An ultra-low temperature preservation box based on a variable frequency cascade refrigeration system

Abstract

The application relates to a super-low-temperature preservation box based on a variable-frequency cascade refrigeration system and belongs to the technical field of refrigeration. The super-low-temperature preservation box comprises a high-temperature stage compressor, a condenser, a condensing fan, a high-temperature stage drying filter, a high-temperature stage capillary tube, an intermediate heat exchanger, a gas-liquid separator, a low-temperature stage compressor, an oil separator, a low-temperature stage drying filter, a low-temperature stage capillary tube, an evaporator, a super-low-temperature preservation box body, an expansion tank and a dew prevention pipe; the high-temperature stage compressor is connected with the dew prevention pipe, the condenser, the high-temperature stage drying filter, the high-temperature stage capillary tube, the intermediate heat exchanger and the gas-liquid separator in sequence; and the high-temperature stage compressor and the low-temperature stage compressor are variable-frequency compressors. The application has the advantages of fast cooling speed, low energy consumption, small stable operation noise, small temperature fluctuation in the box body and reduced failure rate of the compressor.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of refrigeration, and relates to an ultralow-temperature preservation box based on a variable-frequency cascade refrigeration system. BACKGROUND

[0002] Ultralow temperature refers to cooling to below-80 DEG C, and is mainly used for the preservation of biological samples and scientific research experiments in the fields of medicine and electronics. In order to achieve low temperature below-80 DEG C, a cascade refrigeration system is used.

[0003] At present, a fixed-speed compressor is used in the existing ultralow-temperature preservation box on the market. Although the fixed-speed compressor can meet the temperature and cooling rate requirements of the ultralow-temperature preservation box, the effect is poor in energy saving and noise reduction. In order to achieve a certain cooling rate, a compressor with large cooling capacity is usually selected, and the temperature is maintained in the set range through the start-stop mode of the compressor after reaching the temperature. On the one hand, the selected compressor has large cooling capacity, high energy consumption and large noise; on the other hand, the frequent start-stop of the compressor will cause large temperature fluctuation in the box, and the service life of the compressor will be greatly shortened. SUMMARY

[0004] Therefore, the purpose of the application is to provide an ultralow-temperature preservation box based on a variable-frequency cascade refrigeration system.

[0005] In order to achieve the above purpose, the application provides the following technical scheme:

[0006] An ultralow-temperature preservation box based on a variable-frequency cascade refrigeration system, which comprises a high-temperature stage compressor 1, a condenser 2, a condensing fan 3, a high-temperature stage drying filter 4, a high-temperature stage capillary tube 5, an intermediate heat exchanger 6, a gas-liquid separator 7, a low-temperature stage compressor 8, an oil separator 9, a low-temperature stage drying filter 10, a low-temperature stage capillary tube 11, an evaporator 12, an ultralow-temperature preservation box body 13, an expansion tank 14 and a dew prevention pipe 15.

[0007] The high-temperature stage compressor 1 is connected with the dew prevention pipe 15, the condenser 2, the high-temperature stage drying filter 4, the high-temperature stage capillary tube 5, the intermediate heat exchanger 6 and the gas-liquid separator 7 in sequence.

[0008] The gas-liquid separator 7 is connected with the high-temperature stage compressor 1.

[0009] The condensing fan 3 is arranged on one side of the condenser 2.

[0010] The condenser 2 is further connected with the oil separator 9.

[0011] The oil separator 9 is connected with the low-temperature stage compressor 8 and the intermediate heat exchanger 6.

[0012] The low-temperature stage compressor 8 is connected with the condenser 2.

[0013] The intermediate heat exchanger 6 is further connected with a low-temperature stage dry filter 10, a low-temperature stage capillary tube 11, an evaporator 12 and an expansion tank 14 in sequence;

[0014] The evaporator 12 is arranged on the ultra-low-temperature storage box body 13;

[0015] The high-temperature stage compressor 1 and the low-temperature stage compressor 8 are variable frequency compressors.

[0016] Optionally, the condenser 2 is divided into two parts, one part is a high-temperature stage condenser, and the other part is a low-temperature stage pre-cooler;

[0017] The refrigerant in the high-temperature stage compressor 1 is compressed into a high-temperature and high-pressure gas, enters the condenser 2 through the anti-dew tube 15, and is cooled into a high-temperature and high-pressure liquid. The refrigerant is condensed through the high-temperature stage condenser of the condenser 2, throttled through the high-temperature stage capillary tube 5, and then becomes a low-temperature and low-pressure refrigerant gas-liquid two-phase mixture, i.e. a high-temperature stage refrigerant, and then enters the intermediate heat exchanger 6 to cool the refrigerant in the low-temperature stage compressor 8. The refrigerant absorbs heat and evaporates into a gaseous state, returns to the high-temperature stage compressor 1 through the gas-liquid separator 7, and is compressed into a high-temperature and high-pressure gas.

