Many people are familiar with vacuum freeze-drying, but perhaps fewer know about vacuum phase drying. So today, let's take a look together at what vacuum phase drying is.
Vacuum air phase drying is mainly applied to dry large transformers and other electrical equipment, abbreviated as VPD.
The moisture in transformers mainly comes from the insulating materials, with an initial moisture content as high as 6% to 8%, and the standard moisture content after drying is 0.5% to 0.1%. According to the drying theory, the moisture in transformer insulation materials all exists in the form of capillary adsorption. The drying process is to drive the moisture inside the insulating material of the equipment to migrate from the inside to the outside by creating a partial pressure difference of water vapor between the interior and the surrounding space of the insulating material. The greater the pressure difference and the higher the temperature, the faster the drying speed.
The traditional vacuum drying of transformers is carried out by heating and drying with air as the heat carrier medium or transformer oil as the heat carrier medium. Vacuum phase drying, on the other hand, utilizes the vapor phase of special kerosene as the heat carrier medium for heating and takes advantage of the phase shift thermal mechanism. Its basic principle is: It is achieved by heating and evaporating kerosene in a vacuum state, turning it into high-temperature kerosene vapor. When it encounters a cooler workpiece in a vacuum tank, it condenses, releasing latent heat of condensation (approximately 306.6kJ/kg), thereby heating the body of the container. The condensed kerosene is collected and reheated for evaporation. This cycle repeats multiple times, causing the temperature of the workpiece to continuously rise. The moisture within it also constantly evaporates into water vapor, which is drawn away by the vacuum system, ultimately achieving the goal of thorough drying.
Vacuum air phase drying is superior to traditional drying methods mainly due to the following characteristics:
Kerosene vapor is used as the heat transfer medium. Its saturated pressure is much lower than that of water but higher than that of transformer oil. Therefore, water can be smoothly removed during the heating stage. After the heating is completed, the kerosene can evaporate quickly without affecting the diffusion coefficient of water inside the material during the high vacuum stage.
2. In addition to heating insulating materials through convective heat transfer, kerosene vapor also undergoes phase change condensation and heat release on the surface of the insulating materials. The condensed kerosene then undergoes film heat transfer on the insulating surface. The heat exchange efficiency is higher than that of any other heating method, thus the heating speed is fast.
3. Vacuum phase drying raises the heating temperature from 110℃ to 130℃ without causing aging of insulating materials. However, at this time, the partial pressure of water vapor also increases to approximately 98kPa, the diffusion coefficient of water within the insulating material increases, and the evaporation rate of water is greatly accelerated.
4. The condensed kerosene can easily penetrate the surface and interior of the insulating material, accelerating the heat conduction of the insulating material itself and making the deep temperature distribution of the insulating material uniform. This is of vital importance for drying.
5. Condensed kerosene, as an excellent cleaning solvent, can wash away the dirt and dust on the transformer, which is a unique feature of coal gas phase drying. This feature makes vacuum air phase drying particularly suitable for handling the repair of transformers.