adoption in the early 1960s when government buildings in Washington, D.C.,
embraced them for creating cold water
for perimeter induction cooling. Induction
systems supply cold primary air to the
space, requiring colder water from the
chiller. Chillers in those days had a coefficient of performance (COP) of about
4.0, with high-flow (velocity) smooth-bore
tubes, low tube-counts and one-pass
evaporators to reduce pressure drop.
In the 1970s, variable-air-volume (VAV)
systems made the colder chilled water
used for induction systems unnecessary.
Why Series? Given the chiller’s relatively low efficiency
Multiple-stage or series chillers pro- by today’s standards, it made sense to
vide rigorous, stable cooling at extreme raise temperatures.
conditions. Series chillers are standard VAV systems were widely adopted
chillers that are piped or lined up in a because they save energy and adapt to
series, which allows the system to use less unknown cooling loads. VAV systems are
energy to cool. still the most popular choice for delivering
Figure 1. Series counterflow chiller arrangement equalizes lift performed by each compressor,
minimizing the energy needed to create high lift.
gal, screw and absorption chillers; and
air-cooled and water-cooled chillers. The
district cooling business model is also
key – overall chiller plant efficiency goes
directly to the bottom line.
More and more chiller plants are
selecting low-flow, low-temperature, yet
highly efficient chillers (see sidebar).
Because pump energy is proportional to
the cube of the flow, even incremental flow
reductions quickly result in net positive
cash flow. But when flow goes down, temperature must also, favoring series chillers.
A typical maximum acceptable
chiller pressure drop is 25 ft of water.
Even after adding more tubes to reduce
pressure drop, two chillers in series might
use twice that, because each chiller has
twice the amount of water going through
it. In traditional primary-secondary systems, constant flow through the chillers
equals constant pressure drop and a constant pump energy.
Today, variable-primary systems
send variable amounts of water flow
through the chillers to limit the effects of
pressure drop at many load conditions.
Variable-primary systems are made possible by the latest chiller technology and
control advancements. Series evaporators
reduce the need for a bypass in variable-primary systems because the higher initial water flow allows for higher flow
reductions before reaching the minimum
flow for the chiller.
Courtesy Trane Commercial Systems.
When chillers are lined up in parallel,
each individual chiller must provide the
coldest water required for the entire system. In series, each subsequent chiller in
the process can operate more efficiently
and provide colder water. It also uses less
energy for high ambient wet-bulb conditions, which are common in the Middle
East and China.
Common reasons why some designers do not go with series are fear of
something ‘new,’ lack of redundancy and
higher water-pressure drop. All of these
reasons are well-understood, and current
chiller designs compensate appropriately.
conditioned air; however, series chillers
offer additional benefits because chillers
have almost doubled in energy efficiency.
History of Series Chillers
Series chillers saw widespread
System designers are finding that
large chiller plants can be more adaptive
and efficient by installing multiple chillers
rather than one or two large, field-erected
chillers. In plants with more chillers,
redundancy is easily created through
parallel banks of upstream and downstream chillers. Different amounts of
upstream and downstream chillers can
meet the load, so if one chiller is being
serviced, its duty can be spread out to
many other chillers.
Chiller Technology and Its
Impact on Design
The most efficient centrifugal
chillers today have COP of more than 7.0
– which is more than 75 percent higher
than chillers used in the first series
Chiller technology has had a sizeable
impact on the design of chilled-water
plants. The chiller’s responsive control
and multiple-stage stability allows for
more versatile designs. One example is
variable flow through standard centrifugal
chillers, a design discouraged by manufacturers just a few years ago.
Today’s advanced controls offer features that incorporate improvements to
chiller capabilities and variable speed
pumps. Improved tube designs and
extensive testing in manufacturers’ testing
labs have cut minimum water velocities in
half, leading to better turndown and higher
pump energy savings.
Chiller efficiency is dependent upon
several variables – two of them are capacity
(tons) and lift (chiller internal differential
temperature). Multiple-stage centrifugal
chillers have the ability to create high lift,
which is roughly equivalent to the difference between the leaving condenser and
leaving evaporator temperatures.
Series or Parallel?
Chiller plants with series evaporators but parallel condensers are not rec-