Automatic control system for automatic maintenance of microclimate in vegetable storages

. The paper presents an automatic control system for automatic maintenance of the desired microclimate in the storage, controlled by a computer. The state of the microclimate in the vegetable store is monitored with the product and channel temperature sensors and internal and external humidity sensors. This system will automatically control the ventilation parameters at different times of the year, and regulate humidity conditions.


Introduction
Vegetable preservation strongly depends on the microclimate created in the vegetable store.Different vegetables require different storage conditions.Temperature, humidity, and carbon dioxide concentration have the strongest effect on vegetable preservation.Comfortable storage conditions for root crops, vegetables and fruits are created with the microclimate control and temperature, humidity, and air content control systems [1][2][3][4][5][6][7].
The vegetable store ventilation control system includes: duct fans to ensure the main ventilation of the vegetable store; heated ceiling anti-condensate fans to prevent condensation on the ceiling structures of the vegetable store; valves with actuators and valve position sensors that regulate the air supply from the street and the air agitation inside the vegetable store; automatic control units for the vegetable store ventilation system, including a control unit and switching units for the actuating elements of the ventilation system (fans, valves, etc.); air temperature and humidity sensors, inside and outside the vegetable store, product temperature sensors, carbon dioxide concentration sensors (desired) and other elements.

Method
To automatically maintain the desired microclimate in the storage, an automatic control system is used, which is controlled by a computer; using product and channel temperature sensors, and internal and external humidity sensors, it monitors the state of the microclimate in the vegetable storage room [1,[8][9][10][11][12][13].
The mathematical description of thermal processes in the mass of stored products can be determined analytically from the equation for the dynamics of heat exchange between stored vegetables and ventilated air [14][15][16][17].
Heat transfer in the bulk of piece agricultural products is a complex physical phenomenon.The temperature on the surface of the product is determined not only by the intensity of heat removal from the surface but also by its removal from the internal volume of the tuber product.This component of heat is formed as a result of biochemical processes [18][19][20][21].
The heat transfer control, taking into account internal sources and heat costs for the water evaporation from the product, can be written as follows: where С -is the volumetric heat capacity of tubers, J/( 3 ℃); V is the volume of the layer of stored products, m 3 ; p is the porosity (pore space) of the layer of stored products: for potatoes, it is 0.38 ... 0.45; К 0 is the volume of tubers, m 3 ; θ is the temperature of tubers, ℃; t is time, s; q is the amount of heat released in volume V of product per one second, J/s;  и is the amount of heat spent on the water evaporation w (kg/s) with the heat content of water vapor i (J/kg), J/s; α is the volume heat transfer coefficient, J/(m 3 ℃); θB is the air temperature in the inter-tuber space, ℃.
We transfer components  and  и to the left-hand side of equation ( 1) and take    out of the brackets.Then Since all the quantities in brackets on the left-hand side of equation ( 2) have the dimension of volumetric heat capacity [J/(m 3 ℃)], it can be represented as: where С р is the calculated volumetric heat capacity of the layer of potato.
Passing to the operator form and transferring the terms containing θ to the left-hand side, we obtain: Then the transfer function of the process can be written as follows: where Т = С р /.The heat balance equation for air passing through a layer of tubers with thickness h has the following form: where С в is the volumetric heat capacity of air, J/(m 3 •℃); v is the air velocity equal to its quantity (m 3 ) passing through the cross section of the product layer (m 2 ) in 1 s, m/s.
It can be seen from equations ( 2) and ( 6) that the intensity of temperature change in the mass of the product depends on the velocity of the passage of the supply air, the thickness of the layer h of the tuber bulk, the porosity of the layer p, and the initial temperatures θ of the tubers and θB of the air.
Experience shows that the temperature of the supplied air and in the bulk of tubers is not the same along the height of the layer.The layers of tubers cool quickly at the air inlet and 4-5 times slower at the exit from the four-meter layer of the bulk of the potato.The highest temperature of the mass of the stored product is observed at a depth of 0.4 ... 0.6 m from the surface of the bulk.
The thermophysical properties of the tuber bulk depend on its temperature and type.Because of the listed features, it is difficult to accurately determine the result of the coupled solution to equations ( 3) and ( 6).The transfer functions of thermal processes in the mass of stored products can be determined from the acceleration curves.
It was stated that at air supply L< 50 m 3 /(t • h), i.e., per 1 ton of tubers, the transfer function can be expressed as follows: and at L< 50  3 /(t • h) With an increase in air supply from 50 to 250 m 3 /(t • h), the value of the transfer coefficient k decreases from 0.03 to 0.008.The transfer coefficient k shows by how many degrees the temperature in the tuber bulk decreases in 1 h when supplying 1 m 3 of air per 1 t of tubers.The time constants also depend on the air supply: at  ≤< 50  3 /(t • h) Т= 7...8 h, at L > 50...250  3 /(t • h) Т 1 = 8 … 6 ℎ,  2 2 = 2 … 1,6 ℎ.When the ventilation is off, the temperature of the mass of the stored product rises due to self-heating.Transfer function of the mass of the product during self-heating without heat removal is: where   is the transfer coefficient showing by how many degrees the temperature of the mass of the product rises for 1 hour of self-heating without heat removal: for root crops, it is   = 0,14, for cabbage   = 0,13.
The transfer function of the upper zone of the vegetable store can be determined from the differential heat balance equation: where С is the specific heat of air, /( • ℃); G is the mass of air in the upper zone, kg; θ is the air temperature in the upper zone, ℃; q is the heat release of the product, J/s; a is the heat transfer coefficient of air to the fences, J/( 2 •  • ℃); F is the surface area of the fence,  2 ; G B is the specific air flow at the inlet to the upper zone, kg/s; θ0 is the fence temperature, ℃; θB is the air temperature at the entrance to the upper zone, ℃.
If, due to the lack of numerical values of the quantities included in equation ( 10), it is difficult to find an analytical and quantitative expression for the transfer function, then the experimental acceleration curve should be plotted and the transfer function of the upper zone should be determined from it.It is expressed by three components (according to the number of parallel operating excitations): For a typical vegetable store for 1000 tons, we can take  1 = 0,3 ,  2 = 0,5 ,  3 = 0,2 ,  1 = 2,3 ℎ,  2 = 0,12 ℎ,  3 = 0,04 ℎ .
In all vegetable stores with automatic microclimate control, a mixing device is used.As a control object, this device can be described by the heat balance equation in increments: where  н and  р are the temperatures of the outdoor and recirculation air, respectively, ℃; ∆ н = −∆ р is the increment of mixed quantities of outdoor and recirculation air, kg/s; ∆ п is the temperature increment, ℃;  п is the specific consumption of supply air, kg/s.
Taking into account the above relations, equation ( 11) can be represented as: from where it is possible to determine the transfer function of the mixing device as an instantaneous (inertialess) element: The resulting mathematical expressions can be used when choosing elements of control devices, as well as when setting up regulators for automatic climate control systems in potato storage.

