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Saturday, August 23, 2014

Turbomolecular High-Vacuum Pump Employs Floating-Suspension Technology

A new turbomolecular vacuum pump is the first in a new series of high-vacuum pumps featuring floating-suspension technology developed and patented by Agilent Technologies Inc., based in Santa Clara, Calif.

Floating-suspension technology in the TwisTorr 304 FS can be used in a variety of applications and markets, including academic and government research and the analytical, industrial, and nanotech/semiconductor industries, according to Agilent.

“Work is already under way to incorporate this new technology and its benefits into Agilent’s next-generation gas chromatography/mass spectrometry systems, and we will continue our active collaboration with customers to identify their expanding needs and develop new advances in turbo pump design,” says Giampaolo Levi, vice president and general manager of Agilent’s Vacuum Products Division.

Floating suspension system in turbomolecular pump brings benefits in reliability and energy efficiency.

Floating suspension system in turbomolecular pump brings benefits in reliability and energy efficiency.

Running at 60,000 rpm, the TwisTorr pumps up to 250 L/s for nitrogen gas, while drag stages ensure high pumping speed and compression ratios for hydrogen and helium. Previous Agilent pumps and competing pumps, according to the company, were in the range of 1 x 104 to 1 x 105 for hydrogen compression, compared with 1.5 x 106 for the TwisTorr 304 FS. Compression ratio is a measure of pumping effectiveness, as it compares outlet or exhaust pressure to starting or inlet pressure of a gas.

In the high-vacuum range, the light gases such as helium and hydrogen become a larger proportion of the gas load, so the specific compression ratio performance of a high-vacuum pump with respect to these gases is of interest in choosing the most effective (and rapid) means of achieving the vacuum required.

The new pump also reportedly provides dramatic improvements in reliability versus previous Agilent pump designs, based on environmental testing of shock, vibration, and thermal behavior (operating and non-operating).

Agilent Floating Suspension (AFS) is a new generation of rotor suspension technology designed to produce more predictable behavior in the spinning rotor in terms of radial stiffness and bearing preload. AFS consists of a couple of components, one for each bearing, which is made up of two metal rings with silicone rubber vulcanized in between. The outer ring is flanged to the pump body, while the inner ring is press-fit to the bearing’s outer ring.

It is designed to guarantee bearing alignment due to the geometrical precision of the ground rings mounted on the same piece of aluminum (the pump body). The radial stiffness achieved optimizes dynamic rotor behavior and critical speed positioning while minimizing acoustical noise. The lower AFS assembly is aligned as an axial spring to provide constant preload to the bearing and secure axial rotor positioning.

AFS thus is said to offer two advantages over other approaches: 1) radial stiffness of the suspension and bearing axial preload are stable over time (reduced incidence of misalignment), and 2) the simplest mounting scheme requires only two components to provide radial support, axial support, and bearing preload — instead of the four to five components required by other designs.

Outer appearance of Agilent's TwisTorr 304 FS.

Outer appearance of Agilent’s TwisTorr 304 FS.

Floating-suspension technology also minimizes vibration and acoustical noise while providing optimal working conditions for bearings as well as stability for demanding applications and instruments. Further, the bearing design and dry lubrication of the suspension system permits installation in any orientation.

The new pump also features high foreline tolerance. The foreline is the exhaust of a high-vacuum pump. High-vacuum pumps cannot exhaust directly into atmospheric pressure. Even though the pump compresses gas effectively, the pressure of the compressed gas ready to be exhausted is still well below atmospheric pressure. Therefore, high-vacuum pumps must be assisted by a “primary” or “medium” vacuum pump (such as a scroll or rotary vane pump), whose effective range begins around atmospheric pressure and extends down to overlap with the high-vacuum pump.

For a high-vacuum pump, foreline tolerance is a descriptor of the ability of the pump to exhaust. That is, the higher the foreline (pressure) tolerance, the easier it is for the partnering primary pump to work. And often, higher foreline tolerance in the high-vacuum pump allows the use of a smaller, less expensive primary pump.

Low power consumption and low operating temperature are also achieved by the TwisTorr pump, according to Agilent. Two principle conditions are at work: more constant (and efficient) pumping, and smaller required mass of the compact rotor/stator combination. In the TwisTorr drag section, the pumping effect is created by a spinning rotor disk, which transfers momentum to the gas molecules. These molecules are forced to follow the specific spiral groove design (TwisTorr stages) on the stator.

This ensures a constant local pumping speed inside the channel and avoids reverse pressure gradients (and therefore higher or more variable resistance), thereby minimizing power consumption. The double-sided spiral groove design on the TwisTorr stators combines centripetal and centrifugal pumping action in series, greatly reducing the size of the drag section.


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