Vacuum pump pumping speed and vacuum unit configuration of several issues

First, the pump pumping speed is defined as a certain suction pressure, the unit of time by the pump port was removed gas volume. A complete vacuum system, no matter what the application, should have a vessel or chamber to be evacuated, a vacuum unit, a vacuum pump, connecting pipes, valves, cold traps, etc. . The pipes, valves, cold traps and other components as part of the vacuum system, have a certain impediment to the flow of gas. Conversely, they have a certain ability to guide the flow of gas, which capacity is called conductance. This is a very important concept in gas flow, which is defined as the flow rate per unit of pressure differential. The natural flow of gas always flows from high pressure to low pressure. When any of the above components flows at a pressure P1 and P2, respectively, and the amount of gas flowing through it is Q,

U = Q / (P1-P2)

The conductance of different vacuum system components can be determined by means of calculations, simulations, measurements and the like, which, besides the geometry, are related to the gas flow conditions. The flow of different components can be in series and parallel.

The flow of different components in parallel

Vacuum pump is to extract the gas in the vacuum vessel, but often the pump suction port can not be directly connected with the vessel being drawn, due to process needs or reduce the degree of contamination of the vacuum unit with oil vapor pollution must pass through the cold trap, The valve and the pipeline can be connected with the evacuated container. Since each vacuum component has a definite conductance, it can be said that the pump must pass through a certain conductance to be connected with the evacuated container. As shown in the figure, the pump and the vacuum The connecting pipe between the chambers can include cold traps and valves and the like. Assuming that the conductance between the pump and the vacuum chamber is U, the pump must pass through the flow guide U to pump the vacuum chamber and its pumping capacity to be limited. In this case, the suction of the container should really be of a vacuum The pumping chamber at the effective pumping speed S0. If the nominal pumping speed of the pump is S, then according to the law of conservation of flow when the gas is flowing steadily, the relations satisfied by S0, S and U can be derived

The above equation is called the vacuum basic equation, which is based on the basic rules of vacuum system design.

According to the basic vacuum equation, two extreme results can be mathematically obtained. That is, when the conductance 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 when the conductance U is very small, the effective pumping speed S0 of the vacuum chamber is approximately equal to the conductance U. The above results may be more physically understandable, the gas drawn from the suction port of the vacuum chamber must be guided through the flow guide U (pipes, valves, etc.) to be pumped by the vacuum pump, except that the evacuated gas flows from the evacuation port of the vacuum chamber to the pump The process of mouth movement is from high pressure to low pressure flow, and from the pump port is forced flow from low pressure to high pressure based on some kind of suction principle. If the flow guide U is very large, that is, the amount of gas passing through it is not limited, then the pumping capacity of the pump depends on the size of its own pumping speed, which is the same as the pump port directly connected to the vacuum chamber. But if the pump pumping speed is very large, which is relative to the pump pumping speed U is very small, then the pump's actual pumping capacity does not depend on its pumping speed depends on the size of the gas through the guide U Capability, the value of conduction is exactly pump effective pumping speed S0.

In order to maximize pump pumping capacity, to maximize the flow guide U is the most effective way, but often difficult to achieve. And blindly increase pumping speed is more impractical. Therefore, the day using a large flow guide and selection of day pumping speed pump is worth the trade-off. From the basic vacuum equation we can see that the effective pumping speed S0 with S or U are monotonically increasing function. The description of the vacuum basic equation is not esoteric, but not as simple as everyone can know, so in many applications, the user often neglects the guide of pumping speed restrictions, which led to the application of the vacuum effect Affected.

Second, for a no leakage, there is no deflation of the vacuum system, such as the volume of the vacuum chamber V, the effective pumping speed of the vacuum chamber S0, then with the extraction process, the vacuum chamber pressure with time follow the following changes

Where P0 is the pressure at t = 0, that is, the initial pressure, t = V / S0 is called the time constant.

The above law reveals that the pressure in the vacuum chamber is reduced by an order of magnitude after each passage of time. Obviously, the smaller the pressure t is, the faster the pressure drops. When V is constant, the larger the effective pumping speed S0 is, the smaller it is.

However, none of the vacuum systems are leak-tight and deflation-free, and deflation is always present even though it is truly airtight. In fact, equation (3) reflects the pressure of the pump during evacuation of the gas in the vacuum chamber The law of change. When the pressure is high, the influence of system leakage and deflation volume on the relative space gas volume is negligible, and it can be considered that the condition of airtightness and deflation is approximately satisfied, that is, the law of (3) can be approximated Set up When the pressure is low, the system leakage and deflation can not be ignored or even become the main gas load, the deviation of the law of (3), manifested in the pressure drop into a slow, the general pressure of this change in 0.5Pa or so, so a typical vacuum system evacuating process pressure drops quickly, to a certain pressure began to slow down. Since a qualified vacuum system has strict requirements on leak rate, deflation is a major factor that affects system pressure reduction. Deflation is a slow process. Even if baking and other strengthening measures are used to achieve a predetermined Pressure, often go through a long time.

