The vacuum generator is a new, efficient, clean, economical and small vacuum component that uses positive pressure air source to generate negative pressure, which makes it easy and convenient to obtain negative pressure where there is compressed air or where positive and negative pressure is required in a pneumatic system. Vacuum generators are widely used in the fields of machinery, electronics, packaging, printing, plastics and robots in industrial automation. The traditional use of vacuum generators is to cooperate with vacuum suction cups to carry out the adsorption and handling of various materials, especially suitable for the adsorption of fragile, soft, thin non-ferrous, non-metallic materials or spherical objects. In such applications, A common feature is that the required pumping capacity is small, the vacuum degree is not required high, and it works intermittently. The author believes that the analysis and research on the suction mechanism of the vacuum generator and the factors affecting its working performance are of practical significance to the design and selection of positive and negative air pressure circuits.
1. Operating principle of vacuum generator
The working principle of the vacuum generator is to use the nozzle to spray compressed air at a high speed, forming a jet at the nozzle outlet and generating entrainment flow. Under the effect of entrainment, the air around the nozzle outlet is constantly sucked away, so that the pressure in the adsorption chamber drops below atmospheric pressure, forming a certain degree of vacuum. As shown in Figure 1.
Figure 1 Schematic diagram of vacuum generator working principle
It can be seen from hydrodynamics that the continuity equation for incompressible air gas (gas entering at low speed can be approximately regarded as incompressible air)
A1v1= A2v2
Where, A1, A2 ---- sectional area of pipeline, m2
V1, v2 - airflow velocity, m/s
It can be seen from the above formula that the flow rate decreases with the increase of cross section; The flow velocity increases with the decrease of cross section.
For horizontal pipeline, the Bernoulli ideal energy equation of incompressible air is
P1+1/2ρv12=P2+1/2ρv22
Where P1, P2 ---- corresponding pressure at section A1, A2, Pa
V1, v2 --- corresponding velocity at section A1 and A2, m/s
ρ ---- air density, kg/m2
It can be seen from the above formula that when 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 atmospheric pressure, that is, negative pressure. Therefore, the negative pressure can be obtained by increasing the flow rate to generate suction.
According to the nozzle outlet Mach number M1 (the ratio of outlet velocity to local sound velocity), vacuum generators can be divided into subsonic nozzle type (M1<1), sonic nozzle type (M1=1) and supersonic nozzle type (M1>1). Both subsonic nozzle and sonic nozzle are contraction nozzles, while supersonic nozzle type must be expansion nozzle after contraction (Laval nozzle) In order to obtain the maximum suction flow or the maximum pressure at the suction inlet, the vacuum generators are designed as supersonic nozzles.
2. Analysis of suction performance of vacuum generator
2.1 Main performance parameters of vacuum generator
① Air consumption: refers to the flow qv1 from the nozzle.
② Suction flow: refers to the air flow qv2 sucked from the suction port. When the suction port is open to the atmosphere, its suction flow is the largest, which is called the maximum suction flow qv2max.
③ Pressure at the suction inlet: recorded as Pv. When the suction inlet is completely closed (such as the suction cup sucking the workpiece), that is, when the suction flow is zero, the pressure in the suction inlet is the lowest, recorded as Pvmin.
④ Suction response time: suction response time is an important parameter indicating the working performance of the vacuum generator. It refers to the time from the opening of the reversing valve to reaching a necessary vacuum degree in the system circuit.
2.2 Main factors affecting the performance of vacuum generator
The performance of the vacuum generator is related to many factors, such as the minimum diameter of the nozzle, the shape of the contraction and diffusion tubes, the path diameter and its corresponding position, and the pressure of the air source. Figure 2 shows the relationship curve between the pressure at the suction inlet, suction flow, air consumption and supply pressure of a vacuum generator. It shows that when the supply pressure reaches a certain value, the pressure at the suction inlet is low, and then the suction flow reaches the maximum. When the supply pressure continues to increase, the pressure at the suction inlet increases, and then the suction flow decreases.
① Characteristic analysis of the maximum suction flow qv2max: the qv2max characteristic of an ideal vacuum generator is required to be within the range of common supply pressure (P01=0.4 -- 0.5MPa), and qv2max is at the maximum value, and changes gently with P01.
② Characteristic analysis of the pressure Pv at the suction inlet: the ideal Pv characteristic of the vacuum generator is required to be within the range of the common supply pressure (P01=0.4 -- 0.5MPa), Pv is at the minimum value, and changes gently with Pv1.
