A domestic deep freeze dryer based on two-stage compound refrigeration and a temperature control method thereof

Abstract

The application relates to the technical field of freeze-drying equipment, and particularly discloses a household deep-freezing freeze-drying machine based on two-stage composite refrigeration and a temperature control method, which comprises a refrigeration system, a drying cavity, a vacuum system and a temperature control system. The refrigeration system adopts a two-stage composite refrigeration closed cycle in series connection of an R600a low-pressure stage circuit and a CO2 transcritical high-pressure stage circuit, the high-pressure stage compressor has a working pressure of 8-15 MPa, and can stably output deep-freezing refrigeration capacity. The drying cavity is provided with a heat insulation structure composed of vacuum heat insulation and aerogel, and has low refrigeration capacity loss. The temperature control system comprises a three-stage temperature sensor array and a self-adaptive PID controller, can collect the surface temperature of a cold trap, the temperature of a material tray and the ambient compensation temperature, and can execute stepwise temperature reduction and dynamic heat compensation control in combination with the vacuum degree of the cavity, so that the defects of insufficient deep-freezing capacity and low temperature control precision of traditional household freeze-drying machines are broken through, stable deep-freezing at-70 DEG C and below and high-precision temperature control within a range of plus or minus 1 DEG C are realized, the freeze-drying quality and system energy efficiency are improved, and the household scene demand is met.

Description

Technical Field

[0001] This invention relates to the field of freeze-drying equipment technology, specifically to a household deep freeze dryer based on two-stage composite refrigeration. Background Technology

[0002] Household freeze dryers, as a type of food preservation and sample processing equipment that can retain the nutritional components, flavor, and form of materials to the greatest extent, have been widely used in home life, small laboratories, and other scenarios in recent years. Their core principle is to use vacuum freeze drying technology to allow the moisture in the material to directly sublimate from solid to gaseous in a low-temperature vacuum environment, avoiding the loss of nutrients and damage to the form of the material caused by high-temperature heating.

[0003] With the increasing demand for home freeze-drying, users are placing higher demands on the quality of freeze-dried materials, especially for heat-sensitive fruits and vegetables, biological samples, etc. Equipment needs to provide a cryogenic environment of -70℃ and below to reduce damage to the cell structure of the materials by ice crystals and improve the rehydration and nutrient retention rate of the freeze-dried materials. However, existing home freeze dryers generally use a single-stage vapor compression refrigeration cycle, with the refrigerant mostly being a single working fluid. Limited by the thermodynamic characteristics of single-stage refrigeration, its minimum refrigeration temperature can usually only reach -40℃ to -50℃, making it difficult to meet cryogenic requirements. Furthermore, when pursuing lower temperatures, the compressor compression ratio of a single-stage refrigeration system increases significantly, leading to a significant decrease in system energy efficiency, poorer operational stability, and long-term high-load operation can easily cause compressor wear, shortening the equipment's lifespan.

[0004] In terms of thermal insulation performance, traditional household freeze dryers often use a single polyurethane insulation layer or a simple vacuum sandwich structure for the drying chamber, resulting in low thermal insulation efficiency and significant loss of cooling capacity. This not only further exacerbates the energy consumption burden of the refrigeration system but also easily leads to uneven temperature distribution inside the chamber, causing excessive local temperature deviations, which in turn affects the uniformity of material freeze-drying and causes defects such as incomplete freeze-drying and shape collapse in some materials.

[0005] The temperature control system is the core unit for ensuring the freeze-drying effect. Most existing household freeze dryers use single-point temperature sensors to collect the temperature of the cavity or material surface. The feedback dimension is limited and cannot comprehensively reflect the synergistic relationship between the cold trap's cooling status, the actual material temperature, and changes in ambient temperature, resulting in a lack of comprehensiveness and accuracy in control decisions. Furthermore, their control methods are mostly fixed-parameter PID control or simple on / off control, unable to adaptively adjust control parameters based on the magnitude of temperature deviation, the rate of deviation change, and vacuum fluctuations, resulting in low temperature control accuracy (typically ±3~5℃) and significant temperature regulation lag. In addition, existing equipment lacks coordinated control logic between the refrigeration system, vacuum system, and heating unit, making it difficult to achieve coordinated adjustment of stepped cooling, dynamic heating, and vacuum stability. This fails to adapt to the differentiated needs of each stage of the freeze-drying process, further affecting the quality of the freeze-dried material and the system's operating efficiency.

[0006] The stability of the vacuum system is also a key factor limiting the performance of household freeze dryers. Most existing equipment only has a basic vacuum pump and lacks a dedicated pressure-stabilizing and buffering structure. During the material sublimation stage, the chamber pressure is prone to drastic changes with fluctuations in the vacuum pump's pumping rate and the rate of gas production during material sublimation, making it impossible to maintain a stable vacuum range. This vacuum instability directly leads to fluctuations in the material sublimation rate, not only prolonging the freeze-drying cycle but also potentially causing problems such as surface hardening and internal moisture residue, severely impacting the freeze-drying effect.

[0007] In summary, existing household freeze dryers have significant shortcomings in terms of deep freezing capacity, heat insulation performance, temperature control accuracy, vacuum stability, and system synergy, making it difficult to meet users' actual needs for high-quality deep freeze drying. Therefore, developing a household deep freeze dryer with strong deep freezing capacity, high heat insulation efficiency, precise temperature control, stable vacuum, and efficient synergy among all systems has become an important development direction for the industry. Summary of the Invention

[0008] The purpose of this invention is to provide a household deep cryogenic dryer based on two-stage composite refrigeration and a temperature control method. It aims to solve the technical problems of insufficient deep cryogenic capacity, low temperature control accuracy, poor vacuum stability, low system energy efficiency, and poor coordination among units in existing household freeze dryers. By matching the working fluid and pressure of the refrigeration system, optimizing the composite insulation structure, coordinating multiple dimensions of sensing and control, and designing a pressure-stabilizing vacuum system, a stable deep cryogenic environment of -70℃ and below can be achieved in household scenarios. At the same time, the temperature control accuracy, vacuum stability, and system energy efficiency are improved, ensuring the quality of freeze-dried materials.

[0009] The household freeze dryer of this invention includes a refrigeration system, a drying chamber, a vacuum system, and a temperature control system. These systems are coupled and work together to form a complete freeze-drying technology solution. The refrigeration system provides the cooling capacity for the entire device, employing a low-pressure stage circuit using R600a refrigerant and a high-pressure stage circuit using a CO2 transcritical cycle. The low-pressure stage circuit and the high-pressure stage circuit are connected in series to form a two-stage composite refrigeration closed loop. The operating pressure of the low-pressure stage compressor in the low-pressure stage circuit is set to 0.3–0.8 MPa, and the operating pressure of the high-pressure stage compressor in the high-pressure stage circuit is set to 8–15 MPa. The matching design of this working fluid and pressure is not a conventional choice in this field, but is specifically optimized for the -70℃ cryogenic demand and system energy efficiency requirements of household deep freeze-drying: the thermophysical properties of R600a refrigerant are adapted to medium and low temperature compression refrigeration, and the working pressure of 0.3~0.8MPa can ensure the efficient operation of the low-pressure stage compressor, provide a stable intermediate cooling temperature for the high-pressure stage circuit, and avoid the efficiency drop and overload operation caused by the high-pressure stage compressor suction overheating; CO2 refrigerant has excellent cryogenic performance under transcritical cycle, and the working pressure of 8~15MPa is the key range for CO2 to achieve transcritical cycle and stably output -70℃ and below cooling capacity. If the pressure is below 8MPa, the target cryogenic temperature cannot be reached, and if the pressure is above 15MPa, the system pipeline will be under excessive pressure, reducing the operational safety and drastically increasing the compressor energy consumption. The series structure of the two-stage loop enables the gradual transfer of cooling capacity. Combined with precise pressure matching, the refrigeration system can meet the needs of cryogenic cooling while taking into account operating efficiency and stability. It breaks through the performance bottleneck of single-stage refrigeration systems in the cryogenic range. Compared with existing single-stage refrigeration household freeze dryers, the cooling capacity and energy efficiency are significantly improved.

[0010] The drying chamber is the core area for freeze-drying materials. It features a composite insulation structure made of vacuum insulation and aerogel. This composite insulation structure combines the advantages of both vacuum insulation and aerogel insulation, significantly improving insulation efficiency and reducing cold loss compared to existing single polyurethane insulation or simple vacuum interlayer insulation structures. The vacuum insulation layer effectively suppresses cold loss caused by air convection and conduction heat transfer, while the nano-aerogel felt has extremely low thermal conductivity, maintaining excellent insulation performance even under fluctuating vacuum conditions. This combined composite structure achieves efficient insulation within the chamber wall thickness required for miniaturized household equipment, reducing the cold energy consumption of the refrigeration system, improving overall energy efficiency, and ensuring uniform temperature distribution within the drying chamber. This avoids uneven freeze-drying results caused by localized temperature deviations, providing structural support for maintaining a stable cryogenic environment.

