temperature cycle includes a low-temperature compressor 1, a low-temperature four-way valve 2, a low-temperature heat exchanger 3 and a low-temperature main valve 4. Optionally, the low-temperature heat exchanger 3 is a fin heat exchanger.

[0056] Optionally, the D port of the low-temperature four-way valve 2 is connected to the exhaust port of the low-temperature compressor 1, the E port of the low-temperature four-way valve 2 is connected to one end of the low-temperature heat exchanger 3, the S port of the low-temperature four-way valve 2 is connected to the inlet of the low-temperature compressor 1, the C port of the low-temperature four-way valve 2 is connected to one end of the first path of the intermediate heat exchanger 31, and the other end of the first path of the intermediate heat exchanger 31 is connected to the other end of the low-temperature heat exchanger 3 through the low-temperature main valve 4.

[0057] Optionally, the low-temperature cycle further includes a low-temperature gas-liquid separator 5 and an economizer 6. The low-temperature gas-liquid separator 5 is provided on the pipeline between the inlet of the low-temperature compressor 1 and the S port of the low-temperature four-way valve 2.

[0058] Optionally, the other end of the first path of the intermediate heat exchanger 31 is first connected to the first port of the economizer 6, the second port of the economizer 6 is connected to the low-temperature stage main valve 4, the second port of the economizer 6 is also connected to the low-temperature stage auxiliary valve 7, the other end of the low-temperature stage auxiliary valve 7 is connected to the third port of the economizer 6, and the fourth port of the economizer 6 is connected to the air supply port of the low-temperature stage compressor 1.

[0059] Optionally, the low-temperature cycle also includes a low-temperature high-pressure switch 8, a low-temperature high-pressure sensor 9, and a low-temperature low-pressure switch 10. The low-temperature high-pressure switch 8 and the low-temperature high-pressure sensor 9 are disposed on the pipeline between the exhaust port of the low-temperature compressor 1 and the D port of the low-temperature four-way valve 2. The low-temperature low-pressure switch 10 is disposed on the pipeline between the inlet of the low-temperature compressor 1 and the low-temperature gas-liquid separator 5.

[0060] Optionally, the low-temperature stage cycle further includes a low-temperature stage needle valve 12 , which is provided on the pipeline between the low-temperature stage gas-liquid separator 5 and the S port of the low-temperature stage four-way valve 2 .

[0061] In order to detect the temperature at various locations in the low-temperature stage cycle, the low-temperature stage cycle optionally also includes a low-temperature stage intake temperature sensor 11, a pre-valve temperature sensor 13, a post-valve temperature sensor 14, a coil temperature sensor 15, and a low-temperature stage exhaust temperature sensor 16. The low-temperature stage intake temperature sensor 11 is disposed on the pipeline between the low-temperature stage gas-liquid separator 5 and the S port of the low-temperature stage four-way valve 2, and is located downstream of the low-temperature stage needle valve 12. The pre-valve temperature sensor 13 is disposed on the pipeline between the low-temperature stage main valve 4 and the low-temperature stage heat exchanger 3. The post-valve temperature sensor 14 is disposed on the pipeline between the economizer 6 and the intermediate heat exchanger 31. The coil temperature sensor 15 is used to detect the coil temperature of the low-temperature stage heat exchanger 3, and the low-temperature stage exhaust temperature sensor 16 is used to detect the temperature on the exhaust pipeline of the low-temperature stage compressor 1.

[0062] Optionally, the high temperature stage cycle includes a high temperature stage compressor 17, a high temperature stage four-way valve 18, a high temperature stage heat exchanger 19 and a high temperature stage main valve 20. Optionally, the high temperature stage heat exchanger 19 is a shell and tube heat exchanger.

[0063] Optionally, the D port of the high-temperature four-way valve 18 is connected to the exhaust port of the high-temperature compressor 17, the S port of the high-temperature four-way valve 18 is connected to the inlet of the high-temperature compressor 17, the E port of the high-temperature four-way valve 18 is connected to one end of the second path of the intermediate heat exchanger 31, the other end of the second path of the intermediate heat exchanger 31 is connected to one end of the high-temperature heat exchanger 19 via the high-temperature main valve 20, and the other end of the high-temperature heat exchanger 19 is connected to the C port of the high-temperature four-way valve 18.

[0064] Optionally, the high temperature cycle further includes a high temperature gas-liquid separator 21 , which is provided on the pipeline between the inlet of the high temperature compressor 17 and the S port of the high temperature four-way valve 18 .

[0065] Optionally, the high-temperature cycle also includes a high-temperature high-pressure switch 22, a high-temperature high-pressure sensor 23, a high-temperature low-pressure switch 24, and a high-temperature low-pressure sensor 25. The high-temperature high-pressure switch 22 and the high-temperature high-pressure sensor 23 are disposed on the pipeline between the exhaust port of the high-temperature compressor 17 and the D port of the high-temperature four-way valve 18. The high-temperature low-pressure switch 24 and the high-temperature low-pressure sensor 25 are disposed on the pipeline between the inlet of the high-temperature compressor 17 and the high-temperature gas-liquid separator 21.

[0066] Optionally, the high temperature stage cycle further includes a high temperature stage needle valve 27 , which is provided on the pipeline between the high temperature stage gas-liquid separator 21 and the S port of the high temperature stage four-way valve 18 .