Results and discussion
The designs of fruit and vegetable stores have much in common.A requirement of the technological process of fruit storage is the necessity to cool the product and accurately maintain the relative humidity of the air.Therefore, the automation scheme for fruit storage equipment includes control systems for air-cooling units and steam supply for air humidification in the chambers.
In fruit storage, the concentration of carbon dioxide is maintained at a level significantly higher than in the atmospheric air, reaching 1% or more.At that, the oxygen content decreases, and the nitrogen content increases.These circumstances improve the storage conditions of fruits.The CO2 content is controlled by passing circulating air through limewater or by burning the gas with a controlled air supply.The gas mixture thus obtained, enriched with nitrogen, is cooled and fed into the storage.The recommended storage temperature is less than 5°C, but it should be higher than the freezing temperature of the fruit; it must be maintained with high accuracy.The control of the moisture content of the gas mixture is also of great importance since it determines the loss of water by stored fruits and controls the content of ethylene gas emitted by the fruits.
Active ventilation makes it possible to maintain optimal temperature and humidity conditions in storage facilities.At the same time, it ensures the removal of water from the surface of vegetables, and respiration products leading to the development of pathogens in the mass of vegetables.
Air is supplied to the mass of the stored product using supply ventilation systems equipped with centrifugal or axial fans.The mode of operation of the ventilation system depends on the outdoor temperature, type, and weight of the stored product.To reduce the temperature of the stored product, outside air is blown by a fan through the supply shaft through the ventilation duct into the product mass.At unacceptably low or high temperatures of the outdoor air, the fan drives the internal (recirculation) air through the product; the supply chamber is closed by a valve at this time.Automatic humidity control is rarely used due to the lack of sensors operating at a relative air humidity of more than 90%.If necessary, the humidity is controlled manually, including exhaust fans.

Conclusions
For fruit storage with a capacity of 1000 to 3000 tons, a set of electrical equipment was developed that provides automatic control of the microclimate in fruit storage chambers, control of the operation of condenser and evaporation equipment, operation control and emergency protection of compressors of refrigeration machines, signaling about the operating modes of the equipment (Fig. 1).One set can automatically control two to four cameras.An obligatory condition for storing vegetables is a special temperature regime.For this, refrigeration art is used.Recirculation of air in a vegetable store requires the installation of a ventilation system.This system will automatically control the ventilation parameters at different times of the year, and regulate humidity conditions.The microclimate monitoring and control system of the vegetable store allows for controlling the temperature and humidity, simultaneously controlling the operation of the refrigeration and ventilation systems.Refrigeration units for vegetable stores automatically maintain the required temperature; their operating system controls the function of the compressor, air coolers, and condensers.The monitoring function of modern refrigeration equipment will allow the owner of a vegetable store or a service organization to remotely control the temperature regime and the operation of refrigeration units; and, in case of an emergency, quickly take the necessary measures.
The microclimate system controls all the necessary technological equipment: shutters, EC fans, valves, jet fans, heaters, dosing devices, and shut-off valves for the gaseous medium.Placement of additional equipment: humidifiers, ozonizers, and refrigerators allows for increasing the shelf life and improving the quality of products.