Know All About Diffusion Pump Part-1 Q.1 How does a diffusion pump work? Ans. A diffusion pump works on the principle of gas diffusion to create vacuum. In a diffusion pump, diffusion pump oil is heated to its boiling temperature. Vapor of this oil is compressed in a vertically tapering hollow cone as it moves up. Simultaneously this oil vapor escapes from jets along the height of hollow cone. Oil vapor escaping at great speed traps air and then collides with water cooled wall of Diffusion Pump and moves down. Trapped air is released at bottom of Diffusion Pump. This creates differential pressure in diffusion pump. Pressure at top is minimum (high Vacuum) and at bottom is maximum (low Vacuum) from bottom of diffusion pump the air is sucked by rotary pump.
Q.2 Is a diffusion pump made only in cylindrical shape? Ans. Diffusion Pump has been successfully made in rectangular shape also. In fact rectangular shaped Diffusion Pump has an advantage that its inlet connected to Vacuum Chamber can be of larger area and Diffusion Pump itself can be adjusted to occupy less space. This also works with Diffusion Pump Oil as a working medium just like cylindrical Diffusion Pump. Q.3 I understand that Turbo molecular pump (TMP) is an alternative to diffusion pump. Is it better or worse than diffusion pump? Ans. Turbo molecular pump works on the principal of positive displacement. In this pump a series of inclined blades are mounted on a shaft which rotates at very high speed. These are called Rotors. Fixed blades or stators are fixed in such a way that Rotors and Stators alternate. Because of rotation of shaft and incline rotors push air down to next row of Stators. Stators too are inclined in same direction and push this air to next row of Rotors and so on to exit, where a Rotary Pump draws away the air. Though this pump creates good vacuum and can be used in place of Diffusion Pump but its applications are limited to critical applications like semiconductor coatings. This is because of its high cost, more requirement of maintenance and the fact that since this pump cannot be made of large size because of mechanical considerations, a diffusion pump can be replaced only by a couple of Turbo molecular pumps. Diffusion Pump on the other hand has no moving parts and requires almost no maintenance other than Diffusion Pump Oil change, is cheaper in cost and can be built in almost any size.
Turbo Molecular Pump