Fruit and vegetable warehouse liquid nitrogen fresh-keeping control system and control method

Abstract

The invention relates to the technical field of industrial control, and particularly discloses a liquid nitrogen fresh-keeping control system and method for a fruit and vegetable warehouse, and the method comprises the following steps: S1, obtaining the current temperature of the fruit and vegetable warehouse, and calculating the ideal usage amount of liquid nitrogen based on the optimal storage temperature of fruits and vegetables; s2, calculating a single dosage, adding liquid nitrogen at a preset adding point, updating the position of the adding point, continuing adding, and repeating the operation; s3, judging whether the fruits and vegetables are frozen on the basis of hardness testing, and if the fruits and vegetables are frozen, adjusting an adding method of liquid nitrogen; and S4, calculating the average value of the fruit and vegetable skin temperature, and reducing the fruit and vegetable skin temperature to the optimal storage temperature by using a carbon dioxide-compressor cooling method. According to the liquid nitrogen fresh-keeping control system and method for the fruit and vegetable warehouse, large ice crystals are prevented from being generated in fruits and vegetables in the liquid nitrogen fresh-keeping process, and therefore the freshness, taste and nutritional ingredients of the fruits and vegetables are guaranteed to the maximum extent.

Description

Technical Field

[0001] The present invention relates to the technical field of industrial control, and particularly relates to a liquid nitrogen fresh-keeping control system and control method for a fruit and vegetable warehouse. Background Art

[0002] Liquid nitrogen is a colorless, odorless, and low-viscosity transparent liquid with a boiling point of -196°C under normal pressure. Utilizing the low-temperature characteristics of liquid nitrogen, fruits and vegetables can be quickly cooled to an appropriate fresh-keeping temperature range (generally -18°C to -25°C), thereby inhibiting the respiration, metabolism of fruits and vegetables, and the growth and reproduction of microorganisms, achieving the purpose of long-term fresh-keeping.

[0003] When liquid nitrogen comes into contact with fruits and vegetables, it can quickly absorb a large amount of heat, causing the fruits and vegetables to reach a frozen state in a short time. This rapid freezing process can effectively reduce the formation of ice crystals. In a general freezing process, larger ice crystals will damage the cell structure of fruits and vegetables, resulting in the loss of nutrients and a deterioration in taste. However, the tiny ice crystals formed by liquid nitrogen quick-freezing cause less damage to the cell structure of fruits and vegetables, thus maintaining the original taste and nutritional value of fruits and vegetables.

[0004] In the prior art, although liquid nitrogen fresh-keeping can quickly freeze food, if the freezing speed is too fast or uneven, it may cause larger ice crystals to form inside the fruits and vegetables, damaging the cell structure of the fruits and vegetables and affecting the taste and texture of the fruits and vegetables. Therefore, a liquid nitrogen fresh-keeping control system and control method for a fruit and vegetable warehouse are needed to avoid the formation of larger ice crystals inside the fruits and vegetables during the liquid nitrogen fresh-keeping process, preventing the loss of nutrients and the deterioration of taste of the fruits and vegetables. Summary of the Invention

[0005] The purpose of the present invention is to provide a liquid nitrogen fresh-keeping control system and control method for a fruit and vegetable warehouse to solve the above technical problems.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] A liquid nitrogen fresh-keeping control method for a fruit and vegetable warehouse includes the following steps:

[0008] Step S1: Obtain the current temperature W of the fruit and vegetable warehouse, and calculate the ideal usage amount M of liquid nitrogen based on the optimal storage temperature W best of the fruits and vegetables;

[0009] Step S2: Calculate the single usage amount m = M / N, where N represents the preset number of usages. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen with a mass of m at a preset addition point. Update the position of the addition point and continue to add liquid nitrogen with a mass of m. Repeat the above steps until the total usage amount of liquid nitrogen is equal to the ideal usage amount.

[0010] Step S3: Determine whether the fruits and vegetables are damaged by freezing based on the hardness test. If the fruits and vegetables are damaged by freezing, adjust the liquid nitrogen addition method, and the liquid nitrogen addition method includes:

[0011] Calculate the proportion B of the frozen fruits and vegetables in the total amount of fruits and vegetables, and update the single-use amount of liquid nitrogen. Calculate the proportion Bs of the frozen fruits and vegetables in the total amount of fruits and vegetables after the single-use amount is updated. When the proportion Bs ≤ 5%, stop the adjustment.

