How Venturi Tube Works

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The principle of the vacuum generating device, that is, the Venturi tube. The Venturi tube is the abbreviation of Venturi tube. The principle of the Venturi effect is that when the wind blows through the barrier, the air pressure near the port above the leeward side of the barrier is relatively low, resulting in adsorption and the flow of air. The principle of the venturi tube is actually very simple. It changes the air flow from thick to thin to speed up the gas flow rate, so that the gas forms a "vacuum" zone behind the outlet of the venturi tube. When this vacuum zone is close to the workpiece, it will produce a certain adsorption effect on the workpiece. As shown in the figure, A-compressed air inlet B-nozzle C-muffler D-adsorption chamber inlet Compressed air enters from the inlet A of the venturi tube, and a small part is discharged through the nozzle B with a small cross-section. As the cross-section gradually decreases, the pressure of the compressed air increases, and the flow rate also increases. `At this time, a vacuum is generated in the inlet of the D adsorption chamber, causing the surrounding air to be sucked into the venturi, and the compressed air flows into the diffusion chamber to increase the flow rate of the gas, and then the air flow oscillation is reduced through the silencer device. The vacuum generator is a new, efficient, clean, economical, small vacuum component that uses a positive pressure air source to generate negative pressure. This makes it very easy and convenient to obtain negative pressure in places with compressed air, or in places that require both positive and negative pressure in a pneumatic system. Vacuum generators are widely used in industrial automation machinery, electronics, packaging, printing, plastics and robotics and other fields. The traditional use of vacuum generators is to cooperate with suction cups to adsorb and transport various materials. It is especially suitable for adsorbing fragile, soft, thin non-ferrous, non-metallic materials or spherical objects. In this type of application, a common feature is that the required air pumping volume is small, the vacuum degree is not high and it works intermittently. The main performance parameters of the vacuum generator ① Air consumption: refers to the flow rate qv1 flowing out from the nozzle. ②suction 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. ③Pressure at the suction port: Recorded as Pv. When the suction inlet is completely closed (such as a suction cup sucking the workpiece), that is, when the suction flow is zero, the pressure in the suction port is the lowest, recorded as Pvmin. ④ Suction response time: The adsorption 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 the time when a necessary vacuum degree is reached in the system loop. The main factors affecting the performance of vacuum generators. 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 diameter and its corresponding position, and the pressure of the air source. Figure 2 is the relationship curve between the pressure at the suction port, suction flow rate, air consumption and supply pressure of a vacuum generator. 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 reaches the maximum. When the supply pressure continues to increase, the pressure at the suction port increases, and the suction flow rate decreases. ①Characteristic analysis of maximum suction flow qv2max: The ideal qv2max characteristics of the vacuum generator require that within the common supply pressure range (P01=0.4---0.5MPa), qv2max is at the maximum value and changes slowly with P01. ②Characteristic analysis of the pressure Pv at the suction port: The ideal Pv characteristics of the vacuum generator require that Pv be at the minimum value within the common supply pressure range (P01=0.4---0.5MPa), and change smoothly with Pv1. ③Under 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 a more ideal matching relationship between the pressure at the suction inlet and the suction flow rate, multi-stage vacuum generators can be designed to be connected in series. ④The length of the diffuser tube should 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 diffuser tube. But the pipe is too long. The pipe wall friction loss increases. Generally, it is more reasonable for plumbers to use 6 to 10 times the diameter of the pipe. In order to reduce energy loss, an expansion section with an expansion angle of 6°---8° can be added to the outlet of the diffuser straight pipe. ⑤The adsorption response time is related to the volume of the adsorption chamber (including the diffusion chamber, adsorption pipe and suction cup or closed cabin volume, etc.), and the leakage amount of the adsorption surface is related to the required suction inlet pressure. 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 also shorter. ； If the adsorption volume is large and the adsorption speed is fast, the nozzle diameter of the vacuum generator should be larger. ⑥On the premise of meeting the usage requirements, the vacuum generator should reduce its air consumption (L/min). The air consumption is related to the supply pressure of compressed air. The greater the pressure, the greater the air consumption of the vacuum generator. Therefore, when determining the pressure duty at the suction port, attention should be paid to the relationship between the supply pressure of the system and the air consumption. Generally, the pressure at the suction port generated by a vacuum generator is between 20kPa and 10kPa. At this time, if the pressure on the supply gauge increases, the pressure at the suction port will no longer decrease, but the air consumption will increase. Therefore, reducing the pressure at the suction inlet should be considered in terms of controlling the flow rate. ⑦Sometimes it is difficult to obtain a lower pressure at the suction port due to the shape or material of the workpiece, and the pressure at the suction port increases due to air being sucked in from the edge of the suction cup or through the workpiece. In this case, it is necessary to correctly size the vacuum generator so that it can compensate for the increase in suction pressure caused by leakage. Since it is difficult to know the effective cross-sectional area when leaking, a simple test can be used to determine the pressure increase 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 the workpiece, vacuum generator, suction cup and vacuum gauge. The leakage amount can be easily known by checking the display reading of the vacuum gauge and the performance curve of the vacuum generator. When considering leakage, the characteristic curve of the vacuum generator is very important to correctly identify the vacuum generator. Leakage is sometimes unavoidable. Here’s how to size your vacuum generator when there is a leakage:: The size of the vacuum generator can be found by adding the nominal suction flow rate and leakage flow rate. Methods to increase the suction flow rate of vacuum generators Vacuum generators are divided into high vacuum type and high suction flow type. As reflected in Figure 3, the former curve has a large slope and the latter is flat. When the diameter of the nozzle throat is constant, to obtain a high vacuum, the suction flow rate must be reduced, and to obtain a large suction flow rate, the pressure at the suction inlet must be increased. In order to increase the suction flow of the vacuum generator, a multi-stage expanded pressure pipe can be designed. The three-stage diffuser tube vacuum generator increases the suction flow by two and a half times. If two three-stage diffuser tube vacuum generators are connected in parallel, the suction flow will be doubled. The vacuum generator is a small and economical vacuum generating component that is used in places with positive pressure air sources to greatly simplify the vacuum circuit. Therefore, it is helpful to reduce the manufacturing cost of the machine, to improve the reliability of the machine, to achieve high speed and automation of the machine, and has broad application prospects.