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Roughing pump

Vacuum pump

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 1x10−3 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 1x10−3 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
Rotary Vane	Low ultimate pressure
Low cost
Long pump life	Backstreams oil
Produce hazardous waste
Rootes Lobe	Very high pumping speed	Frequent maintenance
Requires a purge gas
Requires a backing pump
Must be absolutely horizontal
Rotary Piston	High volume
Low cost	Noise
Vibration
Safety valve
Scroll	Clean
Low "dry" ultimate pressure
Easily servicable
Quiet
Evolved from air conditioning compressor so technology is well known	Limited bearing life
Limited scroll life
Permeable to small gases
Not hermetically sealed
Clean applications only
Diaphragm	Low cost
Quiet
Easily serviced	Low pumping speed
High ultimate pressure
Frequent service required
Hook & Claw	No backstreaming
Low ultimate pressure	Expensive
Screw Rotor	Low ultimate pressure
Less maintenance than Hook & Claw	Expensive
Dry Piston	Low ultimate pressure	Expensive
Sorption	Clean
No moving parts	Requires Liquid nitrogen
Requires regeneration
Limited 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.

Info: Wikipedia Source

This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page.

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