5. Microwave Drying industrial applications

Microwave heating in the 1970s, China's electronics industry subordinate units established Institute of Applied microwave, promote the microwave heating technology. Some military enterprises successfully developed 5kw, 10kw, 20kw magnetron, klystron, trial 20kw, 40kw industrial microwave ovens, wood, food, pharmaceutical, chemical, ceramics and other industrial applications sector, to obtain a higher economic efficiency.

(1) microwave food quick drying
Microwave heating method for the moisture content of dried vegetables less than 20% of the "greens", high-efficiency than the traditional method 10 times. Microwave heating and drying seaweed food, its flavor and little difference between fresh seaweed. Fried foods by microwave for final drying, not only fuel efficient, but also get a light and delicious food low oil content, such as instant noodles and other foods.

(2) food, dried fruit microwave puffing
Protein-based feed supplements added starch, salt, spices and other mixed-expanders containing mixing, forming, pre-drying and then microwave heating to produce carbon dioxide and water vapor to get loose texture of food. Apple slices, pre-dried (dry moisture of 92% to 20% moisture), after drying by microwave heating and then rapidly cooled, and then get the same flavor of fresh apple apple crisp.

(1) microwave cooking meat processing
Microwave heating cooking meat, with an average energy saving of 63%, the nutritional content of meat are not affected. As 2.2kw microwave oven barbecue 2450MHz frequency just 2.8min, meat bones can reflect the microwave, it is easy to cooked meat inside.

(2) microwave sterilization of food preservation
80% of urban residents in China has family home microwave ovens, very few people know the leftovers in the microwave heating 1 ~ 2min, general bacteria will be killed.

(3) Microwave Drying herbs sterilization
Chinese herbal medicine is a national treasure of China, but whether Pieces or water pills, pill, because its content is more bacteria, so the change of foreign exchange export restrictions. We all know that herbs are plant roots, stems and leaves. After picking through the sun, the old drying room with washing and drying, so the higher the number of bacteria, 50000-100000 per Dakota. The pharmaceutical cobalt 60 irradiation of bacteria dropped to about 10,000. Microwave heating and drying, not only shorten the time many (such as drying room for an old-fashioned pill 6h, and continuous microwave drying only 22min), but also reduce the number of bacteria an order of magnitude (10 000 bacteria can be reduced to 1000 or so), Chinese medicine components are not affected. Bolus good color, low shrinkage, especially herbs.

(4) Microwave Drying of the ceramic body
Two continuous microwave drying ceramic body, drying time is shorter than the old method 2/3 (down to the 1.5h 0.5h), and uniform moisture content of the ceramic body ministries, wrinkled surface without anesthesia. Glazed, burning manufactures high yield, good quality.

(5) Microwave Drying of wood
A high-tech enterprises in Nanjing Northeast timber, the company offers a lot of sets of microwave heating and drying equipment, with 915MHz, 60kw tunnel microwave dryer mahogany, birch and other heating and drying, to obtain very gratifying results.

 (6) Drying field dyes, polymers and other materials, the microwave drying technology come in handy
According to data analysis, some dyes and auxiliaries during drying, environmental protection has become a prominent problem, such materials smaller particle size (<100μm) and with vivid colors, even a small amount of dust on the environment will cause significant pollution, so the drying process exhaust dust collector to be very careful, it normally takes two dust collector, and the second stage is often used bag filters or wet scrubbing dust collector, the wash water to be recycled, without direct emissions, otherwise cause secondary pollution on the environment. If the pre-pre-dried (moisture content 15% control) were granulated (particle size diameter by commodity), and then the particle dyes microwave heating drying, the product can be dried evenly, and can improve the color, solubility, diffusivity, and adsorption and other physical properties. Because microwave heating and drying can strictly control the drying temperature and time to obtain the desired product polymorphs thereof.

