Control method of heat pump drying system and heat pump drying system

Abstract

The present application relates to the technical field of heat pump drying, and particularly provides a control method of a heat pump drying system and the heat pump drying system, aiming at solving the problem that the existing heat pump drying system cannot guarantee defrosting effect and heat supply at the same time. To this end, the control method comprises: acquiring an outdoor environment temperature; when the outdoor environment temperature is lower than a preset environment temperature, controlling a fixed-frequency compression system to start and run; judging whether the heating capacity of the fixed-frequency compression system meets the required heating capacity; and according to the judgment result, selectively controlling a variable-frequency compression system to enter a heating mode or a refrigeration defrosting mode. By adopting double-system independent operation, the present application can switch different operation modes according to different environment temperatures, so that the heat pump drying system can output heat while defrosting, thereby guaranteeing the defrosting effect to the maximum and not reducing the system heat output, so as to guarantee the baking temperature of the baking house.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of heat pump drying, and particularly provides a control method of a heat pump drying system and the heat pump drying system. BACKGROUND

[0002] Flue-cured tobacco is the main material for making cigarettes, and the preparation process of flue-cured tobacco is a process of controlling the moisture of tobacco to make it dehydrate and dry in time.

[0003] In the tobacco curing process, a heat pump dryer is generally used to provide heat source. When the heat pump dryer operates in winter, frost is easily formed due to the small air volume at the bottom of the heat exchanger, and defrosting is not easy. The current defrosting method is to stop the heat pump dryer from starting when the ambient temperature and the coil temperature of the outdoor unit are lower than a certain set temperature to avoid frost formation. However, in this case, although the defrosting effect can be ensured, the heat in the curing barn will rapidly decay because the heat pump dryer does not start, and the curing temperature in the curing barn cannot be guaranteed.

[0004] Therefore, there is a need in the art for a new technical solution to solve the above problems. SUMMARY

[0005] The present application aims to solve the above technical problems, i.e., to solve the problem that the existing heat pump drying system cannot guarantee both defrosting effect and heat supply.

[0006] In a first aspect, the present application provides a control method of a heat pump drying system, the heat pump drying system comprising a fixed-frequency compression system and a variable-frequency compression system, the fixed-frequency compression system being capable of realizing a heating cycle, and the variable-frequency compression system being capable of realizing a heating cycle and a refrigeration defrosting cycle respectively, the control method comprising: acquiring an outdoor ambient temperature; when the outdoor ambient temperature is lower than a preset ambient temperature, controlling the fixed-frequency compression system to start and run; judging whether the heating capacity of the fixed-frequency compression system meets the required heating capacity; and according to the judgment result, selectively controlling the variable-frequency compression system to enter a heating mode or a refrigeration defrosting mode.

[0007] In the specific embodiment of the control method of the heat pump drying system, the fixed-frequency compression system comprises a fixed-frequency compressor, a first outer heat exchanger and a first inner heat exchanger connected in sequence to form a heating circulation loop, the variable-frequency compression system comprises a variable-frequency compressor, a second outer heat exchanger, a second inner heat exchanger and a four-way valve, the variable-frequency compressor is connected with the second outer heat exchanger and the second inner heat exchanger through the four-way valve to form a heating circulation loop and a refrigeration defrosting circulation loop respectively, the first outer heat exchanger and the second outer heat exchanger share an outer fan, the first inner heat exchanger and the second inner heat exchanger share an inner fan, and the step of "selectively controlling the variable-frequency compression system to enter a heating mode or a refrigeration defrosting mode according to a judgment result" specifically comprises: when the heating capacity of the fixed-frequency compression system meets the required heating capacity, judging whether the heat pump drying system meets refrigeration defrosting conditions; if yes, controlling the variable-frequency compression system to enter the refrigeration defrosting mode.

[0008] In the specific embodiment of the control method of the heat pump drying system, the step of "controlling the variable-frequency compression system to enter the refrigeration defrosting mode when the heating capacity of the fixed-frequency compression system meets the required heating capacity" specifically comprises: controlling the inner fan and the outer fan to run at a preset minimum wind speed respectively and controlling the variable-frequency compressor to run at a highest operating frequency; acquiring an outlet air temperature and a dry-bulb temperature; when the outlet air temperature is lower than the dry-bulb temperature, controlling the inner fan to stop running for a first preset time and gradually reducing the operating frequency of the variable-frequency compressor until the outlet air temperature is higher than the dry-bulb temperature.

