Variable pressure vacuum drying system
Variable pressure vacuum drying system is a new vacuum drying technology developed in cooperation with foreign countries, which has advanced level in the world. It adopts the vapor phase drying technology with water vapor as the heat carrier. In the hot air heating stage, it uses the water vapor evaporated by the dried products in the drying tank and the filled dry air as the heat transfer medium to conduct the heat exchange between the heater and the dried products. The heating uniformity is good and the heating efficiency is high. The drying process of step air extraction is adopted, which has high heat transfer efficiency, short drying cycle and fully automatic operation. The equipment has the advantages of superior drying performance and low operating cost. It is mainly used for vacuum drying of transformers and transformers of 220kV, 110kV and below.
The system has the following characteristics:
Variable pressure vacuum drying technology is a vapor-phase drying technology with water vapor as the medium, which has high heat transfer efficiency, obviously shortens the drying cycle, uniform temperature of transformer body and good consistency of drying degree;
It has the function of on-line automatic judgment of transformer vacuum drying end point;
The design of vacuum system is reasonable, the mechanism is simple and the performance is excellent;
High degree of automation, the whole process of drying process is monitored by programmable controller, completely excluding the influence of human factors on the drying quality, automatic or manual operation of the equipment can be switched at any time, stable and reliable operation;
Save energy consumption, no environmental pollution, no danger of flammability and explosion, and meet the industrial requirements of modern society;
The investment intensity of the equipment is small and the operation and maintenance are simple.
The drying cycle is 30% - 50% shorter than the traditional process
The drying quality is good, and the final dry pressure is better than 100Pa
30% - 50% energy saving
There are four stages of vacuum drying by variable pressure method:
1) Warm up phase. In this stage, the tank is in a low vacuum, and the temperature of the winding and the core can rise quickly and evenly by using the convection heat transfer of water vapor with large heat capacity. In the preheating process, heating evaporation and water removal are carried out continuously, and the pressure in the tank changes from 80000 PA to 40000 PA, so as to ensure that the water vapor is in an unsaturated state in the drying tank, which eliminates the possibility of water vapor condensation on the surface of the core. Such a cyclic process can make the temperature in the tank evenly distributed and continuously evaporate the moisture in the deep insulation material. At this stage, when the core temperature rises to 70 ℃, 35% ~ 45% of the water in the dried product can be removed. Adjust the holding time according to the environmental humidity to prevent the iron core from rusting.
2) Transition phase. In this stage, the vacuum gradually decreased from eighty thousand PA to ten thousand PA. At this time, there is not much water to be removed, mainly to gradually improve the vacuum degree and core temperature to prepare for the main stage.
3) Backbone phase. It is also called pressure swing dehydration stage. After preheating stage and transition stage, when the core temperature reaches the predetermined value, vacuum to 10000 PA, and turn to the pressure swing dehydration stage. Pressure swing dehydration process: that is, close the main valve after vacuumizing for a period of time. In this way, the main purpose is to pump out the water vapor when vacuumizing, the vacuum degree will rise, and the temperature of the body will fall. When the main valve is closed, the moisture in the insulating layer evaporates again, reducing the vacuum degree. These water vapor also participate in convective heat transfer, replenishing the heat loss of the body due to water vaporization, which makes the body temperature rise. In the process of pressure swing dehydration, water vapor is removed and heat is transferred, which makes water continuously evaporate from the body. In this stage, the core temperature is further increased to over 74 ℃, and the vacuum degree is gradually increased to about 500 PA. At this stage, 45% ~ 50% of the water content of the dried substance can be removed.
4) Final stage. In the stage of pressure swing dehydration, the vacuum degree of the drying tank will gradually increase due to a large amount of water evaporation and being pumped out. When the core temperature reaches the specified value and the vacuum degree reaches the starting pressure of final drying (500 PA), the final drying stage will be started. At this time, the drying tank continuously vacuumizes, and the residual moisture in the insulation layer is continuously evacuated until the end of drying, and the vacuum degree finally reaches the end pressure of final drying (50 PA). Stop vacuumizing and start to enter the terminal to judge the preparation time. At the end of the preparation time, the vacuum degree was measured as P1, and the final dry end pressure was P0. Then, the terminal judgment value Q = p1-p0 - pressure rise of the empty tank for 900 seconds (20 PA). When q is less than 40 PA / ton, it is determined to be qualified.