[0011] The thermal insulation structure specifically includes a nano-aerogel felt wrapped around the wall of the drying chamber and a metal foil layer wrapped around the outside of the nano-aerogel felt. The nano-aerogel felt serves as the main thermal insulation layer and undertakes the main thermal insulation function. The metal foil layer can not only reflect radiative heat transfer, further reducing the heat exchange between the chamber and the outside, but also play a role in shaping and protecting the nano-aerogel felt, preventing the aerogel felt from being damaged or falling off during equipment transportation and use, ensuring the long-term stability and thermal insulation effect of the thermal insulation structure, and solving the problems of easy damage and large radiative heat transfer loss of a single aerogel layer.

[0012] The vacuum system is used to provide and maintain the vacuum environment required for freeze-drying in the drying chamber. It includes a rotary vane pump and a pressure stabilizing line. The rotary vane pump, as the vacuum power source, has a continuous and stable pumping capacity, which can continuously extract the gas and water vapor generated by the sublimation of materials inside the drying chamber, providing the basic pumping capacity for the chamber to reach a high vacuum range of 10-50 Pa. The pressure stabilizing line works in conjunction with the rotary vane pump to buffer the airflow and balance the internal pressure of the drying chamber. During the material sublimation stage, the material will continuously generate water vapor, which can easily cause the chamber pressure to rise instantaneously. The pumping rate of the rotary vane pump also fluctuates slightly. The pressure stabilizing line can effectively suppress the pressure fluctuations caused by these factors and avoid sudden increases or decreases in vacuum. The rotary vane pump is responsible for pumping the vacuum level in the chamber to the target range, while the pressure stabilizing pipeline is responsible for maintaining the stability of the vacuum level. The synergistic cooperation between the two enables the vacuum level in the drying chamber to be stably maintained within the range of 10 to 50 Pa during the material sublimation stage. Compared with existing vacuum systems without a pressure stabilizing structure, the vacuum stability is greatly improved. A stable vacuum environment is the key to ensuring a uniform sublimation rate of the material and achieving high-quality freeze drying. At the same time, it avoids frequent start-ups and shutdowns of the refrigeration system and vacuum system due to vacuum fluctuations, further improving the system operating efficiency.

[0013] The temperature control system is used to achieve precise control of the drying chamber temperature and coordinated scheduling of various systems. It includes a three-level temperature sensor array and an adaptive PID controller. The three-level temperature sensor array collects the surface temperature of the cold trap, the temperature of the material tray, and the environmental compensation temperature. Compared to existing single-point temperature acquisition methods, three-level temperature measurement achieves multi-dimensional temperature sensing: the cold trap surface temperature reflects the cooling output status of the refrigeration system; the material tray temperature directly reflects the actual temperature of the freeze-dried material; and the environmental compensation temperature eliminates the interference of ambient temperature changes on the chamber temperature. These three dimensions of temperature data provide comprehensive and accurate feedback for control decisions, solving the problems of single-point temperature measurement feedback being limited in scope, susceptible to interference, and exhibiting control lag. The temperature control system is configured to execute stepped cooling control and dynamic heat replenishment control based on the collected temperature signals and the vacuum level of the drying chamber. By incorporating the vacuum level into the control input parameters, it achieves linked control of temperature and vacuum, aligning with the highly coupled characteristics of temperature and vacuum in the freeze-drying process, making the control strategy more suitable for the actual needs of the freeze-drying process.

[0014] The two-stage compound refrigeration closed loop consists of a low-pressure stage compressor, an intercooler, a high-pressure stage compressor, a condenser, a throttling device, and an evaporator connected sequentially via piping. The evaporator is attached to the outside of the drying chamber and forms a tight thermal coupling with it, continuously absorbing heat from inside the drying chamber through heat conduction. This structural design achieves efficient transfer of cooling capacity from the refrigeration system to the drying chamber. The tight thermal coupling between the evaporator and the drying chamber increases the heat exchange area and improves heat conduction efficiency, allowing the deep-cold cooling capacity generated by the refrigeration system to be quickly and evenly transferred to the inside of the drying chamber, rapidly reducing the chamber temperature. This also avoids unnecessary losses of cooling capacity during transfer, further improving the utilization efficiency of the refrigeration system. This provides structural support for achieving deep-cold environments of -70℃ and below. Compared to a built-in evaporator design, this external structure also facilitates equipment maintenance and cleaning, making it more suitable for the needs of household appliances.

[0015] The adaptive PID controller is the core control unit of the temperature control system, executing two control steps, A1 and A2. Step A1 involves data acquisition and processing. The adaptive PID controller acquires the cold trap surface temperature T1, material tray temperature T2, and ambient compensation temperature T3 from the three-stage temperature sensors in real time, calculates the temperature difference ΔT = T_SV - T2 and ΔT1 = T1 - T3, and acquires the vacuum level of the drying chamber in real time. By calculating the temperature difference, the deviation between the actual temperature and the set temperature, as well as the temperature deviation between the cold trap and the environment, is quantified, providing a quantitative basis for subsequent parameter adjustment and control output. Step A2 involves parameter adjustment and control output, where the adaptive PID controller adjusts the parameters based on the temperature difference ΔT, ΔT1, and vacuum level. The system dynamically adjusts PID control parameters and, based on these adjusted parameters, controls the operating frequency of the low-pressure stage compressor, the power of the supplementary heating element, the frequency of the vacuum pump, and the output power of the fan in real time. This control logic achieves coordinated control of multiple parameter inputs and multiple actuator outputs. It combines temperature deviation, deviation change trends, and vacuum level as the basis for PID parameter adjustment, enabling the PID control parameters to adaptively adjust according to the system's operating status. Compared to fixed-parameter PID control, this significantly improves control accuracy and system response speed. Furthermore, through coordinated control of the low-pressure stage compressor, supplementary heating element, vacuum pump, and fan, it achieves global linkage of refrigeration, supplementary heating, vacuum, and heat dissipation, ensuring a high degree of matching between the operating status of each system and the requirements of the freeze-drying process.

[0016] Furthermore, the adaptive PID controller employs a segmented adjustment strategy for dynamically adjusting PID control parameters. When ΔT > 5℃, the PID proportional coefficient P is increased. Increasing the proportional coefficient P improves the system's response speed, rapidly reduces temperature deviation, and quickly brings the material temperature closer to the set temperature, suitable for the rapid cooling process in the pre-freezing stage. When ΔT < 1℃, the PID integral coefficient I is increased. Increasing the integral coefficient I effectively reduces the system's steady-state error, ensuring the material temperature is precisely stabilized near the set temperature, improving temperature control accuracy, suitable for the deep cryogenic steady-state operation stage. When the change in ΔT1, Δ(ΔT1), exceeds the preset threshold, the PID derivative coefficient D is increased. Increasing the derivative coefficient D can suppress temperature fluctuations in advance, enhancing system stability and preventing large fluctuations in material temperature caused by sudden changes in cold trap temperature or ambient temperature. This segmented adaptive PID parameter adjustment strategy uses different adjustment methods for different temperature deviation states, balancing system response speed, temperature control accuracy, and operational stability. Compared to conventional single PID parameter adjustment, it is more suitable for the temperature control requirements of different stages in the freeze-drying process.

[0017] In the real-time control stage, the adaptive PID controller adjusts the low-pressure stage compressor frequency MV1, the supplementary heating tube power MV2, the vacuum pump frequency MV3, and the fan output power MV4 based on the adjusted PID parameters, so that the temperature difference ΔT approaches 0, ensuring that the material temperature accurately tracks the set temperature and the vacuum degree. Maintaining a pressure of 10–50 Pa ensures a stable vacuum environment required for freeze drying. The fan output power MV4 is proportional to the magnitude of ΔT1. When the temperature difference between the cold trap and the environment ΔT1 increases, the fan output power increases synchronously to accelerate the heat dissipation efficiency of the cold trap and prevent excessive frost buildup in the cold trap from affecting the cooling output. When ΔT1 decreases, the fan output power decreases synchronously to reduce the fan's energy consumption and achieve dynamic matching between heat dissipation and cooling output. This ensures the efficiency of the refrigeration system while reducing the energy consumption of auxiliary equipment.

[0018] Based on the aforementioned household deep freeze dryer, this invention also proposes a corresponding temperature control method applied to the aforementioned household deep freeze dryer. Through a phased control strategy, it achieves coordinated temperature and vacuum control throughout the freeze-drying process, ensuring freeze-drying effect and system operating efficiency. This method includes four steps, S1 to S4. S1 is the system startup and data acquisition step, which starts the refrigeration system and vacuum system. A three-stage temperature sensor array collects the cold trap surface temperature T1, material tray temperature T2, and environmental compensation temperature T3 in real time, while simultaneously acquiring the vacuum level of the drying chamber in real time. The adaptive PID controller receives the temperature and vacuum signals and calculates the temperature difference ΔT = T_SV - T2 and ΔT1 = T1 - T3. This step provides real-time and accurate quantitative data for subsequent control decisions, ensuring the pertinence and effectiveness of the control strategy. At the same time, it enables the synchronous start-up of the refrigeration system and the vacuum system, preparing for the freeze-drying process.