[0009] In the specific embodiment of the control method of the heat pump drying system, the step of "selectively controlling the variable-frequency compression system to enter a heating mode or a refrigeration defrosting mode according to a judgment result" further comprises: when the heating capacity of the fixed-frequency compression system does not meet the required heating capacity, first controlling the variable-frequency compression system to enter the heating mode, and then judging whether the heat pump drying system meets refrigeration defrosting conditions; if yes, controlling the variable-frequency compression system to enter the refrigeration defrosting mode.

[0010] In the specific embodiment of the control method of the heat pump drying system, the step of "controlling the variable-frequency compression system to enter the refrigeration defrosting mode when the heating capacity of the fixed-frequency compression system does not meet the required heating capacity" specifically comprises: controlling the inner fan and the outer fan to stop running, controlling the fixed-frequency compressor to stop running for a second preset time and controlling the variable-frequency compressor to run at a highest operating frequency; acquiring an outlet air temperature and a dry-bulb temperature; when the outlet air temperature is lower than the dry-bulb temperature, controlling the inner fan and the outer fan to continue to stop running until the outlet air temperature is higher than the dry-bulb temperature.

[0011] In the specific embodiment of the control method of the heat pump drying system, the step of "controlling the variable frequency compression system to enter the refrigeration defrosting mode when the heating capacity of the fixed frequency compression system does not meet the required heating capacity" further comprises: when the outlet air temperature is higher than the dry-bulb temperature, controlling the inner fan and the outer fan to run at a preset minimum air speed to a set air speed, respectively.

[0012] In the specific embodiment of the control method of the heat pump drying system, the step of "judging whether the heat pump drying system meets the refrigeration defrosting condition" specifically comprises: obtaining the coil temperature of the first outer heat exchanger and / or the second outer heat exchanger; and when the coil temperature is lower than a preset coil temperature, judging that the variable frequency compression system meets the refrigeration defrosting condition.

[0013] In the specific embodiment of the control method of the heat pump drying system, the control method further comprises: when the outdoor environment temperature is higher than a preset environment temperature, controlling the variable frequency compression system to start and run in the heating mode; when the variable frequency compressor runs at the highest operating frequency and the heating capacity of the variable frequency compression system does not meet the required heating capacity, controlling the fixed frequency compression system to start and run; judging whether the sum of the heating capacities of the variable frequency compression system and the fixed frequency compression system meets the required heating capacity; and if yes, controlling the variable frequency compressor to gradually reduce the operating frequency to stop.

[0014] In the specific embodiment of the control method of the heat pump drying system, the step of "judging whether the sum of the heating capacities of the variable frequency compression system and the fixed frequency compression system meets the required heating capacity" further comprises: if no, controlling the variable frequency compressor to run at a variable frequency, so that the sum of the heating capacities of the variable frequency compression system and the fixed frequency compression system meets the required heating capacity.

[0015] In the second aspect, the application further provides a heat pump drying system comprising a controller configured to be capable of executing the above-mentioned control method.

[0016] In the case of adopting the above technical solution, by adopting the dual-system independent operation, different operating modes can be switched according to different environment temperatures, so that the heat pump drying system can perform heat output while defrosting, which can maximize the defrosting effect while not reducing the system heat output, so as to ensure the baking temperature of the baking house. BRIEF DESCRIPTION OF DRAWINGS

[0017] The preferred embodiments of the application will be described below with reference to the accompanying drawings, in which:

[0018] Figure 1 is a working principle diagram of the heat pump drying system of the application;

[0019] Figure 2is the main step flow chart of the control method of the heat pump drying system of the present application;

[0020] Figure 3 is the step flow chart of the first case of the control method of the heat pump drying system of the present application;

[0021] Figure 4 is the step flow chart of the second case of the control method of the heat pump drying system of the present application;

[0022] Figure 5 is the step flow chart of the third case of the control method of the heat pump drying system of the present application;

[0023] LIST OF REFERENCE NUMERALS

[0024] 1, fixed frequency compressor; 2, variable frequency compressor; 3, four-way valve; 4, outdoor unit; 5, indoor unit; 6, liquid storage tank; 7, electronic expansion valve. DETAILED DESCRIPTION

[0025] The preferred embodiments of the present application will be described below with reference to the accompanying drawings. It should be understood by those skilled in the art that these embodiments are only used to explain the technical principles of the present application, and are not used to limit the protection scope of the present application. Those skilled in the art can make adjustments according to the needs in order to adapt to specific application occasions.

[0026] It should be noted that in the description of the present application, the ordinal numbers "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance.

[0027] In addition, it should also be noted that in the description of the present application, although the steps of the control method of the present application are described in a specific order in this application, these orders are not limiting, and those skilled in the art can perform the steps in different orders without deviating from the basic principles of the present application.

[0028] First refer to Figure 1 , which is the working principle diagram of the heat pump drying system. As shown in Figure 1As shown, the heat pump drying system includes a fixed frequency compression system and a variable frequency compression system, the fixed frequency compression system can realize a heating cycle, the variable frequency compression system can realize a heating cycle and a refrigeration defrosting cycle respectively, the fixed frequency compression system includes a fixed frequency compressor 1, a first outer heat exchanger and a first inner heat exchanger connected in sequence to constitute a heating cycle loop, the variable frequency compression system includes a variable frequency compressor 2, a second outer heat exchanger, a second inner heat exchanger and a four-way valve 3, the variable frequency compressor 2 is connected with the second outer heat exchanger and the second inner heat exchanger through the four-way valve 3 to constitute a heating cycle loop and a refrigeration defrosting cycle loop respectively, the heating cycle loop and the refrigeration defrosting cycle loop realize forward and reverse flow switching of refrigerant through switching of the four-way valve 3 to realize switching of a heating mode and a refrigeration defrosting mode. The first outer heat exchanger and the second outer heat exchanger share an outer fan and the pipelines are not communicated with each other, the first inner heat exchanger and the second inner heat exchanger share an inner fan and the pipelines are not communicated with each other, the first inner heat exchanger, the second inner heat exchanger and the inner fan constitute an indoor unit 5, the first outer heat exchanger, the second outer heat exchanger and the outer fan constitute an outdoor unit 4, the outdoor unit 4 is connected with the indoor unit 5, a liquid storage tank 6 is connected between the fixed frequency compression system and the variable frequency compression system, the refrigerant amount entering the respective systems is adjusted by controlling an electronic expansion valve 7 through respective suction and discharge pressures and suction and discharge temperatures. The highest operating frequency of the variable frequency compressor 2 is less than the output frequency of the fixed frequency compressor 1, that is, the heating capacity of the variable frequency compression system is less than the heating capacity of the fixed frequency compression system.

[0029] Reference will now be made to Figure 2 The figure is the main step flow chart of the control method of the heat pump drying system. As Figure 2 The control method of the present application includes the following steps:

[0030] S1, obtaining an outdoor environment temperature;

[0031] S2, when the outdoor environment temperature is lower than a preset environment temperature, controlling the fixed frequency compression system to start and run;

[0032] S3, judging whether the heating capacity of the fixed frequency compression system meets the required heating capacity;

[0033] S4, according to the judgment result, selectively controlling the variable frequency compression system to enter a heating mode or a refrigeration defrosting mode.

[0034] Exemplarily, the preset environment temperature is 10℃. When the outdoor environment temperature is lower than 10℃, since the heating capacity of the fixed frequency compression system is higher than that of the variable frequency compression system, the fixed frequency compression system is preferentially used to output heating capacity, and the variable frequency compression system supplements the heating capacity output by the fixed frequency compression system when the heating capacity output by the fixed frequency compression system cannot meet the required heating capacity.

[0035] In step S2, the method of "judging whether the heating capacity of the fixed-frequency compression system meets the demand heating capacity" is specifically: whether the actual temperature rising rate in the curing room meets the set temperature rising rate, for example, the user sets the curing room to rise 1℃ per 30min in the temperature rising stage, if the temperature rising requirement is not met after 30min, it is judged that the heating capacity of the fixed-frequency compression system does not meet the demand heating capacity, otherwise, it is met; or, since the dry-bulb temperature will decay after the dehumidification is completed, if the dry-bulb temperature does not reach the dry-bulb temperature before the decay after the set time (for example, 10min), it is judged that the heating capacity of the fixed-frequency compression system does not meet the demand heating capacity, otherwise, it is met.

[0036] In step S4, the step of "selectively controlling the variable-frequency compression system to enter the heating mode or the refrigeration defrosting mode according to the judgment result" specifically includes:

[0037] S41, when the heating capacity of the fixed-frequency compression system meets the demand heating capacity, judging whether the heat pump drying system meets the refrigeration defrosting condition;

[0038] S42, if yes, controlling the variable-frequency compression system to enter the refrigeration defrosting mode.

