Molecular Pump Working and Theory

Gaede and then Langmair developed the idea of a molecular pump.

Theory of Molecular Pump

Whenever a surface moves very near to a static surface, then the gaseous molecules in between the space of these two surfaces get a motion along with the motion of the moving surface. This phenomenon happens due to the viscous property of the gaseous molecules. For this phenomenon to happen the distance between the static and moving surfaces must be less than 0.03 mm. Practically the force acting on the gaseous molecules to move them is the viscous force. As a result, the gaseous molecules achieve the velocity of the moving surface.

Actually, in a low pressure, the average mean free paths for the movement of the molecules are quite longer than the perpendicular distance between the surfaces. As a result, the chances of collisions between the molecules become quite low in this space. On the other hand, the collisions of the molecules with the moving surface becomes dominating. Because of that, the gas molecules obtain the speed of the moving surface.

Basic Construction of a Molecular Pump

The inner wall of a hollow metallic cylinder serves as the static surface of the molecular pump. We call it the stator of the pump. There is a solid cylinder fitted inside the hollow cylinder. The diameter of the solid cylinder is so chosen that the gap between the inner wall of the stator and the periphery of the solid cylinder becomes very narrow. During the operation of the molecular pump, the solid cylinder rotates. This is the reason we call it the rotor of the pump.

Molecular Pump

The stator consists of one inlet and one outlet. There is a slot in the inner periphery of the stator between the inlet and outlet. Consequently, it makes a wider space between the outlet and inlet compared to other portions between peripheries of the stator and rotor. An auxiliary motor coupled with the shaft of the rotor rotates the rotor at a high speed. The speed of the rotor is maintained by more than 5000 RPM.

Working Principle of Molecular Pump

The direction of the rotation of the rotor is such that its outer surface always moves from inlet to outlet on the wider space section of the molecular pump. For that reason, in our model of the molecular pump, the rotation must be clockwise.

Now we connect a manometer in between the inlet and outlet of the pump. During rotation of the rotor at required high speed, we can clearly observe and measure the pressure difference between the inlet and outlet of the diffusion pump with the help of that manometer. The difference between the Mercury levels in the manometer indicates the pressure difference between the inlet and outlet of the pump.

Equation of Pressure Difference in a Molecular Pump

Let us consider P1 of the pressure at the inlet and P2 is the pressure at the outlet.

This equation is according to the kinetic theory of gases.

Here η is the coefficient of viscosity.
L is the peripheral distance between the inlet and outlet within the wider space between the stator and the rotor.
h is the distance between peripheries of the stator and rotor in between the inlet and outlet.
ω is the angular velocity of the rotor.


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