1, Turbocharged turbopump has the ability to provide ultra-clean, hydrocarbon-free vacuum environment, so it is often used by users. However, users occasionally find turbomolecular pumps can not provide a hydrocarbon-free vacuum environment. After the investigation found that the source of hydrocarbons, 50% is caused by operational errors, such as the turbo molecular pump front-stage pump for the oil seal rotary vane pump, there is no control of backflow of the safety valve, the unreasonable deflation process Will cause reflux of oil vapor, undesirable safety valve can also cause oil pollution. In order to realize the requirement of no hydrocarbon in the system, it is necessary to know some basic knowledge of the turbomolecular pump's compression ratio and how to pump the pump if the fore pump is not a dry pump. 2, the compression ratio Turbomolecular pump compression ratio is the ratio of the pressure at the preceding stage pipe (exhaust port) and the pressure at the intake port. Due to the different molecular weight of the gas to be pumped, the compression ratio of the pump to the various gases is also different. The relationship between the square root of the gas molecular weight M and the compression ratio K is shown in Fig. 6. The compression ratio of hydrogen to the pump is small, typically about 1000, so that if the hydrogen pressure in the foreline is 1 x 10-7 Torr (13.33 μPa), then the hydrogen pressure at the inlet is 1000 times smaller, which is 1 × 10-10 Torr (13.33nPa). Since hydrogen is the major residual gas in ultra-high vacuum systems, the hydrogen compression ratio is a key factor in determining the ultimate pressure of a turbomolecular pump. Turbomolecular pumps are relatively large for large molecular weight gases, such as those for hydrocarbon molecules, typically above 1012. The ratio varies depending on the pump and the molecular weight. Due to the difference in the foreline pump and other factors, the partial pressure of the hydrocarbons in the foreline of the turbo-molecular pump ranges from 10-4 Torr (13.33 mPa) to 10-6 Torr ( 133.3 [mu] Pa), under which pressure the partial pressure of hydrocarbons at the pump inlet will be 1012 times lower, ie 10-16 Torr (13.33 fPa) or lower. This pressure is almost infinitely small, beyond the measurable range, even the most sensitive mass spectrometer is difficult to measure. 3. Inflation Measures (1) Why Inflation When the turbomolecular pump is shut down or running extremely slow, the pump will no longer have a large enough compression ratio (pressure gradient within the pump) to prevent hydrocarbons present on the foreside from passing through the turbine blades Return to the vacuum chamber. This phenomenon is called anti-proliferation or molecular regurgitation. Under static conditions, when the pressure in the entire system is equalized, the partial pressure of the oil is usually 10-4 ~ 10-6 Torr (or 13.33mPa ~ 133.3μPa) on the foreline side and finally reaches the pump inlet. When the pump is turned off, properly aerating the pump is an effective measure to control the flow of oil molecules and to keep the vacuum chamber free of hydrocarbons. When the pump is stopped, the hydrocarbon backflow quickly passes through the pump into the vacuum chamber, and if the system remains under vacuum, the hydrocarbons will stick to the surface of the clean blades and vacuum chamber. When the system is subsequently operated, it will be extremely difficult to remove the adhering hydrocarbons. On the other hand, if the pump is filled with dry nitrogen or dry air while the turbomolecular pump is out of operation, the dry gas will provide a protective layer of gas to the exposed surface, and after the system is inflated, the refluxed hydrocarbon The compound, due to its ability to mix with the charged gas, has a very low adhesion capacity and has a very small percentage of hydrocarbons in the gas mixture and can be quickly evacuated on the next evacuation. (2) Delayed Inflation Although it is common practice for the pump to be inflated after the pump is disconnected from the power supply, it is not uncommon for turbopumps to decelerate gradually after the power is turned off. If the pump is decelerating for a few seconds or minutes Inflation will be better. During pump deceleration to 30% ~ 50% of its normal speed, the pump can still play the role of suction and compression. The vacuum chamber can effectively be placed in a vacuum state and hydrocarbon reflux can be prevented. Delayed inflation can also allow enough time to close the valve, in the case of frequent power outages, delayed inflation is useful. To delay inflation, the foreline vacuum must be maintained within the range of 1 to 1000 μmHg (1 × 10 -3 mmHg to 1 mmHg or 133.3 to 133.3 Pa) so there must be a vacuum valve between the turbo-molecular pump and foreline pump or Inside the foreline pump is a control valve that isolates the turbo molecular pump from the foreline pump when the power supply is interrupted. Otherwise, the foreline is inflated by the forepump and results in oil contamination. (3) Where to inflate The inflator on the anterior stage of the turbo-molecular pump forces the hydrocarbons to flow immediately through the turbo-molecular pump into the vacuum chamber. On the other hand, if the turbomolecular pump is inflated on the suction side, the purpose of covering the surface with the cleaning gas can be achieved. And the gas flow to the turbomolecular pump (from top to bottom), can temporarily stop and delay the hydrocarbon reflux, and some turbo-molecular pump in the middle of the compression stage between the inflatable, it and inflatable control of hydrocarbons in the suction side Backflow is almost equally effective. Inflatable at compression stage is especially superior in ultra-high vacuum systems. Because intermediate inflation does not require a costly metal-sealed, baked inflation valve. (4) How to inflate If the turbopump is placed in a clean, dry environment, the air inside the chamber can be filled. However, the location of the inflatable entrance must be carefully chosen. If the inflation inlet is located close to the outlet of the oil-sealed rotary vane pump, the vapor contained in the gas will inevitably contaminate the vacuum system, and if the air is wet, Time, so to be filled with dry nitrogen or air filtered through the dryer. In addition, it is not always necessary to fill the gas under atmospheric pressure. If the pressure of the gas is several Torr (several hpa), the dry gas is enough to control the hydrocarbon reflux. 