Vacuum System Selection Knowledge
Vacuum units classification ?
Vacuum units are generally named according to the type of main pump, and then can be classified according to product characteristics, such as oil-bearing vacuum units and oil-free vacuum units; more commonly, according to the scope of vacuum application, they can be divided into low vacuum units, medium vacuum pump units, high vacuum units and ultra-high vacuum units.
The low vacuum unit has high working pressure and large exhaust capacity. It is mostly used for vacuum transportation, vacuum impregnation, vacuum filtration, vacuum degassing of vacuum dryer, etc. Main pumps commonly used in low vacuum units, reciprocating pumps, water ring pumps, oil seal mechanical pumps, screw vacuum pumps, wet Roots pumps, jet pumps, molecular sieve adsorption pumps and other Direct-Air vacuum pumps can be competent, but according to the cleanliness, humidity and other conditions of the pumped gas, dust collectors, gas-water-oil-water separators, drying wells and other facilities need to be equipped. 。
Medium vacuum unit is suitable for various vacuum systems requiring large pumping speed and obtaining medium vacuum, such as coating machine, vacuum smelting, vacuum heat treatment, medicine, chemical industry, electrical welding and other industries. In the past, the combination of Roots Vacuum Pump + Oil Seal Mechanical Pump, Roots Vacuum + Water Ring Vacuum Pump and Double Roots Vacuum Pump was the most common. With the rise of oil-free vacuum pumps such as screw vacuum pumps, oil-free vacuum units such as Roots vacuum pumps and screw vacuum pumps are gradually increasing, and their adaptability is wider.
High vacuum unit works in the state of molecular flow, the working pressure range is 10-2-10-6 Pa, with small displacement and high pumping speed. Generally, the main pump is assumed by diffusion pump, molecular pump, titanium sublimation pump and cryogenic condensation pump. Because they can not work directly in the atmosphere, they must be equipped with front pump, pre-pumping pump and so on. In special cases, they need to be equipped and maintained to prevent pressure fluctuation. Pumps and gas storage tanks, etc.
The working pressure range of ultra-high vacuum unit is 10-6-10-10 Pa. The limit vacuum of the main pump should be above 10-7-10-8 Pa and the corresponding pumping speed should be guaranteed in the working pressure range, such as sputtering ion pump, turbomolecular pump, etc. Moreover, the material of the system composition should be baked at 200-450 C and low leakage rate and outgassing rate should be guaranteed. Knowledge of Vacuum System Selection?
What gases can the vacuum system pump?
Gas produced in the process of vacuum system: It includes the gas emitted from material in the process and the gas introduced in the process. At the same time, the gas evaporated by liquid or solid in the vacuum system is also included. The calculation of the amount of this gas varies with the process and the material being treated.
Atmospheric gases originally existed in vacuum system: vacuum container chamber and vacuum pipeline in vacuum system. These gases originally contain a certain amount of atmospheric gases. In the initial stage of pumping, they are the main gas load of vacuum system, and they are also the earliest pumped out by the system.
Leakage gases in vacuum system: Leakage gases include gases that leak into the vacuum system through the vacuum sealing connection and various leakage channels. For a given vacuum system, the amount of gas leaked is a constant. Different applications of vacuum systems and different limiting pressures also impose strict limits on the amount of leakage gas.
Gases released from the surface desorption of various materials in vacuum system: At atmospheric pressure, some gases will be adsorbed and dissolved on the surface of materials in vacuum system. Under negative pressure, this part of the gas will be released again. Its discharge rate is related to material properties, treatment process and material surface condition.
The atmosphere outside the vacuum system penetrates into the gas in the system through the wall material: the gas also dissolves and permeates in the solid. Therefore, the atmosphere will permeate part of the gas into the vacuum system through the structure material of the container wall. This kind of penetration may not be considered in general metal systems, but some applications in glass vacuum systems or thin-walled metal systems will need to consider the impact of gas penetration. Knowledge of Vacuum System Selection?
Ten aspects should be paid attention to in the process of structural design and construction of vacuum system:
1. Seamless steel pipes and sheets with standard material selection can reduce welding structure and improve air tightness of vacuum components. For the system components which need to be welded, steel with good weldability should be selected.
2. Reasonable design of welding structure and improvement of welding process quality. Welding is an important procedure in the construction of vacuum system. Welding structure should avoid the voids accumulated in the weld seam in vacuum. If there are dirt accumulation or voids, it will not only cause difficulties in cleaning, but also become a slow exhaust gas source in the vacuum system. If there is dead space in the weld, it will be difficult to find the leak.
3. Avoid isolation holes (air bags) or open them for rapid vacuum extraction. The air bag will become the source of slow exhaust, increasing the exhaust time.
4. It is better to electroplate, polish and oxidize the components and inner walls of the vacuum system to reduce surface bleeding. Different vacuum environments have different requirements for the inner wall roughness. In principle, the smoother the inner surface of components and shells in the vacuum system is, the better (generally, the inner wall roughness Ra of high vacuum is 6.3-3.2; the inner wall roughness Ra of medium vacuum is about 12.5); and the inner wall of ultra-high vacuum requires polishing to achieve a very smooth surface. In addition, it should be noted that the rusty metal surface is very harmful to vacuum.
Fifth, flange joints are welded first and then processed to ensure size, roughness and air tightness. It is easy to cause flange deformation during welding, so at present, flange is processed after welding in our country, which not only meets the requirements of size and roughness, but also ensures the reliability of sealing when two flanges are connected.
6. Vacuum rubber sealing rings working at high temperatures can be protected by water-cooled structures to prevent damage or shorter service life of components.
7. Vacuum system components and the shell wall of the vacuum chamber should be of a certain thickness to ensure that they have sufficient strength to ensure that they will not deform under the action of internal and external forces (meet internal and external pressure, and do not deform under three atmospheric pressures). In practical application, circular structure is generally preferable for vacuum vessels, while convex structure is preferable for end caps.
8. Ensure the dynamic sealing performance of the rotating or moving parts entering the vacuum chamber from outside. In addition to choosing a good sealing structure, the shaft or rod must meet the roughness requirements to prevent axial scratches on the shaft and rod. Such scratches can cause gas leakage, reduce the vacuum of the system, and are not easy to be found.
9. The arrangement of measuring gauge seats in vacuum systems should follow the following principles: a. The measuring gauge should not be placed in places with more sealing surfaces. Because every sealing surface can not guarantee absolute air leakage, where the sealing surface is concentrated, it must be easy to leak, and the measurement value may not be accurate. B. Vacuum hygiene must be guaranteed everywhere in the inner wall of the regulation, otherwise the measurement will be inaccurate. C. Regulations should be connected as close as possible to the measured area in order to reduce measurement errors.
10. The water jacket structure on the shell of the vacuum chamber should ensure that the water flow is smooth and unimpeded, and that no stagnant water can occur, resulting in local overheating. Therefore, the position of the inlet and outlet pipes should be one at a time, and a flow barrier should be set up to make the water flow along a certain route.
vacuum system product picture: