4 about the effect of microwave vacuum drying

Microwave vacuum is used more and more widely. Here are several important elements on the dull effect of microwave vacuum:

1.Product varieties and details

Product varieties and conditions vary greatly, microwave vacuum dryer process is not fixed. In fact, in the boring process of a microwave vacuum.

The interior of the material is gradually formed in a loose and porous shape, and its internal thermal conductivity is weakened, that is, the material gradually becomes a bad thermal conductor. With the boring process of the microwave vacuum,

The internal temperature will be higher than the external, the larger the material volume, the larger the temperature gradient in the surface, the internal heat conduction cannot balance the temperature difference caused by the microwave, so that the temperature gradient can reach an unbearable level.

Therefore, the material should be treated in advance to a smaller grain or sheet in order to improve the role of boring. Powdered products are unique when the microwave is boring.

When they are stacked together, they should not be regarded as many small particles, but as a whole, requiring special attention to the temperature difference in the material layer. Generally, when the material appears in a larger form.

When the material is close to the deceleration boring period, the microwave power should be reduced, so as to reduce the temperature difference between the surface and the surface, but the opposite effect is to extend the boring moment.

2. Vacuum of the equipment

The lower the pressure, the lower the boiling point temperature of the water, and the water diffusion speed of the guess.

The fine size of the vacuum degree in the microwave vacuum resonator is firstly limited by the breakdown electric field strength, because in the vacuum condition, the gas molecules are easily ionized by the electric field.

And the breakdown field strength of air and water vapor decreases with the pressure; the lower the electromagnetic wave frequency, the smaller the gas breakdown field strength.

Gas breakdown is most likely to occur in the microwave energy coupling port and the field strength set within the cavity. The onset of breakdown and discharge not only consumes microwave energy.

And it will damage the parts and occur large microwave reflection, shorten the service life of the magnetron. If the breakdown discharge occurs on the surface of the food, it will burn the food.

Generally, the field strength of 20kV / m can breakdown the food (the dielectric constant is different). So it's very important to choose the right vacuum, the higher the vacuum, the better.

The high vacuum degree not only increases the energy consumption, but also increases the possibility of breakdown and discharge.

3.Equipment power

Microwave has the characteristics of material selective heating. Water is a very strong polar molecular substance, more vulnerable to microwave action and heat,

Therefore, the higher the water content, the easier it is to absorb microwave, the faster the heat; when the water content decreases, its ability to absorb microwave also decreases accordingly.

Generally in the early stage of boring, the moisture content is higher, the input microwave power can be figure 1 microwave power on the boring effect is higher, can use continuous microwave heating.

At this time, most microwaves can be absorbed by water, and in the process of constant speed and evaporation, during the decrease of water, less microwave energy will be needed.

4. Equipment working time

The selection of microwave vacuum is very important, which is also affected by many elements. With pineapple pulp as the material.

The influence of the microwave on the effect of the pineapple powder is discussed. Under the vacuum of 98.2~99.2kPa, the moisture content of the material in the dry stage changes little.

This is because the water molecules inside the material have not fully absorbed a lot of microwave energy, insufficient heat source; with the boring continue, the polar molecules inside the material sensation.

More energy is converted into heat, promoting the movement of water molecules, and the water content of the material changes greatly. In the late period of microwave vacuum,the material gradually formed a loose porous,

The internal thermal conductivity begins to weaken, and the water content also tends to stabilize. In addition, boring moments are also affected by the moisture content of the product. If general boring products, moisture content can be controlled in 3%~5%, if the requirement is as low as 1% or below, boring moment need to be extended accordingly.