Abstract
Abstract. To ensure the high quality of the materials being dried, one of the most promising ways to dehydrate vegetable raw materials is drying under the action of electromagnetic radiation (microwave drying). Microwave drying is widely used in various industries, in particular, in the food and woodworking industries. (Research purpose) Calculation of the microwave pulse and pause duration; their experimental determination, as well as the determination of the moisture removal rate at the pulse moment and the depth of pulse penetration into the layer. (Materials and methods) When drying in a microwave field, the gradient of moisture content in the material prevents the moisture movement towards the surface, internal cracks can be formed as well. Therefore, the combined methods of drying can yield the best results. The pulse duration has been calculated by the permissible increment of the grain temperature, the pause duration has been determined by assuming that during the pulse, moisture from the caryopsis kernel is pushed out and cooled under isothermal conditions by an air flow. (Results and discussion) It has been confirmed that at a microwave pulse energy duration of 4, 6, and 10 s, the speed of blow-off and blowdown of the layer with external air was 0.5 m per second. When grain is cooled by natural convection, the pulse time is 10 s, the pause time is 1, 2, 3, and 5 min. For the blow-off mode, the pulse exposure time was 6 and 10 s, that of blow-off - 0.5, 1.0, and 1.5 minutes. The maximum duration of the microwave pulse was determined by the flow density of allowable grain heating and the fraction of heat required for the evaporation of moisture when heated to 20-25 degrees. (Conclusions) The pause duration is determined by the grain cooling time to the temperature preceding the pulse. It has been experimentally established that the calculated values of the pulse and pause duration with an accuracy of 15 percent for grain with a moisture content of 20-24 percent at a microwave flow density of 0.7 kilowatts per square meter, at a depth of the microwave energy penetration into the grain by 70 percent, do not exceed 20-22 mm, and the moisture removal rate is 0.1-0.15 percent.
Highlights
Calculated by the permissible increment of the grain temperature, the pause duration has been determined by assuming that during the pulse, moisture from the caryopsis kernel is pushed out and cooled under isothermal conditions by an air flow. (Results and discussion) It has been confirmed that at a microwave pulse energy duration of 4, 6, and 10 s, the speed of blow-off and blowdown of the layer with external air was 0.5 m per second
When grain is cooled by natural convection, the pulse time is 10 s, the pause time is 1, 2, 3, and 5 min
The maximum duration of the microwave pulse was determined by the flow density of allowable grain heating and the fraction of heat required for the evaporation of moisture when heated to 20-25 degrees. (Conclusions) The pause duration is determined by the grain cooling time to the temperature preceding the pulse
Summary
(Цель исследования) Рассчитать длительность СВЧ-импульса, паузы; экспериментально определить их, а также величину влагосьема при импульсе и глубину проникновения его в слой. (Результаты и обсуждение) Подтвердили, что при длительности импульса СВЧ-энергии 4, 6 и 10 с, скорость обдува и продувки слоя наружным воздухом составляла 0,5 м в секунду. При охлаждении зерна естественной конвекцией время импульса – 10 с, время паузы – 1, 2, 3 и 5 мин. Максимальную длительность СВЧ-импульса определили плотностью потока допустимой теплоты нагрева зерна и долей теплоты, пошедшей на испарение влаги при нагреве до 20-25 градусов. Экспериментально подтвердили расчетные значения длительности импульса и паузы с погрешностью 15 процентов на зерне влажностью 20-24 процента при плотности СВЧ-потока 0,7 кВт на квадратный метр; глубина проникновения в зерновку 70 процентов СВЧ-энергии выше 20-22 мм, а влагосьем – 0,1-0,15 процента.
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