A vacuum generator is a new type of high-efficiency, clean, economical, and small vacuum component that uses a positive pressure source to generate negative pressure. This requires both positive and negative pressure in a place with compressed air or in a pneumatic system. Getting negative pressure in the place becomes very easy and convenient. Vacuum generators are widely used in industrial automation in the fields of machinery, electronics, packaging, printing, plastics and robotics. The traditional use of vacuum generators is the combination of vacuum suction cups for the adsorption and handling of various materials, especially for the adsorption of fragile. Soft, thin non-ferrous, non-metallic or spherical objects. A common feature in this type of application is the small amount of pumping required, the low vacuum requirements and intermittent operation. The author believes that the pumping mechanism of the vacuum generator and the analysis and study of the factors affecting its working performance have practical significance for the design and selection of positive and negative pressure gas paths.

1, the working principle of the vacuum generator

The working principle of the vacuum generator is to use a nozzle to spray compressed air at a high speed, and to form a jet at the outlet of the nozzle to generate a entrainment flow. Under the action of entrainment, the air around the outlet of the nozzle is continuously sucked away, so that the adsorption chamber The pressure inside is reduced to below atmospheric pressure to form a certain degree of vacuum.

It is known from fluid mechanics that for the continuity equation of incompressible air gas (the gas can be approximated as incompressible air at low speed)

A1v1= A2v2

In the formula, A1, A2----the cross-sectional area of the pipe, m2

V1, v2----air flow rate, m/s

As can be seen from the above formula, the cross section is increased, the flow velocity is decreased, the cross section is decreased, and the flow velocity is increased.

For horizontal pipelines, the Bernoulli ideal energy equation for incompressible air is

P1+1/2ρv12=P2+1/2ρv22

Where P1, P2----the corresponding pressure at section A1, A2, Pa

V1, v2----the corresponding flow velocity at section A1, A2, m/s

ρ----air density, kg/m2

It can be seen from the above equation that the flow rate increases and the pressure decreases. When v2>>v1, P1>>P2. When v2 is increased to a certain value, P2 will be less than one atmospheric pressure, that is, a negative pressure is generated. Therefore, the flow rate can be increased to obtain a negative pressure, and suction is generated.

According to the Mach number M1 of the nozzle outlet (the ratio of the outlet flow velocity to the local sound velocity), the vacuum generator can be divided into a subsonic tube type (M1<1), a sound velocity nozzle type (M1=1) and a supersonic nozzle type ( M1>1). The subsonic nozzle and the sonic nozzle are both shrinking nozzles, and the supersonic nozzle type must be the first contracted and expanded nozzle (ie Laval nozzle). In order to obtain the maximum suction flow rate or the pressure at the highest suction port The vacuum generators are all designed as supersonic nozzles.