[0019] S2 represents the stepped cooling control step in the pre-freezing stage. Upon entering the pre-freezing stage, the temperature control system executes stepped cooling control: the adaptive PID controller adjusts the operating frequency of the low-pressure stage compressor and the high-pressure stage compressor based on ΔT. First, it lowers the drying chamber temperature to the preset temperature at a preset rapid cooling rate, then switches to a preset slow cooling rate to continue cooling to -70℃ and maintain stability. Stepped cooling is a key design feature adapted to the freezing characteristics of materials. Rapid cooling allows materials to quickly pass through the maximum ice crystal formation zone, reducing damage to the material's cellular structure from ice crystals. Slow cooling ensures that the internal temperature of the material drops uniformly to -70℃, avoiding excessive temperature differences between the material's surface and interior, which can lead to problems such as freezing cracks and poor rehydration after freeze-drying. Compared to existing single-rate cooling methods, this better ensures the quality of the freeze-dried material. Simultaneously, by adjusting the operating frequencies of the high- and low-pressure stage compressors, precise control of the cooling output is achieved, providing a guarantee of cooling capacity for the stepped cooling process.

[0020] S3 is the vacuum and heat replenishment coordinated control step in the sublimation stage. Upon entering the sublimation stage, the vacuum system maintains the vacuum level in the drying chamber within the range of 10–50 Pa through a rotary vane pump and pressure equalization via a pressure stabilizing pipeline. The adaptive PID controller adjusts the vacuum level based on ΔT, ΔT1, and the vacuum level. The system dynamically adjusts the power of the supplementary heating tubes and the output power of the fan. The sublimation stage is the core stage of material freeze-drying. Ice crystals in the material directly sublimate into water vapor under vacuum. This process requires heat absorption. The dynamically adjusted heating tubes provide the necessary heat for sublimation, preventing insufficient heat from causing a slow sublimation rate. Simultaneously, the supplementary heating power is dynamically adjusted based on temperature deviation and vacuum level to prevent excessive heating from causing the material temperature to rise and the ice crystals to melt. The dynamic adjustment of the fan output power ensures the heat dissipation efficiency of the cold trap, ensuring that the water vapor generated during sublimation can quickly condense in the cold trap, maintaining vacuum stability within the cavity. This step achieves a high degree of synergy between vacuum stability and dynamic supplementary heating, ensuring the continuous and stable sublimation process.

[0021] S4 represents the coordinated control step during the steady-state operation phase. During this phase, the adaptive PID controller continuously adjusts the PID parameters, compressor load, heating power, and fan power, coordinating with the refrigeration and vacuum systems to maintain the drying chamber in a stable operating state. In the steady-state operation phase of the freeze-drying process, a cryogenic environment of -70℃ and a vacuum environment of 10–50 Pa need to be maintained stably over a long period. This step, through continuous adjustment by the adaptive PID controller, compensates in real-time for interference caused by factors such as ambient temperature and material state, ensuring that the operating state of each system remains optimal. Simultaneously, it achieves global coordination of refrigeration, heating, vacuum, and heat dissipation, maximizing system energy efficiency and reducing energy consumption in household appliances while ensuring the freeze-drying effect.

[0022] In this temperature control method, the preset rapid cooling rate during the pre-freezing stage is -2℃ / min, the preset temperature is -40℃, and the preset slow cooling rate is -0.5℃ / min. This parameter matching was obtained through extensive testing and optimization of the cavity structure, refrigeration capacity, and freezing characteristics of common materials (such as fruits, vegetables, meat, and small biological samples) of the household freeze dryer. It can ensure the freezing quality of the materials while taking into account the cooling efficiency, and avoid system overload caused by excessively fast cooling rate or excessively slow freezing cycle caused by excessively slow cooling rate.

[0023] The power adjustment range of the supplementary heating tube in the sublimation stage is 50-300W. This adjustment range can cover the heat requirements of different materials and different sublimation stages, realize the fine adjustment of supplementary heating power, and avoid insufficient or excessive supplementary heating. At the same time, the output power of the fan is proportional to the size of ΔT1, realize the dynamic matching of heat dissipation efficiency, and ensure the efficient operation of the refrigeration system.

[0024] When operating at a steady state of -70℃, the freeze dryer has a temperature control accuracy of ±1℃ and an energy efficiency coefficient (COP) of ≥2.8. Compared with existing household freeze dryers, the temperature control accuracy is significantly improved, ensuring that the material temperature remains stable near the set cryogenic temperature. The stable vacuum degree combined with high-precision temperature control further improves the quality of freeze-dried materials. An energy efficiency coefficient (COP) of ≥2.8 means that the system's refrigeration efficiency is greatly improved, reducing the operating energy consumption of household equipment and better meeting the energy-saving requirements of household equipment.

[0025] This invention relates to a home-use deep freeze dryer and its temperature control method. Through the coordinated optimization design of the refrigeration system, drying chamber, vacuum system, and temperature control system, it achieves the technical goals of stable deep freezing at -70℃ and below, high-precision temperature control of ±1℃, stable vacuum of 10~50Pa, and high-efficiency operation with COP≥2.8 in home scenarios. The various systems are coupled and work together, breaking through the performance bottleneck of existing home-use freeze dryers. The working fluid and pressure matching design of the two-stage composite refrigeration system provides core cold energy guarantee for deep freezing. The heat insulation structure of vacuum insulation and aerogel composite reduces cold energy loss. The adaptive PID control combining three-stage temperature sensing and vacuum degree achieves high-precision, multi-dimensional coordinated control. The vacuum system of rotary vane pump and pressure stabilizing pipeline ensures the stable vacuum environment required for freeze drying. The staged temperature control method is highly compatible with the freeze drying process, ensuring the quality of the freeze-dried materials. Compared with existing household freeze dryers, this invention has achieved significant improvements in deep freezing capability, temperature control accuracy, vacuum stability and system energy efficiency. At the same time, the structural design is adapted to the miniaturization, ease of maintenance and low energy consumption requirements of household equipment, and has good practicality and promotion value.

[0026] Compared with the prior art, the present invention has the following advantages and beneficial effects: (1) This invention uses a two-stage composite refrigeration closed loop with R600a low-pressure stage circuit and CO2 transcritical high-pressure stage circuit connected in series, and with precise pressure matching of 0.3~0.8MPa and 8~15MPa, it breaks through the low temperature bottleneck of traditional single-stage refrigeration, and can stably achieve a deep cryogenic environment of -70℃ and below, effectively reducing the damage of ice crystals to the cell tissue of materials. It is especially suitable for freeze-drying materials with strict low temperature requirements such as heat-sensitive fruits and vegetables and small biological samples, and greatly improves the rehydration, morphological integrity and nutrient retention rate of the freeze-dried materials.

[0027] (2) The drying chamber of the present invention adopts a composite heat insulation structure composed of nano aerogel felt and metal foil layer. The extremely low thermal conductivity of nano aerogel felt combined with the radiative heat barrier effect of metal foil layer, combined with vacuum insulation characteristics, greatly reduces cold loss. Compared with traditional single heat insulation structure, the heat insulation efficiency is significantly improved. It not only reduces the energy consumption burden of refrigeration system, but also ensures the temperature uniformity inside the chamber, avoids uneven freeze drying of materials due to local temperature deviation, and at the same time helps the system energy efficiency coefficient COP≥2.8, which is suitable for household energy saving needs.

[0028] (3) The vacuum system of the present invention can maintain the vacuum level in the sublimation stage of the material stably within the range of 10 to 50 Pa through the synergistic effect of continuous pumping by the rotary vane pump and the pressure equalization buffer of the pressure stabilizing pipeline. This effectively suppresses the fluctuation of pumping rate and the sudden rise and fall of pressure caused by the sudden change of gas production during material sublimation. The stable vacuum environment ensures that the material sublimation rate is uniform, avoids defects such as surface hardening and internal moisture residue, and shortens the freeze-drying cycle, thereby improving the consistency of freeze-dried quality.

[0029] (4) The three-level temperature sensor array of the present invention comprehensively collects the surface temperature T1 of the cold trap, the temperature T2 of the material tray and the environmental compensation temperature T3, and provides multi-dimensional feedback for control decision-making in combination with the vacuum degree signal, thus solving the one-sided problem of traditional single-point temperature measurement; the adaptive PID controller adjusts the P / I / D parameters in segments, adjusts P when ΔT>5℃, adjusts I when ΔT<1℃, and adjusts D when ΔT1 changes abruptly, and accurately controls the four actuators such as the frequency of the low-pressure stage compressor and the power of the supplementary heating tube, so as to realize the precise linkage of stepped cooling and dynamic supplementary heating, so that the temperature control accuracy reaches ±1℃ when running at -70℃ steady state, which exceeds the accuracy level of traditional fixed parameter control.

[0030] (5) The temperature control method of the present invention is designed in stages according to pre-freezing, sublimation and steady-state operation. In the pre-freezing stage, the temperature is rapidly reduced to -40℃ at -2℃ / min and then slowly reduced to -70℃ at -0.5℃ / min. This not only quickly passes through the maximum ice crystal formation zone, but also ensures that the internal temperature of the material is uniform. In the sublimation stage, the heating power in the range of 50 to 300W is dynamically adjusted and the fan output power is proportional to ΔT1, providing stable heat supply and heat dissipation guarantee for the sublimation process. The entire process realizes the coordinated control of refrigeration, vacuum, heating and heat dissipation without manual intervention, which is suitable for the use scenarios of non-professionals in the home.

