Colleagues, does anyone know how to pair a vacuum pump?

Generally, a relatively rough guideline is that the ratio of pre-stage pumps to booster pumps should be 1:3. Some foreign manufacturers, who have confidence in their own process designs, opt for a ratio of 1:5 or even 1:6. Basically, one principle is followed: the compressed gas produced by the booster pump must be discharged in a timely manner by the pump ahead of it, so as to create a favorable pressure difference between the inlet and outlet for the booster pump.

1. The pumping speed of a pump is defined as the volume of gas that is drawn through the pump’s outlet per unit time, under a certain suction pressure. A complete vacuum system, for any application, should include a container or chamber that needs to be evacuated, a set of vacuum equipment, which may be a vacuum pump, as well as connecting pipes, valves, cold traps, and so on. Pipes, valves, cold traps, etc., as components of a vacuum system, all exert a certain degree of resistance to the flow of gases. Conversely, they all possess a certain ability to conduct gas flow, and this ability is known as conductance. This is a very important concept in gas flow; it is defined as the flow rate under a unit pressure difference. The natural flow of gas always occurs from areas of high pressure to those of low pressure. For any of the aforementioned components, when the pressures at its two ends are P1 and P2 respectively, and the volume flow rate of the gas passing through it is Q, then the conductance of that component is given by U = Q/(P1 – P2). The conductances of various vacuum system components can be determined through calculations, simulations, measurements, etc. Besides being related to their geometric shape, they also depend on the flow state of the gas. The flow conductances of different components can be connected in series and parallel. 　　A vacuum pump is used to remove gases from a vacuum chamber. However, the pump’s suction port often cannot be connected directly to the chamber that needs to be evacuated. Due to process requirements or in order to reduce contamination in vacuum systems caused by oil vapors, it is necessary to use cold traps, valves, and pipes to establish a connection to the chamber. Since each vacuum component has a specific flow rate, it can be said that the pump must pass through certain flow channels in order to connect to the chamber. As shown in the diagram, the connection pipeline between the pump and the vacuum chamber may include cold traps and valves. Assuming the flow conductance between the pump and the vacuum chamber is U, the pump must pass through this flow conductance U in order to evacuate the vacuum chamber; its pumping capacity is thus limited. In this case, what is truly significant for the evacuation of the container is the effective pumping speed S0 at the evacuation port of the vacuum chamber. If the nominal pumping speed of the pump is S, then, based on the law of conservation of flow rate for steady flow of gases, the relationship among S0, S, and U can be derived. The above equation is known as the fundamental equation of vacuum; it is the basic principle upon which vacuum system design is based. 　　Based on the fundamental equations of vacuum, two extreme results can be derived mathematically: namely, when the flow rate U is very large, the effective pumping speed S0 of the vacuum chamber can be approximated equal to the pumping speed S of the pump ； When the pumping speed S of the pump is very large, or the flow conductance U is very small, the effective pumping speed S0 of the vacuum chamber is approximately equal to the flow conductance U. The above results are physically more understandable: the gas drawn out from the vacuum chamber’s exhaust port must pass through flow channels U (i.e., pipes, valves, etc.) before it can be drawn away by the vacuum pump. The movement of the gas from the exhaust port of the vacuum chamber to the pump inlet is a flow from high pressure to low pressure, whereas its removal from the pump inlet is a forced flow from low pressure to high pressure, based on certain pumping principles. If the flow rate U is very high, meaning that there is no limit to the amount of gas that can pass through it, then the pumping capacity of the pump is determined by its own pumping speed, just as when the pump outlet is directly connected to the vacuum chamber. However, if the pumping speed of the pump is very high, which means that the flow conductance U relative to the pumping speed is very small, then the actual pumping capacity of the pump is not determined by its pumping speed but rather by the ability of the gas to pass through the flow conductance U; the value of this flow conductance corresponds exactly to the effective pumping speed S0 of the pump. 　　To maximize the pumping capacity of the pump, increasing the flow conductance U to the greatest extent possible is the most effective method, but it is often difficult to achieve. Moreover, simply increasing the pumping speed of the pump is even less practical. Therefore, it is worth considering the use of a flow coefficient with a high daytime value and a pump with a high pumping speed during the day. From the basic equations of vacuum, it can be seen that the effective pumping speed S0 is a monotonically increasing function of both S and U. The basic equations of vacuum are not overly complex, but they are also not simple enough to be considered common knowledge for everyone. As a result, in many application areas, users often overlook the limitation that flow rate imposes on the pumping speed, which significantly affects the effectiveness of vacuum technology applications. II. For a vacuum system that has no leaks and from which no gas is released, with a volume of V and an effective pumping speed of S0, the pressure inside the vacuum chamber changes over time according to the following pattern during the pumping process. Here, P0 represents the pressure at t=0, that is, the initial pressure, while t = V/S0 is known as the time constant. 　　