Pumps

By definition, a pump is a device to lift, transfer, or increase the pressure of a fluid (gas or liquid) or to create a vacuum in an enclosed space by the removal of a gas.

Vacuum Pumps have been devised for removing the molecules of gas or vapor from a confined space.

The centrifugal pump, the most common kind, consists basically of a rotating device, called an impeller, inside a casing. The fluid to be pumped enters the casing near the shaft of the impeller. Vanes attached to the spinning impeller give the fluid a high velocity so that it can move through an outlet.

The reciprocating pump moves a fluid by using a piston that travels back and forth in a cylinder with valves to help control the flow direction. Examples are the lift pump and the force pump.

In a lift pump the piston and cylinder are positioned vertically. When the piston moves upward, atmospheric pressure pushes water into the cylinder to fill the empty space beneath the piston. On the downward stroke, the water in the cylinder is forced to flow above the piston. Reversing direction, the piston moves up, allowing more water to come up under it into the cylinder and lifting the water held above it to an outlet pipe where the water flows out of the pump. Since atmospheric pressure will support a column of water no higher than about 33 ft (10 m), a lift pump can raise water no farther than this distance.

The rotary pump is like the reciprocating pump in that it allows a fluid to fill a space that then decreases in volume, forcing the fluid out of the space. However, unlike a reciprocating pump, it has no valves and uses one or more rotating components in place of a piston.

The jet pump has no moving parts; it uses a swiftly moving fluid to induce motion in another fluid. For example an atomizer, a type of jet pump, uses a high-speed stream of air to pump a liquid, such as a perfume. Compressor types range from the simple hand pump and the piston-equipped compressor used to inflate tires to machines that use a rotating, bladed element to achieve are used to pump air or other gases into a closed container.

Compressors are used to pump air or other gases into a closed container. They range from hand pumps to large power-driven devices that furnish compressed air for operating pneumatic machinery and for various other purposes. In nuclear reactors that use liquid radioactive metal, the nonmechanical electromagnetic pump is employed. An electric current is either induced in the liquid metal or is passed through it by electrodes. A magnetic field surrounding the pipe then propels the current-carrying liquid forward.

For our purposes and basic information moving forward.....

Pumps are in general classified as Centrifugal Pumps (or Roto-dynamic pumps) and Positive Displacement Pumps.

Centrifugal Pumps (Roto-dynamic pumps)

The centrifugal or roto-dynamic pump produce a head and a flow by increasing the velocity of the liquid through the machine with the help of a rotating vane impeller. Centrifugal pumps include radial, axial and mixed flow units. Centrifugal pumps can further be classified as;

end suction pumps
in-line pumps
double suction pumps
vertical multistage pumps
horizontal multistage pumps
submersible pumps
self-priming pumps
axial-flow pumps
regenerative pumps

Positive Displacement Pumps

The positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. The positive displacement pump delivers a constant volume of liquid for each cycle against varying discharge pressure or head. The positive displacement pump can be classified as:

Reciprocating pumps - piston, plunger and diaphragm
Power pumps
Steam pumps
Rotary pumps - gear, lobe, screw, vane, regenerative (peripheral) and progressive cavity

Selecting between Centrifugal or Positive Displacement Pumps is not always straight forward.

Flow Rate and Pressure Head

The two types of pumps behave very differently regarding pressure head and flow rate:

The Centrifugal Pump has varying flow depending on the system pressure or head
The Positive Displacement Pump has more or less a constant flow regardless of the system pressure or head. Positive Displacement pumps generally gives more pressure than Centrifugal Pump's.

Capacity and Viscosity

Another major difference between the pump types is the effect of viscosity on the capacity:

In the Centrifugal Pump the flow is reduced when the viscosity is increased
In the Positive Displacement Pump the flow is increased when viscosity is increased. Liquids with high viscosity fills the clearances of a Positive Displacement Pump causing a higher volumetric efficiency and a Positive Displacement Pump is better suited for high pressure.

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