[0031] (6) The two-stage composite refrigeration cycle of the present invention is formed by connecting components such as low-pressure stage compressor and intercooler in sequence. The evaporator and drying chamber are tightly thermally coupled, and the cold energy transfer is efficient. The modular design of each system, the pressure stabilizing pipeline of the vacuum system and the adaptive logic of the temperature control system have improved the stability of the equipment operation. After long-term verification, it can achieve continuous fault-free operation, and is easy to maintain and has a long service life, fully meeting the actual needs of household deep freeze drying. Attached Figure Description

[0032] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings: Figure 1 This is a three-dimensional structural diagram of the household deep freeze dryer of the present invention; Figure 2 This is a cross-sectional schematic diagram of the deep freeze dryer of the present invention; Figure 3 For the present invention Figure 2 Enlarged structural diagram at point A in the diagram; Figure 4 This is a schematic diagram of the overall system structure of the household deep freeze dryer of the present invention; Figure 5 This is a schematic diagram of the refrigeration system of the present invention; Figure 6 This is a flowchart of the temperature control algorithm of the present invention.

[0033] The attached diagram shows the markings and corresponding component names: 1- Stainless steel cavity; 2- Multi-layer shelf; 3- Nano aerogel felt; 4- Vacuum insulation panel; 5- Aluminum foil reflective layer; 6- Galvanized steel plate; 7- Double-layer hollow tempered glass observation window. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments and accompanying drawings. The illustrative embodiments and descriptions of this invention are for illustrative purposes only and are not intended to limit the invention. It should be noted that this invention is already in the actual research and development stage.

[0035] Example 1 like Figure 1-5 As shown, a household deep freeze dryer based on two-stage composite refrigeration. This embodiment provides a household cryogenic dryer based on two-stage composite refrigeration. The whole machine includes four core units: refrigeration system, drying chamber, vacuum system, and temperature control system. Each unit works together through pipelines and control circuits to achieve the technical effects of cryogenic environment at -70℃ and below, stable vacuum of 10~50Pa, and high-precision temperature control of ±1℃.

[0036] Refrigeration system The refrigeration system is the core cooling capacity output unit of the equipment. It adopts a two-stage composite refrigeration closed-loop structure with an R600a low-pressure stage circuit and a CO2 transcritical high-pressure stage circuit connected in series. The low-pressure stage compressor is a fully enclosed rotary compressor with a rated operating pressure of 0.3-0.8 MPa and R600a refrigerant, which is suitable for compression refrigeration in the medium and low temperature range and provides a stable intermediate cooling temperature for the high-pressure stage circuit. The high-pressure stage compressor is a fully enclosed scroll compressor with a rated operating pressure of 8-15 MPa and CO2 refrigerant, which realizes deep cryogenic cooling capacity output under transcritical cycle.

[0037] The two-stage compound refrigeration closed loop consists of a low-pressure compressor, an intercooler, a high-pressure compressor, a condenser, a throttling device, and an evaporator connected sequentially via high-pressure resistant copper pipes. The specific operating process is as follows: Low-pressure stage circuit: The low-pressure compressor (rated working pressure 0.3~0.8MPa, refrigerant R600a) compresses the low-temperature and low-pressure R600a refrigerant gas and discharges it to the intercooler.

[0038] Intercooling: The high-temperature and high-pressure R600a gas enters the intercooler and exchanges heat with the low-temperature liquid refrigerant after throttling, and is cooled to the intermediate temperature. At the same time, some of the liquid refrigerant absorbs heat and evaporates to form saturated R600a gas, which continues to flow to the inlet of the high-pressure compressor, providing stable intercooling conditions for the high-pressure stage and avoiding overheating of the high-pressure stage intake.

[0039] High-pressure stage circuit: The high-pressure compressor (rated working pressure 8-15MPa, refrigerant CO2) receives saturated R600a gas from the intercooler and further compresses it to form high-temperature and high-pressure transcritical CO2 gas, which is discharged into the condenser.

[0040] Condensation and throttling: The high-temperature and high-pressure CO2 gas is cooled and condensed in the condenser, and then split into two paths: one part is throttled by the throttling device and enters the intercooler to provide cooling capacity; the other part is throttled again and enters the evaporator to absorb heat from the drying chamber.

[0041] Heat absorption and evaporation: The low-temperature, low-pressure refrigerant entering the evaporator absorbs heat from the drying chamber and evaporates, enabling the chamber to achieve deep cooling at -70°C and below. The evaporated refrigerant gas then flows back to the inlet of the low-pressure compressor, completing a closed loop.

[0042] The evaporator is attached to the outer wall of the drying chamber and tightly thermally coupled through a high-temperature vulcanized silicone pad, ensuring that the cooling capacity continuously and efficiently absorbs the heat inside the drying chamber through heat conduction, thus achieving efficient deep cooling.

[0043] Drying chamber The drying chamber is the core unit for freeze-drying materials. It is made of food-grade stainless steel and integrally stamped. The effective volume of the chamber is 10-20L, suitable for household freeze-drying needs. The chamber walls are equipped with a vacuum insulation and aerogel composite insulation structure, from the inside out: Stainless steel cavity 1: Main load-bearing structure, with built-in storage rack / material tray support; Nano aerogel felt 3: 20-30mm thick, thermal conductivity ≤0.018W / (m・K), wrapped around the entire circumference of the cavity wall, undertaking the main heat insulation function; Vacuum insulation panel 4: Sealed sandwich structure, the vacuum degree inside the sandwich is maintained below 1×10-2Pa, suppressing air convection and conduction heat transfer; Aluminum foil reflective layer 5: 0.1mm thick, wrapped around the outside of the aerogel felt, reflects radiative heat transfer, with a radiative heat blocking efficiency of ≥95%, and at the same time plays a role in shaping and protecting the aerogel layer; 6. Galvanized steel sheet: Equipment outer shell, providing overall protection for the internal heat insulation structure; Double-glazed tempered glass observation window 7: It is opened in the cavity door, with a vacuum drawn between the double-glazed glass and filled with desiccant. The glass thickness is 8mm and the hollow layer thickness is 10mm, which reduces heat leakage from the window and ensures uniform heat insulation of the cavity as a whole.

[0044] The cavity is equipped with multiple shelves 2: used to place material trays and support materials to be freeze-dried, meeting the freeze-drying needs of household materials in multiple batches and of multiple categories.

[0045] Vacuum system The vacuum system provides the high vacuum environment required for freeze drying in the drying chamber, including a rotary vane pump, a pressure regulating line, a vacuum sensor, and a drain / pressure relief valve. The rotary vane pump is a small oil-sealed rotary vane vacuum pump with a rated pumping rate of 2L / s. It can continuously extract air and water vapor generated by material sublimation inside the drying chamber, providing basic pumping capacity for the chamber to reach a vacuum range of 10-50Pa. A pressure stabilizing pipeline is connected in series between the rotary vane pump and the drying chamber, including a buffer chamber, a throttle valve, and a bellows. The buffer chamber has a volume of 1L and can buffer airflow pulsations during the pumping process. The throttle valve can adjust the airflow rate, and the bellows absorbs pipeline vibration. The three work together to achieve airflow buffering and pressure balance, avoiding sudden increases / decreases in chamber pressure caused by sudden changes in the rate of gas generation from material sublimation and fluctuations in the pumping rate of the rotary vane pump. The vacuum sensor is a capacitive vacuum gauge, installed on the top of the drying chamber, with a measurement range of 0-1000Pa and a measurement accuracy of ±1Pa. It collects the chamber vacuum level in real time and transmits it to the temperature control system. The drain / pressure relief valve is a connection module at the bottom of the chamber, used to release pressure and drain condensate after freeze-drying, facilitating equipment maintenance and cleaning.

[0046] Temperature control system The temperature control system is the core of the equipment's intelligent control, including a three-level temperature sensor array, an adaptive PID controller, and an actuator drive module, which realizes multi-dimensional temperature acquisition, adaptive parameter adjustment, and coordinated control of various actuators.

[0047] All three temperature sensor arrays use Pt100 platinum resistance temperature sensors with a measurement accuracy of ±0.1℃. The parameters T1, T2, and T3 are as follows: T1: Cold trap surface temperature sensor, attached to the surface of the evaporator cold trap, to collect the cold trap temperature with an accuracy of ±0.1℃; T2: Material tray temperature sensor, embedded in the material tray inside the drying chamber, to collect the actual temperature of the material with an accuracy of ±0.1℃; T3: Environmentally compensated temperature sensor, installed inside the equipment housing, to collect the ambient temperature around the equipment with an accuracy of ±0.1℃; All three sensors collect temperature signals at a sampling frequency of 1Hz and transmit them to the adaptive PID controller.

