One important point that is often ignored is that the pumping arrays within the body of the pump fill most fo the space and act as construction to high pressure gas flow during the roughing period.
The reason for belaboring these points is that each type of pump will have different requirements for roughing/backing pumps. The same considerations will come into play when evaluating their varying abilities to handle process gas loads.
One source of confusion in the differences of these pumps is that they tend to look alike from the outside. The inside, though, is where the crucial differences reside.
The turbomolecular pump has been available for many years and is classified along with diffusion pumps as momentum transfer pumps. In this simplest form, the turbo pump is a rotating circular disk with a number of angled blades machined into it. As the disk rotates, the blades impact the incoming gas molecules and transfer the mechanical energy of the blades into gas molecule momentum that is directed from the inlet through a fixed stator with gas transfer holes.
The gas is than compressed from stage to stage through the pump to the exhaust port, where it is drawn off by the backing pump which is tasked with maintaining a particular exhaust pressure. In general, this pressure must be below 100 millitorr with 10-20 millitorr as common averages.
Turbomolecular pumps are easily capable of pumping into the 10-10torr range on a suitable system.
Molecular drag pumps vary from turbo pumps in that the momentum transfer from mechanical motion to gas motion is not by impact but by impacting the motion of a rapidly rotating solid surface to the gas molecules between a fixed and rotating surface.
A particular performance difference between molecular drag pumps and turbo pumps is that a drag pump will start at higher inlet pressure, and does not require as low a backing pressure for full operation.
Additionally, molecular drag pumps will have much lower pumping speed (roughly 1/8-1/3) than a turbomolecular pump with equivalent inlet dimensions.
Turbo/drag pumps are an elegant design compromise between turbomolecular and molecular drag pumps. Essentially, they are a combination of both turbo and drag pump. This is really a case of attempting to get the best attributes of both types of pump at the same time and in the same package. For most applications, this is successfully achieved.
This elegant compromise plays out to provide both the pumps ability to provide a low ultimate pressure along with the drag pumps ability to operate with a higher backing pressure.
This allows a diaphragm or piston type oil-free roughing/backing pump to be used which solves a number of applications problems in terms of both cleanliness and pump size.
Copyright 2000-2006 All Rights Reserved A&J Vacuum Services, Inc.
All Trademarks and brand names mentioned herain are the property of their respective owners
