THERMAL AND STRUCTURAL CALCULATION OF AN IMPROVED INFRARED DRYER FOR DRYING MULBERRY FRUITS

ABSTRACT

In order to obtain high-quality dried products, the purpose of this study is to improve an efficient and economical unit for drying mulberry fruits using infrared radiation and convection. Based on the scientific and experimental studies carried out, the infrared drying unit is improved. This drying unit allows faster drying time, does not depend on weather conditions, and maintains the quality of the product.

АННОТАЦИЯ

Для получения высококачественных сушеных продуктов целью данного исследования является усовершенствование эффективной и экономичной установки для сушки плодов тутовника с использованием инфракрасного излучения и конвекции. На основе проведенных научных и экспериментальных исследований, усовершенствована инфракрасная сушильная установка. Данная сушильная установка позволяет ускорить время сушки, не зависит от погодных условий, поддерживается качества продукта.

Keywords: IR radiation, convection, drying, dehydration.

Ключевые слова: ИК излучения, конвекция, сушка, обезвоживание.

For engineering calculations of various drying systems, drying curves are used in practice, showing the change in speed and temperature. Drying curves are obtained experimentally, since it is impossible to calculate the heating time constant without knowing the geometric parameters and the outer surface area of the mulberry fruit. However, an assessment of the drying kinetics of a specific raw material allows us to proceed to the next stage in the improvement of radiation dryers - thermal calculation. The thermal calculation of the IR installation is preceded by the task of determining the power of radiant energy generators, their geometric dimensions and location in the drying installation in relation to the raw material being processed. At the Tashkent State Technical University, the concept of infrared drying plant technology was developed, which was adapted to the needs of micro, small and medium-sized enterprises in the Republic of Uzbekistan. On fig. 1 shows the design of the dryer, which is developed on the basis of analyzes, existing techniques and technologies for drying mulberry fruits [1].

1-case; 2-shelves (racks for pallets); 3-door; 4 windows for monitoring the drying process; 5-leg installation; 6-mesh pallets; 7-infrared emitters; 8-thermocouples; 9-fan; 10-control unit; 11-lock
Figure 1. Advanced IR dryer

Thermal design of an advanced infrared dryer. To solve the problem of creating energy-efficient and environmentally friendly drying plants for food products, P.D. Lebedev proposed a differential heat balance equation, which is valid for calculating the technological process of drying raw materials and bodies of any configuration [2].

The heat balance equation for the conditions of uniform heating over the thickness of the irradiated body, in which the energy absorbed by the irradiated raw material over time , will be spent on heating it, transferring heat by convection and radiation to the surrounding space and evaporating moisture from it, has the following form:

             (1)

here  is the coefficient of absorption of radiation by the irradiated raw material;  is irradiation density , W/m;  and  are the area of the irradiated and total surfaces of the raw material, m;  is time from the beginning of exposure to infrared radiation, h;  is mass of processed raw materials (mulberry fruits), kg; and  is temperatures of raw materials and ambient air, °С;  is heat capacity of the irradiated raw material, kcal/kg deg;  is reduced degree of emissivity of the irradiated raw materials and internal enclosures of the drying plant;  is coefficient of heat transfer by convection, kcal/m² h deg;  and  are temperature of raw materials and surrounding surfaces, °C;  is the rate of evaporation of the substance (initial intensity), kg/m² h;  is index of radiation absorption by the cocoon, 1/m;  is depth of raw material permeability by infrared flow from its outer surface, m.

The ratio of total heat transfer (convection and radiation) to convective losses is assumed to be constant due to minimal heat losses due to the low heating temperature of the irradiated raw material:

                                                       (2)

The amount of heat loss due to heat transfer over time is approximately determined by the formula below:

                                    (3)

here is the total heat transfer coefficient, kcal/m² h deg.

In practical terms, the value of the overall coefficient, α ranges from 16 to 20 kcal/m² h·deg. With approximation, the heat balance of the irradiated raw material is also determined, since the moisture evaporation rate is considered constant and equal to the average intensity . The equation can be represented as:

                            (4)

here  is the ratio of the areas of the total surface and its irradiated part;

 is the ratio of the total surface area of the irradiated raw material to its volume m²/m³;  is specific gravity of the irradiated raw material, kg/m.

Dividing the variables in the resulting differential equation and substituting on it:

                                                (5)

                                                      (6)

The final form of the heat balance equation for the irradiated raw material is:

                                                (7)

Integration of the obtained expressions over from  to and over from a given initial temperature  to the final temperature gives an expression for the corresponding heating time :

                                             (8)

From the resulting heat balance equation, one can also derive an equation for the heating kinetics of an irradiated body:

                                           (9)

Determination of the main characteristics according to these dependencies will help to calculate the energy consumption, which depends on the radiation density and the location of the infrared generator in the installation:

                                                      (10)

here  is energy illumination or radiation density, W/m;  is emitter power, W;  is distance between infrared emitters, m;  is source efficiency coefficient, depending on the degree of space filling by the irradiated raw material and on the ratio of the chamber length  to  is the distance from the emitter to the irradiated surface of the raw material (in practical conditions it varies within 0.7…0.85).

Multiple reflection coefficient :

                                               (11)

here  is the reflection coefficient of the camera;  is the reflection coefficient of the product irradiation surface; is the fraction of the stream reflected by the camera.

Then the energy consumption for drying will be expressed by the equation:

                                                   (12)

here η is the energy efficiency of the emitter.

Structural calculation of an advanced infrared dryer. The main advantage of the infrared drying process is the higher rate of moisture removal compared to other drying methods. This advantage is due to the action of the flow of radiant thermal energy, which penetrates to a depth of 0.1...2.0 mm of the processed raw material.

