The Working Principle of Molecular Pump

- Jan 19, 2018-

The pumping mechanism of molecular pump is different from the mechanical pump is pumped by the volume change of the pump cavity, the use of the principle of momentum transfer, the molecular orientation flow is discharged outside the pump, so as to achieve the purpose of pumping.

Molecular vacuum pump is the use of high-speed rotating rotor to transfer energy to the gas molecules, so that making it compression and evacuation. It has the following types:

Traction molecular pump: The gas molecules and high-speed rotor collision to obtain momentum, which is sent to the exit, therefore, it is a momentum transfer pump.

Turbo molecular pump: The pump houses a grooved disk or a bladed rotor that rotates between the stator disks (or stators). The circumferential line speed of the rotor is very high. This kind of pump usually works in the state of molecular flow.

Compound molecular pump: It is a compound molecular vacuum pump which is series combination with a turbo type and a traction type.

The molecular pump rotates at high speed under the drive of the motor, and uses the frequency conversion power to drive the motor and the purpose is to get a higher speed. In the molecular flow area, the gas molecules collide with the surface of the blade rotating at a high speed, and the momentum is transmitted to the gas molecules, so that the partial gas molecules to generate directional flow on the rigid body surface, and expelled the pump to achieve the purpose of gas extraction. Usually with high-speed movement of rigid body surface to carry gas molecules and make it move in a certain direction is called the phenomenon of molecular traction, using this phenomenon made of vacuum pump called traction molecular pump.

The advantage of the traction molecular pump is the short start-up time, high compression ratio in the molecular flow state, and can remove all kinds of gases and vapors, and is especially suitable for pumping heavier gases. However, due to the small pumping speed of the traction molecular pump, small sealing clearance, poor reliability of work, easy mechanical failure and difficult manufacturing, in fact it is rarely used. Later on the traction molecular pump was improved, and the turbo molecular pump appeared.

There are multi-groups of interphase traction wheels vanes and fixed wheel vanes in a turbo molecular pump, and there are many vanes on each wheel that are tilted at a certain angle. The actual turbo molecular pump is composed of cascaded blade arrangement in multiple stages, in terms of rotor plate, stator plate, rotor plate to followed by alternating arrangement, the rotating blade is similar to the effect of the electric fan blade, and can pump the gas from one side to the other side. Improving the speed of the molecular pump is beneficial to improving the pumping speed of the molecular pump. Due to the limitation of the rotate speed of the blades, it is more difficult to pump the vacuum if the gas molecules move faster.

Turbo molecular pump is also a mechanical vacuum pump, which is pumped through the combination of high-speed rotating multistage turbine rotor blades and stationary blades. It produces a high compression ratio for the extracted gas in the molecular flow area, so as to achieve the desired vacuum performance. The ultimate vacuum of the turbo molecular pump is higher than the diffusion pump, up to 10-8 pa. Normal working requires a certain degree of pre vacuum. Its vacuum level varies slightly according to the pump. It is usually between 1-200pa, and mechanical pump can be used as a front stage pump. Because of the high speed of the turbo molecular pump, usually driven by the IF motor and the IF power supply frequency between 300-400HZ.

Water cooling is generally used in the turbo molecular pump.

The compound molecular pump is a series combination of turbo molecular pump and traction molecular pump. It integrates the advantages of the two pumps, and has a large pumping speed and a high compression ratio within a wide pressure range of 10-6~1pa, which greatly improves the outlet pressure of the pump.