What is the difference between Centrifugal and Positive Displacement Pumps?

Most chillers need a pump, but many people are unaware of why, or which pump to choose. This guide has been developed to support you in making an appropriate choice for your requirements.

Centrifugal Pumps

What is a Centrifugal Pump and How Does It Work?

Centrifugal pumps are designed to move fluid through the transfer of rotational energy from one or more driven rotors, known as impellers. Impellers consist of several curved vanes.

They are designed for use with liquids with relatively low viscosity, that pour like water or a very light oil. Centrifugal pumps may be used with slightly more viscous liquids at temperatures closer to ambient (or above), but additional horsepower must be added, as centrifugal pumps become less efficient with even minimal increases in viscosity. Centrifugal pumps will also require an increase in horsepower when pumping liquids that are more dense than water.

Fluid enters the impeller at its axis (also known as the eye). The impeller is situated on the opposite side of the pump to the eye and is connected, through a drive shaft, to a motor, which is rotated at high speed. This rotational movement forces fluids out through the impeller vanes and into the pump casing.

There are two main types of pump casing design for centrifugal pumps – volute and diffuser casings. Applied Thermal Control use diffuser casings for all units requiring a centrifugal pump. In a diffuser casing, fluid pressure increases as fluid is expelled between a set of stationary vanes surrounding the impeller.

What Are the Benefits of a Centrifugal Pump?

Centrifugal pumps are usually specified for higher flows and for pumping lower viscosity liquids.

It is possible to “throttle” the flowrate of a centrifugal pump over a wider range. This can be done through using a discharge valve (less energy efficient option) or a Variable Frequency Driver (VFD) to slow the pump and motor speed down. However, throttling has limitations. Centrifugal pumps should not be throttled below the ‘minimum safe flowrate’ as indicated by the manufacturer for longer than a minute or so. If this is ignored, recirculation within the pump may cause excessive heat build-up. Too much throttling can also cause excessive shaft deflection, increasing wear on bearings and seals in the pump.

What Are the Limitations of a Centrifugal Pump?

The efficient operation of centrifugal pumps relies on the constant, high speed rotation of the impeller.

Centrifugal pumps function most efficiently at the centre of the curve. If operating too far to the right or left, it is likely that pump life will reduce due to shaft deflection or increased cavitation. If operating a centrifugal pump at any point other than the Best Efficiency Point (BEP), a positive displacement pump should be considered. When operating a centrifugal pump outside of its BEP, a larger motor will be required, increasing initial cost and energy consumption costs.

Centrifugal pumps are better suited to low pressure, high capacity, pumping of low viscosity liquids. High viscosity oils will cause excess wear and overheating to centrifugal pumps, leading to damage and premature failure.  

With high viscosity fluids, there is greater resistance and higher pressure is needed to maintain a specific flow rate, which can make centrifugal pumps inefficient.

Centrifugal pumps are unable to provide suction when dry so must always be primed with the pumped fluid. This means that centrifugal pumps are not suited to applications where flow is not continuous.

Centrifugal pumps may be unsuitable in cases where consistent flow is important – if the pressure within the circuit is variable, a centrifugal pump will produce a variable flow.  

Positive Displacement Pumps

What is a Positive Displacement Pump and How Does It Work?

Positive displacement pumps are designed to move fluid by repeatedly enclosing a fixed volume of fluid and transporting it through the system mechanically. Positive displacement pumps used by Applied Thermal Control move fluid through a cyclic pumping action driven by rotary vanes. Other designs of positive displacement, or PD, pump may be driven by pistons, screws, gears, rollers or diaphragms.

Most positive displacement pumps can be placed into two categories:

  • Reciprocating Positive Displacement Pumps
  • Rotary Positive Displacement Pumps

Reciprocating positive displacement pumps work through cycles of reciprocation, where pistons, plungers or diaphragms move backwards and forwards. A really simple example of a reciprocating pump can be seen in a bicycle pump.

Rotary positive displacement pumps rotating cogs, gears or vanes to move fluids rather than a backward and forward motion. The rotating element of the pump forms a liquid seal with the pump casing and allows suction to be created at the inlet of the pump. This causes fluid to be drawn into the pump, where it is enclosed within the cogs, gears or vanes and transferred to the discharge. Applied Thermal Control use rotary vane pumps in units that require a positive displacement pump. Rotary vane pumps use a set of moveable vanes that are mounted in an off-set rotor. The vanes are able to maintain a close seal against the casing wall and effectively transfer the trapped fluid to the discharge port. The pumps within Applied Thermal Control units are self-priming, constructed of stainless steel, and produce low levels of vibration and pulsation.

What are the benefits of a positive displacement pump?

Positive displacement pumps are better suited to handling higher viscosity fluids and are able to operate at high pressures and relatively low flows efficiently. Where centrifugal pumps tend to lose flow as fluid viscosity increases, the flow of a positive displacement pump increases. This happens because higher viscosity fluids are better able to full the clearances of the pump, resulting in higher volumetric efficiency.

Where metering is an important factor, positive displacement pumps are more accurate.

Positive displacement pumps produce a relatively consistent flow, regardless of pressure. This makes them ideal for applications with variable pressure conditions.

Positive displacement pumps can be operated at any point of the curve, with the volumetric efficiency improving at higher speed portions of the curve.  This occurs because volumetric efficiency is affected by slip. At low speed, the percentage of slip is higher than at high speeds.

Positive displacement pumps are better suited to pumping shear sensitive fluids than centrifugal pumps, especially when operating at low speeds.

Because positive displacement pumps create a vacuum, they are capable of creating a suction lift. This means that the chiller does not have to be on the same level as the application, making a positive displacement pump a versatile option.    

What are the limitations of a positive displacement pump?

Positive displacement pumps are generally more complex than centrifugal pumps in their construction, making them more difficult to maintain.

Positive displacement pumps are unable to generate the high flow rates that are characteristic of centrifugal pumps.

Due to the cyclic action of reciprocating pumps, the fluid may pulse, accelerating during the compression phase and slowing down during the suction phase. As a result of this, vibrations can occur within the water circuit, which can impact the performance of the application. For example, in electron microscopy, where pulsing can impact image quality. This can be reduced through employing some form of damping or smoothing, or by using two (or more) pistons, plungers, or diaphragms going through alternating phases of reciprocation.

Due to the high pressures created by positive displacement pumps, there should be some form of pressure relief on either the pump or discharge line in case of blockage. All ATC chillers manufactured with a positive displacement pump has an adjustable bypass as standard.

Comparison Table

Property  Centrifugal Pump Positive Displacement Pump
Effective viscosity range                              Efficiency decreases with increasing viscosity                                                   Efficiency increases with increasing viscosity
Pressure tolerance

Flow varies with changing pressures

Efficiency decreases at both lower and higher pressures

Flow insensitive to changes in pressure

Efficiency increases with increasing pressure

Priming Required Not required
Flow (at constant pressure) Constant Pulsing
Design Choice at ATC                                            At ATC, we use centrifugal pumps with a diffuser casing in our designs At ATC, we use rotary vane positive displacement pumps in our units.

Applied Thermal Control Ltd.
39 Hayhill Industrial Estate, Barrow upon Soar, Leicestershire, LE12 8LD.
Telephone: +44 (0) 1530 83 99 98
E-Mail: sales@app-therm.com
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