Cascade unit defrosting control method and cascade unit

Abstract

The invention relates to the technical field of cascade units, in particular to a defrosting control method of a cascade unit and the cascade unit, and the defrosting control method of the cascade unit comprises a frequency control mode and / or an opening degree control mode. The frequency control mode comprises the steps that the high-temperature-stage condensation saturation temperature T1 and the high-temperature-stage heat exchanger return water temperature T2 are detected, the defrosting frequency F is calculated, F = A * T1-B * T2, 1.2 Hz / DEG C < = A < = 1.8 Hz / DEG C and 0.2 Hz / DEG C < = B < = 0.8 Hz / DEG C are met, and the frequency of the low-temperature-stage compressor is adjusted according to the defrosting frequency. The opening degree control mode comprises the steps that the low-temperature-level high-pressure saturation temperature T3 and the upstream temperature T4 of the downstream of the low-temperature-level main valve are detected, when T3-T4 is smaller than or equal to 0 DEG C, the opening degree of the low-temperature-level main valve is adjusted to be small every first preset duration t1, when T3-T4 is larger than 0 DEG C and smaller than or equal to 1 DEG C, the opening degree of the low-temperature-level main valve is adjusted to be small every second preset duration t2, and when T3-T4 is larger than 1 DEG C, the opening degree of the low-temperature-level main valve is not changed.

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 defrosting process of conventional cascade units, the frequency and opening of the low-temperature compressor are set to a fixed value. However, due to the change in the return water temperature of the high-temperature stage, the capacity of the low-temperature stage does not match the capacity of the high-temperature stage, which can easily lead to high-pressure or low-pressure protection of the high-temperature 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 defrosting control method for a cascade unit is provided, comprising:

[0006] Frequency control mode: detect the high-temperature stage condensation 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 satisfies 1.2Hz / ℃≤A≤1.8Hz / ℃, the second auxiliary coefficient B satisfies 0.2Hz / ℃≤B≤0.8Hz / ℃, 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 front 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, and 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, and 0<D<C≤100.

[0014] Optionally, the first preset time length t1 and the second preset time length 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 also 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 starting the frequency control mode and / or the opening control mode.

[0019] Optionally, before switching the mode, the following steps are also 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, and the defrost control method of the cascade unit is applied. 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 defrosting 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 condensation saturation temperature T1 and the return water temperature T2 of the high-temperature stage heat exchanger, calculating the defrosting 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 according to the defrosting frequency F. The opening control mode includes: detecting the low-temperature stage high-pressure saturation temperature T3 and the valve front temperature T4 downstream of the low-temperature stage main valve, when T3-T4≤0℃, the low-temperature stage main valve adjusts the opening degree every first preset time t1, when 0℃<T3-T4≤1℃, the low-temperature stage main valve adjusts the opening degree every second preset time t2, and when T3-T4>1℃, the opening degree of the low-temperature stage main valve remains unchanged. 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 operation of the cascade unit.

[0030] The present invention also provides a cascade unit, which uses the above-mentioned defrost control method for the 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. The cascade unit uses the above-mentioned defrost control method for the cascade unit to improve the stability and reliability of the unit operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a schematic diagram of a defrosting control method for a cascade unit provided in an embodiment of the present invention;

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

[0033] In the figure:

[0034] 1. Cryogenic compressor; 2. Cryogenic four-way valve; 3. Cryogenic heat exchanger; 4. Cryogenic main valve; 5. Cryogenic gas-liquid separator; 6. Economizer; 7. Cryogenic auxiliary valve; 8. Cryogenic high-pressure switch; 9. Cryogenic high-pressure sensor; 10. Cryogenic low-pressure switch; 11. Cryogenic suction temperature sensor; 12. Cryogenic needle valve; 13. Valve front temperature sensor; 14. Valve rear temperature sensor; 15. Coil temperature sensor; 16. Cryogenic 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 in conjunction with the accompanying drawings and implementation methods. It is to be understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for ease of description, only the parts related to the present invention are shown in the accompanying drawings, rather than all of them.

[0036] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection or a detachable connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

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

[0038] In the defrosting process of a conventional cascade unit, the frequency and opening of the low temperature stage compressor are set to a fixed value. However, due to the change in the return water temperature of the high temperature stage, the capacity of the low temperature stage does not match the capacity of the high temperature stage, which easily leads to high pressure or low pressure protection of the high temperature stage, affecting the stability and reliability of the unit operation. In order to balance the capacity of the low temperature stage with the capacity of the high temperature stage and improve the stability and reliability of the cascade unit operation, this embodiment provides a defrosting 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. The frequency of the low-temperature stage compressor 1 can be adjusted by turning on the frequency control mode, and the opening control mode can be adjusted by turning on the opening of the low-temperature stage main valve 4.

