How is the hot air vacuum drying method conducted?

The hot air vacuum drying method is currently a widely adopted process. In this method, hot air with a temperature ranging from 105 to 130°C is introduced into a vacuum tank to heat the apparatus, ensuring uniform heating of its interior and raising the temperature to achieve the evaporation of moisture.

For large-capacity transformers, the heating and vacuum extraction processes need to be alternated. For instance, initially, 40 hours of hot air heating is followed by 10 to 15 hours of vacuum extraction; then, 10 to 20 hours of heating is succeeded by another 10 to 15 hours of vacuum extraction. The number of cycles depends on the voltage level, with higher voltage levels requiring more cycles. This is due to the increased number of insulation components and thicker wire bundling in ultra-high voltage transformers, resulting in smaller oil passage clearances.

When the internal temperature reaches a certain level, a significant amount of moisture evaporates, resulting in higher humidity in the oil gap. Continuing to introduce hot air becomes difficult, causing a noticeable drop in insulation temperature and minimal effectiveness of the hot air circulation heating. Under these circumstances, performing vacuum extraction reduces the pressure, leading to rapid evaporation of moisture in insulation components and oil gaps. This, in turn, decreases the water vapor concentration in the insulation. After reaching a certain level, resuming hot air heating helps maintain a moderate temperature drop inside the transformer, which quickly recovers. This process achieves effective drying.

The schematic diagram of the vacuum pipeline system is shown in Figure 41-1. Due to the high vacuum requirements (10-133Pa) for the vacuum tank, a two-stage vacuum pump should be equipped in the vacuum pipeline. This ensures the desired vacuum level is achieved while minimizing the time needed for vacuum extraction. Typically, an H-9 slide valve vacuum pump is suitable for the primary pump, and a ZJ-1200 mechanical booster pump is recommended for the secondary pump. To guarantee the seal integrity of the entire vacuum pipeline system, various specifications of high vacuum valves (such as GI and GIQ types) should be selected. To prevent humid air from entering the vacuum pump and condensing into water, a condenser is specially designed, and water-oil separators are placed before and after the pump during vacuum extraction.

To ensure the hot air introduced into the tank is clean, an air filter should be installed in front of the pump.

LuShan, est. 1975, is a Chinese professional manufacturer specializing in power transformers and reactors for 49 years. Leading products are single-phase transformer, three-phase transformers, DC inductors, AC reactors, filtering reactor, expoxy resin high-voltage transformer and intermediate, high-frequency products. Our transformers and reactors are widely used in 10 application areas: rapid transit, construction machinery, renewable energy, intelligent manufacturing, medical equipment, coal mine explosion prevention , excitation system, vacuum sintering, central air conditioning.

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