[0012] Step S4: Calculate the average value W of the surface temperature of the fruits and vegetables. ave If the average value W ave > W best , then use the carbon dioxide-compressor cooling method to reduce the surface temperature of the fruits and vegetables to the optimal storage temperature W. best .

[0013] Preferably, in the step S1, the method for calculating the ideal usage amount M of liquid nitrogen based on the optimal storage temperature W of the fruits and vegetables specifically includes: best Calculate the temperature difference ΔW = W - W.

[0014] Calculate the temperature difference ΔW = W - W. best , calculate the heat consumption Q = c1ΔW according to the temperature difference ΔW, where c1 represents the specific heat capacity of air, and calculate the ideal usage amount of liquid nitrogen. Wherein, c2 represents the heat of vaporization of liquid nitrogen.

[0015] Preferably, in the step S2, the preset number of uses N ≥ 4, and make the time consumed for adding liquid nitrogen under different numbers of uses N equal to the preset specified time.

[0016] Preferably, in the step S2, the method for updating the position of the addition point specifically includes:

[0017] Divide a circular area with the distance between the center of the circle and the addition point as the radius. The center of the circle is the center point of the fruit and vegetable placement formation. Rotate the addition point clockwise by θ degrees along the boundary of the circular area to obtain a new addition point, where θ = 2π / N.

[0018] Preferably, in the step S3, the method for determining whether the fruits and vegetables are damaged by freezing based on the hardness test specifically includes:

[0019] Preset k detection points on the surface of the fruits and vegetables, apply a preset value of pressure to the detection points, and obtain the depression distance L of the fruit and vegetable surface into the interior at this time. If there is a detection point where the depression distance L is greater than or equal to the preset judgment distance, it is determined that the fruits and vegetables are damaged by freezing.

[0020] Preferably, in the step S3, another method for determining whether the fruits and vegetables are damaged by freezing specifically includes:

[0021] Obtain the ethylene content Y in the air before adding liquid nitrogen. After adding liquid nitrogen with a mass of m, update the ethylene content Ys in the air. If the updated ethylene content Ys > Y + η, it is determined that the fruits and vegetables are damaged by freezing, where η represents the preset ethylene content fluctuation value.

[0022] Preferably, in the step S4, the carbon dioxide - compressor cooling method includes:

[0023] Add a preset volume of carbon dioxide to the fruits and vegetables, measure the surface temperature of the fruits and vegetables at this time. If the surface temperature of the fruits and vegetables is greater than the optimal storage temperature, use a compressor to reduce the surface temperature of the fruits and vegetables to the optimal storage temperature.

[0024] A liquid nitrogen fresh - keeping control system for a fruits and vegetables warehouse, including:

[0025] Calculation module: Obtain the temperature W of the current fruits and vegetables warehouse, and calculate the ideal usage amount M of liquid nitrogen based on the optimal storage temperature W of the fruits and vegetables; best Calculate the ideal usage amount M of liquid nitrogen;

[0026] Cooling module: Calculate the single - time usage amount m = M / N, where N represents the preset number of usage times. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen with a mass of m at the preset addition point. Update the position of the addition point and continue to add liquid nitrogen with a mass of m. Repeat the above steps until the total usage amount of liquid nitrogen is equal to the ideal usage amount;

[0027] Adjustment module: Judge whether the fruits and vegetables are damaged by freezing based on hardness testing. If the fruits and vegetables are damaged by freezing, adjust the liquid nitrogen addition method. The liquid nitrogen addition method includes:

[0028] Calculate the proportion B of the frozen - damaged fruits and vegetables in the total amount of fruits and vegetables, and update the single - time usage amount of liquid nitrogen; Calculate the proportion Bs of the frozen - damaged fruits and vegetables in the total amount of fruits and vegetables after updating the single - time usage amount. When the proportion Bs ≤ 5%, stop the adjustment;

[0029] Compensation module: Calculate the average value W of the surface temperature of the fruits and vegetables; ave If the average value W; ave > W; best Use the carbon dioxide - compressor cooling method to reduce the surface temperature of the fruits and vegetables to the optimal storage temperature W; best .