[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 compressor 1 is adjusted to 90 Hz, that is, in the frequency control mode, the frequency of the low-temperature 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 compressor 1 is adjusted to 30 Hz, that is, in the frequency control mode, the frequency of the low-temperature compressor 1 is not lower than 30 Hz.

[0042] Optionally, the frequency of the low-temperature compressor 1 is adjusted according to the rounded result of the defrost frequency F. Rounding the calculated result can facilitate adjustment and reduce the accuracy requirement on 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 valve inlet temperature T4 downstream of the low-temperature stage main valve 4. When T3-T4≤0℃, the low-temperature stage main valve 4 adjusts the opening degree to a smaller value every first preset time t1. When 0℃<T3-T4≤1℃, the low-temperature stage main valve 4 adjusts the opening degree to a smaller value every second preset time t2. When T3-T4>1℃, the opening degree of the low-temperature stage main valve 4 remains unchanged. That is, when the difference between the low-temperature stage high-pressure saturation temperature T3 and the valve inlet temperature T4 is less than or equal to 1℃, the opening degree of the low-temperature stage main valve 4 needs to be reduced. When the difference between the low-temperature stage high-pressure saturation temperature T3 and the valve inlet temperature T4 is greater than 1℃, it can be considered that the capacities of the low-temperature stage and the high-temperature stage are balanced, and the opening degree does not need to be adjusted.

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

[0045] Optionally, the first preset time length t1 and the second preset time length t2 are both 15s, that is, the adjustment frequency of the opening is the same in both cases. Optionally, the value of C is 5, that is, the current opening is reduced by 5% each time it is adjusted. Optionally, the value of D is 3, that is, the current opening is reduced by 3% each time it is adjusted.

[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 temperature before the valve is less than or equal to 0°C, the frequency of adjusting the opening degree is faster, and when the difference between the low temperature stage high pressure saturation temperature and the temperature before the valve is greater than 0°C but still less than or equal to 1°C, the frequency of adjusting the opening degree 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 cryogenic main valve 4 is greater than 500P, the opening of the cryogenic main valve 4 is adjusted to 500P, that is, in the opening control mode, the opening of the cryogenic main valve 4 is not higher than 500P. Optionally, when the adjustment target of the opening of the cryogenic main valve 4 is less than 200P, the opening of the cryogenic main valve 4 is adjusted to 200P, that is, in the opening control mode, the opening of the cryogenic 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 also be turned on separately. In this embodiment, the frequency control mode and the opening control mode are turned on synchronously to obtain the best adjustment effect. Of course, by adjusting the frequency of the low-temperature compressor 1 separately, adjusting the opening of the low-temperature main valve 4 separately, or adjusting the frequency of the low-temperature compressor 1 and the opening of the low-temperature main valve 4 at the same time, 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 operation of the cascade unit.

[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, wherein 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 formally switching to the defrosting state.

[0051] Preparation mode: The low-temperature compressor 1 is shut down after reducing the frequency to 40Hz, and the high-temperature compressor 17 is shut down. After the fifth preset time t5, the low-temperature main valve 4 is opened to the first defrost initial opening, and the high-temperature main valve 20 is opened to the second defrost initial opening, and then the switching mode is entered. Optionally, the first defrost initial opening and the second defrost initial opening are both 400P. Optionally, the fifth preset time t5 is 60s.

[0052] Switching mode: The low-temperature four-way valve 2 and the high-temperature four-way valve 18 are both switched to the defrosting connection mode, and the cascade unit enters the defrosting state. Then after the third preset time t3, the low-temperature compressor 1 is turned on and the frequency is increased to 45Hz, the high-temperature compressor 17 is turned on, and the low-temperature compressor 1 continues to increase the frequency to 50Hz, maintaining the fourth preset time t4, and then starting the frequency control mode and / or the opening control mode. Optionally, the third preset time t3 is 5s. Optionally, the fourth preset time t4 is 30s.

