Roughing pump

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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[edit]

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 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
Dry Roughing Pumps
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

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