Centrifugal vs. Centrifugal Pumps positive displacement pumps.
Pumps are generally classified as Centrifugal Pumps (or Rotodynamic pumps) and Positive Displacement Pumps.
Centrifugal pumps (rotodynamic pumps)
A bomba Centrifugal or rotodynamic produces a head and flow by increasing the speed of the liquid through the machine with the help of the rotating vane rotor. Centrifugal pumps include radial, axial and mixed flow units.
Centrifugal pumps can be further 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
A positive displacement pump operates by alternating filling a cavity and displacing a given volume of liquid. A positive displacement pump delivers a constant volume of liquid for each cycle regardless of discharge pressure or head.
The positive displacement pump can be classified as:
- Alternative pumps – piston, piston and diaphragm
- Power Bombs
- steam pumps
- Rotary pumps – gear, lobe, screw, vane, regenerative (peripheral) and progressive cavity
Selection between centrifugal or positive displacement pumps
Selecting between a centrifugal pump or a positive displacement pump is not always straightforward.
Flow Rate and Pressure Head
The two types of pumps behave very differently in relation to head and flow:
- The Centrifugal Pump has a variable flow rate depending on the system pressure or head.
- The Positive Displacement Pump has a more or less constant flow, regardless of the pressure or head of the system. Positive displacement pumps generally make more pressure than centrifugal pumps.
Capacity and Viscosity
Another major difference between pump types is the effect of viscosity on capacity:
- In a Centrifugal Pump the flow is reduced when the viscosity is increased
- In a positive displacement pump, flow is increased when viscosity is increased
High viscosity liquids fill the gaps of Positive Displacement Pumps causing greater volumetric efficiencies and Positive Displacement Pumps are best suited for higher viscosity applications. A centrifugal pump becomes very inefficient even at modest viscosity.
Mechanical Efficiency
Pumps behave differently also considering mechanical efficiency.
- Changing system pressure or head has little or no effect on the flow rate in a positive displacement pump
- Changing the system pressure or head can have a dramatic effect on the flow rate in a centrifugal pump
Positive Liquid Suction Head – NPSH
Another consideration is the Net Positive Suction Head-NPSH.
- In a Centrifugal Pump, the NPSH varies depending on the flow rate determined by the pressure
- In a Positive Displacement Pump, the NPSH varies depending on the flow rate determined by the speed. Reducing positive displacement pump pump speed reduces NPSH
Source: https://www.engineeringtoolbox.com/classification-pumps-d_55.html