[0048] like Figure 6As shown, the adaptive PID controller uses an industrial-grade microcontroller with a built-in dedicated control algorithm and executes the following control logic: Data acquisition and calculation: The controller receives the T1, T2, and T3 signals from the three-level temperature sensors and the P_e signal from the vacuum sensor in real time, and automatically calculates the temperature difference ΔT=T_SV-T2 (T_SV is the set target temperature, default -70℃) and ΔT1=T1-T3, with a sampling period of 1 second; Adaptive PID parameter adjustment: The PID control parameters are dynamically adjusted based on the real-time values ​​of ΔT and ΔT1. When ΔT > 5℃, increase the PID proportional coefficient P, P = P + ΔP, ΔP > 0, to improve the system response speed and quickly pull back the temperature deviation. When ΔT < 1℃, increase the PID integral coefficient I, I = I + ΔI, ΔI > 0, reduce steady-state error, and ensure temperature control accuracy; When the change in ΔT1, Δ(ΔT1), is greater than the preset threshold, the PID differential coefficient D is increased, D=D+ΔD, ΔD>0, to suppress temperature fluctuations in advance and enhance system stability. Multi-actuator coordinated control: Based on the adjusted PID parameters, the controller outputs four control quantities MV1-MV4: MV1: Low-pressure stage compressor frequency, 20~60Hz. MV1 output is dynamically adjusted by ΔT to make ΔT approach 0. MV2: Power of supplementary heating tube, 50~300W, MV2 output, supplementary heating starts when ΔT>2℃, power is dynamically adjusted; MV3: Vacuum pump frequency, 10–50 Hz. MV3 output depends on the cavity vacuum level. Adjust the vacuum level to maintain it within the range of 10–50 Pa. MV4: Fan output power. The output of MV4 is proportional to the size of ΔT1. The larger ΔT1 is, the higher the output power, thus achieving dynamic matching between heat dissipation efficiency and cooling output. Closed-loop feedback control: Based on the judgment logic of "|ΔT|≤1?", the controller collects the temperature and vacuum signals after the actuator adjustment in real time, and executes the above steps in a loop to form closed-loop control until ΔT approaches 0 and the vacuum stabilizes at 10~50Pa, meeting the steady-state operation requirements.

[0049] The supplementary heating element is a stainless steel electric heating element, installed on the inner side wall of the drying chamber, with a rated power of 300W, achieving stepless power adjustment from 0 to 300W; the fan is a small axial flow fan, installed on the outside of the condenser, to dissipate heat for the refrigeration system, achieving stepless adjustment of output power; the actuator drive module is the connection unit between the controller and each actuator, realizing the amplification and conversion of control signals, ensuring the accuracy of actuator action.

[0050] Example 2 like Figure 6 As shown, a temperature control method for a household deep freeze dryer based on two-stage composite refrigeration is presented. This embodiment provides a temperature control method for a household deep freeze dryer based on two-stage composite refrigeration, applied to the household deep freeze dryer described in Embodiment 1. This method formulates a phased control strategy according to the different requirements of the pre-freezing stage, sublimation stage, and steady-state operation stage of the freeze-drying process, achieving global coordination of temperature, vacuum, heat replenishment, and heat dissipation. Specifically, it includes the following steps: S1. System Startup and Data Acquisition The refrigeration and vacuum systems are activated. A three-stage temperature sensor array collects real-time data on the cold trap surface temperature T1, material tray temperature T2, and ambient compensation temperature T3 at a sampling frequency of 1Hz. The vacuum sensor obtains the real-time vacuum level of the drying chamber at a sampling frequency of 1Hz. The temperature and vacuum signals mentioned above are transmitted to the adaptive PID controller via the 485 communication protocol. After receiving the signals, the controller automatically calculates the temperature difference ΔT=T_SV-T2 (with the target temperature T_SV=-70℃) and ΔT1=T1-T3, providing a quantitative basis for subsequent control decisions. In this step, the refrigeration system and the vacuum system are started synchronously. The rotary vane pump first runs at full load to evacuate the vacuum of the drying chamber to below 50Pa, preparing for the pre-freezing stage.

[0051] S2, Pre-freezing stage - stepped cooling control During the pre-freezing stage, the temperature control system implements a stepped cooling control strategy. The adaptive PID controller adjusts the operating frequency of the low-pressure stage compressor (MV1) and the high-pressure stage compressor based on the real-time value of ΔT to control the cooling capacity output. Rapid cooling stage: According to the parameter adjustment logic of "ΔT>5?→True", the controller controls the high and low pressure stage compressors to operate at 80%~100% load, and at a preset rapid cooling rate of -2℃ / min, the internal temperature of the drying chamber is reduced from room temperature to -40℃ (preset temperature). During this stage, ΔT>5℃, the controller increases the PID proportional coefficient P to quickly reduce the temperature deviation, so that the material can quickly pass through the maximum ice crystal formation zone and reduce the damage of ice crystals to the material cell structure. Slow cooling stage: According to the parameter adjustment logic of "ΔT<1?→True", when the cavity temperature drops to -40℃, the controller automatically switches the cooling rate and controls the high and low pressure stage compressors to operate at 30%~50% load, with a preset slow cooling rate of -0.5℃ / min, to continue to reduce the cavity temperature from -40℃ to -70℃ and keep the temperature stable. During this stage, ΔT<1℃, the controller increases the PID integral coefficient I to reduce steady-state error and ensure that the cavity temperature drops to the target cryogenic temperature evenly. This avoids problems such as freezing cracks and poor rehydration caused by excessively rapid cooling due to large temperature differences between the material surface and the interior.

[0052] S3, Sublimation Stage - Vacuum and Heat Refill Coordinated Control During the sublimation stage, the vacuum system continuously pumps air through a rotary vane pump and uses a pressure stabilizing pipeline to buffer and equalize the pressure, maintaining the vacuum level of the drying chamber stably within the range of 10–50 Pa. This provides a stable vacuum environment for the sublimation of ice crystals in the material. According to the control logic of MV3, the controller adjusts the vacuum level accordingly. Adjust the vacuum pump frequency to maintain the vacuum level within the target range; The adaptive PID controller is based on ΔT, ΔT1, and vacuum degree. Based on real-time values, dynamically adjust the power of the supplementary heating element (MV2) and the output power of the fan (MV4): Reheating control: According to the MV2 output logic of "ΔT>2?→True", when ΔT>2℃, the controller starts reheating and adjusts the power of the reheating tube in real time according to the material sublimation rate. The adjustment range is 50~300W, which provides the latent heat required for material sublimation, avoids the sublimation rate being too slow due to insufficient heat, and prevents excessive reheating from causing the material temperature to rise and the ice crystals to melt, ensuring that the sublimation process continues and is stable. Heat dissipation control: The controller controls the fan output power MV4, which is proportional to the magnitude of ΔT1. The larger ΔT1 is, the higher the fan output power. By adjusting the fan speed, the heat dissipation efficiency of the condenser in the refrigeration system is ensured, the condensation effect on the surface of the evaporator cold trap is ensured, and the water vapor generated by the sublimation of materials is captured in time to maintain the stability of the cavity vacuum.

[0053] S4. Steady-state operation phase - global coordinated regulation Entering the steady-state operation stage, the freeze-drying process enters the continuous sublimation stage, requiring long-term stability in a -70℃ cryogenic environment and a 10-50Pa vacuum environment. The adaptive PID controller executes a continuous coordinated adjustment strategy. The controller is based on ΔT, ΔT1 and The system continuously and dynamically adjusts the PID control parameters (P / I / D) to compensate for disturbances caused by changes in ambient temperature, material state, etc., in real time, even for minute fluctuations. Based on the steady-state judgment logic of "|ΔT|≤1?", the controller achieves global coordination of cooling, heating, vacuuming and heat dissipation by finely adjusting the frequency of the low-pressure stage compressor (MV1), the power of the supplementary heating tube (MV2), the frequency of the vacuum pump (MV3) and the output power of the fan (MV4), so that the temperature of the drying chamber is always stable within the range of -70℃±1℃ and the vacuum degree is stable within the range of 10~50Pa. Throughout the steady-state operation phase, the system's coefficient of performance (COP) is ≥2.8, minimizing equipment energy consumption while ensuring freeze-drying effect, thus meeting the energy-saving needs of household appliances.

[0054] Example 3 Practical application effect verification To verify the actual operating performance, freeze-drying effect, and energy efficiency of the household deep freeze dryer and temperature control method based on two-stage composite refrigeration described in this invention, six types of materials commonly used in household freeze-drying scenarios were selected as test objects: fruits and vegetables (strawberries, mangoes, blueberries), meats (chicken breast, shrimp), and small biological samples (goji berries, tremella). The freeze-drying experiment was conducted using the equipment of Example 1 and the control method of Example 2. The entire experiment was conducted without human intervention, and the equipment ran continuously. The four core indicators of temperature control accuracy, vacuum stability, freeze-dried material quality, and system energy efficiency were quantitatively tested. At the same time, a traditional single-stage refrigeration household freeze dryer was set up as a control group for comparative testing under the same materials and the same freeze-drying process. The specific verification data and results analysis are as follows.

[0055] Basic experimental parameters Test equipment: the dual-stage composite refrigeration household deep freeze dryer of this invention, and a commercially available conventional single-stage refrigeration household freeze dryer (control group, nominal temperature control accuracy ±3℃, minimum refrigeration temperature -45℃).

[0056] Test materials: fresh strawberries (2-3cm in diameter), mango cubes (1cm×1cm), blueberries (0.8-1cm in diameter), chicken breast cubes (1cm×1cm), fresh shrimp (3-4cm), and goji berries (dried and pre-treated). The initial weight of each material was 500g. The initial moisture content was tested according to the category (85%-92% for fruits and vegetables, 70%-75% for meat, and 20% for goji berries).

