Physical phenomenon of exhaust in vacuum system and its causes
Before the mechanical pump starts to work, the pressure of each part of the system is the same. There is no gas flow. After the pump is started, the gas flow characteristics will change. At first, when the mechanical pump starts, the gas near the air inlet is drawn away, and the pressure at the air inlet is high. It starts to decrease, so the pressure difference between the two ends of the pipeline begins, and the gas continuously flows from the container to the pump along the pipeline. Obviously, the greater the pumping speed, the greater the change in pressure difference. Finally, the pumping speed is limited by the flow resistance of the pipeline and cannot be increased. The gas flow state goes through three situations: viscous flow (low vacuum) where molecules collide frequently, molecular flow (high vacuum) where molecules mainly collide with the tube wall, and transitional flow (medium vacuum) in between. The physical phenomena of the co-pumping process can be described as follows:

1. The pumping process from 760 Torr to 1 Torr: At the beginning of pumping, if the relative humidity of the air towel is high, a strange phenomenon will occur at the moment when the pumping starts, that is, the cooling caused by the sudden drop in pressure The water vapor in the pumped container will condense into mist particles (it can be clearly seen on the glass container). This turbulent flow of mist particles is a sign of high gas pressure and airflow characteristics of large flow.

When the pressure of the pumped container drops from 760 Torr to about 1 Torr, the most noticeable change is the pressure difference between the inside and outside of the container, which is about 1 kg (or 14.7 psi) of force per square centimeter of area. For example, an observation window with a diameter of 15 cm in a coating machine must withstand a force of 376 pounds. For a rectangular screen of a TV picture tube, the force must reach the order of "ton".

Another interesting change in this process is the gradual change in the residual gas composition in the vessel. The container is filled with air before being evacuated. When the degree of vacuum gradually increases, the grease and moisture are released one after another, and they interact with other residual gas components to generate new gases and substances, which fundamentally change the proportion of residual gas components in the container. . Finally, most of the residual gas in the system becomes impurity gas.

2. The pumping process from 1 Torr to 10-4 Torr: the physical properties of the gas also change greatly during this process, that is, the thermal conductivity of the gas, the discharge properties, and the flow properties of the gas (the mean free path of the molecules) properties have changed. The study of changes in these physical properties is also of paramount importance to vacuum technology (for example, using different gas properties at different vacuum levels, vacuum technology can be applied in different sectors of industry).