Pumping speed of the hottest vacuum pump and confi

The pumping speed of the vacuum pump and the configuration of the vacuum unit

the pumping speed of the vacuum pump and the configuration of the vacuum unit:

first, the pumping speed of the pump is defined as the volume of gas pumped out through the pump port per unit time under a certain suction pressure. A complete vacuum system, no matter for any application, should have a container or chamber that needs to be vacuumed, a set of vacuum unit, or a vacuum pump, as well as connecting pipes, valves, cold traps, etc. As the components of the vacuum system, pipes, valves, cold traps, etc. have a certain blocking effect on the flow of gas. On the contrary, they all have a certain conductivity to the flow of gas, which is called flow conductivity. This is a very important concept in the flow of gas, which is defined as the flow rate under unit pressure difference. The natural flow of gas always flows from high pressure to low pressure. When the pressures at both ends of any of the above components are P1 and P2 respectively, and the amount of gas flowing is Q, the flow conductance of this component

u=q/(P1-P2)

the flow conductance of different vacuum system components can be determined by calculation, simulation, measurement and other methods. It is not only related to geometry, but also related to the flow state of gas. The conductance of different components can be connected in series and parallel

the purpose of vacuum pump is to pump out the gas in the vacuum container, but often the extraction port of the pump cannot be directly connected with the pumped container. Due to the needs of technology or to reduce the pollution degree of the vacuum unit polluted by oil steam, it must be connected with the pumped container through cold traps, valves and pipelines. Because each vacuum component has a certain flow guide, it can be said that the pump must be connected with the pumped container through a certain flow guide, As shown in the figure, the connecting pipes between the pump and the vacuum chamber in the figure can include cold traps, valves, etc. Assuming that the flow guide between the pump and the vacuum chamber is u, the pump must pass through the flow guide u to pump air into the vacuum chamber, and its pumping capacity will be limited. At this time, what is really meaningful for the pumping effect of the container should be the effective pumping speed S0 at the suction port of the vacuum chamber. If the nominal pumping speed of the pump is s, then the relationship between S0, s and u can be derived according to the law of flow conservation when the gas flows stably

the above formula is called the basic vacuum equation, which is the basic law based on in the design of the vacuum system

according to the basic vacuum equation, two extreme results can be obtained mathematically, that is, when the flow conductivity u is very large, the effective pumping speed S0 of the vacuum chamber can be approximately equal to the pumping speed s of the pump; When the pumping speed s of the pump is very large, or the flow conductivity u is very small, the effective pumping speed S0 of the vacuum chamber is approximately equal to the flow conductivity U. The above results may be easier to understand physically. The gas pumped from the vacuum chamber suction port must pass through the flow guide u (i.e. pipes, valves, etc.) to be pumped by the vacuum pump, but the movement process of the pumped gas from the vacuum chamber suction port to the pump port is the flow from high pressure to low pressure, and the pumping from the pump port is the forced flow from low pressure to high pressure based on some pumping principle. If the flow guide u is very large, that is, the amount of gas passing through it is unlimited, then the pumping capacity of the pump depends on its own pumping speed, which is the same as that the pump port is directly connected with the vacuum chamber. However, if the pumping speed of the pump is very large, that is, the flow conductivity u is very small relative to the pumping speed of the pump. At this time, the actual pumping capacity of the pump does not depend on its pumping speed, but on the ability of the gas to pass through the flow conductivity U. The value of the flow conductivity is just the effective pumping speed S0 of the pump

in order to give full play to the pumping capacity of the pump, maximizing the flow conductivity u is the most effective method, but it is often difficult to achieve. And blindly increasing the pumping speed of the pump is more unrealistic. Therefore, it is worth weighing the use of a large flow guide in the daytime and the selection of a pump with a large pumping speed in the daytime. From the basic vacuum equation, we can know that the effective pumping speed S0 is a monotonically increasing function with s or U. The content of vacuum basic equation is not profound, but it is not simple enough to be everyone's common sense. Therefore, in many application fields, users often ignore the restriction of flow conductivity on pump speed, which greatly affects the effect of vacuum technology application

II. For a vacuum system without air leakage, the "new material innovation capacity construction project" is proposed. For example, the volume of the vacuum chamber is V and the effective pumping speed of the vacuum chamber is S0. With the pumping process, the pressure in the vacuum chamber follows the following change law with time

where P0 is the pressure when t=0, that is, the initial pressure, and t=v/s0 is called the time constant

the above law reveals that the pressure in the vacuum chamber decreases by an order of magnitude after about time. Obviously, the smaller t is, the faster the pressure decreases. When V is constant, the greater the effective pumping speed S0, the smaller it can be

however, there is no vacuum system without air leakage and deflation. Even if there is no air leakage, deflation always exists. In fact, formula (3) reflects the pressure change law of the pump in the process of pumping out the space gas in the vacuum chamber. When the pressure is high, the air leakage and bleeding volume of the system is relatively small compared with the gas volume in space, and its influence can be ignored. It can be considered that the conditions of no air leakage and no bleeding are approximately satisfied, that is, the law of (3) can be approximately established. When the pressure is low, the air leakage and bleeding volume of the system can not be ignored or even become the main gas load. The law of (3) will deviate, which is manifested in that the pressure drop becomes slow. Generally, the pressure at which this change occurs is about 0.5pa. Therefore, the typical air extraction process of a vacuum system first drops rapidly, and then slows down at a certain pressure. As a qualified vacuum system has strict requirements on its leakage rate, deflation is the main factor affecting the pressure reduction of the system, and deflation is a slow process. Even with baking and other strengthening measures, it often takes a long time to reach a predetermined pressure