[0039] In step S42, the step of "controlling the variable-frequency compression system to enter the refrigeration defrosting mode when the heating capacity of the fixed-frequency compression system meets the demand heating capacity" specifically includes:

[0040] S421, controlling the inner fan and the outer fan to run at the preset minimum air speed respectively and controlling the variable-frequency compressor 2 to run at the highest operating frequency;

[0041] S422, acquiring the outlet air temperature and the dry-bulb temperature;

[0042] S423, when the outlet air temperature is lower than the dry-bulb temperature, controlling the inner fan to stop running for a first preset time and gradually reducing the operating frequency of the variable-frequency compressor 2 until the outlet air temperature is higher than the dry-bulb temperature.

[0043] In the above steps, as Figure 4As shown, when the heating capacity of the fixed-frequency compression system meets the required heating capacity, the variable-frequency compression system does not need to perform heating to supplement the heating capacity, and the fixed-frequency compression system operates normally before the heat pump drying system performs defrosting, and the variable-frequency compression system is not started. When the heat pump drying system needs to perform defrosting, the variable-frequency compression system is directly started and operated in the refrigeration defrosting mode, at this time, the indoor fan and the outdoor fan are controlled to operate at the preset minimum wind speed, the variable-frequency compressor 2 is controlled to operate at the highest operating frequency, and the outlet air temperature and the dry-bulb temperature are detected in real time. When the outlet air temperature is lower than the dry-bulb temperature, the indoor fan is controlled to stop operating for a first preset time (for example, 3 min), and the operating frequency of the variable-frequency compressor 2 is reduced (that is, the evaporation effect of the variable-frequency compression system at the indoor unit 5 is reduced). After 3 min, if the heat pump drying system is still in the defrosting stage, the outlet air temperature and the dry-bulb temperature are compared, if the outlet air temperature is still lower than the dry-bulb temperature, the operating frequency of the variable-frequency compressor 2 is further reduced, and the lowest operating frequency of the variable-frequency compressor 2 is the lowest operating frequency of the variable-frequency compressor 2, until the outlet air temperature is higher than the dry-bulb temperature. Among them, the variable-frequency compressor 2 reduces the operating frequency by 5 Hz as a gear, and the lowest operating frequency of the variable-frequency compressor 2 is 20 Hz.

[0044] In step S4, the step of "selectively controlling the variable-frequency compression system to enter the heating mode or the refrigeration defrosting mode according to the judgment result" further includes:

[0045] S411, when the heating capacity of the fixed-frequency compression system does not meet the required heating capacity, the variable-frequency compression system is first controlled to enter the heating mode, and then it is judged whether the heat pump drying system meets the refrigeration defrosting condition;

[0046] S412, if yes, the variable-frequency compression system is controlled to enter the refrigeration defrosting mode.

[0047] In step S412, the step of "controlling the variable-frequency compression system to enter the refrigeration defrosting mode when the heating capacity of the fixed-frequency compression system does not meet the required heating capacity" specifically includes:

[0048] S4121, controlling the indoor fan and the outdoor fan to stop operating, controlling the fixed-frequency compressor 1 to stop operating for a second preset time, and controlling the variable-frequency compressor 2 to operate at the highest operating frequency;

[0049] S4122, obtaining the outlet air temperature and the dry-bulb temperature;

[0050] S4123, when the outlet air temperature is lower than the dry-bulb temperature, the indoor fan and the outdoor fan are controlled to continue to stop operating until the outlet air temperature is higher than the dry-bulb temperature.

[0051] Among them, step S4123 further includes:

[0052] S41231, when the outlet air temperature is higher than the dry-bulb temperature, control the inner fan and the outer fan to run at a preset minimum air speed to a set air speed, respectively.

