Diffusion pump vacuum pumping system
It should be noted in the operation (or control system design) of the diffusion pump pumping system that if the pumping system is in operation and the vacuum chamber is under atmospheric pressure (i.e. the vacuum chamber is undergoing process preparation, such as loading and unloading materials, handling workpieces, etc.), the high vacuum valve and the coarse pump valve of the pumping system should be closed. Before starting the rough pumping procedure, close the front pipe valve and open the crude drain valve. In a general vacuum system, only a simple coarse valve can be used for rough extraction.
In large vacuum chambers with special requirements, multistep procedures can be used for rough pumping: first, the vacuum chamber can be roughly pumped through a small diameter hole or bypass pipe. The small orifice or bypass branch pipe can limit the velocity of the airflow and prevent turbulence from stirring up the particles on the vacuum chamber substrate. These particles will adhere to the insulating substrate on static electricity. Secondly, when the pressure in the vacuum chamber drops to a lower value, then open the main rough suction valve, you can also set throttle valve in the rough suction pipeline, valve plate gradually open from small to large. Thirdly, if a Roots pump is used in the vacuum system, the pump can be bypassed or idled until the pressure difference between the inlet and outlet is reduced to the safe value of the Roots pump, such as 1000Pa Pa, before the Roots pump and the front mechanical pump are in series pumped. These functions can be performed by simple pressure sensors. The pressure of the vacuum chamber should not be less than 15Pa. After this pressure, the vacuum valve should be closed and the high vacuum valve should be opened after a period of delay. Generally, no coarse pump is used to pump large-scale systems with pressure below 15 Pa. Otherwise, serious mechanical pump oil reflux will occur.
If the diffusion pump system is started from the static state, the diffusion pump must first pass into the cooling water, and then start the mechanical pump. After pumping the gas from the rough suction pipe to a certain pressure, such as 100Pa, the front pipe valve can be opened and the diffusion pump can be electrically heated. After the diffusion pump is pre-pumped to a certain pressure, it closes the front valve and opens the coarse-pumping valve to roughly pump the vacuum chamber. These actions can be automatically controlled by a pre-set vacuum gauge. The heating time of 100 mm or 150 mm diameter diffusion pump is about 15 minutes, and that of 600-800 mm diameter diffusion pump is 30-45 minutes. For a diffusion pump vacuum system with a high oil return rate, when starting a diffusion pump, attention should be paid to avoiding the backflow contamination above the cold trap before cooling the cold trap. When the diffusion pump starts to work, it is better to use local cooling (precooling) or gas flushing to solve this problem. Generally, this step requires operator to manually control.
When the vacuum system shuts down, the high vacuum valve is first closed and the cold trap is heated, so that the diffusion pump pumps out the gas and vapor from the cold trap. If the cold trap is naturally balanced with its surroundings, it will take 4 to 20 hours depending on the structure of the cold trap. This time is greatly shortened by using dry nitrogen to cool the trap of liquid nitrogen storage tank. When the cold trap temperature reaches 0 degrees, the power of the diffusion pump can be cut off. When the diffusion pump is cooled to about 50 C, close the pre-pipeline valve, then the pre-mechanical pump can be stopped and deflated. Finally, the cooling water of the diffusion pump should be switched off. If the diffuser pump needs to be deflated under special circumstances, it should be carried out on the valve above the cold trap, but the pump must be cooled to less than 50 degrees Celsius. Vacuum chamber deflation can not be carried out when the valve of the front pipeline of the diffusion pump is opened. This will cause air to flow back into the diffusion pump through the rough suction pipe and the rough suction pipe, thus bringing a large number of mechanical pump oil vapor into the diffusion pump.
If the pump in the vacuum system is started and shut down every day, gas flushing technology can be used to prevent backflow contamination during the transition process. Instantaneous contamination caused by the occasional start-up or shut-down of a continuous system is minimal, but daily shutdowns greatly increase the contamination effect. Even if the diffusion pump is still in operation, the pump distillate from the well will return to the part between the well and the high vacuum valve.
The typical operation steps of the typical diffusion pump system are as follows:
1. When the system starts up, the cooling water of the diffusion pump is connected first, the temperature setting value of the water temperature relay is adjusted, and then the mechanical pump is turned on for rough pumping.
2. Before starting the mechanical pump, check whether the oil level of the pump meets the requirements and whether the high vacuum valve and the front valve are closed. Open the coarse suction valve. In the medium and small pumping system, only a simple rough pipe is needed. In some large vacuum chambers with special requirements, multistep procedures may be used for rough pumping: first, the vacuum chamber can be roughly pumped through a small diameter hole or bypass pipe, and then the main rough pumping valve can be opened when the pressure drops to no turbulence.
3. When the pressure in the vacuum chamber drops to 100 Pa, the front pipe valve can be opened and the diffuser pump heater can be electrified. If the diffusion pump is already in vacuum, then the diffusion pump can be heated at the same time in step 1. When the diffusion pump is heated by electricity, if the power supply has a voltage regulator, the starting point voltage can be increased appropriately, but when the pump oil is close to the evaporation temperature, the voltage should be adjusted back to the prescribed voltage, so as to speed up the start-up. But it should be noted that too long time will cause cracking of pump oil.
4. Generally, when the system is roughly pumped to about 15-20Pa, close the roughly pumping valve, and open the high vacuum valve after the diffusion pump works normally (that is, the normal injection of oil vapor jet).
5. If the vacuum of the pumped vessel does not increase after a period of normal operation of the diffusion pump, the leakage rate of the system is too high.