Any vacuum system wants to minimize the pumping time, which is related to improving efficiency and reducing energy consumption, but not all vacuum applications have the conditions to shorten the pumping time. Different vacuum applications can be divided into two categories: one is the system does not change the amount of deflation, but only the vacuum requirements; the other is required to fully evacuate the vacuum chamber, that is to go to the deflation rate A certain threshold. These two different types of applications require different pump configurations. For the former kind of application, if the degree of vacuum is above 0.5pa, as long as the time constant is small enough, the time of pumping can be shortened day by day. However, if the vacuum requirements below 0.5Pa, it is necessary to consider the impact of deflation on the pressure changes. Deflation changes slowly over time. Especially in the case of no baking. In order to achieve a higher degree of vacuum in a predetermined short period of time, it is necessary to extract a larger purge volume at a higher pumping speed. In other words, if the deflated quantity is Q, the effective pumping speed of the pump is S0, then the equilibrium pressure P = Q / S0 can be achieved. If the equilibrium pressure is determined, the shorter the time to reach, the greater the effective pump speed required. Evaporation coating is a typical application of this type. Due to the high speed and short time of vapor deposition, the influence of the amount of deflation (ie the influence of the active gas) is not considered. However, the energy of the evaporated particles is low, requiring most of the particles to be deposited onto the workpiece without collisions to ensure the binding force and reduce the 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. Correspondingly, Pressure is about 1 × 10-2Pa, which is the vacuum evaporation coating requirements.

How to achieve this pressure in the shortest possible time, the effective pump pumping speed requirements, the principle is the shorter the time, due to the larger the deflated, the effective pumping speed on the larger requirements. Therefore, the evaporation coating is generally equipped with pumping speed powerful oil diffusion pump unit, the power of dozens of kilowatts, a few minutes to ten minutes can reach the working vacuum, but the system caused by the oil vapor pollution is inevitable, especially Plastic metallized film is easy to yellow. Turbomolecular pump pumping speed can not meet the needs of large-scale evaporation. The pumping speed of the cryogenic pump is the industrial scale coating can not afford. According to the characteristics of the gas load in the gas-drawn space, the use of the molecular booster pump to remove the permanent gas, combined with the low-temperature condensate water trap pump to remove the water vapor, is expected to achieve a new pumping technology for obtaining a clean vacuum with a large pumping speed. When the pressure in the vacuum chamber is above 0.5 Pa, the main gas component is permanent gas, while the main gas component below 0.5 Pa is water vapor (90%). As the molecular booster pump with super-vacuum pumping capacity, from 100Pa to 0.5Pa pumping time is very short, but after the opening of 0.1Pa low-temperature condensate trap pump, the indoor pressure can be reduced in a short time 1 An order of magnitude, to 1 × 10-2Pa. For the 3-5m3 large-scale evaporation coating equipment, it is undoubtedly groundbreaking to dispose 3-4 units of 1000 l / s molecular booster pump and a 5kw low-temperature condensate trap . For the latter application, the change of deflation depends on temperature and time, but not on the pressure of gas-phase space. As long as the pressure is lower than the equilibrium pressure corresponding to the existing adsorption capacity, it is generally satisfied during the evacuation process condition. Therefore, with a strong pumping speed even in a very short period of time to a very low pressure in the room, still can not significantly reduce the amount of deflation in the vacuum chamber, and must be equipped with a suitable pumping speed at a reasonable baking temperature, reasonable The amount of deflated to meet the requirements of the level of technology, which generally go through dozens of minutes of time. Such applications are more typical of the titanium industry sputtering coating and ion plating, rare earth permanent magnet material melting. Among them, the excess of active gas will affect the quality of the film and the quality of the material, so the process has a longer period of finishing pumping process.

For the coating chamber is about 1m3 sputtering or ion plating equipment, the general configuration of 4000 l / s pumping speed vacuum unit, in order to promote the vacuum chamber and the workpiece more quickly deflated, often baked to 300 ℃ temperature. It is worth emphasizing that in the titanium coating, the pumping speed of the pump size, the characteristics of the pump, pumping process and the deposition pressure required between the show a correct relationship. In a coating cycle, the pumping of the vacuum unit can be divided into three phases, namely the fine pumping stage, the glow bombardment and the sputter deposition stage. The purpose of fine pumping is to reduce the amount of deflation in the vacuum chamber, the result depends mainly on the baking temperature and pumping time, and space pressure has little to do, especially the pressure in the same order of magnitude. Therefore, the main pump pumping speed within the appropriate differences, the effect of fine pumping is the same, the vacuum chamber evacuation rate can be reduced to the same level, although the corresponding limit vacuum. In particular, the 1000 l / s molecular boost pump and the 1500 l / s turbomolecular pump have the same effect at this stage of pumping. Glow bombardment stage, the discharge pressure at this time about 2Pa, generally speaking, the main pump pumping capacity is affected, the traditional method of throttling at the expense of pumping speed in exchange for the stability of the pump, diffusion pump and turbine molecules The pump is so, especially the diffusion pump pumping speed greater loss, the corresponding discharge of argon gas flow is also significantly reduced. However, this stage only a large effective pumping speed, large argon flow to get a better bombardment cleaning effect. At this point molecular booster pump is a clear advantage. Typical working pressure during the final sputter deposition phase is 0.5 Pa, and diffusion pumps and turbomolecular pumps still need to be throttled, not to mention that deposition pressures are difficult to stabilize in this case and reduced pumping rates are bound to The partial pressure of the active gas (deflated) reached during the fine drawing clearly rises. In the deflation of a certain amount of circumstances, the partial pressure of active gas determines the level of the quality of the deposited film. The molecular booster pump, which is capable of pumping at full pumping speed, again shows its superiority.

