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Microwave is a form of energy (not heat), but it can be converted into heat in medium. The reaction of materials to microwave can be divided into four situations: (1) penetrating microwave; (2) reflecting microwave; (3) absorbing microwave; (4) partially absorbing microwave.
The biggest characteristic of microwave heating is that microwave is produced inside the heated object. The heat source comes from the inside of the object, and the heating is uniform. It will not cause the phenomenon of "unripe outside coke inside", which is conducive to improving the product quality. At the same time, because "heating inside and outside simultaneously" greatly shortens the heating time, the heating efficiency is high, and it is conducive to improving the product output. The inertia of microwave heating is very small, which can realize the rapid control of temperature rise and fall, and is conducive to the automatic control of continuous production quota.
Generally, in the field of energy processing, most of the materials processed are dielectric materials, and dielectric materials usually absorb microwave energy to varying degrees. Dielectric materials and microwave electromagnetic fields are coupled each other, which will form various power dissipation to achieve the purpose of energy conversion. There are many ways of energy conversion, such as ion conduction, dipole rotation, interface polarization, hysteresis, piezoelectric phenomena, electrostriction, nuclear magnetic resonance and ferromagnetic resonance, among which ion conduction and dipole rotation are the main principles of microwave heating. Microwave heating is a kind of heating method which converts microwave energy into heat energy by absorbing microwave energy, and makes itself totally different from other conventional heating methods. Traditional heating method is based on the principle of heat conduction, convection and radiation to transfer heat from the outside to the material heat. Heat is always transferred from the surface to the inside to heat the material. Thermal gradient inevitably exists in the material, so the material heated is not uniform, resulting in local overheating of the material. Microwave heating technology is different from traditional heating method. It is through the internal dipole molecule of the heated body. High-frequency reciprocating motion produces "internal friction heat" which increases the temperature of the heated material. Without any heat conduction process, the material can be heated both inside and outside at the same time. The heating speed is fast and uniform, and the heating purpose can be achieved only by one-tenth or one-tenth of the energy consumption of the traditional heating mode. Theoretical analysis shows that the amount of heat generated by matter in microwave field is closely related to the type of matter and its dielectric properties, that is, microwave has selective heating characteristics for matter.
The main characteristics of defrosting microwave heating machine:
1. Heating effect of dielectrics. Positive ions in dielectrics exist in pairs with negatively charged electrons nearby. These electrons are closely combined and do not interact with each other. The electric field strength of the dielectrics is zero to the outside world as a whole. If a strong electric field is applied to the dielectric, the positive and negative electrons will be rearranged immediately. If the electric field is alternating and high frequency, the frequent rotation of electrons between molecules will vibrate and heat will be generated by friction.
2. The penetration depth of microwave. Microwave energy is absorbed by dielectric loss when microwave enters dielectrics, and the microwave intensity will gradually weaken. The microwave energy will decay according to certain rules.
3. Selection of microwave heating mode. The absorption and heating of microwave are related to the loss coefficient. If the food with high loss coefficient is mixed with the food with low loss coefficient, the heating effect is a noticeable problem.
3.Microwave heating machine has the characteristics of fast heating speed, small heat loss and easy operation. It can not only shorten the process time, improve productivity, reduce costs, but also improve product quality.
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