The above patterns indicate that every approximately time interval, the pressure inside the vacuum chamber decreases by one order of magnitude. Clearly, the smaller t is, the faster the pressure drops. When V remains constant, the larger the effective pumping speed S0, the smaller can be. 　　However, no vacuum system is leak-free; even if there are no leaks, some gas leakage still occurs. In fact, equation (3) reflects the pattern of pressure changes as the pump removes gas from the volume inside the vacuum chamber. When the pressure is high and the amount of air leakage and release from the system is small relative to the amount of gas in the space, their impact can be ignored; it can be assumed that the conditions of no leakage and no release are approximately satisfied, meaning that the rule in (3) can hold approximately. When the pressure is low and the system’s gas leakage and outgassing amounts are not negligible and even constitute the main source of gas load, the pattern described in (3) deviates; this is manifested by a slower rate of pressure drop. Generally, this transition occurs at a pressure of around 0.5 Pa. Therefore, in a typical vacuum system, the pressure drops rapidly at first, and then slows down at a certain pressure level. Since a qualified vacuum system has strict requirements regarding its leakage rate, gas release is the main factor affecting the reduction of system pressure. Moreover, gas release is a slow process; even with enhanced measures such as baking, it often takes a long time to reach a certain desired pressure level. 　　Any vacuum system aims to minimize the pumping time, as this is related to improving efficiency and reducing energy consumption; however, not all vacuum applications have the conditions necessary to shorten the pumping time. Different vacuum applications can be divided into two main categories: one category involves no consideration of the amount of gas emitted within the system; in this case, only the required level of vacuum matters ； The other category requires sufficient outgassing within the vacuum chamber, meaning that the outgassing rate must be reduced to a certain critical value. These two different types of applications have different requirements for pump configuration. For the former type of applications, where the required vacuum level is above 0.5 Pa, as long as the time constant is sufficiently small, it is possible to significantly reduce the pumping time. However, if the vacuum level is required to be below 0.5 Pa, the effect of gas release on pressure changes must be taken into consideration. The change in gas emission over time is slow. Especially without baking. To achieve a high vacuum level in a relatively short period of time, it is necessary to remove a large amount of gas at a high pumping speed. In other words, if the gas release rate is Q and the effective pumping speed of the pump is S0, then an equilibrium pressure of P = Q/S0 can be achieved. Once the equilibrium pressure is determined, the shorter the time required to reach it, the greater the required effective pumping speed of the pump. Evaporation coating is a typical example of such an application; due to the fast coating speed and short processing time, the influence of gas evolution (i.e., the effect of reactive gases) is not taken into account. However, the energy of evaporated particles is low; thus, it is required that most of these particles deposit onto the workpiece without any collisions, so as to ensure good adhesion and minimize scattering. This necessitates that the mean free path within the vacuum chamber be no less than the distance from the evaporation source to the workpiece. The corresponding pressure level is approximately 1×10⁻² Pa. This is the requirement regarding vacuum level for evaporation coating. 　　How to reach this pressure in the shortest possible time places requirements on the effective pumping speed of the pump. The principle is that the shorter the time, the greater the outgassing rate, and thus the higher the required effective pumping speed. Therefore, evaporation coating typically employs oil diffusion pump systems with high pumping speeds; these systems have power ratings in the tens of kilowatts and can reach the operating vacuum level within a few minutes to over ten minutes. However, oil vapor contamination of the workpieces caused by this system is inevitable; in particular, plastic metallized coatings tend to turn yellow. Currently, the pumping speed of turbomolecular pumps is insufficient to meet the requirements of large-scale evaporation coating. Moreover, cryopumps with high pumping speeds are beyond what industrial-scale coating processes can handle. Based on the characteristics of the gas load in the evacuated space, the use of molecular booster pumps to remove permanent gases, in combination with cryogenic condensation water traps to remove water vapor, is expected to enable a novel pumping process capable of achieving a high pumping speed and a clean vacuum. When the pressure in the vacuum chamber is above 0.5 Pa, the main gas component is inert gases; whereas when it is below 0.5 Pa, the main gas component is water vapor (90%). Due to the superior medium-vacuum pumping capability of the molecular booster pump, the pumping time from 100 Pa to 0.5 Pa is extremely short. By activating the cryogenic condensate capture pump at pressures below 0.1 Pa, it is possible to reduce the pressure inside the chamber by one order of magnitude in a relatively short time, achieving