Selecting the right vacuum pump

Across a whole spread of laboratory applications, the requirement to achieve a deep vacuum is a common theme.

Across a whole spread of laboratory applications, the requirement to achieve a deep vacuum is a common theme.

Physics laboratories, for example, depend on deep vacuum in areas such as the manufacture of semiconductors and in the development of thin film technology, whilst in other laboratory applications, experiments in spectroscopy, surface science studies and gas analysis all make exacting demands of vacuum pumps.

Applications for the deepest vacuums have come to rely on turbomolecular pumps.

The efficiency of the turbomolecular pump is such that it can achieve vacuum levels of better than 10<-4>mbar extremely quickly: a pump cable of operating at 200cu.m/s might evacuate a clean, 1 litre chamber to 10<-4>mbar in the order of half a minute.

A turbo pump with a sufficient compression ratio can go on to achieve vacuums to 10<-8>mbar and better, although with other physical variables coming into play (leakage, levels of cleanliness, etc), these deeper ultimate vacuums can take an order of hours to reach.

The operation of the turbopump, however, is dependent upon a rough vacuum of the order of 1mbar being achieved first, which is the role of the backing pump.

Selection of the right backing pump has a big impact on the overall vacuum system.

It has both size and cost implications for the overall package, and the time taken to achieve the 1mbar vacuum required before the turbopump can operate effectively can be a key consideration.

For example, in the same 1 litre chamber cited above, while the turbo pump might be able to achieve its evacuation to 10-4m bar in just half a minute, a conventional backing pump with a pumping speed of 16 m3/hr might take half an hour to achieve the initial 1mbar pressure required by the turbo pump.

One of the problems with conventional designs of backing pumps is that the curve of flow against pumping speed is non-ideal, with pumping speed falling off significantly as pressures dip below 20mbar.

Overcoming this design weakness can mean employing three or four stage backing pumps to keep 'roughing' times down, but this adds considerably to both the cost and size of the system, whilst the added complexity can have adverse servicing and reliability implications.

Addressing these shortcomings, Welch has developed two innovative new pump designs intended for high vacuum and mass spectrometry applications, providing highly effective backing pumps for hybrid turbomolecular pumps, as well as for molecular drag pumps and gas analysers.

Oil-free, and with low ultimate pressures, the Model 8111 (an oil-free diaphragm pump) and Model 2581 (a dry running Wob-L piston pump) have been developed to offer high pumping speeds below 20mbar, cost effective solutions, and simple designs with minimal and easy maintenance.

Innovative pump technologies The 8111 is designed as an OEM pump and represents a significant development in pumping technology, employing tangential technology and forced valve operation (for which there is a patent pending).

The new valve technology employed is highly efficient, with the suction valve located at the very edge of the pump chamber.

The forced valve operation and tangentially fixed diaphragm minimise dead space.

The result is a highly compact, two stage diaphragm pump which sets benchmark standards for its size by offering a flow rate of 32 l/min and delivering 1mbar ultimate pressure performance comparable to that offered by three or four stage pumps.

As well as its size advantage, the 8111 also offers a significant cost advantage over the three and four stage pumps which would typically be used.

Applications for the 8111 include use on helium leak detectors, mass spectrometers, MVD chambers, electron microscopes, surface science engineering and reaction cell analysers.

Additionally, it will find uses on dental ovens and for degassing polymers.

Other applications include medicine and analytical engineering.

The second design, the 2581, provides the smallest possible footprint for a 100 l/min dry running pump.

The Wob-L piston pump design delivers ultimate pressures below 5mbar, yet has a footprint measuring just 19 by 32cm.

Simplicity is the key to the economical, aluminium design.

As the piston wobbles, air resistance on the upward stroke expands the Teflon seal on the piston, thus increasing the efficiency whilst compensating for the wobble action.

The pump is designed as a backing pump for hybrid turbos, MDPs and gas analysers where quick roughing of a chamber is needed, offering a higher flow rate than an oil-sealed rotary pump that might typically be used.

Achieving such deep vacuums as the 8111 and 2581 pumps offer has traditionally meant using oil lubricated pumps, but this can be undesirable in many applications.

Thus the 8111 and 2581 will meet the needs of applications requiring high vacuum but where the escape of oil vapour into the process could present a significant hazard.

The 8111 and 2581 are the latest in the string of developments of innovative pumping technologies.

With expertise across the spread of pumping strategies, Welch is best placed to provide the most appropriate solution to any pumping application.

And with its own foundry, and with its ability to manufacture pumps and compressors in volume to cover the needs of a wide range of sectors, Welch is readily able to produce ranges of pumps which are highly cost efficient.

A complete range of highly reliable and competitively priced Welch vacuum pumps and systems is available across Europe and from ASF Thomas, along with a correspondingly high level of local service and support.

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