[0018] The refrigerant in the low-temperature stage compressor 8 is compressed into a high-temperature and high-pressure gas, pre-cooled to a certain temperature through the low-temperature stage pre-cooler of the condenser 2, and then enters the intermediate heat exchanger 6 through the oil separator 9. The refrigerant entering the intermediate heat exchanger 6 is cooled by the high-temperature stage refrigerant to form a low-temperature stage refrigerant, and then becomes a low-temperature and low-pressure refrigerant gas-liquid two-phase mixture after throttling through the low-temperature stage capillary tube 11, and then enters the evaporator 12 to evaporate and refrigerate. Finally, it returns to the low-temperature stage compressor 8 in a gaseous state. The expansion tank 14 connected with the low-temperature stage compressor 8 causes pressure fluctuations.

[0019] Optionally, the intermediate heat exchanger 6 is a double-pipe heat exchanger or a plate heat exchanger.

[0020] Optionally, when the ultra-low-temperature storage box is started, it is first determined whether it is first powered on;

[0021] The first power-on determination is determined by the preset temperature value Ta in the box body. When the actual temperature T in the box body is greater than or equal to Ta, the first power-on mode is entered, otherwise the non-first power-on mode is entered.

[0022] When the actual temperature T in the box body is less than Ta, it indicates that the ultra-low-temperature storage box has been running for a period of time, and the compressor is started in a high-speed mode. When the actual temperature T in the box body is greater than or equal to Ta, it indicates that the ultra-low-temperature storage box has not been running for a long time or has been running for a short time before stopping, and the compressor is started in a low-speed mode. Before reaching the target speed, it stays at a first dwell speed and a second dwell speed for a period of time, respectively.

[0023] When the high-temperature stage compressor runs at the target rotating speed for a time t1, the start-up condition of the low-temperature stage compressor is judged; the start-up of the low-temperature stage compressor is judged according to the preset temperature Tb of the intermediate heat exchanger, when the temperature Tex of the intermediate heat exchanger is less than Tb, the low-temperature stage compressor starts up, and after the low-temperature stage compressor starts up, different speed-up modes are selected according to whether it is the first time to be powered on; otherwise, the high-temperature stage compressor is stopped, a fault prompt is output, and the process is exited;

[0024] The target rotating speed of the high-temperature stage compressor and the low-temperature stage compressor is calculated according to the deviation ΔT of the actual temperature T in the box from the set temperature T 设 ;

[0025] When ΔT≥4℃, the target rotating speed F max = F max ;

[0026] When -2℃<ΔT<4℃, the target rotating speed F min = F 设 ;

[0027] F max is the maximum rotating speed, and F min is the minimum rotating speed;

[0028] After the high-temperature stage compressor and the low-temperature stage compressor run at the target rotating speed for a time t2, whether to enter the energy-saving control mode is judged; when the deviation ΔT of the actual temperature in the box from the set temperature is less than 4℃, the energy-saving mode is entered, otherwise, the process continues to run at the target rotating speed;

[0029] After entering the energy-saving control mode, the high-temperature stage compressor takes the temperature Tex of the intermediate heat exchanger as the control condition, and the low-temperature stage compressor takes the temperature in the box as the control condition; for different set temperatures in the box, the intermediate heat exchanger is adjusted to the optimal temperature range;

[0030] When the set temperature T 设 ≥-68℃, the optimal temperature range of the intermediate heat exchanger Tex is T1-T2;

[0031] When the set temperature -78℃ 设 <-70℃, the optimal temperature range of the intermediate heat exchanger Tex is T2-T3;

[0032] When the set temperature -82℃ 设 <-78℃, the optimal temperature range of the intermediate heat exchanger Tex is T3-T4;

[0033] When the set temperature T 设 ≤-82℃, the optimal temperature range of the intermediate heat exchanger Tex is T4-T5;

[0034] T1 represents -28℃, T2 represents -32℃, T3 represents -36℃, T4 represents -38℃, T5 represents -40℃;

[0035] The starting value of the optimal temperature range does not contain the end value, and the end value contains the end value;

