1、 Overview of slide valve vacuum pump

Slide valve vacuum pump, full name of slide valve oil seal mechanical vacuum pump, referred to as slide valve pump, is a kind of variable volume gas transmission pump, and its application scope and conditions are basically the same as rotary vane pump.

Because of its structural characteristics, the capacity of slide valve vacuum pump is much larger than that of rotary vane vacuum pump, so it is more widely used in large vacuum equipment. The basic types of slide valve vacuum pump are divided into single-stage and double-stage. Among them, the limit pressure of small pump in single-stage pump is generally no more than 0.6pa, and that of large pump is no more than 1.3pa (all gas is closed); the limit pressure of double-stage pump is not more than 0.06pa (closing gas). The slide valve vacuum pump with pumping speed greater than 150L / s mostly adopts single-stage form.

Due to the large eccentricity of the rotary mass of the slide valve vacuum pump, there is no good mass balance, so it will produce large vibration during operation. At the same time, it is difficult to realize the complete balance of the inertia force of the slide valve pump because the motion track of the rotating mass center of the sliding valve pump is a complex closed curve. This limits the speed of the slide valve vacuum pump, which is generally controlled at 350-600 R / min, and some can reach more than 1000 R / min.

2、 Working principle and structure of slide valve vacuum pump

1. Working principle of slide valve vacuum pump

The main structure of slide valve vacuum includes pump body, eccentric wheel, slide valve assembly and guide rail. Kelexi, the manufacturer of dry screw vacuum pump, shows its simple working principle in the way of legend.

Knowledge analysis of the most comprehensive slide valve vacuum pump

Figure 1: working principle diagram of simple slide valve vacuum pump

As shown in the figure, the drive shaft concentric with the pump chamber drives the eccentric wheel to rotate, and the eccentric wheel drives the slide valve ring to move, making the slide valve rod slide up and down and swing left and right on the guide rail. The slide valve divides the pump chamber into two parts, a and B. When the drive shaft rotates in the direction shown in the figure, the volume of chamber a increases and the pressure decreases. The gas enters cavity a through the inlet and the slide valve rod (one side of chamber a is opened), which is in the process of suction. When the volume of chamber a reaches the maximum, the air inlet is isolated from chamber a, and the suction process is finished. At the same time, the volume of cavity B is in the process of continuous reduction and compression of gas. When the gas pressure of chamber B reaches the exhaust pressure, open the vent valve of oil seal and start to exhaust. When the slide valve is in the upper position, the exhaust is finished (the volume of chamber a is also at the maximum), and a new cycle begins.

2. Structural characteristics of slide valve vacuum pump

As shown in Figure 1, the main structure of the slide valve vacuum pump includes a series of components such as pump body, eccentric wheel, slide ring, sliding rod and guide rail. Similar to the rotary vane vacuum pump, the slide valve pump is also equipped with a gas ballast valve. The gas ballast hole is opened near the exhaust port or on the end cover to remove the gas containing condensable steam. An oil-gas separator is set at the pump outlet to reduce the pump oil loss and environmental pollution.

The following figure is a typical slide valve vacuum pump structure diagram.

Knowledge analysis of the most comprehensive slide valve vacuum pump

Structure diagram of typical slide valve vacuum pump

In addition to the distinction of single and double stage, the slide valve vacuum pump also has the distinction of single cylinder, double cylinder and three cylinder. Among them, the three cylinder sliding valve pump can better solve the mass balance of the slide valve, improve the vibration of the pump and relatively high speed.

3、 Application points of slide valve vacuum pump

The slide valve vacuum pump can be used alone as well as as the front stage pump of diffusion pump, oil booster pump and roots pump. Compared with rotary vane pump, the slide valve pump is more durable. It is widely used in the fields of vacuum coating, vacuum heat treatment, vacuum sintering, molecular distillation and aviation simulation experiment. However, in the presence of water vapor, dust and other media environment, it is necessary to install a filter device, which can not be used in flammable and explosive gas environment and compressed gas transmission. At the same time, limited by the characteristics of oil sealed mechanical vacuum pump, its applicability is not as good as some dry vacuum pumps in some clean environment fields, such as semiconductor, petrochemical, etc.

4、 Noise source and solution of slide valve vacuum pump

Mechanical impact noise is the main component of the slide valve pump noise. During the normal operation of the slide valve pump, the mechanical impact between the slide valve and the inner wall of the pump body (the junction of the large and small cylinders at the intake side) will cause strong mechanical impact noise. The factors that affect the mechanical impact noise are: the gap between the pump body and the slide valve ring, the clearance between the eccentric wheel and the slide valve ring, and the offset between the pump axis and the large cylinder center of the pump body. In the design, the clearance of the impact part is usually adjusted to reduce or even eliminate the impact, so as to reduce the mechanical impact noise of the slide valve pump.

1. Liquid impact noise and gas-liquid discharge noise are important components of pump noise.

During the compression process of the slide valve pump, the pump oil is driven by the slide valve to rush to the inner wall of the pump cylinder and the slide valve rod near the exhaust hole at high speed, resulting in strong liquid impact noise. And in the exhaust process of the slide valve pump, the strong gas-liquid emission noise will be produced when the lubricating oil and gas pass through the exhaust passage at high speed. These two kinds of noise are most obvious in the end of exhaust. In order to reduce these two kinds of noise, lower speed and smaller inner diameter of pump cylinder can be selected in the design, so as to reduce the speed of liquid impact and gas-liquid discharge speed, which is conducive to reducing noise; when the pump is in normal operation, reduce the amount of oil into the pump, so as to reduce the amount of oil impinging on the inner wall of the pump and the amount of oil discharged, which is conducive to reducing the noise. The noise can also be reduced by improving the flow channel of the exhaust valve, increasing the channel area as much as possible and reducing the speed of lubricating oil passing through the exhaust valve.

2. Bearing noise is not common in slide valve pump noise.

The rotary speed of the slide valve pump is low, the bearing is fully lubricated, and the sound of the bearing is very low when it is in normal operation. Bearing noise mainly refers to the noise generated when the bearing is not running normally. When the pump is in normal operation, there will be a weak noise due to the abnormal noise of the bearing in the process of installation.

3. Cavitation noise is seldom seen in the noise of slide valve pump. For medium and large scale slide valve pump, the pump speed should not be too high, otherwise it is easy to appear cavitation phenomenon and reduce the pump life.

4. The oil tank is also a factor that affects the noise of the slide valve pump.

Steel welded oil tank is easy to cause vibration; cast iron oil tank is not easy to produce vibration, and cast iron itself has sound absorption effect, so using cast iron oil tank can reduce pump noise by 2 ~ 3 dB (a). The built-in oil gas separator in the oil tank can properly increase the blocking and enhance the noise elimination effect without affecting the exhaust pressure. In this way, the noise of the pump can be reduced by 1 ~ 2 dB (a).