③ Under the condition that the suction inlet is completely closed, the relationship between the pressure Pv at the suction inlet and the suction flow rate under specific conditions is shown in Figure 3. In order to obtain an ideal matching relationship between the pressure at the suction inlet and the suction flow rate, multi-stage vacuum generators can be designed to be combined in series.
④ The length of the diffusion tube shall ensure that various wave systems at the nozzle outlet are fully developed, so that approximately uniform flow can be obtained on the exit section of the diffusion tube. However, if the tube is too long, the friction loss of the tube wall will increase. Generally, it is reasonable for the plumber to be 6-10 times of the pipe diameter. In order to reduce energy loss, an expansion section with an expansion angle of 6-8 ° can be added at the outlet of the straight pipe of the diffuser.
⑤ The adsorption response time is related to the volume of the adsorption chamber (including the volume of the diffusion chamber, adsorption pipeline, suction cup or closed chamber, etc.), and the leakage amount of the adsorption surface is related to the pressure at the required suction inlet. For a certain pressure requirement at the suction inlet, the smaller the volume of the adsorption chamber, the shorter the response time; If the pressure at the suction inlet is higher, the adsorption volume is smaller, the surface leakage is smaller, and the adsorption response time is shorter; If the adsorption volume is large and the adsorption speed is fast, the nozzle diameter of the vacuum generator should be larger.
⑥ The air consumption (L/min) of the vacuum generator should be reduced on the premise of meeting the operating requirements. The air consumption is related to the supply pressure of compressed air. The higher the pressure, the greater the air consumption of the vacuum generator. Therefore, when determining the pressure duty at the suction inlet, pay attention to the relationship between the supply pressure of the system and the air consumption, Generally, the pressure at the suction inlet generated by the vacuum generator is between 20kPa and 10kPa. At this time, if the pressure of the water supply meter increases again, the pressure at the suction inlet will not decrease, but the gas consumption will increase. Therefore, reducing the pressure at the suction inlet should be considered from the aspect of flow rate control.
⑦ Sometimes, due to the influence of the shape or material of the workpiece, it is difficult to obtain a lower pressure at the suction inlet. Because air is sucked from the edge of the suction cup or through the workpiece, the pressure at the suction inlet increases. In this case, it is necessary to correctly select the size of the vacuum generator so that it can compensate for the pressure rise at the suction inlet caused by leakage. Since it is difficult to know the effective sectional area when leakage occurs, a simple test can be used to determine the pressure rise at the suction inlet caused by leakage. Since it is difficult to know the effective sectional area when leakage occurs, the leakage amount can be determined through a simple test. The test circuit is composed of workpiece, vacuum generator, suction cup and vacuum gauge. It is easy to know the leakage amount by checking the performance curve of the vacuum generator after the display reading of the vacuum gauge.
When considering the leakage, the characteristic curve of the vacuum generator is very important for the correct determination of the vacuum generator. Leakage is sometimes inevitable. When there is leakage, the method to determine the size of the vacuum generator is as follows: add the nominal suction flow and the leakage flow to find out the size of the vacuum generator.
3. Methods of Improving Suction Flow of Vacuum Generator
3.1 The vacuum generator is divided into high vacuum type and high suction flow type. The former has a large curve slope and the latter has a flat curve. When the throat diameter of the nozzle is fixed, the suction flow must be reduced to obtain high vacuum, and the pressure at the suction inlet must be increased to obtain large suction flow.
3. In order to increase the suction flow of the vacuum generator, multi-stage expansion pressure pipe can be designed. If two three-stage diffuser vacuum generators are used in parallel, as shown in the figure, the suction flow will be doubled.
4. Conclusion
4.1 The vacuum generator is a compact and economical vacuum generating element, which is used in places with positive pressure air source to greatly simplify the vacuum circuit. Therefore, it is conducive to reducing the manufacturing cost of the machine, improving the reliability of the machine, realizing the high-speed and automation of the machine, and has broad application prospects.
4.2 In the process of system design, various performance parameters of the vacuum generator shall be comprehensively considered, and the performance indicators matching the system shall be selected. Generally, the better gas supply pressure is 0.4 --- 0.5 MPa, and the pressure at the suction inlet is generally 20 kPa --- 10 kPa.
Article address: http://www.shruohao.com/jishuzhichi/21.html , vacuum suction hoist, tracheal suction hoist, electric glass sucker, roll carrier, overturning suction hoist, plate suction hoist, glass electric sucker, vacuum tracheal suction hoist, vacuum suction cup hanger, vacuum suction cup manipulator, stacking manipulator, suction cup manipulator, stacking manipulator, vacuum suction cup, electric suction cup, glass suction cup; If you need to reprint, please indicate the source of this article!