[0053] In the above steps, as shown in Figure 5 When the heating capacity of the fixed-frequency compression system does not meet the required heating capacity, the variable-frequency compression system needs to heat to supplement the heating capacity. Before the heat pump drying system needs to be defrosted, the fixed-frequency compression system operates normally, the variable-frequency compression system starts and operates in a heating mode, and the operating frequency of the variable-frequency compressor 2 dynamically changes to supplement the required heating capacity. When the heat pump drying system needs to be defrosted, the variable-frequency compression system switches from the heating mode to the refrigeration defrosting mode, at this time, the inner fan and the outer fan are controlled to stop running, the variable-frequency compressor 2 is controlled to run at the highest operating frequency, and the fixed-frequency compressor 1 is controlled to stop running for a second preset time (for example, 20s), that is, the fixed-frequency compressor 1 is started after 20s, and the variable-frequency compression system suction and discharge pressure sensor is shielded for 30s. After 10s, compare the outlet air temperature with the dry-bulb temperature, when the outlet air temperature is lower than the dry-bulb temperature, control the inner fan and the outer fan to continue to stop running until the outlet air temperature is higher than the dry-bulb temperature, and when the outlet air temperature is higher than the dry-bulb temperature, control the inner fan and the outer fan to run at a preset minimum air speed to a set air speed, respectively, wherein the set air speed is the set air speed of the current baking section in the curing barn.

[0054] In steps S41 and S411, the step of "judging whether the heat pump drying system meets the refrigeration defrosting condition" specifically includes:

[0055] Obtain the coil temperature of the first outer heat exchanger and / or the second outer heat exchanger;

[0056] When the coil temperature is lower than the preset coil temperature, it is judged that the heat pump drying system meets the refrigeration defrosting condition.

[0057] In the above steps, for example, the preset coil temperature is 3℃, that is, when the coil temperature is lower than 3℃, it is determined that the heat pump drying system needs to be defrosted, at this time, the variable-frequency compression system enters the refrigeration defrosting mode, and when the coil temperature is higher than 3℃, it is determined that the heat pump drying system does not need to be defrosted, at this time, the variable-frequency compression system exits the refrigeration defrosting mode.

[0058] The control method of the present application further includes the following steps:

[0059] S5, when the outdoor environment temperature is higher than the preset environment temperature, control the variable-frequency compression system to start and operate in a heating mode;

[0060] S6, when the variable-frequency compressor 2 operates at the highest operating frequency and the heating capacity of the variable-frequency compression system does not meet the required heating capacity, control the fixed-frequency compression system to start and operate;

[0061] S7, judging whether the total heating capacity of the variable frequency compression system and the fixed frequency compression system meets the required heating capacity;

[0062] S71, if yes, controlling the variable frequency compressor 2 to gradually reduce the operating frequency to stop;

[0063] S72, if no, controlling the variable frequency compressor 2 to operate in variable frequency mode, so that the total heating capacity of the variable frequency compression system and the fixed frequency compression system meets the required heating capacity.

[0064] In the above steps, as shown in Figure 3 , exemplarily, when the outdoor environment temperature is higher than 10℃, at this time, only single system operation is needed to heat, the variable frequency compression system is first controlled to start and operate in heating mode, during the operation, the operating frequency of the variable frequency compressor 2 dynamically changes according to the required heating capacity, when the variable frequency compressor 2 operates at the highest operating frequency and the heating capacity of the variable frequency compression system still does not meet the required heating capacity, the fixed frequency compression system is controlled to start and operate. After the fixed frequency compression system operates, if the required heating capacity is met, the variable frequency compressor 2 is controlled to gradually reduce the operating frequency to stop, if the required heating capacity cannot be met only by relying on the fixed frequency compression system, the fixed frequency compression system and the variable frequency compression system need to be operated in cooperation, at this time, the operating frequency of the variable frequency compressor 2 dynamically changes, so that the total heating capacity of the variable frequency compression system and the fixed frequency compression system meets the required heating capacity, that is, the required heating capacity for the drying room to rise in temperature can be met.

[0065] Wherein, the method of “judging whether the heating capacity of the variable frequency compression system meets the required heating capacity” is the same as the method of “judging whether the heating capacity of the fixed frequency compression system meets the required heating capacity”, which will not be repeated here.

[0066] As can be understood by those skilled in the art, in the preferred technical solution of the control method of the present application, by adopting double system independent operation, different operation modes can be switched according to different environment temperatures, so that the heat pump drying system can output heat while defrosting, which can maximize the defrosting effect while not reducing the system heat output, so as to ensure the baking temperature of the drying room.