[0036] After entering the energy-saving mode, the high-temperature stage compressor adjusts the speed every t3 time, and the speed is increased by △F when it is higher than the optimal temperature range, the speed is reduced by △F when it is lower than the optimal temperature range, and the speed is unchanged when it is in the optimal temperature range;

[0037] After entering the energy-saving mode, the low-temperature stage compressor adjusts the speed every t4 time; the adjustment time t4 of the low-temperature stage compressor is less than the adjustment time t3 of the high-temperature stage compressor, so as to avoid the simultaneous adjustment of the speed of the high-temperature stage compressor and the low-temperature stage compressor, which causes the temperature T in the box to be out of adjustment; when △T>2℃, the speed is increased by △F; when -2℃≤△T≤2℃, the speed is unchanged; when △T<-2℃, the speed is reduced by △F.

[0038] The beneficial effects of the present application are:

[0039] 1. Fast cooling rate;

[0040] 2. Low energy consumption;

[0041] 3. Stable operation with low noise;

[0042] 4. Small temperature fluctuation in the box;

[0043] 5. Reducing the failure rate of the compressor.

[0044] Other advantages, objects, and features of the present application will be apparent to those skilled in the art from the following specification, and will be learned from the practice of the present application. The objects and other advantages of the present application can be achieved and obtained by the following specification. BRIEF DESCRIPTION OF DRAWINGS

[0045] In order to make the purpose, technical scheme and advantages of the present application more clear, the preferred detailed description of the present application will be combined with the drawings as follows, wherein:

[0046] Figure 1 It is a flow chart of the cascade refrigeration system;

[0047] Figure 2 It is a compressor speed-up mode diagram;

[0048] Figure 3 It is a system control flow chart.

[0049] Reference numerals: 1, high-temperature stage compressor; 2, condenser; 3, condenser fan; 4, high-temperature stage dry filter; 5, high-temperature stage capillary tube; 6, intermediate heat exchanger; 7, gas-liquid separator; 8, low-temperature stage compressor; 9, oil separator; 10, low-temperature stage dry filter; 11, low-temperature stage capillary tube; 12, evaporator; 13, ultra-low-temperature storage box body; 14, expansion tank; 15, anti-dew tube. DETAILED DESCRIPTION

[0050] The advantages and effects of the present application can be easily understood by those skilled in the art from the description. The present application can also be implemented or applied in other different specific embodiments, and the details in the description can be modified or changed in various ways based on different views and applications without departing from the spirit of the present application. It should be noted that the drawings provided in the following examples only illustrate the basic concept of the present application in a schematic manner, and the features in the following examples and embodiments can be combined with each other without conflict.

[0051] The drawings are only used for illustrative description, and the representation is only a schematic diagram, not a physical diagram, and cannot be understood as a limitation of the present application; in order to better illustrate the embodiments of the present application, some components in the drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art, it is understandable that some known structures and their descriptions in the drawings may be omitted.

[0052] The same or similar reference numerals in the drawings of the embodiments of the present application correspond to the same or similar components; in the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "left", "right", "front", "back" and the like is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore the terms describing the positional relationship in the drawings are only used for illustrative description, and cannot be understood as a limitation of the present application, for those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0053] The refrigerant flow path of the high-temperature stage compression system is: high-temperature compressor 1→anti-dew tube 15→condenser 2→high-temperature stage dry filter 4→high-temperature stage capillary tube 5→intermediate heat exchanger 6→gas-liquid separator 7→high-temperature compressor 1.

[0054] The refrigerant flow path of the low-temperature stage compression system is: low-temperature compressor 8→condenser 2→oil separator 9→intermediate heat exchanger 6→low-temperature stage capillary tube 11→evaporator 12→low-temperature stage compressor 8.

[0055] The refrigerant in the high-temperature stage refrigeration system is compressed by the high-temperature compressor into a high-temperature and high-pressure gas, enters the condenser 2 through the anti-dew pipe 15, and is cooled into a high-temperature and high-pressure liquid. The condenser 2 is divided into two parts, one part is used as a high-temperature stage condenser, and the other part is used as a low-temperature stage pre-cooler, and the two are integrated, which can effectively reduce the volume of the condenser. The refrigerant after condensation is throttled by the high-temperature stage capillary tube 5 into a low-temperature and low-pressure refrigerant gas-liquid two-phase mixture, enters the intermediate heat exchanger 6 to cool the refrigerant in the low-temperature stage system, at this time, the refrigerant is mostly evaporated into a gaseous state by absorbing heat, and returns to the high-temperature stage compressor after passing through the gas-liquid separator 7.

