The core components of the molecular pump are rotating blades and stationary blades (Figure 1). The rotation speed of the rotating blade reaches 1800 to 90000 rpm, the speed at the blade tip reaches several hundred meters per second, which is close to the average speed of the molecule. And gas molecules are transported to the exhaust port by collision with it. When the rotating blade that are set to be subtended rotating at a high speed, the probability of the gas molecules moving from the suction side to the exhaust side is A, and the probability that the gas molecules move from the exhaust side to the suction side is B. The rotating blade hits the gas molecules, then the gas molecules hit the fixed blade and fly to the next rotating blade, finally the gas molecule is ejected to the exhaust port, making A>B to achieve the exhaust function.
Exhaust Velocity of Molecular Pump = Opening Area of Suction Side × 11.6 × A/B.p
In formula, p refers to the increase value of pressure.
The larger of the angle of the blade, the higher of exhaust speed smaller of the compression ratio. If the angle of the blade is small, the exhaust speed is low and the compression ratio is large. The exhaust speed of the molecular pump is nearly not affected by the gas type, and the pump's ultimate pressure can be 10-8 Pa, but the speed of exhaust will be reduced for light molecules/atoms.
As a kind of mechanical pump with high rotation speed, the molecular pump requires high balance and it is very fragile to solid foreign matter. So usually a metal filter is installed at the inlet of it. When the molecular pump working, the blades may be damaged if the pressure suddenly deteriorates.
Fig.1 Structure and schematic diagram of a molecular pump
Conventional molecular pumps use bearings to ensure high speed rotation of the blades. But the new-type molecular pump adopts magnetic levitation structure, which makes the speed of the blade continuously increase, and at the same time the molecular pump is more suitable for the ultra-high vacuum system, also, it prolongs the service life of the molecular pump.
Compared with oil-sealed rotary vane pumps and turbo-type dry pumps, molecular pumps has cleaner work environment, and it can achieve extremely high vacuum levels. However, molecular pumps cannot operate at atmospheric pressure, and it must be used in combination with other rough-out vacuum pumps (oil-sealed rotary vane pumps or turbo dry pumps) that can operate at atmospheric pressure. The molecular pump has complicated structure and high price, and there is vibration during high-speed rotation.