[0067] Hereinafter Figures 3-5 , a possible control process of the present application will be introduced. As shown in Figures 3-5 , a possible complete process of the control method of the present application is:

[0068] S101, acquiring the outdoor environment temperature;

[0069] S102, judging whether the outdoor environment temperature is lower than 10℃;

[0070] If yes, step S103 is executed;

[0071] If no, step S118 is performed;

[0072] S103, controlling the fixed-frequency compression system to start and run;

[0073] S104, judging whether the heating capacity of the fixed-frequency compression system meets the required heating capacity;

[0074] If yes, step S105 is performed;

[0075] If no, step S111 is performed;

[0076] S105, judging whether the heat pump drying system meets the refrigeration defrosting condition;

[0077] If yes, step S106 is performed;

[0078] If no, step S103 is continuously performed;

[0079] S106, controlling the inner fan and the outer fan to run at preset minimum wind speed respectively and controlling the variable-frequency compressor 2 to run at the highest operating frequency;

[0080] S107, obtaining the outlet air temperature and the dry-bulb temperature;

[0081] S108, judging whether the outlet air temperature is lower than the dry-bulb temperature;

[0082] If yes, step S109 is performed;

[0083] If no, step S110 is performed;

[0084] S109, controlling the inner fan to stop running for a first preset time and gradually reducing the operating frequency of the variable-frequency compressor 2;

[0085] S110, controlling the inner fan to continuously run at the preset minimum wind speed and controlling the variable-frequency compressor 2 to run at the current operating frequency;

[0086] S111, controlling the variable-frequency compression system to enter the heating mode;

[0087] S112, judging whether the heat pump drying system meets the refrigeration defrosting condition;

[0088] If yes, step S113 is performed;

[0089] If no, step S111 is continuously performed;

[0090] S113, controlling the inner fan and the outer fan to stop running, controlling the fixed-frequency compressor 1 to stop running for a second preset time, and controlling the variable-frequency compressor 2 to run at the highest operating frequency;

[0091] S114, obtaining the outlet air temperature and the dry-bulb temperature;

[0092] S115, determining whether the air temperature is lower than the dry-bulb temperature;

[0093] If yes, step S116 is performed;

[0094] If no, step S117 is performed;

[0095] S116, controlling the inner and outer air fans to continue stopping operation;

[0096] S117, controlling the inner and outer air fans to operate at a preset minimum air speed to a set air speed, respectively;

[0097] S118, controlling the variable frequency compression system to start and operate in a heating mode;

[0098] S119, when the variable frequency compressor 2 operates at a highest operation frequency and the heating capacity of the variable frequency compression system does not meet the required heating capacity, controlling the fixed frequency compression system to start and operate;

[0099] S120, determining whether the sum of the heating capacities of the variable frequency compression system and the fixed frequency compression system meets the required heating capacity;

[0100] If yes, step S121 is performed;

[0101] If no, step S122 is performed;

[0102] S121, controlling the variable frequency compressor 2 to gradually reduce the operation frequency to stop;

[0103] S122, controlling the variable frequency compressor 2 to operate in a variable frequency mode.

[0104] Based on the above-mentioned embodiments of the control method, the present application further provides a heat pump drying system, comprising a controller configured to be capable of performing the above-mentioned control method.

[0105] So far, the technical solutions of the present application have been described in combination with the preferred embodiments shown in the drawings, but it is easy for those skilled in the art to understand that the protection scope of the present application is obviously not limited to these specific embodiments. Those skilled in the art can make equivalent changes or replacements to the related technical features without departing from the principles of the present application, and the technical solutions after these changes or replacements will all fall within the protection scope of the present application.

Claims

1. A control method of a heat pump drying system, characterized by, The heat pump drying system comprises a fixed-frequency compression system and a variable-frequency compression system, the fixed-frequency compression system can realize a heating cycle, the variable-frequency compression system can realize a heating cycle and a refrigeration defrosting cycle respectively, the fixed-frequency compression system comprises a first outer heat exchanger and a first inner heat exchanger, the variable-frequency compression system comprises a variable-frequency compressor, a second outer heat exchanger and a second inner heat exchanger, the first outer heat exchanger and the second outer heat exchanger share one outer fan, and the first inner heat exchanger and the second inner heat exchanger share one inner fan; The control method comprises: obtaining an outdoor environment temperature; when the outdoor environment temperature is lower than a preset environment temperature, controlling the fixed-frequency compression system to start and run; judging whether the heating capacity of the fixed-frequency compression system meets a required heating capacity; according to the judgment result, selectively controlling the variable-frequency compression system to enter a heating mode or a refrigeration defrosting mode; the step of "according to the judgment result, selectively controlling the variable-frequency compression system to enter a heating mode or a refrigeration defrosting mode" comprises: when the heating capacity of the fixed-frequency compression system meets the required heating capacity, judging whether the heat pump drying system meets refrigeration defrosting conditions; if yes, controlling the variable-frequency compression system to enter the refrigeration defrosting mode; the step of "when the heating capacity of the fixed-frequency compression system meets the required heating capacity, controlling the variable-frequency compression system to enter the refrigeration defrosting mode" comprises: controlling the inner fan and the outer fan to run at a preset minimum wind speed respectively and controlling the variable-frequency compressor to run at a highest operating frequency; obtaining an outlet air temperature and a dry-bulb temperature; when the outlet air temperature is lower than the dry-bulb temperature, controlling the inner fan to stop running for a first preset time and gradually reducing the operating frequency of the variable-frequency compressor until the outlet air temperature is higher than the dry-bulb temperature.