[0056] The refrigerant in the low-temperature stage system is compressed into a high-temperature and high-pressure gas by the compressor, is pre-cooled to a certain temperature by the condenser 2 first, and then enters the intermediate heat exchanger 6 through the oil separator 9. The intermediate heat exchanger is generally a double-pipe heat exchanger or a plate heat exchanger. The refrigerant entering the intermediate heat exchanger is cooled by the high-temperature stage refrigerant, and the cooled low-temperature stage refrigerant is throttled by the low-temperature stage capillary tube 11 into a low-temperature and low-pressure refrigerant gas-liquid two-phase mixture, enters the evaporator 12 to evaporate and cool, and finally returns to the low-temperature stage compressor in a gaseous state. The expansion tank connected with the low-temperature stage compressor makes the pressure of the buffer system fluctuate.

[0057] The high-temperature stage and low-temperature stage compressors of the application are variable frequency compressors. When the ultra-low-temperature storage box is started, it is first determined whether it is the first power-on. The first power-on determination is determined by the temperature preset value Ta in the box, when the actual temperature T in the box is greater than or equal to Ta, the first power-on mode is entered, otherwise the non-first power-on mode is entered.

[0058] The purpose of the first power-on judgment is to control the compressor to start in different speed-up modes. When the actual temperature T in the box is less than Ta, it indicates that the ultra-low temperature storage box has been running for a period of time, and the temperature in the box is low enough, and the pressure in the refrigeration system is not high. At this time, the compressor can start in a high speed-up mode, and there is no need to worry about the compressor speed being too fast, causing the refrigeration system pressure to exceed the standard, causing a strong impact on the compressor, which may cause the compressor to be damaged and reduce the service life of the compressor. Starting in a high speed-up mode can quickly reach the conditions for starting the low-temperature level compressor and improve the cooling rate of the system. When the actual temperature T in the box is greater than or equal to Ta, it indicates that the ultra-low temperature storage box has not been running for a long time or has been running for a short time. At this time, the pressure in the system is still high, and if the compressor starts in a high speed mode, it is likely to cause damage to the compressor. At this time, the compressor starts in a low speed-up mode, and the speed-up time is slow. Before reaching the target speed, it stops at the first and second stop speeds for a period of time. The purpose of stopping is: on the one hand, it gives the system enough time to establish a refrigerant circulation to avoid the compressor speed being too fast, causing the system pressure to exceed the standard; on the other hand, when the compressor starts, a large amount of lubricating oil in the compressor will be sprayed out, and stopping for a period of time is also beneficial to the oil return of the compressor.

[0059] When the high-temperature level compressor runs at the target speed for a time t1, the starting conditions of the low-temperature level compressor are judged. The low-temperature level compressor starts when the intermediate heat exchanger temperature Tex is less than Tb. After the low-temperature level compressor starts, different speed-up modes are selected according to whether it is the first power-on.

[0060] The initial target speed of the high-temperature and low-temperature level compressors is calculated according to the deviation ΔT of the actual temperature in the box from the set temperature.

[0061] Table 1 Compressor target speed calculation table

[0062]

[0063] After the high-temperature and low-temperature level compressors run at the target speed for a time t2, it is judged whether to enter the energy-saving control mode. When the deviation ΔT of the actual temperature in the box from the set temperature is less than 4°C, the energy-saving mode is entered, otherwise the initial target speed is continued,

[0064] After entering the energy-saving control mode, the high-temperature level compressor uses the intermediate heat exchanger temperature Tex as the control condition, and the low-temperature level compressor uses the temperature in the box as the control condition. For different set temperatures in the box, there is an optimal temperature range for the intermediate heat exchanger, and within this temperature range, the efficiency of the refrigeration system is the highest, as shown in Table 2.

[0065] Table 2 optimal temperature range of intermediate heat exchanger Tex

[0066] Set temperature T 设 ]] Optimal temperature range for the intermediate heat exchanger Tex T 设 ≥-68℃ T1 ~ T2 -78°C < T 设 -70°C T2 ~ T3 -82 °C < T 设 -78 °C T3 ~ T4 T 设 ≤-82℃ T4 ~ T5

[0067] T1 represents -28℃, T2 represents -32℃, T3 represents -36℃, T4 represents -38℃, T5 represents -40℃; the start value of the optimal temperature range does not contain the end value, and the end value contains the end value;

[0068] After entering the energy saving mode, the high temperature stage compressor adjusts the rotating speed every t3 time. If the temperature is higher than the optimal temperature range, the rotating speed is increased by △F; if the temperature is lower than the optimal temperature range, the rotating speed is decreased by △F; if the temperature is in the optimal temperature range, the rotating speed is unchanged.

