Several Models of Industrial Combination Drying in China

- May 07, 2019 -

Combination drying is a kind of drying technology with broad prospects for development. It can give full play to the advantages of various drying processes, overcome their shortcomings and make up for their shortcomings by borrowing advantages to achieve high efficiency, low energy consumption and high quality. Because microwave drying is a drying technology completely different from other drying methods, it is also a drying technology which combines most with other drying methods, and is also a drying technology which has been studied most in the world at present. The following are several common combination drying methods combined with microwave, the combination drying of microwave hot air is the most studied one. Because of the long drying time and poor quality of hot air, it is not suitable for drying thermal sensitive materials. The combination of hot air and microwave drying can overcome the above shortcomings. In addition, the cost of microwave drying is higher than that of hot air drying, and microwave drying alone is uneconomical. Hot air drying for materials is from the surface to the internal drying, the direction of temperature gradient and moisture transfer is opposite, while microwave drying is from the internal heating, the direction of temperature gradient and moisture transfer is the same, the combination of the two can achieve the purpose of shortening drying time and reducing costs. There are three ways to combine microwave with hot air drying: (P) < P > When drying from constant speed to deceleration (i.e. material moisture content reaches critical moisture), microwave can be introduced into the dryer to generate heat and vapor pressure inside the material, which can diffuse moisture to the surface of the material and remove it. At this time, the drying speed can be greatly improved by using microwave. It takes a long time for a single drying system to remove a few percentage points of moisture at the lowest efficiency near the end of drying. Microwave can significantly reduce the drying time. Before drying, the moisture content of the material is high. The material can be heated to evaporation temperature by microwave first, then dried by ordinary hot air to remove surface moisture. The drying time can be shortened.

Microwave has the advantages of fast heating speed, short drying time, good selectivity, high energy utilization and easy control, but it is easy to produce burns caused by overheating and food marginal coking, crust and hardening by using microwave alone in food drying. Most of the above phenomena are caused by too high temperature and too fast drying.

P>Vacuum can reduce the evaporation temperature of water, make the material evaporate rapidly at lower temperature, and avoid oxidation, thus improving the drying quality. In the field of medicine, food and chemical industry, there are many heat-sensitive materials that need to be dried rapidly at low temperature. Therefore, the combination of microwave technology and vacuum technology has become a new technology with great development prospects and practical value.

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