ASHRAE Journal - May 2021 - 48

COLUMN ENGINEER'S NOTEBOOK

FIGURE 5 One vs. two loop functional block diagrams. Note how Option 1 has one loop, setpoint and input, while Option 2 has two loops with different inputs but a common setpoint.

Input
Condenser Water
Supply Temperature
Sensor Downstream
from the Bypass
Connection
PI Loop
Output Goes From
0% to 100% as the
Need for Cooling
Increases

Option 1

Interlock
Output = Input if
CW System is ON;
Output = 0% if CW
System is OFF

Bypass Valve
System
0% = Full Bypass
25% = Full Flow to
the Cooling Towers
Variable Speed
Fan-West Cell
30% = ON
40% to 100% =
Minimum Speed to
Full Speed

Setpoint
85°F

Variable Speed
Fan-East Cell
30% = On
40% to 100% =
Minimum Speed to
Full Speed

A More Informed Solution
If you study Table 1 in the context of Option 2, you will
recognize a significant lag in the process still exists due
to the volume of water in the tower cold basins. Using a
strategy based on the basin temperature seems desirable.
Experience indicates this is more complicated than
it seems. One lesson learned in my experience is that
water falling through a cooling tower is like a gentle
rain; it stratifies. More specifically, if a tower fan is off
and then starts, there will be an immediate impact on
the temperature of the water falling off the fill relative
to the temperature of the water entering the fill. But,
because this water is falling gently into the cold basin,
initially the temperature of the water in the top " layer "
will be different from that in the bottom " layer. "
On that project, tower cells installed over a 4 ft (1.2 m)
deep basin taught me this. When I probed its depths by
attaching a weight to my temperature sensor, I discovered stratification.
As a result, I recommend that
* Each cell be controlled based on its cold basin temperature;
48

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Input
Condenser Water
Supply Temperature
Sensor Downstream
from the Bypass
Connection

Option 2

PI Loop
Output Goes From
0% to 100% as the
Need for Cooling
Increases

Interlock
Output = Input if
CW System is ON;
Output = 0% of CW
System is OFF

Bypass Valve
System
0% = Full Bypass
100% = Full Flow
to the Cooling
Towers

Interlock
Output = Input if
CW System is ON;
Output = 0% of CW
System is OFF

Variable Speed
Fan-East Cell
5% = ON
15% to 100% =
Minimum Speed to
Full Speed

Setpoint
85°F
PI Loop
Output Goes From
0% to 100% as the
Need for Cooling
Increases
Input
Roof Level
Condenser Water
Supply Temperature
Sensor Downstream
From the Cooling
Towers

Variable Speed
Fan-West Cell
10% = ON
15% to 100% =
Minimum Speed to
Full Speed

* Measuring the cold basin temperature using a rigid
averaging probe with length matched to the basin's
minimum operating level; and
* If possible, finalize sensor location based on field
testing.
The logic required is like the Option 2 logic, but with
a loop for each cell. All contingencies mentioned for
Option 2 apply.

Conclusion
There are a lot of dynamics to consider when developing logic to control a large condenser water system. That
is part of the fun and fascination associated with control
system design and facility =operations. But as David St.
Clair and Mother Nature point out, success ultimately
involves recognizing and addressing the lags.

References

1. St. Clair, D. 1993. Controller Tuning and Control Loop
Performance, a Primer, 2nd Edition, P. 56. Self-Published. http://www.
straightlinecontrol.com.
2. Taylor, S. 2012. " Optimizing design & control of chilled water
plants, part 5. " ASHRAE Journal (6):56.
3. Peterson, K. 2018. " Avoiding centrifugal chiller surge. " ASHRAE
Journal (11).

https://www.straightlinecontrol.com https://www.straightlinecontrol.com http://www.ashrae.org

ASHRAE Journal - May 2021

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