ASHRAE Journal - May 2021 - 46

COLUMN ENGINEER'S NOTEBOOK

1.4

1.4

1.2

1.2

1.0

1.0

0.8

0.8

kW/ton

kW/ton

FIGURE 3 Chiller kW per ton profiles for the two similar but not identical chillers served by the tower in the life-cycle cost analysis example.

0.6

0.6

0.4

0.4

0.2

0.2

0

0

100

200

300
tons

550 ton VFD Centrifugal, 85°F EWT
550 ton VFD Centrifugal, 75°F EWT
550 ton VFD Centrifugal, 65°F EWT

Enter Transportation Lags

400

500

600

0

0

100

200

300

400

500

600

700

tons
550 ton VFD Centrifugal, 80°F EWT
550 ton VFD Centrifugal, 70°F EWT

600 ton VFD Centrifugal, 85°F EWT
600 ton VFD Centrifugal, 75°F EWT
600 ton VFD Centrifugal, 65°F EWT

600 ton VFD Centrifugal, 80°F EWT
600 ton VFD Centrifugal, 70°F EWT

TABLE 1 Transportation times for the different operating conditions associated with the system illustrated in Figure 1.

The variables discussed to this
point may seem obvious. But the
FLOW
transportation lag may not " jump
CONDITION
RATE
(GPM)
out " until you consider the large
volume of water in the piping and
tower basins. That is how it was
Absorption Chiller Only 2,320
for me the first time I recognized
this phenomenon while commis- Centrifugal Chiller Only 1,120
Modular Chiller Only
200
sioning one of my first central
plant control system designs.
Absoprtion Chiller + 2,520
Modular Chiller
Table 1 illustrates the approxiCentrifugal Chiller + 1,320
mate transportation times for a
Modular Chiller
parcel of water moving between
All Chillers 3,640
the indicated points in our system, which varies with operating
mode. As a result, a tuning solution that works well for
one mode might not work for others.
Focusing our discussion on the most common operating mode (centrifugal chiller plus modular chiller), we
discover that if the tower fan were to cycle off, it might
take 2.4 minutes for a parcel of water that had just fallen
off the fill into the cold basin (Point 2 in Figure 1) to reach
the location of the temperature sensor controlling the
process using the Option 1 strategy (Point 4 in Figure 1).
That means the cooling tower fan cycled off based on a

TRANSPORTATION TIME, MINUTES
BYPASS VALVE
DISCHARGE TO TOWER
TO REMOTE SENSOR
(FIGURE 1, POINTS 1
TO 4, 5,282 GALLONS)

BYPASS VALVE
DISCHARGE TO
TOWER TO COLD
BASIN (FIGURE 1,
POINTS 1 TO 2,
2,109 GALLONS

TOWER BASIN TO
REMOTE SENSOR
(FIGURE 1, POINTS
2 TO 4, 3,173
GALLONS)

TOWER BASIN TO LOCAL
SENSOR (FIGURE 1,
POINTS 2 TO 3, 1,797
GALLONS)

2.28

0.91

1.37

0.77

4.72

1.88

2.83

1.60

26.41

10.55

15.86

8.98

2.10

0.84

1.26

0.71

4.00

1.60

2.40

1.36

1.45

0.58

0.87

0.49

condition that existed more than 2.4 minutes ago-more
because those numbers do not take into account the lag
introduced by the mass of the sensor and well.**
Let's assume the current setpoint is steady at 65°F
(18°C) and that the system is steady state at a load condition, generating a 4°F (2.2°C) rise across the condenser
(69°F [21°C] return temperature). Because the system is
steady state, the fan speed will be in balance at a point
that causes the 69°F (21°C) water flowing into the hot
basins to drop to 65°F (18°C) by the time it falls off the fill
into the cold basin. Thus, the cold basin, and the piping

discussion is also conservative because Table 1 focuses on the common piping and does not include the volume contained in the
connections between the common pipe and the chiller and the condenser barrel.

**This

46

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