Views: 13 Author: Site Editor Publish Time: 2022-09-13 Origin: Site
In conventional vacuum drying, because it is difficult to conduct air convection heat transfer under vacuum conditions, only relying on heat conduction to provide heat energy to materials, the heat transfer speed is slow, the efficiency is low, and the temperature control is difficult.
Microwave heating is a kind of electromagnetic wave heating. Microwaves directly act on materials, so that they can be heated inside and outside at the same time. There is no need to transfer heat through convection or conduction. Therefore, the heating speed is fast, the drying efficiency is high, and the temperature is easy to control.
Microwave vacuum drying equipment is a new type of microwave energy application equipment that combines microwave energy technology and vacuum technology. It combines a series of advantages of microwave and vacuum drying, overcomes the shortcomings of conventional vacuum drying such as long cycle and low efficiency, and improves the work efficiency by 4-10 times compared with conventional methods in the drying process of general materials.
Microwave vacuum drying equipment is a high-tech product integrating electronics, vacuum, mechanics, thermodynamics, program control and other disciplines. It is a new technology and process developed on the basis of in-depth research on physical changes of substances, internal and external heat and mass exchange and moisture transfer process under vacuum conditions.
In industrial production, many articles cannot be dried under high temperature conditions; For example, for some drugs, chemicals, nutritious foods and high-grade Chinese herbal medicines such as ginseng and pilose antler, in order to ensure the product quality, the drying treatment must be carried out at a temperature lower than 100 ℃ or at room temperature. As is known to all, the boiling point of water also decreases when the pressure decreases. For example, at an atmospheric pressure (101.3kPa), the boiling point of water is 100 ℃, while at 0.073 atmospheric pressure (7.37kpa), the boiling point of water is 40 ℃. Under the vacuum condition, heating the object can make the water inside the object evaporate without heating up.
Since it is difficult to conduct air convection heat transfer under vacuum conditions, only relying on heat conduction to provide heat energy to materials. The conventional vacuum drying method has slow heat transfer speed, low efficiency and difficult temperature control. Microwave heating is a kind of radiation heating, in which the microwave directly interacts with the material, so that its inside and outside are heated at the same time. It does not need to transfer heat through convection or conduction. Therefore, the heating speed is fast, the drying efficiency is high, and the temperature is easy to control.
Freeze microwave vacuum drying equipment has been on the market in China for more than 10 years, but most of them are small-scale equipment. Due to the technical difficulty in producing this kind of equipment, this kind of equipment has not been well promoted. In the process of producing microwave vacuum freeze-drying equipment, the most prominent technical problem is the discharge of low-pressure gas: when the gas in the vacuum state is diluted to a certain extent, it will produce high-frequency discharge when external energy is added.
High frequency discharge occurs in low-pressure gas, and the harm is that microwave energy cannot be delivered into the material, microwave reflection is serious, sealing window of microwave equipment is damaged, and then the vacuum state in the microwave oven is damaged. The advantages of microwave vacuum low-temperature drying are high efficiency: conventional vacuum drying equipment uses steam for heating, which needs to be heated from the inside to the outside, and the heating speed is slow, which requires a large amount of coal. Microwave vacuum drying equipment uses electromagnetic wave heating, which does not need heat transfer media, and directly heats the inside of the object. The heating speed is fast. A 1kW microwave can heat water at normal temperature to 100 ° C in 3-5 minutes, avoiding the above disadvantages, Therefore, the drying cycle is greatly shortened and the energy consumption is reduced.
Compared with the conventional drying technology, the work efficiency can be increased by more than four times, and the heating is uniform. Since microwave heating heats materials simultaneously from the inside to the outside, the temperature difference between the inside and the outside of the materials is very small, which will not cause inconsistent internal and external heating in conventional heating, so it is easier to produce expansion effect, which is conducive to crushing and greatly improves the drying quality.