An empty 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 the presence of 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. As shown in Figure 1.
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)
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
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.
2. Analysis of suction performance of vacuum generator
2.1, the main performance parameters of the vacuum generator
1 Air consumption: refers to the flow rate qv1 flowing out of the nozzle.
2 Inhalation flow rate: refers to the air flow rate qv2 drawn from the suction port. When the suction port is open to the atmosphere, the suction flow rate is the largest, which is called the maximum suction flow rate qv2max.
3 Pressure at the suction port: recorded as Pv. When the suction port is completely closed (such as the suction cup sucking the workpiece), that is, when the suction flow rate is zero, the pressure in the suction port is the lowest, which is recorded as Pvmin.
4 Suction response time: The sorption response time is an important parameter indicating the performance of the vacuum generator. It refers to the time from when the reversing valve is opened to the system loop to reach a necessary vacuum.
2.2 Main factors affecting the performance of the vacuum generator
The performance of the vacuum generator is related to the minimum diameter of the nozzle, the shape of the shrinkage and diffusion tube, the diameter and its corresponding position and the pressure of the source. Figure 2 is a graph showing the relationship between the pressure at the suction port of a vacuum generator, the suction flow rate, the air consumption and the supply pressure. The figure shows that when the supply pressure reaches a certain value, the pressure at the suction port is low, and the suction flow rate is at this time. When the supply pressure continues to increase, the pressure at the suction port increases, and the suction flow rate decreases.
1 Characteristic analysis of the maximum suction flow rate qv2max: The ideal qv2max characteristic of the vacuum generator is required to be within the common supply pressure range (P01=0.4---0.5MPa), qv2max is at the maximum value, and the change with P01 is gentle.
2 Characteristic analysis of pressure Pv at the suction port: The Pv characteristic of the ideal vacuum generator is required to be within the range of common supply pressure (P01=0.4---0.5MPa), Pv is at the minimum value, and the change with Pv1 is gentle. .
3 Under the condition that the suction port is completely closed, the relationship between the pressure Pv at the suction port and the suction flow rate under certain conditions is shown in Fig. 3. In order to obtain the ideal matching relationship between the pressure and the suction flow at the suction port, It can be designed as a multi-stage vacuum generator in series combination.
4 The length of the diffuser tube should ensure that the various wave systems at the outlet of the nozzle are fully developed, so that an approximate uniform flow can be obtained at the exit section of the diffusion pipe. However, the pipe is too long and the friction loss of the pipe wall is increased. Generally, the pipeman is 6--10 times of the pipe diameter. In order to reduce the energy loss, an expansion section with an expansion angle of 6°--8° can be added to the outlet of the straight pipe of the diffuser pipe.
5 The sorption response time is related to the volume of the adsorption chamber (including the diffusion chamber, the adsorption tube and the suction cup or the volume of the closed chamber, etc.), and the leakage amount of the adsorption surface is related to the pressure at the desired suction port. For the pressure requirement of a certain suction port, if the volume of the adsorption chamber is smaller, the response time is shorter; if the pressure at the suction port is higher, the smaller the adsorption volume is, the smaller the surface leakage amount is, the shorter the sorption response time is; If the adsorption volume is large and the adsorption speed is fast, the nozzle diameter of the vacuum generator should be larger.
6 The vacuum generator should reduce its gas consumption (L/min) on the premise of meeting the requirements of use. The gas consumption is related to the supply pressure of compressed air. The higher the pressure, the larger the gas consumption of the vacuum generator. Therefore, it is determined. Pay attention to the relationship between the supply pressure of the system and the gas consumption when the pressure is on the suction port. Generally, the pressure at the suction port generated by the vacuum generator is between 20 kPa and 10 kPa. At this time, the pressure on the surface of the watch is increased, the pressure at the suction port is no longer lowered, and the air consumption is increased. Therefore, the pressure at the suction port should be reduced in terms of controlling the flow rate.
7 Sometimes it is difficult to obtain a lower pressure at the suction port due to the shape or material of the workpiece. The pressure at the suction port is increased due to the suction of air from the edge of the suction cup or through the workpiece. In this case, it is necessary to be correct. The size of the vacuum generator is chosen to compensate for the pressure rise at the suction port caused by the leak. Since it is difficult to know the effective cross-sectional area at the time of the leak, a simple test can be used to determine the pressure rise at the suction port caused by the leak. Since it is difficult to know the effective cross-sectional area at the time of leakage, a simple test can be used to determine the amount of leakage. The test circuit consists of a workpiece, a vacuum generator, a suction cup and a vacuum gauge. The reading of the vacuum gauge and the performance curve of the vacuum generator make it easy to know the amount of leakage.
When considering a leak, the characteristic curve of the vacuum generator is very important for the correct determination of the vacuum generator. Leakage is sometimes unavoidable. The method of determining the size of the vacuum generator when there is a leak is as follows: Add the nominal suction flow rate to the leak flow rate to find the size of the vacuum generator.
3. Method for increasing suction flow rate of vacuum generator
3.1. The vacuum generator is divided into a high vacuum type and a high suction flow type. The former has a large slope and the latter is flat. In the case where the diameter of the nozzle throat is constant, in order to obtain a high vacuum, the suction flow rate is inevitably lowered, and in order to obtain a large suction flow rate, the pressure at the suction port is inevitably increased.
3. In order to increase the suction flow of the vacuum generator, a multi-stage enlarged pressure tube design can be adopted. If two three-stage diffuser tube vacuum generators are connected in parallel, as shown, the suction flow rate will be doubled.
4.1. The vacuum generator is a compact and economical vacuum generating component. It is used in places with positive pressure gas source to greatly simplify the vacuum circuit. Therefore, it is beneficial to reduce the manufacturing cost of the machine and improve the reliability of the machine. It is conducive to the realization of high speed and automation of machinery, and has broad application prospects.
4.2. During the system design process, various performance parameters of the vacuum generator should be considered comprehensively, and the performance index matching the system should be selected. Generally, the supply pressure of the gas supply is: 0.4---0.5 MPa, and the pressure at the suction port is generally It is: 20kPa - 10kPa.