ASHRAE Journal - February 2024 - 40

TECHNICAL FEATURE
Variable Speed Chiller
Two decades ago, the use of VSDs in chiller
applications was restricted by various factors,
including integration issues between compressors
and VSDs, high implementation costs, inadequate
information on performance characteristics, low
reliability and undeveloped control methods.8
Moreover, phase-change processes in the refrigerant
loop complicated the situation further. However,
10
8
6
4
2
10
8
6
4
2
Variable Speed Compressor
lpm and BHP stands for brake horsepower in kW.
(1)
(2)
FIGURE 3 COP of centrifugal chiller. Left: Constant speed compressor. Right: Variable speed compressor.
Constant Speed Compressor
20
40
60
Load Factor (%)
80
100
20
40
in recent years, various chiller manufacturers have
successfully addressed these challenges and produced
and validated variable speed chillers.
As illustrated in Figure 3, for turbo (centrifugal) chillers,
constant speed chillers exhibit an increase in energy
efficiency at higher load factors. Conversely, variable
speed chillers experience a decline in coefficient
of performance (COP), peaking at a load factor of
approximately 60%. Nevertheless, when considering
the same capacity, the overall COP of variable speed
chillers is higher than that of constant speed chillers.
Additionally, their energy efficiency is particularly
impressive between 40% and 80% load factors.
Therefore, data centers that use variable speed chillers
can optimize energy consumption by ensuring that each
chiller's load factor remains between 40% and 80%,9
while also operating HSP and quantity control, which
includes redundant chillers.
Pump Inverter
Speed changes are used to predict the impact on the
performance of a centrifugal pump. As the speed varies,
the affinity laws apply to all relevant points on the pump
characteristic curve. Hence, the resulting modification
in pump performance due to a change in pump speed
can be determined from Equations 1 and 2, where Np
represents the pump speed in lpm, QP is the flow rate in
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60
Load Factor (%)
When using a variable flow chiller, the flow rate of
chilled/condenser water fluctuates in response to
the load factor. As the flow rate and
pump speed are proportional, the
pump's speed and flow rate decrease
proportionally when the load factor
decreases. Consequently, in accordance
with Equation 2, power consumption does
not decrease linearly with speed but
rather as a cube. Data centers equipped
with pump inverters can optimize energy
use by reducing the flow rate along with
the chiller's load factor. In such cases, the
80
100
HSP (cooling system) can be operated by running spare
chilled/condenser water pumps.
Fan Inverter
The implementation of VSD/VFD is equally applicable
to fans for air movement and pumps for fluid
movement, both of which are well-defined by the
affinity laws. This has already been demonstrated in
numerous studies.10 This method relies on the alteration
of the fan performance curve that occurs when the fan
speed is adjusted, which can be measured using a set
of formulas outlined in the fan law. As the fan speed
decreases, the input power also decreases proportionally
to the cube of the reduced speed. The variable speed
method permits precise airflow control that closely
corresponds to the system or load curve, enabling the
fan to achieve the desired results with minimal input
power. The fan law is described by Equations 3 and 4,
where NF represents the speed of the fan in rpm, QF
represents the airflow rate in m3/h, and HP represents
the input power in kW.
(3)
(4)
COP
COP

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ASHRAE Journal - February 2024

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