ASHRAE Journal - June 2014 - 70

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based on leaving condenser water temperature eliminates the need to make adjustments for changes in the
rate of water flow through the condenser over a range of
possible flow rates.
As shown in Figure 2, the natural curve of a variable
speed chiller when operating with a fixed chilled water
supply temperature is a simple concept. It is the locus
of points of highest chiller operating efficiency at various condenser water temperature and load conditions.
Notice that the best efficiency (lowest kW/ton) for the
variable speed chiller when the leaving condenser water
temperature is 85°F (29°C) is achieved at about 63%
load. This is the point on the natural curve for that condensing water temperature. The natural curve is developed by connecting the points of highest efficiency for
each condensing water temperature.
A similar natural curve can be constructed for applications where chilled water supply temperature is
variable. Because the natural curve of a chiller represents the highest operating efficiency curve of the
chiller, it is clear that for maximum chiller operating
efficiency, chillers should be sequenced to operate
as closely as possible to their natural curves. Figure
2 shows that the natural curve, and therefore, the
loading at which optimum operating efficiency is
achieved for a variable speed chiller, is not at or near
its full capacity at any condenser water temperature
condition.
Figure 2 is typical for many variable speed chillers.
Therefore, to operate variable speed chillers most efficiently, the sequencing methodology needs to be different
than that normally employed for constant speed chillers. This is one reason why variable speed chillers do not
achieve optimum performance when mixed with, or operated the same as, constant speed chillers in a chiller plant.

Condenser Pumps and Cooling Towers
Another way to improve variable speed chiller efficiency is to take advantage of the fact that the performance of variable speed chillers is more sensitive to
reductions in condenser water temperatures (Figure
1). A significant amount of research and development
has been done on tower optimization strategies.3,4,5
However, there has not been a strong emphasis on operating cooling tower fans and condenser pumps more
effectively in multi-tower plants. This is because constant speed chillers are not as significantly affected by
70

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Figure 3: Effect of wet bulb on cooling tower approach.

Figure 4: Comparison of conventional and variable speed condenser pump and
tower fan operation.

reductions in condenser water temperatures as are variable speed chillers.
Currently, most chiller plant operating strategies seek
to keep the water flow rate through condenser bundles
constant. Before network-based digital controls were
available, constant condenser water flow was desired
because chiller controls were reactive, and changes in
condenser conditions could affect the stability of the
stand-alone chilled water temperature control provided
with each chiller.
The operation of tower fans varies from project to project. Many chiller plants operate tower fans to provide a
specific tower leaving water temperature. When variable
speed chillers are applied, the efficiency improvements
that can accrue from lower condensing water temperatures have led some to recommend a low leaving tower
water temperature setpoint as an optimization strategy.
Others adjust the leaving tower water setpoint based
on outdoor wet-bulb temperature or other criteria.
However, nearly all tower fan and condenser pumping
strategies shed condenser pumps and tower fans, usually in parallel with the shedding of chillers, as the load

ASHRAE Journal - June 2014

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