Several Problems Worth Paying Attention to When the Edward Turbo Pump Runs
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In order to achieve the requirement of hydrocarbon-free in the system, in the case that the foreline pump is not a dry pump, it is necessary to understand the compression ratio of the turbo-molecular pump and some basic knowledge of how to inflate the pump.
2. Compression Ratio The compression ratio of a turbomolecular pump refers to the ratio of the pressure in the foreline (at the exhaust port) to the pressure at the intake port. Due to the different molecular weight of the pumped gas, the pump has different compression ratios for various gases. The relationship between the square root of the molecular weight of gas M and the compression ratio K is shown in Fig. 6. The compression ratio of the pump to hydrogen is very small, typically about 1000, so if the pressure of hydrogen in the foreline is 1×10-7 Torr (13.33 μPa), then the hydrogen pressure at the inlet is 1000 times smaller, ie, 1×10 −10 Torr (13.33 nPa). Since hydrogen is the main residual gas in ultra-high vacuum systems, the hydrogen compression ratio is a key factor in determining the ultimate pressure of the turbomolecular pump.
The turbomolecular pump is quite large for large molecular weight gases, such as those for hydrocarbon molecules, and is generally higher than 1012. This ratio varies according to different pumps and different molecular weights. Because of the difference in the fore pump and other factors, the partial pressure of the hydrocarbon in the foreline of the turbomolecular pump is 10-4 Torr (13.33 mPa) to 10-6 Torr ( Between 133.3 μPa), under this condition, the partial pressure of hydrocarbon at the inlet of the pump will be reduced by 1012 times, ie, 10-16 Torr (13.33 fPa) or less. This is almost infinitely small pressure, beyond the measurable range, even the most sensitive mass spectrometer is difficult to measure.
3. Inflation measures (1) Why inflate When the turbomolecular pump is turned off or running extremely slowly, the pump will no longer have a sufficiently large compression ratio (pressure gradient in the pump) to prevent hydrocarbons on the fore stage side from passing through the turbine blades. Recirculate the vacuum chamber. This phenomenon is called anti-diffusion or molecular reflux. Under static conditions, when the pressure of the entire system is balanced, the partial pressure of the oil on the foreline side is usually 10-4 to 10-6 Torr (or 13.33 to 133.3 μPa) and will eventually reach the inlet of the pump. When the pump is turned off, properly inflating the pump is an effective measure of controlling the backflow of oil molecules and keeping the vacuum chamber free of hydrocarbons.
When the pump is turned off, hydrocarbon backflow quickly passes through the pump into the vacuum chamber. If the system is still under vacuum, hydrocarbons will stick to the surfaces of the clean blades and vacuum chamber. When the system is subsequently operated, it will be extremely difficult to remove the sticking hydrocarbons. On the other hand, when the turbomolecular pump is stopped, if the pump is filled with dry nitrogen or dry air, the dry gas will provide a protective layer of gas to the exposed surface, and after the system is inflated, the refluxed hydrocarbon Since the compound is mixed with the charged gas, its adhesive ability is very weak, and the proportion of hydrocarbons in the mixed gas is extremely small, and it can be pumped away very quickly on the next evacuation.
(2) Delayed charging Although it is common for the pump to inflate the pump after the power is turned off, it is common for the pump to slow down gradually if the turbomolecular pump is turned off, if the delay is several seconds or several minutes. Refilling will be better. When the pump decelerates to 30% to 50% of its usual speed, the pump still functions as a suction and compression. It can effectively keep the vacuum chamber under vacuum and prevent the backflow of hydrocarbons. Delayed inflation can also allow sufficient time for the valve to close. In the event of frequent power outages, delayed inflation is useful. In order to delay inflation, the fore vacuum must be maintained within the range of 1 to 1000 μm Hg (1×10-3 mmHg to 1 mmHg or 133.3 to 133.3 Pa), so there must be a vacuum valve between the turbomolecular pump and the foreline pump, or A control valve is installed inside the foreline pump. This valve should isolate the turbomolecular pump from the foreline pump when the power supply is interrupted. Otherwise, the foreline pump and the foreline are inflated and cause oil contamination.
(3) Where to inflate The inflator on the front side of the turbomolecular pump can force the hydrocarbon immediately into the vacuum chamber through the turbomolecular pump. On the other hand, if it is inflated on the suction side of the turbomolecular pump, the purpose of covering the surface with clean gas can be achieved. The flow of gas to the turbomolecular pump (from top to bottom) temporarily prevents or retards the backflow of hydrocarbons, and some turbomolecular pumps inflate between the compression stages, and it controls the hydrocarbons on the suction side. Reflow is almost as effective. In ultra-high vacuum systems, it is particularly advantageous to inflate at the compression stage. Because the intermediate inflation does not require an expensive metal-sealed bakeable inflation valve.
(4) How to Inflate If the turbomolecular pump is placed in a clean, dry environment, it can be filled with air. However, the location of the inflatable inlet must be carefully selected. If the aeration inlet is located close to the outlet of the oil-sealed rotary vane pump, the injected gas will contain oil vapors that will inevitably contaminate the vacuum system, and if the air is wet, in order to reduce the need to pump gas later. The time is then to be filled with dry nitrogen or air filtered by a dryer. In addition, it is not always necessary to charge the gas under atmospheric pressure. If the pressure of the gas is a dry gas of a few Torr (several hPa), it is sufficient to control the reflux of the hydrocarbon.