a pressure of 1×10-2 Pa. For large evaporation coating equipment with a capacity of 3–5 m3, equipping it with 3–4 molecular booster pumps rated at 1000 liters per second and one low-temperature condensate collection pump with a power output of 5 kW enables the implementation of the aforementioned pumping process, which is undoubtedly groundbreaking. For the latter type of applications, since the amount of gas released varies depending on temperature and time, and has little relation to the pressure in the gas phase space, it is sufficient as long as the pressure is below the equilibrium pressure corresponding to the existing adsorption capacity; this condition is generally met during the evacuation process. Therefore, even if the space pressure is reduced to very low levels in a short period of time using a high pumping speed, this does not significantly reduce the gas emission rate inside the vacuum chamber. It is necessary to use an appropriate pumping speed, and at a reasonable baking temperature, to bring the gas emission rate down to the level required by the process – a process that generally takes several dozen minutes. Typical applications in this category include sputtering and ion plating in the titanium and gold industry, as well as the melting of rare earth permanent magnet materials. Among them, excessive reactive gas can affect the quality of the membrane layer and the quality of the material; therefore, a prolonged degassing process is included in the manufacturing process. 　　For sputtering or ion plating equipment with a coating chamber of approximately 1 m³, a vacuum pump unit with a pumping speed of 4,000 l/s is typically employed. To facilitate faster outgassing from the vacuum chamber and workpieces, the chamber is often heated to a temperature of 300°C. It is worth emphasizing that in titanium plating, there is a reciprocal relationship among the pumping speed of the pump, the pump’s characteristics, the pumping process, and the desired deposition pressure. During a coating cycle, the pumping action of the vacuum system can be divided into three stages: the high-vacuum stage, the glow discharge and sputtering deposition stage. The purpose of precision pumping is to reduce the gas emission from the vacuum chamber; the resulting effect mainly depends on the baking temperature and the pumping time, and has little to do with the pressure in the chamber, especially when the pressures are within the same order of magnitude. Therefore, within an appropriate range of differences in the pumping speed of the main pump, the effect of precise pumping is the same, and the gas release rate in the vacuum chamber can be reduced to the same level, despite the different ultimate vacuums involved. Specifically, the molecular pump with a flow rate of 1000 liters per second and the turbo molecular pump with a flow rate of 1500 liters per second achieve the same pumping efficiency at this stage. During the glow discharge phase, since the discharge pressure is around 2 Pa at this time, the pumping capacity of the main pump is generally affected. Traditionally, a throttling method is used to sacrifice the pumping speed in order to ensure the stable operation of the pump; this applies to both diffusion pumps and turbo molecular pumps. In particular, diffusion pumps experience a greater loss in pumping speed, and as a result, the flow rate of argon gas during discharge also decreases significantly. However, at this stage, only a large effective pumping speed and a high argon flow rate can achieve better bombardment cleaning effects. In this regard, molecular boosters have a clear advantage. During the final sputtering deposition stage, the typical operating pressure is 0.5 Pa; the diffusion pump and turbo molecular pump still need to be throttled. Not only is it difficult to stabilize the deposition pressure under such conditions, but the reduced pumping rate inevitably causes the partial pressure of the active gases (outgassing) to rise significantly during the high-vacuum purification process. With a constant amount of gas released, the partial pressure of the reactive gas determines the impact on the quality of the deposited film layer. The molecular booster pump, which can achieve stable pumping at full flow rate, once again demonstrates its superiority. III. Different vacuum systems require different levels of vacuum. Therefore, it often has to be accomplished by a set of vacuum units. That is, vacuum pumps operating in different pressure ranges are connected in series; the vacuum pump on the high-vacuum side is capable of achieving the vacuum level required by the system, while the vacuum pump on the low-vacuum side discharges directly into the atmosphere. Obviously, the simplest vacuum unit is a vacuum pump that discharges directly into the atmosphere. However, high-vacuum systems generally require three-stage units, while medium-vacuum systems usually require two-stage units. It is difficult to form an effective high-vacuum unit using one high-vacuum pump and one low-vacuum pump. 　　