ASHRAE Journal - March 2021 - 47

COLUMN ENGINEER'S NOTEBOOK

FIGURE 3 Campus central plant industrial makeup water system sketches: existing (top) vs. solution implemented by owner (bottom).

Condenser
Evaporator
Condenser
Evaporator
Condenser
Evaporator
ET

CT Makeup Water
Solenoid

PRV

ICW
Campus
CHWS
Campus
CHWR

Existing Central Plant Makeup Water System

1

Condenser
Evaporator
Condenser
Evaporator
Condenser
Evaporator

caused the pressure in the CHW
lines to fall from the design pressure of ~30 psi or ~2.1 bar (207 kPa)
to as low as ~20 psi or ~1.4 bar
(138 kPa), per observations from the
plant operators on the day of our
hot call. Figure 2 includes our pressure analysis.
Per our analysis, an industrial
cold water makeup pressure of
~30 psi or ~2.1 bar (207 kPa) at the
central plant is sufficient to permit
the AHUs at the research facility to
bleed and burp lines to purge air
from the CHW piping. However,
when the makeup water pressure
at the central plant drops to less
than ~23 psi or ~1.6 bar (159 kPa),
then one of two conditions are
present for air to enter the lines
and degrade the chilled water coil
performance. (See the Expansion
Tank Buffer Failure section below
for the second condition.)
Our hydraulic model of the building also shows that under peak
flow conditions, AHU-1 requires an
additional ~1 psi or ~0.1 bar (7 kPa) of
distribution pressure in the pathway
from Node 2 to Node 2-A when compared to the pathway from Node 2
to Node 2-B, which may explain the
biased accumulation of air observed
within AHU-1.

CT Makeup Water
Solenoid
ET

2

PRV

ICW

PRV

ICW
Campus
CHWS
Campus
CHWR

Central Plant Makeup Water System Revision Implemented By Owner

Expansion Tank Buffer Failure
Why did the drop in CHW makeup water pressure
affect the system pressure? Shouldn't the expansion
tank have provided a buffer against those fluctuations?
Under normal circumstances, it is expected that a sudden or temporary reduction in available makeup water
pressure would not impact the operation of the closedloop chilled water system, because the expansion tank
would maintain pressure, and the backflow prevention
valve would prevent water from leaving the system
through the makeup line.
Discussion with the central plant operators

disclosed that an adjacent building project caused a
significant loss of chilled water volume on the day the
research facility suffered the loss of cooling capacity.
The combination of water volume loss with reduced
available makeup water pressure yielded the hydraulic conditions causing the symptoms observed in this
case study.

Recommendations
As described earlier, our interim solution to increase
chilled water pressure at the building is ill-suited to
a long-term resolution of the problem. Other taller
buildings are also on campus that will eventually
MARCH 2021

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ASHRAE Journal - March 2021

Table of Contents for the Digital Edition of ASHRAE Journal - March 2021

Contents
ASHRAE Journal - March 2021 - Intro
ASHRAE Journal - March 2021 - Cover1
ASHRAE Journal - March 2021 - Cover2
ASHRAE Journal - March 2021 - 1
ASHRAE Journal - March 2021 - Contents
ASHRAE Journal - March 2021 - 3
ASHRAE Journal - March 2021 - 4
ASHRAE Journal - March 2021 - 5
ASHRAE Journal - March 2021 - 6
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ASHRAE Journal - March 2021 - Cover4
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