4, the pre-pipeline safety valve installed If you do not use a reasonable pre-pipeline safety valve, it is possible to contaminate the turbopump and contaminate the vacuum chamber. When the current stage pump is off, the foreline pump will inflate itself back to the exhaust port of the turbo molecular pump. This type of foreline pump will pump the oil from the foreline backflow and then into the vacuum chamber through the turbomolecular pump. This phenomenon is called oil pollution. After being contaminated, the blades of the turbo molecular pump must be under the guidance of the manufacturer Freon to clean. A vacuum valve is installed between the turbo-molecular pump and the foreline pump to prevent backfilling and to close the valve immediately in the event of a power outage. Ideally, it would inflate the inlet of the foreline pump without letting the pump oil back flow into the safety valve. In addition, the safety valve can only be opened if its pressure is substantially equalized or it may cause pressure shocks on the foreline. For example, the foreline pump pressure will be atmospheric at the moment of the power cut, but the foreline may still be at Under vacuum. If the pressure difference between the two sides of the valve is very large, once the power valve will open immediately, the atmospheric pressure of contaminated oil will be washed by the foreline pump to the foreline, which will pollute the system. Therefore, the safety valve requires a certain delay to open the time in order to allow the foreline pump after the valve into the pipeline vacuum, the pressure difference before and after the valve can be opened before the safety valve, in short, to take measures to control the pressure on both sides of the valve Poor, do not make the air back to red. Now many direct-coupled rotary vane pumps have a safety valve in the pump, but the sealing performance of the valve must be guaranteed. Once the valve is shut down, the consequences will be very serious. The turbopump system will be contaminated with oil. This problem may be detected Found beforehand Install a foreline vacuum gauge on the working foreline pump to shut off the pump. If the foreline pressure increase does not increase between 10 and 1000 μmHg (1 × 10-2 and 1torr or 1.333 and 133.3Pa) Validity of the valve If the pressure continues to rise rapidly, reaching the atmospheric pressure directly indicates that the valve has failed. If the valve is well sealed, check if the valve is open immediately or equalize pressure before switching on. Watch for regulation and start the foreline pump. If the regulation hurries to almost atmospheric pressure, it indicates the impact of the reflux steam. 5, operating procedures Turbomolecular pump type and type is varied, each pump mode of operation provided by the manufacturer, the turbocharger pump operation is the simplest, cheapest way is to start the turbopump and the front stage When the turbo-molecular pump accelerates to normal speed, the system is pre-pumped at the same time. In this initial stage of high-pressure pre-pumping, it is impossible to return the oil vapor. Because at this time the system gas in a viscous flow or laminar flow state, the density of the exhaust gas can block any hydrocarbon molecules back to the turbomolecular pump direction. When the molecular flow state is reached, the turbomolecular pump has entered normal speed operation, preventing reflux of the oil vapor when the pump is operating at high compression ratio. Turbomolecular pump front stage pump by a Newton switch control, two pumps can start and stop at the same time. In some fast-cycling systems, there is not enough time for the turbomolecular pump to reach normal operating speed in each cycle because of the short time. In this case, the turbo-molecular pump can not be turned on periodically with the duty cycle. At this point the turbomolecular pump had to continuous operation, the former pump crude pumping vacuum chamber, turbopump can quickly reach the start pressure, you can quickly open the main valve, turbopumps can be relatively high pressure Work (saving time) can also minimize the backwash of the roughing pipe. If the system requires a rough pipe, improper operation may also cause backflow problems, if the system is coarse pumping to too low pressure, crude pipe will appear within the molecular flow pattern. The roughing pump oil vapor may flow back to the vacuum chamber. This regurgitation occurs when the pressure is below 100-200 μmHg (13.33-26.66 Pa). When the rough vacuum pumping chamber rough shut down the valve, and roughing pump is still running, the roughing pipe pressure drop will appear molecular flow state. Hydrocarbon reflux may occur and condense on the roughing valve cap or valve seal. When the roughing valve is opened again, it is possible for the condensate to leave the bonnet or seal and back toward the vacuum chamber. During the next high vacuum cycle these oil vapors will migrate into the vacuum chamber and become contaminated. In order to solve the problem of backflow caused by rough pumping, it is mainly to avoid the occurrence of molecular flow in the upstream pipeline to prevent oil vapor backflow. When the foreline pressure reaches the previous level, close the roughing valve to inflate the roughing pipe (with an inflation valve and a controllable leak), and then close the roughing pump. because