4. If the pre-pipeline is equipped with a safety valve, if a rationally operated pre-pipeline safety valve is not used, it may pollute the turbomolecular pump and contaminate the vacuum chamber. When the current stage pump is de-energized and stopped, the foreline pump will inflate itself to the exhaust port of the turbomolecular pump. This kind of foreline pump inflates the oil in the foreline and then passes through the turbomolecular pump into the vacuum chamber. This phenomenon is called oil pollution. After being contaminated, the turbomolecular pump blades must be used under the guidance of the manufacturer. Freon to clean.
A vacuum valve is installed between the turbomolecular pump and the foreline pump to prevent reverse inflation and close the valve immediately when the power is cut off. Ideally, it will be able to inflate the inlet of the foreline pump without dipping the pump back into the safety valve. In addition, the safety valve can only be opened when its pressure is substantially equal, otherwise there may be problems of pressure impact on the foreline, for example, the pressure of the foreline pump will be atmospheric pressure at the moment of power off, but the foreline can still be in the pipeline. Under vacuum conditions. If there is a large pressure difference between the two sides of the valve, once the current-carrying valve opens immediately, the gas under atmospheric pressure containing contaminated oil will be pumped by the foreline pump to the preceding pipeline, which may contaminate the system. Therefore, the safety valve needs a certain delay in opening time, so that the foreline pump evacuates the pipe behind the valve into a vacuum, and the pressure difference before and after the valve equalizes before the safety valve can be opened. In short, a method is adopted to control the pressure on both sides of the valve. Bad, do not make the air flow back. Many direct-connected rotary vane pumps are equipped with safety valves in the pump, but the valve sealing performance must be ensured. Once the valve is not closed, the consequences are quite serious. The turbomolecular pump system will be contaminated with oil. This problem may be detected. Discovered in advance. Install a foreline vacuum regulator on the running foreline pump and turn off the pump if the fore-stage pressure increase value is between 10 and 1000 μm Hg (1×10-2 to 1 torr or 1.333 to 133.3 Pa) and no longer increases. If the valve is effective, if the pressure continues to rise quickly, direct atmospheric pressure will indicate the failure of the valve. If the seal of the valve is good, check whether the valve opens immediately or waits for the pressure to equalize before opening. Observe the regulation and open the foreline pump. If the regulation immediately jumps to almost atmospheric pressure, it indicates the impact of refluxing steam.
5. Operating Procedures Since the types and models of turbomolecular pumps are various, the operating mode of each type of pump is provided by the manufacturer. The simplest and cheapest way to operate the turbomolecular pump is to start the turbomolecular pump and the front stage at the same time. The pump, when the turbomolecular pump accelerates to normal speed, also pre-exhausts the system. In this initial stage of high-pressure pre-extraction, the reflux of oil vapor is not possible. Because the gas in the system is in a viscous or laminar state at this time, the density of the exhaust gas is large, and any hydrocarbon molecule can be blocked from flowing back toward the turbomolecular pump. When the molecular flow state is reached, the turbomolecular pump has been operated at a normal speed. When the pump operates at a high compression ratio, the backflow of oil vapor is prevented. <
The turbopump’s foreline pump is controlled by a pushbutton switch that can be started and stopped simultaneously.
In some fast-cycling systems, there is not enough time for the turbo-molecular pump to reach normal operating speed within a short cycle of each cycle. In this case, the turbomolecular pump cannot be turned on periodically with the work cycle. At this point the turbomolecular pump has to operate continuously. The foreline pump roughs out the vacuum chamber and the turbopump's starting pressure can be reached very quickly. The main valve can be opened quickly. The turbomolecular pump can be under relatively high pressure. Work (saving time) also minimizes backflow in rough piping.
If the system needs rough piping, improper operation may also cause reflux problems. If the system is rough pumped to too low pressure, the molecular flow state will appear in rough piping. Then, the oil vapor of the rough pump may flow back to the vacuum chamber. This reflux occurs when the pressure is less than 100-200 μmHg (13.33-26.66 Pa).
When the vacuum chamber is rough pumped moderately, the crude pumping valve is closed, and when the rough pump continues to operate, the pressure in the rough pumping pipeline decreases and the molecular flow state occurs. Hydrocarbons may flow back and condense on the rough valve cover or the valve seal. When the rough valve reopens, the condensate may leave the bonnet or seal and return to the side of the vacuum chamber. In 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 foreline pipeline so as to prevent the backflow of oil vapor. When the pre-stage pressure is reached in the current pipeline, close the rough valve to inflate the rough pipeline (with an inflation valve and a controllable leak) and close the rough pump. As a result, no molecular flow can occur in rough piping, preventing backflow.
When turbomolecular pumps are used, it is usually not necessary to use rough piping. For example, in a rapid-circulation system, rough pumping pipelines are not available, and those turbomolecular pumps that are resistant to atmospheric shock can be used. The vacuum chamber can be evacuated directly from atmospheric pressure.
The above discussion is a matter to be noted when turbo-molecular pumps are operated with oil-based fore pumps. In the current development of different types of dry fore-stage pump can be used as the turbo-molecular pump's foreline pump to solve the problem of reflux of the oil system, to achieve a clean vacuum, if you can use the current kind of atmospheric pressure The new molecular pump that has been pumped to high vacuum works better
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