Main Types of Pumps: Centrifugal and Positive Displacement

Chelsey Bipat
Author : Chelsey Bipat
May 12, 2021
5 Minutes Read
  
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    Pumps simplify the transportation of water and other fluids, making them very useful in all types of buildings - residential, commercial and industrial. For example, fire pumps provide a pressurized water supply for firefighters and automatic sprinklers, water booster pumps deliver potable water to upper floors in tall buildings, and hydronic pumps are used in HVAC systems that use water to deliver space heating and cooling.

    There are many pump designs, but most types can be classified into centrifugal and positive displacement pumps. This article will provide an overview of each pump type, comparing their performance features.


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    Centrifugal Pumps

    centrifugal pump

    A centrifugal pump uses an impeller, which has curved blades that accelerate the fluid outward the when rotating. Impellers are normally driven by an electric motor or combustion engine, and their movement produces suction at the pump inlet, pulling water inside.

    Based on the type of water flow they produce, centrifugal pumps can be classified into three sub-types. The flow pattern is determined by both the impeller shape and the pump’s construction.

    SUB-TYPE

    DESCRIPTION

    PERFORMANCE

    Axial Flow Pump

    Also known as a propeller pump, it produces  water flow along the impeller shaft direction.

    High flow rate
    Low pressure

    Radial Flow Pump

    This pump type produces flow in a direction perpendicular to the shaft (90° angle).

    Low flow rate
    High pressure

    Mixed Flow Pump

    This pump type combines radial and axial flow, producing a conical flow pattern around the shaft.

    Medium flow rate
    Medium pressure

    Centrifugal pumps are the most common type, since they are suitable for handling water and relatively easy to manufacture. These pumps also tend to have a lower price, since mass production has brought down their manufacturing cost.

    Positive Displacement Pumps

    shutterstock_1422280976

    Positive displacement pumps move a fixed amount of fluid at regular intervals. They are built with internal cavities that fill up at the suction side, to be discharged with higher pressure at the outlet. Based on how fluid is displaced, positive displacement pumps can be reciprocating or rotary.

    SUB-TYPE

    DESCRIPTION

    EXAMPLES

    Reciprocating Pump

    Flow is established inside a cavity that expands and contracts, such as a piston. Water moves into the cavity during expansion, and is forced out during contraction, while the flow direction is controlled by using check valves.

    Bladder
    Diaphragm
    Peristaltic
    Piston / plunger

    Rotary Pump

    This pump type uses a rotor that traps water in cavities, releasing it at the outlet. These cavities can be the spaces between gear teeth or screw threads, among other configurations.

    Some designs use more than one shaft, but the principle is the same: the rotor shape is designed to capture “pockets” of water and displace them in the intended direction.

    Gear
    Screw
    Progressing cavity
    Rotary lobe
    Rotary vane

    How Do Centrifugal and Positive Displacement Pumps Compare?

    Both pump types move fluids in a specified direction, but accelerating a fluid continuously is not the same as displacing it in fixed amounts. As a result, there are important performance differences between both pump types.

    PUMP TYPE

    CENTRIFUGAL PUMP

    POSITIVE DISPLACEMENT PUMP

    Effect of system pressure

    When the system exerts a greater pressure against the pump, flow is reduced.

    Flow is constant, regardless of system pressure. The pump simply produces more pressure in response.

    Effect of fluid viscosity

    A higher fluid viscosity reduces flow rate, and efficiency drops.

    A higher fluid viscosity increases flow rate.

    When pumps are driven by an electric motor, a variable frequency (VFD) can achieve major energy savings, by reducing motor RPM when the full pump speed is not required. However, you must make sure to check compatibility: some pumps are incompatible with VFDs, while others allow speed control above a minimum RPM value.

    VFDs and other pump controls can be combined with NEMA Premium Efficiency motors to achieve the lowest possible operating cost.

    Conclusion

    Pumps have multiple applications, and they are widely used in residential, commercial and industrial buildings. As a result, a wide variety of pump designs has been developed. However, variety can also lead to confusion, and using the wrong pump for an application can cause energy waste or even equipment damage.

    To ensure that all your building systems use the correct pump types, the best recommendation is working with qualified MEP engineers. They can also help you integrate VFDs and other control features during the design phase, saving thousands of dollars in power bills over time.

    Tags centrifugal pump | pump types | positive displacement pump

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