FAQ of microwave energy application technology

Question 1: what is microwave? A: microwave is electromagnetic wave like radio wave, infrared ray and visible light Microwave refers to electromagnetic wave with frequency of 300mhz-300kmhz, that is, electromagnetic wave with wavelength between 1m and 1mm Microwave frequency is higher than general radio wave frequency, which is also called "UHF electromagnetic wave" Question 2: how is microwave generated? A: microwave energy is usually obtained by DC or 50MHz AC through a special device There are many kinds of microwave devices, but they are mainly divided into two categories: semiconducting devices and vacuum devices The electric vacuum device is a device which uses the electron to complete the energy transformation by moving in the vacuum, or called the electron tube There are magnetron, multicavity klystron, microwave triode, quadrupole, traveling wave tube, etc At present, magnetron and klystron are mainly used in microwave heating field, especially in industrial application Question 3: what are the frequencies of microwave applications? A: because microwave is widely used, especially in the field of communication, in order to avoid mutual interference, the International Radio Regulatory Commission has made specific regulations on frequency division The frequencies allocated to industry, science and medicine are 433 MHz, 915 MHz, 2450 MHz, 5800 MHz and 22125 MHz, which are used separately from the communication frequency At present, 915 MHz and 2450 MHz are commonly used for industrial heating in China The choice of microwave frequency and power depends on the shape, material and moisture content of the heated material Question 4: what is the principle of microwave drying equipment? A: the dielectric material is composed of polar molecules and nonpolar molecules Under the action of electromagnetic field, these polar molecules change from the original random distribution state to the orientation according to the polarity arrangement of electric field Under the action of high frequency electromagnetic field, these orientations change continuously according to the frequency of alternating electromagnetic field, which causes the movement and friction of molecules and generates heat At this time, the field energy of the alternating electric field is converted into the heat energy in the medium, which makes the medium temperature rise continuously This is a popular explanation for the microwave heating Zui Question 5: what is the mechanism of microwave sterilization equipment? A: the mechanism of microwave sterilization lies in that, like any biological cell, bacteria and adults are a condensed medium composed of complex compounds such as water, protein, nucleic acid, carbohydrate, fat and inorganic substance Among them, water is the main component of biological cells, accounting for 75-85% All kinds of physiological activities of bacteria can only be carried out with the participation of water In the process of bacterial growth and reproduction, the absorption of various nutrients is completed through the diffusion, infiltration and adsorption of cell membrane Under the action of a certain intensity of microwave field, insects and bacteria in the material will also relax due to molecular polarization, and absorb microwave energy to heat up Because they are condensed matter, the intermolecular force intensifies the conversion of microwave energy to thermal energy Thus, proteins in vivo are affected by both nonpolar thermal movement and polar rotation, and their spatial structure changes or destroys, thus denaturing proteins After denaturation, the solubility, viscosity, expansibility, permeability and stability of protein will change obviously, but the biological activity will be lost On the other hand, the non thermal effect of microwave energy plays a special role in the process of sterilization that conventional physical sterilization does not have It is also one of the causes of bacterial death Question 6: what is the microwave penetration capability? A: the penetration ability is the ability of electromagnetic wave to penetrate into the medium When electromagnetic wave enters from the surface of the medium and propagates in the medium, as the energy is continuously absorbed and converted into heat energy, the energy it carries decreases exponentially with the distance to the surface of the medium The transmission depth is defined as: the internal power density of the material is 1 / E of the surface energy density or 36.8% of the calculated depth D the heating depth of microwave is much greater than that of infrared heating, because the wavelength of microwave is nearly a thousand times of the infrared wavelength Infrared heating is only surface heating, and microwave heating is in-depth internal heating Question 7: what is microwave selective heating? Answer: the absorption loss of different materials to microwave is different, that is, the characteristics of selective heating, which is beneficial to the drying process Because the absorption loss Zui of water molecules to microwave is large, the microwave power absorbed in the part with high water content is more than that with low water content, so the drying rate tends to be the same Question 8: why is microwave heating called internal heating? A: conventional heating (such as flame, hot air, electric heat, steam, etc.) uses heat conduction, convection, and heat radiation to transfer heat to the surface of the object to be heated first, and then through overheating conduction to gradually increase the center temperature (commonly known as external heating) It needs a certain heat conduction time to make the central part reach the required temperature, and it needs a longer time for objects with poor heat conductivity Microwave heating belongs to the internal heating mode Electromagnetic energy directly acts on the medium molecules to convert into heat, and the transmission performance makes the internal and external medium heat at the same time, without heat conduction, and the internal lack of heat dissipation conditions, resulting in the internal temperature higher than the external temperature gradient distribution, forming the driving internal water The vapor pressure difference of the surface penetration accelerates the evaporation rate of water Especially for the food with water content below 30%, the speed can be shortened hundreds of times, and the uniform drying can be achieved in a short time Question 9: what is the microwave absorption capacity of various substances? A: microwave heating is the heating of dielectric materials due to the loss of electric field energy The dielectric constant ε R and dielectric loss tangent TG δ of different dielectric materials are different, so the thermal effect of microwave electromagnetic field is different A substance composed of polar molecules can absorb microwave energy well Water molecules are extremely polar, which is a good medium for microwave absorption Therefore, any matter containing water molecules must absorb microwave The other is composed of nonpolar molecules, which do not or seldom absorb microwave Such substances include polytetrafluoroethylene, polypropylene, polyethylene, polysulfone, plastic products, glass, ceramics, etc they can pass through microwave without absorbing microwave Such materials can be used as containers or supports for microwave heating or as sealing materials In microwave field electricity, the magnitude P of microwave power absorbed by dielectric is directly proportional to the frequency f, the square of electric field strength e, the dielectric constant ε R and the tangent value TG δ of dielectric loss That is: P = 2 π f · E2 · ε R · V · TG δ question 10: how about the dehydration efficiency of microwave drying equipment? A: theoretically, 1.39 kg of water can be vaporized by microwave power per kWh Due to line loss, cavity efficiency and other factors, the actual effect is 0.8-1.1 kg / kWh The user can estimate the required microwave power according to the difference between the initial water content of the material to be treated and the final water content of Zui and the requirements of the output.