[0069] After entering the energy saving mode, the low temperature stage compressor adjusts the rotating speed every t4 time. The adjusting time t4 of the low temperature stage compressor is less than the adjusting time t3 of the high temperature stage compressor, so as to avoid the temperature T in the box being out of control due to the simultaneous adjustment of the rotating speed of the high and low temperature stage compressors. When the deviation △T between the actual temperature and the set temperature is greater than 2℃, the rotating speed is increased by △F; when -2℃≤△T≤2℃, the rotating speed is unchanged; when △T<-2℃, the rotating speed is decreased by △F.

[0070] Through the above control mode, the high and low temperature stage compressors can run at full load during the cooling process, and the refrigerating capacity is greater than that of the fixed speed compressor, so the cooling rate is fast. When the set temperature range is reached, the high and low temperature stage compressors enter the energy saving mode control, and the rotating speed and power of the compressor are reduced to the minimum under the premise of meeting the temperature control requirement, so the energy consumption is the lowest. The rotating speed of the compressor is reduced, and the corresponding noise is also reduced. At this time, the refrigerating capacity of the compressor is smaller than that of the fixed speed compressor, which avoids the problem that the system pressure is too high during the start of the fixed speed compressor, which may cause damage to the compressor and reduce the service life of the compressor. At the same time, due to the absence of frequent start and stop, the fluctuation degree of the temperature in the box is also greatly reduced.

[0071] Finally, it should be pointed out that the above embodiments are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the purpose and scope of the technical solutions, which should be covered in the scope of the claims of the present application.

Claims

1. An ultra-low temperature storage box based on a variable frequency cascade refrigeration system, characterized by: The ultra-low temperature storage box includes a high-temperature stage compressor (1), a condenser (2), a condensing fan (3), a high-temperature stage dryer filter (4), a high-temperature stage capillary tube (5), an intermediate heat exchanger (6), a gas-liquid separator (7), a low-temperature stage compressor (8), an oil separator (9), a low-temperature stage dryer filter (10), a low-temperature stage capillary tube (11), an evaporator (12), an ultra-low temperature storage box body (13), an expansion tank (14) and an anti-dew tube (15); The high-temperature stage compressor (1) is sequentially connected to the anti-dew tube (15), the condenser (2), the high-temperature stage dryer filter (4), the high-temperature stage capillary tube (5), the intermediate heat exchanger (6), and the gas-liquid separator (7); The gas-liquid separator (7) is then connected to the high-temperature stage compressor (1); The condensing fan (3) is arranged on one side of the condenser (2); The condenser (2) is also connected to the oil separator (9); The oil separator (9) is simultaneously connected to the low-temperature stage compressor (8) and the intermediate heat exchanger (6); The low-temperature stage compressor (8) is connected to the condenser (2); The intermediate heat exchanger (6) is also sequentially connected to the low-temperature stage dryer filter (10), the low-temperature stage capillary tube (11), the evaporator (12) and the expansion tank (14); The evaporator (12) is arranged on the ultra-low temperature storage box body (13); The high-temperature stage compressor (1) and the low-temperature stage compressor (8) are variable frequency compressors; When the ultra-low temperature storage box is powered on, it first judges whether it is the first power-on; The first power-on judgment is made based on the preset temperature Ta inside the box. When the actual temperature T inside the box ≥ Ta, it enters the first power-on mode, otherwise it enters the non-first power-on mode; When the actual temperature T inside the box < Ta, it means that the ultra-low temperature storage box has been running for some time, and the compressor starts in the high speed-up mode. When the actual temperature T inside the box ≥ Ta, it means that the ultra-low temperature storage box has not been running for a long time or has stopped soon after running for a short time, and the compressor starts in the low speed-up mode. Before reaching the target speed, it stays at the first stay speed and the second stay speed for a period of time respectively; When the high-temperature stage compressor runs at the target speed for time t1, it judges the starting condition of the low-temperature stage compressor; The start of the low-temperature stage compressor is judged based on the preset temperature Tb of the intermediate heat exchanger. When the temperature Tex of the intermediate heat exchanger < Tb, the low-temperature stage compressor starts. After the low-temperature stage compressor starts, it selects different speed-up modes to run according to whether it is the first power-on; otherwise, it controls the high-temperature stage compressor to stop, outputs a fault prompt and exits; The target speed of the high-temperature compressor and the low-temperature compressor is determined by the actual temperature T in the box and the set temperature T 设 The deviation △T is calculated as: When △T≥4℃, the target speed F=F max ; When -2℃<△T<4℃, the target speed F max is the maximum speed, F min is the minimum speed; After the high-temperature stage compressor and the low-temperature stage compressor run at the target speed for t2 time, it judges whether to enter the energy-saving control mode; when the deviation △T between the actual temperature inside the box and the set temperature < 4°C, it enters the energy-saving mode judgment, otherwise it continues to run at the target speed; After entering the energy-saving control mode, the high-temperature stage compressor uses the temperature Tex of the intermediate heat exchanger as the control condition, and the low-temperature stage compressor uses the temperature inside the box as the control condition; for different set temperatures inside the box, the intermediate heat exchanger is adjusted to the optimal temperature range; When the set temperature T 设 When the temperature is ≥-68℃, the optimal temperature range of the intermediate heat exchanger Tex is T1~T2; When the set temperature is -78℃ <T 设 When the temperature is less than -70℃, the optimal temperature range of the intermediate heat exchanger Tex is T2 to T3; When the set temperature is -82℃ <T 设 When the temperature is less than -78℃, the optimal temperature range of the intermediate heat exchanger Tex is T3~T4; When the set temperature T 设 When the temperature is ≤-82℃, the optimal temperature range of the intermediate heat exchanger Tex is T4~T5; T1 means -28℃, T2 means -32℃, T3 means -36℃, T4 means -38℃, and T5 means -40℃; The starting value of the optimal temperature range does not include the endpoint value, and the ending value includes the endpoint value; After entering the energy-saving mode, the high-temperature compressor adjusts its speed every t3 time. If the temperature is higher than the optimal temperature range, the speed increases by △F; if the temperature is lower than the optimal temperature range, the speed decreases by △F; within the optimal temperature range, the speed remains unchanged; After entering the energy-saving mode, the low-temperature compressor adjusts its speed every t4 time; the low-temperature compressor speed adjustment time t4 is less than the high-temperature compressor adjustment time t3, to avoid the high-temperature compressor and the low-temperature compressor adjusting their speeds at the same time, causing the temperature T in the box to be out of balance; when △T>2℃, the speed increases by △F; when -2℃≤△T≤2℃, the speed remains unchanged; when △T<-2℃, the speed decreases by △F.

