Vacuum generator working principle and structural parameter diagram

How the vacuum generator works

It uses a nozzle to inject compressed air at high speed to form a jet at the nozzle outlet, producing an entrainment flow. Under the action of entrainment, the air around the nozzle outlet is continuously sucked away, reducing the pressure in the adsorption chamber to atmospheric pressure. Below, a certain degree of vacuum is formed, as shown in the figure.



From fluid mechanics, it can be known that the continuity equation for incompressible air gas (gas moving at low speed can be approximately considered as incompressible air)

A1v1= A2v2

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

v1, v2----air flow velocity, m/s

It can be seen from the above formula that as the cross-section increases, the flow velocity decreases;as the cross-section decreases, the flow velocity increases.

For horizontal pipelines, according to Bernoulli’s ideal energy equation of incompressible air:

P1+1/2ρv12=P2+1/2ρv22

In the formula, P1, P2----corresponding pressure at cross-section A1, A2, Pa

v1, v2----corresponding flow velocity at cross-section A1, A2, m/s

ρ----density of air, kg/m2

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

According to the nozzle exit Mach number M1 (the ratio of the exit flow velocity to the local sound speed), vacuum generators can be divided into subsonic tube types (M1<1), sonic nozzle types (M1=1) and supersonic nozzle types ( M1>1). Subsonic nozzles and sonic nozzles are both shrinking nozzles, while supersonic nozzles must first shrink and then expand (i.e. Laval nozzles). In order to obtain the maximum suction flow or the highest suction inlet pressure , the vacuum generators are all designed as supersonic nozzle type.

Vacuum generator structural parameter diagram


The vacuum generator is mainly composed of a nozzle and an expansion tube. From the above, it can be seen that the function of the nozzle is to convert the energy of compressed air into kinetic energy to generate supersonic airflow;the function of the expansion tube is to decelerate the supersonic airflow to reduce the noise when exhausting gas. It can be known from gas dynamics that when the air flow flows in a one-dimensional steady isentropic flow, the relationship between the flow cross-sectional area and the speed of the air flow should satisfy the formula:






f is the flow cross-sectional area;M is the Mach number, M=v/A;v is the speed of the airflow;A is the local sound speed.


It can be seen from the above formula that when the flow velocity increases, whether the flow cross-sectional area expands or shrinks depends on the positive and negative of (M2-1), that is, M>1 or M<1. When the flow velocity is less than the local sonic acceleration, M is less than 1, (M2-1) is negative, and the sign of df is opposite to that of dv. Therefore, when the flow velocity increases, the airflow cross-sectional area should decrease. When the flow speed is equal to the speed of sound, M=1, then M 2-1=0, that is, df=0, at which time the airflow cross-sectional area reaches the minimum value. When the flow speed is greater than the local speed of sound, that is, supersonic speed, M is greater than 1, (M 2- 1) is positive, and the sign of df is the same as that of dv. Therefore, when the flow speed increases, the cross-sectional area of ​​the airflow should expand.

Therefore, in order to obtain supersonic airflow, the cross-sectional shape of the nozzle should first shrink when accelerating less than the speed of sound, reach a minimum when it is equal to the speed of sound, and then gradually expand at supersonic speed, that is, a Laval nozzle structure that gradually contracts and expands;at the same time, The reason is that in order to decelerate the supersonic airflow, the cross-sectional shape of the expansion tube should first shrink when the supersonic speed decelerates. When the flow speed is equal to the speed of sound, that is, when M = 1, the cross-section is the smallest, and then when the flow speed is smaller than the speed of sound, the cross-section gradually expands. Expand structure. This is a simplified structural diagram of the nozzle and expansion tube of the vacuum generator. The nozzle tapering angle A is recommended to be 120° to 140°, the tapering angle B is recommended to be 14° to 16°;the expansion tube tapering and expanding angles C and H are recommended Take 6°~8°.


Vacuum generator structure diagram