[0030] The beneficial effects of the present invention:

[0031] 1. The method of adding liquid nitrogen in batches. By reasonably planning the injection volume and injection time interval of liquid nitrogen each time, it can not only avoid the occurrence of fruit and vegetable freezing damage caused by too rapid temperature reduction, but also ensure that the temperature in the warehouse continues to drop steadily, gradually reach and maintain within the optimal fresh-keeping temperature range required for fruits and vegetables, thereby improving the fresh-keeping effect of fruits and vegetables, extending their shelf life, reducing losses caused by improper storage, and ensuring the economic benefits of merchants and the usage experience of consumers.

[0032] 2. In order to accurately judge which liquid nitrogen addition method can effectively reduce the freezing damage behavior to fruits and vegetables, an evaluation system is constructed. In this evaluation system, the key index of the proportion of frozen fruits and vegetables in the total amount of fruits and vegetables is used to dynamically adjust and update the single-use amount of liquid nitrogen in a timely manner. Continuously adjust and optimize the single-use amount of liquid nitrogen to maximize the integrity of fruits and vegetables.

[0033] 3. Introduce the carbon dioxide-compressor cooling method. Add a preset volume of carbon dioxide gas to the space where fruits and vegetables are stored and start the compressor equipment to extract heat from the air around the fruits and vegetables, so that the surface temperature of the fruits and vegetables gradually decreases until it reaches the preset optimal storage temperature value. The presence of carbon dioxide and the appropriate low-temperature environment work together to inhibit the respiration intensity of fruit and vegetable cells and maintain their freshness, taste, and color. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present invention will be further described below with reference to the accompanying drawings.

[0035] Figure 1 It is a schematic flow chart of a method for controlling the preservation of fruits and vegetables in a warehouse using liquid nitrogen according to the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0037] Please refer to Figure 1 As shown, the present invention is a method for controlling the preservation of fruits and vegetables in a warehouse using liquid nitrogen, including the following steps:

[0038] Step S1: Obtain the temperature W of the current fruit and vegetable warehouse, and calculate the ideal usage amount M of liquid nitrogen based on the optimal storage temperature W of the fruits and vegetables best

[0039] ​Step S2: Calculate the single - use dosage \(m = M / N\), where \(N\) represents the preset number of uses. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen with a mass of \(m\) at a preset addition point. Update the position of the addition point and continue to add liquid nitrogen with a mass of \(m\). Repeat the above steps until the total usage amount of liquid nitrogen is equal to the ideal usage amount;

[0040] Step S3: Based on the hardness test, determine whether the fruits and vegetables are damaged by freezing. If the fruits and vegetables are damaged by freezing, adjust the liquid nitrogen addition method. The liquid nitrogen addition method includes:

[0041] Calculate the proportion \(B\) of the frozen - damaged fruits and vegetables in the total amount of fruits and vegetables, and update the single - use dosage of liquid nitrogen Calculate the proportion \(Bs\) of the frozen - damaged fruits and vegetables in the total amount of fruits and vegetables after the single - use dosage is updated. When the proportion \(Bs\leq5\%\), stop the adjustment;

[0042] Step S4: Calculate the average value \(W\) of the surface temperature of the fruits and vegetables ave , if the average value \(W\) ave \(>W\) best , then use the carbon dioxide - compressor cooling method to reduce the surface temperature of the fruits and vegetables to the optimal storage temperature \(W\) best .

[0043] It should be noted that in actual operation, when using liquid nitrogen for fruit and vegetable preservation, two problems are often faced. First, when all the liquid nitrogen is poured into the fruit and vegetable warehouse at one time, due to the extremely strong low - temperature effect and extremely low temperature of liquid nitrogen, the surrounding temperature will drop rapidly and sharply. This rapid temperature change exceeds the range that the fruits and vegetables can bear, resulting in serious damage to the internal structure of the fruits and vegetables, and then causing the freezing damage phenomenon. Once the fruits and vegetables are damaged by freezing, their cell tissues will be irreversibly damaged, which will not only affect the taste, color and nutritional components of the fruits and vegetables, but also greatly shorten their preservation period, resulting in relatively large economic losses.

[0044] By controlling the usage amount and addition point of liquid nitrogen, while ensuring that the surface temperature of the fruits and vegetables is reduced to the specified temperature, it is also possible to avoid generating more ice crystals, reduce the probability of serious damage to the internal structure of the fruits and vegetables, not only avoid the occurrence of freezing damage to the fruits and vegetables caused by too rapid temperature reduction, but also ensure that the temperature in the warehouse drops continuously and stably, gradually reaches and maintains within the optimal preservation temperature range required by the fruits and vegetables, thereby improving the preservation effect of the fruits and vegetables, extending their shelf life, and reducing the losses caused by improper storage.

