Microwave equipment manufacturers recommend vacuum microwave drying equipment technology for you

Microwave vacuum drying technology

Microwave belongs to electromagnetic wave, which has a series of electromagnetic wave characteristics such as interference, diffraction, transmission and reflection. Microwave energy is transmitted by electromagnetic waves. For matter in microwave field, microwave will produce reflection, absorption and penetration. The microwave emitted from the magnetron enters the microwave resonant cavity loaded with the heated material through the excitation cavity and waveguide, and oscillates back and forth in the resonant cavity to heat the material.

The difference between microwave and traditional heating technology is that it relies on high frequency electromagnetic oscillation to initiate molecular motion and heat the material as a whole. Because of the evaporation of external water or the heat exchange with the external medium, the external temperature is lower than the internal temperature. Thus, a temperature gradient is formed from the inside to the outside, and water always tends to move from high temperature to low temperature in the material.

The evaporation of water is a process of moving from the inside to the outside, so the mass transfer and heat transfer inside the material are in the same direction, which makes the microwave a good drying heat source. In the food industry, heat sensitive materials need to be dried quickly at low temperature, and the combination of microwave technology and vacuum technology has become a new technology with application value.

Drying time:

The selection of microwave vacuum drying time is very important and influenced by many factors.

1. The microwave power of the system should match the material quantity. That is to say, according to the requirement of evaporation capacity, the system device with appropriate size should be equipped.

2. The drying difficulty of the material itself. That is, the affinity level of material molecules with water molecules or corresponding solvents.

3. Requirements for moisture content of finished products. For general drying products, the moisture content can be controlled at 3 ~ 5%. If the requirement is as low as 1% or less, the drying time should be extended accordingly.

4. The type of dissolving benefit. Different solvents need different drying time and degree of difficulty, and some solvents may need to cooperate with the corresponding recovery system.

Size and characteristics of materials:

Microwave is penetrating and can dry relatively large materials, but the drying time is also changed. In fact, in the process of microwave vacuum drying, the porous material gradually forms, and its internal thermal conductivity begins to weaken, that is, the material gradually becomes a poor thermal conductor.

With the microwave vacuum drying process, the internal temperature will be higher than the external temperature. The larger the material volume is, the greater the internal and external temperature gradient will be. The internal heat conduction can not balance the temperature difference caused by microwave, which makes the temperature gradient reach an unacceptable level. Therefore, unless there is a special reason, the material should be pretreated to smaller particles or flakes in order to improve the drying effect.

When the material must appear in a larger form, it is necessary to reduce the microwave power when the material is close to the deceleration drying period, so as to effectively reduce the internal and external temperature difference, but the opposite effect is to prolong the drying time. According to the drying data of different sizes of materials, the deceleration drying time will be extended from 10-15 minutes to 25-40 minutes, which is acceptable in most cases.

Powder products have their unique characteristics in microwave drying. When they are stacked together, they should not be regarded as many small particles, but as a large whole. At this time, special attention should be paid to the temperature difference between the inside and outside of the material layer.

Uniformity of microwave heating:

The uniformity of microwave heating affects the drying speed of each part of the material and the quality uniformity of the finished product after drying.

For a single material, the microwave is heated as a whole, and there is no case that the outside is heated first, and then the inside is heated gradually, so as to reflect the thermal uniformity. However, when observing the material in the whole heating cavity, the microwave resonates in the cavity in multiple modes.

The more modes formed by microwave, the more uniform the heating condition is. However, the number of modes formed by microwave is affected by many factors, such as the size and form of the cavity, the position and quantity of the coupling port, the amount of material and so on. The homogenization of these conditions can improve the uniformity of microwave heating, but it is impossible to achieve a completely consistent heating temperature. Because the microwave is reflected back and forth in the resonant cavity, no matter how improved, there are still areas with relatively dense peaks and nodes, and the electric field energy will not be completely consistent. To achieve a consistent drying effect, unless the material itself is in motion, it can help to change the microwave resonance mode, so as to improve the thermal uniformity.

Material mixing and drying:

Materials with different characteristics or the same material with different moisture content should not be dried together. It is generally recommended to dry only one material at a time, because the dielectric constant of different products is different, and the evaporation rate of materials with different moisture content is also different. It is obviously very difficult to achieve the same drying effect in Z.

Of course, there are exceptions, for example, when two different mixed powders are dried, and the powder is in the condition of Ji fine and fully mixed, it can be regarded as the same material. This principle can also be applied to liquids or pastes.

Drying efficiency and economy:

Efficiency is a significant trend of microwave drying method Z. it may take 20-30 hours to treat with freeze-drying method, but it only takes 1-2 hours to replace with microwave vacuum drying method, which greatly improves the efficiency and related comprehensive benefits. It is worth mentioning that some materials containing alcohol or other solvents cannot be frozen at all.

At the same time, the microwave vacuum drying method also has a certain potential in energy saving. Through a lot of practice and comprehensive calculation, an approximate energy consumption value can be provided. The power consumption is about 1.6 degrees per kilogram of water evaporated. For products with higher added value, this is obviously acceptable, while for some products with relatively low added value, it needs to be calculated from the comprehensive cost.

In addition, for fruits, vegetables, agricultural and sideline products with moisture content of 90-95%, it is uneconomic and inefficient to remove all moisture at the beginning by microwave. The correct procedure is to reduce the moisture content of the product to 30 ~ 60% in advance before removing the residual moisture by microwave vacuum drying.

System security:

It should be noted that the system can not run without load, that is, the heating chamber must be filled with load. If there is no material in the cavity, the microwave will turn back to the coupling port after many times of reflection, which will produce the superposition of electric field near the coupling port and form a very high standing wave. On the other hand, there should be no metal object in the cavity, because the metal object will disturb the distribution of electric field strength in the cavity, which may cause non resonance and expand standing wave.

Another important point we must pay attention to is to strictly prevent microwave leakage. Microwave shielding or attenuation should be considered in the parts of the system where the inlet and outlet are connected with the outside world, and the microwave will not interfere with the work of other detection elements.