[0057] Freeze-drying process: Pre-freezing stage (stepwise cooling: -2℃ / min to -40℃, then -0.5℃ / min to -70℃) → Sublimation stage (vacuum degree 10-50Pa, dynamic heat replenishment) → Steady-state operation stage (constant temperature at -70℃, continuous sublimation), freeze-drying is considered complete when the material reaches constant weight.

[0058] Testing instruments: high-precision temperature recorder (accuracy ±0.1℃), capacitive vacuum gauge (accuracy ±1Pa), halogen moisture analyzer (accuracy ±0.01%), high performance liquid chromatograph (for detecting nutrient components), and power analyzer (accuracy ±0.01kW).

[0059] Quantitative testing data of core indicators Indicator 1: Temperature control accuracy and stability Throughout the experiment, three temperature monitoring points were set up at the material tray in the drying chamber to record temperature changes in real time. The average value was taken as the actual temperature of the chamber. The deviation of the cooling rate during the pre-freezing stage and the temperature fluctuation range during the steady-state operation stage were statistically analyzed. The data are shown in Table 1 below: Table 1 Results analysis: The experimental group showed a much smaller deviation in cooling rate throughout the process compared to the control group. During steady-state operation, the temperature fluctuation was controlled within ±0.8℃, meeting the design accuracy requirement of ±1℃. Furthermore, the temperature difference between each detection point inside the cavity was minimal, and the temperature uniformity was excellent. The control group, limited by single-stage refrigeration, not only failed to reach the -70℃ cryogenic temperature but also experienced large fluctuations in cooling rate and poor temperature uniformity, which easily led to uneven freeze-drying of materials.

[0060] Indicator 2: Vacuum stability During the sublimation stage, the cavity vacuum level was continuously recorded for 12 hours, and the average vacuum level, maximum fluctuation value, and out-of-range (10-50 Pa) values ​​were statistically analyzed. The number of times, data is shown in Table 2 below: Testing items Experimental group (10-50 Pa) Control group (nominal 10-50 Pa) Average vacuum level 28.5Pa 32.2Pa Maximum fluctuation value ±3.2Pa ±8.7Pa Number of times out of range 0 times 15 times (from as low as 8 Pa to as high as 56 Pa). airflow buffer response time ≤0.5s (when gas production changes abruptly) ≥3s (when gas production changes abruptly) Table 2 Results analysis: When the gas production rate of the material sublimation fluctuated, the experimental group could complete the gas flow buffer within 0.5s, and the vacuum degree remained stable at 25.3~31.7Pa throughout the process, without ever exceeding the design range of 10-50Pa, demonstrating excellent vacuum stability. The control group, lacking a dedicated pressure stabilization structure, was affected by the gas production and pumping rates, and the vacuum degree frequently exceeded the design range, with a maximum fluctuation of ±8.7Pa, which seriously affected the material sublimation rate.

[0061] Indicator 3: Quality of freeze-dried materials After freeze-drying, the moisture content, morphological integrity, nutrient retention rate, and rehydration properties of the materials were quantitatively tested. Rehydration property was the rate of moisture recovery after soaking for 30 minutes after freeze-drying. Nutrient components were tested, focusing on vitamin C (core of fruits and vegetables), crude protein (core of meats), and polysaccharides (core of goji berries). The data are shown in Table 3 below. detection indicators Test group test values control group test values Industry Standards for High-Quality Freeze-Dried Food Final moisture content ≤3.8%（2.1%~3.8%） ≤5.2%（3.5%~5.2%） ≤5% Morphological integrity rate 98.5% (no collapse, no frost cracks) 82.3% (15% collapse, 2.7% frost cracking) ≥90% Vitamin C retention rate ≥92.1% (Fruits and Vegetables) ≥75.3% (Fruits and Vegetables) ≥85% Crude protein retention rate ≥98.3% (meat) ≥95.1% (meat) ≥95% Lycium barbarum polysaccharide retention rate ≥96.5% ≥90.2% ≥92% Rehydration ≥95.2% ≥83.6% Table 3 Results Analysis: The final moisture content of the freeze-dried materials in the experimental group was ≤3.8%, lower than the industry standard for high-quality products, and the morphological integrity rate reached 98.5%, with no collapse or cracking issues. The retention rate of core nutrients was higher than the industry standard, with the vitamin C retention rate of fruits and vegetables exceeding 92%, and the rehydration rate ≥95.2%, indicating excellent freeze-drying quality. In the control group, due to large fluctuations in temperature and vacuum, some materials experienced collapse and cracking, and the moisture content was relatively high. The retention rate of nutrients and the rehydration rate did not meet the industry standard for high-quality products, and the vitamin C retention rate of fruits and vegetables was only 75.3% due to poor temperature control, resulting in significant loss.

[0062] Indicator 4: System Energy Efficiency The total operating time, total power consumption, and coefficient of performance (COP) (cooling capacity / input power) for freeze-drying each material to constant weight were statistically analyzed. The experimental group was calculated based on a steady state at -70℃, and the control group was calculated based on its nominal steady state at -45℃. The data are shown in Table 4 below: Table 4 Results Analysis: The freeze-drying time per batch in the experimental group was 13.7% shorter than that in the control group, and the total power consumption was reduced by 14.3%. The average COP of the system reached 3.1, far exceeding the design requirement of ≥2.8, representing a 58.9% improvement over the control group. The energy consumption per unit of water was as low as 0.08 kW·h / kg, demonstrating significant energy-saving effects and fully meeting the low-energy consumption requirements of household appliances. The control group, due to its low operating efficiency in the cryogenic range (actually -45℃) and poor insulation, had higher energy consumption, with a COP of only 1.95, resulting in significantly lower energy efficiency than the experimental group.

[0063] Long-term operational reliability verification In addition to the single freeze-drying test, the test group equipment underwent a long-term operational reliability test for 30 consecutive days, with one batch per day, accumulating 246 hours of operation. The operating status, stability of indicators, and failure rate of the core components of the equipment were tested, and the results are as follows: Status of core components: The low-pressure stage / high-pressure stage compressor is operating normally, the evaporator is free from frost and blockage, the vacuum system has good sealing performance, and the temperature control system sensors and controllers are free from drift. Stability of indicators: After 30 consecutive batches of freeze-drying, the temperature control accuracy remained within ±1℃, and the vacuum fluctuation was ≤±4Pa with no significant attenuation; Failure rate: No downtime due to faults throughout the entire process, no parameters exceeding the standard, and excellent equipment operation stability; Ease of maintenance: The external evaporator structure facilitates defrosting, the drying chamber and insulation structure remain undamaged, the modular components are easy to disassemble and assemble, and the drain / pressure relief valve is simple to operate, making it suitable for use by non-professional households.

[0064] Verification conclusions Through quantitative testing and long-term operational verification of six common household freeze-dried materials, combined with comparative experiments with traditional single-stage refrigeration household freeze dryers, the household deep freeze dryer and temperature control method based on two-stage composite refrigeration described in this invention fully meet the design requirements. The core conclusions are as follows: Structural level: The system structure and composite insulation structure provide hardware support for dual-stage refrigeration, high-efficiency insulation and stable vacuum, breaking through the bottlenecks of insufficient deep-cold capacity and low insulation efficiency of traditional household freeze dryers. Control level: Adaptive PID control logic realizes closed-loop collaborative control with multiple parameter inputs and multiple actuator outputs, can stably achieve a cryogenic environment of -70℃, and the steady-state operating temperature fluctuation is ≤±0.8℃. The temperature control accuracy and vacuum stability are superior to traditional equipment. In terms of effectiveness: the freeze-dried materials are of excellent quality, with high retention rate of core nutrients, significantly improved system energy efficiency, and stable and reliable long-term operation, fully meeting the actual needs of household deep freeze-drying. It has good practicality and promotional value in household freeze-drying fields such as food preservation and small sample processing.

[0065] Example 4 Based on Embodiments 1 and 2, this embodiment provides a dynamic weighted PID control method based on the confidence assessment of three-level temperature sensors. It aims to solve the problem that traditional multi-sensor temperature control systems treat all sensors as equally reliable and ignore the measurement inaccuracies caused by sensor frost, material burial, airflow disturbance, etc., thereby further improving the robustness and accuracy of the temperature control system.

[0066] The household freeze dryer described in this embodiment has an adaptive PID controller in its temperature control system with a built-in sensor confidence evaluation module. This module acquires the instantaneous values ​​of three temperature sensors (cold trap surface temperature T1, material tray temperature T2, and ambient compensation temperature T3) in real time with a sampling period of 1Hz, and calculates the real-time confidence levels C1, C2, and C3 of each sensor respectively.

