Working principle of reciprocating vacuum pumps

A reciprocating vacuum pump is a machine that draws in gas by using the reciprocating motion of a piston inside a cylinder. The principle of a reciprocating vacuum pump is similar to that of a reciprocating pump; however, since its working medium is gas, when the piston reaches the outer dead center, pressure is exerted to expel as much gas as possible from the cylinder, so as to prevent excessive residual gas from causing the piston to move inward. During suction, this gas expands and reduces the amount of gas that can be drawn in. At the same time, the structure of its intake and exhaust valves also differs from that of the inlet and outlet valves in reciprocating pumps. Figure 5-22 shows a reciprocating vacuum pump with a planar slide valve valve train. As the eccentric wheel 2 mounted on the main shaft rotates, it drives the control rod 3 to move back and forth, thereby causing the planar slide valve in the slide valve valve train 1 to move back and forth as well. The deflection angle of the eccentric wheel spindle is 90 degrees. . 1 Spool valve mechanism ； 2 eccentric wheels, 3 control levers ； 4-Cooling water inlet ； 5 Cooling water outlet ； 6 Inlet ports ； 7 Exhaust port: The structural feature of the reciprocating vacuum pump is its planar slide valve valve train, which serves to achieve pressure balance. The ratio of exhaust pressure to intake pressure is called the pressure ratio. To achieve a vacuum level of 95%, when the suction pressure is 38×133.32 Pa and the discharge pressure is 760×133.2 Pa, the pressure ratio Φ ≈ 2. At such a high pressure ratio, its volume coefficient drops to very low levels, resulting in a pumping rate that approaches zero. Therefore, structural measures must be taken to increase the volume coefficient. This measure generally employs a pressure balance method. The planar slide valve valve train of a reciprocating vacuum pump is shown in Figure 5-23. The slide valve is equipped with an inverted “Ⅱ”-shaped air passage. When the piston reaches its extreme position, the flat slide valve is in the middle position, and the high-pressure gas that has not been discharged from the cylinder’s clearance flows through this “Ⅱ”-shaped passage into the working chamber on the other side of the piston. In other words, this “Ⅱ”-shaped passage reduces the pressure on the exhaust side, allowing a significant amount of the gas that has not been completely discharged to flow to the other side where it can be expelled. In this way, the efficiency of the vacuum pump’s cylinder is improved, and the pressures on both sides of the piston’s dead center are balanced. The reciprocating vacuum pump is a dry-type vacuum pump suitable for pumping non-corrosive gases that do not contain solid particles; however, it has a complex structure and requires extensive maintenance. In recent years, some manufacturers have made improvements to this, replacing the planar slide valve shown in Figure 5-23 with an automatic spring-driven intake and exhaust valve similar to those used in compressors. In this way, the structure can be simplified, the rotational speed can be increased, and the size of the machine can be reduced.