Fore Vacuum for Turbomolecular Pumps

A turbomolecular pump operates effectively only when its foreline pressure remains well below atmospheric levels. The roughing/backing pump maintains this fore vacuum, continuously removing gases that the turbo pump compresses but cannot fully discharge on its own. Without a properly sized and selected backing pump, the turbomolecular pump cannot reach its designed compression ratio, which can lead to reduced performance, overheating, or even shutdown.

Using an oil-free diaphragm pump for fore-vacuum generation is often preferred for analytical and semiconductor processes, as it significantly reduces the risk of hydrocarbon contamination and supports clean, dependable vacuum generation.

If your high vacuum system operates without significant process gas load, the ultimate vacuum of the backing pump can be assumed to equal the available backing vacuum. In this case, the pump’s volume flow rate mostly affects pump-down time. However, if there is a continuous gas load (for example, from process gases, outgassing, or leaks) the backing pump must have sufficient speed to ensure the turbomolecular pump’s maximum permissible backing pressure is not exceeded.

Reliable operation demands a roughing pump that can maintain stable foreline pressure even under variable gas loads. For this reason, dry vacuum pumps such as oil-free diaphragm pumps with high pumping speed close to their ultimate vacuum are an excellent choice for many laboratory and analytical setups.

When selecting a roughing pump for your turbo pump, several performance factors are important:

• Ultimate Vacuum: The pump must reach a vacuum low enough to meet the backing pressure requirements of the turbomolecular pump, often down to around 0.3 mbar, depending on your specific system.
• High Pumping Speed: The pump should deliver high pumping speed even near its ultimate vacuum to shorten pump-down time and maintain a safe foreline pressure during operation.
• Low Power Consumption: Energy efficiency helps keep operating costs under control.
• Anti-Suckback Protection: Low leakage rates and reliable anti-suckback design prevent contamination or venting if power is interrupted.
• High Reliability: Look for pumps designed for continuous, around-the-clock operation.
• Condensate Tolerance: Good resistance to vapors and condensates extends service life and reduces maintenance.
• Stable Operation: Ultimate vacuum stability and infrequent service intervals help maintain consistent conditions for the turbomolecular pump.
• Compact Design: A small footprint, low weight, and minimal vibration support easy integration into your setup.

Oil-free diaphragm pumps are an excellent choice for many applications where clean vacuum and hydrocarbon-free pumping are required. They deliver reliable performance and simple operation with minimal maintenance. Rotary vane pumps are another option when higher pumping speeds or lower ultimate vacuums are needed, but they do carry the risk of oil backstreaming into the system. Scroll pumps can also be a suitable oil-free alternative for higher speeds. Many operators prefer dry roughing pumps when contamination control and minimal maintenance are priorities. 

Selecting the right vacuum pump type depends on your specific requirements for vacuum level, cleanliness, and system compatibility.