[0045] In addition, it is necessary to consider the length of the actual physical operation process in the time dimension. From the start of the liquid nitrogen addition operation to achieving the desired cooling effect and completing a series of adjustments, a certain amount of time will inevitably be consumed. During this period, due to the influence of the external environment and the heat exchange between the fruits and vegetables themselves and the surrounding environment, the temperature of the fruit and vegetable epidermis will fluctuate to a certain extent, which will cause the actual temperature of the fruits and vegetables at this time to deviate from its most ideal storage temperature range.

[0046] To address this situation and ensure the quality and freshness preservation effect of fruits and vegetables during storage, a temperature control treatment method, namely the carbon dioxide - compressor cooling method, needs to be introduced. Specifically, a preset volume of carbon dioxide gas will be accurately added to the space where the fruits and vegetables are stored. As an inert gas, the appropriate presence of carbon dioxide in the storage environment of fruits and vegetables can not only adjust the atmosphere but also inhibit the growth of microorganisms on the surface of fruits and vegetables and the rate of oxidation reactions to a certain extent.

[0047] By using the carbon dioxide - compressor cooling method for subsequent adjustments, there are two benefits. First, through this temperature control method, it can ensure that the epidermis temperature of fruits and vegetables remains stable at the best state throughout the storage period, avoiding problems such as the decline in the quality of fruits and vegetables and rotting caused by excessive temperature fluctuations.

[0048] Second, the combined effect of the presence of carbon dioxide and a suitable low - temperature environment can significantly inhibit the respiration intensity of fruit and vegetable cells. After adding carbon dioxide, the epidermis temperature of the fruits and vegetables is measured in real - time. If the monitoring data shows that the epidermis temperature of the fruits and vegetables is still greater than its optimal storage temperature, the compressor equipment needs to be started for cooling until the pre - set optimal storage temperature value is reached, which helps to extend the freshness preservation period of the fruits and vegetables and keep key quality indicators such as their freshness, taste, color, and nutritional components at a relatively high level.

[0049] In another preferred embodiment of the present invention, based on the optimal storage temperature W of the fruits and vegetables best The method for calculating the ideal usage amount M of liquid nitrogen specifically includes:

[0050] Calculate the temperature difference ΔW = W - W best , calculate the heat consumption Q = c1ΔW according to the temperature difference ΔW, where c1 represents the specific heat capacity of air, and calculate the ideal usage amount of liquid nitrogen where c2 represents the heat of vaporization of liquid nitrogen.

[0051] It is worth noting that when using the laws of thermodynamics to calculate the specific amount of liquid nitrogen to be used, that is, in a closed system, heat will neither be generated out of thin air nor disappear without reason. It will only flow from the high - temperature area to the low - temperature area, or be transferred and redistributed among the various parts of the system.

[0052] In a specific environment, the fruit and vegetable warehouse is approximately regarded as a closed system, which can reduce the calculation amount. Based on this principle, the ideal usage amount of liquid nitrogen can be accurately calculated according to the required temperature reduction. Specifically, first determine the temperature difference between the current environment temperature of the fruits and vegetables and their optimal storage temperature. This temperature difference is the key starting point for the entire calculation. Because it is this temperature difference that causes heat to be transferred between the system (i.e., the warehouse and the fruits and vegetables stored therein) and liquid nitrogen.

[0053] When liquid nitrogen is introduced into the system, the liquid nitrogen begins to absorb the surrounding heat and vaporizes from the liquid state to the gaseous state by itself. The heat of vaporization of liquid nitrogen represents the amount of heat required to be absorbed by unit mass of liquid nitrogen during the vaporization process. After obtaining the value of the heat of vaporization of liquid nitrogen, according to the heat change amount corresponding to the required temperature reduction and combining with the heat of vaporization of liquid nitrogen, a mathematical model is established and calculated. The ideal usage amount of liquid nitrogen is obtained through thermodynamic calculation formulas, ensuring that while meeting the cooling requirements, the usage efficiency of liquid nitrogen can be maximized, avoiding waste of resources and adverse effects such as overcooling of fruits and vegetables caused by excessive use of liquid nitrogen.

[0054] In another preferred embodiment of the present invention, the preset number of usage times N≥4, and the time consumed for adding liquid nitrogen under different numbers of usage times N is equal to the preset specified time.