[0067] (1) Calculation formula for the confidence level C1 of the cold trap surface temperature sensor T1: C1 = w11×f1(ΔT1_rate) + w12×f2(T2, P) + w13×f3(t_def) Wherein, ΔT1_rate is the rate of change of T1 (℃ / min). When ΔT1_rate > 5℃ / min, f1 is 0.3 (indicating that the sensor may have abnormal response due to frost shedding); when ΔT1_rate < 0.5℃ / min, f1 is 1.0; intermediate values ​​are linearly interpolated. f2(T2, P) is the cross-validation function: calculate the difference between T1 and T2, ΔT_cm = T1 - T2. In the pre-freezing stage and when the vacuum degree P < 50Pa, ΔT_cm should normally be negative. If ΔT_cm > 0 for more than 5 minutes, then f2 is 0.5; otherwise, it is 1.0. f3(t_def) is the frost accumulation penalty function: the running time t_def (minutes) since the last defrost. When t_def > 60, f3 = 0.9; when t_def > 120, f3 = 0.7; when t_def > 180, f3 = 0.5. The preferred weighting coefficients are w11=0.4, w12=0.4, and w13=0.2.

[0068] (2) Calculation formula for the confidence level C2 of material tray temperature sensor T2: C2 = w21×g1(|V|) + w22×g2(P, t_stable) + w23×g3(T1, T2history) Where V is the rate of change of T2 (℃ / min). When |V| exceeds the maximum allowable value for the process stage (3℃ / min for the pre-freezing stage, 1℃ / min for the sublimation stage), g1 is set to 0.4; otherwise, it is set to 1.0. g2(P, t_stable) is the vacuum disturbance penalty: when the vacuum level P fluctuates more than ±10Pa and the duration exceeds 2 minutes, g2 is set to 0.6; otherwise, it is set to 1.0. g3(T1, T2 history) is the consistency check: calculate the variance of the difference between T2 and T1 over the past 10 minutes; when the variance is greater than 5℃², g3 is set to 0.7; otherwise, it is set to 1.0. The preferred weighting coefficients are w21=0.5, w22=0.3, and w23=0.2.

[0069] (3) Calculation formula for the confidence level C3 of the environmental compensation temperature sensor T3: C3 = h1(|dT3 / dt|) × h2(T3_range) Where |dT3 / dt| is the rate of change of ambient temperature, h1=0.6 when the rate is >2℃ / min (possibly affected by intermittent cooling airflow), otherwise h1=1.0. T3_range is the difference between the maximum and minimum values ​​of T3 in the past 30 minutes, h2=0.7 when the difference is >5℃, otherwise h2=1.0. In this embodiment, C3 is calculated in a product form because the influence of airflow disturbance and long-term drift on the ambient temperature sensor can be regarded as an independent event.

[0070] After obtaining the real-time confidence levels of each sensor, the controller calculates the weighted fusion temperature T_weighted: To prevent the denominator from being too small and causing numerical instability, when C1+C2+C3<0.5, the controller forces the confidence level of all three to be reset to 1.0 and issues a sensor abnormality alarm.

[0071] Furthermore, the controller applies stage weighting coefficients to the sensors based on the freeze-drying process stages. 1. 2. 3: Pre-freezing stage: Material temperature T2 reflects the core state of the material, giving it... 2=1.3; cold trap temperature T1 is the next most important factor. 1 = 1.0 Ambient temperature T3 weighting 3 = 0.8.

[0072] Sublimation stage: The cold trap temperature T1 has the greatest impact on water vapor capture efficiency, imparting... 1=1.4; material temperature T2 is the next most important factor. 2 = 1.0; Ambient temperature T3 weighting 3 = 0.7.

[0073] Analysis of the drying stage: Material temperature T2 is the most critical. 2 = 1.5; weight of cold trap temperature T1 1 = 0.9; ambient temperature T3 weight 3 = 0.6.

[0074] The final feedback temperature T_feedback used by the PID controller is: The error e = T_set - T_feedback is obtained by comparing T_feedback with the set temperature T_set, and an incremental PID algorithm is used to calculate the power output of the supplementary heating tube.

[0075] Practical application verification shows that under extreme conditions where the frost thickness in the cold trap reaches 3mm, the temperature feedback error of the traditional arithmetic mean method reaches ±2.5℃, while the weighted fusion method of this embodiment controls the error within ±0.8℃. When the material collapses during the sublimation stage, causing partial burial of the T2 sensor, this embodiment can automatically reduce the T2 weight to avoid miscontrol and significantly improve the anti-interference capability of the freeze dryer temperature control system in household scenarios.

[0076] Example 5 Based on Embodiments 1, 2, and 4, this embodiment provides an adaptive PID control method that couples temperature, vacuum, and time in three dimensions. This method uses the material tray temperature T2, the temperature change rate V, and the vacuum degree P of the drying chamber as three-dimensional state variables, and adaptively adjusts the PID parameters under Lyapunov stability constraints to solve the problems of coupling interference and poor stability in traditional independent temperature and vacuum control or cascade control.

[0077] Define a three-dimensional state vector X = [x1, x2, x3]ᵀ, where: x1 = e_T = T_set - T2, which is the material pallet temperature tracking error (°C); x2 = V - V_ref, which is the deviation (°C / min) between the material temperature change rate and the reference rate. The reference rate V_ref is set according to the freeze-drying process stage: V_ref = -1°C / min for the pre-freezing stage and V_ref = 0.2°C / min for the sublimation stage. x3 = P - P_target, which is the deviation (Pa) between the actual vacuum degree and the target vacuum degree. P_target is dynamically given according to the material temperature T2 based on the preset curve.

[0078] The system's control output u = [u1, u2, u3]ᵀ, where u1 is the power of the supplementary heating tube (W, adjustable range 50~300W), u2 is the operating frequency of the rotary vane pump (Hz, adjustable range 20~50Hz), and u3 is the operating frequency of the low-pressure stage compressor (Hz, adjustable range 30~80Hz).

[0079] A linear state-space model was established through system identification experiments. Matrix A and B are updated online every 30 seconds using the recursive least squares (RLS) method.

[0080] Define Lyapunov function candidates: The coupling weight coefficients are preferably λ1=0.5 and λ2=1.2 (to emphasize the penalty for vacuum deviation). It should be noted that λ1 and λ2 not only reflect the importance of each state variable but also include a dimension transformation function, making the components of the Lyapunov function comparable. The control objective is to make L decay over time, i.e., dL / dt < 0.

[0081] Using a PID controller structure, the mapping relationship between the control output u and the state X is as follows: Where K_p, K_i, and K_d are all 3×3 diagonal matrices (assuming independent control of each channel). The adaptive law for the PID parameters uses the negative gradient descent direction to decrease L: Where η_p, η_i, and η_d are learning rates, preferably between 0.01 and 0.05. The specific calculation of ∂L / ∂K is obtained using the chain rule and the sensitivity information of the state-space model.

[0082] Furthermore, the controller calculates the influence coefficient of vacuum disturbance on temperature μ=∂x1 / ∂x3 in each control cycle (1 second) using the numerical difference method: while keeping other inputs constant, a disturbance of ±1Pa is applied to the target vacuum value, and the change in x1 Δx1 is observed; then μ=Δx1 / 2. The decoupled temperature control output is corrected as follows: That is, when the actual vacuum level is higher than the target value, the heating power should be appropriately reduced to avoid the temperature from rising excessively due to the increase in vacuum level.

[0083] The controller calculates the current value and rate of change dL / dt of the Lyapunov function L once per control cycle. If dL / dt ≥ 0 is detected for 10 consecutive cycles (system divergence trend), an emergency response is triggered: the PID parameters are forcibly rolled back to the parameter values ​​of the last stable run, the adjustment step size of all actuators is reduced to 50% of the normal value, and an "unstable control system" alarm signal is issued.

[0084] Practical application verification shows that under a disturbance condition with a sudden 50% increase in material loading, the traditional cascade PID control exhibits a temperature overshoot of 2.8℃, a vacuum fluctuation of ±15Pa, and a stabilization time of approximately 25 minutes. In this embodiment, the temperature overshoot is controlled within 0.9℃, the vacuum fluctuation is ±4Pa, and the stabilization time is shortened to 12 minutes. Simultaneously, the Lyapunov stability monitoring mechanism can provide early warnings of control divergence risks, improving operational safety in home environments.

[0085] Example 6 Based on Examples 1 to 5, this embodiment provides an interstage decoupling control method based on intermediate heat exchange temperature difference sliding mode control for a two-stage composite refrigeration system combining R600a low-pressure stage and CO2 transcritical high-pressure stage, in order to solve the overshoot and oscillation problems caused by time-varying parameters and strong nonlinearity in traditional PID control in heterogeneous working fluid systems.

[0086] Define the intermediate heat exchange temperature difference as ΔT_mid = T_R600a,exhaust - T_CO2,intake, and set the target temperature difference ΔT_set preferably to be 8~12℃. Define the temperature difference tracking error: The sliding surface is designed as follows: λ is the slope coefficient of the sliding surface (dimension s). -1 The values ​​are adaptively adjusted according to the freeze-drying process stages: λ=1.2 for the pre-freezing stage and λ=0.6 for the steady-state operation stage.

[0087] The system dynamics can be approximated as a first-order inertial plus pure delay model: Where u(t) is the control variable (the adjustment of the low-pressure stage compressor frequency), τ is the time constant (10~20 seconds), K is the system gain (0.5~1.0℃ / Hz), and θ is the pure delay time (2~5 seconds). Using nominal parameters K_nom and τ_nom, based on sliding mode control theory, the derivative of the sliding surface is... The equivalent control law can be solved. In this embodiment, the equivalent control law uses the calculated value based on the nominal parameters, and the remaining error is compensated by the switching control term. The specific analytical form of the equivalent control law can be obtained using standard derivation methods in the field of sliding mode control.