Due to the large number of reflections from the walls of the advanced infrared dryer, the infrared rays can be almost completely absorbed. The heat transfer coefficient in this case is assumed to be much higher. Thus, a large amount of heat is transferred per unit surface area of the dried product per unit time. This advantage makes it possible to significantly speed up the drying process of mulberry fruits [3].

The material balance is designed to determine the amount (flow rate) of evaporated moisture and the flow rate of the drying agent and consists of dried material and gas flows. To determine the hourly output of a dryer, it is necessary to establish the annual output of the dryer for the finished product. Then the hourly output of the dryer is  (kg/h):

 kg/h                                              (13)

here  is annual productivity of the drying plant for finished products, kg; is the number of hours of operation of the device per day; is the number of working days in a year.

If during the drying process there is an irretrievable loss of material, then the hourly productivity is calculated with the following correction:

 kg/hour                                            (14)

here  - coefficient taking into account the release of finished products (taken in the range from 0.95 to 0.99).

The amount of moisture removed (in kg/h) is determined using the material balance equation:

 kg/h                                       (15)

here  is initial moisture content of the material, %;  is the final moisture content of the material, %.

Then the productivity of the plant for drying raw materials will be (in kg/h):

 kg/h                                                  (16)

During the drying process, the mass of absolutely dry matter () does not change if there are no other losses, i.e. (in kg/h):

 kg/h                              (17)

Where

                                              (18)

In this case, the moisture content of the material will be:

• initial humidity:

                                               (19)

• final humidity:

                                               (20)

Calculation of the heat and mass transfer surface of the drying chamber. To determine the dimensions of the device, it is necessary to calculate the surface of the material through which heat and moisture are transferred, or the duration of the processing of the material, respectively [4].

The following ratio applies to any dryer:

 h.                                        (21)

here  is the amount of material simultaneously filling the dryer in the drying zone, kg;  is the average integral residence time of the material in the drying zone, h;

Calculation of the overall dimensions of the installation. For drying mulberries, a cabinet-type convective infrared dryer is used.

The mass of the dried product at the outlet of the dryer  (kg/h) is calculated by the following formula:

                                            (22)

The mass flow rate  (kg/h) for the drying process is determined by the formula below:

 kg/h                                           (23)

here W is the amount of moisture evaporated when the product is in the drying zone, kg/h; l is air consumption for evaporation of 1 kg of moisture, kg/kg:

 kg/kg                                            (24)

here  and are the moisture content of the air at the inlet and outlet of the drying chamber, respectively, g/kg.

The volume of consumed (consumed) air  (m³/s) is calculated by the following formula:

                                   (25)

here  is specific volume of air, m³/kg; %;  ­is outdoor air temperature, °С;  is the gas constant;  is relative humidity of the outside air,  is saturated vapor pressure at t₀, Pa.

Heat consumption Q (J/h) is determined as follows:

  J/h                                                 (26)

here  is specific heat consumption per 1 kg of evaporated moisture, J/kg;

 J/kg                                             (27)

here and  are the enthalpy of humid air before and after infrared heating (determined from the Ramzin diagram).

Drying unit pallet area (m²):

 m²                                                 (28)

here is the specific productivity of the installation for a dry product, kg/(m² h).

Total length of drying plant pallets  (m):

 m.                                                 (29)

here  is the width of the pallet, m.

These calculations make it possible to evaluate installations for drying agricultural products, including mulberry fruits, when improving infrared drying installations, since an improved installation must ensure accurate compliance with the drying mode parameters for uniform drying of raw materials throughout the entire volume of the dryer. camera. The regime parameters of drying include the most favorable conditions for temperature, radiation wave length, material humidity and air velocity [5].

Conclusions

1. The design of the convective infrared drying plant for drying mulberry fruits has been improved. And also, the parameters, overall dimensions and performance of the proposed rational pilot industrial convective drying plant with IR heating were determined.
2. The thermal and structural design of the improved infrared dryer has been performed. In the future, it is advisable to obtain mathematical models of the drying rate of mulberry fruits, depending on the influence of controlled and uncontrolled factors on the drying process. Mathematical models will make it possible to develop an automatic control system for the infrared drying process.
3. It has been established that the advanced infrared convection dryer is able to dry products from 83-91% moisture to 16-25% moisture at a drying temperature of 65 ºС, 70 ºС and 75 ºС. Thus, this installation can be effectively used in micro, small and medium-sized enterprises of the Republic of Uzbekistan.
4. Unlike other processing methods, infrared drying allows you to get a final product that retains almost all of its valuable physical and chemical properties. Energy consumption for the drying process in an infrared dryer is up to 10-12 times lower than in other types of installations.

References:

1. Abhijit T., Сафаров Ж.Э., Султанова Ш.А. Исследование процесса сушки плодов тутовника “Развитие науки и технологий” // Научно–технический журнал №6/2021, С. 200-205.
2. Лебедев П.Д. Аналитические и численные процедуры построения решений некоторых задач управления. Дисс. кан. тех. наук. 2009. с.150.
3. Gündoğdu M., Kan T., Canan I. Bioactive and Antioxidant Characteristics of Blackberry Cultivars from East Anatolia. Turkish Journal of Agriculture and Forestry, 2016, 40(3): 344-351.
4. Abhijit T., Сафаров Ж.Э., Султанова Ш.А. Моделирование процесса сушки пищевого сырья // Universum: технические науки: электрон. научн. журн. 2021. 11(92).
5. Tarawade A., Safarov J.E., Sultanova Sh.A. Mathematical modeling of the drying process of capillary porous material / International scientific and technical journal. Innovation technical and technology Vol.1, №.3. 2020.