[0055] It can be understood that the purpose of the preset number of usage times is to reduce the occurrence of frost damage to fruits and vegetables caused by pouring excessive liquid nitrogen at one time, and the specified time consumed for adding liquid nitrogen is to reduce the influence of time on the cooling effect of liquid nitrogen.

[0056] In another preferred embodiment of the present invention, the method for updating the position of the adding point specifically includes:

[0057] Divide a circular area with the distance between the center of the circle and the adding point as the radius. The center of the circle is the center point of the fruit and vegetable placement formation. Rotate the adding point clockwise by θ degrees along the boundary of the circular area to obtain a new adding point, where θ = 2π / N.

[0058] It should be noted that by continuously and stably rotating the container or storage space for storing fruits and vegetables and simultaneously adding liquid nitrogen at a precisely controlled rate and amount, an extremely effective temperature control mechanism is constructed, thereby maximizing the prevention of frost damage to fruits and vegetables caused by liquid nitrogen. This rotation action promotes the continuous dynamic circulation and mixing process of the air in the warehouse and the microenvironment around the fruits and vegetables. During this process, the low temperature carried by the liquid nitrogen is not concentrated on a certain point or a certain local area, but is evenly dispersed with the flow of air and gradually penetrates into the entire storage space. At the same time, the combination of this rotation and adding liquid nitrogen can also achieve the goal of uniform cooling of fruits and vegetables.

[0059] In another preferred embodiment of the present invention, the method for judging whether fruits and vegetables are damaged by freezing based on hardness testing specifically includes:

[0060] Preset k detection points on the epidermis of the fruits and vegetables, apply a preset value of pressure to the detection points, and obtain the depression distance L of the fruit and vegetable epidermis inward at this time. If there is a detection point where the depression distance L is greater than or equal to the preset judgment distance, it is judged that the fruits and vegetables are damaged by freezing.

[0061] It should be noted that k representative and evenly distributed detection points are carefully preset on the epidermis of the fruits and vegetables. These detection points can comprehensively and accurately reflect the state information of the fruit and vegetable epidermis in different parts. When it is necessary to detect whether the fruits and vegetables are damaged by freezing, a professional pressure application device is used to apply a preset value of pressure to each detection point. After applying the pressure, a high-precision displacement sensor is used to obtain the depression distance L of the fruit and vegetable epidermis inward at this time. This depression distance L intuitively reflects the deformation degree of the fruit and vegetable epidermis when subjected to pressure. In order to accurately judge whether the fruits and vegetables are damaged by freezing, a judgment distance threshold is preset. If the actually measured depression distance L is greater than or equal to this preset judgment distance threshold, it can be clearly judged that the fruits and vegetables are damaged by freezing. Because when the fruits and vegetables are damaged by freezing, their internal cell structure will be damaged, resulting in changes in the mechanical properties such as toughness and elasticity of the epidermis. As a result, under the action of the same pressure, the depression degree of the epidermis will be greater than that in the normal state and exceed the preset judgment distance threshold.

[0062] In another preferred embodiment of the present invention, another method for judging whether fruits and vegetables are damaged by freezing specifically includes:

[0063] Obtain the ethylene content Y in the air before adding liquid nitrogen. When adding liquid nitrogen with a mass of m, update the ethylene content Ys in the air. If the updated ethylene content Ys > Y + η, it is judged that the fruits and vegetables are damaged by freezing, where η represents the preset ethylene content fluctuation value.

[0064] It can be understood that when the fruits and vegetables are damaged by freezing, a series of chemical reactions will occur inside them. Among them, the ethylene content will increase. It is like a signal indicator, indicating whether the fruits and vegetables have suffered from freezing damage. Specifically, when freezing damage occurs, the biochemical balance inside the fruit and vegetable cells is broken. The originally stable order of life activities becomes disordered, and some enzymatic reactions deviate from the normal track. At the same time, the energy metabolism inside the cells is also affected, providing additional energy support for ethylene synthesis or reducing the inhibitory factors for its synthesis.

[0065] Ethylene, as a plant hormone, plays a crucial role in the ripening, senescence, and stress response of fruits and vegetables. Under normal circumstances, ethylene maintains a relatively stable level within fruits and vegetables, regulating the orderly progress of physiological processes. However, when freeze injury occurs and its content rises abnormally, it will further accelerate the physiological disorder and quality deterioration process of fruits and vegetables.