Third, different vacuum systems require different degrees of vacuum. So often it must be done by a vacuum unit. That is, the vacuum pumps working in different pressure ranges are connected in series. The vacuum pump on the high vacuum side can reach the required vacuum degree of the system, while the vacuum pump on the low vacuum side is directly discharged into the atmosphere. Obviously the simplest vacuum unit is a direct-on-atmosphere vacuum pump. But the high vacuum system generally needs three units, the vacuum generally needs two units. A high vacuum pump and a low vacuum pump make it difficult to make an efficient high vacuum unit.

There are several reasons for this. The continuity of traffic is one of them. High vacuum pump has a pre-pressure limit, that is, before the level above a certain pressure, the pump can not work properly. When the current stage pump reaches this critical pressure, the pumping speed will often be reduced, so that the exhaust flow of the foreline pump may be smaller than that of the main pump. Such inconsistency of flow breaks the requirement of continuity of flow and inevitably Will cause the vacuum unit can not work properly. However, if the vacuum pump in the high and low then connect a vacuum pump, you can play the role of up and down, continuous flow, and the pump can work in the best condition. Roots pump can work in the vacuum range, is the most suitable, it is also called Roots booster pump, due to its compression ratio is not high, just to connect a few Pa to a few hundred Pa range. When the three high-vacuum unit into a higher degree of vacuum, due to the main pump exhaust flow was significantly reduced, then only rely on a smaller fore pump to maintain the continuity of pumping, in practice this Is often used method, this can reduce unit energy consumption. Another reason that high-vacuum units often require a third-stage unit is due to the high vacuum pump's suction pressure limitations. Pumps have initial work pressure, the traditional high vacuum pump is in the range of a few Pa. Therefore, the pump must be pre-pumped to the pressure of the main pump to start working. However, it is often a long time to pump the foreline pump to this pressure because the pumping speed of the pump decreases with the pressure drop, especially for the vacuum unit with periodic pumping, Time is required, the longer the pre-pumping time, the longer the time to enter the work of vacuum, so the increase of a vacuum pump with a pre-stage low vacuum pump, pump can be achieved in a relatively short period of time the pressure can work, so you can Make the system enter the work pressure as soon as possible, have guaranteed the use efficiency of the apparatus.

Roots pump and oil booster pump can be used as a medium vacuum pump, molecular booster pump has a very high compression ratio, which in addition to making it a clean vacuum but also has excellent high vacuum performance, but also in the vacuum range is super The pumping capacity. This makes the molecular booster pump the only vacuum pump with medium and high vacuum performance at present, so only need to cooperate with a low vacuum pump to form a high vacuum unit comparable to the third-stage unit. Specifically, since the molecular booster pump has high pressure resistance, the pre-stage pump can be easily placed in a high-flow state. However, the high pressure of the molecular booster pump slows down the pre-pump load of the foreline pump. Molecular booster pump can work at 100-50Pa, the foreline pump from the atmosphere to this pressure, basically follow the pressure drop of each order of an order of magnitude, so the unit can have a high pumping efficiency. Simplifying high-vacuum units, eliminating Roots pumps is another boon to molecular booster pumps. For larger high-vacuum applications, the pre-pumping capacity of the fore-stage pump can also be appropriately enhanced to further shorten the pumping time. Since the pre-pumping time is very short compared to the entire exhausting process, the foreline pump has a long service life Short, so can serve as a pre-pumping multiple sets of equipment, and this is often very realistic. This makes large-scale application of the vacuum unit is greatly simplified. Vacuum applications in some applications, the need to enter the 10-1Pa range, which Roots pump two-stage unit is often difficult to achieve, and the use of two Roots cascade connected three-stage unit can increase the degree of vacuum to enter an order of magnitude 10-1Pa, so the vacuum application is also commonly used three-stage unit. As the molecular booster pump in 10-1Pa can be full pumping speed, it is also possible to replace the two stage vacuum pump Roots Roots pump. In general, the long-term work in the vacuum pressure of the low end of the Roots pump, molecular booster pump can be completely replaced. Roots pumps that work long hours in the medium-vacuum high-end pressure range should be relatively small, because front-stage pumps in this pressure range often have a strong pumping speed. This macroscopically predicted the future of the molecular booster pump to replace the Roots pump.

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