Composition of the diffusion pumping system
Composition of the diffusion pumping system :
The diffusion pump system mainly consists of diffusion pump, cold trap, gate valve and mechanical pump. For example, in a vacuum system with low oil return rate, there is no need to configure a liquid nitrogen cold trap, only a water-cooled baffle plate is needed. In applications where the oil return rate is strictly required, a liquid nitrogen cold trap should be installed in the system, and a local water cooling baffle may be used. The liquid nitrogen cold trap is mainly used to capture the water vapor and the light fraction reflux in the pump working fluid. The net pumping rate of air at the entrance of vacuum chamber for a 150 mm diffusion pump set is about 1000L/s, and the pumping rate of water vapor by the liquid nitrogen cold trap is as high as 4000L/s. A well designed and manufactured diffusion pump system can reduce the oil return rate from the working fluid of the pump in the vacuum chamber to a very low level, even smaller than that from other sources of organic matter.
Diffusion pump system:
In a smaller vacuum system, a mechanical pump can be used alternately for the coarse vacuum chamber and as the front-stage pump of the diffusion pump. When roughing, the pressure in the partitioned preceding pipeline will increase. If the roughing time is less than 15 minutes, the pressure increase is usually not obvious. For large vacuum systems, such as diffusion pump pumping systems with a diameter of 400 mm or larger, a Roots pump unit with a mechanical pump as the front pump is often used for rough pumping of the vacuum chamber. In the rough extraction process, a smaller maintenance pump can be used as the front pump of the diffusion pump. When the vacuum chamber is pumped to a high vacuum (after the inlet pressure of the diffusion pump is reached), the Roots pump is switched to the front stage of the diffusion pump to ensure that the pumping system has the maximum exhaust capacity.
The heat conduction gauge for control and fault protection is installed on the vacuum chamber and the front pipeline respectively. The ionization gauge is installed on the vacuum chamber or on the short suction pipe close to the vacuum chamber. The auxiliary ionization gauge is arranged between the cold trap and the high vacuum main valve. At present, there is no auxiliary ionization gauge in the general vacuum system, but this gauge is sometimes very useful. It can be used for the most direct diagnostic measurement of the diffusion pump, that is, measuring the no-load pressure of the diffusion pump, and this measurement can only be carried out with the gauge installed under the main valve (high vacuum valve).
Large cold traps should generally be equipped with an ionization gauge interface, in order to facilitate the installation of the ionization gauge, but also in high vacuum valve by one end of the diffusion pump with a spare gauge interface. When designing and installing a high vacuum valve, it is best to keep the valve body in a vacuum all the time, thus reducing the volume and surface area exposed to the atmosphere during each operation cycle and thus shortening the extraction time. During the installation of the gauge, all the inlets of the ionization gauge and the heat conduction gauge should be facing downward or toward the appropriate side in order to prevent dropping into the particles and affecting the measurement accuracy.
Some attachments listed above are often unnecessary in standard systems. The leak detection interface located on the front pipeline provides convenience for leak detection of vacuum chamber, well and diffusion pump. The leak detector can be connected to the system when the system works, so that the leak detection can get the highest sensitivity and the fastest response speed.
The throttle valve can be installed on the inlet side of the cooling pipe of the diffusion pump to save water and increase the oil temperature of the injection stage. Coarse suction pipe wells are often used in diffusion pump pumping systems with high oil return rate. If maintained properly, the coarse suction wells can reduce the migration of mechanical pump oil to vacuum chamber.