ASHRAE Journal - May 2021 - 44

COLUMN ENGINEER'S NOTEBOOK

Setpoint Dynamics
The setpoint itself is frequently in a state of flux
to optimize the plant. This moving target becomes
another variable impacting control process stability,
introducing lags via the resetting sensing mechanism
and the network dynamics transmitting the resetting information to the primary control process. The
absorption chiller adds a " wrinkle " to the plant in
our example. With its mix of technologies, this plant
requires a different setpoint strategy that varies with
the mix of chillers online.

Efficiency Dynamics
Condenser water reset strategies are a trade-off
between improving compressor kW per ton at the cost
of increased tower fan energy. On a day with design heat
rejection and wet-bulb temperature, the cooling tower
fans will operate at full speed to deliver the design water
supply temperature. If the wet-bulb temperature drops,
the towers could deliver colder water if the fan speed
remained constant (Figure 2).
Reset strategies leverage this characteristic, spending fan energy to save compressor energy on off-design
days, which is not a " one-size-fits all " affair. For example, in a recent life-cycle cost analysis I worked through,
a tower selected to reject 9,365,000 Btu/h (2745 kW)
and deliver 85°F (29°C) water with a 74.2°F (23.4°C)
wet-bulb temperature, targeting lowest first cost, had a
40 hp (30 kW) fan motor. A tower selected for the same
conditions but targeting best life-cycle cost had a 15 hp
(11 kW) fan motor.
The cost premium had a 6.9 year simple payback,
which was acceptable to the owner given the 15-plus
year anticipated remaining plant life.II But because of
the difference in fan energy spent to save compressor
energy, the reset schedule for the plant with the lowest
first-cost tower will be different from the one used for
the best life-cycle cost tower.

FIGURE 2 Typical cooling tower performance (blue dot = design).

Cold Water Temperature (°F)

a way to bring the chiller online if it is starting with
tower basins full of cold water due to their tendency to
approach the ambient wet-bulb temperature when idle.
For both machines, it allows supply temperature to be
tempered under low ambient wet-bulb conditions when
the natural draft of the tower provides too much cooling.

90
88
86
84
82
80
78
76
74
72
70
68
58

12°F Range
10°F Range
8°F Range
Design Point

Predicted Performance
Fan Motor Alternative = Full Speed, 30 bhp
Flow Rate = 3,300 U.S. gpm (100% of Design)
60

66 68 70 72 74 76
Wet-Bulb Temperature (°F)

82 84

These Performance Curves are Based on Constant Fan Power

Wait, There's More!
The chiller kW/ton profile for the life-cycle cost analysis varied with condenser water temperature, was nonlinear and was machine-dependent (similar, but not
identical chillers [Figure 3]).
The amount of fan energy you could afford to spend
to save chiller energy varied with the tower selection, the chiller load,the combination of machines
online and the ambient wet-bulb temperature. A
reset schedule that was crafted without taking all
the variables into consideration might not deliver as
intended.
For math-phobics like me, that's really intimidating.
Fortunately, the building systems know the answer; you
just need to ask, which was the plan for this project. We
provided the structure for a reset schedule in the control
logic and then used functional testing and trend data to
establish the optimal setpoints after verifying the basic
functionality of the new tower cells.

Control Strategies-Option 1
Returning to the case study, a very common approach
to controlling a tower bypass valve is to sequence it with
the fans based on the temperature supplied to the plant.
This approach was specified for the plant discussed
here. This seems extremely logical: you need to maintain the desired temperature at Point 4 (Figure 1), and you
have three mechanisms to accomplish it. Sequencing
the bypass valves and fans to maintain setpoint seems to
make sense.

IIIn this case, the space and structure required for the physically larger tower associated with best life-cycle cost were not a concern.
That is not always the case.

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M AY 2021

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ASHRAE Journal - May 2021

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