Microwave drying application requires attention to several issues and countermeasures

    Using high energy conversion efficiency, easy to achieve automatic control, small footprint, you can shut down at any time according to the actual production of microwave heating and drying equipment and other measures, can be avoided using the general conventional furnace or boiler heating occurs when the products and environmental pollution , security risks, high maintenance costs, life is short, difficult to control working temperature and other issues. However, if used improperly, especially microwave drying chamber structure and load configuration unreasonable, will lead to microwave energy reflection, reducing efficiency, uneven heating of materials, product quality is difficult to guarantee and other issues. The latest research results show that the microwave drying process in the form of load, ventilation and ventilation or direction for drying uniformity, product quality and energy efficiency of microwave drying equipment, etc. have a great impact.

    1 air direction and the air volume to the choice of rice and brown thick layer of microwave drying results show that, in the microwave drying proper ventilation is necessary, the ventilation direction of wind of the size if appropriate, will affect the drying uniformity, energy efficiency drying process and other important performance indicators. According to the dry material layer ventilation and microwave radiation in the direction of the direction of the relationship between the ventilation mode can be divided into downstream ventilation, ventilation and cross-flow ventilation countercurrent three forms [Note: countercurrent flow ventilation is ventilation flow in the material layer the direction opposite to the direction of microwave radiation material, i.e. bottom-up flow of air, while the microwave radiation from top to bottom; downstream ventilating air ventilation means in the material layer of the material flow direction and microwave radiation in the same direction, that is, since the down flow (radiation); cross-flow ventilation is ventilation air in the direction of flow of the material layer and microwave radiation in the direction perpendicular to the material.

    Thick layer of microwave drying rice experimental results show that the downstream ventilation can improve the uniformity of drying, energy efficiency, and the quality of the material to be dried. Which is mainly reflected in the dry material layer moisture and temperature distribution is more uniform.

    In the state of the downstream ventilation microwave drying rice after 90min thick the temperature and water content distribution typical curves. Expressed in its downstream ventilation mode with the thick layer of brown rice microwave dry entertainment features. Rice and brown rice due to have similar characteristics, trapped in the thick layer of rice moisture content of the temperature distribution curve of the phase spoon thick layer of rice and water in a temperature distribution curve is very similar rates. From the highest point to the brown layer at a depth of 0.05m, as the depth increases, the temperature will increase, but the rate of moisture content decreases. Although the intensity of the microwave heat energy at the highest point theory is the strongest, but the temperature is relatively low. At a depth of 0.0 5-0.07m interval, the temperature reached the highest value, the moisture content has reached a minimum value. The maximum rate of increase of temperature with the efficiency and reduce the rate of a maximum depth of 0.05 is within the range 0.0 7m. Ventilation air and the heat transfer between brown in this range have reached equilibrium. Continues to increase with depth, the temperature will decrease, because the depth of more than 0.07m, the microwave energy will become increasingly smaller heat. Thus, in depths greater than 0.130m, the drying capacity will become increasingly weak. Thick layer of rice microwave drying there is a similar situation. Thick layer of rice in the microwave drying process, there is indeed an effective dry thickness, drying the effective thickness of about 0.130m.

    Downstream of the thick layer of microwave drying air has two effects: First, the grain layer is evaporated out of the water to the dryer, the other - an effect that heat transfer. Circulating air in the care of the state, the air of the highest intensity of heat from the microwave energy to the thickness of the material layer, the top layer depths, so that the heat required for drying is evenly distributed. According to the theory of microwave transmission, brown thick layer of the intensity distribution of the microwave energy to be negative exponential curve. Microwave drying rice experimental results also show that the non-ventilated conditions, verifying the grain thick layer of the temperature distribution of drought negative exponential curve shape, the thickness of the top layer of the highest temperature. When the downstream ventilation, as described above, the top of the microwave energy is transmitted to the downstream of the air ventilation layer is deep brown, so not the highest temperature at the top. And thick layer of relatively high temperature zone extends to depths of thick layers. Ventilation in the thick layer of brown rice downstream microwave drying temperature and moisture distributions spoon plays a very important role in nature. In fact, the aforementioned microwave drying thick brown dry thickness of the effective state in the downstream ventilation plant will increase.

    For a thick layer of brown rice and microwave drying, if the microwave power is controlled 0.05-0.09kW/kg speed is controlled within the range 0.12-0.20m / s range, you can guarantee that there are no fissuring and germination rate and decreased quality problems.

    2 the microwave drier for reflection problems limited by the characteristics of the microwave transmission in the microwave drying chamber and drying of the structure design and the actual operation of the energy reflection problems inevitably exist.

    Spread evenly load conditions of experimental research results show that, with the amount of material in the drying chamber increases (ie as the drying bed material thickness increases), the power reflection coefficient decreases. In the rice tile load conditions, when the material thickness of less than 10mm, the power reflection coefficient in the 0.42-0.52 range. If the rice tile load conditions. Material thickness is greater than 20mm, or brown tile load conditions, the material thickness is greater than 17mm, the power reflection coefficient will be less than 0.15. This change in the power reflection coefficient is because as the number of changes in the material bed drying, will change the impedance of the load, thereby enabling the drying chamber also change the power reflection coefficient. Drying air to the power reflection coefficient exists to reach the minimum threshold value of the material thickness. For rice and brown rice, the tile load conditions, the critical values ​​of 20mm and 17mm.

    Experimental studies also showed that due to the different variety of materials with different dielectric constants, so that drying air in the case of the same amount of materials exhibit different power reflection properties. Therefore, the same amount of material and the microwave power condition, the dielectric loss constant of the microwave energy absorbed by the material more reflected less, so the drying chamber is relatively small power reflection coefficient.

3 microwave drying chamber loading patterns related to the impact on the energy efficiency of the floating layer and spread evenly load conditions rice and brown rice in the microwave drying process of the energy efficiency results suggest that if drying methods and processes will be reasonable, microwave drying rice and brown rice can achieve high energy efficiency, even higher than the traditional method of drying can be achieved. Arranged in the microwave drying process in order to play a certain time interval post-irradiation effects, intermittent irradiation effects or retarding effect, can effectively improve the microwave energy efficiency of the drying process. Intermittent drying process was significantly higher than the energy efficiency of the drying process is not intermittent energy efficiency. Load the form of microwave energy efficiency of the drying process is also influenced, microwave drying should adopt the form of thin load.

 

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