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A positive displacement pump causes a liquid or
gas to move by trapping a fixed amount of fluid
and then forcing (displacing) that trapped volume
into the discharge pipe. The periodic fluid
displacement results in a direct increase in
pressure. A positive displacement pump can be
further classified as either
* a rotary-type (for example the rotary vane),
* lobe pump similar to oil pumps used in car
engines, or
* the Wendelkolben pump or the helical twisted
Roots pump.
Roots-type pumps
The low pulsation rate and gentle performance of
this Roots-type positive displacement pump is
achieved due to a combination of its two 90°
helical twisted rotors, and a triangular shaped
sealing line configuration, both at the point of
suction and at the point of discharge. This design
produces a continuous and non-vorticuless flow
with equal volume. High capacity industrial "air
compressors" have been designed to employ this
principle as well as most "superchargers" used on
internal combustion engines.
Reciprocating-type pumps
Reciprocating-type pumps use a piston and cylinder
arrangement with suction and discharge valves
integrated into the pump. Pumps in this category
range from having "simplex" one cylinder, to in
some cases "quad" four cylinders or more. Most
reciprocating-type pumps are "duplex" (two) or
"triplex" (three) cylinder. Furthermore, they are
either "single acting" independent suction and
discharge strokes or "double acting" suction and
discharge in both directions. The pumps can be
powered by air, steam or through a belt drive from
an engine or motor. This type of pump was used
extensively in the early days of steam propulsion
(19th century) as boiler feed water pumps. Though
still used today, reciprocating pumps are
typically used for pumping highly viscous fluids
including concrete and heavy oils.
Compressed-air-powered double-diaphragm pumps
Another modern application of positive
displacement pumps are compressed-air-powered
double-diaphragm pumps, commonly called SandPiper
or Wilden Pumps after their major manufacturers.
They are relatively inexpensive and are used
extensively for pumping water out of bunds, or
pumping low volumes of reactants out of storage
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