There are several reasons for this. The continuity of flow is one of them. High-vacuum pumps all have a limit on the pressure that can be applied to the preceding stage; that is, if the pressure in that preceding stage exceeds a certain value, the pump cannot operate properly. When the foreline pump reaches this critical pressure, its pumping speed often decreases. As a result, the exhaust flow rate of the foreline pump may become lower than that of the main pump. This discrepancy in flow rates violates the requirement for flow continuity and inevitably prevents the vacuum system from functioning properly. However, by connecting an intermediate vacuum pump between the high-vacuum pump and the low-vacuum pump, it can serve as a link between them, ensuring a continuous flow of gas while allowing each pump to operate in its optimal condition. Roots pumps can operate in the medium vacuum range and are the most suitable for this purpose; hence they are also known as Roots booster pumps. Given their relatively low compression ratio, they are ideally suited for applications involving pressures ranging from a few Pa to several hundred Pa. When a triple-stage high-vacuum system reaches a high level of vacuum, the exhaust flow rate of the main pump decreases significantly; in such cases, a smaller pre-pump is sufficient to maintain continuous pumping. This is a method commonly used in practice, as it helps to reduce the energy consumption of the system. Another reason why high-vacuum systems often require a three-stage unit is due to the limitations on the suction pressure of high-vacuum pumps. Pumps all have a starting operating pressure, with conventional high-vacuum pumps having values in the range of a few Pa. Therefore, the primary pump must be pre-evacuated to this pressure before the main pump can start operating. However, it often takes a relatively long time for the foreline pump vented directly to the atmosphere to reach this pressure, as the pumping speed of the pump decreases as the pressure drops. This is especially true for vacuum systems with periodic pumping cycles, where there are requirements regarding the time needed to reach the operating vacuum level. The longer the pre-pumping time, the longer it takes to attain the desired vacuum level. Therefore, adding a medium-vacuum pump in conjunction with the low-vacuum foreline pump enables the system to reach a pressure at which the main pump can operate in a shorter period of time. This allows the system to quickly reach its operating vacuum level, thereby ensuring the efficient utilization of the equipment. 　　Both Roots pumps and oil booster pumps can be used as medium vacuum pumps. Molecular booster pumps have an extremely high compression ratio, which not only enables them to achieve a clean vacuum but also provides excellent high vacuum performance; at the same time, they possess strong pumping capacity in the medium vacuum range as well. This makes the molecular booster pump the only vacuum pump available today that possesses medium to high vacuum performance; therefore, by combining it with a low vacuum pump, it is possible to create a high vacuum system whose performance is comparable to that of a three-stage system. Specifically, because molecular booster pumps have a high pressure tolerance, they enable the preceding pump to operate easily at high flow rates ； The molecular booster pump has a high intake pressure, which alleviates the pre-pumping burden on the foreline pump. Molecular booster pumps can operate at pressures ranging from 100 to 50 Pa. The pre-pump reduces the pressure from atmospheric levels to these values, following roughly a pattern in which the pressure drops by one order of magnitude per unit of time; as a result, the system can achieve very high pumping efficiency. Another advantage of the molecular booster pump is the simplification of high-vacuum systems, as it eliminates the need for rotary vane pumps. For larger high-vacuum application equipment, it is also possible to enhance the pre-pumping capacity of the forepump appropriately to further reduce the pumping time. Since the pre-pumping time is very short compared to the entire evacuation process, the forepump operates for a short duration as well; therefore, it can be used for pre-pumping multiple sets of equipment, which is often very practical. This greatly simplifies the vacuum units used in large-scale applications. In certain medium-vacuum applications, a vacuum level of 10-1 Pa is required, which is often difficult to achieve with two-stage Roots pumps. However, a three-stage setup using two Roots pumps in series can increase the vacuum level by an order of magnitude, allowing it to reach 10-1 Pa; therefore, three-stage pumps are also commonly used in medium-vacuum applications. Since the molecular booster pump can achieve full pumping speed at 10-1 Pa, it can also replace the two-stage Roots pump in three-stage vacuum systems. Generally speaking, rotary vane pumps and molecular booster pumps can completely replace pumps that operate for long periods at low pressure levels in the medium vacuum range. Rotary vane pumps that operate for extended periods in the high-end pressure range of medium vacuum are relatively rare, as pump stages used in this pressure range typically possess strong pumping speeds. This macroscopically predicts the prospect of molecular booster pumps replacing Roots pumps.

I also want to ask, does the ratio you mentioned refer to the pumping speed ratio?

It makes no sense to discuss combinations without considering the evacuation medium – you’re not evacuating air, after all It’s like an ultimate vacuum – a theoretical value that has no meaning for selection purposes ；

The pumping speed should be set such that the mass flow rate remains constant; however, it is necessary to take into account changes in volumetric efficiency caused by factors such as compression ratio and pressure difference, as these can in turn affect the mass flow rate； For first- and second-stage pumps, the compression ratio is dynamic; it depends on the actual required operating pressure, the suction medium, etc

What’s the compression ratio—what are the numbers? Could you be more specific? Thank you

The compression ratio is not fixed; it is related to the rotational speed, pressure difference, and the gas itself, thereby affecting the volumetric efficiency! It involves many factors; it’s by no means the case that a unit can be designed simply by determining a few ratios as is commonly thought……