any vacuum system hopes to shorten the extraction time as much as possible, which is related to improving efficiency and reducing energy consumption, but not all vacuum applications have the conditions to shorten the extraction time. Different vacuum applications can be divided into two categories: one is not to consider the amount of bleeding in the system, but only the requirements of vacuum degree; The other is to require sufficient bleeding in the vacuum chamber, that is, the bleeding rate should be reduced to a certain critical value. These two different applications have different requirements for pump configuration. For the former application, if the vacuum degree is required to be above 0.5pa, as long as the time constant is small enough, the pumping time can be shortened in the daytime. However, if the vacuum degree is required to be less than 0.5pa, the influence of bleeding on pressure change must be considered. The amount of bleeding changes slowly with time. Especially without baking. In order to achieve a higher vacuum degree in a predetermined short time, it is necessary to pump out a larger amount of gas at a larger pumping speed. That is to say, if the bleeding volume is Q and the effective pumping speed of the pump is S0, the equilibrium pressure p=q/s0 can be achieved. If the balance pressure is determined, the shorter the time it reaches, the greater the effective pumping speed of the pump is required as the number of product varieties in the wood product packaging box continues to increase. Evaporation coating is a typical application of this kind. Due to the fast evaporation speed and short time, the influence of outgassing volume (i.e. the influence of active gas) is not considered. However, the low energy of evaporated particles requires that most particles deposit on the workpiece without collision, so as to ensure the bonding force and reduce scattering. This requires that the average free path in the vacuum chamber is not less than the distance from the evaporation source to the workpiece, and the corresponding pressure is about 1 × PA, which is the vacuum requirement of evaporation coating

how to reach this pressure in the shortest possible time puts forward requirements for the effective pumping speed of the pump. The principle is that the shorter the time is, the greater the amount of bleeding is, the greater the effective pumping speed is required. Therefore, the evaporation coating is generally equipped with an oil diffusion pump unit with strong pumping speed, with a power of dozens of kW, and the working vacuum can be reached in a few to ten minutes, but the oil vapor pollution caused by the system to the workpiece is inevitable, especially the plastic metallized film is easy to turn yellow. At present, the pumping speed of turbo molecular pump can not meet the needs of large-scale evaporation plating. And the large pumping speed of the cryogenic pump can not be borne by the industrial scale coating. According to the characteristics of the gas load in the pumped space, the permanent gas is pumped by the molecular booster pump, and the water vapor is pumped by the low-temperature condensate trap pump, which is expected to achieve a new pumping process with high pumping speed and clean vacuum. When the pressure in the vacuum chamber is above 0.5pa, the main gas component is permanent gas, while the main gas component below 0.5pa is steam (90%). Because the molecular booster pump has super medium vacuum pumping capacity, the pumping time from 100Pa to 0.5pa is very short, and using the low-temperature condensate trap pump after 0.1pA can reduce the indoor pressure by an order of magnitude to 1 in a relatively short time × Pa。 For M3 large evaporation coating equipment, the above extraction process can be realized by configuring a 1000 l/S molecular booster pump and a 5kW low-temperature condensate capture pump, which is undoubtedly groundbreaking. For the latter kind of applications, because the change of bleeding volume depends on temperature and time, and has little relationship with the pressure in the gas phase space, as long as the pressure is lower than the equilibrium pressure corresponding to the existing adsorption volume, this condition is generally met in the process of pumping. Therefore, with a strong pumping speed, even if the space pressure is reduced to a very low level in a very short time, it still cannot significantly reduce the amount of bleeding in the vacuum chamber. Instead, a suitable pumping speed must be configured to make the amount of bleeding reach the size of the water shape required by the process in a reasonable time under a reasonable baking temperature, which generally takes dozens of minutes. Typical applications of this kind include sputtering and ion plating in titanium industry, smelting of rare earth permanent magnet materials, etc. Among them, excessive active gas will affect the quality of film and material, so there is a long-term fine extraction process in the process

for sputtering or ion coating equipment with a coating chamber of about 1m3, a vacuum unit with a pumping speed of 4000 L/S is generally configured. In order to make the vacuum chamber and workpiece deflate faster, it is often baked to a temperature of 300 ℃. It is worth emphasizing that in titanium gold plating, there is a positive relationship between the pump speed, pump characteristics, pumping process and the required deposition pressure. In a coating cycle, the pumping of the vacuum unit can be divided into three stages, namely, fine pumping stage, glow bombardment and sputtering deposition stage. The purpose of fine pumping is to reduce the amount of bleeding in the vacuum chamber. The result mainly depends on the baking temperature and pumping time, and has little to do with the space pressure, especially when the pressure is within the same order of magnitude. Therefore, if the pumping speed of the main pump is within an appropriate difference, the effect of fine pumping is the same, and the bleeding rate in the vacuum chamber can be reduced to the same level, although the corresponding limit vacuum is different. Specifically, 1000 l/S molecular booster pump and 1500 L/s turbomolecular pump have the same pumping effect at this stage. In the glow bombardment stage, because the discharge pressure is about 2PA at this time, generally speaking, the pumping capacity of the main pump is affected. Traditionally, throttling method is used to sacrifice the pumping speed in exchange for the stable operation of the pump. This is the case with diffusion pump and turbomolecular pump, especially the pumping speed loss of diffusion pump is greater, and the discharge argon flow is also significantly reduced. However, at this stage, only with a large effective pumping speed and a large argon flow can a better effect of bombardment cleaning be obtained. At this point, molecular booster pump has obvious advantages. Typical working pressure in the final sputter deposition stage