Editing Vacuum pump (section)

=== Positive displacement pump ===<!-- This section is linked from [[Vacuum]] -->
[[Image:L-Pumpe2.png|thumb|The manual water pump draws water up from a well by creating a vacuum that water rushes in to fill. In a sense, it acts to evacuate the well, although the high leakage rate of dirt prevents a high quality vacuum from being maintained for any length of time.]]
[[Image:Two moving spirals scroll pump.gif|frame|Mechanism of a scroll pump]]

A partial vacuum may be generated by increasing the volume of a container. To continue evacuating a chamber indefinitely without requiring infinite growth, a compartment of the vacuum can be repeatedly closed off, exhausted, and expanded again. This is the principle behind a '''positive displacement pump''', for example the manual water pump. Inside the pump, a mechanism expands a small sealed cavity to reduce its pressure below that of the atmosphere. Because of the pressure differential, some fluid from the chamber (or the well, in our example) is pushed into the pump's small cavity. The pump's cavity is then sealed from the chamber, opened to the atmosphere, and squeezed back to a minute size.<ref name="VCH" /><ref name=":3" />

More sophisticated systems are used for most industrial applications, but the basic principle of cyclic volume removal is the same:<ref>{{Cite book |last=Ekenes |first=Rolf N. |url=https://books.google.com/books?id=h1jfwLceRA4C&pg=PA139 |title=Southern marine engineering desk reference |date=2009 |publisher=Xlibris Corp |isbn=978-1-4415-2022-7 |location=United States |pages=139–40 |oclc=757731951}}</ref><ref>{{Cite book |last=Coker |first=A. Kayode |url=https://books.google.com/books?id=N8RcH8juG_YC&pg=PA562 |title=Ludwig's applied process design for chemical and petrochemical plants. Volume 1 |date=2007 |publisher=Elsevier Gulf Professional |others=Ernest E. Ludwig |isbn=978-0-08-046970-6 |edition=4th |location=Amsterdam |page=562 |oclc=86068934}}</ref>
* [[Rotary vane pump]], the most common
* [[Diaphragm pump]], zero oil contamination
* [[Liquid ring]] high resistance to dust 
* [[Reciprocating compressor|Piston pump]], fluctuating vacuum
* [[Scroll compressor|Scroll pump]], highest speed dry pump
* [[Archimedes' screw|Screw pump]] (10 Pa)
* [[Wankel engine|Wankel]] pump
* External vane pump
* [[Roots blower]], also called a booster pump, has highest pumping speeds but low compression ratio
* Multistage Roots pump that combine several stages providing high pumping speed with better compression ratio
* [[Toepler pump]]
* [[Lobe pump]]

The base pressure of a [[rubber]]- and [[plastic]]-sealed piston pump system is typically 1 to 50 kPa, while a scroll pump might reach 10 Pa (when new) and a rotary vane oil pump with a clean and empty metallic chamber can easily achieve 0.1 Pa.

A positive displacement vacuum pump moves the same [[volume]] of gas with each cycle, so its pumping speed is constant unless it is overcome by backstreaming.