Construction of mechanical vacuum pump

In order to obtain low vacuum, the most commonly used mechanical pump is oil sealed rotary pump, which is called vacuum pump for short. This kind of mechanical pump is widely used because of its high limit vacuum (0.133 ~ 0.0133pa) and its pumping rate is not very low when the pressure is about 133.3pa.

The structure of a common vacuum pump is shown in Figure 59. The inside of the pump is mainly a round simple stator. There is a precision machined solid circle as the rotor. The rotor is eccentrically arranged above the stator cavity wall to separate the air inlet and exhaust pipes, and plays an air tight role. The two wings s and s' are horizontally embedded in the diameter of the rotor body. They are compressed by a spring clamped in the middle of them (see Fig. 60). S and s' divide the space between the rotor and the stator into three parts. When the piece is boarded at position 60A in figure, the air enters space a from the container to be pumped through tube C. When s leaves with the rotation of rotor, area a in Figure 60b increases. The gas is inhaled through pipe C. When turning figure 60C, s' separates space a from air intake pipe C, and then the rotor continues to turn. The volume of space a gradually reduces. The gas is compressed. Until the pressure is greater than 101325p., the vent valve opens and the gas is expelled. In this way, when the rotor is rotating, the space separated by the two wings continuously aspirates and exhausts, making the evacuated container reach a certain degree of vacuum.

The compression ratio of this pump can be as high as 700:1. Therefore, if there is water gas or other condensable gas in the air to be pumped, when the gas is compressed, swallow gas may condense into liquid and become numerous small drops mixed into the oil. With the internal circulation of the oil in the pump, on the one hand, it will be sent to the pumped container, greatly reducing the vacuum degree of the system; on the other hand, it will damage the quality of the oil, reduce the sealing and lubricating effect of the oil in the pump, and also make the rotor stator wall rusty. In order to solve this problem, in addition to the air inlet in the pump to reduce the compression ratio of air and swallow gas, it is also necessary to install a cold trap or absorption tower before the inlet.

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