2. The ultra-low temperature storage box based on the variable frequency cascade refrigeration system according to claim 1, characterized in that: The condenser (2) is divided into two parts, one part is a high-temperature stage condenser, and the other part is a low-temperature stage precooler; After being compressed, the refrigerant in the high-temperature compressor (1) becomes a high-temperature and high-pressure gas, passes through the anti-condensation pipe (15), and enters the condenser (2) to be cooled into a high-temperature and high-pressure liquid; after being condensed by the high-temperature condenser of the condenser (2), the refrigerant is throttled by the high-temperature capillary tube (5) to become a low-temperature and low-pressure refrigerant gas-liquid two-phase mixture, i.e., the high-temperature refrigerant, and then enters the intermediate heat exchanger (6) to cool the refrigerant in the low-temperature compressor (8); The refrigerant absorbs heat and evaporates into gaseous state, passes through the gas-liquid separator (7), and then returns to the high-temperature stage compressor (1); After being compressed, the refrigerant in the low-temperature compressor (8) becomes a high-temperature and high-pressure gas, which is first pre-cooled to a certain temperature by the low-temperature pre-cooler of the condenser (2), and then enters the intermediate heat exchanger (6) through the oil separator (9); the refrigerant entering the intermediate heat exchanger (6) is cooled by the high-temperature refrigerant to form a low-temperature refrigerant, and then throttled by the low-temperature capillary (11) to become a low-temperature and low-pressure refrigerant gas-liquid two-phase mixture, and then enters the evaporator (12) for evaporation and cooling, and finally returns to the low-temperature compressor (8) in the form of gas, and the expansion tank (14) connected to the low-temperature compressor (8) causes pressure fluctuations.

3. The ultra-low temperature storage box based on the variable frequency cascade refrigeration system according to claim 1, characterized in that: The intermediate heat exchanger (6) is a shell-and-tube heat exchanger or a plate heat exchanger.

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