A roughing pump is any

vacuum pump A vacuum pump is a device that draws gas molecules from a sealed volume in order to leave behind a partial vacuum. The job of a vacuum pump is to generate a relative vacuum within a capacity. The first vacuum pump was invented in 1650 by Otto v ...

(typically mechanical) used to initially evacuate a vacuum system, as a first stage towards achieving high vacuum or

ultra high vacuum Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately ...

. The term "roughing pump" derives from the vacuum range it works in, "rough vacuum", above 1x10⁻³

torr The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ). Historically, one torr was intended to be the same as one "millimeter of mercury ...

(0.1 Pa). Pumps that operate in the high vacuum ranges typically don't operate, or only operate inefficiently, at

atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...

s, 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 A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, t ...

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: {, class="wikitable" , - ! colspan="3" , Oil Sealed Pumps , - ! Type ! Advantages ! Disadvantages , - , 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 , - ! colspan="3" , Dry Roughing Pumps , - ,

Scroll A scroll (from the Old French ''escroe'' or ''escroue''), also known as a roll, is a roll of papyrus, parchment, or paper containing writing. Structure A scroll is usually partitioned into pages, which are sometimes separate sheets of papyrus ...

, 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 Sorption is a physical and chemical process by which one substance becomes attached to another. Specific cases of sorption are treated in the following articles: ; Absorption: "the incorporation of a substance in one state into another of a dif ...

, Clean
No moving parts , Requires

Liquid nitrogen Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wide ...

Requires

regeneration Regeneration may refer to: Science and technology * Regeneration (biology), the ability to recreate lost or damaged cells, tissues, organs and limbs * Regeneration (ecology), the ability of ecosystems to regenerate biomass, using photosynthesis ...

Limited capacity

Sources

The Essentials of a Vacuum System
Hasina Jamal, University of Maryland

References

Roy Schmaus, University of Alberta. Archived fro

on May 27, 2006. Vacuum pumps