2. The control method of a heat pump drying system according to claim 1, characterized in that, The fixed-frequency compression system further comprises a fixed-frequency compressor, the fixed-frequency compressor, the first outer heat exchanger and the first inner heat exchanger are connected in sequence to form a heating cycle loop, the variable-frequency compression system further comprises a four-way valve, and the variable-frequency compressor is connected with the second outer heat exchanger and the second inner heat exchanger through the four-way valve to form a heating cycle loop and a refrigeration defrosting cycle loop respectively.

3. The control method of a heat pump drying system according to claim 2, characterized by, the step of "according to the judgment result, selectively controlling the variable-frequency compression system to enter a heating mode or a refrigeration defrosting mode" further comprises: when the heating capacity of the fixed-frequency compression system does not meet the required heating capacity, first controlling the variable-frequency compression system to enter the heating mode, and then judging whether the heat pump drying system meets the refrigeration defrosting conditions; if yes, controlling the variable-frequency compression system to enter the refrigeration defrosting mode.

4. The control method of a heat pump drying system according to claim 3, characterized in that, the step of "when the heating capacity of the fixed-frequency compression system does not meet the required heating capacity, controlling the variable-frequency compression system to enter the refrigeration defrosting mode" comprises: controlling the inner fan and the outer fan to stop running, controlling the fixed-frequency compressor to stop running for a second preset time and controlling the variable-frequency compressor to run at a highest operating frequency; obtaining an outlet air temperature and a dry-bulb temperature; when the outlet air temperature is lower than the dry-bulb temperature, controlling the inner fan and the outer fan to continue to stop running until the outlet air temperature is higher than the dry-bulb temperature.

5. The control method of a heat pump drying system according to claim 4, characterized in that, The step of "controlling the variable frequency compression system to enter the refrigeration defrosting mode when the heating capacity of the variable frequency compression system fails to meet the required heating capacity" further comprises: controlling the inner fan and the outer fan to run at a preset minimum air volume to a set air volume respectively when the outlet air temperature is higher than the dry-bulb temperature.

6. The control method of a heat pump drying system according to claim 1 or 3, characterized by, The step of "judging whether the heat pump drying system meets the refrigeration defrosting condition" specifically comprises: obtaining the coil temperature of the first outer heat exchanger and / or the second outer heat exchanger; judging that the variable frequency compression system meets the refrigeration defrosting condition when the coil temperature is lower than a preset coil temperature.

7. The control method of a heat pump drying system according to claim 2, characterized by, The control method further comprises: controlling the variable frequency compression system to start and run in the heating mode when the outdoor environment temperature is higher than a preset environment temperature; controlling the fixed frequency compression system to start and run when the variable frequency compressor runs at the highest operating frequency and the heating capacity of the variable frequency compression system fails to meet the required heating capacity; judging whether the sum of the heating capacities of the variable frequency compression system and the fixed frequency compression system meets the required heating capacity; if yes, gradually reducing the operating frequency of the variable frequency compressor to stop.

8. The control method of a heat pump drying system according to claim 7, characterized by, The step of "judging whether the sum of the heating capacities of the variable frequency compression system and the fixed frequency compression system meets the required heating capacity" further comprises: if no, controlling the variable frequency compressor to run at variable frequency to make the sum of the heating capacities of the variable frequency compression system and the fixed frequency compression system meet the required heating capacity.

9. A heat pump drying system comprising a controller, characterised in that, The controller is configured to be capable of performing the control method of any one of claims 1 to 8.

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