[0066] In another preferred embodiment of the present invention, the carbon dioxide - compressor cooling method includes:

[0067] Add a preset volume of carbon dioxide to the fruits and vegetables, measure the surface temperature of the fruits and vegetables at this time. If the surface temperature of the fruits and vegetables is greater than the optimal storage temperature, then use a compressor to reduce the surface temperature of the fruits and vegetables to the optimal storage temperature.

[0068] It is worth noting that this is a compensation mechanism after the change in the surface temperature of fruits and vegetables. Through this regulation method, it can ensure that the surface temperature of fruits and vegetables remains stably maintained at the optimal state throughout the storage period, avoiding problems such as the decline in the quality of fruits and vegetables, rotting, etc. caused by excessive temperature fluctuations. On the other hand, the presence of carbon dioxide and the combined action of a suitable low - temperature environment can significantly inhibit the respiration intensity of fruit and vegetable cells. As one of the main physiological processes for fruits and vegetables to consume their own nutrients, after its excessive inhibition, the consumption rate of their own nutrients by fruits and vegetables will be greatly reduced, thus contributing to extending the fresh - keeping period of fruits and vegetables, maintaining key quality indicators such as their freshness, taste, color, and nutritional components at a relatively high level, and providing strong guarantee for subsequent sales and consumption.

[0069] A liquid nitrogen fresh - keeping control system for a fruit and vegetable warehouse, comprising:

[0070] Calculation module: Obtain the temperature W of the current fruit and vegetable warehouse, and calculate the ideal usage amount M of liquid nitrogen based on the optimal storage temperature W best of the fruits and vegetables;

[0071] Cooling module: Calculate the single - time usage amount m = M / N, where N represents the preset number of usage times. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen with a mass of m at the preset addition point, update the position of the addition point and continue to add liquid nitrogen with a mass of m, repeating the above steps until the total usage amount of liquid nitrogen is equal to the ideal usage amount;

[0072] Adjustment module: Judge whether the fruits and vegetables are damaged by freeze injury based on hardness testing. If the fruits and vegetables are damaged by freeze injury, adjust the liquid nitrogen addition method, and the liquid nitrogen addition method includes:

[0073] Calculate the proportion B of freeze - damaged fruits and vegetables in the total amount of fruits and vegetables, and update the single - time usage amount of liquid nitrogen Calculate the proportion Bs of freeze - damaged fruits and vegetables in the total amount of fruits and vegetables after the single - time usage amount is updated. When the proportion Bs ≤ 5%, stop the adjustment;

[0074] Compensation module: Calculate the average value W of the surface temperature of fruits and vegetables ave , if the average value W ave >W best , then use the carbon dioxide - compressor cooling method to reduce the surface temperature of fruits and vegetables to the optimal storage temperature W best .

[0075] The above has described an embodiment of the present invention in detail, but the content described is only a preferred embodiment of the present invention and cannot be considered as limiting the scope of implementation of the present invention. Any equivalent changes and improvements made within the scope of the application of the present invention shall still fall within the scope covered by the patent of the present invention.

Claims

1. A method for controlling the liquid nitrogen preservation of fruits and vegetables in a warehouse, characterized in that, It includes the following steps: Step S1: Obtain the temperature W of the current fruit and vegetable warehouse, and calculate the ideal usage amount M of liquid nitrogen based on the optimal storage temperature W of fruits and vegetables; best ​ Step S2: Calculate the single - use amount \(m = M / N\), where \(N\) represents the preset number of uses. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen with a mass of \(m\) at a preset addition point. Update the position of the addition point and continue to add liquid nitrogen with a mass of \(m\). Repeat the above steps until the total usage amount of liquid nitrogen is equal to the ideal usage amount; Step S3: Based on the hardness test, determine whether the fruits and vegetables are damaged by freezing. If the fruits and vegetables are damaged by freezing, adjust the liquid nitrogen addition method. The liquid nitrogen addition method includes: Calculate the proportion B of frozen and damaged fruits and vegetables in the total amount of fruits and vegetables, and update the single - use amount of liquid nitrogen. Calculate the proportion Bs of frozen and damaged fruits and vegetables in the total amount of fruits and vegetables after the single - use amount is updated. When the proportion Bs ≤ 5%, stop the adjustment. Step S4: Calculate the mean value W of the fruit and vegetable surface temperature ave , if the mean value W ave > W best , then use the carbon dioxide - compressor cooling method to reduce the fruit and vegetable surface temperature to the optimal storage temperature W best .

