What is vacuum air phase drying? Vacuum air phase drying, also known as coal gas phase vacuum drying, is the main drying process used by transformer manufacturers. Its entire process mainly includes four stages: the preparation stage, the heating stage, the depressurization stage and the high vacuum stage.
Preparation stage. Place the transformer in a vacuum tank to evacuate the air, reducing the pressure in the vacuum tank to below 700Pa. At the same time, heat the kerosene in the evaporator with steam exhaust pipes, raising the temperature inside the evaporator to 115℃. The pressure of the kerosene is the saturated pressure at this temperature, approximately 10,000 Pa.
Heating stage. First, the vacuum system stops working. The kerosene vapor in the evaporator enters the vacuum tank by pressure difference to heat the transformer. After condensation, it is discharged from the bottom of the vacuum tank and returns to the evaporator, forming a cycle.
As the temperature in the vacuum tank rises, the kerosene vapor cannot be completely condensed and accumulates. The moisture in the insulating material also begins to be continuously discharged and accumulates in the vacuum tank in the form of water vapor. In addition, a small amount of air may leak into the tank, causing the pressure inside the vacuum tank to gradually increase. At this point, the vacuum pump needs to be turned on, and the vacuum tank is evacuated through the condensation collection tank to maintain the vacuum degree. The mixed gas enters the condensation system and is separated after condensation. The kerosene then enters the evaporator to form a second cycle. Through these two cycles, the transformer in the vacuum tank is gradually heated.
The duration of the heating stage mainly depends on the insulation structure, insulation weight, and the weight of the vessel body, etc.
The depressurization stage, also known as the low vacuum stage. At this stage, the transportation of kerosene vapor stops, and the evaporator also stops working. The vacuum system extracts the mixed gas in the vacuum tank and simultaneously re-evaporates the kerosene remaining in the insulating material. The condensation system condenses and recovers the kerosene in the extracted gas. The pressure reduction stage and the heating stage can be alternated several times according to the specific conditions of the product to achieve the best drying effect. The ending condition is that under a vacuum pressure below 2500Pa, almost no condensate is generated.
High vacuum stage. At this stage, the main task is to continue using the vacuum system to evacuate the vacuum tank, with the final pressure reaching below 10Pa, to further evaporate the moisture and kerosene in the insulating material and achieve thorough drying. Before the high vacuum stage, more than 95% of the moisture in the insulating material has been removed, so this stage does not take a long time. The temperature of the body and the dew point of the vacuum tank space only need to meet the process requirements. One of the important conditions is that the water discharge rate of the insulating material should be less than 10g per ton of insulating material per hour.