Frequently Asked Questions on Microwave Energy Application Technology

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