There are several types of forevacuum pumps
Rotary vane technology is a popular choice for medium vacuum applications, but will exhibit oil back streaming. This can be an issue if a process is sensitive to contamination, for example precision coating. If the application demands an oil free environment, diaphragm and scroll pumps in particular would be suitable. However, both have lower pumping speeds than a rotary vane pump and the scroll can generate particles from its tip seals and is best suited to benign applications. Moreover, diaphragm pumps have a poor base pressure.
For high pumping speeds with good base pressure, the addition of roots pumps in combination with a rotary vane pump will increase pumping speed, but oil back streaming will still occur. Screw drypumps are an oil free option with high pumping speeds even without an added roots pump and can also handle difficult process applications.
There are several choices available for an additional pump for high vacuum processes:
Both diffusion pumps and turbo pumps offer similar base pressures. As with RV pumps, diffusion pumps are a source of oil contamination, but offer high pumping speeds. Turbomolecular pumps are an oil free option. However, the maximum pumping speed of the largest turbo is only around 5% that of a large diffusion pump.
For processes requiring large throughput in the high vacuum range, similar to a diffusion pump but without the issue of oil contamination, a cryopump where gases are trapped using low temperature might be appropriate. These pumps do not require a backing pump whilst operating, but will need a forevacuum pump for regenerating - the frequency of which would be process dependent.
Related: Explore our range of vacuum pumps on the Leybold product page.