A roughing pump is any vacuum pump (typically mechanical) used to initially evacuate a vacuum system, as a first stage towards achieving high vacuum or ultra high vacuum. The term "roughing pump" derives from the vacuum range it works in, "rough vacuum", above 0.001 torr (0.1 Pa). Pumps that operate in the high vacuum ranges typically don't operate, or only operate inefficiently, at atmospheric pressures, whereas pumps that work efficiently at atmospheric pressure usually cannot produce a vacuum lower than approximately 0.001 torr.
One consideration for choosing a roughing pump is whether the pump uses lubricating oil that's exposed to the vacuum. This concern of "hydrocarbon backstreaming" where pump oil as a gas makes its way into the vacuum chamber, has led to oil-free pump designs on the market.
Types Two main types of roughing pumps are oil-sealed roughing pumps and dry roughing pumps. Within the two categories are various kinds of designs with differences among them based on maintenance issues, initial costs, pump lifespan and vacuum level. Early in their manufacture, dry pumps were significantly more expensive than oil-based pumps, however over time the cost gap has closed.
The advantages and disadvantages of each are as follow: Oil Sealed Pumps Type Advantages Disadvantages Rotary Vane Low ultimate pressureLow costLong pump life Backstreams oilProduce hazardous waste Rootes Lobe Very high pumping speed Frequent maintenanceRequires a purge gasRequires a backing pumpMust be absolutely horizontal Rotary Piston High volumeLow cost NoiseVibrationSafety valveDry Roughing Pumps Scroll CleanLow "dry" ultimate pressureEasily servicableQuietEvolved from air conditioning compressor so technology is well known Limited bearing lifeLimited scroll lifePermeable to small gasesNot hermetically sealedClean applications only Diaphragm Low costQuietEasily serviced Low pumping speedHigh ultimate pressureFrequent service required Hook & Claw No backstreamingLow ultimate pressure Expensive Screw Rotor Low ultimate pressureLess maintenance than Hook & Claw Expensive Dry Piston Low ultimate pressure Expensive Sorption CleanNo moving parts Requires Liquid nitrogenRequires regenerationLimited capacity
Sources The Essentials of a Vacuum System, Hasina Jamal, University of Maryland
References Comparison of Roughing Pumps, Roy Schmaus, University of Alberta. Archived from the original on May 27, 2006.