High Compression Turbopumps: Functional Principle and Application


A major milestone in Pfeiffer Vacuum's history was the invention of the turbopump in 1955. Since then, some 3,400 of the company's employees have been working to continually improve vacuum technology. Most recently, this was achieved through the invention of laser balancing technology. This ensures an even longer service life and significantly lower vibrations and noise emissions. But let's start from the beginning: What is the basis for the functional principle of the classic turbopump? And how do you select the right (backing) pump for your application?

The invention of the turbopump

The first turbopump was invented in 1955. At that time, Dr. Willi Becker had been the head of the technical laboratory at Arthur Pfeiffer Vakuumtechnik GmbH (now Pfeiffer Vacuum) for 13 years. He was concerned with the question of how to prevent the oil from flowing back into the pump housing in oil diffusion pumps. For this purpose, he used a baffle in the form of a rotating fan wheel. Using this setup, gas particles flowed in the direction of the pressure gradient without significant conductance losses. In the opposite direction, backflowing oil molecules were reflected by the rotating fan wheel. This prevented the molecules from reaching the high vacuum side.

In further research, Dr. Becker noticed that this setup not only reduced the oil backflow from the diffusion pump, it also produced a lower total pressure. He then applied a rotor-stator combination and multiple pump stages in series. For his setup, he used the double-flow version – a rotor that was belt-driven to reach a speed of 16,000 rpm. Weighing 62 kg and with a pumping speed of 900 m3/h, the pump was patented in 1956 and was the forerunner of all today's turbopumps. In 1958, it was presented for the first time at the International Vacuum Congress in Namur, Belgium. Without this invention, our modern life would be unthinkable – because without turbopumps, many manufacturing steps for the production of semiconductors as well as countless coating processes would not be possible.

Dr. Willi Becker, 1958 in the laboratory of Arthur Pfeiffer Vakuumtechnik GmbH
Dr. Willi Becker, 1958 in the laboratory of Arthur Pfeiffer Vakuumtechnik GmbH (today Pfeiffer Vacuum)

Functional principle and compression ratio

Functional principle of the turbopump
Figure 1: Functional principle of the turbopump

How does a turbopump work? The transfer of momentum from the rapidly rotating blades to the gas molecules to be pumped is the cornerstone of the pumping action of the arrangement of rotor and stator blades. Molecules hitting the blades are adsorbed there and leave the blade again after a short time. The vane velocity v adds up to the thermal molecular velocity c. The thermal molecular velocity c is the speed at which the molecules leave the pump. Molecular flow must prevail in the pump. Otherwise, the velocity component transferred by the blade would be lost through collisions with other molecules. The mean free path T must therefore be greater than the channel height h. During the pumping of gas, backpressure occurs in kinetic pumps, causing backflow. S0 denotes the pumping speed without backpressure. It decreases with increasing backpressure and reaches the value 0 at the maximum compression ratio K.

Arrangement of rotor and stator blades
Arrangement of rotor and stator blades

The compression ratio K0 can be estimated according to Gaede [1]. For the optically dense blade structure (Figure 1), Gaede's formula applies:
Gaede's formula
Gaede's formula

This is only an excerpt.
You can download the full application report as a PDF file.

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