[0053] like Figure 2 As shown, this embodiment also provides a cascade unit, and the above-mentioned defrosting control method of the cascade unit is applied. 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 arrow in the figure indicates the flow direction of the refrigerant in the cascade unit in the heating state, and the dotted arrow in the figure indicates 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 via the low-temperature main valve 4.

[0057] Optionally, the low temperature cycle further comprises a low temperature gas-liquid separator 5 and an economizer 6. The low temperature gas-liquid separator 5 is arranged 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 main valve 4, the second port of the economizer 6 is also connected to the low-temperature auxiliary valve 7, the other end of the low-temperature 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 compressor 1.

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

[0060] Optionally, the low-temperature stage cycle further includes a low-temperature stage needle valve 12 , which 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 .

[0061] In order to detect the temperature at various locations of the low-temperature cycle, the low-temperature cycle optionally also includes a low-temperature 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 exhaust temperature sensor 16. Among them, the low-temperature intake temperature sensor 11 is arranged on the pipeline between the low-temperature gas-liquid separator 5 and the S port of the low-temperature four-way valve 2, and is located downstream of the low-temperature needle valve 12. The pre-valve temperature sensor 13 is arranged on the pipeline between the low-temperature main valve 4 and the low-temperature heat exchanger 3. The post-valve temperature sensor 14 is arranged 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 heat exchanger 3, and the low-temperature exhaust temperature sensor 16 is used to detect the temperature on the exhaust pipeline of the low-temperature 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 . The high temperature gas-liquid separator 21 is disposed on a 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 further 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 arranged 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 arranged 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 disposed 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] In order to detect the temperature at various locations of the high-temperature stage cycle, the high-temperature stage cycle optionally further includes a high-temperature stage intake temperature sensor 26, an outlet water temperature sensor 28, a return water temperature sensor 29 and a high-temperature stage exhaust temperature sensor 30. Among them, the high-temperature stage intake temperature sensor 26 is arranged 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, and is located downstream of the high-temperature stage needle valve 27. The outlet water temperature sensor 28 is arranged on the outlet water pipeline of the high-temperature stage heat exchanger 19, and the return water temperature sensor 29 is arranged on the return water pipeline of the high-temperature stage heat exchanger 19. The high-temperature stage exhaust temperature sensor 30 is used to detect the temperature on the exhaust pipeline of the high-temperature stage compressor 17.

[0068] The cascade unit applies the defrosting control method of the cascade unit to improve the stability and reliability of the unit operation.

[0069] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A defrosting control method for a cascade unit, characterized in that: include: Frequency control mode: 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, opening control mode: detecting the low-temperature high-pressure saturation temperature T3 and the valve inlet temperature T4 downstream of the low-temperature main valve (4); when T3-T4≤0°C, the low-temperature main valve (4) reduces the opening at a first preset time interval t1; when 0°C<T3-T4≤1°C, the low-temperature main valve (4) reduces the opening at a second preset time interval t2; when T3-T4>1°C, the opening of the low-temperature main valve (4) remains unchanged.

2. The defrosting control method of 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 compressor (1) is adjusted to 30 Hz.

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

4. The defrosting control method of 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 degree of the low-temperature stage main valve (4) is less than 200P, the opening degree of the low-temperature stage main valve (4) is adjusted to 200P.

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

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

7. The defrosting control method for a cascade unit according to any one of claims 1 to 4, characterized in that: Before starting the frequency control mode and / or the opening control mode, the following steps are also included: 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, maintained for the fourth preset time t4, and then the frequency control mode and / or the opening control mode are turned on.

8. The defrosting control method of a cascade unit according to claim 7, characterized in that: Before the switching mode, the following steps are also included: Preparation mode: the low-temperature compressor (1) is shut down after reducing the frequency to 40 Hz, the high-temperature compressor (17) is shut down, 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 the switching mode is entered.

9. The defrosting control method for a cascade unit according to claim 8, 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.

10. A cascade unit, characterized in that: A defrost control method for a cascade unit as described in any one of claims 1 to 9 is applied, wherein the cascade unit comprises a low-temperature stage cycle, a high-temperature stage cycle and an intermediate heat exchanger (31), wherein the low-temperature stage cycle comprises the low-temperature stage compressor (1), a low-temperature stage four-way valve (2), a low-temperature stage heat exchanger (3) and the low-temperature stage main valve (4), and wherein the high-temperature stage cycle comprises a high-temperature stage compressor (17), a high-temperature stage four-way valve (18), the high-temperature stage heat exchanger (19) and a high-temperature stage main valve (20).

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