Frequently Asked Questions about Microwave Energy Application Technology

JasonYoo

Frequently Asked Questions about Microwave Energy Application Technology [Copy link]

Frequently Asked Questions about Microwave Energy Application Technology
Question 1: What is a microwave?
Answer: Microwaves are electromagnetic waves like radio waves, infrared rays, and visible light. Microwaves refer to
electromagnetic waves with a frequency of 300MHz-300KM Hz, that is, electromagnetic waves with a wavelength between 1 meter and 1
mm . The frequency of microwaves is higher than that of ordinary radio waves, and is also commonly referred to as "ultra-high frequency electromagnetic waves."

Question 2: How are microwaves generated?
Answer: Microwave energy is usually obtained by direct current or 50MHz alternating current through a special device. There are
many types of devices that can generate microwaves, but they are mainly divided into two categories:
semiconductor devices and vacuum devices. Vacuum devices are devices that use electrons to move in a vacuum to complete energy conversion,
or they are called electron tubes. Among vacuum devices,
magnetrons, multi-cavity klystrons, microwave triodes, quadrupoles, traveling wave tubes, etc. can generate high-power microwave energy. In the current
microwave heating field, especially in industrial applications, magnetrons and klystrons are mainly
used .
Question 3: What are the frequencies of microwave applications?
Answer: Because microwaves are widely used, especially in the field of communications, in order to avoid mutual interference, the International Radio
Regulatory Committee has made specific
regulations on the division of frequencies. 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 frequencies.
At present, the commonly used frequencies for industrial heating in China are 915 MHz and 2450 MHz. The selection of microwave frequency and power can be determined according to
the shape, material, and moisture content of the heated material . Question 4: What is the principle of microwave heating? Answer: The dielectric material is composed of polar molecules and non-polar molecules. Under the action of the electromagnetic field, these polar molecules change from the original random distribution state to the polarity arrangement orientation of the electric field. Under the action of the high-frequency electromagnetic field, these orientations change continuously according to the frequency of the alternating electromagnetic field. This process causes the movement and mutual friction of the molecules to generate heat. At this time, the field energy of the alternating electric field is converted into heat energy in the medium, causing the temperature of the medium to continue to rise. This is the most popular explanation of microwave heating. Question 5: What is the mechanism of microwave sterilization? Answer: The mechanism of microwave sterilization is that bacteria and adult insects, like any biological cells, are a condensed medium composed of complex compounds such as water, protein, nucleic acid, carbohydrates, fat and inorganic substances. Water is the main component of biological cells, with a content of 75-85%, because all physiological activities of bacteria must be carried out with the participation of , and the growth and reproduction process of bacteria, the absorption of various nutrients is completed through the diffusion, penetration 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 substances, the intermolecular forces intensify the energy state conversion of microwave energy to thermal energy. As a result, the protein in the body is simultaneously affected by non-polar thermal motion and polar rotation, causing its spatial structure to change or be destroyed, and the protein to denature. After the protein is denatured, its solubility, viscosity, swelling, permeability and stability will change significantly, and lose its biological activity. On the other hand, the non-thermal effect of microwave energy plays a special role in sterilization that conventional physical sterilization does not have. It is also one of . Question 6: What is the penetration ability of microwaves? Answer: Penetration ability is the ability of electromagnetic waves to penetrate into the interior of a medium. When electromagnetic waves enter from the surface of a medium and propagate inside it, the energy they carry decays exponentially as they penetrate into the surface of the medium because the energy is constantly absorbed and converted into heat energy. The penetration depth is defined as the depth D from which the power density inside the material is 1/e or 36.8% of the surface energy density. The heating depth of microwaves is much greater than that of infrared heating because the wavelength of microwaves is nearly a thousand times that of infrared wavelengths. Infrared heating is only surface heating, while microwaves are deep internal heating. Question 7: What is selective heating of microwaves? Answer: Materials of different properties have different absorption losses of microwaves, which is the characteristic of selective heating, which is beneficial to the drying process . Because water molecules have the greatest , the parts with high water content absorb more microwave power than the parts with low water content, so the drying rate tends to be consistent . Question 8: Why is microwave heating called internal heating? Answer: Conventional heating (such as flame, hot air, electric heating, steam, etc.) uses heat conduction, convection, and thermal radiation to first transfer heat to the surface of the heated object, and then gradually increase the center temperature through heat conduction (commonly known as external heating). It takes a certain amount of heat conduction time to make the center reach the required temperature, and it takes longer for objects with poor thermal conductivity. Microwave heating belongs to the internal heating method. Electromagnetic energy directly acts on the medium molecules to convert into heat, and the transmission performance makes the inner and outer media of the material heated at the same time, without the need for heat conduction. The lack of heat dissipation conditions inside causes the temperature gradient distribution inside to be higher than the outside, forming a vapor pressure difference that drives the internal moisture to penetrate to the surface, accelerating the migration and evaporation rate of moisture. Especially for foods with a moisture content of less than 30%, the speed can be shortened by hundreds of times, achieving uniform drying in a short time . Question 9: What is the absorption capacity of various substances for microwaves? Answer: Microwave heating is the heat generated by the loss of electric field energy of the dielectric material itself. The dielectric constant εr and dielectric are different, so the thermal effect under the action of microwave electromagnetic field is also different. Substances composed of polar molecules can absorb microwave energy well . Water molecules are extremely polar and are the best medium for absorbing microwaves, so any substance containing water molecules will absorb microwaves. Another type is composed of non-polar molecules, which basically do not absorb or absorb very little microwaves. Such substances include polytetrafluoroethylene, polypropylene, polyethylene, polysulfone, plastic products, glass, ceramics, etc. They can transmit microwaves but do not absorb them. Such materials can be used as containers or supports for microwave heating, or as sealing materials. In the microwave field, the size P of the microwave power absorbed by the medium is proportional to the frequency f and the electric field strength.




























































E squared, dielectric constant εr and dielectric loss tangent tgδ.
That is: P = 2πf·E2·εr·V·tgδ
Question 10: What is the dehydration efficiency of microwaves?
Answer: Theoretically, 1.39 kg of water can be vaporized per kilowatt-hour of microwave power. Due to
factors , the actual effect is 0.8-1.1 kg/
kilowatt-hour. Users can estimate the required microwave power based on the difference between the initial and final moisture content of the material to be processed and the output requirements
.