ASHRAE Journal - June 2020 - 11

LETTERS

Engineers
Notebook:
Optimizing
Campus
Chilled Water
Connections
Thank you to author Nathan Ho
for highlighting the difﬁculties in
achieving high chilled water return
(CHWR) temperatures, or high ∆T,
in a campus district cooling system
(April's "Optimizing Campus Chilled
Water Connections." However, I take
exception to the conclusions that
the "lazy river" conﬁguration is the
best solution to "promote the lowest
campus pumping energy, highest ∆T
and greater operational ﬂexibility
and resiliency."
While, the chilled water ∆T "death
spiral" (when a ∆T control valve is
used to control the building's CHWR
temperature) is a real condition
and is accurately described by Mr.
Ho, there are measures that could
have been taken prior to modifying the existing piping since all the
components were installed already.
Speciﬁcally, the building CHW supply sensor should be used as an
upper limit for the control valve
sequence to prohibit loss of building
temperature and humidity control
especially in part-load conditions.
For example, set the primary
control parameter of the ∆T control
valve to the desired CHWR temperature (~58°F), but have a secondary
control parameter allowing the valve
to throttle until the building CHW
supply temperature increases to a
point where building cooling is lost.
This setpoint is adjustable, but is

ENGINEERS NOTEBOOK
COLUMN

Nathan Ho

Optimizing Campus
Chilled Water
Connections
Campuses and institutions often use centralized plants with chilled water distribution infrastructure to provide cooling to multiple buildings for the various operational and packaging beneﬁts that come with a central utility plant scheme. The way
these buildings connect to the chilled water distribution infrastructure can have a
signiﬁcant impact on the performance of the central plant and on the HVAC systems
in the building. Although several articles have been written about this topic over the
years,1 - 4 the author has recently observed examples where campus chilled water
design fundamentals have been misapplied, so our design community may beneﬁt
from a refresher.
Chilled water distribution pumps located at the central plant are often the largest consumers of pump
energy on a campus due to the entire campus chilled
water ﬂow moving through them. Inspecting the pump
head equation in Figure 1 reveals that an effective way to
reduce their power draw and overall energy consumption is to lower their operating pressure requirements.
This is where design of campus chilled water building
connections comes in.
The most efﬁcient campus pumping scheme involves
operating the central plant chilled water distribution pumps with just enough pressure to overcome
the friction and ﬁtting losses required to pump water
through the distribution piping with a small margin

Steve Tredinnick, P.E.,
Member ASHRAE, Lisle, Ill.

The Author Responds

BY NATHAN HO, P.E., ASSOCIATE MEMBER ASHRAE

Optimizing Pumping Energy Performance

of the alternative technical resources
available to offer guidance.

of additional pressure to achieve a slight positive
pressure delta at the most remote building connections. This approach is described by this author as the
"lazy river" strategy and relies on booster pumps to be
provided at buildings that require more head pressure than the coincident campus distribution pressure available at their connection. Figure 2 shows an
example of a boosted-secondary pumping building
connection. These building booster pumps are effectively operating in series with the central plant chilled
water distribution pumps and are controlled to meet
the pump head pressure requirements speciﬁc to their
local HVAC loads.
Nathan Ho, P.E., is an associate principal and engineering group leader with P2S Inc., in Irvine,
Calif. He is the chair of ASHRAE SPC 110, vice chair of TC 4.3, and member of TC 9.10.

generally ~48°F, depending on the
amount of outside air that is introduced at the air-handling units, etc.
This enhancement to the sequence is
explained in more detail in the 2nd
edition of the ASHRAE District Cooling
Guide and the ASHRAE Owner's Guide
for Buildings Served by District Cooling.
The above enhancement does
not magically increase the CHWR
temperature to the intended design
condition, but is good for at least 2°F
improvement in building ∆T without
drastically affecting the building's
ability to cool. The simple matter is, if
the coil leaving water temperature was
close to the design parameters, the ∆T
control valve would not be required at
all and everyone would be happy.
While I used to love the "lazy
river" ride at water parks when my
kids were younger, I think there
are proven alternatives for campus
building chilled water interconnections. The decoupler could have
been retained and the control valve
sequences been revised. Finally, there
is no panacea or "easy button" for low
CHW ∆T since each campus/system is
unique and may require unique solutions, and designers should be aware

Thank you for sharing your
thoughts.
Evaluation of the fundamental
equations of coil heat transfer presented in the article reveals that
supplying the coldest available water
to a chilled water coil will provide
the highest-possible ∆T performance
for a given set of airside conditions.
This is directly accomplished by
eliminating the blending of warmer
building chilled water return to
raise the building chilled water supply temperature supplied to chilled
water coils; this will not only achieve
better performance, but also yield
more reliable operation as there
are no blending control valves that
could fail or have their operational
parameters adjusted over time.
Recirculation of water requires
additional pumping energy, and
blending the building chilled water
temperature upward will result in
an increase in ﬂow rate through the
coils, which increases building pumping energy consumption; please keep
in mind that lower coil ∆T requires
higher ﬂow to meet a given thermal
demand. Eliminating the decoupler
(aka bridge or common leg) at the
building connection to the campus
distribution places the plant and
building pumps in series, which further reduces campus pumping energy
by enabling buildings that are closer
to the campus plant to use the coincident available campus distribution
pressure rather than wasting available
plant pump head by throttling valves.

J U N E 2020

Nathan Ho, P.E., Member ASHRAE, Irvine, Calif.
ashrae.org

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ASHRAE Journal - June 2020

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