2. The nitrogen preservation control method for a fruit and vegetable warehouse according to claim 1, characterized in that, In the step S1, based on the optimal storage temperature W of fruits and vegetables best , the method for calculating the ideal usage amount M of liquid nitrogen specifically includes: Calculate the temperature difference ΔW = W - W best , calculate the heat consumption Q = c1ΔW based on the temperature difference ΔW, where c1 represents the specific heat capacity of air, and calculate the ideal usage amount of liquid nitrogen Among them, c2 represents the heat of vaporization of liquid nitrogen.

3. A method for controlling the preservation of fruits and vegetables in a warehouse with liquid nitrogen according to claim 1, characterized in that, In the step S2, the preset number of uses \(N\geq4\), and the time consumed for adding liquid nitrogen under different numbers of uses \(N\) is equal to the preset specified time.

4. A method for controlling the liquid nitrogen preservation of fruits and vegetables in a warehouse according to claim 1, characterized in that, In the step S2, the method for updating the position of the addition point specifically includes: Divide a circular area with the distance between the center of the circle and the addition point as the radius. The center of the circle is the center point of the formation of the fruits and vegetables. Rotate the addition point clockwise by \(\theta\) degrees along the boundary of the circular area to obtain a new addition point, where \(\theta = 2\pi / N\).

5. A method for controlling the liquid nitrogen preservation of fruits and vegetables in a warehouse according to claim 1, characterized in that, In the step S3, the method for determining whether the fruits and vegetables are damaged by freezing based on the hardness test specifically includes: Preset \(k\) detection points on the surface of the fruits and vegetables, apply a preset value of pressure to the detection points, and obtain the depression distance \(L\) of the fruit and vegetable surface towards the inside at this time. If there is a detection point where the depression distance \(L\) is greater than or equal to the preset judgment distance, it is determined that the fruits and vegetables are damaged by freezing.

6. The nitrogen preservation control method for a fruit and vegetable warehouse according to claim 1, characterized in that In the step S3, another method for determining whether the fruits and vegetables are damaged by freezing specifically includes: Obtain the ethylene content \(Y\) in the air before adding liquid nitrogen. When adding liquid nitrogen with a mass of \(m\), update the ethylene content \(Y_s\) in the air. If the updated ethylene content \(Y_s>Y + \eta\), it is determined that the fruits and vegetables are damaged by freezing, where \(\eta\) represents the preset ethylene content fluctuation value.

7. A method for controlling the liquid nitrogen preservation of fruits and vegetables in a warehouse according to claim 1, characterized in that, In the step S4, the carbon dioxide - compressor cooling method includes: Add a preset volume of carbon dioxide to the fruits and vegetables, measure the surface temperature of the fruits and vegetables at this time. If the surface temperature of the fruits and vegetables is greater than the optimal storage temperature, reduce the surface temperature of the fruits and vegetables to the optimal storage temperature through a compressor.

8. A liquid nitrogen freshness preservation control system for a fruit and vegetable warehouse, characterized in that, It includes: Calculation module: Obtain the current temperature W of the fruit and vegetable warehouse, and calculate the ideal usage amount M of liquid nitrogen based on the optimal storage temperature W of the fruits and vegetables; best ​ Cooling module: Calculate the single - use amount \(m = M / N\), where \(N\) represents the preset number of uses. Arrange the fruits and vegetables in a preset formation and add liquid nitrogen with a mass of \(m\) at a preset addition point. Update the position of the addition point and continue to add liquid nitrogen with a mass of \(m\). Repeat the above steps until the total usage amount of liquid nitrogen is equal to the ideal usage amount; Adjustment module: Based on the hardness test, determine whether the fruits and vegetables are damaged by freezing. If the fruits and vegetables are damaged by freezing, adjust the liquid nitrogen addition method. The liquid nitrogen addition method includes: Calculate the proportion B of frozen damaged fruits and vegetables in the total amount of fruits and vegetables, and update the single - use amount of liquid nitrogen Calculate the proportion Bs of frozen damaged fruits and vegetables in the total amount of fruits and vegetables after the single - use amount is updated, and stop adjusting when the proportion Bs ≤ 5%; Compensation module: Calculate the mean value W of the surface temperature of fruits and vegetables ave , if the mean value W ave >W best , then use the carbon dioxide-compressor cooling method to reduce the surface temperature of fruits and vegetables to the optimal storage temperature W best .

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