Views: 1 Author: Site Editor Publish Time: 2023-04-01 Origin: Site
Question 1: What is microwave?
Answer: Microwave, like radio waves, infrared rays and visible light, is an electromagnetic wave. Microwave refers to electromagnetic waves with a frequency of 300MHz-300KMHz, that is, electromagnetic waves with a wavelength between 1 meter and 1 millimeter. Microwave frequencies are higher than ordinary radio frequencies and are commonly referred to as "ultra high frequency electromagnetic waves.".
Question 2: How is microwave generated?
A: Microwave energy is usually obtained from direct current or 50 MHz alternating current through a special device. There are many types of devices that can generate microwave, but they are mainly divided into two categories: semiconductor devices and electrical vacuum devices. Electric vacuum devices are devices that use electrons to move in a vacuum to complete energy conversion, or called electron tubes. In electric vacuum devices, magnetrons, multicavity klystrons, microwave triodes, quadrupoles and traveling wave tubes can generate high-power microwave energy. At present, magnetrons and klystrons are mainly used in the field of microwave heating, especially in industrial applications.
Question 3: What are the frequencies of microwave applications?
A: Because microwave applications are extremely widespread, especially in the field of communication, the International Radio Regulatory Commission has made specific regulations on frequency division in order to avoid mutual interference. The frequencies allocated for industrial, scientific and medical use are 433 MHz, 915 MHz, 2450 MHz, 5800 MHz and 22125 MHz, which are used separately from communication frequencies. Currently, the commonly used frequencies for industrial heating in China are 915 MHz and 2450 MHz. The choice 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?
A: Dielectric materials consist of polar and nonpolar molecules. Under the action of electromagnetic fields, these polar molecules shift from their original random distribution state to their orientation according to the polarity of the electric field. Under the action of high-frequency electromagnetic fields, these orientations constantly change according to the frequency of alternating electromagnetic fields, which causes molecular motion and mutual friction to generate heat. At this point, the field energy of an alternating electric field is converted into heat energy in the medium, causing the temperature of the medium to rise continuously. This is the most popular explanation for microwave heating.
Question 5: What is the mechanism of microwave sterilization?
A: The mechanism of microwave sterilization is that bacteria and adults, like any biological cell, are a condensed medium composed of complex compounds such as water, protein, nucleic acid, carbohydrates, fats and inorganic substances. Water is the main component of biological cells, with a content of 75 to 85%, because various physiological activities of bacteria must be carried out with the participation of water. During the growth and reproduction process of bacteria, the absorption of various nutrients is completed through the diffusion, infiltration and adsorption of cell membrane. Under the action of a certain intensity microwave field, insects and bacteria in the material will also relax due to molecular polarization, while absorbing microwave energy to increase temperature. Because they are condensed matter, intermolecular forces intensify the conversion of microwave energy into thermal energy. Thus, the protein in the body is subjected to both nonpolar thermal motion and polar rotation, resulting in changes or destruction of its spatial structure and denaturation of the protein. After protein denaturation, its solubility, viscosity, expansibility, permeability and stability will undergo significant changes and lose 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 the causes of bacterial death.
Question 6: What is the penetration ability of microwave?
A: Penetration ability is the ability of electromagnetic waves to penetrate into the interior of a medium. When electromagnetic waves enter and propagate from the surface of the medium, as energy is constantly absorbed and converted into heat, the energy they carry decays exponentially with the distance they penetrate the surface of the medium. The transmission depth is defined as the depth D calculated from the internal power density of a material being 1/e or 36.8% of the surface energy density. The heating depth of microwave is much greater than that of infrared heating, because the wavelength of microwave is nearly a thousand times the wavelength of infrared. Infrared heating is only surface heating, while microwave heating is deep inside.
Question 7: What is microwave selective heating?
Answer: Materials with different properties have different microwave absorption losses, which is characterized by selective heating, which is beneficial to the drying process. Due to the maximum absorption loss of water molecules to microwave, the parts with high water content absorb more microwave power than the parts with low water content, resulting in a consistent drying rate.
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 central temperature through heat conduction (commonly known as external heating). It requires a certain amount of heat conduction time to reach the desired temperature at the center, while it takes longer for objects with poor heat conductivity. Microwave heating is an internal heating method, in which electromagnetic energy directly acts on the molecules of the medium to convert them into heat, and the transmission performance allows both the internal and external media of the material to be heated at the same time, without the need for heat conduction. However, the internal lack of heat dissipation conditions results in a temperature gradient distribution that is higher than the external temperature, forming a vapor pressure difference that drives the internal water to penetrate into the surface, accelerating the migration and evaporation rate of water. Especially for foods with a water content below 30%, the speed can be reduced by hundreds of times to achieve uniform drying in a short time.
Question 9: What is the absorption capacity of various substances to microwave?
Answer: Microwave heating refers to the loss of electric field energy by the dielectric material itself to generate heat. And the dielectric constants of different dielectric materials ε R and dielectric loss angle tangent value tg δ Is different, so the thermal effect under the action of microwave electromagnetic field is also different. A substance composed of polar molecules that can better absorb microwave energy. Water molecules have a strong polarity and are the best medium for absorbing microwaves. Therefore, substances containing water molecules must absorb microwaves. The other type is composed of nonpolar molecules, which basically do not or rarely absorb microwaves. This type of material includes polytetrafluoroethylene, polypropylene, polyethylene, polysulfone, plastic products, glass, ceramics, etc. They can transmit microwaves without absorbing microwaves. This type of material can be used as a container or support for microwave heating, or as a sealing material. In microwave field electricity, the magnitude P of the microwave power absorbed by the medium is proportional to the frequency f, the square of the electric field strength E, and the dielectric constant ε R and dielectric loss tangent value tg δ。 Namely: P=2 π f • E2 • ε r•V•tg δ
Question 10: What is the dehydration efficiency of microwave?
Answer: Theoretically, 1.39 kilograms of water can be vaporized per kilowatt hour of microwave power. Due to factors such as line loss and cavity efficiency, the actual effect is 0.8-1.1 kilograms per kilowatt hour. The user can estimate the required microwave power based on the difference between the initial and final water content of the material to be processed and the production requirements.