Multi-stage dry vacuum pumps have been commercially available for over 30-35 years. Early versions were generally water-cooled, and in more recent years some air-cooled versions have been introduced. Due to the fact that typically 4-6 stages of compression are required to attain pressures close to what oil-sealed pumps are capable of in just two stages, multi-stage dry pumps create a significant amount of heat and require more cooling, specifically in the latter stages.
The air-cooled versions of these pumps are also considered PoU (point of use) as they require no water or other facilities besides AC power. The purpose of identifying these pumps as air-cooled is that there are also water-cooled versions available.
As far as oil contamination of product, while they are not completely oil-free, dry pumps are the best option. They have lubricating fluid in the gear chambers and/or the bearing chambers. In most cases the gear chamber has oil and the opposing bearing chamber has greased bearings. In either case, the lubricating oil is not in the swept gas stream, meaning that the volatiles being pumped through the vacuum pump generally do not directly contact and contaminate the lubricating fluid. This reduces the chances of the operating oil contaminating the product.
These pumps have produced some effective results (in some applications) that in some cases are more attractive than rotary vane pumps. However, when they fail (which they will), they can result in higher cost rebuilds than vane or scroll pumps, because the rebuild is typically more specialized.
We first installed a multi-stage dry roots pump about 6 years ago in place of a rotary vane pump on vacuum ovens that were used to process shatter. In this case, the customer was not using cold traps on the rotary vane pumps or changing the oil as frequently as required. Their vane pumps would fail in 2-3 months. The multi-stage air-cooled dry roots pump ran for over 8 months with zero maintenance, but like all pumps ingesting volatile compounds, the compounds condensed in the final stages and seized the pump. As a result, the pump required complete disassembly and cleaning. Fortunately, in cases like this, there is oftentimes no damage to the internal components, and the pumps can be rebuilt with no degradation of performance or operating life. We equipped subsequent installs of these pumps in this application with a gas ballast function, and the life of the pumps typically exceeded 14 months with zero maintenance.
That being said, we have seen these pumps utilized on other applications (wiped wall evaporation, for example) with less favorable results (some failures as early as 2 months).