6. microwave dryer

Microwave dryer, microwave source consists of a control cabinet, the resonant cavity (drying chamber), gear, humidity and fans and other components. It can be various forms of the gap or continuous material heating and drying. Microwave power sources are available klystron, a magnetron (5kw, 10kw, 20kw) or low-power microwave tube (1kw, 2kw). Multi-source energy feedback manner, whether it is high-power or low-power microwave tube can design and manufacture of high-power microwave dryer, as with two 20kw magnetron from two holes coupled microwave energy is fed into the drying chamber, it is obtained 40kw dryer. 2kw as 20 points and 20 microwave tube coupling hole fed to the drying chamber can be obtained 40kw of microwave dryers, these two methods to obtain high power dryer, own merits and demerits, but the use of low-power multi-port tube fed have the following advantages: low power tube low price, easy maintenance, single-tube damage can not stop replacement.

Microwave dryer drying chamber (or microwave cavity) can be made into various types, mainly multi-mode microwave oven (rectangular, round cavity standing wave), single-mode resonant cavity traveling wave type, three. Depending on the shape of the heated drying materials designed for different types of drying chamber, such as filaments, fibers, paper available cavity traveling wave, electromagnetic wave propagation in the heater straight or zigzag propagation, flat, V-type waveguides, ridge waveguides bow. Drying chamber powder, paste, granules and other materials can be designed into the box, microwave dryer tunnel, regardless of the microwave cavity, are by their inherent quality to measure the number of its advantages.

Through the characteristic

Where V is cavity volume, m3; S cavity surface area m2; δ, the skin depth, m, δ = / πfμσ, μ permeability, H / m. The formula can tell us the material, shape, size to estimate its value based on Qo of the resonator. Qo is a measure of the cavity loss of basic parameters, the other parameters of the resonance frequency fo is the basic parameters of the cavity resonance characteristics. Characteristic impedance ξo basic parameters of a reference cavity surface electric field strength measurement.

The emphasis here is fo, Qo, ξo are to a certain oscillation modes in terms of their basic values different modes general parameters are different. Cavity required metal conductor material, such as stainless steel, aluminum and the like.

Role in the transmission of microwave dryer, the main transport of materials in and out of the drying chamber, such as conveyor belts, extinguishers, and other media, are required insulator material, such as PTFE, quartz glass, porcelain and other 95.

Microwave heating of materials into and out of the dryer in the drying chamber is provided at the entrances yoke microwave leakage current, leakage amount lower than 2mw / cm2.

In short, microwave heating and drying equipment, small size, simple structure, high efficiency, easy operation and maintenance. Why higher disposable microwave heating drier investment? The main investment is large microwave source, microwave source and control cabinets need of such 5kw (3-5) million, 10kw need (5-7) million. With the development of the electronics industry, low-power microwave source will achieve production, advanced production technology, lower prices and a high efficiency, long life purpose. With a low-power microwave tube-fed multi-source energy of microwave heating drier reasonable prices will fall.

7. Conclusions

This paper proposes a new powder processing process route, namely the pre-pre-dried powder, the final moisture content control in 10% to 15% - and then the bulk of this material were wet granulated roll - for particulate material by microwave heat drying is performed. By analyzing the full text of this process route has the following characteristics:

(1) the entire process without serious dust and pollution of the environment.
(2) drying rate is high, pre-dried to remove the moisture free water, and dried at constant drying stage, the drying stage that is controlled by external factors. It can only change the external variables such as the amount of hot air, medium temperature, high heat can accelerate the drying, etc. (of course, to prevent material defects). Late granules were dried by microwave heating method within the material particles with the surface while the heat, the internal moisture vaporization of a pressure gradient, prompting internal moisture migration to the surface, to accelerate the drying, although the process is affected by internal conditions of controlled microwave heating characteristics change pass heat and mass transfer kinetics condition that the material internal moisture in the positive pressure gradient and temperature gradient, humidity gradient jointly promote, the rapid migration to the surface, and pass the air, humidity and the fan is discharged.
(3) energy saving, loose wet material with a roll press granulation, easily granulated, particle uniformity, high strength, low energy consumption. Pre-drying and post-drying methods of scientific and rational, efficient use of energy.
(4) drying - granulation - drying can arrange one-stop production, easy to automate production and management. It suggested that the state consider relevant units application.