ASHRAE Journal - April 2020 - 74

COLUMN ENGINEERS NOTEBOOK

increased, the chilled
water return temperature
will also eventually begin
to increase after the coil
is no longer able to maintain its design leaving air
temperature. At this point,
the coil cooling capacity
becomes diminished as
the chilled water return
temperature rises.
If operational load
conditions exist where a
higher leaving air temperature is acceptable,
supplying colder chilled
water will achieve higher
coil ΔT than supplying
warmer chilled water
(Figure 11, Page 73).

FIGURE 12 Building campus chilled water diagram with initial observations recorded.

VFD

Status: Off
Speed: 0.0%

T

Campus
CHWS

ΔP

Campus
Supply
Temp:
41.0°F

T
VFD

Reading:
~0 to 1 psi

Campus
CHWR

T

C

Status: Running
Speed: 67.5%

ΔP

Btu Meter
Campus
Return
Temp:
59.0°F

Reading: 58.0%

C

AHU-1
Control
Valve Position:
100%
Open

FM

Campus:
CHW FLow:
0.0 gpm

Campus CHWR Valve
Overridden to 100% Open

T

C

C

Setpoint: 10.0 psi
Reading: 9.9 psi

AHU-2
Control
Valve Position:
100%
Open

Setpoint: 60.0°F
Reading: 58.3%

FIGURE 13 Reconfigured building campus chilled water connection with resulting performance observations.

Case Study-Northern
California University
Example: Tertiary Blending
Vs. Series Building CHW
Pump

VFD

Status: Off
Speed: 0.0%

T

Campus
CHWS

Campus
Supply
Temp:
41.3°F

T
VFD

ΔP

Reading:
~0 to 1 psi

Campus
CHWR

Btu Meter
Campus
Return
Temp:
62.3°F

T

FM

Campus:
CHW Flow:
250 gpm

Bypass
Removed

Reading: 41.5%

C

C

Status: Running
Speed: 86.0%

ΔP

Setpoint: 15.0 psi
Reading: 15.1 psi

C

C
AHU-1
Control
Valve Position:
96.5%
Open

AHU-2
Control
Valve
Position:
58%
Open

T
The author recently had
Campus CHRW Valve
the opportunity to trouManually Locked 100% Open
bleshoot cooling issues
at a building using a campus chilled water decoupled
existing bookstore building connection to the campus
direct connection. He observed firsthand the significhilled water system.
cant impact the design and operation of a building
The initial site observations and diagram of the building
campus chilled water connection can have on building
campus chilled water connection are shown in Figure 12.
performance.
From the observations noted above, the building chilled
The building connection being evaluated is located
water supply is being substantially blended with building
within a bookstore at a university in Northern
return water to supply very warm water to the building
California that had a campus chilled water TES syschilled water coils. The elevated chilled water supply temtem. The bookstore occupants initially reported losperature to the coils eventually led to a degrading chilled
ing the ability to provide cooling during summer con- water ΔT "death spiral" involving starved coils asking for
ditions. Additionally, the loss in cooling capacity was
more water, which ultimately leads to a loss in control of
observed to coincide with testing and balancing activ- supply air temperature and very low chilled water ΔT, as
ities on the chilled water system for an adjacent new- low as 0.3°F (0.2°C) in this particular situation.
construction building. The building operators iniUpon further inspection of the field observation data,
tially believed the solution to this issue was restricting it was noted that the chilled water return temperature
the amount of flow that the new neighboring building control valve was overridden to be fully open under these
could draw from the campus. However, the solution
conditions to promote injection of campus chilled water
ended up being modifying the configuration of the
into the building distribution. However, the essentially
74

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