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.
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
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
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
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
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
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
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
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.
|Property || Centrifugal Pump ||Positive Displacement Pump |
viscosity range ||Efficiency decreases
with increasing viscosity ||Efficiency
increases with increasing viscosity
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. |