ASHRAE Journal - June 2014 - 69

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Figure 1: Performance comparison of constant and variable speed centrifugal chiller.

Figure 1 illustrates two important performance differences between variable and constant speed centrifugal
chillers:
1. When the entering condenser water temperature is
fixed, constant speed centrifugal chillers have a relatively flat operating efficiency over the 50% to 100% load
range while variable speed chillers see an improving
efficiency (lower kW/ton) as the load drops below 100%.
2. The efficiency of a variable speed chiller is more
positively affected by reductions in the condenser water
temperature than a constant speed chiller.
These two performance differences make imperative
a reconsideration of how a plant can be operated most
effectively when variable speed centrifugal chillers are
incorporated in the plant design. The differences can
also help explain how it is possible to configure variable speed plants that provide improved efficiency but
have about the same installed cost as constant speed
plants. This cost issue will be discussed later in this
article.

Chiller Operation
Engineers and plant operators are most familiar with
constant speed chiller operation. Because constant
speed chillers have a relatively flat performance curve
across a range of loading conditions (Figure 1), the focus
of constant speed chiller plant operations has been to
minimize the amount of on-line equipment. This permits chillers to operate as close as possible to their full
capacity to reduce the percent of parasitic load from
condenser water pumps and tower fans that are usually
sequenced with chillers. However, Figure 1 also shows
that variable speed chillers operate most efficiently well

Figure 2: The "natural curve" of a variable centrifugal speed chiller.

below full load. The efficiency improvement can be
substantial.
To determine how to sequence variable speed chillers
for their most efficient operation, a new chiller performance parameter has been developed called the "natural curve"1 of the chiller. Figure 2 illustrates the natural
curve concept. Like Figure 1, the performance curves in
Figure 2 assume a constant chilled water temperature at
all loads. However, in Figure 2 the performance curves
are for equal leaving condenser water temperature
instead of entering condenser water temperature. The
use of leaving condenser water temperature (like the
use of chilled water supply temperature) is a more useful concept in rating chillers because it compensates for
variations in condenser water flow.
For example, if condenser water flow is varied, the
performance of the chiller will remain in accordance
with Figure 2 so long as the leaving condenser water temperature is the parameter monitored to determine performance. If the flow is reduced, a lower entering condenser water temperature must be provided to maintain
the same leaving condenser water temperature. This
results in a higher log mean temperature difference
between the condensing water and the refrigerant in the
condenser that offsets the loss in water-to-tube conductance caused by the lower flow rate.
Conversely, at higher flows a lower temperature difference offsets the increase in the water-to-tube conductance due to the higher flow rate. This characteristic has
been shown to hold for chiller evaporators over ranges of
flow.2 Though the direction of heat flow is opposite in a
chiller condenser, the characteristics of the heat transfer
are essentially the same, and rating chiller efficiency
J u n e 2014

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ASHRAE Journal - June 2014

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