ASHRAE Journal - March 2021 - 46

COLUMN ENGINEER'S NOTEBOOK

tank. Using this strategy to alleviate
the symptoms of low CHW distribution pressure can work as a temporary
measure; however, it is not an ideal
long-term solution, as throttling the
building's CHW return could reduce
the peak chilled water flow capacity available to the building. It can
potentially also place a higher load on
the campus distribution pump, if the
building happens to be a critical path
for setting system pumping pressure
requirements.

FIGURE 2 Facility CHW pipe pressure analysis.

Node #2C: CHW Piping Elevation
At ~12 ft Above Roof Elevation
Automatic Air Vent

CC-1A
CC-1B

CHW Control Valve

CC-1C
Node #2B: CHW Piping Elevation
At ~12 ft Above Roof Elevation
Node #1:
Roof
Elevation
At 45 ft
Above
Grade

CC-2A
CC-2B
CC-2C
Node #2: CHW Piping WYE Before
Branches Take Off to Serve Each AHU
Roof

Next Day-Digging Deeper for
Root Causes

Third Floor

Looking deeper into the cause
of the issue, we investigated three
areas:
* The rooftop air vents that allowed air into the pipes;
* The reason for the drop in campus supply pressure; and
* The failure of the expansion
tank buffer to compensate.

Air Vent Investigation

Second Floor
Campus
CHWS
Campus
CHWR

Grade

Campus Pressure Drop Investigation
Why did the campus CHW pressure to the research
facility suddenly drop? In other words, how was our
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Underground Level 1

Assumptions:
1. Central plant chilled water system makeup water point of connection is located at ~4 ft (1.2 m) above grade and located in
close proximity to the inlets of the campus distribution pumps with negligible pressure loss between the point of connection
and the pump inlets.
2. The grade elevation for the central plant is approximately equal to the grade elevation for the building analyzed within this
case study.
Hydraulic Pressure Analysis:
Inputs: Roof elevation is at 45 ft (13.7 m) above grade; AHU curb height is 1 ft, 6 in. (0.46 m); AHU topmost coil elevation is
~10 ft, 8 in. (3.3 m) above the curb; makeup water point of connection is located at ~4 ft (1.2 m) above grade.
Static Hydraulic Pressure: 45 ft (13.7 m) building height + 1.5 ft (0.46 m) AHU curb + 10.7 ft (3.3 m) top coil elevation above
curb - 4 ft (1.2 m) makeup water point of connection = ~53.2 ft (16.2 m).
Convert to Pressure: 53.2 ft/2.31 ft/psi = ~23 psi
16.2 m/10.2 m/bar = ~1.6 bar
Notes:
1. Best practice is to provide additional pressure at the topmost hydraulic point to provide a margin of safety to maintain
positive conditions during normal operation, typically ~5 psi (0.35 bar).
2. ~23 psi + ~5 psi = ~28 psi (~1.6 bar + 0.35 bar = ~1.95 bar) anticipated required makeup water pressure at central
plant chilled water system makeup water point of connection.

Even with the drop in campus pressure, why did the air vents allow air
into the pipes, essentially operating
opposite to their function? Were they
faulty devices, and, if so, why did all
of them fail in the same way?
We met with the manufacturer's
representative and studied the product literature, and it turns out that
many air vent products on the market are designed to
allow air to enter the pipe when the pipe gauge pressure
goes negative; this is a convenience feature to assist operators in draining a pipe. The vent design assumes that in
normal operation the pipe pressure will always be positive
and that a relative vacuum only occurs when an operator
is attempting to drain the pipe.

First Floor

MARCH 2021

team able to fill and burp the lines during initial startup a year before, and what caused the issue to surface
only now?
Investigating the campus setup, we discovered that
the industrial cold water makeup line that sets the central plant's CHW distribution pipe pressure also serves
as the makeup water line for the central plant's cooling towers. For several days leading up to the recordsetting hot weather day, the outdoor conditions were
exceptionally warm, causing evaporation at the cooling
towers and triggering a consistent demand for substantial amounts of makeup water. The diverted water

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ASHRAE Journal - March 2021

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