Due to the long start-up time of the diffusion pump and the oil booster pump, in the periodically operated equipment, when loading and unloading, in order to shorten the working period without cutting off the power of the diffusion pump and the oil booster pump, the high vacuum valve is the pre-pipeline valve is closed to make the main pump in normal operation. Since the valve always have a very small amount of air leakage and surface deflation, the pressure at the outlet of the main pump will increase over a period of time. If it exceeds the maximum discharge pressure of the main pump and returns to the pump, the oil vapor will oxidize.
In order to solve this problem, one approach is to use the backing pump to continue to pump out the main pump discharge gas, but at this time the amount of gas discharged from the main pump is very small, and there is a phenomenon that the backing pump is overqualified and wastes a lot of energy. So another method may be used, stop the work of the backing pump, close the valve of the foreline, and set a maintenance pump or gas tank at the outlet of the main pump, which can ensure that the gas in the main pump can be discharged and saved energy. Gas tanks cannot be too large, it can only be used on small systems that use diffusion pump as the main pump, while the holding pump can be used on a large main pump system. Another function of the gas storage tank is that some smaller application equipment does not allow vibration in its process, that is, the backing pump must be stopped during the process, at this time the gas tank is used to store gas discharged from the main pump ensure the quality of the processed workpiece during the process.
The calculation of the volume of the gas storage tank for anti-vibration purposes is based on the fact that the gas discharged from the vacuum chamber by the diffusion pump is completely discharged into the gas tank, causing the pressure in the tank to increase by no more than The maximum exhaust pressure at the outlet of the diffusion pump.
The volume of the gas storage tank for the purpose of energy saving and shortening the working period is calculated based on the following considerations: When the backing pump stops working and the diffusion pump is still in normal operation, the diffusion pump discharges the gas to the gas tank. The gas source at this time is the leakage flow rate between the high vacuum valve at the inlet of the diffusion pump and the foreline valve at the outlet of the diffusion pump, and the surface deflation flow in the interval. These flow cause the pressure at the outlet of the diffusion pump increasing, but it cannot exceed the maximum exhaust pressure of the diffusion pump.
The purpose of using the maintenance pump is to save energy. It is set at the outlet of the oil vapor flow pump, generally in parallel with the pre-stage mechanical pump. It is the same type of pump as the backing pump. Experience has shown that maintaining pump pumping speed can be one tenth of the pumping speed of the backing pump. Using a maintenance pump can save a great deal of energy and reduce environmental noise.