ASHRAE Journal - June 2014 - 30

Column EnginEEr's notEbook
Steven T. Taylor

How to Design & Control
Waterside Economizers
By Steven t. taylor, P.e., Fellow aSHrae

ASHRAE/IES Standard 90.11 requires either airside economizers or waterside economizers for most cooling system applications. Air economizers are more common and
almost always more energy efficient. But there are some applications where water
economizers are preferred, such as:
* High-rise office buildings with floor-by-floor air
handlers where access to outdoor air is architecturally
limited.
* Large data centers, where air economizers are not
always physically practical because airflow rates are so
high, or air economizers are precluded by concerns (valid or not) that they can increase particle contamination
risk and cause low humidity levels. Water economizers
can also provide energy savings on par with air economizers on data centers that have hot aisle containment
and warm supply air temperatures, e.g., "cold" aisle supply air temperatures over 80°F (27°C) and hot aisles over
100°F (38°C). These warm air temperatures, in turn, allow chilled water supply temperatures over 65°F (18°C),
readily attainable by cooling towers for most of the year
in most climates.
* Small fan coil systems where airside economizers are
physically impractical or overly expensive.
* Chilled beams, radiant cooling, and other hydronic
systems for which airside economizers do not apply.
Here are some tips for designing cost effective and
energy efficient waterside economizers:

System Design & equipment Selection
* To maximize performance (and also to meet ASHRAE
Standard 90.1 prescriptive requirements), the economizer must be "integrated" with the chillers, meaning the
economizer has to be able to reduce the load on the chillers even if it cannot handle the entire load. This means
the heat exchanger must be in series with, and upstream
of, the chillers as shown in Figure 2, rather than in parallel
with the chillers Figure 1. In the series position the economizer can pre-cool the return chilled water to reduce the
30

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J u n e 2014

load on the chillers even when the chillers must run to
deliver the desired chilled water temperature. The first
costs are basically the same as a nonintegrated economizer, but the number of hours the economizer can be
operational is vastly extended, significantly improving
energy performance and cost effectiveness. Integrated
economizers are also easier to control, as described in
more detail later. There are few, if any, advantages to
nonintegrated economizers.
* As shown in Figure 2, the same cooling towers and
condenser water pumps should be used to serve both
the economizer heat exchanger and the chiller condensers. Some designers provide separate towers and pumps,
at considerable expense because of concerns about low
chiller head pressure due to low condenser water supply temperatures. But head pressure control is easily and
inexpensively addressed by making the condenser isolation valves modulating and controlling them off the head
pressure control signal output that is standard on most
chiller controllers. The valves throttle flow through the
condenser as needed to maintain chiller minimum lift
regardless of how cold the condenser water supply temperature is in economizer mode.
* The capacity and quantity of cooling towers and condenser water pumps remains the same as they would be
without the economizer. For office building applications,
this is intuitively clear: we know that when the economizer
is on, weather will be cold so loads will be well below design
loads; hence only one of the two chillers (in the example
shown in Figure 2) will be needed, freeing the other to supSteven T. Taylor, P.E., is a principal of Taylor Engineering in Alameda, Calif. He is a member of SSPC 90.1 and chair of TC 4.3, Ventilation Requirements and Infiltration.



ASHRAE Journal - June 2014

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Contents
ASHRAE Journal - June 2014 - Cover1
ASHRAE Journal - June 2014 - Cover2
ASHRAE Journal - June 2014 - 1
ASHRAE Journal - June 2014 - 2
ASHRAE Journal - June 2014 - Contents
ASHRAE Journal - June 2014 - 4
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