Pumps play a fundamental role in all types of buildings, since they simplify the movement of fluid substances. Most pumps in residential and commercial locations handle water, and they have applications in many building systems: fire pumps provide a pressurized water supply for firefighters and automatic sprinklers, booster pumps deliver potable water to upper floors in high-rise constructions, and hydronic pumps are important in HVAC systems that deliver heating and cooling with water.

There is a large variety of pump designs, but most types can be classified into centrifugal pumps and positive displacement pumps. This article provides an overview of each type and compares performance features.


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Centrifugal Pumps

Centrifugal pumps use an impeller, which has curved blades that accelerate fluids towards their edge when rotating. The impeller is normally driven by an electric motor or combustion engine, and its movement produces suction at the pump inlet, drawing water inside.

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

SUBTYPE

DESCRIPTION

PERFORMANCE

Axial Flow

Produces flow along the same direction as the impeller shaft, and is also known as propeller pump.

High flow rate
Low pressure

Radial Flow

Produces flow perpendicular to shaft (90° angle).

Low flow rate
High pressure

Mixed Flow

Combines radial and axial flow, producing a conical flow pattern around the impeller 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 be the most affordable, since mass production has brought down their price.

Positive Displacement Pumps

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 the fluid is displaced, positive displacement pumps can be reciprocating or rotary.

SUBTYPE

DESCRIPTION

EXAMPLES

Reciprocating

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

Bladder
Diaphragm
Peristaltic
Piston / plunger

Rotary

Uses a rotor that traps water in cavities, releasing it at the pump 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

Comparison Between Centrifugal and Positive Displacement Pumps

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

OPERATING CONDITIONS

CENTRIFUGAL PUMP

POSITIVE DISPLACEMENT PUMP

Effect of system pressure

Flow is reduced when system pressure increases.

Constant flow. If system pressure increases, the pump produces more pressure as well.

Effect of viscosity

Higher fluid viscosity reduces flow rate. Pumping efficiency drops as viscosity increases.

Higher fluid viscosity increases flow rate.

When pumps are driven by an electric motor, a drastic efficiency increase may be possible with a variable frequency drive (VFD), a device that reduces motor RPM when the full speed is not required. Just make sure you check compatibility with the type of pump you are using: some models are incompatible, and others allow the use of VFDs as long as motor speed is kept above a specified RPM value.

Pump controls can be complemented with NEMA Premium Efficiency motors to achieve the lowest possible operating cost.

Conclusion

Pumps are among the most used devices in modern society, performing various functions in residential, commercial and industrial locations. As a result, a wide range of pump designs has been developed. However, variety can also lead to confusion, and using the wrong pump type for an application can cause energy waste or equipment damage.

To make sure all your building systems use the correct pump types, the best recommendation is working with qualified design professionals. This also allows you to integrate VFDs and other control features during the design phase, saving thousands of dollars in power bills over time.

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