ASHRAE Journal - April 2020 - 69

COLUMN ENGINEERS NOTEBOOK

Note that there is no decoupler,
also known as a common leg. This
ensures minimum pump energy
and maximum chilled water ∆T, as
explained in the case study building
below (See "Case Study-Northern
California University Example:
Tertiary Blending Vs. Series Building
CHW Pump" subhead on Page 74.)
A bypass around the pumps with
a check valve is provided to allow
the plant secondary pumps to meet
building chilled water demand for
buildings close to the plant that have
sufﬁcient differential pressure from
the secondary pumps.
The control logic is to ﬁrst try to
meet ﬂow demand with the plant
secondary pumps and staging on the
building pumps if differential pressure available from the campus distribution is insufﬁcient. The building chilled water pumps shut off if
the lead pump is at minimum speed
for a period of time and building
chilled water differential pressure
is above setpoint, indicating that
the campus secondary chilled water
pumps are providing the required
chilled water differential pressure.
The boosted-secondary pumping example in Figure 2 shows the
building differential pressure sensor in what the author refers to as
the "trim and respond" position.
Locating the building differential
pressure sensor near the discharge
of the building chilled water pumps
provides ﬁrst-cost and operational
beneﬁts by minimizing length of
controls wiring and locating the
sensor in a location that is easily serviceable. The data collected can also
be used to trend the overall building pressure demands, which can
be helpful for troubleshooting and
evaluating building demand history.

To efﬁciently operate the building chilled water pumps with the
building differential pressure
sensor in this conﬁguration, it is
essential that a functional "trim
and respond" algorithm5 is used
to actively reset the building differential pressure setpoint. When

FIGURE 1 Pump power equation.

P=(Q)(H )(SG )
3960()
P=Power, Horse Power
H=Head, ft
Q=Flow, gpm
SG=Speciﬁc Gravity
=Pump Efﬁciency, Decimal

FIGURE 2 Boosted-secondary pumping building connection example with building ∆P sensor in "trim and
respond" position.

VFD

VFD

Campus
CHWS

∆P (Campus

C

C
Btu Meter

Connection)

Campus
CHWR

Building
CHWS

T

∆P (Building
Demand)

C

C

T

Building
CHWR

FM

properly implemented, a "trim and
respond" algorithm uses the differential pressure sensor as a proxy for
building demand, and the physical
location that the sensor is installed
no longer becomes a factor for
determining how low the differential pressure setpoint can be reset to.
Additionally, locating the differential pressure sensor near the building chilled water pumps can reduce
the potential for "lag" between
changes in building demand and
response from the pumps and
enhance the responsiveness of the
system.
Dedicated building pumps can
also provide a margin of operational
resiliency if the campus distribution pumps are unable to or are not
being properly controlled to maintain a positive pressure differential
at the building connection. In this
scenario, a dedicated building pump

FIGURE 3 Waterside heat transfer equation. Note:
conversion coefficients are based on pure water.
Glycol mixtures will need to be corrected for
changes in fluid properties (e.g., specific heat)
based on how much glycol is used.

Q=m×cP×∆T
Q (Btu/h) = 500×gpm×∆T

in a boosted-secondary conﬁguration could be used to "assist" the
campus distribution pumps in overcoming distribution pressure losses
to a limited degree based on available motor horsepower from the
dedicated building pump.

Optimizing Chilled Water ΔT

High temperature differential
between chilled water supply and
return temperatures, ∆T, is desirable for the decreased volumetric
ﬂow rate required to deliver a given
rate of cooling energy. Figure 3 shows
the waterside heat transfer equation
ASHRAE Journal - April 2020

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