ASHRAE Journal - March 2021 - 45

COLUMN ENGINEER'S NOTEBOOK

representing the AHU cooling coils
FIGURE 1 Facility AHU CHW diagram with observation notes.
that we checked in turn.
The following is a summary of our
1, 2
1, 2
Automatic
Air
Vent
initial observations:
CC-1A
CHW Control Valve
* AHU-1 was providing ~67°F
CC-1B
(~19°C) supply air.
CC-1C
* AHU-2 was providing ~60°F
AHU-1
(~16°C) supply air.
1, 2
1, 2
LAT:~67°F
* CHW control valves for both
CC-2A
AHUs were found to be fully open, atCC-2B
tempting to reach the setpoint of 55°F
CC-2C
(12.8°C).
AHU-2
* CHW air vents at both AHUs apLAT:~60°F
peared to be allowing air to ﬂow into
the pipe, acting like a vacuum breaker
Roof
instead of an air vent.
Third Floor
* Cooling coil CC-1A appeared not
Second Floor
to be cooling the air; it was warm to
Grade
First Floor
the touch.
Campus
CHWS
* Cooling coils CC-1B and CC-1C
Campus
CHWR
Underground Level 1
appeared to be actively cooling and
were cool to the touch.
Notes:
* Cooling coil CC-2A appeared to
1. Control valves found in the full-open position with the air vents allowing air to ﬂow into the pipe, acting like a vacuum
breaker instead of an air vent.
be less effective at cooling than coils
2. When control valves were manually throttled toward the closed position, the air vents transitioned from sucking air into
CC-2B and CC-2C.
the pipe to pushing bubbles of air out from the pipe
* Cooling coils CC-2B and CC-2C
plant, as they had their hands full supporting numerappeared to be actively cooling; these were cool to the
ous facilities in the record heat. To provide an immeditouch.
Based on these observations, we hypothesized that air ate stopgap solution for the research facility, our team
performed the same manual throttling procedure at
might be present in the topmost coils, partly displacthe chilled water return isolation valve at the building or blocking the ﬂow of chilled water-or entirely
ing's connection to the campus distribution line; this
blocking it in the case of CC-1A. To test this hypothachieved the similar effect of building up gauge presesis, we manually adjusted the chilled water control
valves toward the closed position, increasing the gauge sure in the CHW piping on the roof, keeping the coils
free of air and allowing the facility's AHUs to function
pressure in the chilled water pipe. This caused the
at design capacity.
automatic air vents to transition from sucking air to
This stopgap gave the campus operators time to
venting water and air bubbles. The observation was
repeatable, suggesting that insufﬁcient gauge pressure investigate the system pressure and make modiﬁcawithin the campus chilled water piping was the culprit. tions and adjustments back at the central plant, while
allowing the critical research facility to continue
This allowed air to enter the highest-elevation chilled
water coils, signiﬁcantly diminishing the AHU's cooling operation.
Readers who are familiar with thermal energy storage
performance.
systems may point out that throttling the building's CHW
Emergency Stopgap Measure
return line connection to the campus distribution piping
At this point it was approaching midday, and the
is essentially what a pressure sustaining valve does in sitcentral plant operators were unavailable to investigate uations in which CHW components are located at elevaand adjust available system pressure at the central
tions above the height of an open thermal energy storage
MARCH 2021

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