[0067] To monitor temperatures at various locations within the high-temperature cycle, the high-temperature cycle optionally includes a high-temperature intake air temperature sensor 26, an outlet water temperature sensor 28, a return water temperature sensor 29, and a high-temperature exhaust gas temperature sensor 30. The high-temperature intake air temperature sensor 26 is located in the pipeline between the high-temperature gas-liquid separator 21 and the S port of the high-temperature four-way valve 18, downstream of the high-temperature needle valve 27. The outlet water temperature sensor 28 is located in the outlet water pipeline of the high-temperature heat exchanger 19, and the return water temperature sensor 29 is located in the return water pipeline of the high-temperature heat exchanger 19. The high-temperature exhaust gas temperature sensor 30 is used to monitor the temperature of the exhaust gas pipeline of the high-temperature compressor 17.

[0068] The cascade unit applies the above-mentioned defrosting control method for the cascade unit, thereby improving the stability and reliability of the unit operation.

[0069] Obviously, the above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the claims of the present invention.

Claims

1. A defrost control method for a cascade unit, characterized in that: Applicable to a cascade unit, the cascade unit comprising a low-temperature stage cycle, a high-temperature stage cycle and an intermediate heat exchanger (31), the low-temperature stage cycle comprising a low-temperature stage compressor (1), a low-temperature stage four-way valve (2), a low-temperature stage heat exchanger (3) and a low-temperature stage main valve (4), and the high-temperature stage cycle comprising a high-temperature stage compressor (17), a high-temperature stage four-way valve (18), a high-temperature stage heat exchanger (19) and a high-temperature stage main valve (20); The defrost control method of the cascade unit includes: Switching mode: the low-temperature four-way valve (2) and the high-temperature four-way valve (18) are both switched to the defrost connection mode, and then after the third preset time t3, the low-temperature compressor (1) is turned on and the frequency is increased to 45 Hz, the high-temperature compressor (17) is turned on, and the low-temperature compressor (1) continues to increase the frequency to 50 Hz, and maintains the fourth preset time t4, and then starts the frequency control mode and / or the opening control mode; The frequency control mode includes: detecting the high-temperature stage condensation saturation temperature T1 and the return water temperature T2 of the high-temperature stage heat exchanger (19), calculating the defrost frequency F, F=A×T1-B×T2, the first auxiliary coefficient A satisfies 1.2 Hz / °C≤A≤1.8 Hz / °C, the second auxiliary coefficient B satisfies 0.2 Hz / °C≤B≤0.8 Hz / °C, and adjusting the frequency of the low-temperature stage compressor (1) according to the defrost frequency F; And / or, the opening control mode includes: detecting the low-temperature stage high-pressure saturation temperature T3 and the valve inlet temperature T4 downstream of the low-temperature stage main valve (4); when T3-T4≤0°C, the low-temperature stage main valve (4) reduces the opening every first preset time t1; when 0°C<T3-T4≤1°C, the low-temperature stage main valve (4) reduces the opening every second preset time t2; when T3-T4>1°C, the opening of the low-temperature stage main valve (4) remains unchanged.

2. The defrost control method for a cascade unit according to claim 1, characterized in that: When the defrosting frequency F is greater than 90 Hz, the frequency of the low-temperature compressor (1) is adjusted to 90 Hz; And / or, when the defrosting frequency F is less than 30 Hz, the frequency of the low-temperature stage compressor (1) is adjusted to 30 Hz.

3. The defrost control method for a cascade unit according to claim 1, characterized in that: The frequency control mode and the opening control mode are started synchronously.

4. The defrost control method for a cascade unit according to claim 1, characterized in that: When the adjustment target of the opening of the low-temperature main valve (4) is greater than 500P, the opening of the low-temperature main valve (4) is adjusted to 500P; And / or, when the adjustment target of the opening of the low-temperature stage main valve (4) is less than 200P, the opening of the low-temperature stage main valve (4) is adjusted to 200P.

5. The defrost control method for a cascade unit according to any one of claims 1 to 4, characterized in that: When T3-T4≤0℃, the opening of the low-temperature main valve (4) is C% of the current number of steps per interval of the first preset time t1. When 0℃<T3-T4≤1℃, the opening of the low-temperature main valve (4) is D% of the current number of steps per interval of the second preset time t2. 0<D<C≤100.

6. The defrost control method for a cascade unit according to claim 5, characterized in that: The first preset time t1 and the second preset time t2 are both 15s; and / or, the value of C is 5; And / or, the value of D is 3.

7. The defrost control method for a cascade unit according to claim 6, characterized in that: Before switching the mode, the following steps are also included: Preparation mode: the low-temperature compressor (1) is stopped after reducing the frequency to 40 Hz, the high-temperature compressor (17) is stopped, and after the fifth preset time t5, the low-temperature main valve (4) is opened to the first defrost initial opening, the high-temperature main valve (20) is opened to the second defrost initial opening, and then enters the switching mode.

8. The defrost control method for a cascade unit according to claim 7, characterized in that: The first defrost initial opening degree and the second defrost initial opening degree are both 400P; And / or, the third preset time length t3 is 5s; And / or, the fourth preset time length t4 is 30s; And / or, the fifth preset time length t5 is 60s.

9. Cascade unit, characterized in that: The defrost control method for a cascade unit as described in any one of claims 1 to 8 is applied.

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