Evacuation of Vacuum Chambers
The evacuation of vacuum chambers, i.e. removing gas out of closed tanks in a given time, is one of the most common applications of vacuum pumps. In general, evacuation is the process of lowering the pressure in a chamber from a starting pressure to a target pressure. In most industrial applications, there are two different types of applications: The one-time pumping for a subsequent process and the cyclic pumping of vacuum chamber.
The one-time pumping of a chamber before running a vacuum process usually starts at atmosphere. Additional gas loads due to desorption and the acceleration of pumps play a special role. In contrast to cyclic operations, the requirements on the pumping time are usually of secondary importance, as they have no direct influence on the cycle times. For example, the pumping to high vacuum conditions is often performed only once, because the process takes a very long time. The use of load locks prevents repeated pumping down to low pressures.
The pumping and venting of chambers in cyclic operations concerns load lock and transfer chambers of process systems, but also the pumping of volumes from packaging machines. In these processes it's crucial to meet the given pumping times. This has to be shorter than the cycle time of the process. Due to the very fast cycle times of many processes, pumping times of a few seconds are not uncommon. Depending on the ratio of the pumping time to the cycle time, some pumps can be loaded higher and thus contribute to a faster pumping.
Physical Effects During Evacuation
The time required to pump out a vacuum vessel is determined by various influences. First and foremost, of course, is the size of the volume to be pumped, the pumping speed of the vacuum pump used and the start and target pressure of the pumping process. Depending on the target pressure or the time available, there are a number of other important factors.
Desorption is the release of molecules and atoms bound to surfaces. In vacuum applications, this is mostly water from the ambient air, but hydrocarbons (e.g. grease or oil) also play a role. The desorption rate varies over time and depends largely on the temperature of the respective surface, the material and its texture. Since only molecules on the surface can change to the gaseous state, the internal structure of the releasing material (e.g. porosity) determines the decay behaviour of the desorption. For example, the decrease in desorption of plastics over time is smaller than that of metal surfaces, since a larger proportion of gas particles must first diffuse from the interior to the surface. While the additional gas loads can often be neglected due to desorption effects in the rough vacuum range, they have a decisive influence on the pumping times into the fine vacuum or below. They also influence the achievable pressure level. In order to accelerate desorption and thus achieve lower pressures, chambers and pumps for target pressures below 1 × 10-6 mbar are often subjected to a heating process. This baking at temperatures between 130°C and 200°C usually takes 24 hours, but can also take several days, depending on the final pressure to be achieved. For load lock applications with short cycle times, heating out is not a solution. As the humidity of the ambient air accounts for a large part of the desorption load, venting of the chambers with pure nitrogen or dry compressed air can significantly reduce the desorption load and thus accelerate pumping. If applicable, prior drying and/or heating of the substrates is another helpful option. For transfer chambers that are pumped out cyclically but do not come into contact with the ambient air and start from a lower pressure level, desorption plays only a minor role.
Leaks are another source of additional and undesirable gas loads. These often occur at joints, but can also be caused by housing or pipeline defects. As with desorption, leaks prolong pump-down times and degrade the minimum achievable pressure. In contrast to desorption loads, however, the leakage rate remains unchanged over time. This can be remedied by leak detection on the system. In addition, the leak rate of a system can be significantly reduced if flange connections are fitted with metal gaskets (CF flanges) instead of O-rings (ISO flanges). This is generally the case for all types of ultra-high vacuum applications.
An essential property to describe the performance of a vacuum system is the pumping speed. This term is synonymous with volume flow. At the same pressure, two equal volumes with different temperatures contain a different mass of gas. Thus, changes in the gas temperature also lead to changes in the mass of gas pumped per unit of time, even if the pumped volume remains the same. Accordingly, the gas temperature in pumping processes must also be taken into account. This is done by balancing the heat flows from and to the gas. Within a vacuum chamber, various incoming and outgoing heat flows lead to temperature changes of the gas. Especially in fast pumping processes, a relatively slow heat conduction via the vessel wall into the gas is opposed to rapid cooling by removing the gas mass. As a result, a strong cooling of the gas can be observed in such applications. Subsequent heating of the flowing gas within pipelines and pump inlets slows down the pumping process. In applications where pumping takes several minutes or more, this effect plays a less important role, as there is sufficient time to reheat the gas by heat conduction through tank walls.
Pipelines connect the chamber with the vacuum pumps. In general, one tries to keep the losses due to pipes as small as possible. This can be achieved on the one hand by shortening the pipe length and on the other by increasing the pipe diameter. In vacuum technology the reciprocal value of loss is conductance. The greater the conductivity of the pipelines, the greater the effective suction capacity available at the chamber. In pumping processes, not only the conductance but also the volume of the pipeline plays an important role. When the pipe diameter is increased, the volume of the pipe increases and so does the total quantity of gas to be pumped. This leads to an optimization problem, especially with time-critical cyclical pumping processes. For load lock applications, it is common practice to separate the pump system and the pipelines using valves that are located close to the chamber and to use the line volume already under vacuum as a buffer. In this case, pipelines with a larger diameter will initially experience a faster pressure drop as a result of the pressure equalization between the vessel volume and the pipeline volume. Depending on the target pressure to be achieved, this advantage can be lost again due to the higher total volume to be pumped as described above.
Especially for pumps for the rough and fine vacuum, machines with variable speed drives are increasingly gaining acceptance. While in particular roots pumps with a fixed speed are only allowed to switch on at a certain pressure, pumps with frequency converters offer the possibility to work from the beginning and to increase their speed dependent on the current load. Frequency converters allow pump systems to be optimally adjusted in terms of pumping time, energy consumption and noise emission. In cyclic operations, pumps with variable speed drives can be overloaded, thus shortening the pumping time, assumed that the actual cycle time is long enough for the pumps to reach their permissible thermal state again during stand-by operation. High-vacuum pumps can generally only be added to the pumping process at a certain low pressure. In any case, they require a certain amount of time after start-up to reach their full performance capacity: with turbomolecular pumps it is the start-up to a certain speed, diffusion pumps must be heated up and cryopumps must be cooled down. In order to save time, it is recommended that the pumps keep on running against a closed valve, even if they are not used at that time. This procedure also has the advantage that the pumps consume relatively little energy when idling compared to starting up.
- Size of the chamber
- Material and quality of chamber surfaces
- Starting pressure: Desorption plays an important role in lock chambers pumped from the atmosphere. For transfer chambers that do not come into direct contact with atmospheric air, the effect is of minor importance.
- Additional gas loads due to leakages
- Change of gas temperature due to expansion and heat transfer between gas and chamber walls
- Conductivity losses due to piping, valves, filters, etc.
- Buffer volume of pre-evacuated pipelines
- Acceleration and deceleration of pumps
- Switching behaviour of pumps for fine and high vacuum