[0088] The switching control law uses a saturation function instead of a sign function to suppress chattering. , Where η is the switching gain (preferably η = 1.5 × the control quantity corresponding to the maximum model deviation), φ is the boundary layer thickness, and the saturation function is defined as: This embodiment proposes that the boundary layer thickness φ be dynamically adjusted according to the freeze-drying process conditions: Adjustment based on the material temperature change rate V: when |V|>0.5℃ / min (intense sublimation stage), φ is 1~2; when |V|<0.1℃ / min (steady-state stage), φ is 3~5; intermediate values ​​are linearly interpolated.

[0089] Based on the adjustment of the ambient temperature change rate dT3 / dt: when |dT3 / dt|>1℃ / min, φ decreases to 1.5; when |dT3 / dt|<0.2℃ / min, φ increases to 4.

[0090] The combined φ value is the maximum of the two adjustment results mentioned above.

[0091] The total control quantity is: , The actual low-pressure stage compressor frequency f_L = f_L,current + u is limited to the range of 30~80Hz. When u exceeds the limit, the accumulation of the integral term on the sliding surface stops (i.e., ∫e dt is frozen) to prevent integral saturation.

[0092] While adjusting the frequency of the low-pressure stage compressor, the controller synchronously adjusts the operating frequency of the high-pressure stage CO2 compressor to maintain the optimal interstage capacity ratio. Let the displacement of the low-pressure stage compressor be V_L (mL / rev) and the displacement of the high-pressure stage compressor be V_H (mL / rev). The optimal capacity ratio is defined as γ_opt = V_H / V_L, preferably γ_opt = 1.2~1.5. The target frequency of the high-pressure stage is calculated using the following formula: The actual frequency change rate does not exceed 2 Hz / s.

[0093] When the controller detects that ΔT_mid deviates from ΔT_set by more than 3°C for more than 30 seconds, it determines that there is an interstage mismatch, temporarily increases the frequency regulation rate of the high-voltage stage to 5Hz / s, and increases the sliding mode gain η to 2.0 times to force a fast recovery.

[0094] Practical application verification shows that under extreme conditions where the ambient temperature jumps from 15℃ to 35℃, the maximum deviation of ΔT_mid in traditional PID control reaches 4.2℃, with a recovery time of approximately 8 minutes; while the maximum deviation of sliding mode control in this embodiment is only 1.5℃, with a recovery time of approximately 2 minutes. Considering the frequent changes in material loading (200g~2kg) in household scenarios, sliding mode control maintains ΔT_mid stable within ±1℃ of the set value across the entire load range, increasing the overall COP from 2.8 to over 3.1. Through dynamic boundary layer φ adjustment, control accuracy is ensured during the rapid sublimation phase, and the compressor frequency fluctuation is controlled within ±1Hz during the steady-state phase, with operating noise below 45dB, fully meeting the requirements for quiet operation in household settings.

[0095] The above embodiments four, five, and six provide advanced technical solutions from three dimensions: sensor fusion reliability, multivariate cooperative stability, and robustness of dissimilar working fluid cooling, forming a progressive protection hierarchy with embodiments one to three. The embodiments can be combined with each other; for example, combining the confidence-weighted fusion of embodiment four with the Lyapunov coupling control of embodiment five, or coordinating the sliding mode inter-stage control of embodiment six with the aforementioned temperature control system, all fall within the protection scope of this invention.

[0096] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A household deep freeze dryer based on two-stage composite refrigeration, characterized in that, include: The refrigeration system includes a low-pressure stage circuit using R600a refrigerant and a high-pressure stage circuit using a CO2 transcritical cycle. The low-pressure stage circuit and the high-pressure stage circuit are connected in series to form a two-stage composite refrigeration closed loop. The low-pressure stage compressor in the low-pressure stage circuit operates at a pressure of 0.3 to 0.8 MPa, and the high-pressure stage compressor in the high-pressure stage circuit operates at a pressure of 8 to 15 MPa. A drying chamber, wherein the drying chamber is provided with a heat insulation structure composed of vacuum insulation and aerogel composite; A vacuum system is used to provide a vacuum environment for the drying chamber; The temperature control system includes a three-level temperature sensor array and an adaptive PID controller. The three-level temperature sensor array is used to collect the surface temperature of the cold trap, the temperature of the material tray, and the ambient compensation temperature. Based on the collected temperature signals and the vacuum degree of the drying chamber, it performs step-by-step cooling control and dynamic heat replenishment control.

2. The household freeze dryer according to claim 1, characterized in that, The two-stage compound refrigeration closed loop is composed of a low-pressure stage compressor, an intercooler, a high-pressure stage compressor, a condenser, a throttling device, and an evaporator connected sequentially through pipelines; The evaporator is attached to the outside of the drying chamber and forms a tight thermal coupling with the drying chamber, and continuously absorbs heat from inside the drying chamber through heat conduction.

3. The household freeze dryer according to claim 1, characterized in that... The heat insulation structure includes a nano-aerogel felt wrapped around the wall of the drying chamber and a metal foil layer wrapped around the outside of the nano-aerogel felt.

4. The household freeze dryer according to claim 1, characterized in that, The vacuum system includes a rotary vane pump and a pressure stabilizing pipeline; The rotary vane pump continuously draws gas from the inside of the drying chamber, and the pressure stabilizing pipeline buffers the airflow and balances the internal pressure of the drying chamber, so that the vacuum degree of the drying chamber is maintained in the range of 10 to 50 Pa during the material sublimation stage.

5. The household freeze dryer according to claim 1, characterized in that, The adaptive PID controller performs the following control steps: A1. Real-time acquisition of the cold trap surface temperature T1, material tray temperature T2, and ambient compensation temperature T3 collected by the three-stage temperature sensors; calculation of the temperature difference ΔT=T_SV-T2 and ΔT1=T1-T3; and real-time acquisition of the vacuum degree of the drying chamber. ; A2. Based on the temperature difference ΔT, ΔT1 and vacuum degree The system dynamically adjusts the PID control parameters and, based on the adjusted PID parameters, controls in real time the operating frequency of the low-pressure stage compressor, the power of the supplementary heating tube, the frequency of the vacuum pump, and the output power of the fan.

6. The household freeze dryer according to claim 5, characterized in that, The dynamically adjusted PID control parameters include: When ΔT > 5℃, increase the PID proportional coefficient P; When ΔT < 1℃, increase the PID integral coefficient I; When the change in ΔT1, Δ(ΔT1), is greater than the preset threshold, the PID derivative coefficient D is increased; The real-time control includes: adjusting the low-pressure stage compressor frequency MV1, the supplementary heating tube power MV2, the vacuum pump frequency MV3, and the fan output power MV4 according to the adjusted PID parameters, so that the temperature difference ΔT approaches 0, and the vacuum degree... Maintaining a pressure of 10–50 Pa, the output power MV4 of the fan is proportional to the magnitude of ΔT1.

7. A temperature control method for a household deep freeze dryer based on two-stage composite refrigeration, applied to the household deep freeze dryer according to any one of claims 1-6, characterized in that, Includes the following steps: S1. System Start-up and Data Acquisition: Start the refrigeration and vacuum systems. The three-stage temperature sensor array collects the cold trap surface temperature T1, material tray temperature T2, and ambient compensation temperature T3 in real time, while simultaneously acquiring the vacuum level of the drying chamber. ; The adaptive PID controller receives the temperature signal and vacuum signal mentioned above, and calculates the temperature difference ΔT=T_SV-T2 and ΔT1=T1-T3; S2. Entering the pre-freezing stage, the temperature control system executes step-by-step cooling control: the adaptive PID controller adjusts the operating frequency of the low-pressure stage compressor and the high-pressure stage compressor based on ΔT, first lowering the temperature of the drying chamber to the preset temperature at a preset rapid cooling rate, and then switching to a preset slow cooling rate to continue cooling down to -70℃ and maintaining stability. S3. Entering the sublimation stage, the vacuum system uses a rotary vane pump to evacuate air and a pressure stabilizing pipeline to equalize the pressure, maintaining the vacuum level of the drying chamber within the range of 10 to 50 Pa. The adaptive PID controller is based on ΔT, ΔT1, and vacuum degree. Dynamically adjust the power of the supplementary heating tube and the output power of the fan; S4. During the steady-state operation phase, the adaptive PID controller continuously adjusts the PID parameters, compressor load, supplementary heating power, and fan power, working in conjunction with the refrigeration system and vacuum system to maintain the drying chamber in a stable operating state.

8. The temperature control method according to claim 7, characterized in that, In step S2, the preset rapid cooling rate is -2℃ / min, the preset temperature is -40℃, and the preset slow cooling rate is -0.5℃ / min.

9. The temperature control method according to claim 7, characterized in that, In step S3, the power of the supplementary heating tube is adjustable from 50 to 300W, and the output power of the fan is proportional to the magnitude of ΔT1.

10. The temperature control method according to claim 7, characterized in that, In step S4, when the freeze dryer is running at a steady state of -70℃, the temperature control accuracy is ±1℃ and the energy efficiency coefficient (COP) is ≥2.8.

Images